CELL CULTURE PROCESS

Abstract: A glycoprotein is produced by a process comprising culturing mammalian host cells expressing nucleic acid encoding said glycoprotein in the presence of (a) a factor that modifies growth state in a cell culture, (b) a divalent metal cation that can adopt and prefers and octahedral coordination geometry, and/or (c) a plasma component. In this process, the occupancy of an N-linked glycosylation site occupied only in a fraction of a glycoprotein is enhanced. Such culturing is preferably carried out at a temperature of between about 30 DEG C and 35 DEG C and/or in the presence of up to about 2mM of a butyrate salt and/or in the presence of a cell-cycle inhibitor.

Description: CELL CU LTURE PROCESS Background ot the Invention Field of the Invention The present inv ention concerns a process tor the production ot gKcoproteins in mammalian cell culture More specificalK the invention prov iαes a process tor producing gK coproteins in mammalian ceils that results in enhanced occupancv ot an N-hnked gK cosv iation site occupied onK in a traction ot a glycoprotein A process for increasing the fraction ot Type I tissue plasminogen activator (t-PA ) in a mammalian cell culture is specificalK disclosed Description of Related Disclosures and Technology GKcoproteins man\ ot w hich have been produced by techniques ot recombinant DNA technology, are ot great importance as diagnostic and therapeutic agents In a eukarvotic cell ironment. gK cosv Iation is attached to a secreted or membrane-spanning protein by co- and post-translational modification Proteins destined tor the cell surface are first co-translationally translocated into the lumen of the endopiasmic reticulum (ER) mediated by a signal sequence at or near the ammo terminus of the nascent chain Inside the ER. the signal sequence is usually removed and a high-mannose core o gosacchaπde unit is attached to the asparagine (N) resιdue(s) present as part of the sequence Asn-X-Ser/Thr. where X is any amino acid except, perhaps, pro ne The efficiency of this co-translational glycosylation step is dependent on the presentation of an appropriate conformation of the peptide chain as it enters the endopiasmic reticulum (Impeπali and O'Connor. Pure & Applied Chem . 70 33-40 ( 1998)) Potential N-hnked glycosylation sites may no longer be accessible after the protein has folded (Korn eld & Kornfeld. Ann Rev Biochem 54 631 -664 ( 1985)) Proteins next move from the ER to the Goigi apparatus where further modifications, such as sulfation and processing of the high-mannose oligosacchaπde chain to a complex-type oligosacchaπde occur and the proteins are directed to their proper destinations N-hnked oligosacchaπdes can have a profound impact on the pharmaceutical properties ot gK coprotein therapeutics (e g . inxno halt-lite and bioactivitv ) Different bioprocess parameters (e g . bioreactortype pH. media composition, and ammonia) hav e been shown to atfect protein glycosy lation significant!} Changes in terminal gl cosylation (sialy Iation and galactosv Iation ) and N-gl can branching are the most frequently observed alterations The Carhohxdratc Stntctui of Tissue Plasminogen Actnatoi Tissue plas inogen activator (t-PA ). a glycoprotein. is a muitidomain seπne protease w hose phv siological role is to convert plasminogen to plasm in and thus to initiate or accelerate the process ot fibπnoly sis Initial clinical interest in t-PA was raised because ot its relatively high activ ity in the presence, as compared to the absence of fibrin Wild-type t-PA is a poor enzvme in the aosence ot fibrin, but the presence of fibrin strikingly enhances its ability to activate plasminogen Recombinant human t-PA is used therapeuticalK as a fibrinolvtic agent in the treatment of acute myocardial imarction and pulmonary embolism both conditions usually resulting trom an obstruction of a blood vessel by a fibrin-containing thrombus In addition to its striking fibrin specificity t-PA. exhibits sev eral further distinguishing characteristics (a) T-PA differs from most seπne proteases in that the single-chain form ot the molecule has appreciable enzvmatic activity Toward some small substrates and toward plasminogen in the absence ot fibrin two-chain t-PA has greater activity than one-chain t-PA In the presence ol fibrin however the two torms ot t-PA are equally active (Rijkene/α/ J Biol Chem 2 " 2920-2925(1982) Limene.α/ Thromb Haemost 6461-68 ^ (1990) Bennett et al I Biol Chem 2665191-5201(1991)) Most other seπne proteases exist as zvmogens and require proteolvtic cleavage to a two-chain torm to release full enzvmatic activity (b) The action ot t-PA and can be inhibited by a serpin PAI-1 (Vaughan etal J Clm invest 84 586-591 (1989) Vnmanaα/ J Biol Chem 2593644-3647(1984)) (c) T-PA binds to fibrin in with a j in the μM range 10 (d) T-PA has a rapid clearance that is mediated bv one or more receptors in the liver (Nilsson ttal Thromb Res 39 511-^21(1985) Bugelski el al Throm Res 53 287-303(1 89) Morton etal J Biol Chem 264 7228-7235(1989)) A substantially pure torm ot t-PA was first produced from a natural source and tested tor in activity bv Collen a al U S Pat No 4752603 issued 21 June 1988 (see also Rijken et al J Biol Chem 2^6 7035 \ (1981)) Pennica et al (Nature Q1 214 ( 1983)) determined the DNA sequence ot t-PA and deduced the amino acid sequence from this DNA sequence (U S Pat No 4766075 issued 23 August 1988) Human wild-tvpe t-PA has potential N-hnked glycosylation sites at amino acid positions 117 184218 and 448 Recombinant human t-PA (ACTIVASE® t-PA) produced bv expression in CHO cells was reported to contain approximately 7% bv weight of carbohydrate consisting of a high-mannose oligosacchaπde at position 117 0 and complex oligosacchaπdes at Asn- 184 and Asn-448 (Vehar et al "Characterization Studies of Human Tissue Plasminogen Activator produced bv Recombinant DNA Technology " Cold Spring Harbor Symposia on Quantitative Biology LI 551-562 (1986)) Position 218 has not been found to be glycosylated in native t-PA or recombinant wild-tvpe t-PA Sites 117 and 448 appear alwavs to be glycosvlated while site 184 is thought to be glvcosvlated onlv in a traction of the ^ molecules The t-PA molecules that are glvcosvlated at position 184 are termed! vpe I t-PA and the molecules that are not glvcosvlated at position 184 are termed Type II t-PA In melanoma-derived t-PA the ratio of Tvpe I to Tvpe II t-PA is about 1 1 The most comprehensive analysis of the carbohydrate structures ot CHO cell-derived human t-PA was carried out bv Spellman etal J Biol Chem 264 14100-14111(1989) who showed that at least 17 different Asn-hnked carbohydrate structures could be detected on the protein These ranged from the high- 0 mannose structures at position 117 to di- tπ- and tetra-antennarv N-acetvllactosamine-tvpe structures at positions 184 and 448 Tvpe I and Type II t-PAs were reported to be N-glv cosv lated in an identical wav at Asn- 117 and Asn- 448 positions when isolated from the same cell line For further details see also Parekh et al Biochemistry 28 7644-7662 (1 89) The specific fibπnolvtic activity of Tvpe II t-PA has been shown to be about 0°o greater than that of Tvpe I t-PA (Einarsson et al Biochim Biophvs Acta 830 1-10 (1 85)) Further increased Tvpe I is ^ correlated with increased half-life (Cole etal Fibπnolvsis 1^-22(1993)) However Type II t-PA which lacks a portion of carbohydrate associated with Type I t-PA as well as desialated t-PA demonstrated a longer T 1 - beta than standard t-PA (Beebe and Aronson Thromb Res "1 11-22(1988)) Analysis of the sequence ot t-PA has identified the molecule as hav ing five domains Each domain has been defined with reference to homologous structural or functional regions in other proteins such as trvpsin chvmotrxpsin plasminogen prothrombin tibronectin ana epidermal growth factor (EGF) These domains have been designated starting at the N-terminus ot the amino acid sequence ot t-PA as the finger (F) domain from amino ^ acid 1 to about ammo acid 44 the growth factor ( G) domain trom about ammo acid 4^ to about amino acid 91 ( based on homologv ith EGF ) the krιngle- 1 ( k l ) domain trom about ammo acid 92 to about amino acid 173 the kπngle-2 (K.2) domain from about amino acid 180 to about amino acid 261 and the seπne protease ( P) domain from about amino acid 264 to the carboxvl terminus at amino acid ^2^ These domains are situated essentially adjacent to each other and are connected bv short 'linker regions These linker regions bring the total number ot amino 0 acids ot the mature polvpeptide to 527 although three additional residues (GK -Ala-Arg) are occasionally tound at the amino terminus This additional tπpeptide is generally thought to be the result of incomplete precursor processing and it is not known to impart functionality Native t-PA can be cleaved between position ^5 and position 276 ( located in the seπne protease domain ) to generate the t o-chain torm ot the molecule Each domain contributes in a different w av to the overall biologically significant properties ot the t-PA ^ molecule Domain deletion studies show that the loss ot the finger growth factor or kπngle-2 domain results in a lower-atfinitv binding of the variant t-PA to fipπn (van Zonneveld et al Proc Natl Acad Sci LSA 8 - 4670- 4677 ( 1986) Verheijen et al EMBO J 5 3525-3530 ( 1986)) howev er more recent results obtained with substitution mutants indicate that the πngle-2 domain is less involved in fibrin binding than earlier expected (Bennett et al supra) The domain deletion studies have implicated the finger and growth factor domains in 0 clearance bv the liver (Collen et al Blood, 71 216-219 (1988) kalvan et al J Biol Chem 263 3971-3978 (1988) Fu et al Thromb Res 50 33-41 (1988) Refino e. -./ Fibπnolvsis 2 30 ( 1988) Larsen e. a/ Blood. 73 1842-1850 (1989) Browne etal J Biol Chem 263 1599-1602 (1988)) The kπngle-2 domain is responsible for binding to lvsine The seπne protease domain is responsible for the enzvmatic activity of t-PA and contains specific regions where mutations were shown to affect both fibrin binding and fibrin specificity (possiblv direct interactions ^ ith fibrin) and other regions where only fibrin specificity is altered (possiblv indirect interactions with fibrin) (Bennett etal supra) Studies with mutants resulting trom site-directed alterations indicate the involvement of the glvcosvlation ot t-PA in clearance (Lau et al Bio Technology 9^3-958 ( 1987) Lau et al Bio Technology 6 "4 ( 1988)) An unglvcosvlated variant of t-PA consisting of the kπngIe-2 and protease domains was described to 0 have a slower plasma clearance than wιld-t\pe t-PA (Martin al Fibπnolvsis 4 (Suppl 3) 9 (Abstract 26) ( 1990)) The effect of altering oligosacchaπde structures at sites 184 and 448 ot t-PA w ere also examined bv Howard 6/ α/ Glvcobiologv I 41 1 -418 ( 1991 ) Hotchkiss et al (Thromb Haemost 60 2^-261 ( 1988)) selectively removed oligosacchaπde residues from the t-PA molecule and demonstrated that the removal of these residues decreased the rate of clearance ot t-PA These researchers and Lau et al (( 1987) supra ( 1988 ) supra) also generated the t-PA variant human t-P was substituted with glutamine) to prevent glvcosv lation at position 1 17 This variant similarly to that obtained bv enzymatic removal of the high- mannose oligosacchaπde at this position exhibited an about two-fold slower clearance rate than wild-tvpe human t-PA See also EP-A 238 304 published 23 September 1 87 and EP-A 227 462 published I JuK 1987 Several reports have suggested that the carbohy drate moieties ot t-PA influence the in v itro activ ltv of this enzvme (E arsson et al supra Opdenakker et al Proc Sci Exp Biol Med 182 248-257 ( 1986 ) ) T-PA is endocvtosed bv mannose receptors ot liver endotheiial cells and bv galactose receptors of parenchv tnai cells Indeed the in \ n o clearance ot recombinant human t-PA produced in mammalian cell cultures was influenced by carbohy drate structures particularly bv the high-mannose ohgosacchaπdes (Hotchkιss e. al supra) A t-P A vaπant (designated TNk t-PA) that has a glvcosv lation site added at amino acid position 103 the native glvcosv lation site removed at ammo acid position 1 17 and the sequence at ammo acid positions 296-299 ot native human t-PA replaced bv A AA has been shown to have increased circulatory half-lite and markedly better fibrin specificitv than w lld-tv pe human t-PA (kevt et al Proc Natl Acad Sci L SA 91 . 