Nakatsuji Y, Miller R H
Department of Neurosciences, Case Western Reserve University, School of Medicine, Cleveland, Ohio 44106, USA.
J Neurosci Res. 2001 Nov 1;66(3):487-96. doi: 10.1002/jnr.1240.
The proliferation of type-1 astrocytes is strongly inhibited by homotypic cell-contact. To examine the mechanisms mediating this inhibition of proliferation, the expression of cell cycle related proteins was compared between exponential growth-phase and contact-inhibited astrocytes. Expression of the cyclin-dependent kinase (Cdk) inhibitor p27Kip1 was upregulated 10-fold in confluent compared with growth-phase cultures. Density-induced expression of p27Kip1 was reversible. When confluent cultures of astrocytes expressing high levels of p27Kip1 were replated at low density, the expression of p27Kip1 decreased rapidly. In contrast to p27Kip1, the expression levels of the cell cycle protein, cyclin A was decreased ten-fold in confluent cultures compared with those in growth phase. In addition, the ratio of hyperphosphorylated to hypophosphorylated retinoblastoma protein (pRb) decreased concomitantly with the increase of p27Kip1 and the decrease of cyclin A levels. These results suggest that increased expression of p27Kip1 and decreased expression of cyclin A underlie the reduction in proliferation of contact inhibited astrocytes. High levels of mitogenic stimulation could transiently override contact-dependent inhibition of astrocyte proliferation. Addition of exogenous epidermal growth factor (EGF) resulted in elevated proliferation at high density and formation of multiple cell layers. Addition of EGF did not substantially alter levels of p27Kip1 or cyclin A, but did elevate the levels of cyclin D1. Such changes in cell cycle protein expression may contribute to elevated cell proliferation seen in reactive gliosis after injury to the adult CNS.
1型星形胶质细胞的增殖受到同型细胞接触的强烈抑制。为了研究介导这种增殖抑制的机制，比较了指数生长期和接触抑制的星形胶质细胞中细胞周期相关蛋白的表达。与生长期培养物相比，汇合培养物中细胞周期蛋白依赖性激酶（Cdk）抑制剂p27Kip1的表达上调了10倍。p27Kip1的密度诱导表达是可逆的。当将表达高水平p27Kip1的星形胶质细胞汇合培养物以低密度重新接种时，p27Kip1的表达迅速下降。与p27Kip1相反，与生长期相比，汇合培养物中细胞周期蛋白cyclin A的表达水平下降了10倍。此外，视网膜母细胞瘤蛋白（pRb）的高磷酸化与低磷酸化之比随着p27Kip1的增加和cyclin A水平的降低而相应降低。这些结果表明，p27Kip1表达增加和cyclin A表达降低是接触抑制的星形胶质细胞增殖减少的基础。高水平的有丝分裂刺激可以暂时克服星形胶质细胞增殖的接触依赖性抑制。添加外源性表皮生长因子（EGF）导致高密度时增殖增加并形成多层细胞。添加EGF并没有实质性改变p27Kip1或cyclin A的水平，但确实提高了cyclin D1的水平。细胞周期蛋白表达的这种变化可能有助于在成体中枢神经系统损伤后反应性胶质增生中观察到的细胞增殖增加。
