Signalling Pathways Regulating the Dephosphorylation of Ser729 in the Hydrophobic Domain of Protein Kinase Cε upon Cell Passage

Abstract

We have recently demonstrated that in quiescent fibroblasts protein kinase C (PKC) ε95 is phosphorylated at Ser729, Ser703, and Thr566 and that upon passage of quiescent cells phosphorylation at Ser729 is lost, giving rise to PKCε87. Ser729 may be rephosphorylated later, suggesting cycling between PKCε87 and PKCε95. Here we show that the dephosphorylation at Ser729 is insensitive to okadaic acid, calyculin, ascomycin C, and cyclosporin A, suggesting that dephosphorylation at this site is not mediated through protein phosphatases 1, 2A or 2B. We demonstrate that this dephosphorylation at Ser729 requires serum and cell readhesion and is sensitive to rapamycin, PD98059, chelerythrine, and Ro-31–8220. These results suggest that the phosphorylation status of Ser729 in the hydrophobic domain at Ser729is regulated independently of the phosphorylation status of other sites in PKCε, by a mTOR-sensitive phosphatase. The mitogen-activated protein kinase pathway and PKC are also implicated in regulating the dephosphorylation at Ser729. We have recently demonstrated that in quiescent fibroblasts protein kinase C (PKC) ε95 is phosphorylated at Ser729, Ser703, and Thr566 and that upon passage of quiescent cells phosphorylation at Ser729 is lost, giving rise to PKCε87. Ser729 may be rephosphorylated later, suggesting cycling between PKCε87 and PKCε95. Here we show that the dephosphorylation at Ser729 is insensitive to okadaic acid, calyculin, ascomycin C, and cyclosporin A, suggesting that dephosphorylation at this site is not mediated through protein phosphatases 1, 2A or 2B. We demonstrate that this dephosphorylation at Ser729 requires serum and cell readhesion and is sensitive to rapamycin, PD98059, chelerythrine, and Ro-31–8220. These results suggest that the phosphorylation status of Ser729 in the hydrophobic domain at Ser729is regulated independently of the phosphorylation status of other sites in PKCε, by a mTOR-sensitive phosphatase. The mitogen-activated protein kinase pathway and PKC are also implicated in regulating the dephosphorylation at Ser729. protein kinase C mitogen-activated protein kinase okadaic acid protein phosphatase 1 protein phosphatase 2A polyacrylamide gel electrophoresis 12-O-teteradecanoylphorbol-13-acetate phosphatidylinositol The PKC1 family of related phospholipid-dependent serine/threonine kinases are involved in the control of many cellular processes, including cell growth and differentiation (1Goodnight J. Mischak H. Mushinski J.F. Adv. Cancer Res. 1994; 64: 159-209Crossref PubMed Google Scholar, 2Dekker L.V. Parker P.J. Trends Biochem. Sci. 1994; 19: 73-77Abstract Full Text PDF PubMed Scopus (919) Google Scholar). To date 11 PKC isoforms have been identified: the conventional PKCs (α, βI, βII, and γ), which are regulated by calcium and diacylglycerol, the novel PKCs (δ, ε, and θ), which are calcium independent but dependent upon diacylglycerol, and the atypical PKCs (λ, ι, and ζ), which are both diacylglycerol- and calcium-independent. Another isoform, PKCμ, is known as protein kinase D (3Nishizuka Y. Nature. 1988; 334: 661-665Crossref PubMed Scopus (3532) Google Scholar). Recent work from many groups has highlighted the importance of phosphorylation in the regulation of PKC activity (4Newton A.C. Curr. Opin. Cell Biol. 1997; 9: 161-167Crossref PubMed Scopus (846) Google Scholar, 5Dutil E.M. Keranen L.M. DePaoli-Roach A.A. Newton A.C. J. Biol. Chem. 1994; 269: 29359-29362Abstract Full Text PDF PubMed Google Scholar, 6Keranen L.M. Dutil E.M. Newton A.C. Curr. Biol. 1995; 5: 1394-1403Abstract Full Text Full Text PDF PubMed Scopus (390) Google Scholar, 7Garcia-Paramio P. Cabrerizo Y. Bornancin F. Parker P.J. Biochem. J. 1998; 333: 631-636Crossref PubMed Scopus (61) Google Scholar). Initially phospholipid-dependent kinase 1 phosphorylates a conserved site on the lip of the catalytic region that corresponds to Thr566 in PKCε (8Le Good J.A. Ziegeler W.H. Parekh D.B. Alessi D.R. Cohen P. Parker P.J. Science. 