PURIFIED CERAMIDE-ACTIVATED PROTEIN KINASE, ASSAY FOR IDENTIFYING AGENTS WHICH ACT ON SAME, AND METHODS OF USING SAID AGENTS

Abstract: A purified membrane-bound ceramide-activated protein kinase having an apparent molecular weight of about 97 kD as determined by SDS polyacrylamide gel electrophoresis is provided. This protein kinase specifically phosphorylates the threonine residue in a Pro-Leu-Thr-Pro containing polypeptide. A method of determining whether an agent is capable of specifically inhibiting the phosphorylation activity of the ceramide-activated protein kinase is also provided. A method of determining whether an agent is capable of specifically stimulating the phosphorylation activity of the ceramide-activated protein kinase is further provided. A method of treating a subject having an inflammatory disorder is further provided. A method of treating a human subject infected with HIV so as to reduce the proliferation of HIV in the human subject is further provided. Finally, a method of treating a subject having a disorder associated with poor stem cell growth is provided. The figure shows the effect of phospholipases on the ceramide-activate protein kinase catalyzed phosphorylation in the presence of TNF- alpha .

Description: PURIFIED CERAMIDE-ACTIVATED PROTEIN KINASE, ASSAY FOR IDENTIFYING AGENTS WHICH ACT ON SAME. AND METHODS OF USING SAID AGENTS This invention was made with support under Grant Nos. ROl- CA-42385 and CA-09512-06 from the National Institute of Health. Accordingly, the United States government has certain rights in the invention. Background of the Invention Throughout this application, various publications are referenced by Arabic numerals. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosure of these publications is hereby incorporated by reference into this application to describe more fully the art to which this invention pertains. Recent investigations have identified a metabolic pathway involving sphingomyelin and derivatives that may be involved in signal transduction (1-8) . This pathway is initiated by the hydrolysis of sphingomyelin to ceramide via the action of a sphingomyelinase. Ceramide may then be deacylated to sphingoid bases, putative inhibitors of protein kinase C (9- 12) , or phosphorylated to the sphingolipid ceramide 1- phosphate by the action of a recently described calcium- dependent ceramide kinase (4, 5, 13) . The biologic role of ceramide 1-phosphate and regulation of the kinase that mediates its synthesis have not yet been determined. This pathway appears specific for ceramide derived from sphingomyelin, as ceramide derived from glycosphingolipids is not converted either to sphingoid bases (14) or to ceramide l-phosphate (4) . Recently, Hannun and coworkers (6-8) have provided evidence that this sphingomyelin pathway may be involved in signal transduction. Tumor necrosis factor (TNF) α, interferon, and 1,25-dihydroxyvitamin D 3 , factors that induce monocytic differentiation of H -60 promyelocytic cells, all stimulate sphingomyelin degradation to ceramide as an early event in cellular activation (6-8) . A synthetic ceramide N- acetylsphingosine could replace these agents in induction of monocytic differentiation of these cells. Furthermore, there have also been numerous reports that TNF and IL-1 stimulate a common set of events in diverse biological systems (60) . Direct evidence for second-messenger function for ceramide has also been shown. Davis and coworkers (15-17) originally showed that sphingosine induced epidermal growth factor receptor (EGFR) phosphorylation on Thr-669 in A-431 human epidermoid carcinoma cells by a mechanism that did not involve protein kinase C. It was demonstrated that sphingosine was rapidly converted to ceramide by these cells and that ceramide induced identical phosphorylation (18) . These studies were interpreted as evidence that ceramide had bioeffector properties, and might mediate, in part, the action of exogenous sphingosine. However, prior to the subject invention, no kinase was identified capable of mediating the effects of ceramide as a second messenger. The subject invention provides a purified ceramide-activated protein kinase which functions as a key element in a sphingomyelin pathway utilizing ceramide as a second messenger. The knowledge that a ceramide-activated protein kinase exists as part of the sphingomyelin pathway enables the treatment of certain disorders by selectively modifying the function of this kinase in appropriate cells. Such disorders where this approach is possible include, by way of example, HIV infection, inflammatory disorders and disorders associated with poor stem cell growth. Accordingly, the subject invention provides methods of treating subjects having such disorders with agents capable of modifying the activity of ceramide-activated protein kinase, and methods of identifying such agents. Summary of the Invention The subject invention provides a purified membrane-bound ceramide-activated protein kinase having an apparent molecular weight of about 97 kD as determined by SDS polyacrylamide gel electrophoresis, which protein kinase is capable of specifically phosphorylating the threonine residue in a Pro-Leu-Thr-Pro-containing polypeptide. The subject invention also provides a method of determining whether an agent is capable of specifically inhibiting the phosphorylation activity of the ceramide-activated protein kinase of the subject invention which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Le - Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro-containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of inhibiting the activity of the ceramide- activated protein kinase; and (c) determining whether the agent inhibits the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically inhibiting the activity of the ceramide- activated protein kinase. The subject invention further provides a method of determining whether an agent is capable of specifically stimulating the phosphorylation activity of the ceramide- activated protein kinase of the subject invention which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro- containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of stimulating the activity of the ceramide-activated protein kinase; and (c) determining whether the agent stimulates the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically stimulating the activity of the ceramide-activated protein kinase. The subject invention further provides a method of treating a subject having an inflammatory disorder which comprises administering to the subject an agent capable of inhibiting the phosphorylation activity of a ceramide-activated protein kinase of T helper cells and macrophage cells of the subject in an amount effective to inhibit said phosphorylation activity, thereby reducing the inflammation associated with the disorder. The subject invention further provides a method of treating a human subject infected with HIV so as to reduce the proliferation of HIV in the human subject which comprises administering to the human subject an agent capable of inhibiting the phosphorylation activity of a ceramide- activated protein kinase of the HIV-infected cells of the human subject in an amount effective to inhibit said activity, thereby reducing the proliferation of HIV in the human subject. Finally, the subject invention provides a method of treating a subject having a disorder associated with poor stem cell growth, which comprises administering to the subject an agent capable of stimulating the phosphorylation activity of a ceramide-activated protein kinase of the stem cells of the subject in an amount effective to stimulate said phosphorylation activity, thereby stimulating stem cell growth. Brief Description of the Figures Figure 1 Kinetics of 32 P, incorporation into the EGFR peptide. Peptide phosphorylation was done in a reaction mixture containing 25 μl of postnuclear supernatant (220 μg of protein) from A-431 cells, 50 μl of EGFR peptide (4 mg/ml in 25 mM Hepes, pH 7.4), and 125 μl of reaction buffer (50 mM Hepes, pH 7.4/20 mM MgCl 2 ) . The reaction was initiated by addition of 50 μl of [7- 32 P]ATP (150 μM final concentration) and terminated by addition of 50 μl of 0.5 M ATP in 90% (vol/vol) formic acid. Samples were spotted on phosphocellulose paper, washed with 1 M acetic acid/4 mM pyrophosphate, and 32 P incorporation was measured by liquid- scintillation counting, as described (17, 25) . A boiled protein blank was subtracted from each data value. The data (means) represent duplicate samples from one of two similar experiments. Figure 2 Mg 2+ concentration-dependence of EGFR peptide phosphorylation. These studies were done as described for Figure 1, using microsomal membrane (7.5 μg/μl) as the source of kinase activity. Reaction mixtures received various concentrations of Mg + (0.1-25 mM final concentration) , and reactions were terminated at 2 minutes. Phosphorylated peptide was isolated by HPLC and quantified by Cerenkov counting. The dimensions of velocity (V) are pmol-min'Vπig of protein "1 . Data represent values derived from one of three similar experiments. Figure 3 Identification of phosphorylated EGFR peptide. Reactions were done, and phosphorylated peptide was quantified as for Figure 2. (Left) HPLC elution profile of samples with or without the EGFR peptide. (Right) Phosphoamino acid analysis of the phosphorylated peptide purified by HPLC. Phosphorylated amino acids (Y, tyrosinase; T, threonine; S, serine) were resolved by one-dimensional thin-layer electrophoresis and identified by ninhydrin staining of carrier phosphoamino acids and autoradiography. Figure 4 Kinetics of ceramide-induced 32 P ; incorporation into EGFR peptide. Peptide phosphorylation was done as described in Figure 2 in the absence (O) or presence (•) of 0.5 μM C 8 - ceramide. Phosphorylated peptide was resolved by HPLC. Values (means) represent data from three experiments. Figure 5 Concentration-dependence of ceramide-induced 32 P t incorporation into EGFR peptide. Peptide phosphorylation reactions were done as described for Figure 2, for 2 minutes, using various concentrations of ceramide (0.001-3 μM) . Phosphorylated peptide was resolved by HPLC. Values (means) represent data from three experiments. Figure 6 Concentration-dependence of sphingosine-induced 32 P, incorporation into EGFR peptide. Peptide phosphorylation reactions were done and analyzed as described in Figure 5, using various concentrations of sphingosine (0.001-3 μM) . Values (means) represent data from two experiments. Figure 7 Kinetics of TNF-o;-induced 32 P, incorporation into EGFR peptide. HL-60 cells were resuspended in RPMI 1640 medium (1 X 10 6 cells per ml) containing 1% FBS for 2 hours before stimulation with TNF-α (30 nM) . At the indicated times, cells were centrifuged at 500 X g for 5 minutes, and the cell pellet was homogenized in buffer, as described. Portions of a microsomal membrane fraction were used in the kinase assay, as described for Figure 2. Values (means) represent data derived from two separate experiments. Figure 8 Concentration-dependence of TNF-α-induced 32 P ; incorporation into EGFR peptide. These studies were done as described for Figure 7 with various concentrations of TNF-α for 60 minutes of stimulation. Values (means) represent data derived from two separate experiments. Figures 9A and 9B TNF-α effects on sphingomyelin (9A) and ceramide (9B) concentrations in a cell-free system. HL-60 cells were grown in RPMI 1640 medium supplemented with 10% bovine calf serum and amino acids (4) . To measure sphingomyelin, cells were resuspended (1 x 10 6 ml "1 ) , labeled for 48 hours in medium with [ 3 H]choline (1 μCi ml "1 ) (57), in serum-free medium containing bovine insulin (5 μg ml "1 ) and human transferrin (5 μg ml "1 ) . After 3 hours, cells were resuspended (150 x 10 6 ml "1 ) in homogenization buffer (50 mM NaF, 5 mM EGTA, and 25 mM Hepes, pH 7.4) , disrupted at 4°C with 150 strokes of a tight-fitting Dounce homogenizer (Fisher Scientific, Pittsburgh, Pennsylvania) , and centrifuged for 5 minutes (500g) . The nuclei-free supernate was first incubated for 5 minutes at 4°C with 30 nM human TNF- α (Genentech, South San Francisco, California) or diluent (50 M Hepes, pH 7.4) . At time zero, 15 μl of supernate (112 μg per incubation) were added to a reaction mixture containing 30 μl of 25 mM Hepes, pH 7.4, 30 μl of 750 μM ATP, and 75 μl of reaction buffer (50 mM Hepes, p 7.4 and 20 mM MgCl 2 ) at 22°C. The reaction was terminate with CHC1 3 :CH 3 0H:HC1 (100:100:1, v/v/v) (3, 4, 13) and 150 μl of balanced salt solution (135 mM NaCl, 4.5 mM KC1, 1.5 m CaCl 2 , 0.5 mM MgCl 2 , 5.6 m glucose, and 10 mM Hepes, pH 7.2) containing 20 mM EDTA. Lipids in the organic phase extract were subjected to alkaline methanolysis to remove glycerophospholipids (4) . Sphingomyelin recovery in the nuclei-free supernate was 93% of that in intact cells. measure of 10 6 cell equivalents of supernate contained 50 μg of protein. Sphingomyelin was resolved by thin-layer chromatograhy (TLC) with CHC1 3 :CH 3 OH:CH 3 COOH:H 2 0 (25:15:4:1.5) as solvent, identified by iodine vapor staining and quantified by liquid scintillation spectrometry (1, 8, 56) . Ceramide was quantified with the diacyglycerol kinase reaction (4, 57) . Values (mean) are derived from triplicate determinations from one experiment representative of three similar studies for sphingomyelin and four similar studies for ceramide. Figures 10A and 10B Effect of TNF-α on ceramide-activated protein kinase activity. HL-60 cells were incubated in serum-free medium and homogenized (as in Figures 9A and 9B) . After an initial incubation with TNF-α, 15 μl of nuclei-free supernate (112 μg per incubation) were added to a reaction mixture containing 30 μl of EGFR peptide (4 mg ml "1 in 25 mM Hepes, pH 7.4), 30 μl of [7-P 32 ] ATP (750 μM, 4000 dpm pmol "1 ) , and 75 μl of reaction buffer (40) . The reaction was terminated by adding 30 μl of 0.5 M ATP in 90% formic acid. Phosphorylated peptide was first run on a C 18 Sep-Pak cartridge, then resolved by C 18 reverse-phase HPLC (Waters, Milford, Massachusetts) , with a linear gradient of acetonitrile. The peptide eluted at 