Regulation of protein kinase Cδ downregulation by protein kinase Cε and mammalian target of rapamycin complex 2

Abstract

Phosphorylation and dephosphorylation of PKCs can regulate their activity, stability and function. We have previously shown that downregulation of PKCδ 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 PKCδ downregulation. A brief treatment of HeLa cells with phorbol 12,13-dibutyrate (PDBu) induced phosphorylation of PKCδ 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 PKCδ in HeLa but not in HeLa/CP cells. The PKC inhibitor Gö 6983 inhibited PDBu-induced downregulation of PKCδ, decreased Thr505 phosphorylation and increased PKCδ tyrosine phosphorylation at Tyr-311 site. However, knockdown of c-Abl, c-Src, Fyn and Lyn had little effect on PKCδ downregulation and Tyr311 phosphorylation. Pretreatment with the phosphatidylinositol 3-kinase inhibitor Ly294002 and mTOR inhibitor rapamycin restored the ability of PDBu to downregulate PKCδ in HeLa/CP cells. Knockdown of mTOR and rictor but not raptor facilitated PKCδ downregulation. Depletion of PKCε also enhanced PKCδ downregulation by PDBu. These results suggest that downregulation of PKCδ is regulated by PKCε and mammalian target of rapamycin complex 2 (mTORC2).

Upregulation of PKCη by PKCε and PDK1 involves two distinct mechanisms and promotes breast cancer cell survival

“…Figure 2 shows that while the PKC activator PDBu induced downregulation of PKCα, -δ and -ε, it caused upregulation of PKCη. Consistent with the notion that PKC activity is required for PKC activator-induced downregulation of PKCs [5, 6, 24], the general PKC inhibitor Gö 6983 prevented PDBu-induced downregulation of PKCα and PKCδ. However, Gö 6983 was able to induce PKCη downregulation in PDBu-treated cells.…”

Novel regulation of protein kinase C-η

“…Since cross-regulation of PKC isozymes by other PKC family members has been suggested by several studies [11,12,31], we examined if the knockdown of a particular PKC isozyme affects PKCη level. As shown in Fig.…”

“…For example, PKCε rather than PDK1 was shown to phosphorylate PKCδ and PKCε at the activation loop whereas PKCδ induced autophosphorylation as well as transphosphorylation of PKCε at the hydrophobic motif [12]. We recently reported that depletion of PKCε enhanced PDBu-induced downregulation of PKCδ in HeLa cells [11]. Since the general PKC inhibitor Gö 6983 but not the conventional PKC inhibitor Gö 6976 induced PKCη downregulation (Fig.…”

“…It is generally believed that the priming phosphorylation of PKC occurs at the activation loop by phosphoinositide-dependent kinase-1 (PDK1) and is followed by autophosphorylation at the turn and the hydrophobic motifs [6]. Recent studies, however, suggest that PKCs may also be transphosphorylated by other members of the PKC family [3,6,7,11]. For example, PKCδ has been shown to be transphosphorylated by PKCε and vice versa [12].…”

The role of PKC isoforms in the inhibition of NF-κB activation by vitamin K2 in human hepatocellular carcinoma cells

“…However, recent studies indicated that conventional PKCs can be phosphorylated at the TM and HM by the mammalian target of rapamycin complex 2 [48,49]. More recently, PKCδ, one of the novel PKCs, was found to be transphosphorylated at the A-loop by another novel PKC, PKCɛ, in place of PDK1 [50].…”