1998; 281: 2042-2045Crossref PubMed Scopus (971) Google Scholar, 9Chou M.M. Hou W. Johnson J. Graham L.K. Lee M.L. Chen C.-S. Newton A.C. Schaffhausen B.S. Toker A. Curr. Biol. 1998; 8: 1069-1077Abstract Full Text Full Text PDF PubMed Google Scholar). Phosphorylation at this site is important for the enzymatic activity of PKC (4Newton A.C. Curr. Opin. Cell Biol. 1997; 9: 161-167Crossref PubMed Scopus (846) Google Scholar). Two further phosphorylation sites have been identified in the C-terminal region of the enzyme at the turn and hydrophobic motifs (4Newton A.C. Curr. Opin. Cell Biol. 1997; 9: 161-167Crossref PubMed Scopus (846) Google Scholar, 5Dutil E.M. Keranen L.M. DePaoli-Roach A.A. Newton A.C. J. Biol. Chem. 1994; 269: 29359-29362Abstract Full Text PDF PubMed Google Scholar, 6Keranen L.M. Dutil E.M. Newton A.C. Curr. Biol. 1995; 5: 1394-1403Abstract Full Text Full Text PDF PubMed Scopus (390) Google Scholar). Phosphorylation at the turn motif (Ser703 in PKCε) is believed to be mediated through autophosphorylation (5Dutil E.M. Keranen L.M. DePaoli-Roach A.A. Newton A.C. J. Biol. Chem. 1994; 269: 29359-29362Abstract Full Text PDF PubMed Google Scholar). There is some debate as to whether the hydrophobic site (Ser729 in PKCε) becomes phosphorylated as a result of autophosphorylation or by a separate kinase. Phosphorylation at this hydrophobic site may be modulated by PKCζ and appears to be sensitive to rapamycin (10Parekh D.B. Ziegler W. Parker P.J. EMBO J. 2000; 19: 496-503Crossref PubMed Scopus (508) Google Scholar, 11Parekh D. Ziegler W. Yonezawa K. Hara K. Parker P.J. J. Biol. Chem. 1999; 274: 34758-34764Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). Although phosphorylation at these two C-terminal sites is not essential for the catalytic activity of PKC, they seem to regulate the stability of the enzyme (12Zhang J. Wang L. Schwartz J. Bond R.W. Bishop W.R. J. Biol. Chem. 1994; 269: 19578-19584Abstract Full Text PDF PubMed Google Scholar, 13Gysin S. Imber R. Eur. J. Biochem. 1997; 249: 156-160Crossref PubMed Scopus (18) Google Scholar, 14Bornancin F. Parker P.J. Curr. Biol. 1996; 6: 1114-1123Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar, 15Cazaubon S. Bornancin F. Parker P.J. Biochem. J. 1994; 301: 443-448Crossref PubMed Scopus (118) Google Scholar, 16Orr J.W. Newton A.C. J. Biol. Chem. 1994; 269: 27715-27718Abstract Full Text PDF PubMed Google Scholar). PKCε is the only isoform that has oncogenic potential (17Mischak H. Goodnight J. Kolch W. Martiny-Baron G. Schaechtle C. Kazanietz M.G. Blumberg P.M. Pierce J.H. Mushinski J.F. J. Biol. Chem. 1993; 268: 6090-6096Abstract Full Text PDF PubMed Google Scholar, 18Cacace A.M. Guadagno S.N. Krauss R.S. Fabbro D. Weinstein I.B. Oncogene. 1993; 8: 2095-2104PubMed Google Scholar) that may be mediated through its interaction with Raf 1 kinase (19Cacace A.M. U.effing M. Phillip A. Han E.K.-H. Kolch W. Weinstein I.B. Oncogene. 1996; 13: 2517-2526PubMed Google Scholar, 20Ueffing M. Lovric J. Philipp A. Mischak H. Kolch W. Oncogene. 