30% acetonitrile, as determined by monitoring Cerenkov radiation in l-ml fractions. Background activity was subtracted from each point. (10A) Kinetics of TNF-α (30 nM) -stimulated EGFR peptide phosphorylation. Values (mean) represent data from four experiments. (10B) Concentration dependence of EGFR peptide phosphorylation at 5 minutes of stimulation with TNF-α (0.01 to 30 nM) . Values (mean) represent data derived from duplicate points in two experiments. The SEM of the values in (10A) was 18% and the mean range of values in (10B) was 3%. Figure 11 Effect of phospholipases on ceramide-activated protein kinase activity. Nuclei-free supernates, prepared as in Figures 9A and 9B, were first incubated with TNF-α (3 nM) or added directly to reaction mixtures that contained various phospholipases; sphingomyelinase (SMase) (1 x 10 "3 U ml "1 , S. aureus) , phospholipase Aj (PlAj) (3.8 x 10" 2 and 3.8 x 10" 1 U ml" 1 , Vipera ruselli) , phospholipase C (PLC) (3.8 x 10" 2 U ml" l , Bacillus cereus) and phospholipase D (PLD) (3.8 x 10" 2 U ml" 1 , Streptomyces chromocuscus) . Peptide phosphorylation was measured as in Figures 10A and 10B. Control value represents peptide phosphorylation in the absence of phospholipases or TNF-α. Values (mean ± SEM) represent data derived from duplicate samples in three experiments. *P < 0.001 compared to control. Figure 12A and 12B IL-1S effects on sphingomyelin levels in EL4 cells. Time course (12A) and dose response (12B) . Cells were grown to growth arrest (1-1.5 x 10 6 cells ml "1 ) in DME/Ham's F12 medium (1:1, v/v) containing 10% horse serum and for 48 hours [ 3 H] choline (lμCi ml "1 ) . On the day of an experiment, cells were resuspended back into the same medium at 10 x 10 6 cells ml "1 and stimulated with 40 ng ml "1 IL-lβ fcr the indicated times (12A) or for 30 minutes with increasing concentrations of IL-lβ (12B) . Human IL-lβ may be obtained using methods well known to those skilled in the art. Reactions were terminated with CHC1 3 :CH 3 0H:HC1 (100:100:1) containing 10 mM EDTA (82) . Lipids in the organic phase extract were dried under N 2 and subjected to mild alkaline hydrolysis (0.1 M methanolic KOH for 1 hour at 37°C) to remove glycerophospholipids. Spingomyelin was resolved by thin-layer chromatography (TLC) using CHC1 3 :CH 3 0H:CH 3 COOH:H 2 0 (60:30:8:5) as solvent, identified by iodine vapor staining, and quantified by liquid scintillation spectrometry. As previously reported, the use of [ 3 H] choline as a measure of sphingomyelin content was validated by simultaneous phospholipid phosphorus measurements (62) . Each value represents the mean ± SEM of triplicate determinations from four experiments in (12A) , and one representative of four similar studies performed in triplicate in (12B) . Figure 13 Effect of IL-lβ on ceramide levels in EL-4 cells. Cells were stimulated as in Figures 12A and 12B with IL-lβ (40 ng ml "1 ) and ceramide contained within the organic phase extract quantified enzymatically using the E. coli diacylglycerol kinase reaction (57) . Lipids were resolved by TLC using CHCl 3 :CH 3 OH:CH 3 COOH (65/15/5) as solvent, autoradiographed and quantified by liquid scintillation spectrometry. Each value represents the mean ± SEM of triplicate determinations from 10 experiments. Figure 14 Effect of IL-lβ on ceramide-activated protein kinase activity. Cells (30 x 10 6 ml "1 ) , handled as in Figures 12A and 12B, were stimulated with IL-lβ (10 ng ml "1 ) and homogenized at 4°C with a Dounce homogenizer in buffer (25 mM HEPES, pH 7.4, 5 mM EGTA, 50 mM NaF and 10 μg/ml each of leupeptin and soybean trypsin inhibitor) . Homogenates were centrifuged at 500 x g for 5 minutes to remove nuclei and at 200,000 x g for 30 minutes to prepare microsomal membranes. Membranes were resuspended into homogenizing buffer (2.2 μg membrane protein μl "1 ) . For assay of kinase activity, the reaction mixture contained 20 μl of microsomal membrane, 40 μl EGFR peptide (4 mg ml "1 in 25 mM Hepes, pH 7.4) and 100 μl buffer (50 mM HEPES, pH 7.4, 20 mM MgCl 2 ) (40) . Phosphorylation was initiated at 22°C by addition of 40 μl [7- 32 P] ATP (100 μM final concentration) and terminated at the indicated times by addition of 40 μl of 0.5 M ATP in 90% formic acid. Phosphorylated peptide was eluted from a Cu Sep pak cartridge (Millipore) , lyophilized, and resolved by C 18 reverse phase HPLC using a linear gradient of acetonitrile. The peptide eluted at 30% acetonitrile as determined by measuring Cerenkov radiation in 1 ml fractions. All assays were performed under conditions determined as linear for time and enzyme concentration. Enzyme activity was determined from the percent conversion of substrate to product and the specific radioactivity of [γ. 32 P ] ATP. Baseline kinetic analyses revealed a maximum reaction velocity of 12.5 pmol min "1 mg "1 of microsomal membrane protein and Michaelis constants (K, of 70 μM ATP and 0.15 mg/ml for EGFR peptide. For most studies, 100 μM ATP was used to maintain a high 32 P specific radioactivity (4000 dpm pmol "1 ) , although qualitatively similar results were obtained with 500 μM ATP. Ceramide and sphingosine (10 nM to 1 μM) enhanced kinase activity to 1.5-2.5 of control. Values (mean ± range) represent duplicate determinations from two experiments. Figures 15A and 15B IL-lβ effects on sphingomyelin and ceramide levels (15A) and ceramide-activated protein kinase activity (15B) in a cell- free system. Nuclei-free supernates, prepared as in Figure 14, were incubated for 10 minutes at 4°C with IL-lβ (10 ng ml" 1 ) or diluent (DME:F12 with 10% horse serum) to allow for ligand-receptor interaction. Thereafter, supernates (300 μg incubation "1 in 25 μl) were added to a reaction mixture (total volume 250 μl) as described in Figure 14. For studies measuring lipid levels, incubations were stopped by extraction of lipids into an organic phase and resolved as described in Figures 12A, 12B and 13. For studies measuring kinase activity, incubations contained EGFR peptide and [ 32 P]ATP, and phosphorylated peptide was quantified as described in Figure 14. Background activity was subtracted from each point. Values (mean) represent data from two experiments for sphingomyelin performed in triplicate, three experiments for ceramide performed in triplicate, and five experiments for ceramide-activated protein kinase activity performed in duplicate. Figures 16A and 16B Fractionation of protein kinase activity toward the EGFR peptide (16A) and PKC (16B) activity by anion exchange chromatography. Membrane proteins (1 mg) prepared from HL-60 cells were solubilized in 1% Triton X-100 and chromatographed on a 0.2 ml DE52 anion exchange column. The proteins bound to the column were eluted with 1 ml washes of NaCl in a stepwise concentration gradient. Fractions were assayed for ceramide-activated protein kinase activity by phosphorylation of the EGFR peptide and for PKC activity by histone phosphorylation, as described in Experimental Details. Data are representative of three experiments. Figure 17 SUBSTITUTESHEET Phosphorylation of yelin basic protein by fraction containing activity toward the EGFR peptide. Portions o membrane proteins (5 ml/assay) prepared from HL-60 cells b fractionation on a DE52 anion exchange column were incubate with MBP (12.5 mg/assay) in the presence of [7- 3 P]ATP (50 m final concentration) . Phosphorylated MBP was resolved on 15% SDS-polyacrylamide gel and autoradiographed as describe in Experimental Details. Data are representative of fiv experiments. Figure 18 Isolation of the kinase activity toward the EGFR peptide b isoelectric focusing (IEF) . The flow-through fraction o the DE52 column containing kinase activity toward the EGF peptide was subjected to IEF. The pH was measured and kinas activity of each fraction was analyzed as described i Experimental Details. Lane 1 is the flow-through fractio of the DE52 column mixed with ampholytes (2%) , Triton X-100 (1%) and glycerol (5%) . Lanes 2-20 represent fractions obtained by IEF. The molecular weight of phosphorylated MBP is 18.5 kD. Data represent one of five identical experiments. Figure 19 Phosphorylation of a 97 kD protein in IEF fractions containing activity toward the EGFR peptide. Fraction 12 obtained by isoelectric focusing as above was incubated with [γ- 32 P]ATP (50 mM final concentration) in the presence (+) or absence (-) of MBP (1 mg ml "1 ) for 15 min and electrophoresed. The phosphorylated MBP was allowed to run off the gel. Data represent one of three separate experiments. Figure 20 SUBSTITUTE SHEET Renaturation of the kinase activity detected by isoelectric focusing. Intact membrane (125 mg) , cytosol (250 mg) and flow-through fraction (100 mg) of the DE52 column were electrophoresed on a 10% SDS-polyacrylamide gel. The kinase activity was renatured and autophosphorylation was performed as in Experimental Procedures by incubation of the gel for 1 h in a reaction buffer containing [7- 32 P]ATP. The gel was then autoradiographed. Data represent one of four experiments. Figure 21 Enhancement of 97 kD protein kinase activity by ceramide. HL-60 cells (1 x 10 6 ml "1 ) were treated with C8-ceramide for 15 min at 37°C. Membrane proteins (300 mg/lane) from untreated and ceramide-treated cells were subjected to SDS-polyacrylamide gel electrophoresis. Kinase activity was renatured and autophosphorylation and autoradiography of the gel were performed as in Figure 20. Lane 1, control; lane 2, 0.03 mM C8-ceramide; lane 3, 0.3 mM C8-ceramide; lane 4, 3 mM C8-ceramide. Data are representative of six experiments. Figure 22 Enhancement of 97 kD protein kinase activity by TNFα. HL-60 cells (1 x 10 6 ml "1 ) were pre-incubated in serum-free media for 2 h, then treated with TNFα for 20 min at 37°C. Membrane proteins (170 mg/lane) from untreated and TNF-treated cells were subjected to SDS-polyacrylamide gel electrophoresis. Kinase activity was renatured as in Figure 20 and autophosphorylation was performed for varying lengths of time. C, Control; T, TNFα-stimulated cells. Data are representative of 3 experiments. Figure 23 Enhancement of MBP phosphorylation by renatured kinase from TNF-stimulated cells. Membrane proteins (170 mg/lane) from untreated and TNF-treated cells were subjected to SDS-PAGE and kinase activity was renatured as in Figure 22. Autophosphorylation was allowed to proceed for 10 min. Gel slices were then excised from regions corresponding to 97 D and 75-90 kD, crushed, and incubated for 1 h with 10 ml MBP (5 mg ml '1 ) and 40 ml kinase reaction mixture in the presence of [7- 32 P]ATP (50 mM final concentration) . Reactions were terminated by addition of 10 ml Laemmli buffer, boiling for 2 min and centrifugation to precipitate gel particles. The supernatants were electrophoresed and autoradiography was performed as in Figure 17. Detailed Description of the Invention Specifically, the subject invention provides a purified membrane-bound ceramide-activated protein kinase having an apparent molecular weight of about 97 kP as determined by SPS polyacrylamide gel electrophoresis, which protein kinase is capable of specifically phosphorylating the threonine residue in a Pro-Leu-Thr-Pro-containing polypeptide. As used herein, "purified" means free of any other protein kinases. For example, the purified membrane-bound ceramide- activated protein kinase may include the protein kinase, membrane fragments, other non-kinase proteins, and a suitable buffer. Alternatively, the purified membrane-bound ceramide-activated protein kinase may include only the protein kinase bound by a membrane and a suitable buffer. By way of example, the membrane-bound ceramide-activated protein kinase of the subject invention may be purified by (a) solubilizing the protein kinase from the membrane, (b) separating the protein kinase from strong anions, and from protein kinase C and MAP kinases by PE52 anion exchange chromatography, (c) performing preparative SPS-gel electrophoresis based on conditions determined from a denaturation/renaturation reaction, (d) performing a high resolution isoelectric focussing using a Rotofor apparatus, (e) performing strong anion exchange chromatography by HPLC, (f) performing hydrophobic column chromatography by HPLC, and (g) performing continuous elution electrophoresis, thereby purifying the protein kinase. The purified protein kinase may then be affixed to a membrane for proper kinase function. As used herein, "ceramide-activated" means having activity which is accelerated by the presence of ceramide. Specifically, the protein kinase of the subject invention is capable of phosphorylating certain protein substrates (e.g. human epidermal growth factor receptor) if the kinase is membrane-bound, and is in the presence of Mg +2 and ATP. However, the rate at which the protein kinase phosphorylates its protein substrate is increased by the presence of ceramide. The purified protein kinase of the subject invention comprises a single peptide chain having an apparent molecular weight of approximately 97 kD as determined by SPS polyacrylamide gel electrophoresis. There are numerous means of determining the molecular weight of a particular protein, some methods yielding slightly differing molecular weights for the same protein. For example, an earlier measurement of the molecular weight of the protein kinase of the subject invention was approximately 95 kP. The 97 kP molecular weight was determined using a denaturation/renaturation procedure well known to those skilled in the art. Briefly, the method involves running the protein of interest on a denaturing gel having substrate embedded therein, washing the gel, allowing the protein to renature, assaying for protein activity in situ thereby locating the protein on the gel, and comparing the location of the protein on the gel with that of molecular weight markers, thereby determining the molecular weight of the protein. As used herein, "specifically phosphorylating" means phosphorylating the threonine residue in a Pro-Leu-Thr-Pro- containing polypeptide without phosphorylating other amino acid residues which ordinarily serve as phosphate acceptors (e.g. serine and tyrosine) . As used herein, "polypeptide" means a single chain of amino acid residues. Accordingly, a