Abstract:
Methods of utilizing the arrestin-2/sTAM-1 complex as a therapeutic target. The methods include treating cells of a living organism to mediate an interaction between arrestin-2 and STAM-1 adapter protein molecules, wherein the interaction is characterized by the arrestin-2 adapter protein molecule directly binding to the STAM-2 adapter protein molecule. Pharmacological agents can be identified for therapeutic uses by determining whether the pharmacological agent disrupts the interaction between the arrestin-2 and STAM-1 adapter protein molecules.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/348,724, filed May 26, 2010, the contents of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support by National Institutes of Health (NIH) grant GM075159. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention generally relates to the use of the arrestin-2/STAM-1 complex as a therapeutic target, for example, to identify and develop pharmacological agents capable of treating medical diseases, such as the treatment of metastasis in cancer patients and myocardial infarction. 
         [0004]    Chemokines are a family of small cytokines, or proteins, that are secreted by cells of certain organisms, and in particular the cells of all vertebrates. Chemokines interact with G protein-linked transmembrane receptors, or chemokine receptors, found on the surfaces of their target cells. Of interest to the present invention are the CXC family of chemokines (α-chemokines), and in particular the CXC chemokine receptors (CXCR) to which CXC chemokines bind. 
         [0005]    The CXC chemokine receptor 4 (CXCR4), a G protein-coupled receptor (GPCR), upon activation by its cognate ligand stromal-cell derived factor-1α (SDF-1α/CXCL12), is known to be rapidly internalized and targeted into the degradative pathway by a ubiquitin-dependent mechanism. See Marchese, A., and Benovic, J. L., Agonist-promoted ubiquitination of the G protein-coupled receptor CXCR4 mediates lysosomal sorting, J. Biol. Chem. 276, 45509-45512 (2001); Shenoy, S. K., McDonald, P. H., Kohout, T. A., and Lefkowitz, R J., Regulation of receptor fate by ubiquitination of activated beta 2-adrenergic receptor and beta-arrestin, Science 294, 1307-1313 (2001); and Marchese, A., Raiborg, C., Santini, F., Keen, J. H., Stenmark, H., and Benovic, J. L., The E3 ubiquitin ligase AIP4 mediates ubiquitination and sorting of the G protein-coupled receptor CXCR4, Dev. Cell 5, 709-722 (2003). Activation by CXCL12 induces rapid and transient phosphorylation of serine residues 324 and 325 within the carboxyl-terminal tail (C-tail) of CXCR4, thereby promoting binding to the E3 ubiquitin ligase atrophin-I interacting protein 4 (AIP4) via a novel WW-domain mediated interaction culminating in ubiquitination of vicinal lysine residues (Marchese et al. (2003); Bhandari, D., Robia, S. L., and Marchese, A., The E3 ubiquitin ligase atrophin interacting protein 4 binds directly to the chemokine receptor CXCR4 via a novel WW domain-mediated interaction, Mol. Biol. Cell. 20, 1324-1339 (2009)). This is followed by internalization of CXCR4 onto early endosomes where the ubiquitin moiety serves as a sorting signal to direct the receptor to lysosomes for proteolysis (Marchese and Benovic (2001); Marchese et al. (2003)). 
         [0006]    In general, the ubiquitin moiety on ubiquitinated receptors interacts with ubiquitin binding domains (UBD) found in several proteins of the endosomal sorting complex required for transport (ESCRT) machinery (Raiborg, C., and Stenmark, H., The ESCRT machinery in endosomal sorting of ubiquitylated membrane proteins, Nature 458, 445-452 (2009); Shields, S. B., Oestreich, A. J., Winistorfer, S., Nguyen, D., Payne, J. A., Katzmann, D. J., and Piper, R., ESCRT ubiquitin-binding domains function cooperatively during MVB cargo sorting, J. Cell Biol. 185, 213-224 (2009)). The ESCRT machinery is made up of four distinct protein complexes (ESCRT 0-III) that act in a sequential and coordinated manner to target ubiquitinated receptors into multivesicular bodies, which then fuse with lysosomes where degradation occurs. Recruitment into this pathway takes place by the initial recognition of the ubiquitinated receptor by ESCRT-0, which then subsequently recruits ESCRT-I to the endosomal membrane, followed by recruitment of ESCRT II and III, culminating in proper execution of the sorting process (Williams, R. L., and Urbe, S., The emerging shape of the ESCRT machinery, Nat. Rev. Mol. Cell Biol. 8, 355-368 (2007); Raiborg and Stenmark (2009)). Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) is understood to be a critical element of ESCRT-0 and has been shown to mediate down regulation of several cell surface signaling receptors (Bache, K. G., Brech, A., Mehlum, A., and Stenmark, H., Hrs regulates multivesicular body formation via ESCRT recruitment to endosames, J. Cell Biol. 162, 435-442 (2003); Kanazawa, C., Morita, E., Yamada, M., Ishii, N., Miura, S., Asao, H., Yoshimori, T., and Sugamura, K., Effects of deficiencies of STAMs and Hrs, mammalian class E Vps proteins, on receptor downregulation, Biochem. Biophys. Res. Commun. 309, 848-856 (2003); Abella, J. V., Peschard, P., Naujokas, M. A., Lin, T., Saucier, C., Urbe, S., and Park, M., Met/Hepatocyte growth factor receptor ubiquitination suppresses transformation and is required for Hrs phosphorylation, Mol. Cell Biol. 25, 9632-9645 (2005); Hasdemir, B., Bunnett, N. W., and Cottrell, G. S., Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) mediates post-endocytic trafficking of protease-activated receptor 2 and calcitonin receptor-like receptor, J. Biol. Chem. 282, 29646-29657 (2007)). One such cell surface signaling receptor is CXCR4 (Marchese et al. (2003)). The ubiquitin moiety on CXCR4 is thought to interact with the ubiquitin interacting motif (UIM) found in HRS, thereby targeting CXCR4 into the degradative pathway. 
         [0007]    Together with HRS, signal-transducing adaptor molecule (STAM) forms ESCRT-O, STAM was originally identified as an adaptor protein involved in cytokine signaling (Takeshita, T., Arita, T., Asao, H., Tanaka, N., Higuchi, M., Kuroda, H., Kanecko, K., Munakata, H., Endo, Y., Fujita, T., and Sugamura, K.; Cloning of a novel signal-transducing adaptor molecule containing an SH3 domain and ITAM, Biochem, Biophys, Res. Commun. 225, 1035-1039 (1996); Takeshita, T., Arita, T., Higuchi, M., Asao, H., Endo, K., Kuroda, H., Tanaka, N., Murata, K., Ishii, N., and Sugamura, K.; STAM, signal transducing adaptor molecule, is associated with Janus kinases and involved in signaling for cell growth and c-myc induction, Immunity 6, 449-457; (1997). Two STAM isoforms exist, STAM-1 and STAM-2, which share 53% amino acid identity and may be redundant in their function (Lohi, O., Poussu, A., Merilainen, J., Kellokumpu, S., Wasenius, V. M., and Lehto, V. P., EAST, an ipidermal growth factor receptor- and Eps 15-associated protein with Src homology 3 and tyrosine-based activation motif domains, J. Biol. Chem., 273, 21408-21415 (1998); Endo, K., Takeshita, T., Kasai, H., Sasaki, Y., Tanaka, N., Asao, H., Kikuchi, K., Yamada, M., Chenb, M., O&#39;Shea, J. J., and Sugamura, K., STAM2, a new member of the STAM family, bindign to the Janus kinases, FEBS Lett, 477, 55-61 (2000); Pandey, A., Fernandez, M. M., Steen, H., Blagoev, B., Nielsen, M. M., Roche, S., Mann, M., and Lodish, H. F., Identification of a novel immunoreceptor tyrosine-based activation motif-containing molecule, STAM2, by mass spectrometry and its involvement in growth factor and cytokine receptor signaling pathways, J. Biol. Chem., 275, 38633-38639 (2000); Yamada, M., Ishii, N., Asao, H., Murata, K., Kanazawa, C., Sasaki, H., and Sugamura, K., Signal-transducing adaptor molecules STAM1 and STAM2 are required for T-cell development and survival, Mol. Cell Biol., 22, 8648-8658 (2002). Similar to HRS, STAM also binds to ubiquitin and may act in concert with HRS to recruit ubiquitinated receptors for lysosomal sorting (Asao, H., Sasaki, Y., Arita, T., Tanaka, N., Endo, K., Kasai, H., Takeshita, T., Endo, Y., Fujita, T., and Sugamura, K., Hrs is associated with STAM, a signal-transducing adaptor molecule, Its suppressive effect on cytokine-induced cell growth, J. Biol. Chem., 272, 32785-32791 (1997); Takata, H., Katao, M., Denda, K., and Kitamura, N., A hrs binding protein having a Src homology 3 domain is involved in intracellular degradation of growth factors and their receptors, Genes Cells 5, 57-69 (2000); Bache, K. G., Raiborg, C., Mehlum, A., and Stenmark, H., STAM and Hrs are subunits of a multivalent ubiquitin-binding complex on early endosomes, J. Biol. Chem., 278, 12513-12521 (2003b); Kanazawa et al., (2003). STAMs may also modulate endosomal sorting by virtue of their ability to interact with endosomal associated deubiquitinating enzymes AMSH (associated molecule with the SH3 domain of STAM) and UBPY, which may modulate the ubiquitination status of both receptors and/or the sorting machinery (McCullough, J., Clague, M. J., and Urbe, S., AMSH is an endosome-associated ubiquitin isopeptidase, J. Cell Biol., 166, 487-492 (2004); Bowers, K., Piper, S. C., Edeling, M. A., Gray, S. R., Owen, D. J., Lehner, P. J., and Luzio, J. P., Degradation of endocytosed epidermal growth factor and virally ubiquitinated major histocompatibility complex class I is independent of mammalian ESCRTII, J. Biol. Chem., 281, 5094-5105 (2006); McCullough, J., Row, P.e., Lorenzo, O., Doherty, M., Beynon, R., Clague, M. J., and Urbe, S., Activation of the endosome-associated ubiquitin isopeptidase AMSH by STAM, a component of the multivesicular body-sorting machinery, Curr. Biol., 16, 160-165 (2006); Row, P. E., Prior, L. A., McCullough, J., Clague, M. J., and Urbe, S., The ubiquitin isopeptidase UBPY regulates endosomal ubiquitin dynamics and is essential for receptor down-regulation, J. Biol. Chem., 281, 12618-12624 (2006); Kong, C., Su, X., Chen, P. I., and Stahl, P. D., Rin1 interacts with signal-transducing adaptor molecule (STAM) and mediates epidermal growth factor receptor trafficking and degradation, J. Biol. Chem., 282, 15294-15301 (2007); Ma, Y. M., Boucrot, E., Villen, J., Affar el, B., Gygi, S. P., Gottlinger, H. G., and Kirchhausen, T., Targeting of AMSH to endosomes is required for epidermal growth factor receptor degradation, J. Biol. Chem., 282, 9805-9812 (2007). Recently, STAMs have been implicated in endoplasmic reticulum to Golgi trafficking, possibly via their interaction with coat protein II proteins (Rismanchi, N., Puertollano, R., and Blackstone, C., STAM adaptor proteins interact with COPII complexes and function in ER-to-Golgi trafficking, Traffic 10, 201-217 (2009). However, their role in GPCR trafficking and signaling is believed to be relatively unknown. 