1997; 15: 2921-2927Crossref PubMed Scopus (71) Google Scholar). PKCε is also unique in having actin and Golgi-binding domains (21Prekeris R. Mayhew M.W. Cooper J.B. Terrain D.M. J. Cell Biol. 1996; 132: 77-90Crossref PubMed Scopus (228) Google Scholar, 22Prekeris R. Hernandez R.M. Mayhew M.W. White M.K. Terrain D.M. J. Biol. Chem. 1998; 273: 26790-26798Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar, 23Csukai M. Chen C.-H. De Matteis M.A. Mochly-Rosen D. J. Biol. Chem. 1997; 272: 29200-29206Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 24Lehel C. Olah Z. Petrovics G. Jakob G. Anderson W.B Biochem. Biophys. Res. Commun. 1996; 223: 98-103Crossref PubMed Scopus (13) Google Scholar, 25Lehel C. Olah Z. Jakon G. Anderson W.B. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1406-1410Crossref PubMed Scopus (123) Google Scholar). PKCε from fibroblasts migrates on SDS-PAGE as two distinct forms, with molecular sizes of 95 and 87 kDa (PKCε 95 and PKCε 87) that differ in their intracellular localization. In quiescent cells the PKCε95 form predominates, whereas after passage PKCε87 becomes the major species. We have recently reported that these forms differ in their phosphorylation status at Ser729 (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). Thr566 and Ser703 are phosphorylated in both these forms of PKCε, and the protein has complete N and C termini (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). The formation of PKCε87 upon cell passage is not due to new protein synthesis. We have therefore suggested that a phosphatase responsible for dephosphorylation of Ser729 is activated upon cell passage. Removal of the Ser729 phosphate from the hydrophobic domain may reduce the stability of the enzyme, rendering it more accessible to phosphatase attack and potentially to degradation (27Sweatt J.D. Atkins C.M. Johnson J. English J.D. Roberson E.D. Chem S.-J. Newton A. Klann E. J. Neurochem. 1998; 71: 1075-1085Crossref PubMed Scopus (51) Google Scholar). Alternatively, the change in localization of PKCε on passage may make it accessible to a Ser729 phosphatase. Therefore regulation of phosphorylation at Ser729 may prove to be yet another level of control for PKC. The regulation of how PKC becomes dephosphorylated has not been well studied. Most work has been carried out using TPA to induce activation that leads to dephosphorylation and degradation. Our system involving cell passage allows us to examine the control of the putative Ser729-specific phosphatase rather than to look at the complete dephosphorylation of the protein. Our results suggest that dephosphorylation of PKCε is a two-stage process. A specific phosphatase removes the phosphate at Ser729 followed by either the removal of other phosphate groups or rephosphorylation at Ser729 and recycling of the enzyme to the 95-kDa form. In this study we have examined how the dephosphorylation of PKCε at Ser729 in 3T3 and 3T6 fibroblasts is regulated. We present evidence that the dephosphorylation of Ser729 is not mediated by protein phosphatases 1, 2A, and 2B but is dependent upon serum and cell readhesion, and that MAPK and mTOR (mammalian target ofrapamycin) pathways are involved. PKC is also important in regulating the phosphorylation status of Ser729. Cells were from the European Collection of Animal Cell Cultures (Porton Down, UK) and the William Dunn Cell Bank (Oxford, UK). Cell culture plastic was from Nunc (Life Technologies, Inc.). All other cell culture reagents were from Life Technologies, Inc. except serum, which was from PAA Laboratories (Linz, Austria). All other chemicals were from Sigma, and chemical inhibitors were from Calbiochem (Nottingham, UK) unless otherwise stated. Nitrocellulose membrane (Hybond C) was from Amersham Pharmacia Biotech. Dried milk powder was Marvel (Premier Beverages, Stafford, UK). BCA reagents were from Pierce. The polyclonal PKCε and PKCδ antibodies used for Western blotting and immunoprecipitations were generated to the C-terminal peptide sequence by Professor N. Groome (Oxford Brookes University, UK) as previously described (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). Peroxidase-conjugated secondary antibodies were from Sigma. 