Pro-Leu-Thr-Pro-containing polypeptide may be the polypeptide Pro-Leu-Thr-Pro or a larger peptide containing this amino acid sequence. The subject invention also provides a method of determining whether an agent is capable of specifically inhibiting the phosphorylation activity of the ceramide-activated protein kinase of the subject invention which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Leu- Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro-containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of inhibiting the activity of the ceramide- activated protein kinase; and (c) determining whether the agent inhibits the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically inhibiting the activity of the ceramide- activated protein kinase. As used herein, the term "agent" includes both protein and non-protein moieties. For example, the agent may be a ceramide analog or an antibody directed against a portion of the ceramide-activated protein kinase of the subject invention. As used herein, "capable of specifically inhibiting" means capable of reducing the phosphorylation activity of the ceramide-activated protein kinase of the subject invention by at least two-fold, but not capable of reducing the phosphorylation activity of a non-ceramide-activated protein kinase. As used herein, a "non-ceramide-activated protein kinase" is a protein kinase whose phosphorylation activity is not altered in the presence of ceramide. An example of a non-ceramide-activated protein kinase is protein kinase C. As used herein, "phosphorylation activity" means the rate at which a protein kinase phosphorylates its substrate. Accordingly, the phosphorylation activity of the ceramide- activated protein kinase of the subject invention means the rate at which the protein kinase phosphorylates the threonine residue in a Pro-Leu-Thr-Pro-containing polypeptide substrate. As used herein, conditions which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent are simply conditions in which Michaelis-Menten enzyme kinetics are observed. Specifically, Michaelis-Menten enzyme kinetics are observed when the enzyme concentration is low in comparison with that of the substrate, i.e. the enzyme concentration is rate-limiting, and the enzyme reaction has not yet approached completion. Quantitatively determining the number of threonine residues which are specifically phosphorylated may be achieved by measuring the kinase reaction rate while Michaelis-Menten kinetics are observed, and from the rate measurement, calculating the number of threonine residues which are specifically phosphorylated. Such methods of calculation are well known to those skilled in the art. An example of the method of the subject invention is provided infra. A rate-limiting amount of membrane-bound ceramide-activated protein kinase is contacted with X μg of polypeptide containing the amino acid sequence Pro-Leu-Thr- Pro, and having Y moles of threonine residues in the Pro- Leu-Thr-Pro sequence, together with an agent under conditions which would permit the phosphorylation of 0.1 x Y moles of threonine residues in the absence of the agent. In the presence of the agent, 0.05 x Y moles of threonine residues are phosphorylated. The agent is shown not to inhibit protein kinase C (a non-ceramide-activated protein kinase) activity using a histone III, substrate assay well known to those skilled in the art. Accordingly, the agent specifically inhibits the activity of the ceramide-activated protein kinase. In one embodiment of the subject invention, the polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro is human epidermal growth factor receptor. The subject invention further provides a method of determining whether an agent is capable of specifically stimulating the phosphorylation activity of the ceramide- activated protein kinase of the subject invention which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro- containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of stimulating the activity of the ceramide-activated protein kinase; and (c) determining whether the agent stimulates the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically stimulating the activity of the ceramide-activated protein kinase. As used herein, the term "agent" includes both protein and non-protein moieties. For example, the agent may be a ceramide analog, an antibody directed against a portion of the ceramide-activated protein kinase of the subject invention, tissue necrosis factor α or interleukin I. As used herein, "capable of specifically stimulating" means capable of increasing the phosphorylation activity of the ceramide-activated protein kinase of the subject invention by at least two-fold, but not capable of increasing the phosphorylation activity of a non-ceramide-activated protein kinase. In one embodiment of the subject invention, the polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro is human epidermal growth factor receptor. An example of the method of the subject invention is provided infra. A rate-limiting amount of membrane-bound ceramide-activated protein kinase is contacted with X μg of polypeptide containing the amino acid sequence Pro-Leu-Thr- Pro, and having Y moles of threonine residues in the Pro- Leu-Thr-Pro sequence, together with an agent under conditions which would permit the phosphorylation of 0.1 x Y moles of threonine residues in the absence of the agent. In the presence of the agent, 0.2 x Y moles of threonine residues are phosphorylated. The agent is shown not to stimulate protein kinase C (a non-ceramide-activated protein kinase) activity using a histone III, substrate assay well known to those skilled in the art. Accordingly, the agent specifically stimulates the activity of the ceramide- activated protein kinase. The subject invention further provides a method of treating a subject having an inflammatory disorder which comprises administering to the subject an agent capable of inhibiting the phosphorylation activity of a ceramide-activated protein kinase of T helper cells and acrophage cells of the subject in an amount effective to inhibit said phosphorylation activity, thereby reducing the inflammation associated with the disorder. In the preferred embodiment of the subject invention, the subject is a human. The inflammatory disorder may be rheumatoid arthritis, ulcerative colitis, graft versus host disease, lupus erythematosus or septic shock. In the practice of the subject invention, the administering of the agent may be effected or performed using any of the various methods known to those of skill in the art. For example, the administration may comprise administering intravenously, intramuscularly or subcutaneously. Further in the practice of the subject invention, the amount of agent effective to inhibit the phosphorylation activity of ceramide-activated protein kinase of T helper cells and macrophage cells of the subject means an amount capable of inhibiting the phosphorylation activity by at least two¬ fold. This amount may be calculated using any of the various methods known to those of skill in the art. The subject invention further provides a method of treating a human subject infected with HIV so as to reduce the proliferation of HIV in the human subject which comprises administering to the human subject an agent capable of inhibiting the phosphorylation activity of a ceramide- activated protein kinase of HIV-infected cells of the human subject in an amount effective to inhibit said activity, thereby reducing the proliferation of HIV in the human subject. In the practice of the subject invention, the administering of the agent may be effected or performed using any of the various methods known to those of skill in the art. For example, the administration may comprise administering intravenously, intramuscularly or subcutaneously. Further in the practice of the subject invention, the amount of agent effective to inhibit the phosphorylation activity of ceramide-activated protein kinase of the HIV-infected cells of the human subject may be calculated using any of the various methods known to those of skill in the art. The subject invention further provides a method of determining whether a human subject is infected with HIV which comprises obtaining a sample of cells from the human subject, said cells being susceptible to infection by HIV, contacting the sample of cells with an agent capable of stimulating the phosphorylation activity of a ceramide- activated protein kinase of the cells of the sample in an amount effective to stimulate said phosphorylation activity and thereby stimulating the proliferation of any HIV present in the cells, detecting in the resulting sample the presence of any HIV, the presence of HIV indicating that the human subject is infected with HIV. As used herein, the "sample" may be obtained from blood or any other bodily fluid known to contain HIV in HIV-infected individuals. The agent capable of stimulating the phosphorylation activity of a ceramide-activated protein kinase may be interleukin-I. As used herein, detecting the presence of HIV may be performed according to any of the various methods known to those skilled in the art. Such methods include, but are in no way limited to, immunoassays against the HIV coat proteins. Further in the practice of the subject invention, the amount of agent effective to stimulate the phosphorylation activity of ceramide-activated protein kinase of the cells of the sample means an amount capable of stimulating the phosphorylation activity by at least two-fold. This amount may be calculated using any of the various methods known to those of skill in the art. Finally, the subject invention provides a method of treating a subject having a disorder associated with poor stem cell growth, which comprises administering to the subject an agent capable of stimulating the phosphorylation activity of a ceramide-activated protein kinase of the stem cells of the subject in an amount effective to stimulate said phosphorylation activity, thereby stimulating stem cell growth. In the preferred embodiment of the subject invention, the subject is a human. Also, in the preferred embodiment of the subject invention, the disorder associated with poor stem cell growth is aplastic anemia. In one embodiment of the subject invention, the agent is interleukin-I. The interleukin-I may be interleukin-Iβ. Further in the practice of the subject invention, the amount of agent effective to stimulate the phosphorylation activity of ceramide-activated protein kinase of the stem cells of the subject means an amount capable of stimulating the phosphorylation activity by at least two-fold. This amount may be calculated using any of the various methods known to those of skill in the art. This invention will be better understood by reference to the Experimental Petails which follow, but those skilled in the art will readily appreciate that the specific experiments detailed are only illustrative of the invention as described more fully in the claims which follow thereafter. Experimental Details I - Characterization of a ceramide-activated protein kinase; Stimulation by tumor necrosis factor a A. Abstract Recent investigations have identified a signal-transduction system involving sphingomyelin and derivatives. In this paradigm, sphingomyelin hydrolysis by a sphingomyelinase generates ceramide, which may be converted to the protein kinase C inhibitor sphingosine or to ceramide 1-phosphate. Ceramide may have second-messenger function because it induces epidermal growth factor receptor phosphorylation, presumably on Thr-669 in A-431 cells. The present study describes a kinase that may mediate ceramide action. With a 19-amino acid epidermal growth factor receptor peptide containing Thr-669, a membrane-bound activity that phosphorylated the peptide was detected in A-431 cells. Activity was linearly related to ATP (0.3-300μM) and peptide concentration (0.02-1 mg/ml) , possessed a physiologic pH optimum (pH 7.0-7.4), and was Mg 2+ -dependent. Other cations - Ca 2+ , Mn 2+ , and Zn 2+ - were ineffective. Natural and synthetic ceramide induced time-and concentration-dependent enhancement of kinase activity. Ceramide (0.5 μM) increased kinase activity 2-fold by 30 s, and activity remained elevated for at least 15 minutes. As little as 0.001 μM ceramide was effective, and 1 μM ceramide induced maximal phosphorylation. Sphingosine was similarly effective. Because tumor necrosis factor (TNF) α rapidly induces sphingomyelin hydrolysis to ceramide during monocytic differentiation of HL-60 cells, its effects on kinase activity were assessed. Kinase activity was increased 1.5- fold at 5 minutes and 2-fold at 2 hr in membranes derived from TNF-stimulated cells. The effective concentration range was 3 pM-30 nM TNF. Exogenous ceramide induced a similar effect. In sum, these studies demonstrate the existence of an unusual Mg 2+ -dependent ceramide-activated protein kinase that may mediate some aspects of TNF-α function. B. Background The present studies were done to identify the kinase that mediated the effect of ceramide on EGFR phosphorylation. The substrate used was a synthetic peptide derived from the amino acid sequence around Thr-669 of the EGFR. These studies demonstrate that A-431 human epidermoid carcinoma cells and HL-60 cells contain a ceramide/sphingosine- activated protein kinase. Further, this kinase is stimulated by TNF-α, which elevates the cellular ceramide level and induces phosphorylation of several proteins (19- 24) , including the EGFR, as an early event in cellular activation. These studies provide initial evidence for a sphingolipid-activated, protein kinase-mediated signaling system. C. Experimental Procedures 1. Materials Ceramide (type III) , sphingosine, palmitic acid, cholera toxin, hexamethylene bisacetamide, re inoic acid, butyrate, leupeptin, and buffers were from Sigma. Fetal bovine serum (FBS) was from GIBCO. [7- 32 P] ATP (3000 Ci/mmol; 1 Ci = 37 GBq) was from New England Nuclear. P81 phosphocellulose paper was from Whatman. Liquid scintillation solution (Liquiscint) was from National Piaghostics (Sommerville, NJ) . HPLC grade solvents were from Fisher. The EGFR peptide (amino acids 663-681, NH 2 -Glu- eu-Val-Glu-Pro-Leu- Thr-Pro-Ser-Gly-Glu-Ala-Pro-Asn-Gin-Ala-Leu-Leu-Arg-COOH) was synthesized by using an Applied Biosystems model 431A machine and purified by reverse-phase HPLC. C 8 -ceramide (N- octanoylsphingosine; C 8 -cer) and TNF α may be prepared according to methods well known to those of skill in the art. TNFα is also commercially available. 