Roles of PKC isoforms in the induction of apoptosis elicited by aberrant Ras

Abstract

Emerging evidence indicates that suppression of protein kinase C (PKC) renders the susceptibility of cells expressing mutated ras to apoptosis. Although the effort has been made, the underlying molecular mechanisms are not fully understood. Here, using shRNAs or PKC inhibitor, we demonstrate that the concurrent suppression of PKC α and β induces cells ectopically expressing v-ras to undergo apoptosis. In this apoptotic process, PKC δ is upregulated and translocated from the cytosol to the nucleus. The activated PKC δ associates with and phosphorylates p73 to initiate apoptosis. In this apoptotic process, Akt appears to be downstream of oncogenic Ras. Moreover, overexpression of PKC δ, without co-suppression of PKC α and β, is not apoptotic to the cells, suggesting that PKC δ and α/β function oppositely to facilitate cells harboring v-ras to survive. Thus, our study demonstrates that PKC α and β are necessary for sustaining the homeostasis in cells containing a hyperactive Ras. The abrogation of these two isoforms switches on the p73-regulated apoptotic machinery via the activation of PKC δ.

Induction of mitotic catastrophe by PKC inhibition in Nf1-deficient cells

“…It was reported that in the absence of PKC, aberrant Ras became unstable and subsequently triggered cell death program. [19][20][21] However, little is known about whether PKC signaling is essential for Nf1 deficient cells to survive or if the blockade of PKC could sensitize the defect cells to apoptosis. In this study, we demonstrated the occurrence of a synthetic lethal interaction between mutated Nf1 and loss of PKC.…”

“…Previously, we showed that PKC δ functioned as a tumor suppression to promote apoptosis in cancer cells harboring oncogenic ras, when the conventional PKC isoforms were suppressed. 19 Therefore, it is possible that PKC δ is upregulated or activated and further acts in synergy with Akt to mediate Chk1 phosphorylation under Nf1 deficient condition. The investigation of Chk1 activation in our experimental setting is under way.…”

“…[19][20][21][22] However, it remained unclear whether Nf1 deficient cells would be susceptible to apoptosis in the absence of PKC. Therefore, human Nf1 deficient ST8814 cells were used in this study.…”

“…[19][20][21] These led us first to test PKC expression. A comparable amount of PKC was expressed in ST8814 or ST/Nf1 cells (data not shown).…”

“…Using small hairpin RNA (shRNA), we demonstrated that PKC α, β, or δ could be either pro-or anti-apoptotic, depending upon cell types or stimuli. [19][20][21] It has also been reported that the stability of Nf1 protein was regulated by PKC and Nf1 deficient cells were highly sensitive to PKC inhibitors. 22 Recently, using genomewide siRNA high-throughput screens, it revealed a diverse set of proteins whose depletion selectively impaired the viability of cells Mutations of tumor suppressor Nf1 gene deregulate Ras-mediated signaling, which confers the predisposition for developing benign or malignant tumors.…”