3670-3674 ( 1994)) Cells expressing tPA-6 a molecule composed of the kπngle-2 and seπne protease domains ot t-PA process it into two glvcotorms a monoglvcosvlated form with Asn-448 occupied, and a diglycosvlated torm with Asn-448 and Asn 184 occupied ( Berg et al Blood 81 1312- 1322 ( 1993 )) Plasminogen exists in two glycoforms The more glvcosvlated form commonly referred to as 'plasminogen- 1 " "plasminogen 1." or "Tvpe 1 plasminogen. has a gaiactosamine-based oligosacchaπde attached at amino acid position 345 (Thr345) and a complex glycosamine-based oligosacchaπde at amino acid position 288 (Asn288) of a native human plasminogen molecule The less glvcosvlated form, commonly referred to as "plasmιnogen-2" "plasminogen II," or" Type 2 plasminogen." has a single oligosacchaπde chain attached at amino acid position 345 (Thr345) (Hayes and Castelhno. J Biol Chem . 254( 18) 8772-8776. 8777-8780 (1979). Lijnen et al Eur J Biochem 120 149- 154 ( 1981 ). Takeda et al Thrombosis Research 39 289-296 ( 1985)) Other glycoproteins display ing variable site occupancy (variations in N- and O-glycosylation site- occupancy) include granulocvte-macrophage colonv-stimulating factor (Okamoto et al Archives of Biochemistry and Biophy sics 286 562-568 ( 1991 )) ιnterteron-gamma (Curlmg - • . -// Biochem J 2^2 333-337 ( 1990)) protein C (Miletich and Broze J Biol Chem 265 1 1397-1 1404 ( 1990)) and ιnterleukιn-2 Glvcosv lation ot gamma- mterteron w as stable throughout an optimized culture design strategy using ted-batch cultures w ith exposure to glucose starvation possibK leading to a dramatic change in glvcosv Iation efficiency (Xie et al Biotechnoi Bioeng . 56 577-582 ( 1997)) Different factors have been discussed to be potentially responsible for variable site-occupancv including availability of do chol-phosphate and nucleotide sugars (Nvberg e. al Biotechnoi Bioeng , 62 336-34^ ( 1998)) glvcosv ltransterase activity (Hendπckson and lmpeπali. Biochemistry 34 9444-9450 ( 1995 ) kaufman et al Biochemistry 33 9813-9819 ( 19941) and variable glvcosv lation site accessibility due to competition with protein folding (Hoist et al The EMBO J X 3538-3546 ( 1996) lmpeπah Ace Chem Res 30 452-459 ( 1997) Shelikotf et al Biotechnoi Bioeng ^0 73-90 ( 1996)) Anv of these factors could be influenced bv cell culture conditions T-PA site-occupancv usually varies within a rather narrow range (- 5%) Studies of Ohzosacchan ltransf rase Metal Cations m G cos\ Iation Asparagine-linked glvcosvlation involves the enzvme-catalvzed modirication ot an asparagine side chain in a nascent poKpeptide with a tπ-antennarv tetradeca-saccharide moietv This first committed step in the biosynthesis ot N-linked gKcoproteins is catalyzed bv oligosaccharvltransterase a heteromeπc membrane- associated enzvme complex tound in the lumen ot the endopiasmic reticulum of eukarvotic cells See Imperiali supra Allen a at J Biol Chem 270 4797-4804(1995) Sharma--/-// Eur J Biochem 116 101-108(1981) SilbersteinandGilmore The FASEB Journal U) 849-858(1996) umar a at Biochem Mol Biol Intl 36817- 826 (1995) Bause a al Biocnem J 31 9^9-985 ( 1995) Xu and Coward Biochemistry 36 14683-14689 (1997) kumarc. -// Biochem Biophvs Res Comm 247 524-529(1998) Watte 1 /-./ Curr Op Struct Biol 7 652-660(1997) For optimal activity ol igosaccharvltransterase requires a small amount ot manganese divalent ion but other divalent metal cations with an octahedral coordination geometr y will support transfer although at reduced rates (Hendπckson and Imperiali supra kaufman £. al supra kumarc;-// Biochem -v. Mol Biol International 36 817-826(1995)) 7 " /7g Role of Temperature in Mammalian C ell Cultures To simulate normal bodv environment fermentor temperature in cultivating mammalian cells is controlled almost exclusively at 37°C This dogma is so widely accepted that so far little attention has been paid to varying temperature in the cell culture process The scarce literature data suggest that reduced fermentor temperature results in improved cell viability and shear resistance, higher cell density and titer in batch cultures and a reduction in glucose/lactate metabolism (Chuppae α/ Biotechnoi Bioeng 55 328-338(1997)) Specifically Reuvenve/α/ J Immunol Methods.86 53-59 (1986) studied the effect of temperatures in the range of 28°C to 37°C on batch hvbπdoma cell cultures They found that although at lower temperatures the cell viability was improved this was accompanied by a decrease in glucose uptake and a decrease in the specific antibody production Therefore in this particular case lower temperatures did not enhance the overall pertoimance ot the cell culture process Sureshkuman and Mutharasan Biotechnoi Bioeng 3~ 92-295 (1991) investigated the effect of the temperature range of 29°C to 42 C on the cell culture process and tound that maximum cell density was achieved at33°C In contrast the glucose uptake and specific lactate production rates were dramatically lower at 33 C than at 39 °C These results showed that the optimal temperatures tor growth and productivity may considerably differ While the viability increase at temperatures below 37°C appears to be a general phenomenon the effect of temperature on specific productivity has been shown to be cell-line dependent (Chuppa et al supra) Weidemann et al Cvtotechnologv 1 111-116 (1994) cultivated adherent recombinant babv hamster kidney (BHk) ceils at temperatures between 30 and 37 C The low-temperature cultivation in batch and repeated batch mode in a two-liter bioreactor showed a lower growth rate and a lower glucose consumption rate d e less lactate production) On the other hand the maximum cell density and productivity were not affected bv the temperature reduction kretzmer et al Cultivation Temperature - Effect on Cell Culture Processes and Their Optimization (American Chemical Society Meeting San Francisco CA) abstract 138 presented April 16 1997 disclosed the effect ot cultivation temperature on ceil culture processes and their optimization, but apparently no specific glycosy lation analysis It has been suggested that reduced termentor temperatures might have other advantages related to product quality and integrity but the effect ot low temperatures on product quality, and in particular, on protein glvcosv Iation. has been scarcely studied Chuppa / -// supra, have reported that fermentation temperature did not have a significant etfect on the sialic acid content ot gK coproteins Although the total sugar content was somewhat lower at 37°C than at 34 °C or 35 5 C. the authors viewed this difference as "not substantial However. S Pat No 5 705.364 described preparing gKcoproteins by mammalian cell culture wherein the sialic acid content of the glycoprotein produced was controlled over a broad range ot values bv manipulating the cell culture environment, including the temperature The host cell was cultured in a production phase of the culture by adding an alkanoic acid or salt thereof to the culture at a certain concentration range, maintaining the osmolality ot the culture at about 250 to about 600 mOsm. and maintaining the temperature ot the culture between about 30 and 35 °C Bahr-Davidson. "Factors Affecting Glycosylation Site Occupancy of ASN- 184 ot Tissue-Type Plasminogen Activator Produced in Chinese Hamster Ovary Cells." A Dissertation submitted to the Department ot Chemical Engineering and the Committee ot Graduate Studies of Stanford University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy. May 1995, investigated the effects ot temperature on glycosylation site occupancy and reported that site occupancy was increased by exposing cells to 26°C (see pages 50-51 ) Hormonal and Other Treatments to Influence Gl\ cosylation The effect of various additives such as components of plasma to the culture media on protein production and glycosylation has been studied in the literature, for example, the effects of hormonal treatments on membrane glycosylation in rat kidney brush broder membranes (Mittal et al . Indian J Exper Biol . 34 782-785 (1996)) Studies of Muc-1 mucin expression established the hormonal basis for mRNA expression (Parry et al J Cell Sci . 101 191 -199 ( 1992)) Thvroid hormone regulation of alpha-lactalbumm w ith differential glvcosv lation has been reported (Ziska et al . Endocrinology . 123 2242-2248 (1988)) The cellular response to protein N-glycosv Iation was increased in the presence of thvroxine. insulin, and thrombm. and the effect was dose-dependent (Oliveira and Baneηee. J Cell Phvsiol . 144 467-472 ( 1990)) Thvroxine was tound to induce changes in the glvcosv lation pattern of rat alpha-fetoprotein (Naval et al Int J Biochem . 18 1 15- 122 ( 1986)) In addition to hormonal treatments, glutathione and glucose-6-phosphate dehydrogenase deficiency increased protein glvcosvlation (Jain, free Radical Biology & Medicine. 24 197-201 ( 1998)) Thvrotropin was found to control o gosacchary Itransterase activity in thyroid cells (Desruisseau et al Mol Cell Endocπnol . 122 223-228 ( 1996)) The addition ot glucose and tπ-iodothyromne (T ) to a medium producing a pro-urokmase derivative improved productivity ( et al Cvtotechnologv . 19 1 - 10 ( 1996)) Also tucosv Itransterase activity in the rat small intestine was responsive to hydrocortisone regulation during the suckling period (Bioi et al Biochim Biophvs Acta 1 133 206-212 ( 1992)) Hvdrocortisone treatment also induced quantitative alterations in glycosylation of mouse mammary tumor virus precursors (Maldarel and Yagi JNCI 77 1 109- 1 1 15 ( 1986)) Glvcosv Iation ot cellular glycoconiugates m a carcinoma cell line was enhanced bv a retinoic acid (Sacks et al GK coconiugate J 1 3 791 -^96 ( 1996)) Further retinoic acid had reversible effects on glv cosaminoglv can synthesis during differentiation of HL-60 leukemia cells ( Reiss et al Can Res 45 2092-2097 ( 1985)) Additionally retinoic acid, as well as hvdrocortisone yyas tound to modulate glycosaminogivcan synthesis ot human malignant keratinocvtes ( Reiss et al J Inv est Dermatol 86 683-688 ( 1986)) The competition betw een folding and glv cosv lation in the endopiasmic reticulum has been described ( Hoist et al supra) as has acute heat shock inducing the phenomenon ot prompt glycosylation (Jethmalani et al . J Biol Chem . 