2. Cell Culture A-431 human epidermoid carcinoma cells were grown in monolayer culture in a 1:1 mixture of Pulbecco's modified Eagle's medium (PMEM) and Ham's F-12 medium containing 10% FBS and were harvested by trypsinization according to methods well known to those skilled in the art (18) . HL-60 cells were grown in suspension culture in RPMI 1640 medium containing 10% FBS and supplements, according to methods well known to those skilled in the art (3) . On the day of an experiment HL-60 cells were resuspended (1 x 10 6 cells per ml) in RPMI 1640 medium/1% FBS for 2 hours before stimulation with lipid activators and differentiating agents. 3. Membrane Preparation Cells (3xl0 7 /ml) were homogenized with a tight-fitting Pounce homogenizer at 4°C in buffer (25 mM Hepes, pH 7.4/5 mM EGTA/50mM NaF/leupeptin at 10 μg/ml) according to methods well known to those skilled in the art (17) . The homogenate was centrifuged at 500 x g for 5 minutes, and the postnuclear supernatant was centrifuged at 200,000 x g for 30 minutes. The microsomal membrane pellet was resuspended (7.5 μg of membrane protein per μl for A-431 cells and 1.5 μg/μl for HL-60 cells) in homogenizing buffer. Membranes were prepared fresh daily. 4. Assay of Kinase Activity For most experiments, the reaction mixture contained 25 μl of microsomal membrane or postnuclear supernatant, 50 μl of EGFR peptide (4 mg/ml in 25 mM Hepes, pH 7.4) and 125 μl of buffer (50 mM Hepes, pH 7.4/20 mM MgCl 2 ) (17). Phosphorylation was initiated at 22°C by addition of 50 μl of [7- 32 P] ATP (150 μM final concentration; 4000 dpm/pmol) . For studies with lipid activators, ceramide and other lipids were dried under N 2 and resuspended in the kinase assay buffer by bath sonication for 2 minutes at 37°C. The reaction was terminated at the indicated times by addition of 50 μl of 0.5 M ATP in 90% formic acid. Unless otherwise indicated, all assays were done under conditions determined as linear for time and enzyme concentration. Enzyme activity was determined from the transfer of 32 P from the y position of ATP to EGFR peptide and the specific radioactivity of [7- 32 P] ATP. Phosphorylated peptide was quantified by two separate methods. For initial studies, samples were spotted on phosphocellulose paper, washed in 1 M acetic acid/4 mM pyrophosphate and subjected to liquid scintillation counting, according to methods well known to those skilled in the art (25) . Values obtained from a boiled blank or a sample lacking peptide were subtracted from each determination. Alternatively, HPLC was done according to methods well known to those skilled in the art (17) . For these studies, samples were first applied to a C 18 Sep-Pak cartridge and eluted with 99.9% acetonitrile/0.1% trifluoroacetic acid. The eluates were lyophilized, resuspended in 6 M guanidine hydrochloride/200 mM Tris, pH 8.5 and applied to a C 18 reverse-phase column (Pynamax, 4.6 mm i.d., Rainin, Woburn, MA). The peptide was eluted with a linear gradient (1% per minute) of acetonitrile at a flow rate of 1 ml/minute and was detected by measuring the Cerenkov radiation associated with 1-ml fractions. 5. Phosphoamino Acid Analysis To determine which amino acid was phosphorylated, phosphoamino acid analysis of the peptide was done. The phosphopeptide peak obtained by HPLC was subjected to partial acid hydrolysis (1 hr at 110°C in 6 M HC1) . The hydrolysates were dried, resuspended in 250 μl of water, and applied to a Powex AG1-X8 column (Bio-Rad) . Amino acids were eluted with 0.5 M HC1, dried, and analyzed by thin- layer electrophoresis, according to methods well known to those skilled in the art (26) . Individual phosphoamino acids were identified by ninhydrin staining of carrier phosphoamino acids and by autoradiography. 6. Other Procedures Protein was measured by the method of Bradford (27) . 7. Statistics Statistical analysis was performed by t test and linear regression analysis by the method of least squares. P. Results Pavis and coworkers (15, 17) showed that addition of sphingosine to A-431 cells enhanced phosphorylation of the EGFR on Thr-669. Subsequently, the subject experiments show that sphingosine was rapidly converted to ceramide in these cells and that exogenous ceramide induced identical effects (18) . To investigate the kinase that mediated ceramide action, a synthetic peptide corresponding to the sequence around Thr-669 was used as substrate. Initial studies were done to determine the kinetics of phosphorylation of the EGFR peptide. The conditions for this assay were adapted from Pavis and coworkers (17, 25) . Briefly, postnuclear supernatant was used as a source of enzyme activity, and samples were spotted on phosphocellulose paper to measure phosphorylated peptide. The kinetics of phosphorylation appeared biphasic. Initial rapid incorporation of 32 P into peptide for 10 minutes was followed by incorporation at a slightly reduced rate for as long as 30 minutes (Figure 1) . Subsequent studies were done to optimize the assay. Kinase activity was found by Lineweaver-Burke analysis to be linearly related (r=0.98) to substrate concentration for ATP (0.3-300μM) and EGFR peptide (0.02-1 mg/ml) at 5 minutes of stimulation. Apparent K,,, values for ATP of 15 μM and for EGFR peptide of 0.25 mg/ml were derived. Apparent V ^ values ranged from 100-200 pmol-min ' Vmg of protein "1 . All subsequent studies were done with 150 μM ATP and EGFR peptide at 4 mg/ml. Under these conditions, substrate concentration was not rate limiting. An additional set of studies assessed the pH optimum for the kinase activity. There was no measurable activity at pH values <5. Thereafter, peptide phosphorylation increased to a maximum at pH 7-7.4 and rapidly dropped to undetectable levels at pH 8. Hence, this kinase appears active within the physiologic pH range. The divalent cation requirement for kinase activity was also investigated. In the presence of EGTA (1 mM) alone, peptide phosphorylation did not occur. Mg 2+ induced dose-dependent peptide phosphorylation (Figure 2) . As little as 0.1 mM Mg 2+ increased kinase activity to 4 pmol-min "1 (mg of protein" '), and maximal activity occurred with 10 mM Mg 2+ ; the EP 50 was -3.5 mM. An increase in Mg 2+ to 25 mM did not further increase activity. Mn + (1-10 mM) , Zn 2+ (1-10 mM) , and Ca 2+ (0.001-lOmM) did not support kinase activity toward the EGFR peptide. These studies indicate that this kinase activity is Mg 2+ -dependent. Cell-fractionation studies were done to compare levels of kinase activity in the postnuclear supernatant, cytosol, and membrane. Activity detected in the postnuclear supernatant was equally divided between membrane and cytosolic fractions. Only membrane activity was enhanced by ceramide (see below) . A more specific method for detection of phosphorylated peptide used reverse-phase HPLC. Peptide was eluted with a linear gradient of acetonitrile, and fractions were monitored for Cerenkov radiation. A peak of Cerenkov radiation was eluted at 30% acetonitrile in samples containing peptide but was absent when the peptide was omitted from the reaction mixture (Figure 3, Left). Phosphoamino acid analysis of the eluate demonstrated the presence of [ 3 P]phosphothreonine (Figure 3, Right) . These studies indicate that of the two potential phosphorylation sites contained within the EGFR peptide, corresponding to Thr-669 and Ser-671, only threonine-669 served as substrate. To determine whether ceramide and sphingosine enhance EGFR peptide phosphorylation, these lipids were added to a reaction mixture containing peptide and membrane. Ceramide (0.5 μM) stimulation of EGFR peptide phosphorylation was evident by 30 s (Figure 4) and demonstratable for at least 15 minutes. Ceramide (0.001-3 μM) enhanced peptide phosphorylation in a concentration-dependent manner at 2 minutes of stimulation (Figure 5) . As little as 1 nM ceramide was effective, and a maximal effect to 2.1-fold of control occurred with 1 μM ceramide; the EP J0 was »«30 nM. Synthetic C 8 -cer and natural ceramide (Sigma type III) were similarly effective. As with basal phosphorylation, ceramide-enhanced phosphorylation occurred exclusively on the threonine residue of the EGFR peptide. Figure 6 shows that sphingosine also stimulated EGFR peptide phosphorylation to a level 1.6 fold of control at 2 minutes of stimulation. The concentration-dependence of this stimulatory effect was similar to that of ceramide. In contrast, palmitic acid, the predominant fatty acid in natural ceramide, failed to increase EGFR peptide phosphorylation. TNF-α has been shown to increase cellular levels of ceramide within minutes of activation of HL-60 cells, and a synthetic ceramide replaced the requirement of TNF-α in monocytic differentiation of these cells (6) . Hence, studies were done to determine whether TNF-α treatment of HL-60 cells activated a kinase similar to that detected in A-431 cells. For these studies, cells were stimulated with TNF-α, and then membranes were isolated and used to assess kinase activity toward the EGFR peptide. Figure 7 demonstrates kinetics of the effect of 30 nM TNF-α, a maximally effective concentration for generation of ceramide and monocyte differentiation of these cells (6) . Cellular stimulation for as little as 5 minutes increased membrane-bound kinase activity to 1.5-fold of control, and activity continued to increase for as long as 2 hours to 2.2-fold of control. The effect of TNF-α was concentration-dependent when measured at 60 minutes of stimulation (Figure 8) . As little as 3 pM TNF-α increased activity to 1.1-fold of control, and a maximal effect of 1.8-fold of control occurred with 30 nM TNF-α; the EP J0 was «-200 pM. Additional studies assessed the effect of the cell-permeable synthetic ceramide, C 8 -cer, on enzyme activity. In three separate studies, addition of as little as 0.3 μM C 8 -cer to the medium of HL-60 cells increased kinase activity in membranes derived from stimulated cells to l.2-fold of control, and a maximal effect was achieved with 10 μM C 8 -cer to 1.5-fold of control. This value was quantitatively similar to that obtained with a maximal concentration of TNF-α in this set of studies. Sphingosine was similarly as effective as C8- cer. Kinase activity was not stimulated by maximally effective concentrations of other HL-60 differentiating agents, including cholera toxin (10 nM) , retinoic acid (0.5 μM) , and butyrate (0.5 mM) (28, 29). In sum, these studies demonstrate that HL-60 cells, like A-431 cells, contain a ceramide-activated protein kinase and that TNF-α, which generates ceramide as an early event in cellular activation, enhances kinase activity. E. Piscussion Pavis et al. (15, 17) originally demonstrated that sphingosine stimulated phosphorylation of the EGFR on Thr- 669 in A-431 cells. It is shown here that sphingosine was rapidly converted to ceramide in these cells and that ceramide induced identical effects. To investigate the kinase that mediated ceramide action, the present studies used a 19-amino acid synthetic peptide corresponding to the sequence around Thr-669 of the EGFR as a substrate. These studies have demonstrated that A-431 cells contain a Mg + - dependent kinase activity with a physiologic pH optimum that was stimulated by ceramide in a time- and concentration- dependent manner. This kinase has some distinctive features. It appears exclusively membrane-bound, does not utilize Ca 2+ as cofactor, and is also activated by the protein kinase C inhibitor sphingosine. These features distinguish this kinase from any other known protein kinase. A similar activity was detected in HL-60 cells and was enhanced rapidly by TNF-α, which elevates ceramide (but not sphingosine) levels, as an early event in cellular activation. Several studies have demonstrated that TNF-α stimulates protein phosphorylation as a proximal event in cellular stimulation (19-24) . A variety of substrates have been identified, including a 28-kPa stress protein in bovine aortic endothelial cells (21) , the eukaryotic initiation factor 4E (22, 23), an uncharacterized 26-kPa cytosolic protein in U937 human monoblastoid cells (20) , and the EGFR (24) . In most of these studies serine/threonine phosphorylation of these proteins was seen, and different investigators have suggested that the cAMP-dependent protein kinase (30) , protein kinase C (31, 32) , or some other protein kinase mediates TNF action (33) . The present studies strongly suggest that another serine/threonine protein kinase, in some systems, mediates TNF action. The amino acid sequence surrounding Thr-669 of the EGFR is unusual, containing three proline residues within a span of 9 amino acids. This unusual structure has no homology to the consensus substrate sequences for any of the major protein kinases (34, 35) . In fact, Gill and coworkers (36) reported that a peptide corresponding to residues 662-673 of the EGFR failed to serve as substrate for a variety of purified protein kinases in vitro, including the cAMP- dependent protein kinase, protein kinase C, calcium- calmodulin-dependent protein kinase, and S6 kinase. Only casein kinase II and glycogen synthase kinase 3 demonstrated significant activity toward this substrate, but the peptide proved to be a poor substrate for both of these kinases, as evidenced by high Km values. Glycogen synthase kinase 3 has a known preference for proline-rich substrates, which may account for the low level of activity detected in these studies (37) . The region corresponding to Thr-669 of the EGFR is located between the transmembrane domain and the ATP-binding site within the catalytic domain. This region also contains Thr- 654, the major protein kinase C phosphorylation site, and the region, in general, is considered to be involved in modulation of receptor function (38) . Mutational removal of Thr-669 has been shown to alter receptor down-regulation and substrate