Kawada M, Yamagoe S, Murakami Y, Suzuki K, Mizuno S, Uehara Y
Department of Bioactive Molecules, National Institute of Health, Tokyo, Japan.
Oncogene. 1997 Aug 7;15(6):629-37. doi: 10.1038/sj.onc.1201228.
While most untransformed cells require substrate attachment for growth (anchorage dependence), the oncogenic transformed cells lack this requirement (anchorage independence) and are often tumorigenic. However, the mechanism of loss of anchorage dependence is not fully understood. When rat normal fibroblasts were cultured in suspension without substrate attachment, the cell cycle arrested in G1 phase and the cyclin-dependent kinase inhibitor p27Kip1 protein and its mRNA accumulated. Conditional expression of oncogenic Ras induced the G1-S transition of the cell cycle and significantly shortened the half-life of p27Kip1 protein without altering its mRNA level. Inhibition of the activation of mitogen-activated protein (MAP) kinase by cyclic AMP-elevating agents and a MEK inhibitor prevented the oncogenic Ras-induced degradation of p27Kip1. These results suggest that the loss of substrate attachment induces the cell cycle arrest through the up-regulation of p27Kip1 mRNA, but the oncogenic Ras confers anchorage independence by accelerating p27Kip1 degradation through the activation of the MAP kinase signaling pathway. Furthermore, we have found that p27Kip1 is phosphorylated by MAP kinase in vitro and the phosphorylated p27Kip1 cannot bind to and inhibit cdk2.
虽然大多数未转化的细胞需要附着于底物才能生长（锚定依赖性），但致癌转化细胞则缺乏这种需求（锚定非依赖性），并且通常具有致瘤性。然而，锚定依赖性丧失的机制尚未完全了解。当大鼠正常成纤维细胞在无底物附着的悬浮液中培养时，细胞周期停滞在G1期，细胞周期蛋白依赖性激酶抑制剂p27Kip1蛋白及其mRNA积累。致癌性Ras的条件性表达诱导细胞周期的G1-S转变，并显著缩短p27Kip1蛋白的半衰期，而不改变其mRNA水平。通过环磷酸腺苷升高剂和MEK抑制剂抑制丝裂原活化蛋白（MAP）激酶的激活，可阻止致癌性Ras诱导的p27Kip1降解。这些结果表明，底物附着的丧失通过上调p27Kip1 mRNA诱导细胞周期停滞，但致癌性Ras通过激活MAP激酶信号通路加速p27Kip1降解，从而赋予锚定非依赖性。此外，我们发现p27Kip1在体外被MAP激酶磷酸化，磷酸化的p27Kip1不能结合并抑制cdk2。