269 23603-23609 ( 1994)) There is a need for increasing the increasing glvcosv Iation site occupancy in gKcoproteins having multiple gly cotorms to produce glycoprotein therapeutics of consistent product quality For example, there is a need to increase the traction of Ty pe I t-PA in the t-PA production process Such increase in site-occupanc generates t-PA with activity more closely resembling the international human t-PA standard, and thus more closely resembling human t-PA Tv pe 1 t-PA is also more soluble than Tv pe II which may be of some value in processing steps Further, increased Tv pe 1 is correlated w ith increasing circulatory halt-life, as noted above Summary ot the Invention It has been found that during the production ot a wild-type glycoprotein. namely human t-PA, in mammalian cells, namely Chinese Hamster Ovary (CHO) cells, use ot certain divalent metals, hormones, or factors that manipulate cell-cycle distribution to control or influence glycosylation significantly increases site occupancy of a glycosylation site of the glycoprotein For example, decreasing the cultivation temperature from 37°C to about 30-35 °C in the production phase significantly enhances the occupancy of the glycosylation site at amino acid position 184, and thereby increases the ratio of Type I t-PA to Type II t-PA Specifically, decreasing the temperature from 37 to 33 or 31 °C increased t-PA site-occupancy up to 6% Temperatures below 37°C are expected similarly to facilitate the occupancy ot not-easily-accessible N-hnked glycosylation sites in other glycoproteins Accordingly , temperature can be used as a sensitive tool for fine tuning the ratio of variously glvcosv lated forms of gK coproteins having one or more N-linked glycosy lation sites occupied oniv in a traction of the protein In addition, other env ironmental factors, including those that manipulate the culture s growth state, and correspondingly cell-cycle distribution, such as but rate oi a cell-cycle inhibitor that increases the proportion of cells in the GO G 1 phase such as quinidine. plasma components such as thy roid hormones, and or certain divalent metal cations significantly elevated the t-PA Type 1 content (about 1 -2 5° o) compared to control conditions, and are expected to act similarly with respect to other gKcoproteins In contrast, addition ot the relevant nucieoside precursor molecules (e g . undine, guanosine. mannose) did not result in improved site-occupancv In one aspect the invention concerns a process tor producing a gly coprotein comprising culturing mammalian host cells producing the glycoprotein (i e , cells expressing nucleic acid encoding the glvcoprotein) in the presence ot (a) a factor that modifies growth state in a cell culture, (b) a divalent metal cation that can adopt and prefers an octahedral coordination geometry, or (c) a plasma component, whereby the occupancy ot an N- lmked glvcosvlation site occupied onlv in a fraction of the gly coprotein is enhanced in the glycoprotein so produced Preferably, the factor is a cell-cycle inhibitor that blocks cells in the GO/G l phase, a buty rate salt, and/or a temperature of the culture of between about 30 and 35 . the divalent cation is manganese or iron, and the plasma component is a hormone Preferably, the cell culture procedure includes a growth phase, followed by a transition phase and a production phase in a preferred embodiment, in the growth phase the mammalian host cells ai e cultured at about 37°C. whereupon, during the transition phase, the temperature is lowered to between about 30°C and 35 "C The host cells preferably are CHO cells, and the glycoprotein preferably is t-PA In another aspect, the invention provides a process for producing human t-PA comprising culturing CHO cells expressing nucleic acid encoding said t-PA in a serum-free medium in a production phase at a temperature of between about 30°C and 35 °C and in the presence of about 0 to 2 mM of a butyrate salt, whereby the occupancy ot an N-hnked glycosy lation site occupied only in a fraction ot t-PA is enhanced in the t-PA so produced In a still further aspect, the invention supplies a process tor producing human t-PA comprising culturing CHO cells expressing nucleic acid encoding said t-PA in a serum-free medium in a growth phase at a temperature of about 37-40°C. wherein said medium comprises from about 10 nM to 100 uM of a divalent metal cation that can adopt and prefers an octahedral coordination geometry, culturing said cell in a transition phase at a temperature of about 37-40°C. and culturing said cell in a production phase wherein after about 48 hours into the production phase the temperature is lowered to between about 30°C and 35 °C and about 0 75 to 1 5 mM of a butyrate salt is added to the medium, whereby the occupancy of an N-hnked glycosylation site occupied only in a fraction of t-PA is enhanced in the t-PA so produced In this process, a plasma component such as a thyroid hormone, e g., thyroxine or tπ-iodothyronine, or a cell-cycle inhibitor that blocks cells in the G0/G 1 phase such as quinidine is optionally added to the culture medium before or duπng the growth phase Hence, the process herein facilitates the production of a preferred glycoform of a glycoprotein. such as Type I t-PA, in a mammalian cell culture, and also increases the ratio of preferred to non-preferred glycoproteins, such as the ratio of Type I to Type II t-PA. in a mammalian cell culture Brief Description of the Drawings Figure 1 shows a depiction of Type I t-PA and Type II t-PA and a chromatogram thereof Figure 2 shows the percentage of Type I human t-PA in cell cultures of CHO cells cultivated at various scales at 37 °C Figure 3 shows graphs of the percentage of Type I human t-PA in cell cultures of CHO cells at 3 " ' 0 C as a function of 12K-fermentatιon run time, with each graph line representing a different run Figures 4A and 4B show the percentage of Type 1 human t-PA in cell cultures of CHO cells cultivated on a laboratory scale (in 25 cm " T-flasks and 100-ml spinner flasks- Fig 4A I or in a 5-liter bioreactor (Fig 4B) for 5-7 days at 33 °C relative to control (cell culture kept at 37°C) Figure 5 shows the percentage of Type I human t-PA in cell cultures of CHO cells cultivated in T-flasks at 37°C for 3-4 days wherein sodium butyrate is added in the amount indicated at the time of inoculation The values are from triplicate experiments Figure 6 shows the percentage of Type I human t-PA in cell cultures ot CHO cells cultivated in 60-mm culture dishes at 37°C for 4-6 days wherein temperature changes with or w ithout sodium butyrate are compared (wherein 37°C 1 - means control at 37°C with no butyrate. 37°C 1 IX is at 3 " C w ith 0 75 mM butyrate. 37°C12X ιs at 37°C w ith I 5 mM but rate 33 C l - is at 33 D C w ith no butyrate 33 C l 1 \ is at 33 °C with 0 75 mM butyrate 33 °C 12X ιs at 33 °C with 1 5 m VI butyrate. 3 1 J C1- is at 3 1 °C with no buty rate. 31 C 1 I X is at 3 \ ' C with 0 75 mM butyrate. and 3 PC 12X is at 3 1 C w ith 1 5 M butyrate ) Figure 7 shows percentage of Type I human t-PA in cell cultures ot CHO cells cultivated in 5-liter bioreactors after 5-7 days, wherein temperature changes with or without sodium butvrate are compared (where control is at 37 3 C w ithout buty rate. 33 "C is at 33°C w ithout buty rate. 2X butr is at 37°C w ith 1 5 mM butyrate. and 33 °C 2X is at 33 °C w ith 1 5 mM butvrate) Figure 8 shows the percentage of Ty pe I human t-PA in cell cultures ot CHO cells cultiv ated in termentors over time at 37°C in w hich sodium butyrate was added to a concentration ot 0 75 mM at approximately 48 hours. and the percentages ot cells in GO G 1 phase at the corresponding time points These results reflect the averages of three separate cultures Figure 9 shows the percentage ot Ty pe I human t-PA in cell cultures ot CHO cells cultivated in T-flasks at 37 °C for 3-4 dav s wherein at the time of inoculation no cell-cy cle inhibitor is added (control) thymidine (250 / g/mL is added, or quinidine (90 uM) is added The values are trom triplicate experiments Figures 10A and 10B show the percentage of Ty pe I human t-PA in cell cultures ot CHO cells cultivated in 60-mm culture dishes at 3 7 for 4-6 days wherein at the time of inoculation 3 nM of MnCl 2 is added (control), or 10 nM. 100 nM. 1 -<M. 10 «M, or 100 uM MnCN is added The values are expressed in triplicate for Fig 10A and as an average of triplicates for Fig 10B Figure 1 1 shows the percentage of Type I human t-PA in cell cultures of CHO cells cultivated in 60-mm culture dishes at 37 °C for 4-6 days wherein at the time of inoculation no ferric citrate is added (control), or 10 μM ferric citrate. 50 μM ferric citrate, or 100 μM ferric citrate is added Figure 12 shows the percentage of Type 1 human t-PA in cell cultures of CHO cells cultivated in 60-mm culture dishes at 37°C for 4-6 days wherein the cells are cultivated in the presence of increased amounts of specified nucleotide sugar precursor molecules Figures 13A and 13B show the percentage of Ty pe I human t-PA in cell cultures ot CHO cells cultivated in 60-mm culture dishes at 3^° C for 7 davs wherein at the time ot inoculation no hormone is added (control), or 1 nM. l O nM. or l OO nM ot tπ-ιodothvronιne (Tπιod ) or ot thyroxιne (Thy rox ) is added The values are expressed in triplicate for Fig 13A and as an average ot triplicates for Fig 13B Detailed Description of the Invention I Definitions As used herein, the word "enhanced" as it relates to occupancy of an N-hnked glycosy lation site occupied only in a fraction of the glycoprotein refers to relative value obtained by practicing the current invention v ersus control value obtained by not using the parameters of this inv ention The value is calculated based on the percentage of gly cosv Iation sites occupied at the particular position ot the gl coprotein in question versus a baseline value, which is determined w ithout using the factors, cations or plasma components herein claimed For example t-PA is secreted as a mixture ot two maior glycoforms Type I (all three N-glycosy Iation sites are occupied) and Type II (Asn- 184 is not occupied), and an enhanced occupancy lev el means an increased site occupancv such that the mixture has increased amounts of Tv pe 1 relative to Type II t-PA versus the control This occupancy level can be measured, for example, by using reversed-phase high-pressure liquid chromatography ( RP-HPLC) to elute the fragments ot the different types ot glycoprotein (the types having different glycosylation site occupancy levels) and integrating the peak areas tor each type ot glycoprotein to determine relative quantities The most t pical way to express these quantities is by the percentage of the higher-occupancv ty pe of glycoprotein to total types of glycoprotein A specific example ot an assay used to measure enhancement for Ty pe I Ty pe II t-PA is set forth beiovy in Example 1 As used herein. ' gly coprotein ' refers generally to peptides and proteins having more than about ten amino acids and at least one gly cosy lation site that is occupied only in a traction ot the glycoprotein product, I e . they display variable site-occupancv or variations in N- and O-glycosy Iation site-occupancy The glycoproteins may be homologous to the host cell or preferably , they are heteroiogous. i e . foreign, to the host cell being utilized, such as a human protein produced bv a CHO cell Preferably, mammalian gKcoproteins (gKcoproteins that were originally derived from a mammalian organism ) are used, more preferably, those which are directly secreted into the medium, and most preferably , those w herein N-glycosylation site-occupancy is involved The specifically -preferred g coproteins herein are t-PA. plasminogen. interteron-gamma. Protein C. IL-2. and CSF for example. GM-CSF The more preferred glycoproteins are t-PA or plasminogen. and the most preferred is t-PA. more notably human t-PA The terms "tissue plasminogen activator " , and "t-PA " refer to human extrinsic (tissue-type) plasminogen activator having fibπnolytic activity that typically has a structure with five domains (finger, growth factor, kπngle- 1. kπngle-2. and protease domains), but nonetheless may have fewer domains or may have some of its domains repeated if it still functions as a thrombolytic agent and retains the N-hnked glycosylation sites at positions 1 17. 