Regulation of protein kinase C delta downregulation by protein kinase C epsilon and mammalian target of rapamycin complex 2.

Phosphorylation and dephosphorylation of PKCs can regulate their activity, stability and function. We have previously shown that downregulation of PKC delta by tumor promoting phorbol esters was compromised when HeLa cells acquired resistance to cisplatin (HeLa/CP). In the present study, we have used these cells to understand the mechanism of PKC delta downregulation. A brief treatment of HeLa cells with phorbol 12,13-dibutyrate (PDBu) induced phosphorylation of PKC delta at the activation loop (Thr505), turn motif (Ser643), hydrophobic motif (Ser662) and Tyr-311 sites to a greater extent in HeLa/CP cells compared to HeLa cells. Prolonged treatment with PDBu led to downregulation of PKC delta in HeLa but not in HeLa/CP cells. The PKC inhibitor Gö 6983 inhibited PDBu-induced downregulation of PKC delta, decreased Thr505 phosphorylation and increased PKC delta tyrosine phosphorylation at Tyr-311 site. However, knockdown of c-Abl, c-Src, Fyn and Lyn had little effect on PKC delta downregulation and Tyr311 phosphorylation. Pretreatment with the phosphatidylinositol 3-kinase inhibitor Ly294002 and mTOR inhibitor rapamycin restored the ability of PDBu to downregulate PKC delta in HeLa/CP cells. Knockdown of mTOR and rictor but not raptor facilitated PKC delta downregulation. Depletion of PKC epsilon also enhanced PKC delta downregulation by PDBu. These results suggest that downregulation of PKC delta is regulated by PKC epsilon and mammalian target of rapamycin complex 2 (mTORC2).