Abstract
Mutation or loss of function of the von Hippel-Lindau (VHL) tumor suppressor gene is regularly found in sporadic renal cell carcinomas (RCC), well vascularized malignant tumors that characteristically overexpress vascular permeability factor/vascular endothelial growth factor (VPF/VEGF). The wild-type VHL (wt-VHL) gene product acts to suppress VPF/VEGF expression, which is overexpressed when wt-VHL is inactive. The present study investigated the pathways by which VHL regulates VPF/VEGF expression. We found that inhibition of protein kinase C (PKC) represses VPF/VEGF expression in RCC cells that regularly overexpress VPF/VEGF. The wt-VHL expressed by stably transfected RCC cells forms cytoplasmic complexes with two specific PKC isoforms, zeta and delta, and prevents their translocation to the cell membrane where they otherwise would engage in signaling steps that lead to VPF/VEGF overexpression. Other experiments implicated mitogen-activated protein kinase (MAPK) phosphorylation as a downstream step in PKC regulation of VPF/VEGF expression. Taken together, these data demonstrate that wt-VHL, by neutralizing PKC isoforms zeta and delta and thereby inhibiting MAPK activation, plays an important role in preventing aberrant VPF/VEGF overexpression and the angiogenesis that results from such overexpression.
Highlights
Gene predispose to a rare hereditary cancer syndrome characterized by the development of renal cell carcinoma (RCC) as well as other tumors including central nervous system hemangioblastomas, pheochromocytomas, and retinal angiomas [1,2,3,4,5,6,7,8]
Inhibition of VPF/VEGF Expression by Different Protein Kinase Inhibitors—Earlier work had shown that the wt-von Hippel-Lindau (VHL) gene suppresses VPF/VEGF expression in RCC and that genistein, a potent inhibitor of protein tyrosine kinases, inhibits hypoxia-induced VPF/VEGF overexpression [19]
These findings suggested that wildtype VHL (wt-VHL) regulated VPF/VEGF expression by interacting with a signaling pathway that involved protein tyrosine kinases
Summary
Gene predispose to a rare hereditary cancer syndrome characterized by the development of renal cell carcinoma (RCC) as well as other tumors including central nervous system hemangioblastomas, pheochromocytomas, and retinal angiomas [1,2,3,4,5,6,7,8]. We present evidence in favor of this hypothesis, demonstrating that 1) PKC inhibitors repress the overexpression of VPF/VEGF characteristic of RCC cells, 2) wt-VHL protein complexes selectively with PKC and ␦, preventing the translocation of these PKC isoforms to the cell membrane and thereby interrupting a signaling cascade that involves MAP kinase, and 3) PKC ␦, but not PKC , complexes directly with wt-VHL protein.
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