specificity (36) . This region is also highly conserved in the v-erbB and neu oncogene products and may represent a site for phosphorylation of these proteins by ceramide-activated protein kinase. In sum, these studies characterize a ceramide-activated protein kinase activity in A- 31 and HL-60 cells. Evidence has been presented that this kinase is activated by TNF-α, which triggers the generation of ceramide as an early event during cellular stimulation. Hence, this kinase may mediate, in whole or in part, signal transduction by TNF-α in some systems. In this paradigm, binding of TNFα to its cell-surface receptor stimulates a neutral plasma membrane- bound sphingomyelinase that cleaves sphingomyelin to yield ceramide. Ceramide would then enhance kinase activity, resulting in the phosphorylation of specific substrates. II - Tumor Necrosis Factor-α Activates the Sphingomyelin Signal Transduction Pathway in a Cell-Free System A. Abstract The mechanism of tumor necrosis factor (TNF) -α signaling is unknown, however, TNF-α signaling most likely involves sphingomyelin hydrolysis to ceramide by a sphingomyelinase and stimulation of a ceramide-activated protein kinase. In a cell-free system, TNF-α induced a rapid reduction in membrane sphingomyelin content and a quantitative elevation in ceramide concentrations. Ceramide-activated protein kinase activity also increased. Kinase activation was mimicked by addition of sphingomyelinase but not by phospholipases Aj, C, or P. Reconstitution of this cascade in a cell-free system demonstrates tight coupling to the receptor, suggesting that this is a signal transduction pathway for TNF-α. B. Experimental Procedure and Piscussion Sphingomyelin can be metabolized to generate molecules that have various functions within the cell (1-6) . Ceramide, which is generated by sphingomyelinase action, can be deacylated to sphingoid bases (1, 14) , which are potential inhibitors of protein kinase C (9-11) or phosphorylated to ceramide 1-phosphate (4) by a ceramide kinase (5, 13) . Ceramide appears to have bioeffector properties (7, 8, 18) . Cell-permeable ceramide analogs stimulate monocytic differentiation of human leukemia (HL-60) cells (7, 8) and the phosphorylation of the epidermal growth factor receptor (EGFR) at Thr 669 in A431 human epidermoid carcinoma cells (18) . TNF-α activates a neutral sphingomyelinase to generate ceramide in HL-60 cells, and it was postulated that this initiated TNF-α action (6) . A ceramide-activated protein kinase with a synthetic peptide derived from the amino acid sequence surrounding Thr 669 of the EGFR (residues 663 to 681) was defined (40) . Kinase activity was membrane- associated, Mg 2+ -dependent, and activated by natural or synthetic ceramide in a concentration-and time-dependent manner. This ceramide-activated protein kinase activity was rapidly increased in membranes derived from HL-60 cells treated with TNF-α. The present studies were undertaken to evaluate coupling of this sphingomyelin pathway to stimulation of the TNF receptor in a cell-free system. The binding of TNF-α to its receptor is detectable within 2 minutes and maximal by 5 to 10 minutes at 4°C in membranes derived from HL-60 cells (41) . Therefore, supernates from HL-60 cells, collected after a low-speed centrifugation to remove nuclei, were first incubated with TNF-α for 5 minutes at 4°C to allow the formation of TNF-receptor complexes. Thereafter, reactions were initiated by warming supernates to 22°C in a reaction mixture containing adenosine triphosphate (ATP) and Mg 2+ at pH 7.4. These conditions were adopted to allow for activation of neutral sphingomyelinase (1, 42) . Under these conditions, TNF-α induced a time- and concentration-dependent reduction in sphingomyelin content (Figure 9A) . The effect of TNF-α was evident at 1 minute and maximal by 7.5 minutes. Sphingomyelin concentrations decreased 27% from a control concentration of 10.4 ± 0.5 (mean ± SEM) to 7.6 ± 0.2 nmol per milligram (nmol mg "1 ) of supernate protein (P < 0.001) . In contrast, the concentration of sphingomyelin in control incubations did not change. Concentrations of TNF-α of 300 pM were effective, with a maximal effect at 3 nM TNF-α [effective dose (EP 50 ) - 500 pM] . Under the same conditions, ceramide increased quantitatively from 1.8 ± 0.3 to 4.0 ± 0.5 nmol mg' 1 (Figure 9B) . This effect was detectable at 1 minute (P<0.001) and maximal by 7.5 minutes. Thus, 2.8 nmol of sphingomyelin per milligram of supernate protein were lost for each 2.2 nmol of ceramide per milligram of supernate protein that was generated. Similar kinetics of sphingomyelin degradation and ceramide generation were determined in intact HL-60 cells (n=3) , confirming previous studies (6) . Other choline-containing lipids, including phosphatidyl-choline, lysophosphatidylcholine, sphingosylphosphorylcholine (1), and 1,2-diacylglycerol were not affected by TNF-α. Thus, TNF-α activated a neutral sphingomyelinase in a cell-free system, which resulted in the generation of the potential second messenger ceramide. The effect of TNF-α on ceramide-activated protein kinase activity was assessed. Nuclei-free supernates contain ceramide-activated protein kinase activity that can phosphorylate EGFR peptide with a maximum velocity (V, ^ ) of 50 to 100 pmol per minute per milligram (pmol min^mg "1 ) of protein and a Michaelis constant (K,,, of 15 μM) for ATP and 0.25 mg ml "1 for peptide (40). Ceramide (0.001 to 3 μM) enhances kinase activity to a maximum of two-fold of the control (40) . TNF-α, which increased ceramide concentrations, similarly enhanced kinase activity in intact cells (40) . For studies assessing the effect of TNFα in broken cell preparations, nuclei-free supernates were incubated under conditions sufficient for stimulation of neutral sphingomyelinase in a reaction mixture that also contained EGFR peptide and 7- 32 P-labeled ATP. Phosphorylated peptide was resolved by high-performance liquid chromatograph (HPLC) and quantified by Cerenkov counting (40) . Kinase activity was calculated from the specific activity of [ - 32 P] ATP and incorporation of 32 P into EGFR peptide. Background activity was subtracted from each point. TNF-α (30 nM) treatment enhanced kinase activity (PcO.001) in a time-dependent manner (Figure 10A) . TNF-α stimulation of kinase activity was evident by 1 minute and demonstrable for at least 10 minutes. If the initial incubation with TNF-α at 4°C was omitted and TNF was added directly to the reaction mixture at 22°C, the reaction was delayed. Under these conditions, enhancement of activity by TNF-α did not occur for 2 minutes, presumably until after TNF-receptor complexes had formed. TNF-α enhanced kinase activity in a concentration-dependent manner at 5 minutes (Figure 10B) . TNF-α was effective at 10 pM and had a maximal effect at 3 nM; the EP J0 was « 300 pM TNF-α. This is similar to the EP J0 of 200 pM for stimulation of ceramide- activated protein kinase by TNF-α in intact cells (40) . TNF-α enhanced kinase activity in a total of 20 separate studies. Guanosine triphosphate (GTP) and guanosine-5' -0- (3-thiotriphosphate) (GTP-7-S) (0.25 to 200 μM) did not affect kinase activity. To demonstrate that the effect of TNF-α is mediated by sphingomyelin hydrolysis to ceramide, a sphingomyelinase or a phospholipase (Aj, C, or P) was added to the kinase reaction mixture and measured EGFR peptide phosphorylation was measured. For some studies, the reaction mixture contained free Ca 2+ (1 mM) , which did not affect results. Control activity reflects several TNF-α-independent protein kinases that are known to phosphorylate EGFR peptide on Thr 669 . Exposure of the nuclei-free supernates to sphingomyelinase (1 x 10 "3 U ml "1 ) from Staphylococcus aureus for 5 minutes induced and increase in kinase activity comparable to that induced by TNF-α (1 nM) (Figure 11) . This concentration of sphingomyelinase stimulates a two-fold elevation in ceramide levels in HL-60 cells (3, 4). Concentrations of phospholipases Aj, C, and P, which were 40- to 400-fold higher than sphingomyelinase and which are effective for phospholipid hydrolysis under conditions used in these assays (43) , did not enhance kinase activity. Hence, the effect of TNF-α in broken cell preparations was mimicked by a sphingomyelinase but not by other phospholipases. The mechanism of coupling of the TNF receptor to sphingomyelinase is unknown. Neutral sphingomyelinase appears to be ubiquitous in mammalian cells and is externally oriented in the plasma membrane (44) . Similarly, sphingomyelin is preferentially localized to the outer leaflet of the plasma membrane (45) . This colocalization of receptor, phospholipase, and substrate at the plasma membrane suggests that ceramide is generated at this site. The exact intracellular site of the ceramide-activated protein kinase has not yet been investigated. However, preliminary evidence suggests it is an intrinsic membrane protein (40) . In this regard, ceramide-activated protein kinase would not have to be present in the outer leaflet of the plasma membrane for signaling to occur, as ceramide can redistribute across a membrane bilayer (46) . Ceramide-activated protein kinase may be a member of an emerging family of serine/threonine protein kinases that includes microtubule-associated protein 2 (MAP2) kinase [extracellular signal-regulated kinase (ERK1) ] (35, 47, 48), EGFR threonine (ERT) kinase (49) , glycogen synthase kinase-3 (35, 47, 48) and p34 cdc2 -containing proline-directed and histone HI kinases (49, 50). The substrates for these kinases appear to have a minimal recognition sequence, X- Ser/Thr-Pro-X, in which the phosphorylated site is flanked by a COOH-terminal proline residue (50, 51) and X can be any amino acid. Substrates for this class of kinases include EGFR, proto-oncogene products Jun and Myc, tyrosine hydroxylase, histone HI, glycogen synthase, synapsin I, and protein phosphatase inhibitor II (37, 49-51) . TNF-induced, proline-directed phosphorylation of these proteins has not yet been demonstrated. The X-Ser/Thr-Pro-X sequence is different from consensus substrate sequences for other major serine/threonine kinases, including cyclic adenosine monophosphate (cAMP) - and cyclic guanosine monophosphate (cGMP) -dependent-proteinkinases, Ca 2+ /calmodulin-dependent- protein kinase, and ribosomal S6 protein kinase (49) . In fact, these kinases have limited activity toward this proline-containing sequence (50) . It has been proposed that various distinct signaling systems, including protein kinases A and C, phospholipases Aj and C, the EGFR tyrosine kinase, and a novel serine kinase, may mediate TNF-α action (19) . It is clear that no single second messenger pathway can account for the entirety of the reported biologic effects of TNF-α. The role of the sphingomyelin pathway in events other than monocytic differentiation has not been investigated nor has the relation to these other signaling systems. This issue is further complicated by the recent cloning of two distinct TNF receptor forms of 55 kP and 75 kP (52-55) with homologous extracellular domains with dissimilar intracellular portions. In sum, the rapid kinetics of activation of the sphingomyelin pathway by TNF-α in intact cells, the ability of cell-permeable ceramide analogs to bypass receptor activation and mimic TNF-α action, and the reconstitution of this cascade in a cell-free system provide strong support for the notion that this pathway serves to couple TNF receptor activation to cellular stimulation. Hence, these studies suggest that TNF-α activates a plasma membrane-bound neutral sphingomyelinase to generate at the second messenger ceramide, which stimulates the ceramide-activated protein kinase to phosphorylate a distinct set of protein substrates, thereby altering their function. Ill - Interleukin-IS Signals Through the Sphingomyelin Pathway in Intact EL-4 Cells and in a Cell-Free System A. Abstract The mechanism of interleukin-1 (IL-1) signaling is unknown. Recent investigations demonstrated that tumor necrosis factor-α utilizes a signal transduction pathway involving sphingomyelin hydrolysis to ceramide and stimulation of a ceramide-activated protein kinase. In intact EL-4 thymoma cells, IL-lβ similarly stimulated rapid reduction in sphingomyelin and elevation in ceramide levels, and enhanced ceramide-activated protein kinase activity. This cascade was also activated by IL-lβ in a cell-free system demonstrating tight coupling to the receptor. Further, exogenous sphingomyelinase but not phospholipases A^ C or P, replaced IL-lβ to stimulate IL-2 secretion in combination with phorbol ester. These studies demonstrate that IL-lβ signals through the sphingomyelin pathway. B. Experimental Methods and Piscussion Hydrolysis of sphingomyelin to ceramide at the plasma membrane by a neutral sphingomyelinase may initiate a cascade that functions in signaling (6-8, 18, 40, 58, 59) . Evidence has been provided that ceramide may stimulate a serine/threonine kinase termed ceramide-activated protein kinase to transduce the signal (18, 40, 59) . Ceramide- activated protein kinase is membrane-bound, Mg +2 -dependent and defined by its capacity to phosphorylate a synthetic peptide derived from the amino acid sequence surrounding Thr 669 of the epidermal growth factor receptor (EGFR) . Ceramide-activated protein kinase may be a member of an emerging family of proline-directed serine/threonine kinases that includes the extracellular-signal regulated (also referred to as mitogen-activated) and P34" 102 kinases (47) . Substrates for these kinases contain the minimal recognition sequence, X-Ser/Thr-Pro-X, in which the phosphorylated site is flanked on its carboxy terminus by a proline residue and X can be any amino acid. Evidence has been provided that tumor necrosis factor (TNF) - α may utilize the sphingomyelin pathway for signaling (6, 40, 59) . TNF stimulates this pathway early during HL-60 cell differentiation into monocytes (6, 59) and synthetic ceramide analogs bypass receptor activation and directly induce differentiation (7) . Further, this cascade has been reconstituted in a cell-free system comprised of extracts of HL-60 cells, demonstrating tight coupling of this pathway