Signalling pathways regulating the dephosphorylation of Ser(729) in the hydrophobic domain of protein kinase C epsilon upon cell passage

We have recently demonstrated that in quiescent fibroblasts protein kinase C (PKC) epsilon (95) is phosphorylated at Ser(729), Ser(703), and Thr(566) and that upon passage of quiescent cells phosphorylation at Ser(729) is los t, giving rise to PKC epsilon (87). Ser(729) may be rephosphorylated later, suggesting cycling between pKC epsilon (87) and PKC epsilon (95). Here we show that the dephosphorylation at Ser(729) is insensitive to okadaic acid, calyculin, ascomycin C, and cyclosporin A, suggesting that dephosphorylati on at this site is not mediated through protein phosphatases 1, 2A or 2B, W e demonstrate that this dephosphorylation at Ser(729) requires serum and ce ll readhesion and is sensitive to rapamycin, PD98059, chelerythrine, and Ro -31-8220, These results suggest that the phosphorylation status of Ser(729) in the hydrophobic domain at Ser(729) is regulated independently of the ph osphorylation status of other sites in PKC epsilon, by a mTOR-sensitive pho sphatase, The mitogen-activated protein kinase pathway and PKC are also imp licated in regulating the dephosphorylation at Ser(729).