184, and 448. At minimum, the t-PA consists of a protease domain that is capable of converting plasminogen to plasmin. and an N-terminal region believed to be at least partially responsible for fibrin binding, and retains the elinked glycosylation sites at positions corresponding to amino acid positions 1 17. 184. and 448 of wild-type human t-PA. The retention of these glycosylation sites is due to the fact that variable site occupancy of recombinant and melanoma-derived wild-type t-PA leads to production of two variants, designated as "Type 1 t-PA " and "Type II t-PA " . respectively Type I t-PA contains N-hnked oligosacchaπdes at positions 1 17. 184. and 448 Type II t-PA contained N-hnked oligosacchaπdes at positions 1 17 and 448. See Fig 1 It will be understood that natural alle c variations exist and can occur among individuals, as demonstrated bv one or more amino acid differences in the ammo acid sequence of t-PA ot each individual The terms "wild-type human tissue plasminogen activator" "w lid-type human t-PA" "native human tissue plasminogen activator. " and ' native human t-PA". where "human t-PA " may be abbreviated as " ht-PA ' refer to native-sequence human t-PA. I e . that encoded by the cDN A sequence reported in U S Pat No 4 766.075. issued 23 August 1988 Amino acid site numbers or positions in the t-PA molecule are labeled in accordance w ith L S Pat No 4.766.075 The t-PA may be from any native source In addition, the t-PA may be obtained from any recombinant expression system, including, tor example. CHO cells or human embryonic kidney 293 cells As used herein, references to various domains of t-PA mean the domains of wild-type human t-PA as hereinabove defined, and functionally equivalent portions of human t-PA having amino acid alterations as compared to the native human t-PA sequence, or of (native or variant) t-PA from other sources, such as bat tissue plasminogen activator ( bat-PA ) I hus. as used herein tne term ' protease domain" refers to the region extending trom ammo acid position 264 to amino acid position 52 " inclusiv e, ot the mature torm ot wild-ty pe human t-PA. and to tunctionally equivalent portions ot human t-P A hav ing amino acid alterations as compared to the native human t-PA sequence, or of t-PA trom other sources, such as bat-PA As used herein, "factor that modifies grow th state in a cell culture ' refers to a factor that increases the proportion ot cells in the GO G l phase ot growth such as a cell-cv cle inhibitor that causes cells to accumulate or Clocks the cells in the GO'G l phase Such tactors manipulate cell cy cle distribution to control or influence gly cos lation Such a factor may affect glv cosv lation in mechanisms bev ond growth state, but are defined herein as affecting at least the growth state As used herein, a "cell-cy cle inhibitor that blocks cells in the GO G l phase of growth" is a molecule that causes cells to accumulate in the GO G l phase ot growth This can be determined by cell cy cle analy sis, I e . uniform suspensions of nuclei are stained w ith propidium iodide (PI ) using the detergent-try psin method of V indelov et al Cv tometrv . 3 323-327 ( 1 83 ) to determine relative cellular DNA content by flow cy tometπc analy sis Events are gated using doublet discrimination to exclude doublets, and the data are modeled using VlodFit LT Cell Cy cle Analy sis softw are ( Verity Software House ) A preferred such inhibitor herein is quinidine By "butyrate" or "butyrate salt" is meant any corresponding salt of butyric acid, such as sodium butyrate or potassium butyrate By "phase" is meant a certain phase of culturing of the cells as is well recognized by the practitioner For example, "growth phase" of the cell culture refers to the period of exponential cell growth (the log phase) where cells are generally rapidly dividing. During this phase, cells are cultured for a period of time, usually between 1 -4 days, and under such conditions that cell growth is maximized The growth cycle for the host cell can be determined for the particular host cell envisioned without undue experimentation During the growth phase, cells are cultured in nutrient medium containing the necessary additives generally at about 30-40°C. preferably about 37°C. in a humidified, controlled atmosphere, such that optimal growth is achieved for the particular cell line Cells are maintained in the growth phase tor a period of betw een about one and four days, usually bet een about two and three days "Transition phase" of the cell culture refers to the period of time during w hich culture conditions tor the production phase are engaged During the transition phase environmental factors such as temperature are shifted from growth conditions to production conditions "Production phase" of the cel l culture refers to the period of time during which cell growth has plateaued During the production phase, logarithmic cell growth has ended and glycoprotein production is primary During this period of time the medium is general ly supplemented to support continued glycoprotein production and to achieve the desired glycoprotein product By "divalent metal cation that can adopt and prefers an octahedral coordination geometry ' is meant a metal cation with two valencies that is capable of. and actually shows preference for. adopting an octahedral coordination geometry Such cations are also characterized in that ohgosacchary Itransterase can function d e . is activated) in their presence Examples of such metal ions include manganese ( Mn~ ). ιron (Fe " ). calcium ( Ca~ ). - 1 I - and magnesium ( Mg ~ ) Div alent cations that show preterences tor other coordination geometries, including nickel (N'r 1 - ), copper (Cu - ). cadmιum (Cd 2- ). and zιnc (Zn 2- ). fail to activate the enzyme and at high concentrations also competitively inhibit activ ity in the presence ot manganese Hence, these latter cations are excluded from the definition By "plasma component" is meant a constituent ot normal plasma This would include growth promoters and tumor-promoting agents tor endothehal cell growth, regulators ot differentiation of epithelial tissues, glucagon. hepaπn. phorbol myπstate acetate. PRL. thy roglobulm. 8Br-cAMP thrombin. vitamin A and its derivatives ( retinoids such as retinoic acid, e g . beta-all-trans retinoic acid), glutathione. steroids such as corticosterone. cortisol. and corticoids. e g . glucocorticoids such as hydrocortisone. and hormones, preferably those that are vital hormones ot metabolism such as estrogen, insulin, and thyroid hormones, e g . thv roxine and tn-iodothy ronine (T 3 ) The thyroid hormones are preferred, and most preferably thvroxine and tπ-iodothyronine Since some serum, including fetal calf serum, contains thyroid hormones and the thyroid hormone binding protein at nanomolar levels, it is preferred to use serum-free medium, particularly if thyroid hormones are employed to enhance site- occupancy The terms "cell culture medium " "culture medium." and "fermentation medium" refer to a nutrient solution used for growing mammalian cells that typically provides at least one component from one or more of the following categories 1 ) an energy source, usually in the form of a carbohydrate such as glucose, 2) all essential amino acids, and usually the basic set of twenty amino acids plus cysteine. 3) vitamins and/or other organic compounds required at low concentrations. 4) free fatty acids; and 5) trace elements, where trace elements are defined as inorganic compounds or naturally-occurring elements that are typically required at very low concentrations, usually in the micromolar range The cell culture medium is generally "serum free" when the medium is essentially free of serum from any mammalian source (e g fetal bovine serum (FBS)) By "essentially free" is meant that the cell culture medium comprises between about 0-5° o serum, preferably between about 0- 1 ° o serum, and most preferably between about 0-0 1 ° o serum Advantageously, serum-tree "defined" medium can be used, w herein the identity and concentration of each of the components in the medium is known (/ e an undefined component such as bovine pituitary extract (BPE) is not present in the culture medium ) In the context of the present invention the expressions "cell", "cell line", and "cell culture" are used interchangeably, and all such designations include progeny Thus, the words "transformants" and "transformed (host) cells" include the primary subiect ceil and cultures derived therefrom w ithout regard for the number of transfers It is also understood that all progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations Mutant progeny that have the same function or biological activity as screened tor in the originally transtormed cell are included Where distinct designations are intended, it will be clear from the context The term "mammalian host cell", "host cell", "mammalian cell", "mammalian recombinant host cell." and the like, refer to cell lines derived from mammals that are capable ot growth and survival when placed in either monolayer culture or in suspension culture in a medium containing the appropriate nutrients and growth tactors The necessary growth factors tor a particular cell line are readi determined empirically w ithout undue experimentation, as described, tor example in Mammalian Cell Culture Mather. J P ed (Plenum Press N Y . 1984). and Barnes and Sato. Cell 22 649 ( 1980) Ty pically, the cells are capable ot expressing and secreting large quantities of a particular gly coprotem of interest into the culture medium Examples ot suitable mammalian host cells w ithin the context ot the present invention may include CHO cells ( FP 1 17 159 published August 29 1989 L S Pat Nos 4.766.075 4.853 330. 