to the TNF receptor (59) . The present studies were performed because of numerous reports that TNF and IL-1 stimulate a common set of events in diverse biologic systems (60) . The murine thymoma EL-4 cell line is a well-defined IL-1 responsive cell line that expresses functional IL-1 receptors (61, 62) . Upon stimulation with IL-1, these cells up-regulate the IL-2 receptor and secrete IL-2 (62) . Initial studies were designed to investigate the effects of IL-lβ on cellular sphingomyelin content. Cells, grown in Pulbecco's Modified Eagle's (PME)/Ham's F-12 medium containing 10% horse serum and [ 3 H]choline (1 μCi ml "1 ) , were resuspended back into the same medium at 10 x 10 6 cells ml" 1 and stimulated with IL-lβ. IL-lβ is commercially available. Under these conditions, IL-lβ induced time- and concentration-dependent sphingomyelin hydrolysis (Figure 12 A, B) . A maximally effective concentration of IL-lβ, 40 ng ml "1 , induced a detectable reduction in-sphingomyelin content by 2 minutes from a baseline of 800 ± 14 pmol 10 6 cells "1 (mean ± SEM) and the level decreased to 648 ± 16 pmol 10 6 cells "1 (p<0.005) at 30 minutes. Concentrations of IL-lβ of 0.01 ng ml "1 were effective, with a maximal effect at 10 ng ml *1 [effective dose (EP 50 ) « 2 ng ml "1 (Figure 12B) ] . A similar reduction in sphingomyelin content after IL-1 stimulation was determined by direct measurement of phosphorous content (63) . In contrast, the content of phosphatidylcholine, the other major choline-containing phospholipid, was unchanged. Under the same conditions, IL-lβ induced a statistically significant increase in ceramide content (Figure 13) . Ceramide increased from 360 to 403 pmol 10 6 cells "1 at 2 minutes (p<0.005) and to a maximum of 450 pmol 10 6 cells "1 at 15 minutes. In separate studies (n=4) , a statistically significant increase in ceramide content was evident by 30 seconds. Maximally effective concentrations of other agents known to stimulate EL-4 cells (65, 66) including 12-0- tetradecanoylphorbol-13-acetate (TPA) , concanavalin A, epinephrine and an anti-CP3 antibody failed to elicit a ceramide response (n=5) . Hence, sphingomyelinase activation appeared specific for stimulation by IL-lβ. Subsequent studies assessed whether IL-lβ also enhanced ceramide-activated protein kinase activity. EL-4 cells were found to contain a membrane-bound ceramide-activated protein kinase activity similar to that reported in A431 human epidermoid carcinoma cells and HL-60 cells (40, 59) . Activity was measured by the transfer of 32 P from the y- position of ATP to EGFR peptide (AA 663-681 of the EGFR) . The effect of IL-lβ on kinase activity was determined using microsomal membranes derived from cells stimulated with IL- lβ. IL-lβ enhanced kinase activity in a time- and concentration-dependent manner. In cells treated with 10 ng ml" 1 IL-lβ, a maximally effective concentration, an increase in kinase activity was detectable at 30 seconds (data not shown) and maximal at 2 minutes (Figure 14, p<0.005). Activity increased to 2.1-fold of control from 5 to 11 pmol per minute per milligram (pmol min "1 mg "1 ) and then gradually declined over 15 minutes. Concentrations of IL-lβ of 0.03 ng ml "1 were effective, with a maximal effect at 10 ng ml "1 [effective dose (ED 30 ) « 2 ng ml "1 ] . This is the same range of concentrations found effective for sphingomyelin hydrolysis. Stimulation by IL-lβ was detected in a total of 10 experiments. Cytosolic fractions of EL-4 cells also contained kinase activity toward EGFR peptide of 2.6 ± 0.3 (mean ± range) pmol min "1 mg "1 . Cytosolic activity, which represents proline-directed protein kinase activities other than ceramide-activated protein (68) , was not enhanced by II-lβ during these studies. Further, protein kinase C activity as determined by phosphorylation of lysine-rich histone (Sigma Chem. Co., type III-S) (69) was not enhanced in either membrane or cytosolic fractions. Early kinetics of activation of a potential signaling system provide some support that the pathway might be involved in the signaling process. However, signal transduction pathways are highly regulated and often interrelated (70) . Hence, activation of one system often results in rapid activation of another. To provide additional support for tight coupling of the sphingomyelin pathway to activation of the IL-lβ receptor, studies were performed with subcellular fractions derived from EL-4 cells. For these studies, supernates, collected after a low-speed centrifugation to remove nuclei, were first incubated with IL-lβ for 10 minutes at 4°C to permit formation of IL-1 receptor complexes (59) . Thereafter, reactions were initiated by warming supernates to 22°C in a reaction mixture containing Mg 2+ at pH 7.4. These conditions were adopted to allow for activation of endogenous neutral sphingomyelinase (1, 42) . For studies measuring kinase activity, reaction mixtures also contained [ 32 P] ATP and EGFR peptide. Under these conditions, IL-lβ stimulated a rapid reduction in sphingomyelin content and a quantitative increase in ceramide content (Figure 15A) . In separate studies, a statistically significant reduction in sphingomyelin content (n=10) and elevation in ceramide content (n=6) were detected at 1 minute of stimulation (p<0.005 vs. control) . Ceramide- activated protein kinase activity also increased (Figure 15B) . These effects were quantitatively similar to those determined in the intact cells. Hence, the effect of IL-lβ to activate the sphingomyelin pathway was also observed in a cell-free system. To determine whether the sphingomyelin pathway mediated the biologic response to IL-lβ, direct activation of the sphingomyelin pathway with exogenous sphingomyelinase (59) was compared to stimulation by IL-lβ. For these studies, cells were treated with IL-lβ, sphingomyelinase and/or phorbol ester and, after 24 h IL-2 secreted into the media was measured. As previously reported (62, 65, 71, 72), IL- lβ (1-30 ng ml" 1 ) alone did not induce detectable IL-2 secretion (Table 1) , nor did TPA (1-20 ng ml "1 ) alone. However, in combination IL-lβ (10 ng ml "1 ) and TPA (20 ng ml " l ) stimulated secretion maximally. Sphingomyelinase alone also failed to stimulate IL-2 secretion, but again, in combination with TPA, induced secretion. Concentrations of sphingomyelinase between 5 x 10 "5 U ml "1 and 1 x 10" 1 U ml "1 ) were effective. In separate studies (n=2) , sphingomyelinase (1 x 10" 3 U ml *1 ) induced secretion at all concentrations of TPA from 0.5 to 20 ng ml "1 . This concentration of sphingomyelinase induced an increase in ceramide content quantitatively similar to that induced by maximally effective concentrations of IL-lβ, and has previously been shown to mimic TNF action in HL-60 cells (59) . In contrast, phospholipases (PL) Aj, C and D at concentrations 10-50 times higher than maximally effective sphingomyelinase, did not stimulate IL-2 secretion alone or in combination with TPA. Hence, the effect of IL-1 to co-stimulate IL-2 secretion in EL-4 cells was mimicked by activation of the sphingomyeline pathway with sphingomyelinase. Table I Induction of IL-2 secretion by IL-1 and sphingomyelinase. EL4 cells (1.5 x 10 6 ml "1 ) were treated with IL-lβ (10 ng ml" i ) , sphingomyelinase (SMase, Staphylo coccus aureus) , PLA 2 ( Vipera ruselli) , PLC (Bacillus cereus) and PLD ( Streptomyces chromofuscus) at the indicated concentrations, in the absence or presence of TPA (20 ng ml" 1 ) . Boiled sphingomyelinase preparations had no activity. Culture supernates were harvested at 24 h and assayed for secreted IL-2 using an anti-mouse IL-2 ELISA kit (Genzyme Corp.) according to the manufacturer's instructions. The lower limit of sensitivity of this assay was 15 pg IL-2 and the assay was linear up to 960 pg IL-2. These data (mean ± range) represent duplicate determinations from 2 experiments. Diluent ±TPA Control nd* nd IL-lβ (10 ng ml "1 ) nd 261 ± 2 SMase (lxlO " u ml "1 ) nd 313 ± 60 PLA 2 (l-5xl0 "2 u ml "1 ) nd nd PLC (l-5xl0 "2 u ml *1 ) nd nd PLD (1-5X10 "2 u ml "1 ) nd nd *nd, not detectable Although signaling for IL-1 has been ascribed to various protein kinases including protein kinases A and C and a novel serine/threonine protein kinase (30, 73-77) , no coherent picture has emerged to account for all of the data. Two distinct IL-1 receptors of 60 kDa and 80 kDa have recently been cloned (78-80) . The receptors are homologous in their extracellular binding domains but have little homology in their cytoplasmic portions. In fact the 60 kDa receptor has only a short intracellular portion. There is no empiric or structural evidence suggesting that these receptors themselves might serve as protein kinases (78-80) . In addition, there is no homology between these receptors and any protein known to be involved in signal transduction. The present studies define a new mechanism by which the IL-1 receptor might activate a protein kinase. Preliminary studies with the human natural killer-like cell line, YT (81) , demonstrate that IL-1 also induces rapid generation of ceramide in this system. Despite the often reported similarities in action of TNF-α and IL-1 there is limited primary sequence homology between their receptors. Hence, the mechanism by which these two cytokines activate the sphingomyelin signal transduction pathway is not readily apparent. In sum, these studies provide evidence that the effects of IL-lβ may be mediated by the sphingomyelin signal transduction pathway. In this paradigm, ligand binding to the receptor activates a neutral sphingomyelinase hydrolysing sphingomyelin to ceramide. Neutral sphingomyelinase appears to be ubiquitous in mammalian cells and like sphingomyelin is externally oriented in the plasma membrane (44) . This co-localization of receptor, phospholipase and substrate at the plasma membrane suggests that ceramide is generated at this site. Ceramide, which can redistribute across a lipid bilayer, then stimulates ceramide-activated protein kinase which phosphorylates a specific subset of cellular proteins thereby altering their function. IV - Renaturation and TNFα Stimulation of a 97 kD Ceramide- Activated Protein Kinase A. Summary Recent investigations identified a new signal transduction pathway, termed the sphingomyelin pathway, which may mediate the action of tumor necrosis factor (TNF)α and interleukin- lb. This pathway is initiated by hydrolysis of sphingomyelin to ceramide by a neutral sphingomyelinase and stimulation of a ceramide-activated Ser/Thr protein kinase. Recent investigations demonstrated that kinase activity is proline-directed, recognizing substrates in which the phosphoacceptor site is followed by a proline residue. Until now, the kinase has been defined only as a membrane-bound activity capable of phosphorylating a peptide derived from the sequence surrounding Thr669 of the epidermal growth factor receptor (EGFR) . In these experiments, the kinase was quantitatively extracted from membrane with detergent and separated from protein kinase C by anion exchange chromatography and isoelectric focusing. Ceramide-activated protein kinase was resolved as an exclusively membrane-bound, 97 kD protein with a pi of 7.05. Kinase activity toward the EGFR peptide co-purified with activity toward a generic proline-directed substrate, myelin basic protein (MBP) . Kinase activity was reconstituted by a denaturation-renaturation procedure and demonstrated activity towards self (autophosphorylation) and exogenous substrate (MBP) . Autophosphorylation occurred exclusively on serine residues. These activities were enhanced to seven-fold of control by ceramide and TNFα. These data provide additional evidence for a role for ceramide- activated protein kinase in signal transduction for TNFα. B. Introduction The sphingomyelin pathway is a signal transduction pathway mediating the action of the cytokines tumor necrosis factor (TNF)α and interleukin-1 (IL-l)b (59). In this paradigm, receptor stimulation initiates hydrolysis of plasma membrane sphingomyelin to ceramide by the action of a neutral sphingomyelinase. Ceramide then acts as a second messenger, stimulating a membrane-bound serine/threonine kinase, termed ceramide-activated protein kinase, thereby, transducing the cytokine signal (58) . Three lines of evidence support the notion that the sphingomyelin pathway mediates cytokine action. First, the sphingomyelin pathway is activated within seconds to minutes by TNF in human leukemia (HL-60) cells and by IL-1 in human dermal fibroblasts and mouse EL4 T-helper cells. Second, cell-permeable ceramide analogs can bypass receptor activation and directly mimic the effect of these cytokines. In this regard, synthetic ceramide analogs have been shown to stimulate differentiation of HL-60 cells into monocytes (6) , phosphorylation of the epidermal growth factor receptor (EGFR) at a specific site, Thr669, in human A431 epidermoid carcinoma cells (18) , induce cyclooxygenase gene expression in fibroblasts (83) and IL-2 secretion in EL4 cells. Third, the effects of TNF and IL-1 on this cascade of events have been reconstituted in cell-free extracts indicating tight coupling of this pathway to the respective receptors (59) . Recently, Wiegmann et al. (90) transfected the 55 kD TNF receptor into the pre-B cell line 70Z/3 which contains no TNF receptor. Under these conditions, transfection conferredTNF-induced sphingomyelin hydrolysis onto these cells. A key element in this signal transduction cascade is stimulation of ceramide-activated protein kinase. Until now, this activity has only been defined as a membrane-bound activity capable of phosphorylating a peptide derived from the amino sequence surrounding Thr669 of the EGFR (amino acids 663-681) . This is the same site recognized by the mitogen-activated protein (MAP) kinases (also known as extracellular signal-regulated kinases or ERKs) . Hence ceramide-activated protein kinase may be a member of the emerging family of serine/threonine kinases, termed proline-directed protein kinases, which recognize substrates containing the motif X-Ser/Thr-Pro-X. Ceramide-activated protein kinase activity is Mg 2+ -dependent and has a physiologic pH optimum. Activity has been found in every cell type has so far been examined including HL-60 cells, A431 cells and EL4 cells. The purpose of the present experiments was to characterize this activity more definitively. In the present studies ceramide-activated protein kinase has been solubilized from HL-60 cell membranes, separated from protein kinase C (PKC) by anion exchange chromatography, partially purified by isoelectric focusing (IEF) and defined as a single band of 97 kD using a denaturation-renaturation protocol. Further, renatured kinase activities towards self (autophosphorylation) and exogenous substrate (myelin basic protein) are markedly enhanced by TNFα and ceramide consistent with the proposition that this kinase is involved in signal transmission. C. Experimental Procedures 1. Reagents Buffers, lipids (phosphatidylserine and diolein) , lysine-rich histone (type Ills) , insulin, transferrin, leupeptin, soybean trypsin inhibitor, hexokinase, glucose-6-phosphate dehydrogenase, phosphoamino acid standards and bovine myelin basic protein (MBP) were purchased from Sigma Chemical Co. HPLC grade solvents were from Fisher Scientific. [7- 3 P]ATP (3000 Ci/mmol) was from NEN/Du Pont. Fetal bovine serum was obtained from GIBCO-BRL. P81 phosphocellulose paper and DE52 anion exchange resin were from Whatman. The EGFR peptide was synthesized as described (40) . C8-ceramide may be readily prepared by those skilled in the art. TNFα was provided by Genentech, and is commercially available. 