Marhin W W, Hei Y J, Chen S, Jiang Z, Gallie B L, Phillips R A, Penn L Z
Department of Microbiology, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada.
Oncogene. 1996 Jan 4;12(1):43-52.
Cyclin D1 can bind and phosphorylate the product (pRb) of the retinoblastoma gene (RB-1) and recent evidence suggests pRb, in turn, may regulate cyclin D1 protein expression. In transformed cell lines, loss of pRb activity strongly correlates with a decrease in cyclin D1 protein expression, and conversely, introduction of pRb can induce cyclin D1 promoter activity. We show here that pRb does not regulate cyclin D1 directly as basal and serum-stimulated levels of cyclin D1 protein and kinase activity are similar in wildtype and pRb-deficient primary mouse embryonic fibroblasts (MEFs). These observations suggest that the suppression of cyclin D1 in pRb-minus tumour cell lines requires both loss of pRb and at least one additional genetic event. We have determined that constitutive, ectopic Myc expression in pRb-deficient, but not wildtype, MEFs suppresses cyclin D1 protein expression and kinase activity. Regulation is evident at either the level of RNA or protein expression. Phenotypically, pRb-deficient MEFs consistently exhibited a delayed growth response in comparison to wildtype MEFs. This growth delay is abrogated in pRb-deficient MEFs which are expressing ectopic Myc protein, coincident with the loss of cyclin D1 protein expression. Moreover, these cells exhibit an increased proliferative capacity, and they no longer show contact inhibition. Our results support a cross-regulatory mechanism between Myc, pRb and cyclin D1 and suggest a novel role for cyclin D1 in tumorigenesis.
细胞周期蛋白D1能结合并磷酸化视网膜母细胞瘤基因（RB-1）的产物（pRb），最近有证据表明，pRb反过来可能调节细胞周期蛋白D1的蛋白表达。在转化细胞系中，pRb活性的丧失与细胞周期蛋白D1蛋白表达的降低密切相关，相反，引入pRb可诱导细胞周期蛋白D1启动子活性。我们在此表明，pRb并不直接调节细胞周期蛋白D1，因为在野生型和pRb缺陷型原代小鼠胚胎成纤维细胞（MEF）中，细胞周期蛋白D1蛋白的基础水平和血清刺激水平以及激酶活性是相似的。这些观察结果表明，在pRb缺失的肿瘤细胞系中，细胞周期蛋白D1的抑制需要pRb的缺失和至少一个额外的遗传事件。我们已经确定，在pRb缺陷型而非野生型MEF中组成性异位表达Myc可抑制细胞周期蛋白D1蛋白表达和激酶活性。这种调节在RNA或蛋白表达水平上均很明显。从表型上看，与野生型MEF相比，pRb缺陷型MEF始终表现出生长反应延迟。在表达异位Myc蛋白的pRb缺陷型MEF中，这种生长延迟被消除，这与细胞周期蛋白D1蛋白表达的丧失相一致。此外，这些细胞表现出增殖能力增强，并且不再表现出接触抑制。我们的结果支持Myc、pRb和细胞周期蛋白D1之间的交叉调节机制，并表明细胞周期蛋白D1在肿瘤发生中具有新的作用。
