Boxem M, van den Heuvel S
Massachusetts General Hospital Cancer Center, Building 149, 13th Street, Charlestown, MA 02129, USA.
Development. 2001 Nov;128(21):4349-59. doi: 10.1242/dev.128.21.4349.
We have investigated the regulation of cell-cycle entry in C. elegans, taking advantage of its largely invariant and completely described pattern of somatic cell divisions. In a genetic screen, we identified mutations in cyd-1 cyclin D and cdk-4 Cdk4/6. Recent results indicated that during Drosophila development, cyclin D-dependent kinases regulate cell growth rather than cell division. However, our data indicate that C. elegans cyd-1 primarily controls G1 progression. To investigate whether cyd-1 and cdk-4 solely act to overcome G1 inhibition by retinoblastoma family members, we constructed double mutants that completely eliminate the function of the retinoblastoma family and cyclin D-Cdk4/6 kinases. Inactivation of lin-35 Rb, the single Rb-related gene in C. elegans, substantially reduced the DNA replication and cell-division defects in cyd-1 and cdk-4 mutant animals. These results demonstrate that lin-35 Rb is an important negative regulator of G1/S progression and probably a downstream target for cyd-1 and cdk-4. However, as the suppression by lin-35 Rb is not complete, cyd-1 and cdk-4 probably have additional targets. An additional level of control over G1 progression is provided by Cip/Kip kinase inhibitors. We demonstrate that lin-35 Rb and cki-1 Cip/Kip contribute non-overlapping levels of G1/S inhibition in C. elegans. Surprisingly, loss of cki-1, but not lin-35, results in precocious entry into S phase. We suggest that a rate limiting role for cki-1 Cip/Kip rather than lin-35 Rb explains the lack of cell-cycle phenotype of lin-35 mutant animals.
我们利用秀丽隐杆线虫体细胞分裂模式高度不变且已被完全描述的特点，研究了其细胞周期进入的调控机制。在一项基因筛选中，我们鉴定出了细胞周期蛋白D（cyd-1）和细胞周期蛋白依赖性激酶4（cdk-4）中的突变。近期研究结果表明，在果蝇发育过程中，细胞周期蛋白D依赖性激酶调控的是细胞生长而非细胞分裂。然而，我们的数据表明，秀丽隐杆线虫的cyd-1主要控制G1期进程。为了研究cyd-1和cdk-4是否仅通过克服视网膜母细胞瘤家族成员对G1期的抑制作用来发挥功能，我们构建了双突变体，完全消除了视网膜母细胞瘤家族以及细胞周期蛋白D - 细胞周期蛋白依赖性激酶4/6的功能。秀丽隐杆线虫中唯一的视网膜母细胞瘤相关基因lin-35 Rb失活，显著减轻了cyd-1和cdk-4突变动物的DNA复制和细胞分裂缺陷。这些结果表明，lin-35 Rb是G1/S期进程的重要负调控因子，可能是cyd-1和cdk-4的下游靶点。然而，由于lin-35 Rb的抑制作用并不完全，cyd-1和cdk-4可能还有其他靶点。Cip/Kip激酶抑制剂为G1期进程提供了额外的控制水平。我们证明，lin-35 Rb和cki-1 Cip/Kip在秀丽隐杆线虫中对G1/S期的抑制作用不重叠。令人惊讶的是，cki-1缺失而非lin-35缺失会导致过早进入S期。我们认为，cki-1 Cip/Kip而非lin-35 Rb的限速作用解释了lin-35突变动物缺乏细胞周期表型的原因。