         [0008]    It has been recently shown that arrestin-2 mediates endosomal sorting of CXCR4 (Bhandari, D., Trejo, J., Benovic, J. L., and Marchese, A., Arrestin-2 interacts with the ubiquitin-protein isopeptide ligase atrophin-interacting protein 4 and mediates endosomal sorting of the chemokine receptor CXCR4, J. Biol. Chem., 282, 36971-36979 (2007). Non-visual arrestins, arrestin-2 and arrestin-3 (also known as β-arrestin1 and β-arrestin2, respectively), are generally known for their ability to regulate GPCR desensitization, internalization and signaling (Moore, C. A., Milano, S. K., and Benovic, J. L., Regulation of receptor trafficking by GRKs and arrestins, Ann. Rev. Phy., 69, 451-482 (2007), although their role in endosomal sorting remains relatively unexplored. Arrestin-2 interacts with and co-localizes with AIP4 on early endosomes, where it targets CXCR4 for lysosomal sorting (Bhandari et al., 2007). In addition to mediating ubiquitination of CXCR4 at the plasma membrane, AIP4 also interacts with and mediates ubiquitination of HRS, likely on endosomes. However, the function of the ubiquitin moiety remains unknown (Marchese et al., 2003). How arrestin-2 may integrate with AIP4 and HRS to carry out CXCR4 sorting into the degradative pathway remains poorly understood. 
         [0009]    It is believed that others have used pharmacological agents that directly target CXCR4 to antagonize agonist (CXCL12) evoked CXCR4 signaling mediated events. A major disadvantage of this approach is that directly targeting CXCR4 is not specific, as it would modulate all intracellular signaling cascades activated by CXCR4. A major caveat with this approach is that it has the potential of producing unintended consequences, such as deleterious side-effects. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    The present invention provides methods of utilizing the arrestin-2/STAM-1 complex as a therapeutic target. 
         [0011]    According to a first aspect of the invention, a method is provided that includes treating cells of a living organism to mediate an interaction between an arrestin-2 adapter protein molecule and a STAM-1 adapter protein molecule, wherein the interaction is characterized by the arrestin-2 adapter protein molecule directly binding to the STAM-1 adapter protein molecule. The treatment preferably involves subjecting a cell of the living organism to a pharmacological agent, and then determining whether the pharmacological agent modulates, for example, disrupts or enhances, the interaction between the arrestin-2 adapter protein molecule and the STAM-1 adapter protein molecule. 
         [0012]    According to a second aspect of the invention, a method is provided that involves identifying a pharmacological agent to treat metastasis of a cancer in living organisms. The method includes treating cells of a living organism with the pharmacological agent, and then determining whether the pharmacological agent disrupts an interaction (binding) between an arrestin-2 adapter protein molecule and a STAM-1 adapter protein molecule of cells of the living organism. If the pharmacological agent disrupts the interaction, the method may further entail treating a second living organism with the pharmacological agent to treat metastasis of a cancer in the second living organism, for example, by decreasing CXCR4 levels and/or inhibiting CXCL12-evoked cell migration in the second living organism. 
         [0013]    A technical effect of the invention is the ability to interact arrestin-2 with the ESCRT machinery to modulate endosomal sorting of CXCR4. In particular, an interaction between the adaptor proteins arrestin-2 and STAM-1 has been identified that enables the arrestin-2/STAM-1 complex to be used as a therapeutic target to modulate CXCR4 levels and to modulate CXCL12-evoked cell migration, which can be extended to use of the arrestin-2/STAM-1 complex to identify and develop novel pharmacological agents capable of targeting the arrestin-2/STAM-1 interaction for therapeutic intervention. In a particular example, the arrestin-2/STAM-1 interaction may be blocked or otherwise disrupted, which can have therapeutically beneficial effects, for example, in the treatment of metastasis in cancer patients, and particularly cancers that have elevated levels of CXCR4 in the tumor cells. 
         [0014]    Data obtained from investigations leading to the invention have indicated that the arrestin-2/STAM-1 complex serves to negatively regulate the cellular levels of CXCR4 upon activation with its cognate ligand (CXCL12), in other words, stabilizes CXCR4 levels in cells. In particular, interaction regions have been mapped between STAM-1 and arrestin-2 in both proteins, and over-expression of these regions in cells has been shown to disrupt the interaction and accelerate CXCR4 degradation. Over-expression of these regions has also been shown to inhibit CXCL12 evoked cell migration, while leaving signaling to extracellular regulated kinases 1 and 2 intact. As such, the arrestin-2/STAM-1 complex potentially represents a highly useful cellular target to decrease CXCR4 levels and to modulate cell migration by intentionally mediating the interaction between arrestin-2 and STAM-1, while leaving a subset of the intracellular signaling cascades and other functions of CXCR4 intact. As such, targeting the arrestin-2/STAM-1 complex may be particularly useful to inhibit migration of tumor cells, and thus metastasis, in patients who have cancers in which CXCR4 levels are elevated. The interface mediating the interaction between arrestin-2 and STAM-1 may be further useful as a target to develop and identify pharmacological agents that may disrupt the interaction between arrestin-2 and STAM-1, with the goal of using them as therapeutics to treat diseases in which reducing CXCR4 level/signaling and migration would be beneficial. 
         [0015]    Prior art methods of modulating CXCR4 signaling have directly targeted CXCR4, thereby affecting all intracellular signaling pathways activated by CXCR4 and thus may have many unintended consequences. In contrast, the present invention targets the recently discovered arrestin-2/STAM-1 protein complex that shows specificity to a subset of CXCR4 related signaling and functional events. Therefore, another advantage of the invention is that side effects or unintended consequences are likely to be minimized by targeting the arrestin-2/STAM-1 complex. Also, by targeting the intracellular arrestin-2/STAM-1 complex, accelerated agonist-promoted degradation of CXCR4 occurs and cell migration can be inhibited. No other targets and/or agents that have this dual effect on CXCR4 degradation/migration are believed to be known. In addition, the capability to modulate both of these endpoints with a high degree of specificity would be particularly important outcomes for treating tumor metastasis. 
         [0016]    The use of pharmacological agents that target the arrestin-2/STAM-1 complex may also be applicable to the treatment of other aspects related to cancer, such as tumor cell invasion, proliferation and angiogenesis. Additional potential uses of the arrestin-2/STAM-1 complex include the treatment of HIV/AIDS infection, WHIM (wart, hypogammaglobulinemia, infection, and myelokathexis) syndrome and opioid induced hyperalgesia. In each of these diseases, by targeting the arrestin-2/STAM-1 complex and enhancing CXCR4 degradation, decreased cellular levels of CXCR4 may reduce HIV infection, decrease CXCR4 signaling observed in WHIM syndrome patients and reduce pain in patients who suffer from opioid-induced hyperalgesia. In addition, targeting the arrestin-2/STAM-1 may be beneficial to treat highly metastatic cancers that are not CXCR4-dependent, such as those that have increased or amplified epidermal growth factor receptor expression. 
         [0017]    Other potential therapeutic uses for targeting the arrestin-2/STAM-1 complex include the treatment of patients who suffer from cardiac and lung ischemia. Immediately after a cardiac ischemic event, cells in the heart release SDF-1α (the cognate ligand of CXCR4). Release of SDF-1α appears to mobilize progenitor cells in the bone marrow to travel to the ischemic site in the heart, where they initiate cardiac tissue repair in an attempt to restore cardiac function. The bone marrow-derived cells express CXCR4 and travel to the site of injury in response to the presence of SDF-1α released after the ischemic event. Increasing the mobilization of cells and improving their motility in response to SDF-1α could potentially increase the mobilization of bone marrow derived cells to the site of injury, with the potential for enhancing the repair mechanisms and benefiting individuals who suffer from cardiac ischemia following a heart attack. In that the invention identifies the arrestin-2/STAM-1 complex as a therapeutic target to modulate CXCR4 levels and CXCL12-evoked cell migration, a beneficial effect of enhancing the interaction may be the ability to improve cell mobility and increase the ability of bone marrow-derived progenitor cells to travel to the heart to initiate tissue repair. As such, while disrupting/blocking the arrestin-2/STAM-1 interaction is believed to be therapeutically beneficial for certain treatments, such as in the treatment of cancer, enhancing/promoting the interaction may be therapeutically beneficial as a treatment for cardiac ischemia. 