5.1 85.259 Lubiniecki et al in Adv ances in Animal Cell Biology and Technology for Bioprocesses Spier et al . eds ( 1989) pp 442-4 1 ). for example. CHO derivatives such as CHO - DHFR ( Lrlaub and Chasιn Proc Natl Acad Sci L A. " 4216 ( 1980)) CHO-k l DUX B 1 1 ( Simonsen and Levinson. Proc Natl Acad Sci L S A. 80 2495-2499 ( 1983 ). Lrlaub and Chasm, supra), and dp 12 CHO cells (EP 307.247 published 15 Vlarch 1989). rat my eloma YB2 3 oAg20 (WO 86 00127 published April 1 1985) mouse C 127 fibroblasts ( Reddy et al DNA. 6 461 -472 ( 1987)). mouse sertoli cells (TM4 Mather. Biol Reprod . 23 243-251 ( 1980)) human cerv ical carcinoma cells (HELA. ATCC CCL 2). human lung cells (W 138. ATCC CCL 75). human liver cells ( Hep G2. HB 8065 ). mouse mammary tumor ( V1MT 060562 ATCC CCL51 ). TRI cells (Mather et al . Annals N Y Acad Sci . 383 44-68 ( 1982)). MRC 5 ceils. FS4 cells, and human melanoma cells (Browne et al Thromb Hae ost . 54 422-424 ( 1985 )). Preferred host cells include CHO-k l DUX B l 1 and dpl2.CHO cells The CHO cells developed for large-scale production of t-PA are maintained cryogenically in a MCB/working cell bank (WCB) system as described by W iebe et al , in Large Scale Mammalian Cell Culture Technology, Lubiniecki. ed.. (Marcel Dekker: New York. 1990). pp 147- 160 DHFR+ CHO-K1 cells transfected with DNA encoding human t-PA have been deposited at the American Type Culture Collection, Manassas. Virginia (ATCC). and are available under accession number CCL 61 A sample of another t-PA-producing CHO cell line (CHO cell line 1 - 1515) has been deposited under ATCC accession number CRL 9606 The latter cell line was reported to result in human t-PA levels approaching 50 pg'cell/day II Modes for Carry ing Out the Invention General and Specific Features of Invention It has been discovered that utilizing a factor that modifies growth state in a cell culture (such as a cell-cvcle inhibitor, a buty rate salt, or low eπng the temperature during the production ot a gly coprotem in a mammalian cell culture from 37°C to about 30-35 C). or utilizing a divalent metal cation that can adopt and prefers an octahedral coordination geometry , or utilizing a plasma component, enhances the occupancy ot the glycosy lation site at a selected and desired amino acid position of the wild-type glycoprotein For example, this lowering ot temperature enhances the occupancy of the glycosylation site at amino acid position 184 ot wild-type human t-PA and accordingly, increases the latio ot Type I t-PA to Type II t-PA The ability to adjust, and increase, the lype 1 to Type II t-PA ratio is significant, since it enables the manufacturer to produce a recombinant protein in which this ratio closely approximates the ratio present in native t-PA (about 1 1 ) In addition, the ratio of Type 1 to Type II t-PA affects the solubility and clearance rate of t-PA. and there is ev idence that an increased Type 1 t-PA concentration somewhat increases the circulatory half-life of t-PA It is knovyn that the high-mannose oligosacchaπde at amino acid position 1 1 " is primarily responsible for the rapid clearance of native human t-PA When this oligosaccharide is removed it has been observ ed that Tvpe 1 t-PA has a longer half- lite than Tvpe II t-PA indicating that there is a secondarv mechanism on which the extra oligosaccharide present on Tvpe I t-PA has a positive effect The experimental findings herein can be extended to other gKcoproteins that ( like t-PA) have at least one glvcosvlation site that is occupied onlv in a traction ot the glvcoprotein product If the factor is a butvrate salt generally the butvrate is present in a concentration ot up to about 2 mM more preferably about 0 35 to 2 mM still more preferably about 0 75 to 1 -> mM The concentration thereof to be selected w ithin this range depends mainlv on the temperature to which the culture is sub|ected and the tvpe of glvcoprotein Hence if for t-PA the temperature remains at about 37 C or is lowered to about 3.3 " the butvrate concentration is preferably lower than about 1 5 mM and more preterablv is about 0 3 to 1 mM most preterablv 0 75 mM However if for t-PA the temperature is lowered to about 30-3 1 °C preterablv the butyrate concentration is about 1 -2 mM more preferably about 1 5 mM This illustration shows tnat more than one ot these tactors mav be operating or present in the cell culture In a preferred aspect the temperature low eπng and/or butvrate addition take(s) place ouπng the production phase ot the growth cv cle In such a scenario the temperature is lowered to between about 30 and 35 °C and/or a butvrate salt is added about 48 hours into the production phase The production phase is suitably preceded bv a growth phase and a transition phase ot growth cvcle During the growth phase the temperature is preterablv kept at about 37°C and/or during the transition phase the temperature of the culture is preferably lowered to between about 30 α C and 35 °C more preferably about 31-33 °C, and most preferably about 31 °C Alternatively or additionally to the factor(s) above the cells are cultured in the presence of a divalent cation as defined above The choice of divalent cation to use as well as the specific concentration thereof depends, inter alia on the type of glycoprotein being produced and the metal cations and other components already present in the culture medium and their respective concentrations For example if the glycoprotein has a number of thio groups it is preferred to use a thiophilic metal such as manganese and iron with iron being the most thiophihc metal In contrast if the glycoprotein contains more oxygen groups, then the oxophi c cations magnesium and calcium are preferred If calcium ion is already present in sufficient quantities in the medium it is not ty pically used tor the purposes herein and a different metal cation is used Further the size of the metal cation mav have an influence with iron and magnesium being smaller and calcium and manganese being larger Steπc effects due to sulfur groups on the glvcoprotein mav dictate a cation of smaller ionic radius The preferred divalent metal cation herein is manganese magnesium or iron more preferably manganese or iron and most preferably manganese The divalent metal cation is preferably present in the culture medium during the v\hole cultivation time and at least is added during the growth phase The concentration thereof generally ranges between about 10 n and 150 uM preterablv from about 10 nm to 100 uM for manganese and from about 20 uM to 100 uM for iron In another alternative alone or together with the divalent cation and or factor a plasma component is present during the culturing The plasma component is tvpicallv present in an amount trom about 1 nM to 1 5-20 μM depending mainlv on the tvpe ot glvcoprotein being produced the type of plasma component utilized and the scale ot fermentation For example if the plasma component is a thvroid hormone it is preferably present in an amount ot about 1 - 150 nM preterablv about 10- 100 nM If the plasma component is glutathione it is preterablv present at about I - 10 μM and if h vdrocortisone it is preferably present at about 5- 15 nM preterablv about 10 nM Preterablv the plasma component is a hormone more preterablv a thvroid hormone and most preterablv thvroxine or tπ-iodothvronine The degree ot site-occupancv to be achieved tor the glvcoprotein is balanced against the desired degree ot secretion ot the glvcoprotein w hich is generally taken into account in selecting which factors and other "> components to emplov and at w hat concentrations or temperatures For example site-occupancv is generally controlled in a select range ot about -5° 0 w ithout affecting t-PA secretion C ell C iiltw e Pr cedures According to the present invention mammalian cells are cultured to produce a desired glv coprotein product In choosing a host cell for the production ot the glvcoprotein within the context of the present invention 0 it is important to recognize that different host cells have characteristic and specific mechanisms tor the translational and post-translational processing and modification (e g glvcosvlation cleavage) of the proteins expressed Appropriate cell lines should be chosen to ensure that the desired post- translational modifications are possible Alternatively host cells can be modified to express a desired gene product required tor the specific post- translational modification 5 In particular the mammalian cells that express the desired glvcoprotein should express or be manipulated to express the particular enzymes such that under suitable conditions the appropriate post-translational modification occurs in The enzvmes include those enzymes necessary for the addition and completion of N- and O-linked carbohydrates such as those described in Hubbard and lvatt Ann . Rev Biochem , 50 555-583 ( 1981 ) for N-hnked oligosacchaπdes The enzymes optionally include ohgosacchary ltransferase, alpha-glucosidase I. alpha-glucosidase II. ER alpha( l 2)mannosιdase Golgi alpha-mannodase I N-acetylyglucosam my ltransferase I Golgi alpha- mannodase ll N-acetylyglucosaminyltransferase ll alpha( l 6)fucosyltransferase β( l ,4)galactosyltransferase. and an appropriate sialyltransferase For culturing the mammalian cells that express the desired glycoprotein and are capable of adding the desired carbohydrates in specific position and linkage numerous culture conditions can be used paving particular 5 attention to the host cell being cultured Suitable culture conditions tor mammalian cells are well known in the art (J Immunol Methods 56 221-234 ( 1983)) or can be easily determined bv the skilled artisan (see for example Animal Cell Culture A Practical Approach 2nd Ed Rickwood D and Hames B D eds (Oxford Universm Press New Y ork 1992)) and varv according to the particular host cell selected The mammalian cell culture ot the present invention is prepared in a medium suitable tor the particular 0 cell being cultured The nutrient solution mav optionally be supplemented with one or more components trom anv of the follow ing categories 1 ) plasma components as defined above and/or growth factors such as for example insulin transferπn and EGF 2 ) salts and buffers such as for example sodium chloride calcium magnesium phosphate and 5 HEPES 3 ) nucleosides and bases such as for example adenosme thvmidine and hvpoxanthme 4) protein and tissue hv drolvsates 5 ) antibiotics such as GENTAMYCIN™ drug and 6 ) hpids such as linoleic or other tatty acids and their suitable carriers Any other necessary supplements may also be included at appropriate concentrations that would be known to those skilled in the art Commercially available media such as Ham's F 10 ( Sigma). Minimal Essential Medium (MEM. Sigma). RPM I- 1640 (Sigma), and Dulbecco's Modified Eagle's Medium ( DMEM. Sigma) are exemplary nutrient solutions In addition, anv of the media described in Ham and Wallace. Meth Enz . 58 44 ( 1979). Barnes and Sato. Anal Biochem , 102 255 ( 1980). L S Patent Nos 4 767.704. 