Regulation of the viability of Nf1 deficient cells by PKC isoforms

Abstract

Suppression of protein kinase C (PKC) is known to be synthetically lethal with ras mutations in various types of cancer cells. The studies also showed that blockade of PKC affected the viability of Nf1 deficient cells. Since PKC family consists of more than 10 isoforms, our study aimed at identifying which isoform(s) played the crucial role in sensitizing Nf1 deficient cells to apoptosis. Using genetic and chemical PKC inhibitors, we demonstrated that the concurrent inhibition of PKC α and β induced Nf1 deficient ST or 96.2 cells, but not SNF02.2 cells with a normal Nf1 or ST cells ectopically expressing Nf1 effective domain gene, to apoptosis. In this process, PKC δ in Nf1 deficient cells, but not in ST/Nf1 cells, was upregulated and translocated to the nucleus. Furthermore, caspase 3 was cleaved and cytochrome c was released to the cytosol. Thus, it appeared that PKC δ and α/β are the crucial components for sustaining the aberrant Ras signaling and further viability of Nf1 deficient cells. The abrogation of these two isoforms activated their opponent PKC δ for switching on the caspase 3-governed apoptotic machinery.

Suppression of PKC causes oncogenic stress for triggering apoptosis in cancer cells

“…The inhibition of PKC triggered apoptosis in cultured cells expressing v-ras or cancer cells harboring mutated ras [ 13 – 17 ]. However, the underlying mechanisms by which oncogenic ras induces apoptosis remain unclear.…”

“…The increased amount of the GTP-bound Ras was detected in all cells expressing aberrant K-ras , in comparison that only the baseline level of active Ras was revealed by the antibody in HPNE cells. Our previous reports demonstrated that murine fibroblasts ectopically expressing mutant ras were susceptible to apoptosis after the co-suppression of PKC α and β isoforms [ 13 – 17 ]. Therefore, the susceptibility of the pancreatic cancer cells to the co-inhibition of PKC α/β was studied.…”

“…In the process of the malignant transformation, oncogenic Ras interacts and further switches on various downstream effector pathways, which triggers phosphorylation chain reactions and activates transcriptional factors. Despite the essential involvement in cell growth and differentiation, hyper-active Ras was shown to be able to induce apoptosis when some of its effector or supporting factors are suppressed [ 13 – 17 ]. For example, the treatment of PKC inhibitors is able to sensitize transformed cells ectopically expressing oncogenic ras or human cancers harboring mutant K-ras to apoptosis [ 48 , 49 ].…”

“…The roles of some of PKC isoforms in the regulation of cell growth or death are rather controversial, depending upon cell types or cellular contexts. For example, PKC α, β, δ possessed dual roles in the regulation of cancer development or programmed cell death [ 16 , 17 , 48 ], which reflect the complexity of these isozymes. Cross-talks among PKC isoforms or with other intracellular signal transducers were often occurred in different subcellular compartments, which guided cells to participate different cellular activities.…”

“…Using siRNA screening, it has uncovered kinase-specific vulnerabilities in tumors harboring mutated ras [ 11 , 12 ]. Furthermore, studies (including ours) demonstrated that loss of PKC (in particular the phorbol ester-dependent subgroup of PKC isoforms), together with Ras mutations, were synthetically lethal [ 13 – 17 ]. Since about 30–40% of human tumors contain an oncogenic ras and attempts to directly target mutated Ras proteins with small molecular weight inhibitors have proved to be unsuccessful [ 18 ], the research focus is now on identifying targeting pathways that function downstream of or parallel with oncogenic Ras.…”