         [0018]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  contains representative blots from one of three ( FIG. 1A-FIG .  1 C) independent experiments, and illustrates interactions between arrestin-2 and ESCRT-0. In  FIG. 1A , equimolar amounts (about 134 nM) of GST (glutathione S-transferase) immobilized on glutathione-Sepharose resin and GST-arrestin-2 were incubated with lysates from HEK293 cells transiently transfected with FLAG-STAM-1, FLAG-STAM-2 or FLAG-HRS. Bound proteins were detected by immunoblotting using the anti-FLAG M2 antibody. In  FIG. 1B , equimolar amounts (about 117 nM) of GST-STAM-1, GST-STAM-2 and GST immobilized on glutathione-Sepharose resin were incubated with purified arrestin-2 (about 212 nM). Bound arrestin-2 was detected using an anti-arrestin-2 monoclonal antibody. In  FIGS. 1A and 1B , blots were stripped and reprobed using an anti-GST antibody to determine the levels of the GST fusion proteins used in the binding assay. In  FIG. 1C , lysates from HeLa cells either transiently transfected with HA-arrestin-2, HA-arrestin-3 or empty vector (pcDNA3) were incubated with antibodies to immunoprecipitate transfected as described below. Immunoprecipitates (IP) and lysates were analyzed by SDS-PAGE and immunoblotting as indicated. 
           [0020]      FIG. 2  contains representative blots from one of three independent experiments, and illustrates the regulation of the arrestin-2/STAM-1 interaction with CXCR4. In  FIG. 2A , HeLa cells transiently transfected with HA-arrestin-2 were serum starved as described below, followed by treatment with 30 nM CXCL12 for about thirty to about sixty minutes. Cell lysates were subject to immunoprecipitation using monoclonal anti-HA and isotype control antibodies. Immunoprecipitates and lysates were analyzed by SDS-PAGE and immunoblotting to detect endogenous STAM-1 and HA-arrestin-2. Immunoblots were subject to densitometric analysis and the bar graph represents the average STAM-1 binding±S.E.M. normalized to the level of HA-arrestin-2 in the immunoprecipitates. STAM-1 binding to arrestin-2 was significantly increased upon agonist treatment as compared to vehicle. Data were analyzed by one-way ANOVA followed by a Bonferroni&#39;s post hoc test (*p&lt;0.05). In  FIG. 2B , STAM-1 is preferentially ubiquitinated upon CXCR4 activation. HEK293 cells co-transfected with HA-CXCR4, FLAG-STAM-1 or FLAG-STAM-2 and HA-ubiquitin were treated with 100 nM CXCL12 for 30 min. FLAG-STAM-1/2 were immunoprecipitated using an anti-FLAG antibody, followed by SDS-PAGE and immunoblotting to detect incorporated HA-ubiquitin. Blots were stripped and re-probed for FLAG-STAM-1/2 to assess loading. Cell lysates were analyzed for the presence of HA-CXCR4. 
           [0021]      FIG. 3  contains representative micrographs from three independent experiments (bars=20 μm), and illustrates the co-localization of Arrestin-2, STAM-1 and CXCR4 on early endosomes. In  FIG. 3A , serum-starved HEK293 cells expressing HA-CXCR4-YFP were treated with 30 nM CXCL12 or vehicle for about 30 minutes. Cells were fixed, permeabilized and double stained with anti-STAM-1 (red) and anti-EEAI (blue). White puncta in the merged images represents co-localization between all three proteins. The percent co-localization between CXCR4-YFP and STAM-1 was quantified as described below. The bar graph represents the percent co-localization between CXCR4-YFP and STAM-1 in vehicle and SDF treated cells±S.E.M. from 10 cells. Data were analyzed by Student t-test *p&lt;0.0001. In  FIGS. 3B , C and D, serum-starved HeLa cells were treated with about 30 nM CXCL12 or vehicle for about 30 minutes Cells were fixed, permeabilized and triple stained with anti-STAM-1 (green), anti-EEA1 (blue) and anti-CXCR4 (red) ( FIG. 3B ), triple stained with anti CXCR4 (red), anti-arrestin-2/3 (green) and anti-EEA1 (blue) ( FIG. 3C ); and HeLa cells expressing YFP-STAM-1 were double stained with arrestin-2/3 (red) and EEA1 (blue) ( FIG. 3D ). White puncta in the merged images represent co-localization between all three proteins. Co-localization between CXCR4 and STAM-1 ( FIG. 3B ; 20%), CXCR4 and arrestin ( FIG. 3C ; 30.7%), and YFP-STAM-1 and arrestin-2 ( FIG. 3D ; 26%). were quantified as described below. Inset represents 4-8× the size of the boxed region. DIC (differential interference contrast) images are shown. 
           [0022]      FIG. 4  show data represent the mean±S.E.M. from three independent experiments, and illustrates that STAM-1 negatively regulates CXCR4 degradation. HEK293 cells stably expressing HA-CXCR4 were transfected with control (GAPD) and STAM-1 siRNA as described below. Cells were treated with vehicle (PBS containing about 0.01% BSA) or about 30 nM CXCL12 for about three hours and receptor levels were determined by immunoblotting followed by densitometric analysis. The bar graph represents the average amount of CXCR4 degraded ±S.E.M. from three independent experiments (*p&lt;0.05, unpaired t-test). In  FIG. 4B , CXCR4 recycling was measured in HEK293 cells transfected with FLAG-CXCR4 and siRNA as described for  FIG. 4A . Surface receptors were labeled with the M1 anti-FLAG antibody followed by treatment with about 30 nM of CXCL12 for about forty-five minutes in DMEM containing about 0.1% BSA, about 20 mM HEPES (pH 7.4) and about 1 mM Ca2+. Antibody remaining on the cell surface was stripped by two rapid washes with Ca2+/Mg2+ free PBS containing about 0.04% EDTA. Cells were then incubated in DMEM containing about 1 mM Ca2+ and about 10 μM AMD3100 (CXCR4 antagonist) and incubated at about 37° C. for about thirty to about sixty minutes. The amount of antibody reappearing on the cell surface was quantified by ELISA as described below, and used as an indicator of receptor recycling. Bars represent the percentage of internalized receptor that recycled ±S.E.M. from three independent experiments. In  FIG. 4C , bars represent the percentage of cell surface receptors internalized in cells treated with CXCL12 as compared with vehicle treated cells. The error bars represent S.E.M. from three independent experiments. In  FIG. 4D , HeLa cells were transfected with GAPD and AMSH siRNA and treated and analyzed as in A. 
           [0023]      FIG. 5  contains representative blots from one of three independent experiments, and illustrates that the STAM-1 GAT domain is both necessary and sufficient for arrestin-2 binding. In  FIG. 5A , STAM-1 truncation mutants are represented schematically. Binding to GST-arrestin-2 is represented by (+) and (−) on the right as assessed by data shown in  FIG. 11 . In  FIG. 5B , equimolar amounts (about 600 nM) of GST-arrestin-2 and GST were incubated with lysates from HEK293 cells transiently transfected with FLAG-tagged full-length-STAM-1 or STAM-1-ΔGAT. In  FIG. 5C , equimolar amounts (about 117 nM) of GST-STAM-1-GAT and GST were incubated with lysates from HEK293 cells transiently transfected with FLAG-tagged arrestin-2. In  FIGS. 5B and 5C , bound proteins were detected by immunoblotting, followed by staining blots with Ponceau-S to assess the amount of GST fusion protein used in the binding assay. 
           [0024]      FIG. 6  illustrates that the expression of the GAT domain disrupts the arrestin-2/STAM-1 interaction and accelerates CXCR4 degradation. In  FIG. 6A , Lysates from HeLa cells co-transfected with HA-arrestin-2 and FLAG-STAM-1-GAT (SI-GAT) or empty vector (PCMV) were incubated with anti-HA and IgG control antibodies. Immunoprecipitates were analyzed by immunoblotting to detect bound endogenous STAM-1 and lysates were analyzed to assess expression of the various constructs.  FIG. 6A  contains representative blots from one of three independent experiments. In  FIG. 6B , HA-CXCR4 degradation was assessed in HEK293 cells expressing FLAG-STAM-1-GAT or empty vector (pCMV) as described below.  FIG. 6C  graphically represents the percent of receptor degraded. Error bars represent S.E.M. from three independent experiments. Data were analyzed by two-way ANOVA and followed by a Bonferroni&#39;s post hoc test. (*p&lt;0.0001). 
           [0025]      FIG. 7  contains representative blots from one of three independent experiments, and illustrates mapping of the STAM-1 binding domain on arrestin-2.  FIG. 7A  schematically represents arrestin-2 truncation mutants used in the binding studies. Binding between GST-STAM-1 and HA-tagged arrestin-2 truncation mutants is shown as weak (+), intermediate (++) and strong (+++) on the right of the graph. In  FIG. 7B , equimolar amounts (about 234 nM) of GST-arrestin-2, GST-Arr2-(25-161) and GST were incubated with lysates from HEK293 cells transiently transfected with FLAG-tagged STAM-1 and empty vector (PCMV-10). In  FIG. 7C , equimolar amounts (about 276 nM) of GST-STAM-1, GST-STAM-1-GAT and GST alone were incubated with lysates from HEK293 cells transiently transfected with FLAG-Arr-2-(25-161). In  FIG. 7C , bound proteins were detected by immunoblotting using an anti-FLAG antibody and blots were stained with Ponceau-S to assess the amount of GST-tagged protein used in the binding assay. 
           [0026]      FIG. 8  illustrates that the expression of Arr2-(25-161) disrupts the STAM-1/arrestin-2 interaction and accelerates CXCR4 degradation. In  FIG. 8A , lysates were prepared from HeLa cells co-transfected with T7-STAM-1, HA-arrestin-2 and increasing amounts (about 0.1 μg and about 2.5 μg) of FLAG-Arr2 (25-161). Lysates were divided into equal aliquots and incubated with either an anti-T7 polyclonal antibody or protein G agarose alone (control). Immunoprecipitates were analyzed by immunoblotting to detect bound HA-arrestin-2 and endogenous HRS and lysates were analyzed to assess the expression of the various constructs.  FIG. 8A  shows representative blots from one of three independent experiments. In  FIG. 8B , HA-CXCR4 degradation was assessed in HEK293 cells expressing FLAG-Arr2-(25-161) or empty vector (PCMV) as described below.  FIG. 8C  is a graphical representation of percent receptor degraded. Error bars represent S.E.M. from three independent experiments. Data were analyzed by two-way ANOVA and followed by a Bonferroni&#39;s post hoc test. (*p&lt;0.0001). Shown are representative blots from one of three independent experiments. 