4.657.866 4 2" 762. 5.122.469 or 4.560.655. WO 90 03430. and WO 87 00195 may be used as culture media Anv ot these media may be supplemented as necessary w ith the components as mentioned above The use of a special medium lacking animal serum (serum-tree medium ) is preferred to avoid interference or counter-action trom components of the serum with one or more of the factors, divalent metal cations, plasma components, or other ingredients employed in accordance w ith the present invention Moreover, the concentration ot the amine groups should be sufficiently high to keep t-P \ in solution as the concentration increases This can be achieved by using greater than about 1 mM lvsine concentrations, by the presence of HEPES. or by the use of sufficiently high ammonium chloride concentrations, although any other amine or ammonium source will do If the goal is to produce t-PA in substantially single-chain form, the culture medium (just as the medium used in the subsequent recovery and purification steps) contains a protease inhibitor, such as aprotimn. alpha- 1 antitrypsin. alpha-2 macroglobulin, soybean trypsin. etc Preferably, aprotimn is employed at a concentration of about 5 to 100 KlU/ml, more preferably about 10 KlU/ml in the t-PA production medium. In a particularly preferred embodiment, the mammalian host cell is a CHO cell, preferably CHO-K 1 DUX B l 1 The necessary nutrients and growth factors for the medium, including their concentrations, for a particular cell line, are determined empirically without undue experimentation as described, for example, in Mammalian Cell Culture. Mather, ed. (Plenum Press NY. 1984) and by Barnes and Sato. CeH. 22 649 ( 1980) A suitable medium contains a basal medium component such as a DMEM/HAM F- 12-based formulation (for composition of DMEM and HAM F12 media and especially serum-free media, see culture media formulations in American Type Culture Collection Catalogue of Cell Lines and Hybridomas. Sixth Edition. 1988. pages 346-349). w ith modified concentrations ot some components such as amino acids, salts, sugar, and v itamins, and optionally containing glycine. hypoxanthine, and thymidine. recombinant human insulin, hydrolvzed peptone, such as PRIMATONE HS™ or PRIMATONE RL™ (Sheffield. England), or the equivalent, a cell protective agent, such as PLURONIC F68™ or the equivalent pluronic polyol. GENTAMYCIN™ and trace elements The formulations ot medium as described in U S Pat No 5.122.469. characterized by the presence of high levels ot certain ammo acids as well as PS-20 as described below , are particularly appropriate The glycoproteins of the present invention may be produced by grow ing cells which express the desired glycoprotein under a variety of cell culture conditions For instance, cell culture procedures tor the large- or small- scale production of glycoproteins are potentially useful within the context ot the present invention Procedures including, but not limited to. a fluidized bed bioreactor. hollow fiber bioreactor. roller bottle culture, or stirred tank bioreactor svstem mav be used, in the later two sv stems, with or w ithout microcarπers and operated alternatively in a batch, fed-batch or continuous mode In a preterred embodiment the cell culture ot the present inv ention is performed in a stirred tank bioreactor sy stem and a ted-batch culture procedure is employed In the preterred ted-batch culture the mammalian host cells and culture medium are supplied to a culturing vessel initially and additional culture nutrients are fed continuously or in discrete increments to the culture during culturing. w ith or vy ithout periodic cell and or product harvest before termination ot culture The ted-batch culture can include tor example a semi-continuous ted batch culture, wherein periodically w hole culture ( including cells and medium ) is removed and replaced bv fresh medium Fed-batch culture is distinguished from simple-batch culture in vyhich all components tor cell culturing (including the cells and all culture nutrients) are supplied to the culturing vessel at the start of the culturing process Fed-batch culture can be further distinguished from pertusion culturing insofar as the supemate is not removed trom the culturing vessel during the process ( in pertusion culturing. the cells are restrained in the culture bv. e g . filtration, encapsulation anchoring to microcarπers etc and the culture medium is continuously or intermittently introduced and removed trom the culturing vessel) Further the cells ot the culture may be propagated according to any scheme or routine that may be suitable for the particular host cell and the particular production plan contemplated Thereiore the present invention contemplates a single-step or multiple-step culture procedure In a single-step culture the host cells are inoculated into a culture environment and the processes of the instant invention are employed during a single production phase of the cell culture Alternatively, a multi-stage culture is envisioned In the multi-stage culture cells may be cultivated in a number of steps or phases For instance, cells may be grown in a first step or growth phase culture wherein cells, possibly removed from storage, are inoculated into a medium suitable for promoting growth and high viability The cells may be maintained in the growth phase for a suitable period of time by the addition of fresh medium to the host cell culture According to a preferred aspect of the invention, fed-batch or continuous cell culture conditions are devised to enhance growth ot the mammalian cells in the growth phase ot the cell culture In the growth phase cells are grown under conditions and tor a period of time that is maximized for growth Culture conditions such as temperature. pFI dissolved oxygen (DOi). and the like, are those used ith the particular host and will be apparent to the ordinarily -skilled artisan Generally, the pH is adjusted to a level between about 6 5 and 7 5 using either an acid (e g . COi ) or a base (e g Na-iCO or NaOH) A suitable temperature range for culturing mammalian cells such as CHO cells is between about 30 to 40°C and preferably about 37°C and a suitable DOτ is between 5-90° o of air saturation At a particular stage the cells may be used to inoculate a production phase or step of the ceil culture Alternatively as described above the production phase or step mav be continuous w ith the inoculation or grovyth phase or step According to the present invention the cell-culture env ironment duπng the production phase of the cell culture is controlled In a preterred aspect the production phase of the cell culture process is preceded by a transition phase ot the cell culture in v\ hιch parameters tor the production phase of the cell culture are engaged T-PA production in mammalian e g CHO. cells ty pically employ s α semi-continuous process hereby cells are culture in a "seed-tram" for various periods of time and are periodically transferred to inoculum fermentors to initiate the cell-amplification process en route to t-PA production at larger scale Thus, ceils used tor rt-PA production are in culture for various periods ot time up to a maximum predefined cell age The parameters of the cell culture process such as seed density pH DOτ and temperature duπng culture duration ot the production culture operating conditions ot harv est, etc are a function ot the particular cell line and culture medium used, and can be determined empirically , w ithout undue experimentation Following the polypeptide production phase, the gly coprotein ot interest is recovered trom the culture medium using techniques which are w ell established in the art The gly coprotem ot interest preterablv is recovered from the culture medium as a secreted poK peptide. although it also may be recovered from host cell lysates As a first step the culture medium or lysate is centπfuged to remove paniculate cell debris The gly coprotein thereafter is purified trom contaminant soluble proteins and poly peptides. w ith the following procedures being exemplary ot suitable purification procedures bv fractionation on immunoatfinity or lon- exchange columns, ethanol precipitation reverse phase HPLC. chromatography on silica or on a cation-exchange resin such as DEAE, chromatotocusing. SDS-PAGE, ammonium sulfate precipitation, gel filtration using, for example. SEPHADEX G-75™, and protein A SEPHAROSE™ columns to remove contaminants such as IgG A protease inhibitor such as pheny l methyl sulfonyl fluoride (PMSF) also may be useful to inhibit proteolytic degradation duπng purification One skilled in the art will appreciate that purification methods suitable for the glycoprotein of interest may require modification to account for changes in the character of the glycoprotein upon expression in recombinant cell culture Also of utility within the context of the present invention are purification techniques and processes that select for the carbohydrates of the invention Such techniques include, for example, ion-exchange soft gel chromatography or HPLC using cation- or anion-exchange resins, wherein the more acidic or more basic traction is collected, depending on which carbohydrate is being selected for In a preferred embodiment CHO cells capable ot producing ht-PA are grown as a suspension in a CHO medium to a predeterm ined cell density The cell suspension may be concentrated bv cross-flow filtration Activ e ht-PA is produced subsequently by the CHO cells suspended in the serum-tree expression medium The ht-PA thus produced is secreted by the CHO cells into the expression medium and mav be separated trom it bv standard techniques Several techniques may be used tor recovery ot the t-PA For example at the end ot the culture, tangential flow filtration, including high-pressure-tangential flow filtration, can be used to remov e the medium containing t-PA from the cells The cell culture supernatants may concentrated, diafiltered. and loaded onto an affinity column capable of specific binding of t-PA ty pically a ivsine affinity column Under the chromatography conditions employed t-PA adheres selectively to the affinity column from w hich it can be recovered and subiected to further purification In the diafiltration step, the supernatant of the cell culture on a dialysis membrane may be diafiltered w ith a dialy sis buffer comprising propy lene glycol. the solution obtained by diafiltration may be loaded on an affinity column capable ot selective binding ot t-PA. and t-P A mav be eluted trom the affinity column w ith a bufter at a pH of about 5 0 to about 9 0 Tne affinity column preterablv is a lysine affinity column, which is preferably eluted at a pH of about 5 0 to about 8 5. more preferably trom about 6 0 to about 8 5 Lysine affinity columns are well know n in the art and are commercially available Suitable columns include Ly sine CPG ( Bioprocessing ). ECH Ly sme CL ™ ( Pharmacia), and Ly sine Hy per D l M ( Biosepra ) The gel is preferably equilibrated vv ith a 50 mM NaτHPθ 4 or 2 HP0 4 solution i pH 7 5 ) prior to loading ot the t-PA solution The elution buffer t pically contains 200 mM arginine. and 