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.
Highlights
IntroductionNeurofibromin (the protein product of Nf1) shares sequence similarities with the catalytic domain of p120 Gap [1, 2]
Neurofibromin shares sequence similarities with the catalytic domain of p120 Gap [1, 2]
Human Nf1 deficient ST or SNF96.2 cells and SNF02.2 cells expressing a functional Nf1 or ST/Nf1 cells were treated with HMG (1-O-methyl-racglycerol, a protein kinase C (PKC) inhibitor) and the occurrence of apoptosis was analyzed by DNA fragmentation assay (Figure 1A)
Summary
Neurofibromin (the protein product of Nf1) shares sequence similarities with the catalytic domain of p120 Gap [1, 2]. Inactivation of Nf1 caused an increase in Ras activity in murine or some human tumor cells [3,4,5]. It appears that loss of function of Nf1 provided the growth advantage conferred by aberrant Ras signaling [1, 6,7,8]. The GAP activity of Nf1 has profound implications in the pathology and complications of Neurofibromatosis type 1. This common familial tumor predisposition syndrome is inherited in an autosomal dominant manner, and the common abnormality of this genetic defect is the formation of peripheral nerve tumors [9,10,11,12]. Approximate 30% of Neurofibromatosis type 1 patients develop malignant peripheral nerve sheath tumor (MPNST) that are often clinically resistant to conventional therapies
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