           [0027]      FIG. 9  illustrates that disrupting the STAM-1/arrestin-2 interaction inhibits HRS ubiquitination but does not effect on CXCR4 and STAM-1 ubiquitination. In  FIGS. 9A and 9B , HEK293 cells stably expressing HA-CXCR4 were transfected with FLAG-ubiquitin and STAM-1-GAT domain or pCMV. In  FIG. 9B , HeLa cells were transfected with HA-ubiquitin, T7-STAM-1 and STAM-1-GAT or pCMV. In  FIG. 9C , cells were transfected as in  FIG. 9A , except T7-HRS was also transfected. Cells were serum starved and treated with about 30 nM CXCL12 for about thirty to about sixty minutes, followed by immunoprecipitation and immunoblotting to detect incorporated ubiquitin as described below. Shown are representative blots from six ( FIG. 9A ) or three ( FIGS. 9B and 9C ) independent experiments. 
           [0028]      FIG. 10  schematically represents a proposed mechanism for the role of the STAM-1/arrestin-2 complex in endosomal sorting of CXCR4. CXCR4 is ubiquitinated by the E3 ubiquitin ligase AIP4 at the plasma membrane, after which it is internalized onto early endosomes, although ubiquitination is not required for this process. Endosomes ubiquitinated CXCR4 is recognized by HRS, likely by an interaction involving the ubiquitin moiety (red) on CXCR4 and the UIM of HRS, and possibly via an interaction with arrestin-2. Arrestin-2 then interacts with STAM-1, which serves to recruit AIP4 culminating in the ubiquitination of HRS. It is speculated that this may trigger a conformational change in HRS induced by an interaction between the ubiquitin moiety (blue) and the internal UIM. CXCR4 is subsequently committed to downstream interactions with ESCRT-I-III, while arrestin-2, STAM-1, AIP4 and auto-inhibited HRS are recycled such that another round of sorting can take place. 
           [0029]      FIGS. 11A through 11E  represent equimolar amounts (about 134 nM) of GST-arrestin-2 and GST immobilized on glutathione-Sepharose resin were incubated with lysates from HEK293 cells transiently transfected with various FLAG-STAM-1 constructs. Bound proteins were detected by immunoblotting using the anti-FLAG M2 antibody, followed by staining with Ponceau-S ( FIGS. 11B-E ) or immunoblotting for GST ( FIG. 11A ) to assess the amount of GST fusion proteins used in the binding assays. Shown are representative blots from one of three independent experiments. 
           [0030]      FIGS. 12A and 12B  represent equimolar amounts (about 117 nM) of GST-STAM-1 and GST immobilized on glutathione-Sepharose resin were incubated with lysates from HEK293 cells transiently transfected with HA-arrestin-2 constructs. Bound proteins were detected by immunoblotting using the anti-HA antibody, followed by staining with Ponceau-S to assess the amount of GST fusion proteins used in the binding assay. Shown are representative blots from one of three independent experiments. 
           [0031]      FIG. 13A  represents data obtained when EGFR (epidermal growth factor receptor) degradation was assessed in HeLa cells transfected with FLAG-STAM-1-GAT, FLAG-Arr-2-(25-161) or pCMV. Cells were treated with about 100 ng/ml EFG for about one hour, followed by immunoblotting as described below. Shown are representative immunoblots from one of three independent experiments.  FIG. 13B  is a bar graph that represents the amount of EGFR degraded as compared to vehicle treated cells±S.E.M. from three independent experiments. Data were analyzed by one-way analysis of variance and were found not to be significantly different. 
           [0032]      FIG. 14  contains a table identifying primers used for generating DNA constructs used in investigations leading to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The chemokine receptor CXCR4, a G protein-coupled receptor, is targeted for lysosomal degradation via a ubiquitin-dependent mechanism that involves the endosomal sorting complex required for transport (ESCRT) machinery. The following reports an investigation which showed that arrestin-2 interacts with ESCRT-0, a protein complex that recognizes and sorts ubiquitinated cargo into the degradative pathway. In particular, STAM-1 (but, notably, not related STAM-2) interacts directly with arrestin-2 and co-localizes with CXCR4 on EEA1 positive early endosomes. Depletion of STAM-1 by RNAi and disruption of the arrestin-2/STAM-1 interaction accelerates agonist-promoted degradation of CXCR4, suggesting that STAM-1 via its interaction with arrestin-2 negatively regulates CXCR4 endosomal (lysosomal) sorting via ubiquitination of HRS. The investigation provided mechanistic insight into the role that arrestin-2 has on targeting CXCR4 into the degradative pathway and furthered an understanding of the complex molecular events that mediate endosomal sorting of GPCRs. Interestingly, disruption of the STAM-1/arrestin-2 interaction blocks agonist-promoted ubiquitination of HRS, but not CXCR4 and STAM-1 ubiquitination. Data from the investigation described below suggest a mechanism whereby arrestin-2, via its interaction with STAM-1, is able to modulate CXCR4 sorting by regulating the ubiquitination status of HRS. 
         [0034]    Provided below are descriptions of materials and methods utilized in the investigation. 
         [0035]    The following cell lines, antibodies and reagents were obtained and used in the investigation. HEK (Human embryonic kidney) 293 cells (obtained from Microbix of Toronto, Canada) and HeLa cells (American Type Culture Collection) were maintained in a Dulbecco&#39;s modified Eagles medium (DMEM; Hyclone) supplemented with 10% fetal bovine serum (FBS; HyClone Laboratories, Logan, Utah USA). HRS (M-79) rabbit polyclonal, the β-arrestin1/2 rabbit polyclonal (H-290) and mouse monoclonal (21-B1) antibodies were obtained from Santa Cruz Biotechnology (Santa Cruz, Calif. USA). Anti-GST monoclonal antibody and gluthathione Sepharose 4B resin were obtained from GE Healthcare (Buckinghamshire, UK). Anti-CXCR4 antibody previously described in Marchese and Benovic (2001). STAM-1 and AMSH polyclonal antibodies were obtained from ProteinTech Group (Chicago, Ill. USA). Arrestin-2 and anti-EEA1 monoclonal antibodies were obtained from BD Biosciences (San Jose, Calif. USA). Anti-HA polyclonal and monoclonal antibodies were obtained from Covance (Berkeley, Calif. USA). Anti-FLAG M2, M1, and M2-horse radish peroxidase conjugated monoclonal antibodies, FLAG polyclonal antibody, Alkaline Phosphatase conjugated anti-mouse antibody, and AMD3100 were obtained from Sigma (St. Louis, Mo. USA). An alkaline phosphatase substrate kit was obtained from Bio-Rad (Hercules, Calif. USA). Anti-T7 goat polyclonal antibody was obtained from Abcam (Cambridge, Mass. USA). Anti-epidermal growth factor receptor mouse monoclonal antibody was obtained from StressGen (Ann Arbor, Mich. USA). Anti-actin monoclonal antibody was obtained from MP Biomedicals (Aurora, Ohio USA). Stromal cell-derived factor-1α (CXCL12) and epidermal growth factor were obtained from PeproTech (Rockyhill, N.J. USA). Alexa-Fluor 635-conjugated goat anti-mouse, Alexa-Fluor 594-conjugated anti-rat, Alexa-Fluor 488-conjugated goat anti-rabbit and Alexa-Fluor 568-conjugated goat anti-rabbit antibodies were obtained from Molecular Probes (Eugene, Oreg. USA). The siRNA for GAPD, STAM-1 (GAACGAAGAUCCGAUGUAU) and AMSH (siGENOME SMARTpool D-012202) were obtained from Dharmacon RNA Technologies (Lafayette, Colo. USA). 
         [0036]    The following DNA constructs obtained and used in the investigation were HA-CXCR4, FLAG-ubiquitin, HA-CXCR4-YFP, HA-arrestin-3 and HA-arrestin-2 constructs, as previously described in (Bhandari et al., 2007). Primers used for generating all constructs are listed in a table attached hereto as  FIG. 14 . For STAM-1 truncation mutants (1-195, 1-269, 1-390, 391-540, 337-540, 270-540, 212-540, 144-540), full-length STAM-1 in 3× FLAG-pCMV-10 was amplified by PCR using primers flanking various regions of STAM-1 as indicated above and harboring 5′ and 3′ HindIII and XbaI restriction enzyme sites, respectively. PCR fragments were digested and ligated into the HindIII and XbaI sites of 3×-FLAG pCMV-10 (Sigma). For STAM-1-ΔGAT, the region encompassing amino acid residues 343-377 was deleted by two-step PCR with mutually annealing overlapping primers and flanking primers based on 3× FLAG-pCMV-10. Amplified product was digested and ligated into HindIII and XbaI sites of 3× FLAG-pCMV-10 and pGEX-4T2 (GE Healthcare). For STAM-1-GAT, amino acid residues 296-380 were amplified by PCR from full-length FLAG-STAM-1 and cloned into the HindIII and XbaI sites of 3× FLAG-pCMV-10 and EcoRI and XhoI sites of pGEX-4T2. For arrestin-2-(25-161) constructs, amino acid residues 25-161 were amplified by PCR from HA-arrestin-2-(1-161) and cloned into HindIII and XbaI sites of 3× FLAG pCMV-10 and SmaI and XhoI sites of pGEX-4T2, respectively. For YFP-STAM-1, full-length STAM-1 was amplified from FLAG-STAM-1 and cloned into the HindIII and KpnI sites of pEYFP-C1 vector (Clontech, Mountain View, Calif.). The sequence of all constructs was verified by sequencing. 