3T3 and 3T6 cells were maintained in Dulbecco's modified Eagle's medium with serum in a at as described previously (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). Cells were by with followed by with Cells were in medium as culture and to for unless otherwise stated. Cells were with inhibitors at described in the were in unless otherwise and with control were than cells were for and was after passage. PKCε and PKCδ and to protein were by Professor D. Mochly-Rosen was by Professor S. of 3T3 and 3T6 cells were with and were after were or were was by Western blotting for the PKCδ was through the PKCδ the were using and were and by Western In results were with Cells were from with was using the BCA was and of protein were on SDS-PAGE to and for PKCε as described previously (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). Cells were in 1 acid, were with protein at for 1 on a of cell protein were with and this was with protein with the was in and and by Cells were in Dulbecco's modified Eagle's medium (Life Technologies, for 1 of Amersham Pharmacia was for Cells were medium and and was in the Cells were at and PKCε were as described were by and were in for 1 and for in Pharmacia and to All are of at independent In quiescent cells PKCε95 predominates, whereas after passage serum with readhesion PKCε87 becomes the major form We have recently that PKCε95 and PKCε87 differ in their phosphorylation at Ser729 and that the formation of PKCε87 is the result of dephosphorylation of PKCε95 at Ser729 (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). is that the in PKCε protein 1 is the result of of with PKCε87 with PKCε95 the polyclonal PKCε used in these is the C-terminal region of PKCε which Ser729. to other Western is by that that new PKCε of passage is after passage Western show that PKCε95 and becomes the PKC form 1 after cell passage 1 is of PKCε this 1 this that PKCε95 1 of passage is through the rephosphorylation of PKCε87. quiescent cells are medium and to for formation of PKCε 87 is not cells are of serum, PKCε87 is in result that some in serum for the formation of PKCε87 upon cell passage. at both PKCε95 and PKCε87 these cells are for to E. Science. PubMed Scopus Google PKCε95 becomes the form as is in cells to and of serum to cells in not the formation of passage of these cells is also the serum is to quiescent fibroblasts not These suggest that serum is but not for the formation of PKCε87 upon cell passage. is also for the formation of passage. is by the that quiescent fibroblasts are in serum medium or to formation of PKCε87 is These results the for both serum and readhesion in PKCε87 We have through that 3T3 and 3T6 fibroblasts both serum and readhesion to and not be that of fibroblasts not PKCε87 formation not the cell from is but not for PKCε 87 formation it is that other are also important in regulating the dephosphorylation of Ser729. may be the of and in the that upon cell passage and the actin is through D of quiescent PKCε87 is The not the of cells with okadaic acid and and inhibitors G. A or cyclosporin A and ascomycin of G. A not the formation of PKCε87 upon passage C not that these protein phosphatases are not involved in the removal of the Ser729 phosphate and that phosphatase is this We that PKCε87 formation upon cell passage. is that of the activity of the Ser729 phosphatase. Alternatively, the inhibitors may be further dephosphorylation of PKCε87 at Ser703 and PKCε87. In we a PKCε formation of is by the G. Bornancin F. R. Parker P.J. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar) have that dephosphorylation of be only by and have that another protein insensitive to may be involved. These have also described the importance of in the dephosphorylation of that at dephosphorylation G. P. C. Bornancin F. Parker P.J. Biochem. J. 1999; PubMed Scopus Google Scholar). We that at to on the of PKCε87 In to it the formation of PKCε87. of fibroblasts with the and A. C. M. PubMed Scopus Google Scholar) the level of PKCε87 upon cell passage may in the activation of a putative Ser729 more this of the degradation of PKCε87. of cells with the inhibitors and J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google and G. R. M.M. A. R. C. L. J. G. Cancer Res. 1994; Google not also in formation of PKCε87 upon cell passage has been demonstrated to a in PKC phosphorylation through its activation of phospholipid-dependent kinase 1 P. Cabrerizo Y. Bornancin F. Parker P.J. Biochem. J. 1998; 333: 631-636Crossref PubMed Scopus (61) Google Scholar, Good J.A. Ziegeler W.H. Parekh D.B. Alessi D.R. Cohen P. Parker P.J. Science. 1998; 281: 2042-2045Crossref PubMed Scopus (971) Google Scholar). Our also in the control of PKCε dephosphorylation and degradation. of fibroblasts with PKCδ PKCδ and PKCε 87 formation upon cell passage of the domain of PKCδ has been to be a specific of PKCδ M. D.R. S. Cancer Res. 1999; Google Scholar, D.R. S. Oncogene. 1999; PubMed Scopus Google Scholar). with a this domain formation of PKCε87 upon cell passage PKCδ and were also used to PKCδ in fibroblasts M. Mochly-Rosen D. Res. 1999; Scopus Google Scholar, Mochly-Rosen D. 1998; PubMed Scopus Google Scholar). A PKCδ peptide upon whereas control and of PKCε on the formation of PKCε 87 upon cell passage. of the and activity of these was through of PKC isoform localization to membrane and not The of PKCε87 upon passage is cells in the of PD98059, a L. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google and rapamycin, of mTOR J. Google and and by or PKC inhibitors J. Biochem. Biophys. Res. Commun. PubMed Scopus Google and W. E. G. J. Chem. 35: PubMed Scopus Google not suggest a for the MAPK and PKC in the control of PKCε87 formation on cell passage. of protein kinase A, and kinases on PKCε87 formation upon suggesting that these pathways are not important in this process. The are in of inhibitors on PKCε87 formation on cell on PKCε87 and and in a new TPA of cells conventional and novel PKC their dephosphorylation and degradation G. Bornancin F. R. Parker P.J. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, G. P. C. Bornancin F. Parker P.J. Biochem. J. 1999; PubMed Scopus Google Scholar). quiescent fibroblasts are with TPA for formation of PKCε 87 is this is at a level with cell passage TPA be with the PKC and not with the with upon cell this be by with rapamycin and not suggesting that the MAPK and mTOR pathways are also important We have previously that fibroblasts are is a change in the phosphorylation status of PKCε and that this is with a change in the intracellular localization of the protein. in quiescent has a localization and is phosphorylated at Ser703, and Ser729. PKCε87 has a and is phosphorylated at Thr566 and Ser703 (26England K. Rumsby M. Biochem. J. 2000; 35: 219-226Google Scholar). We have suggested that on cell passage a PKCε Ser729 phosphatase may be The hydrophobic (Ser729 in PKCε) site is conserved in PKCs and in the of kinases (10Parekh D.B. Ziegler W. Parker P.J. EMBO J. 2000; 19: 496-503Crossref PubMed