50 mM NasHPO_ | θr φH 7 5) Preferably , the elution buffer additionally contains propy lene gly col in a concentration ot about 2 5 to about 20° o After the foregoing recovery/initial purification steps, the feed stream containing 0 5 to 3 0 mg ml t-PA simultaneously generally contains about 0 05 to 5 ng ml ot DNA as determined by a DNA dot blot assay using ~"" P- labeled DNA derived trom the same cell line, resulting in a calculated clearance of approximately 2 10 -fold (depending on the source of the ly sine resm and on the wash conditions used) To further reduce the level of DNA in the product to less than one picogram per human dose, a specific ion-exchange step may be incorporated into the purification procedure, using commercially available ion-exchange columns, such as DE-52™ column (Whatman ) or DEAE-SEPHAROSE FAST FLOW ™ column ( Pharmacia) The purification protocol further includes additional steps that inactivate and/or remove retroviruses that might potentially be present in the cell culture fluid of continuous mammalian cell lines. A significant number of viral clearance steps are available, including additional ultrafiltration/diafiltration steps, treatment with chaotropes such as urea or guanidine. pH extremes, detergents, heat, chemical deπvatization. such as formaldehyde, proteases, conventional separation, such as ion-exchange or size exclusion chromatography, organic solvents, etc. The particular step(s) chosen for viral removal is/are not critical aspect(s) of the present invention, and need to meet the following criteria for t-PA. 1 t-PA must be stable under the treatment conditions while the target virus must be sensitive to the treatment, and 2 the "window of clearance" must be maximum The "window of clearance" is defined for this purpose as the ratio of initial virus titer (spike) in the process fluid prior to the treatment to virus titer after the treatment of the process fluid The recombinant human t-PA recovered and purified following the toregoing protocol typically is at least about 97-99 9° o pure (depending on the lysine resm ) It necessary , further purification can be achieved by additional steps, such as cation-exchange chromatography Accordingly , the product is suitable tor therapeutic applications Various variants of native human t-PA can be purified by essentially the same procedure, and other glycoproteins may be purified by procedures used for their wild-type counterparts, using procedures well-known in the art The present invention is further illustrated by the following, non-limiting examples It is noted that the method of the present invention is equally applicable to the production of other glycoproteins having more than one glycoform in mammalian cell cultures, and the modifications that might become necessary in the course of the adaptation ot the exemplified method to the production ot different gKcoproteins are well within the skill of an ordinarx artisan EXAMPLE 1 Temperature shirt in the production ot rht-PA Materials and Methods CHO cells The CHO cell line used as the mammalian host cell line w as derived from CHO-k l ( ATCC No CCL61 CHO-k l ) and is a CHO-k l mutant dihv drotolate reductase ( DHFR )-defιcιent cell line named CHO- l DUX-B l 1 ( DHFR ) ( obtained from Dr L Chasm of Columbia Univ ersity Simonsen and Levinson. supra Urlaub and Chasm, siφi ) PS-20 basal medium The components ot this medium are listed in Table 1 below TABLE 1 Component Concentration ( mg Calcium chloride, anhydrous 11661 Cupπc sultate. pentahydrate 00012 Ferric nitrate nonahydrate 005 Ferrous sultate. heptahv drate 0417 Potassium chloride 7590 Magnesium sultate. anhy drous 48835 Magnesium chloride anhydrous 14305 Sodium phosphate, monobasic monohydrate 625 Sodium phosphate, dibasic, anhydrous 7102 Zinc sulfate. heptahvdrate 04315 Linoleic acid 0294 Lipoic acid (DL thioctic acid) 0735 Putrescine. dihydrochloπde 05635 Sodium pyruvate 3850 Alanine 31 15 Arginine, monohydrochloπde 7805 Asparagine, monohydrate 5253 Aspartic acid 4655 Cysteine. monohydrochloπde. monohydrate 12292 Cystine. dihydrochloπde 31285 Glutamic acid 5145 Glutamine 16060 Histidine monohydrochloπde. monohy drate 943b Isoleucme 6629 Leucine 9835 Lysine. monohvdrochloπde 200 5 Methionine 3068 Phenylalanine 5036 Proline 12075 Seπne Threomne 8915 Tr ptophan 1514 Tyrosine disodium salt, dihy drate 79125 Va ne 8795 Biotin 00256 D-Calcium pantothenate 368 Cho ne chloride 5086 Cy anocobalamin ( B 12 ) 476 Folic acid 655 ι-Inosιtol 6660 Nicotinamide 21295 Pyπdoxal. monohydrochioπde 2000 Pyπdoxme. monohydrochloπde 0 217 Riboflavin 0 3330 Thiamine. monohv drochloπde 3 190 Glucose 4301 0 Sodium bicarbonate 2440 0 Sodium chloride 5900 0 Pluronic F-68 Prill 1000 0 HEPES 2383 0 Phenol Red 8 10 For convenience, the solid ingredients ot the medium may be combined together w ith the amino acids, and this mixture may be stored as a single unit Ty pe 1 Type II t-PA assav 1 Thavy cell culture supernatant sample (If whole broth, spin out cells in centrifuge ) 2 Add 2 ul freshlv thawed plasminogen to 400 ul ot sample 3 Incubate at 37 " C tor 60 minutes 4 Vdd 20 ul freshly-thawed 1 DTT and 400 ul 8 M guanidine-HCl 50 mM TRIS 3 2 mM EDTA solution 5 Incubate at 37 D C for 15 minutes 6 Transfer to vial and load 250 μl for assay on HP1090™ HPLC using the following conditions ZORBAX™ C8 column at 40°C, Monitoring of eluents by fluorescence (excitation at 275 nm. emission at 340 nm). Running each sample with the following 70-mιnute method where eluent A is 0 1 % tπfluoroacetic acid (TFA) in water and eluent B is 0.1 % TFA in acetonitπle 0 to 5 mm - 75% A (and 25% B) 5 to 35 mm - a linear gradient from 75% A to 60% A 35 1 to 45 mm - 0% A 45 mm to 70 min - 75% A to re-equilibrate the column Tv pe I Type II fragments elute after approximately 25 minutes, and peak areas are integrated to determine relative quantities Protocol Recombinant ht-PA-producing CHO cells were carried in spinner flasks passaged every 3 or 4 day s (at a density of 0 1 % packed cell volume ( PCV )) in selective medium (PS-20 basal medium supplemented w ith 500 mM methotrexate 10 mg L recombinant human insulin (rh-insulin ). 0 1 ml L trace elements, and 0 05 ml L lipid- ethanol ) Sufficient culture yyas removed to seed 15 ml ot medium at 0 2% PCV . and placed in a 50 ml sterile Falcon tube The culture was centπfuged for 10 minutes at 700- 1000 rpm and the supernatant was poured off Upon addition ot 15 ml of fresh selective medium the culture w as agitated gently to resuspend cells Fiv e ml ot culture was placed in three T-25 flasks (25 cm- T-flasks ) The caps vy ere left loosened to allow equilibration w ith incubator atmosphere and the flasks w ere placed in a 33 G C or 1 "C incubator w ith 5% carbon dioxide After 5 to 8 days ot incubation, the cells were counted using a hemacvtometer and or bv checking packed cell volume, and viability was checked using try pan blue The culture was removed trom the flasks, centπfuged for 10 minutes at 2000-2200 rpm and the supernatant yy as assav ed for rht-P A glvcosvlation Alternativ ely to the T 2^ flasks the cells w ere cultured using 60-mm cell culture plates in triplicates Supernatants were frozen at -20 C or -70 C until the Tvpe I II t-PA analysis took place For spinner experiments the foregoing protocol was used except the cells were passaged into 200 ml of fresh medium (with initial PCV of 0 1 %) in a 2 0-ml spinner flask The caps vyere closed tightlv on the flask yyhich was then placed in the 33 C or 1 C incubator on a magnetic stir plate at 60 rpm The m lni-termentor experiments w ere performed under standard t-PA production conditions in fed-batch mode as noted herein m 5-liter stirred tank bioreactors (Applikon Foster Citv CA) The temperature was shifted to 33 C or 3 1 °C on dav 2 ot the production phase For control experiments the foregoing experiments were followed except that incubation took place at 37°C Further experiments vy ere performed as above in 12-k fermentors over the course of 200 hours and the percentage ot Type I t-PA vyas assessed For the experiments herein and below culture conditions were usually changed on dav 1 (e g addition ot different media components ) and dav 2 (e g temperature shift) Results t-PA site occupancy at Asn- 184 was tound to be relative! v consistent across a vaπetv ot scales (T-flask spinner and 80-L 400-L 2000-L and 12 000-L fermentors) (Figure 2) and from run to run in production The factors that can affect site occupancy include those factors affecting oligosacchaπde-dolichol availability (dohchol phosphate pids andhormones) factors affecting protein translation elongation rate (e g . cyclohexamide) factors affecting oligosaccharyltransferase activitv or protein folding rate (e g dithiothreitol), and factors acting through unknown mechanisms, such as time in culture Illustrating the lattermost phenomenon. Figure 3 shows graphs of the percentage of Type I human t-PA in cell cultures of CHO cells at 37°C as a function of 12k-fermentatιon run time with each graph line representing a different run These results show that site-occupancv increases over the course of a batch culture (over culture length) Figure 4 shows that reducing temperature increases t-PA site occupancy Specifically Figure 4A show s the results of laboratorv-scale experiments performed for 5-7 davs (T-flasks and spinner flasks ) In the control experiments vy here in the production phase the temperature remained 3 °C the product contained about 38° o Tv pe 1 t-PA In contrast in the experiments where in the production phase the temperature was lowered to 33 C the t- PA product obtained contained about 43-46% Tvpe I t-PA Figure 4B shows the results of bioreactor experiments indicating that the lower temperature similarly v ields a higher percentage of 1 vpe 1 t-PA EX AMPLE 2 Butv rate addition in the production of rht-PA Mater ials and Methods The quantity of ceils needed for a 0 2% packed cell volume seed density was -entπfuged at approximately 700 x g tor 10 minutes and resuspended in 25 cm " T-flasks in the appropriate tresh medium tor each test case The T-flasks were set up in triplicate with 5 mL in each flask and incubated for 3 to 4 davs at 37°C and 95% aιr'5% COτ Sodium butvrate was added to a concentration ot 0 375 mM 0 75 mM or 1 ~> mM at the time ot inoculation for the 25 cm " T-flasks Cells were also cultured in sp ner flasks at volumes ot 100 ml w ith similar butvrate concentrations to the 1 -flask cases f or tetmentor experiments butv rate w as added on the second dav ot the experiments T-flask cultures w ere also carried out at 33 C and 3" C w ith no butvrate additions The data shovy n tor the T-flask cultures at 33 C are from tw o triplicate experiments ( n=6 ) T-PA site-occupancv was analy zed using the method of Example 1 Results Figure 5 show s that the presence ol sodium butv rate at concentrations ot 0 375 to 1 5 mVl at the time ot inoculation increased t-PA site-occupancv in the T-flasks at 37 °C The same increased effect w as observ ed for the spinner flask and fermentoi experiments Figure 6 show s that tor the 60-mm culture plate experiments temperature shifts to 33 °C and 3 1 C had the largest effect