         [0037]    The following GST-fusion protein binding assays were obtained and used in the investigation.  Escherichia coli  BL21 cells transformed with GST-fusion protein constructs or empty vector (pGEX-4T2) were grown overnight in Luria Broth (LB) containing about 100 μg/ml ampicillin. The following day, cultures were diluted (about 3.7%) and grown to an OD 600  of about 0.35 to about 0.40 at about 37° C., followed by induction with about 0.1 mM IPTG (isopropyl-1-thio-β- D -galactopyranoside) for about one hour at about 18° C. Cells were then pelleted by centrifugation and resuspended in about 1 mL binding buffer (about 20 mM Tris-Cl (pH 7.4), about 150 mM NaCl, about 0.1% Triton X-100, about 1 mM dithiothreitol, about 10 μg/ml leupeptin, about 10 μg/ml aprotinin, about 10 μg/ml pepstatin-A), followed by sonication and centrifugation. Clarified lysates were incubated with glutathione-Sepharose 4B resin for about one hour, washed and resuspended in binding buffer. Samples were analyzed by SDS-PAGE and stained with Gel-Code blue to estimate the protein amounts by comparing the samples to known amounts of purified bovine serum albumin (Roche, Fraction V). For binding assays, equimolar amounts of purified GST-fusion proteins were incubated with about 100 μl clarified cell lysate of HEK293 cells expressing the desired construct for about two to about four hours at about 4° C. For binding experiments using purified arrestin-2, GST fusion proteins were incubated with about 500 ng arrestin-2 in about 100 μl binding buffer for about one hour at about 4° C. Following incubation, samples were washed three times with binding buffer, eluted in 2× sample buffer by boiling for about 10 minutes, and bound proteins were detected by SDS-PAGE followed by immunoblotting. 
         [0038]    The following degradation assay was obtained and used in the investigation. HEK293 cells stably expressing HA-CXCR4 or HeLa cells expressing endogenous levels of CXCR4 grown on 10-cm dishes were transfected with about 100 nM STAM-1, AMSH or GAPD siRNA using Lipofectamine 2000 transfection reagent (Invitrogen of Carlsbad, Calif. USA). To assess the role of STAM-1 and arrestin-2 minigene constructs on CXCR4 degradation, HEK293 cells grown on 10-cm dishes were co-transfected with about 1 μg HA-CXCR4 and about 9 μg FLAG-STAM-1-GAT, FLAG-arrestin-2-(25-161) or empty vector (pCMV-10) using TransIT-LTI transfection reagent (Mirus of Madison, Wis. USA). About twenty-four hours later, cells were passaged onto poly-L-lysine (about 0.1 mg/ml, Sigma) coated 24-well plates (HEK293 cells) or 6 well plates (HeLa cells) and grown for an additional eighteen to twenty-four hours. Cells were washed once and incubated with DMEM containing about 10% FBS and about 50 μg/ml cyclohexamide to stop protein synthesis for about fifteen minutes at about 37° C. Cells were then incubated with the same medium-containing vehicle (about 0.5% BSA) or about 30 nM CXCL12 for about one, two and three hours. Cells were washed and collected in about 300 μl 2× sample buffer, sonicated and receptor amounts were determined by SDS-PAGE followed by immunoblotting using an anti-HA monoclonal antibody or anti-CXCR4 antibody, as previously described (Marchese, A., Ubiquitination of chemokine receptors, Methods Enzymol, 460, 413-422 (2009). To assess EGFR degradation, HeLa cells grown on six well plates were transfected with about 3 μg FLAG-STAM-1-GAT, FLAG-arrestin-2-(25-161) or empty vector (pCMV-10) using TransIT-LT1 transfection reagent. Forty-eight hrs following transfection cells were incubated with DMEM containing 10% FBS and 50 μg/ml cyclohexamide to stop protein synthesis for 15 min at about 37° C. Cells were then incubated with the same medium containing vehicle (0.5% BSA) or 100 ng/ml EGF for 1 hr. Cells were processed as described above for CXCR4 degradation. 
         [0039]    The following coimmunoprecipitation studies were used in the investigation. HeLa cells were transiently transfected with HA-Arrestin-2, HA-arrestin-3 or empty vector alone (pcDNA3) using TransIT-LT1 transfection reagent. About forty-eight hours later, cells were collected in an approximately 1.5 mL immunoprecipitation buffer (about 20 mM Na 2 PO 4  (pH 6.5), about 150 mM NaCl, about 1% (v/v) Triton-X 100, about 10 μg/ml leupeptin, about 10 μg/ml aprotinin, about 10 μg/ml pepstatin A) and incubated at about 4° C. for about thirty minutes. Cells were sonicated, centrifuged and clarified lysates were incubated with an anti-HA monoclonal antibody or isotype control antibody to immunoprecipitate HA-tagged arrestin-2/3 followed by immunoblotting to detect bound endogenous STAM-1 and HRS. Endogenous arrestins were immunoprecipitated from HeLa cells using an anti-arrestin2/3 mouse monoclonal or isotype control antibody followed by immunoblotting to detect bound endogenous STAM-1 and HRS. To assess the effect of the STAM-1-GAT minigene on the interaction between STAM-1 and arrestin-2, lysates from HeLa cells transfected with HA-arrestin-2 and FLAG-STAM-1-GAT or pCMV were incubated with an anti-HA or isotype control antibody and immunoprecipitates were analyzed for the presence of endogenous STAM-1. To assess the effect of the arrestin-2-(25-161) minigene on the interaction between STAM-1 and arrestin-2, HeLa cells transfected with T7-STAM-1, HA-arrestin-2 and FLAG-arrestin-2-(25-161) or pCMV were incubated with an anti-T7 polyclonal antibody and immunoprecipitates were analyzed for the presence of HA-arrestin-2 and endogenous HRS. 
         [0040]    The following confocal Immunofluorescence microscopy techniques were used in the investigation. HEK293 cells transiently transfected with HA-CXCR4-YFP were passaged onto poly-L-lysine coated coverslips and allowed to grow for about twenty-four hours. HeLa cells were used to examine the distribution of endogenous CXCR4. Cells were washed once with warm DMEM containing about 20 mM HEPES (pH 7.5) and incubated in the same medium for about three to about four hours at about 37° C. Cells were treated with about 30 nM CXCL12 or vehicle for about thirty minutes, fixed with about 3.7% paraformaldehyde and then permeabilized with about 0.05% (w/v) saponin for about ten minutes, similar to a protocol previously described in Bhandari et al. (2007). Cells were co-incubated with STAM-1, EEA1 or arrestin2/3 antibodies. Endogenous CXCR4 in HeLa cells was stained with rat anti-CXCR4 monoclonal antibody. Briefly, after permeabilization and fixation, cells were incubated with about 1% BSA in about 0.05% saponin-PBS for about thirty minutes at about 37° C., followed by incubating with primary antibody for about one hour at about 37° C. Primary antibodies for STAM-1 and EEA1 were used at about 1:100 dilution and against CXCR4 and arrestin2/3 was used at an approximately 1:50 dilution. Cells were washed five times with 0.05% saponin-PBS, followed by incubating with appropriate Alexa-Fluor conjugated secondary antibodies for about thirty minutes at about 37° C. Finally cells were washed with PBS and fixed again with about 3.7% formaldehyde-PBS, and then mounted onto glass slides using mounting media containing DAPI. Samples were analyzed using a Zeiss LSM 510 laser scanning confocal microscope equipped with a Plan-Apo 63×/1.4 oil lens objective. Images were acquired using a 1.4 megapixel cooled extended spectra range RGB digital camera set at 512×512 resolution. Acquired images were analyzed using ImageJ software (version 1.41o) and the amount of co-localization between proteins was determined using the colocalization plug-in feature of MAG Biosystems Software (7.6.2.0). 
         [0041]    The following ubiquitination assays were obtained and used in the investigation. For CXCR4 ubiquitination, HEK293 cells stably expressing HA-CXCR4 grown on 10-cm dishes were transfected with about 3 μg FLAG-ubiquitin. About eight hours later, cells were transfected either with about 10 μg FLAG-STAM-1-GAT, FLAG-Arr2-(25-161) or empty vector (pCMV). The next day, cells were passaged onto 6-cm dishes and allowed to grow for an additional twenty-four hours. The following day, cells were serum starved in DMEM containing about 20 mM HEPES for about three hours and then treated with about 30 nM SDF for about thirty minutes, washed once on ice with cold PBS and collected in an approximately 1 mL lysis buffer (about 50 mM Tris-Cl (pH 7.4), about 150 mM NaCl, about 5 mM EDTA, about 0.5% (w/v) sodium deoxycholate, about 1% (v/v) NP-40, about 0.1% (w/v) SDS, about 20 mM NEM, about 10 μg/ml each of leupeptin, aprotinin and pepstatin A). Samples were transferred into microcentrifuge tubes and placed at about 4° C. for about thirty minutes, sonicated, followed by centrifugation to pellet cellular debris. Clarified cell lysate was incubated with an anti-HA polyclonal antibody and the immunoprecipitates were analyzed by SDS-PAGE followed by immunoblotting using an anti-FLAG antibody conjugated to HRP. 
         [0042]    To detect HRS ubiquitination, HEK293 cells stably expressing HA-CXCR4 were transfected with about 3 μg FLAG-ubiquitin. About eight hours later cells, were co-transfected with about 8 μg FLAG-STAM-1-GAT or empty vector (pCMV-10) and about 2 μg T7-tagged HRS. About twenty-four hours later, cells were passaged onto poly-L-lysine coated 6-cm dishes and the next day cells were serum starved for about four to about five hours in DMEM containing about 20 mM HEPES and were treated with about 30 nM SDF or vehicle alone for about thirty to about sixty minutes. Cells were washed with cold PBS and collected in an approximately 1 ml ubiquitination buffer (about 20 mM Tris-Cl (pH 7.5), about 150 mM NaCl, about 1% Triton-X 100, about 5 mM EDTA, about 20 mM NEM, about 10 μg/ml leupeptin, about 10 μg/ml aprotinin and about 10 μg/ml pepstatin-A), incubated for about thirty minutes at about 4° C., sonicated and clarified by centrifugation. HRS was immunoprecipitated using an anti-HRS polyclonal antibody and immunoprecipitates were analyzed by SDS-PAGE followed by immunoblotting to detect ubiquitinated HRS using an anti-FLAG antibody conjugated to HRP. 