Scopus (508) Google Scholar). is therefore essential to how phosphorylation and dephosphorylation in the hydrophobic domain is Here we have examined some that are important in the control of Ser729 phosphorylation We have that the removal of the phosphate at Ser729 from PKCε95 requires both serum and not in the of serum or from E. Science. PubMed Scopus Google and it is therefore that PKCε87 formation is dependent upon of quiescent or cells the cell we have that the cell is not for this cells are and with serum is formation of PKCε87. is that PKCε87 formation is mediated through both of and readhesion of the cells to the in the of Our that the the formation of PKCε87 in quiescent cells this of cells after passage of the PKCε has domain (21Prekeris R. Mayhew M.W. Cooper J.B. Terrain D.M. J. Cell Biol. 1996; 132: 77-90Crossref PubMed Scopus (228) Google Scholar, 22Prekeris R. Hernandez R.M. Mayhew M.W. White M.K. Terrain D.M. J. Biol. Chem. 1998; 273: 26790-26798Abstract Full Text Full Text PDF PubMed Scopus (106) Google and we therefore that PKCε to actin may be activated by either phosphatase activity or to a putative Ser729 phosphatase. Alternatively, of the actin through passage or D the localization of PKCε, it to attack by a Ser729 phosphatase. Our show that the formation of PKCε87 upon passage is not dependent upon the more well protein and we that the formation of PKCε87 upon cell passage is by rapamycin, of mTOR has recently been implicated in the control of phosphorylation of the hydrophobic site in PKCε and PKCδ (10Parekh D.B. Ziegler W. Parker P.J. EMBO J. 2000; 19: 496-503Crossref PubMed Scopus (508) Google Scholar, 11Parekh D. Ziegler W. Yonezawa K. Hara K. Parker P.J. J. Biol. Chem. 1999; 274: 34758-34764Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). These that this is mediated through the activation mTOR of a phosphatase specific for the hydrophobic site rather than through regulation of a kinase. Our this and also suggest that this phosphatase is not or and also with the and the level of PKCε87 upon cell passage. The for these is that these inhibitors are further dephosphorylation and degradation of PKCε 87 that PKCε95 dephosphorylation in two the removal of the phosphate at Ser729 by a specific followed by dephosphorylation at Thr566 and Ser703 by phosphatase. has been that dephosphorylated forms of PKC are more to degradation S. Imber R. Eur. J. Biochem. 1997; 249: 156-160Crossref PubMed Scopus (18) Google Scholar, 14Bornancin F. Parker P.J. Curr. Biol. 1996; 6: 1114-1123Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar, 15Cazaubon S. Bornancin F. Parker P.J. Biochem. J. 1994; 301: 443-448Crossref PubMed Scopus (118) Google Scholar). inhibitors also PKCε87 formation upon passage. may be through of a kinase that PKCε Ser729, a including PKCζ M.M. Hou W. Johnson J. Graham L.K. Lee M.L. Chen C.-S. Newton A.C. Schaffhausen B.S. Toker A. Curr. Biol. 1998; 8: 1069-1077Abstract Full Text Full Text PDF PubMed Google Scholar, D.B. Ziegler W. Parker P.J. EMBO J. 2000; 19: 496-503Crossref PubMed Scopus (508) Google Scholar, 11Parekh D. Ziegler W. Yonezawa K. Hara K. Parker P.J. J. Biol. Chem. 1999; 274: 34758-34764Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar, W.H. Parekh D.B. Good J.A. M. Parker P.J. Curr. Biol. 1999; 9: Full Text Full Text PDF PubMed Scopus Google Scholar). Our results show that PKCε95 becomes the form of PKCε 1 of passage. of PKCε95 may be mediated through rephosphorylation at Ser729 of the form at after passage. Our have that is new of PKCε this suggesting that PKCε95 be from the recycling of PKCε87. of Ser729 potentially be mediated through autophosphorylation (4Newton