2. Cells HL-60 cells were grown in suspension culture in RPMI 1640 medium supplemented with 10% fetal bovine serum, penicillin (10,000 units ml "1 ), streptomycin (10,000 mg ml" 1 ) , serine (16 mg ml "1 ), L-asparagine (8.4 mg ml" 1 ) and glutamine (16.7 mg ml" 1 ) as described (3) . Prior to treatment with ceramide or TNFα, cells were resuspended (1 x 10 6 ml "1 ) in serum-free RPMI containing bovine insulin and human transferrin (5 μg ml" 1 each) for 2 h at 37°C. 3. Membrane preparation All procedures were conducted at 4°C. Cells (3 x 10 8 ml "1 ) were homogenized with a tight fitting dounce homogenizer in 25 mM HEPES (pH 7.4) containing 5 mM EGTA, 50 mM NaF, and protease inhibitors (10 mg ml" 1 each of leupeptin and soybean trypsin inhibitor) . The homogenate was centrifuged at 1000 x g for 10 min and the nuclei-free supernatant was centrifuged at 12,000 x g for 7 min. The resulting supernatant was centrifuged at 250,000 x g for 30 min. To eliminate cytosolic contaminants, the membrane pellet was washed with HEPES buffer and recentrifuged at 250,000 x g for 30 min. For solubilization of ceramide-activated protein kinase activity, the following detergents were tested: Triton X-100 (1%) , CHAPS (10 mM) , b-octylglucoside (1.5%) and sodium deoxycholate (1 mM) . Membranes were incubated in detergent for 10 min at 4°C. Particulate material was then removed by centrifugation at 15,000 x g for 10 min prior to assay for kinase activity. 4. Assay of protein kinase C activity The membrane pellet was resuspended in HEPES buffer (25 mM HEPES, pH 7.4, 5 mM EGTA with protease inhibitors) and solubilized with 1% Triton X-100 as described above. DE52 anion exchange chromatography was employed for fractionating proteins by stepwise elution with increasing concentrations of NaCl from 0.05 M to 0.4 M. PKC activity was measured by the transfer of 32 P from [7~ 32 P]ATP to lysine-rich histones using a standard reaction mixture containing 20 mM HEPES (pH 7.5), 10 mM MgClj, 2 mM dithiothreitol, 1 mg ml" 1 lysine-rich histones, 100 mM [7- 32 P] ATP (1000 dpm pmol "1 ) , phosphatidylserine (80 mg ml' 1 ), 1,2-diolein (8 mg ml" 1 ) and portions of the column eluate as described (2) . The final concentration of free Ca 2+ was 0.5 mM. Phospholipid- and Ca + -independent activity was measured in the absence of Ca 2+ , phosphatidylserine and diolein. Portions of the reaction mixture were transferred to P81 phosphocellulose paper, washed extensively and 3 P-labeled histones were quantified by Cerenkov counting. All incubations were performed under conditions determined as linear for time and enzyme concentration. 5. Assay of Ceramide-activated protein kinase The kinase reaction mixture contained 15 ml intact membrane, solubilized membrane or DE52 column fractions, 75 μl assay buffer (50 mM HEPES, pH 7.4, 20 mM MgCl 2 ) and 30 μl ATP (150 μM final concentration for EGFR peptide and 50 mM final concentration for MBP; 4000 dpm pmol "1 ) . The substrate used was 30 μl EGFR peptide (4 mg ml" 1 in 25 mM HEPES, pH 7.4) or 30 μl bovine MBP (5 mg ml "1 ) . For studies of lipid activation, C8-ceramide was dried under N 2 and added to the assay buffer by sonication for 2 min at 37 °C. All assays were performed under conditions determined as linear for time and enzyme concentration. Phosphorylation of the EGFR peptide was terminated by addition of 30 μl of 0.5 M ATP in 90% formic acid. Samples were applied to a C18 Sep-pak cartridge (Millipore) and phosphorylated peptide was eluted with 99.9% acetonitrile/0.1% trifluoroacetic acid. The eluates were lyophilized, resuspended in buffer (6 M guanidine hydrochloride, 200 mM Tris, pH 8.5) and resolved by HPLC using a C18 reverse phase column (Dynamax, 4.6 mm i.d., Rainin) . The peptide was eluted with a linear gradient of acetonitrile and quantified by measuring the Cerenkov radiation associated with 1 ml fractions (17, 40) . Enzyme activity was determined from the transfer of 32 P from [7- 3 P]ATP to the EGFR peptide and from the specific radioactivity of [γ- 32 ] ATP. Phosphorylation of MBP was terminated by addition of 30 μl of 5X Laemmli sample buffer. Samples were boiled for 2 min and electrophoresed on 15% SDS-polyacrylamide gels, followed by autoradiography and Cerenkov counting of labeled bands. 6. Isoelectric focusing of the kinase activity toward the EGFR peptide Membrane extracts prepared from 1 x 10 9 cells were fractionated by DE52 anion exchange chromatography as described above. The flow-through fraction containing protein kinase activity toward the EGFR peptide was subjected to focusing by a Rotofor preparative isoelectric focusing cell (Bio-Rad) . The pH gradient was achieved by pre-focusing a 2% ampholyte solution (pH range 3-10) for 1.5 h. To prevent protein aggregation, Triton X-100 and glycerol were added to final concentrations of 1% and 5%, respectively. Focusing was performed at a constant power of 12 W for 4 h and 20 fractions of 2 ml each were collected. The pH of individual fractions was measured and a 5 μl aliquot from each fraction was analyzed for EGFR peptide phosphorylation and MBP phosphorylation as described above. To assess protein purification after isoelectric focusing, an aliquot of each fraction was applied to a 12% SDS gel and stained with Coomassie blue. Measurement of the protein content in individual fractions was not possible by the method of Bradford (27) due to interference of ampholytes with the reagents in these assays. 7. Renaturation of ceramide-activated protein kinase Microsome membranes were prepared as described above from untreated, ceramide-treated, or TNF-treated cells. For some experiments, the membrane was solubilized with 1% Triton X-100 and chromatographed on a DE52 anion exchange column. Intact membrane and the flow-through column fraction were electrophoresed on a 10% SDS-gel polymerized with or without MBP (50 μg ml "1 ) or the EGFR peptide (500 μg ml "1 ) . Renaturation of the kinase activity was performed according to the method described for calmodulin-dependent protein kinase II (86) . Briefly, the gel was washed with two changes of wash buffer (50 mM Tris, pH 7.4, 5 mM 2-mercaptoethanol) containing 20% isopropanol at room temperature for 1 h, and once in wash buffer without isopropanol for 1 h. Denaturation was accomplished by incubation of the gel in two changes of 6 M guanidine HC1 in wash buffer for 1 h each. Renaturation was accomplished by incubation of the gel overnight at 4°C in wash buffer containing 0.04% Tween-20. The gel was then equilibrated for 10 min at room temperature in kinase reaction mixture (25 mM HEPES, pH 7.4, 10 mM MgCl 2 , 0.5 mM EGTA and 5 mM NaF) . After equilibration, [7„ 32 P]ATP (50 μM final concentration) was added to the reaction mixture for varying lengths of time. Autophosphorylation was terminated by removal of the reaction mixture and the gel was washed with water for 10 min followed by 6 changes of buffer (5% tricholoracetic acid, 1% sodium pyrophosphate) for 2 h. The gel was then autoradiographed. In some cases phosphorylated bands were excised and subjected to Cerenkov counting. 8. Measurement of ATP and depletion of ATP in membranes The endogenous ATP level was measured by an enzymatic assay using hexokinase and glucose-6-phosphate dehydrogenase (88) . For this procedure, an aliquot of membrane (equivalent to 15 x 10 6 cells) was incubated with reaction mixture containing 25 mM HEPES, 200 mM D-glucose, 8 mM MgCl 2 , 1 mM NADP, 3 units of glucose-6-phosphate dehydrogenase, and 0.03 units of hexokinase. Absorbance was read on a spectrophotometer at 340 nm. The limits of detection of ATP by this assay was 7 mM. Additionally, to remove trace amounts of ATP, a membrane fraction equivalent to 15 x 10 6 cells was pre-treated with 0.03 units of hexokinase for 5 min prior to incubation with ceramide. 9. Phosphorylation of the exogenous substrate. MBP, by renatured kinase contained within a polyacrylamide gel Membrane from untreated or TNF-treated cells was electrophoresed and kinase was renatured and autophosphorylated as above. The gel was then washed for 20 min with 4 changes of 50 mM Hepes buffer, pH 7.4. Bands corresponding to 97 kD and to 75-90 kD were excised, crushed and incubated for varying lengths of time with 10 ml MBP (5 mg ml "1 ) and 40 ml kinase reaction mixture in the presence of [7~ 32 P]ATP (50 mM final concentration) . MBP phosphorylation reactions were terminated as above. After centrifugation to pellet gel particles, the supernates were electrophoresed and phosphorylation was measured as above. 10. Phosphoamino acid analysis Phosphoamino acid analysis was carried out by a modification of the method of Boyle et al. (84) . Briefly, autophosphorylated bands were excised from the denaturation-renaturation gel and hydrolyzed in 6 N HCl at 110°C for 1 h. The hydrolysates were dried by Speed-vac overnight, resuspended in dH 2 0 and applied to an AG 1-X8 anion exchange column (Bio-Rad) . Phosphoamino acids were eluted from the column with 0.5 N HCl and analyzed by one-dimensional thin-layer electrophoresis. Individual amino acids were detected by ninhydrin staining and autoradiography. 11. Protein assay Protein content was measured according to the method of Bradford (27) using BSA as a standard. D. Results 1. Solubilization of membrane-bound protein kinase activity toward the EGFR peptide Previous studies showed that exogenous ceramide enhanced phosphorylation of the EGFR immunoprecipitated from A431 cells on a specific site Thr669 (40, 18) . Subsequently, a ceramide-activated protein kinase was defined as a membrane-bound activity capable of phosphorylating a synthetic peptide derived from the amino acid sequence surrounding Thr669 of the EGFR (residues 663 to 681) . Activity was optimal in a buffer containing Mg 2+ at neutral pH. Other cations including Ca + , Mn + and Zn 2+ were incapable of sustaining kinase activity. To extract this activity from membranes, several detergents were tested. For these studies microsomal membranes were treated with detergent, centrifuged to remove particulate material, and recovery of kinase activity was determined by measuring phosphorylation of the EGFR peptide as described. Table II shows that kinase activity toward the EGFR peptide was quantitatively recovered after treatment with the nonionic detergent Triton X-100 or the zwitterionic detergent CHAPS. In comparison, treatment with b-octylglucoside or sodium deoxycholate resulted in 27% and 47% loss of the activity, respectively. For all subsequent studies with solubilized membrane, Triton X-100 was used. Table II Effect of detergents on ceramide-activated protein kinase. Membrane proteins were extracted by incubation of HL-60 cell membranes in the absence (control) or presence of detergents. Extracts were assayed for kinase activity towards the EGFR peptide as described in Experimental Procedures. 2. Fractionation of protein kinase activity toward the EGFR peptide and PKC by anion exchange chromatography In an attempt to separate the other major membrane-bound serine/threonine protein kinase, protein kinase C (PKC) , from the activity phosphorylating the EGFR peptide, the solubilized membrane preparation was subjected to mild anion exchange chromatography using a PE52 column at neutral pH. Membrane proteins were eluted from the column with a stepwise gradient of NaCl. Each fraction was tested for PKC activity by phosphorylation of histone or for ceramide- activated protein kinase activity by phosphorylation of the EGFR peptide. Figure 16B shows that the PKC activity was, as previously described (2) , found entirely in the fractions eluting between 0.05 and 0.1 M NaCl. In contrast, 80% o protein kinase activity toward the EGFR peptide was detecte in the flow-through fraction of the column and 20% was i the 0.2-0.4 M NaCl fractions (Figure 16B) . Hence, under th conditions employed for this assay, the fractions containin PKC activity did not phosphorylate the EGFR peptide. 3. Phosphorylation of myelin basic protein by the fractions containing activity toward the EGFR peptide The ceramide-activated protein kinase apparently recognizes the same minimal substrate motif (X-Ser/Thr-Pro-X) as MAP kinases (mitogen-activated protein kinases) (18) . Since bovine MBP has often been used as a substrate to assess activity of MAP kinases, it was tested as a substrate for the kinase which displays activity towards the EGFR peptide using the fractions separated by anion exchange chromatography. Figure 17 shows that the majority of activity toward MBP was observed in the column flow-through and wash, and a smaller amount of the activity was detected in the fractions eluted with 0.2-0.4 M NaCl. In contrast, little activity, if any, could be detected in fractions eluted with 0.05-0.1 M NaCl (Figure 17) . Hence, the kinase activity toward MBP correlated closely with the activity toward the EGFR peptide. 