Analysis of tumor-derived mutations has led to the suggestion that p16(INK4a), cyclin D1, cdk4, and the retinoblastoma protein (pRB) are components of a regulatory pathway that is inactivated in most tumor cells. Cell...
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Analysis of tumor-derived mutations has led to the suggestion that p16(INK4a), cyclin D1, cdk4, and the retinoblastoma protein (pRB) are components of a regulatory pathway that is inactivated in most tumor cells. Cell cycle arrest induced by p16(INK4a), inhibitor of cyclin D-dependent kinases, requires pRB, and it has been proposed that this G1 arrest is mediated by pRB-E2F repressor complexes. By comparing the properties of primary mouse embryonic fibroblasts specifically lacking pRB-family members, we find that PRE is insufficient for a p16(INK4a)-induced arrest. In addition to pRB, a second function provided by either p107 or p130, two pRB-related proteins, is required for p16(INK4a) to block DNA synthesis. We infer that p16(INK4a)-induced arrest is not mediated exclusively by pRB, but depends on the nonredundant functions of at least two pRB-family members.
Background & Aims: p16(INK4a) is a cell cycle inhibitor and a major tumor-suppressor protein, but the regulation of p16(INK4a) is poorly understood and the physiologic settings in which it exerts its antiprolifera...
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Background & Aims: p16(INK4a) is a cell cycle inhibitor and a major tumor-suppressor protein, but the regulation of p16(INK4a) is poorly understood and the physiologic settings in which it exerts its antiproliferative effects are unknown, A role for p16(INK4a) i, intestinal neoplasia is suggested by the observation that the promoter region is methylated in a subset of human colon tumors, We examined the expression of the protein in specimens representing the full spectrum of neoplastic progression in the human colon and determined whether expressing cells showed evidence of cell cycle inhibition. Methods: We studied p16(INK4a) expression by immunoprecipitation, immunoblotting, reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and immunofluorescence in matched normal and neoplastic colonic tissue from 70 patients. Results: p16(INK4a) expression was very low in normal mucosa, with staining observed in rare epithelial cells at the base of crypts, A distinctly higher expression was found in 4 of 7 aberrant crypt foci, 32 of 36 adenomas, 18 of 28 primary carcinomas, and 5 of 5 metastatic carcinomas, Within each neoplasm p16(INK4a) staining was heterogeneous, with higher expression commonly seen in areas bordering normal tissue. p16(INK4a) Staining correlated inversely with that of Ki67, cyclin A, and the retinoblastoma protein, suggesting that cell cycle progression was inhibited. Conclusions: These results suggest that p16(INK4a) expression begins in the earliest detectable stages of neoplastic progression in the human colon and exerts a continuous, piecemeal constraint on tumor growth.
A missense mutation in the gene coding for the G-protein-activated inwardly rectifying potassium (GIRK) channel, GIRK2, is responsible for apoptosis in the external germinal layer (EGL) of the cerebellum and a nonapop...
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A missense mutation in the gene coding for the G-protein-activated inwardly rectifying potassium (GIRK) channel, GIRK2, is responsible for apoptosis in the external germinal layer (EGL) of the cerebellum and a nonapoptotic death of midbrain dopaminergic neurons in the weaver (wv) mouse, Failure of axonogenesis and migration are considered to be the primary consequences of GIRK2 channel malfunction in the cerebellum, We investigated whether a disruption of the cell cycle precedes the failure of migration and axonogenesis and leads to massive apoptosis, To this end, immunohistochemistry and immunoblotting for PCNA, Cdk4, cyclin. D, cyclin A, and the Cdk inhibitor p27/kip1, as well as ill situ end-labeling for apoptotic DNA fragmentation, were applied to cerebella of P7-P21+/+, wv/+, and wv/wv mice, In +/+ and wv/+ mice, the expression of cell cycle proteins was limited to the outer, premigratory zone of the EGL, Antibodies to p27, a marker of cell differentiation, gave a reverse staining pattern, Due to migration delay, patches of p27-positive cells persisted in the outer EGL in P21 wv/+ mice, On the contrary, marked cell cycle up-regulation and absence of p27 occurred throughout the EGL at all ages in wv/wv mice, indicating an inability to switch off the cell cycle, Mitotic index evaluation showed that cell cycle activation was unrelated to proliferative events. Cell cycle proteins were not expressed in the substantia nigra, suggesting that nonapoptotic death of mature dopaminergic neurons is not preceded by abortive cell cycle reentry, Our data show that abnormalities of the cell cycle in wv/wv cerebellum represent a major and early consequence of GIRK2 channel malfunction and may strongly influence the susceptibility of EGL cells to apoptosis,These observations may help in understanding the pathogenesis of human neurological channelopathies.
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