         [0043]    For STAM-1 ubiquitination experiments, HeLa cells grown in 6-well dishes were co-transfected with about 3 μg T7-STAM-1 and about 40 ng HA-ubiquitin. About eight hours later, cells were transfected with about 3 μg FLAG-STAM-1-GAT or empty vector (pCMV-10). About twenty-four hours later, cells were passed onto poly-L-lysine coated 6-cm dishes and the following day cells were serum starved, treated and processed as described above for HRS ubiquitination using a modified ubiquitination buffer (about 20 mM Na 2 PO 4  (pH 6.5), about 150 mM NaCl, about 1% Triton-X 100, about 20 mM NEM and protease inhibitor cocktail). Tagged STAM-1 was immunoprecipitated using an anti-T7 goat polyclonal antibody and immunoprecipitates were analyzed by SDS-PAGE followed by immunoblotting to detect ubiquitinated STAM-1 using an anti-HA monoclonal antibody. 
         [0044]    The following internalization and recycling assays were obtained and used in the investigation. For measuring internalization and recycling of CXCR4, HEK293 cells grown on 10-cm dishes were co-transfected with FLAG-CXCR4 (about 1 μg) and about 100 nM STAM-1 or GAPD siRNA using Lipofectamine 2000 transfection reagent. The next day, cells were passaged onto poly-L-lysine coated 24-well plates and grown for an additional twenty-four hours. Cells were serum starved for about three to about four hours, placed on ice, washed once with DMEM containing about 0.1% BSA, about 20 mM HEPES and about 1 mM Ca2+ and then incubated in the same medium containing the calcium-dependent MI anti-FLAG antibody for about one hour on ice, which labels cell surface receptors only. Cells were washed and incubated in the same medium containing vehicle or about 30 nM CXCL12 for about forty-five minutes at about 37° C. To remove surface bound antibody, cells were washed three times with Ca2+ and Mg2+-free PBS containing about 0.04% EDTA. Cells were incubated in DMEM containing about 1 mM Ca2+ and the CXCR4 antagonist AMD3100 (about 10 μM) to block any further internalization for about thirty to about sixty minutes at about 37° C. The amount of receptor/antibody that recycled back to the cell surface was quantified by incubating cells with an alkaline-phosphatase conjugated goat anti-mouse IgG antibody. Briefly, cells were washed once with PBS containing about 1 mM Ca2+ and then fixed with about 3.7% paraformaldehyde for about five minutes on ice. Following fixation, cells were washed three times and incubated with alkaline phosphatase conjugated goat anti-mouse antibody diluted in PBS containing about 1% BSA for one hour at room temperature. Cells were then washed with PBS and incubated with p-nitrophenyl phosphate diluted in diethanolamine buffer (Bio-Rad) for about five to about fifteen minutes. Reactions were stopped by adding about 0.4 N NaOH and an aliquot was used to measure the absorbance at 405 nm. Percent receptor recycling was calculated by dividing the amount of receptor internalized by the amount of receptors recovered after incubation at different time intervals. To calculate the percent receptor internalization, the amount of receptor remaining on the cell surface was divided by the total number of receptors present on the cell surface before treatment with agonist. 
         [0045]    Statistical analyses performed in the investigation used GraphPad Prism 4.00 for Macintosh (GraphPad Software, San Diego, Calif.; www.graphpad.com). 
         [0046]    The following describes results that were obtained with the investigation. A first phase of the investigation established that arrestins interact with ESCRT-0. Although it has been previously shown that HRS and arrestin-2 mediate endosomal sorting of CXCR4 into the degradative pathway (Marchese et al., 2003; Bhandari et al., 2007), the molecular mechanisms have remained poorly understood. To gain mechanistic insight into this process, the investigation initially examined whether arrestin-2 interacts with ESCRT-0 by determining if it binds to HRS, STAM-1 or STAM-2. To address this, celilysates prepared from HEK293 cells expressing FLAG-tagged STAM-1, STAM-2 or HRS were incubated with bacterially purified GST-arrestin-2 and GST immobilized on glutathione-Sepharose resin. As shown in  FIG. 1A , arrestin-2 bound to STAM-1 and HRS, but only weakly to STAM-2. To rule out the possibility of an intermediate protein mediating the interaction with STAM-1, similar experiments were performed using purified arrestin-2. As shown in  FIG. 1B , GST-STAM-1, but not GST-STAM-2 or GST, bound to purified arrestin-2, which indicated that the interaction between arrestin-2 and STAM-1 is direct and confirming that arrestin-2 binds poorly to STAM-2. To determine whether arrestin-2 associates with ESCRT-0 in cells, HA-arrestin-2, HA-arrestin-3 or empty vector (pcDNA3) were transfected into HeLa cells followed by immunoprecipitation and immunoblotting to detect the presence of endogenous STAM-1 and HRS. Both STAM-1 and HRS were detected in the immunoprecipitates from cells expressing HA-arrestin-2, suggesting that arrestin-2 associates with HRS and STAM-1 in cells ( FIG. 1C ), while HRS, but not STAM-1, was detected in the HA-arrestin-3 immunoprecipitates ( FIG. 1C ). Similarly, endogenous arrestins also co-immunoprecipitated with endogenous STAM-1 and HRS in HeLa cells. Taken together, these data showed that the interaction between STAM-1 and non-visual arrestins is limited to arrestin-2, and that HRS interacts with both arrestin-2 and arrestin-3. Additionally, the data suggested that arrestin-2 exists in complex with a subpopulation of ESCRT-0 that includes STAM-1 and HRS, but not STAM-2. 
         [0047]    The investigation then examined whether the interaction between arrestin and ESCRT-0 was regulated by activation of CXCR4. HeLa cells, which endogenously express CXCR4, were transfected with HA-arrestin-2 and treated with CXCL12 (about 30 nM) or vehicle (about 0.05% BSA-PBS) for various times, followed by immunoprecipitation of tagged arrestin-2 and immunoblotting to detect bound endogenous STAM-1. Activation of CXCR4 enhanced the interaction between STAM-1 and arrestin-2 as early as about thirty minutes after agonist treatment that persisted up to about sixty minutes ( FIG. 2A ). As STAM has been shown to be ubiquitinated (McCullough et al., 2004), the investigation next assessed whether CXCR4 activation promotes ubiquitination of STAM-1. HEK293 cells transfected with FLAG-tagged STAM-1 or STAM-2 and HA-tagged ubiquitin were treated with CXCL12 (about 100 nM) for about thirty minutes, followed by immunoprecipitation of tagged STAM proteins and immunoblotting to detect incorporation of tagged ubiquitin. As shown in  FIG. 2B , STAM-1 was ubiquitinated by agonist activation of CXCR4, whereas STAM-2 was not ubiquitinated. 
         [0048]    To confirm that arrestin-2 and STAM-1 are found within the same intracellular compartment, the investigation examined their distribution in cells by confocal immunofluorescence microscopy. As shown in  FIG. 3A , in HEK293 cells transfected with YFP-tagged CXCR4 (a construct previously described in Bhandari et al., 2009), CXCR4 was mainly localized to the plasma membrane in vehicle-treated cells, whereas endogenous STAM-1 was mainly localized to punctate vesicles distributed throughout the cytoplasm, many of which also co-localized with EEA1, used here as a marker for early endosomes. In contrast, upon agonist treatment, CXCR4 distributed into an intracellular punctate pattern, indicating that it had internalized into vesicles that also contained STAM-1 and EEA1 ( FIG. 3A , bottom panels). The distribution of endogenous CXCR4 in HeLa cells treated with CXCL12 was also examined for about thirty minutes, revealing that CXCR4 co-localized with endogenous STAM-1 ( FIG. 3B ) and arrestin-2/3 ( FIG. 3C ) on EEA1 positive early endosomes. CXCR4 activation also promoted co-localization of arrestin-2/3 and YFP-tagged STAM-1 on early endosomes in HeLa cells ( FIG. 3D ). Taken together, the data suggested that, upon internalization, CXCR4 appears on early endosomes together with arrestin-2 and STAM-1. 
         [0049]    As the data suggest that STAM-1 has a role in endosomal sorting of CXCR4, the investigation then examined agonist-promoted degradation of CXCR4 in cells that were depleted of STAM-1 by RNA interference. HEK293 cells stably expressing HA-CXCR4 were transfected with control or STAM-1 siRNA, followed by treatment with CXCL12 (about 30 nM) for about three hours and receptor degradation was assessed by immunoblot analysis, as previously described in Marchese et al. (2003). As shown in  FIG. 4A , siRNA mediated depletion of STAM-1 led to a moderate, but statistically significant, increase in CXCR4 degradation, as compared to control siRNA treated cells, suggesting that STAM-1 negatively regulates agonist-promoted degradation of CXCR4. As the amount of receptor that is degraded is in part a function of the rate of receptor internalization and recycling, the effect of depleting STAM-1 on CXCR4 internalization and recycling was also examined. Cell surface FLAG-tagged CXCR4 was labeled with the M1 anti-FLAG antibody on ice in the presence of about 1 mM Ca2+, as the M1 antibody binds to the FLAG epitope in a calcium-dependent manner. Cells were washed to remove unbound antibody and the media was replaced with DMEM containing CXCL12 (about 30 nM) in the continued presence of about 1 mM Ca2+ and placed at about 37° C. for forty-five minutes to allow for internalization of the M1/CXCR4 complexes to take place. Antibody remaining on the surface, mostly representing un-internalized receptor, was removed by incubating cells with PBS containing EDTA (about 0.04%), a calcium-chelating agent. The amount of antibody (receptor) that recycled back to the cell surface was quantified by cell surface ELISA in parallel wells that were incubated at about 37° C. for about thirty to about sixty minutes. In control siRNA treated cells, approximately 20% of internalized CXCR4 recycled back to the cell surface after about thirty to about sixty minutes, similar to what was observed in STAM-1 depleted cells, suggesting that STAM-1 depletion had no effect on recycling of CXCR4 ( FIG. 4B ). In addition, agonist-promoted internalization of CXCR4 was similar in STAM-1 depleted cells, as compared to control siRNA treated cells, suggesting that STAM-1 is not involved in CXCR4 internalization ( FIG. 4C ). 