and increased t-P A site-occupancv gradually up to 6% 0 75 mM butv rate increased the Type 1 content slightly (about 1 %) compared to no butv rate ( Fig 6 ) In contrast a turther increase ot the butyrate concentration ( 1 5 M) lowered site-occupancv at 37 C and 33 °C but increased it at 3 1 °C (Fig 6 ) Figure 7 shows the ettect ot temperature and butvrate on t-PA site-occupancv in 5-liter bioreactors The Tv pe 1 content w as analy zed on dav s 5-7 Decreasing temperature trom 37°C to 33 increased t-PA site occupancy How ever increasing the butvrate concentration trom 0 75 to 1 5 mM decreased the Ty pe 1 content at both temperatures This confirms the data obtained in 60-mm plate experiments ( see Fig 6) EXAMPLE 3 Cell-cycle inhibitor addition in the production of rht-PA Introduction Temperature reduction, culture length, and butyrate addition were found to increase the glycosylation of t-PA at the Asn- 184 site, as noted in Examples 1 and 2 All of these factors correspond with decreased cell growth rate, leading to the hypothesis that glvcosvlation and site-occupancy are cell-cycle dependent This hypothesis was tested by performing two additional experiments reflected in this example using cell-cycle inhibitors (quinidme and thymidine) Materials and Methods Cells were cultured as tor the sodium butyrate experiment described in Example 2 Thvmidme was dissolved in w ater and sterile filtered tor a 33-36X stock solution Qumidine was made up in dimethyl sulfoxide ( DMSO) and filtered to make a 1000- 1 800X stock Thv midme and quimdine w ere added to the cultures at the time ot inoculation to final concentrations of 250 ug niL and 90 μM respectively T-PA site-occupancv was analyzed using the method of Example 1 Results The cell cv cle analysis tor the control the quimdine and the thvmidme is show n in Table 2 as determined bv model F A 1 T3 MOD TABLE 2 Cell-Cvcle Analy sis Figure 8 shows that site occupancy and cell-cy cle position vary similarly over time in a culture Figure 9 show s that quimdme blocks the cells in the GO G l phase (with 67% in the GO G l phase l and results in increased site occupancy as compared to the control w ith no cell-cy cle inhibitor ( 54% in the GO G l phase ), and thy midine causes the cells to accumulate in the S G2 phases (with 33% in the GO G l phase ) and results in decreased site occupancy as compared to the control These results confirmed that factors that increase the proportion of cells in the G0/G 1 phase increase site occupancy EXAMPLE 4 Divalent metal cation, nucleotide sugar precursor, and hormone addition in the production of rht-PA Materials and Methods All experiments were done in 60-mm culture dishes using the procedure described in Example 1 except that a Mn salt. Fe salt, nucleotide sugar precursor, or hormone was added to the growth medium during the growth phase, with Mn or Fe salt or nucleotide sugar precursor added on day 1 All plates were inoculated at a seeding density of 0 1% PCV and a working volume of 6-8 ml All cases were done in triplicates Plates were incubated at 37X in CO j incubators The amounts of MnC or ferric citrate in the growth medium during the growth phase were increased For MnClτ 3 nM was the control with increasing amounts of MnCU at concentrations of 10 nM. 100 nM. 1 uM. 10 u M. and 100 M For ferric citrate, the control was no salt w ith increasing amounts at 10. 50. and 100 μM The amounts of uπdme. adenosine. and guanosine were 0 5 M each and the amount ot mannose was 5 g 1 (guanosine and mannose were combined), and -GHT is selective medium minus glycine. hypoxanthine and thymidine The amounts of tπ-iodothy ronine and thvroxine employed in the culture medium were increased relative to a control with no hormone, with amounts of 1 nM. 10 nM. or l OO nM tπ-iodothy ronine. or 1 nM. l O nM. or 100 nM thv roxine T-PA site-occupancy was analy zed on days 4-6 ( manganese, iron, or nucleotide sugar precursor experiment ) or on day 7 (hormones) using the method described in Example 1 Results The effect of increasing manganese concentration on t-PA site-occupancy (triplicate runs) is shown in Figures 10A and 10B Supplementing the medium with additional manganese over the 3 nM control value significantly increased the t-PA Tv pe 1 content ( improv ed site-occupancv ) about 2 5 A positive titration effect w as observed between 3 nM and 100 nVI No further improvement occurred when mcieasmg the concentration up to 100 uM which is still an effective concentration Ohgosaccharv itransterase requires Mn " ions tor maximal activ itv but other div alent metal cations with an octahedral coordination geometry including Mg Ca and Fe w ill support tianster albeit at reduced rates ( Hendπckson and Imperiali supra and kautman et al supra) Hence the ettect ot Fe2- on t-PA site-occupancv vyas investigated As is evident trom f ig 1 1 the addition of 10- 100 uM Fe " ( ferric citrate) increased t-PA site occupancy gradually up to about 4% Increasing the availability ot nucleotide sugar precursors ( e g nucleosides mannose glvcine thvmidme or hv poxanthine i did not improv e site-occupancv Moreover the addition ot undine and guanosine decreased t-PA site-occupancv about 2% (see Fig 12) The ettect ot thvroid hormones (thv roxine and tπ-iodothvronine ) on t-PA site-occupancv is shown in Figures 13A and 13B These hormones increased site-occupancv about 2% and it is expected that other plasma components as defined above would similarly have such an effect ************ In conclusion, several culture conditions that affect t-PA N-glycosvlation site-occupancy have been identified These factors are potentially useful to further improve product consistency T-PA site occupancy at Asn- 184 is relatively consistent across a variety of conditions, including a variety of scales (T-flask. spinner. 80- liter.400-liter.2000-liter, and 12.000-lιter). and from run to run in production Factors that increase the proportion of cells in the GO/G l phase, such as temperature, butyrate, and cell-cycle inhibitors, increase site occupancy, as do increased amounts of certain divalent metal cations and/or plasma components preferably present during the whole cultivation time The entire disclosures of all citations cited throughout the specification and the references cited therein are hereby expressly incorporated bv reference Although the foregoing refers to particular preterred embodiments it w ill be understood that the present invention is not so limited It w ill occur to those ordinarily skilled in the art that various modifications may be made w ithout diverting trom the ov erall concept ot the invention All such modifications are intended to be w ithin the scope of the present invention

Claims: WH AT IS CLAIMED IS 1 A process for producing a gK coprotem comprising culturing mammalian host cells expressing nucleic acid encoding said glvcoprotein m the presence ot (a) a factor that modifies grovvth state in a cell culture (b) a divalent metal cation that can adopt and prefers an octahedral coordination geometry or c ) a plasma component whereby the occupancy ot an N-hnked glv cosv lation site occupied onlv in a traction ot the glvcoprotein is enhanced in the glv coprotein so produced 2 The process ot claim 1 w herein said factor is a cell-cvcle inhibitor that blocks cells in the GO G 1 phase a butvrate salt or a temperature ot the culture ot between about 30 and 35 X 3 The process ot claim 2 vy herein said cell-cvcle inhibitor is quimdme The process of claim 2 w herein the butvrate salt is present in a concentration of about 0 35 to 2 mM The process of claim 1 w herein more than one factor is present The process of claim 5 wherein the culturing takes place at a temperature of between about 30X and 35X and in the presence of about 0 75 to 1 5 mM of a butyrate salt The process of claim 1 wherein the divalent metal cation is present during the whole cultivation time at a concentration of between about 10 nM and 100 uM The process of claim 7 wherein the divalent metal cation is manganese or iron The process ot claim 1 wherein the plasma component is a thvroid hormone 0 The process ot claim 1 vy herein said mammalian host cell is cultured in a serum-tree medium 1 The process of claim 10 w herein said mammalian host cell is cultured in a production phase ot growth cvcle and said factor is present 2 The process of claim 1 1 w herein the culturing takes place at a temperature between about 30 and 35 C and a butyrate salt is added about 48 hours into the production phase 3 The process of claim 1 1 vy herein said production phase is preceded bv a growth phase and a transition phase of growth cvcle The process ot claim 1 3 wherein during e growth phase the temperature is kept at about 37X The process ot claim 14 w herein during the transition phase the temperature ot the culture is lowered to betw een about 30X and 35 X The process ot claim 15 w herein said low ered temperature is about 31 C The process of claim 1 wherein the host cell is a Chinese hamster ovary (CHO) cell The process ot claim 1 vy herein the gly coprotein is a tissue plasminogen activator (t-PA) or plasminogen The process ot claim 18 wherein the t-PA is human t-PA Glycoprotein produced by the process ot claim 1 A process for producing human tissue-plasminogen activator (t-PA). comprising culturing Chinese hamster ovary cells expressing nucleic acid encoding said t-PA in a serum-free medium in a production phase at a temperature of between about 30X and 35X and in the presence of about 0 to 2 mM of a butyrate salt, whereby the occupancy of an N-hnked glycosylation site occupied only in a fraction of t-PA is enhanced in the t-PA so produced A process for producing human tissue-plasminogen activator (t-PA) comprising culturing Chinese hamster ovary ceils expressing nucleic acid encoding said t-PA in a serum-free medium in a growth phase at a temperature of about 37-40X. wherein said medium comprises from about 10 nM to 100 μM of a divalent metal cation that can adopt and prefers an octahedral coordination geometry culturing said cell in a transition phase at a temperature of about 37-40X. and culturing said cell in a production phase wherein after about 48 hours into the production phase the temperature is lowered to between about 30X and 35 X and about 0 75 to 1 5 mM ot a butyrate salt is added to the medium herebv the occupancy of an N-hnked glycosylation site occupied only in a fraction of t-PA is enhanced in the t-PA so produced The process ot claim 22 w herein a plasma component or cell-cycle inhibitor that blocks cells in the GO G l phase is added to the culture medium before or during the growth phase The process of claim 23 wherein thvroxine tπ-iodoth ronine. or quinidine is added to the culture medium Human tissue-plasminogen activator produced by the process ot claim 21 Human tissue-plasminogen activator produced by the process of claim 22

Inventors: ANDERSEN DANA C, BRIDGES TIFFANY M, GAWLITZEK MARTIN, HOY CYNTHIA A, ANDERSEN, DANA, C., BRIDGES, TIFFANY, M., GAWLITZEK, MARTIN, HOY, CYNTHIA, A.
Applicants: GENENTECH INC, GENENTECH, INC.
Application year: 2000
Main classification: C12N15
IPC codes: C12N5, C12N9, C12N15, C12P21, C12R1