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Protein Kinase C Inhibitor and Irradiation-induced Apoptosis: Relevance of the Cytochrome c-mediated Caspase-9 Death Pathway¹

Department of Radiation Oncology, University Hospital Zurich, CH-8091 Zurich, Switzerland [S. R., C. G., S. B., M. P.]; Laboratory for Biochemistry, Federal Institute of Technology, 8091 Zurich, Switzerland [S. R., K. W.]; Novartis Pharma Inc., 4002 Basel, Switzerland [D. F.]; and Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724 [M. S. S., S. W. L.]

Abstract

Caspases are a family of cysteine proteases that constitute the apoptotic cell death machinery. We report the importance of the cytochrome c-mediated caspase-9 death pathway for radiosensitization by the protein kinase C (PKC) inhibitors staurosporine (STP) and PKC-412. In our genetically defined tumor cells, treatment with low doses of STP or the conventional PKC-specific inhibitor PKC-412 in combination with irradiation (5 Gy) potently reduced viability, enhanced mitochondrial cytochrome c release into the cytosol, and specifically stimulated the initiator caspase-9. Whereas treatment with each agent alone had a minimal effect, combined treatment resulted in enhanced caspase-3 activation. This was prevented by broad-range and specific caspase-9 inhibitors and absent in caspase-9-deficient cells. The tumor suppressor p53 was required for apoptosis induction by combined treatment but was dispensable for dose-dependent STP-induced caspase activation. These results demonstrate the requirement for an intact caspase-9 pathway for apoptosis-based radiosensitization by PKC inhibitors and show that STP induces apoptosis independent of p53.

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