         [0050]    The role of AMSH on agonist-promoted degradation of CXCR4 was also examined. AMSH is a deubiquitinating enzyme that interacts with STAM-1 and negatively regulates endosomal sorting of the epidermal growth factor receptor (EGFR) (see McCullough et al., (2004)). As shown in  FIG. 4D , siRNA mediated depletion of AMSH did not affect agonist-promoted degradation of CXCR4 in HeLa cells, suggesting that AMSH does not regulate endosomal sorting of activated CXCR4. However, CXCR4 levels were elevated in vehicle-treated cells transfected with AMSH siRNA ( FIG. 4D ), suggesting that AMSH may regulate degradation of constitutively internalized CXCR4, similar to what has been recently reported in (Sierra, M. I., Wright, M. H., and Nash, P., AMSH interacts with ESCRT-0 to regulate the stability and trafficking of CXCR4, J. Biol. Chem., jbc.M109.061309, First Published on Feb. 16, 2010, doi: 10.1074/jbc.M109.061309 (2010). Taken together, the data suggested that STAM-1 negatively regulates CXCR4 degradation likely through a mechanism that directly attenuates endosomal sorting. 
         [0051]    The investigation then turned to examining the arrestin-2 binding site on STAM-1. Arrestin-2 was recently reported to positively regulate CXCR4 sorting into the degradative pathway. To gain insight into the function of the arrestin-2/STAM-1 interaction on CXCR4 trafficking, the investigation initially set out to determine the mechanism of the interaction. To accomplish this, the investigation mapped the arrestin-2 binding region on STAM-1 by truncation mutagenesis. STAMs contain multiple domains, characterized by the presence of an amino-terminal VHS domain (Vps27, Hrs, STAM homology), UIM (ubiquitin interacting motif), SH3 (Src homology) domain, ITAM (immunoreceptor based tyrosine activation motif) and a GAT (GGA and TOM1 homologous) domain that partially overlaps with the ITAM (Prag, G., Watson, H., Kim, Y. C., Beach, B. M., Ghirlando, R., Hummer, G., Bonifacino, J. S., and Hurley, J. H., The Vps27/Hse1 complex is a GAT domain-based scaffold for ubiquitin-dependent sorting, Dev. Cell 12, 973-986 (2007); Ren, X., Koer, D. P., Kim, Y. C., Ghirlando, R., Saidi, L. F., Hummer, G., and Hurley, J. H., Hybrid structural model of the complete human ESCRT-0 complex, Structure 17, 406-416 (2009). Several STAM-1 N-terminal and C-terminal truncation mutants were created according to its domain organization, tagged with the FLAG epitope on the amino terminal end ( FIG. 5A ). GST-arrestin-2 and GST immobilized on glutatruone-Sepharose resin were incubated with lysates expressing the various STAM-1 truncation mutants and bound proteins were detected by immunoblotting. The results from these experiments are summarized in  FIG. 5A  and the data are shown in  FIG. 11 . The arrestin-2 binding region was determined to reside between amino acid residues 296-380 on STAM-1. This region encompasses the GAT domain, which has been shown to form two tandem coiled-coil domains (amino-acid residues 301-377) (Prag et al., 2007; Ren et al., 2009). To further confirm that the GAT domain mediates binding to arrestin-2, deletion of the GAT domain completely abrogated STAM-1 binding to arrestin-2 ( FIG. 5B ) and the GAT domain alone fused to GST was able to bind to arrestin-2 ( FIG. 5C ). 
         [0052]    To determine if the interaction between STAM-1 and arrestin-2 is important for CXCR4 trafficking, the investigation initially expressed the GAT domain as a minigene in cells and assessed whether it disrupted the arrestin-2/STAM-1 interaction. HeLa cells transfected with FLAG-SI-GAT and HA-arrestin-2 were subjected to immunoprecipitation using an anti-HA antibody followed by immunoblotting to detect the presence of endogenous STAM-1 in the immunoprecipitates. As shown in  FIG. 6A , expression of the GAT domain disrupted the arrestin-2/STAM-1 interaction. To determine the function of the arrestin-2/STAM-1 interaction on lysosomal targeting of CXCR4, the investigation examined the effect of expressing the GAT domain on CXCR4 degradation. Remarkably, expression of the GAT domain significantly accelerated CXCR4 degradation following agonist treatment as compared to empty vector ( FIGS. 6B and 6C ). Taken together these data suggested that the STAM-1/arrestin-2 interaction negatively regulates CXCR4 sorting to lysosomes. As the STAM-1 GAT domain has been shown recently to bind to HRS and is predicted to be required for the assembly of ESCRT-0 (Ren et al., 2009), it is conceivable that arrestin-2 binding to STAM-1 displaces its interaction with HRS and promotes disassembly of ESCRT-0, which somehow negatively regulates the amount of CXCR4 that is targeted for lysosomal degradation. 
         [0053]    To gain greater insight into this process, the investigation next set out to identify the STAM-1 binding region on arrestin-2 by truncation mutagenesis. Schematic representations of the arrestin-2 truncation mutants used are shown in  FIG. 7A , generally as has been previously described (Bhandari et al., 2007). GST-STAM-1 and GST were incubated with lysates prepared from HEK293 cells expressing various HA-tagged arrestin-2 truncation mutants. The results from these binding experiments are summarized in  FIG. 7A  and the data are shown in  FIG. 12 . Both the N- and C-terminal regions of arrestin-2 bound to GST-STAM1, but not GST, although binding to the N-terminal region appeared to be stronger, suggesting that it represented the main binding region. Further deletion of this region revealed that the STAM-1 binding site on arrestin-2 is between amino acid residues 1-161 ( FIG. 12B ). The investigation next determined if expression of this region as a minigene in cells also disrupted the arrestin-2/STAM-1 interaction. However, when expressed in cells the arrestin-2-(1-161) minigene completely blocked CXCR4 degradation (data not shown). N-terminal lysine residues within arrestin-2 are predicted to serve as phosphosensors and recognize phosphates attached to receptors (Kern, R. C., Kang, D. S., and Benovic, J. L., Arrestin2/clathrin interactionis regulated by key- and C-terminal regions in arrestin2, Biochemistry 48, 7190-7200 (2009), analogous to what has been observed for arrestin-1 (Vishnivetskiy, S. A., Schubert, C., Climaco, G. C., Gurevich, Y. V., Velez, M. G., and Gurevich, V. V. An additional phosphate-binding element in arrestin molecule, Implications for the mechanism of arrestin activation, J. Biol. Chem. 275, 41049-41057 (2000). Therefore the arrestin-2-(1-161) construct may bind to CXCR4 and have a dominant negative effect on CXCR4 internalization. To rule out any effects at the level of internalization, the first twenty-four amino acids from the N-terminus of arrestin-2 were deleted to create arrestin-2-(25-161) and the investigation initially tested the ability of this mutant to bind to STAM-1. As shown in  FIG. 7B , GST fused to arrestin-2-(25-161), but not GST alone, efficiently bound to FLAG-STAM-1 expressed in cells. A FLAG-tagged construct of arrestin-2-(25-161) when expressed in HEK293 cells also bound to GST-STAM-1-GAT, suggesting that the STAM-1/GAT domain binding site on arrestin-2 is located between amino acid residues 25-161 ( FIG. 7C ). 
         [0054]    The investigation next examined whether expression of arrestin-2-(25-161) disrupted the STAM-1/arrestin-2 interaction and modulated CXCR4 degradation. Expression of FLAG-arrestin-2-(25-161) markedly disrupted the interaction between arrestin-2 and STAM-1 ( FIG. 8A ) and significantly accelerated agonist-promoted degradation of CXCR4 ( FIGS. 8B and 8C ), similar to what was observed with the STAM-1 GAT domain ( FIG. 6 ). Taken together these data further indicated that the interaction between STAM-1 and arrestin-2 attenuates CXCR4 trafficking into the degradative pathway. 
         [0055]    Finally, the investigation turned to examining role of the arrestin-2/STAM-1 interaction on the ubiquitination status of CXCR4, STAM-1 and HRS. STAM, through its interaction with several deubiquitinating enzymes, may regulate the ubiquitination status of both cargo and of itself (McCullough et al., 2006; Row et al., 2006). Therefore, one possibility is that the STAM-1/arrestin-2 interaction modulates the ubiquitination status of CXCR4 and STAM-1, thereby facilitating CXCR4 trafficking into the degradative pathway. To examine this possibility, the investigation examined the effect of expressing the GAT domain on the ubiquitination status of both CXCR4 and STAM-1. Surprisingly, expression of the GAT, as compared to empty vector, did not significantly change the ubiquitination status of CXCR4 ( FIG. 9A ) and STAM-1 ( FIG. 9B ), suggesting that the STAM-1/arrestin-2 interaction does not regulate their ubiquitination status. In sharp contrast, expression of the GAT domain blocked CXCR4 mediated ubiquitination of HRS ( FIG. 9C ). Therefore, taken together, the data showed that the STAM-1/arrestin-2 interaction is critical for modulating ubiquitination of HRS, which is likely important for regulating sorting of CXCR4 into the degradative pathway. 
         [0056]    Non-visual arrestins are known for their ability to mediate GPCR desensitization, trafficking and signaling (Moore et al., 2007; Kovacs, J. J., Hara, M. R., Davenport, C. L., Kim, J., and Lefkowitz, R. J., Arrestin development: emerging roles for beta-arrestins ain developmental signaling pathways, Dev. Cell 17, 443-458 (2009). It has been reported that arrestin-2 interacts with AIP4 and mediates endosomal sorting of CXCR4 into the degradative pathway (Bhandari et al., 2007). The investigation reported above extended these findings to provide further mechanistic insight into this unprecedented role of arrestin-2. The data suggested that arrestin-2 mediates multiple interactions with ESCRT-0 on early endosomes, serving to regulate the amount of CXCR4 that is degraded. In view of the results of the investigation, it is believed that arrestin-2 likely links ubiquitinated CXCR4 to ESCRT-0 via an initial interaction with HRS and/or STAM-1. Interestingly, the data revealed that the arrestin-2 interaction with STAM-1 is important for regulating ubiquitination of HRS, which was believed to attenuate HRS sorting function, thereby controlling the extent to which CXCR4 is degraded. Such a mechanism is schematically depicted in  FIG. 10 . 