A.C. Curr. Opin. Cell Biol. 1997; 9: 161-167Crossref PubMed Scopus (846) Google Scholar) or through the activity of another kinase (10Parekh D.B. Ziegler W. Parker P.J. EMBO J. 2000; 19: 496-503Crossref PubMed Scopus (508) Google Scholar, 11Parekh D. Ziegler W. Yonezawa K. Hara K. Parker P.J. J. Biol. Chem. 1999; 274: 34758-34764Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). of PKCδ activity and also PKCε87 upon passage suggesting that PKCδ either PKCε dephosphorylation or PKCε87 dephosphorylation and degradation. of PKCε95 dephosphorylation may be mediated through a Ser729 phosphatase or through the activation of PKCε, its to phosphatase There is evidence in the of a between PKCε and PKCε is growth whereas PKCδ cell growth (17Mischak H. Goodnight J. Kolch W. Martiny-Baron G. Schaechtle C. Kazanietz M.G. Blumberg P.M. Pierce J.H. Mushinski J.F. J. Biol. Chem. 1993; 268: 6090-6096Abstract Full Text PDF PubMed Google Scholar). PKCε to cells from whereas PKCδ cell Cohen Mochly-Rosen D. J. Cell 1999; Full Text PDF PubMed Scopus Google Scholar, Chen C.-H. M. G. Mochly-Rosen D. Proc. Natl. Acad. Sci. U. S. A. U. S. A. 1999; Scholar). The of PKCδ in the regulation of PKCε dephosphorylation in fibroblasts to be further PKCε87 formation upon passage is by PD98059, a the MAPK pathway in the control of PKCε Ser729 serum is for this it is that the MAPK pathway is of in the medium that to cell it that that cell are essential for PKCε87 formation upon cell passage. PKCε phosphorylation at Ser729 is we have demonstrated that PKCε87 upon passage be by the PKC inhibitors and PKCε87 be in quiescent cells through the of TPA for a for another isoform of PKC in the regulation of PKCε Alternatively, PKCε may regulate the activity of a Ser729 phosphatase. PKC inhibitors therefore this phosphatase activity through of Our for PKCε regulation on passage is in We have results in 3T6 fibroblasts and cells suggesting that are not cell Our suggest that at a two-stage is involved in regulating dephosphorylation of PKCε95. of cells or TPA of quiescent cells dephosphorylation at Ser729. is that PKCε is activated upon passage and PKCε has been to be important in the of and other cells Lee J. 1997; Scholar, J. B.S. J. 1997; PubMed Scopus Google Scholar) PKCε is activated upon cell and is for actin The of a phosphate at Ser729 is sensitive to the cell from and readhesion of it is also dependent upon the MAPK has been reported in activated PKC becomes to phosphatase attack (27Sweatt J.D. Atkins C.M. Johnson J. English J.D. Roberson E.D. Chem S.-J. Newton A. Klann E. J. Neurochem. 1998; 71: 1075-1085Crossref PubMed Scopus (51) Google Scholar) and that of the C-terminal may regulate of the kinase to the after activation J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). The of a phosphate at Ser729 is not mediated through or PKCε87 be rephosphorylated at Ser729 and to PKCε95. may a kinase including W.H. Parekh D.B. Good J.A. M. Parker P.J. Curr. Biol. 1999; 9: Full Text Full Text PDF PubMed Scopus Google Scholar) PKCε87 may be further dephosphorylated at sites Ser703 and mediated by phosphatase. dephosphorylation the of PKC and the protein for degradation S. Imber R. Eur. J. Biochem. 1997; 249: 156-160Crossref PubMed Scopus (18) Google Scholar, 14Bornancin F. Parker P.J. Curr. Biol. 1996; 6: 1114-1123Abstract Full Text Full Text PDF PubMed Scopus (149) Google S. Bornancin F. Parker P.J. Biochem. J. 1994; 301: 443-448Crossref PubMed Scopus (118) Google Scholar). is therefore that dephosphorylated PKCε is sensitive to degradation and the We Professor D. Mochly-Rosen for the PKC and Professor S. of for the domain and Professor P. Parker Cancer for the PKCδ