Alexandrow M G, Moses H L
Department of Cell Biology and the Vanderbilt Cancer Center, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6838, USA.
J Cell Biochem. 1998 Sep 15;70(4):528-42.
The function of the c-myc proto-oncogene in cell cycle progression remains unclear. In order to examine the role c-myc may play in cell cycle progression, we have expressed the hormone-inducible MycER protein in the nontransformed, EGF-dependent mouse keratinocyte cell line BALB/MK. We have found that activation of MycER, but not a mutant MycER, Gal4ER, or FosER, leads to an EGF-dependent and hormone-dependent increased incorporation of labeled thymidine only during the S phase of the cell cycle in BALB/MK cells. A possible explanation for the increase in thymidine incorporation comes from flow cytometric analyses that reveal that activation of MycER leads to an increase in the total number of cells that enter S phase after EGF restimulation. Investigation of the intracellular effects of Myc activation shows that the expression of several putative Myc-sensitive proteins, cyclins A, E, and D1, and the E2F-1 protein are unaffected by Myc induction. Interestingly, we find that the histone H1 kinase activity associated with an E2F-1 complex containing Cyclin A and Cdk-2, but not that associated with Cyclin E, in late G1 and early S phases is increased in cells containing hormone-activated MycER, but not FosER. Although the mechanism for this Myc-dependent effect on E2F-1-associated kinase activity is still unknown, it does not appear to involve dissociation of the Cdk inhibitor p27Kip1 from the complexes as suggested by others. However, we have also found that hormone-treated cells actually show more p16INK4A inhibitor associated with another kinase, Cdk-4, as the cells are entering S phase. Altogether, the data suggest that the presence of excessive Myc protein in keratinocytes can stimulate otherwise noncycling cells to enter the cell cycle, and that this effect of Myc involves both positive effects on E2F-1-associated Cdk-2 and negative effects on Cdk-4 in late G1.
c-myc原癌基因在细胞周期进程中的功能仍不清楚。为了研究c-myc在细胞周期进程中可能发挥的作用，我们在未转化的、依赖表皮生长因子（EGF）的小鼠角质形成细胞系BALB/MK中表达了激素诱导型MycER蛋白。我们发现，MycER的激活而非突变型MycER、Gal4ER或FosER的激活，仅在BALB/MK细胞周期的S期导致依赖EGF和激素的标记胸苷掺入增加。胸苷掺入增加的一个可能解释来自流式细胞术分析，该分析表明MycER的激活导致EGF再刺激后进入S期的细胞总数增加。对Myc激活的细胞内效应的研究表明，几种假定的Myc敏感蛋白、细胞周期蛋白A、E和D1以及E2F-1蛋白的表达不受Myc诱导的影响。有趣的是，我们发现，在含有激素激活的MycER而非FosER的细胞中，与含有细胞周期蛋白A和细胞周期蛋白依赖性激酶2（Cdk-2）的E2F-1复合物相关的组蛋白H1激酶活性在G1晚期和S期早期增加，但与细胞周期蛋白E相关的组蛋白H1激酶活性未增加。尽管Myc对E2F-1相关激酶活性的这种依赖性作用机制仍不清楚，但它似乎不像其他人所建议的那样涉及细胞周期蛋白依赖性激酶抑制剂p27Kip1从复合物中解离。然而，我们还发现激素处理的细胞在进入S期时实际上显示出更多与另一种激酶Cdk-4相关的p16INK4A抑制剂。总之，数据表明角质形成细胞中过量Myc蛋白的存在可以刺激原本不进行细胞周期循环的细胞进入细胞周期，并且Myc的这种作用涉及对G1晚期E2F-1相关Cdk-2的正向作用和对Cdk-4的负向作用。