         [0057]    The investigation employed truncation mutagenesis to narrow the arrestin-2 binding region on STAM-1 to the GAT domain and the STAM-1 binding region on arrestin-2 to amino acid residues 25-161. Expression of both of these domains similarly disrupted the arrestin-2/STAM-1 interaction and enhanced against promoted degradation of CXCR4. The data obtained in the investigation were consistent with the notion that the STAM-1/arrestin-2 interaction negatively regulates sorting of CXCR4 into the degradative pathway. This interaction may be specific to modulating CXCR4 and/or GPCR sorting, as EGFR degradation was not altered by expression of the STAM-1 GAT domain and arrestin-2-(25-161) ( FIG. 13 ). Depletion of STAM-1 by siRNA also enhanced CXCR4 degradation, further revealing that STAM-1 negatively regulates CXCR4 endosomal sorting ( FIG. 4A ). In contrast, it had been previously shown that arrestin-2 promotes CXCR4 sorting (Bhandari et al., 2007) which, when considered with the data obtained in the investigation, indicates that arrestin-2 has opposing effects on CXCR4 degradation. This suggests that arrestin-2 likely acts at multiple steps in the sorting process and may initially act upstream of STAM-1 to positively regulate sorting of CXCR4 into the degradative pathway. Arrestin-2 interacts with the C-tail of CXCR4 (Busillo, J. M., Armando, S., Sengupta, R., Meucci, O., Bouvier, M., and Benovic, J. L., Site-specific phosphorylation of CXCR4 is dynamically regulated by multiple kinases and results in differential modulation of CXCR4 signaling, J. Biol. Chem., 285, 7805-7817 (2010), and therefore it is possible that arrestin-2 binds to CXCR4 on endosomes in order to recruit CXCR4 to ESCRT-0, possibly through an interaction with either HRS and/or STAM-1. This is consistent with the data from the investigation that showed that arrestin-2 co-localizes with CXCR4 and STAM-1 on early endosomes upon agonist activation ( FIG. 3 ). Interestingly, a recent study found that Rim8, a  S. cerevisiae  molecule distantly related to mammalian arrestins, may function to directly recruit a putative 7™ receptor to the ESCRT machinery (Herrador, A., Herranz, S., Lara, D., and Vincent, O., Recruitment of the ESCRT machinery to a putative seven transmembrane-domain receptor is mediated by an arrestin-related protein, Mol. Cell Biol. 30, 897-907 (2010). After arrestin-2 initially directs CXCR4 to ESCRT-0, this is likely followed by an interaction with STAM-1 to attenuate CXCR4 degradation. Therefore, the results from the investigation are consistent with a model in which arrestin-2 influences CXCR4 sorting positively and negatively, and it is a balance of these two activities that dictates the extent to which CXCR4 is degraded. 
         [0058]    The investigation led to the question as to how STAM-1 mediates the negative action of arrestin-2 on CXCR4 degradation. As ubiquitination of HRS is markedly reduced by expression of the GAT domain, it is likely that STAM-1 via its interaction with arrestin-2 regulates the ubiquitination status of HRS to control CXCR4 degradation. This suggests that CXCR4 promoted ubiquitination of HRS ( FIG. 9C ; Marchese et al. (2003)) attenuates its sorting activity. HRS contains a UIM that is thought to bind to ubiquitin moieties on cargo to recruit them into the degradative pathway (Hirano, S., Kawasaki, M., Ura, H., Kato, R., Raiborg, C., Stenmark, H., and Wakatsuki, S., Double-sided ubiquitin binding of Hrs-UIM in endosomal protein sorting, Nat. Struct. Mol. Biol. 13, 272-277 (2006). Interestingly, monoubiquitination of UBD containing proteins is thought to induce an intramolecular interaction between the ubiquitin moiety and the internal UBD, which in a protein such as HRS may induce an auto-inhibitory conformation such that it can no longer bind to ubiquitin moieties on cargo (Hoeller, D., Crosetto, N., Blagoev, B., Raiborg, C., Tikkanen, R., Wagner, S., Kowanetz, K., Breitling, R., Mann, M., Stenmark, H., Dikic, I., Regulation of ubiquitin-binding proteins by monoubiquitination, Nat. Cell Biol. 8, 163-169 (2006). As HRS ubiquitination is reduced by expression of the GAT domain, a loss of auto-inhibition likely enhances its sorting function culminating in enhanced degradation of CXCR4. Therefore, CXCR4 promoted ubiquitination of HRS may occur once HRS has completed its sorting function and CXCR4 has been committed to downstream elements of the degradative pathway ( FIG. 10 ). 
         [0059]    The investigation also raised the question as to how arrestin-2/STAM-1 regulates the ubiquitination status of HRS. It was previously shown that arrestin-2 interacts with AIP4 to regulate endosomal sorting of CXCR4 (Bhandari et al., 2007) and that AIP4 mediates agonist-promoted ubiquitination of HRS (Marchese et al., 2003). Therefore it is possible that arrestin-2, together with STAM-1, may serve to bridge AIP4 and HRS in order to facilitate HRS ubiquitination by AIP4. This is consistent with the investigation&#39;s data that showed that expression of arrestin-2-(25-161) also displaces HRS binding to arrestin-2/STAM-1 ( FIG. 8A ). Alternatively, the arrestin-2/STAM-1 complex may regulate HRS deubiquitination. STAM has been shown to interact with deubiquitinating enzymes, such as AMSH and UBPY, which have been shown to regulate the ubiquitination status of cargo (for example, EGFR, protease activated receptor 2) and/or of STAM itself (McCullough et al., 2004; Row et al., 2006; Hasdemir, B., Murphy, J. E., Cottrell, G. S., and Bunnett, N. W., Endosomal deubiquitinating enzymes control ubiquitination and down-regulation of protease-activated receptor 2, J. Biol. Chem. 284, 28453-28466 (2009). However, from the investigation, the arrestin-2/STAM-1 complex does not appear to modulate the ubiquitination status of CXCR4 or STAM-1 ( FIG. 9 ). In addition, depletion of AMSH did not affect agonist-promoted degradation of CXCR4 ( FIG. 4D ), suggesting that it may not be linked to this process, although it does not exclude the possibility that other DUBs may be involved (Row et al., 2006; Shenoy, S. K., Modi, A. S., Shukla, A. K., Xiao, K., Berthouze, M., Ahn, S., Wilkinson, K. D., Miller, W. E., and Lefkowitz, R. J., Beta-arrestin-dependent signaling and trafficking of 7-transmembrane receptors is reciprocally regulated by the deubiquitinase USP33 and the E3 ligase Mdm2. Proc. Nat. Acad. Sci. USA 106, 6650-6655 (2009). Nevertheless, the results of the investigation were consistent with the notion that the arrestin-2/STAM-1 complex mediates ubiquitination of HRS likely via AIP4. 
         [0060]    Interestingly, the investigation appeared to indicate that STAM-2 is excluded from endosomal sorting of CXCR4, since arrestin-2 binds selectively to STAM-1 ( FIG. 1A ). This suggests that CXCR4 sorting is restricted to ESCRT-0 complexes that contain STAM-1 but not STAM-2. It was also observed through the investigation that activation of CXCR4 selectively enhances STAM-1 ubiquitination over STAM-2 ( FIG. 2B ), further supporting the selectivity of STAM-1 towards CXCR4. However, the arrestin-2/STAM-1 interaction may not be linked to STAM-1 ubiquitination ( FIG. 9B ). Presently, the function of STAM-1 ubiquitination on CXCR4 trafficking remains unknown, although it is possible that it may have a role in some other aspect of CXCR4 related functions. Though polyubiquitination of STAM has been linked to its degradation (Row et al., 2006), it is doubtful that CXCR4 regulates STAM-1 stability as no differences were observed in STAM-1 levels in cells treated with CXCL12 (data not shown). 
         [0061]    In another series of investigations, further work was conducted with the minigenes STAM-1(296-380) (referred to as STAM-1-GAT-domain above) and Arr2(25-161) whereby it was shown that when expressed in cells they attenuate cell migration induced by SDF-1α activation of CXCR4. Cell migration was monitored using two distinct commonly in vitro assays: a scratch assay ( FIG. 15 ) and a trans well assay ( FIG. 16 ). The results of both showed that STAM-1(296-380) and Arr2(25-161) expression in cells attenuates CXCR4-mediated cell migration. The data obtained with this investigation provided significant mechanistic insight into the molecular pathways that mediate CXCR4-induced cell migration and establish the STAM-1/arrestin-2 complex as a potential therapeutic target to treat cancer metastasis. 
         [0062]    On the basis of the above, it can be appreciated that the investigation provided a mechanistic insight into the role of arrestin-2 in endosomal sorting of CXCR4 via multiple interactions with ESCRT-0. The investigation revealed that, via an interaction with STAM-1, arrestin-2 serves as an adaptor to regulate endosomal ubiquitination events that are critical for regulating the sorting of ubiquitinated CXCR4 into the degradative pathway, thereby controlling the amount of CXCR4 that is degraded. On this basis, it was concluded that an interaction between the adaptor proteins arrestin-2 and STAM-1 enables the arrestin-2/STAM-1 complex to be used as a therapeutic target to modulate CXCR4 levels and to modulate CXCL12-evoked cell migration. This aspect of the invention can be extended to the use of the arrestin-2/STAM-1 complex to identify and develop novel pharmacological agents capable of targeting the arrestin-2/STAM-1 interaction for therapeutic intervention, for example, to treat metastasis in cancer patients, and in particular patients with cancers that exhibit elevated levels of CXCR4 in the tumor cells. 
         [0063]    Though the invention has been described in terms of observations and results obtained during an investigation in which a particular series of procedures was performed, the scope of the invention is to be limited only by the following claims.