4. Isolation of the kinase activity toward the EGFR peptide by isoelectric focusing To further resolve the protein kinase activity toward the EGFR peptide, the flow-through fraction of the anion exchange column was mixed with 2% ampholytes and subjected to isoeletric focusing at a constant power of 12 W for 4 h. A linear pH gradient was achieved under these conditions. Protein kinase activity towards MBP was observed in fractions 8-15. The majority of the activity was detected in fractions 11 and 12 (Figure 18) . The pi of the protein kinase in fraction 12 was 7.05. Recovery of total activity applied to the focusing apparatus in fractions 8-15 was 30%, and 60% of this activity was contained in fractions 11 and 12. An identical phosphorylation profile was observed when the EGFR peptide was used as a substrate (data not shown) . In addition to phosphorylated MBP, a faint band of 97 kP was observed in fraction 12. This phosphorylated band became even more obvious when a large sample volume was applied to an SPS-polyacrylamide gel and its appearance was independent of the presence of exogenous substrate (Figure 19) . 5. Renaturation of the protein kinase activity isolated by isoelectric focusing To determine whether the 97 kP phosphorylated protein isolated by IEF might represent an autophosphorylatmg kinase, subcellular fractions were first separated by SPS-PAGE and then renaturation of kinase activity was attempted according to the method described for calmodulin-dependent protein kinase II (86) . It was reasoned that if the 97 kP protein was an auto¬ phosphorylatmg kinase, its activity might be reconstituted by this procedure. Alternatively, if the 97 kP protein was a membrane protein that co-focused by IEF with the kinase that recognized MBP and also served as its substrate, it should not be detected, since potential substrates would be physically separated from the kinase by SPS-PAGE prior to renaturation and initiation of phosphorylation. For these studies, samples of intact membrane, flow-through fraction from DE52 anion exchange chromatography of solubilized membrane, and of cytosol were compared. Figure 20 shows that a single band at 97 kP was observed in intact and fractionated membrane samples. In contrast, 4 bands appeared in the cytosolic sample, with Mr values of 57, 44, 42 and 34 kP. The 44/42 kP bands correspond to the molecular weights reported for MAP kinases in these cells. Polymerization of MBP or of the EGFR peptide into the gel did not affect phosphorylation. These results indicate that the 97 kP protein is an autophosphorylatmg protein kinase. Phosphoamino acid analysis showed that phosphorylation occurred on serine residues. It should be noted that the small amount of kinase activity toward MBP that eluted from the PE-52 column from 0.2-0.4 M NaCl renatured as a 57 kD protein. 6. Ceramide enhances the kinase activity detected by renaturation To demonstrate activation of the kinase by ceramide, HL-60 cells were treated at 37°C with C8-ceramide for 15 min prior to preparation of membranes and SDS-PAGE. Figure 21 shows that treatment of intact cells with ceramide (0.03 to 3 mM) induced a dose-dependent enhancement of auto-phosphorylation of the 97 kD protein with a maximal effect at 0.3 mM. At this concentration, ceramide enhanced autophosphorylation four-fold. Incorporation of ceramide into the gel or addition of ceramide to the kinase reaction mixture during in situ phosphorylation did not enhance kinase activity. Alternatively, microsomal membranes were used to assess the effect of ceramide on autophosphorylation of the 97 kD protein kinase. For these studies, membranes were incubated with ceramide, subjected to SDS-PAGE and then denaturation- renaturation was performed. Under these conditions, ceramide enhanced autophosphorylation to the same extent as when added to intact cells (data not shown) . The concentration of ATP in the membranes was below the limit of 7 mM detected by the hexokinase assay. Moreover; the enhancement of autophosphorylation by ceramide was not affected by pre-treatment of the membrane fraction with hexokinase to deplete traces of ATP. These studies provide additional evidence that the membrane-bound 97 kP protein was a ceramide-activated, autophosphorylatmg protein kinase. Furthermore, these investigations show that activation of the kinase by ceramide was independent of ATP. Addition of ceramide to the PE52 flow-through fraction resulted in a small (1.5-fold) increase in phosphorylation of the EGFR peptide under Vmax conditions. It should be noted that solubilization exposes membrane protein to the entirety of membrane constituents and that many of these components, including much of the ceramide, co-purify with the kinase even through isoelectric focusing. Alternately, it is possible that a target other than ceramide-activated protein kinase is the direct target for ceramide action. 7. TNFα enhances the autophosphorylating kinase activity detected by renaturation To demonstrate stimulation of the 97 kP kinase by TNFα, HL-60 cells were treated with TNFα for 20 min prior to preparation of membranes, SPS-PAGE and renaturation of kinase activity. Renatured kinase activity exhibited time-dependent autophosphorylation. Enhancement of autophosphorylation in response to TNFα (1 nM) , was detectable at 2 min and reached four-fold of control at 15 min (Figure 22). Treatment of intact cells with TNFα (0.1 to 10 nM) induced a dose-dependent enhancement of auto¬ phosphorylation of the 97 kP protein with a maximal effect at 10 nM (not shown) . This is the same range of TNF concentrations which have been shown to activate the sphingomyelin pathway in human leukemia (HL-60) cells (59) and stimulate differentiation of these cells into monocytes (6) . Phospho-amino acid analysis showed that kinase autophosphorylation occurred exclusively on serine residues in TNF-stimulated cells as in unstimulated cells. 8. TNFα enhances renatured kinase activity toward the exogenous substrate. MBP To demonstrate that the enhancement of autophosphorylation by TNFα represents a general increase in kinase activity, gel slices corresponding to 97 kP were excised and incubated with the exogenous substrate MBP in kinase reaction mixture containing [7- 32 P]ATP (89) . After termination of reactions, samples were electrophoresed and visualised by autoradiography. Figure 23 shows that stimulation of cells with TNFα resulted in seven-fold enhancement of MBP phosphorylation by the 97 kP kinase. Similar results were obtained with ceramide- reated cells (not shown) . Enhancement of MBP phosphorylation was detectable at 10 min and maximal at 1 h. A brief period of autophosphorylation (approximately 10 min) markedly enhanced the rate of utilization of exogenous substrate. MBP was not phosphorylated by incubation with a gel slice from the region corresponding to 75-90 kP, which would contain PKC. E. Piscussion The present studies identify a 97 kD protein kinase as ceramide-activated protein kinase. Kinase activity was quantitatively extracted from microsomal membrane with Triton X-100 and separated from PKC by anion exchange chromatography. Isoelectric focusing showed that the kinase resolved as a single peak with a pi of 7.05. Co-purification of the kinase activity toward MBP and the EGFR peptide provides further support for the notion that this activity is proline-directed. Like other members of the proline-directed family of protein kinases, ceramide- activated protein kinase undergoes autophosphorylation. Reconstitution of the kinase activity by denaturation- renaturation demonstrated that ceramide-activated protein kinase was a single band of 97 kP and that it was exclusively membrane-bound. Ceramide and TNFα were observed to enhance the kinase activity whether added to intact cells or to a membrane fraction in the absence of ATP. 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SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Sloan-Kettering Institute For Cancer Research (ii) TITLE OF INVENTION: PURIFIED CERAMIDE-ACTIVATED PROTEIN KINASE, ASSAY FOR IDENTIFYING AGENTS WHICH ACT ON SAME, AND METHODS OF USING SAID AGENTS (iii) NUMBER OF SEQUENCES: 2 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Cooper ~ Dunham (B) STREET: 30 Rockefeller Plaza (C) CITY: New York (D) STATE: New York (E) COUNTRY: USA (F) ZIP: 10112 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentln Release #1.24 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: PCT/US93/(not yet known) (B) FILING DATE: 12-NOV-1993 (vii) PRIOR APPLICATION DATA: (A) APPLICATION NUMBER: US 07/976,378 (B) FILING DATE: 13-NOV-1992 (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: White, John P. (B) REGISTRATION NUMBER: 28,678 (C) REFERENCE/DOCKET NUMBER: 4206 -A-PCT/JPW/AJM (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (212) 977-9550 (B) TELEFAX: (212) 977-9809 (C) TELEX: 422523 COOP UI (2) INFORMATION FOR SEQ ID NO:l: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 4 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: unknown (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: protein (vi) ORIGINAL SOURCE: (A) ORGANISM: homo sapien (xi) SEQUENCE DESCRIPTION: SEQ ID NO:l: Pro Leu Thr Pro 1 (2) INFORMATION FOR SEQ ID NO:2 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 19 amino acids (B) TYPE: ammo acid (C) STRANDEDNESS: unknown (D) TOPOLOGY: unknown (ii) MOLECULE TYPE: protein (vi) ORIGINAL SOURCE: (A) ORGANISM: homo sapien (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: Glu Leu Val Glu Pro Leu Thr Pro Ser Gly Glu Ala Pro Asn Gin 1 5 10 15 Ala Leu Leu Arg

Claims: What is claimed is: 1. A purified membrane-bound ceramide-activated protein kinase having an apparent molecular weight of about 97 kP as determined by SPS polyacrylamide gel electrophoresis, which protein kinase is capable of specifically phosphorylating the threonine residue in a Pro-Leu-Thr-Pro-containing polypeptide. 2. A method of determining whether an agent is capable of specifically inhibiting the phosphorylation activity of the ceramide-activated protein kinase of claim 1 which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro-containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of inhibiting the activity of the ceramide-activated protein kinase; and (c) determining whether the agent inhibits the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically inhibiting the activity of the ceramide-activated protein kinase. 3. The method of claim 2, wherein the polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro is human epidermal growth factor receptor. 4. A method of determining whether an agent is capable of specifically stimulating the phosphorylation activity of the ceramide-activated protein kinase of claim 1 which comprises: (a) contacting the protein kinase with a predetermined amount of a polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro, and the agent, under conditions i) which would permit activity of the protein kinase to be linear with respect to time and protein kinase concentration in the absence of the agent, and ii) which would permit the specific phosphorylation by the protein kinase of a predetermined number of the threonine residues in such predetermined amount of the Pro-Leu-Thr-Pro-containing polypeptide in the absence of the agent; (b) quantitatively determining the percentage of such predetermined number of threonine residues which are specifically phosphorylated in the presence of the agent, thereby determining whether the agent is capable of stimulating the activity of the ceramide-activated protein kinase; and (c) determining whether the agent stimulates the activity of a non-ceramide-activated kinase, so as to determine whether the agent is capable of specifically stimulating the activity of the ceramide-activated protein kinase. 5. The method of claim 4, wherein the polypeptide containing the amino acid sequence Pro-Leu-Thr-Pro is human epidermal growth factor receptor. 6. A method of treating a subject having an inflammatory disorder which comprises administering to the subject an agent capable of inhibiting the phosphorylation activity of a ceramide-activated protein kinase of T helper cells and macrophage cells of the subject in an amount effective to inhibit said phosphorylation activity, thereby reducing the inflammation associated with the disorder. 7. The method of claim 6, wherein the inflammatory disorder is rheumatoid arthritis. 8. The method of claim 6, wherein the inflammatory disorder is ulcerative colitis. 9. The method of claim 6, wherein the inflammatory disorder is graft versus host disease. 10. The method of claim 6, wherein the inflammatory disorder is lupus erythematosus. 11. The method of claim 6, wherein the inflammatory disorder is septic shock. 12. A method of treating a human subject infected with HIV so as to reduce the proliferation of HIV in the human subject which comprises administering to the human subject an agent capable of inhibiting the phosphorylation activity of a ceramide-activated protein kinase of HIV-infected cells of the human subject in an amount effective to inhibit said activity, thereby reducing the proliferation of HIV in the human subject. 13. A method of treating a subject having a disorder associated with poor stem cell growth, which comprises administering to the subject an agent capable of stimulating the phosphorylation activity of a ceramide- activated protein kinase of the stem cells of the subject in an amount effective to stimulate said phosphorylation activity, thereby stimulating stem cell growth. 14. The method of claim 13, wherein the disorder associated with poor stem cell growth is aplastic anemia. 15. The method of claim 13, wherein the agent is interleukin-I.

Inventors: GOLDE DAVID W, KOLESNICK RICHARD N, GOLDE, DAVID, W., KOLESNICK, RICHARD, N.
Applicants: GOLDE DAVID W, KOLESNICK RICHARD N, SLOAN KETTERING INST CANCER, GOLDE, DAVID, W., KOLESNICK, RICHARD, N., SLOAN-KETTERING INSTITUTE FOR CANCER RESEARCH
Application year: 1993
Main classification: A61K33
IPC codes: A61K38, A61K45, A61P7, A61P9, A61P29, A61P31, A61P43, C12N9, C12Q1, C12R1