Beijersbergen R L, Carlée L, Kerkhoven R M, Bernards R
Division of Molecular Carcinogenesis, Netherlands Cancer Institute, Amsterdam.
Genes Dev. 1995 Jun 1;9(11):1340-53. doi: 10.1101/gad.9.11.1340.
The orderly progression through the cell cycle is mediated by the sequential activation of several cyclin/cyclin-dependent kinase (cdk) complexes. These kinases phosphorylate a number of cellular substrates, among which is the product of the retinoblastoma gene, pRb. Phosphorylation of pRb in late G1 causes the release of the transcription factor E2F from pRb, resulting in the transcriptional activation of E2F-responsive genes. We show here that phosphorylation of the pRb-related p107 is also cell cycle regulated. p107 is first phosphorylated at 8 hr following serum stimulation of quiescent fibroblasts, which coincides with an increase in cyclin D1 protein levels. Consistent with this, we show that a cyclin D1/cdk4 complex, but not a cyclin E/cdk2 complex, can phosphorylate p107 in vivo. Furthermore, phosphorylation of p107 can be abolished by the overexpression of a dominant-negative form of cdk4. Phosphorylation of p107 results in the loss of the ability to associate with E2F-4, a transcription factor with growth-promoting and oncogenic activity. A p107-induced cell cycle block can be released by cyclin D1/cdk4 but not by cyclin E/cdk2. These data indicate that the activity of p107 is regulated by phosphorylation through D-type cyclins.
细胞周期的有序进程由几种细胞周期蛋白/细胞周期蛋白依赖性激酶（cdk）复合物的顺序激活介导。这些激酶使多种细胞底物磷酸化，其中包括视网膜母细胞瘤基因的产物pRb。G1晚期pRb的磷酸化导致转录因子E2F从pRb释放，从而导致E2F反应基因的转录激活。我们在此表明，与pRb相关的p107的磷酸化也受细胞周期调控。在静止的成纤维细胞受到血清刺激后8小时，p107首先发生磷酸化，这与细胞周期蛋白D1蛋白水平的增加相吻合。与此一致的是，我们表明细胞周期蛋白D1/cdk4复合物而非细胞周期蛋白E/cdk2复合物可在体内使p107磷酸化。此外，p107的磷酸化可通过过表达显性负性形式的cdk4而消除。p107的磷酸化导致其失去与E2F-4结合的能力，E2F-4是一种具有促进生长和致癌活性的转录因子。p107诱导的细胞周期阻滞可被细胞周期蛋白D1/cdk4解除，但不能被细胞周期蛋白E/cdk2解除。这些数据表明，p107的活性通过D型细胞周期蛋白的磷酸化来调节。
