Hu Xiaotang, Cui Dongming, Moscinski Lynn C, Zhang Xiaohong, Maccachero Vivian, Zuckerman Kenneth S
Interdisciplinary Oncology Program, University of South Florida, H Lee Moffitt Cancer Center and Research Institute, Tampa, FL 33161, USA.
Cytokine. 2007 Feb;37(2):155-62. doi: 10.1016/j.cyto.2007.03.009. Epub 2007 Apr 24.
Transforming Growth Factor-beta (TGFbeta) is known to be a negative regulator of G1 cyclin/cdk activity. It is not clear whether TGFbeta has any effect on G2 checkpoint kinases. We have found that TGFbeta downregulated the expression of several G2 checkpoint kinases including cdc2, cyclin B1, and cdc25c without causing cell accumulation in G2/M phases in two human leukemia cell lines. The inhibition was time-dependent with a maximal inhibition being observed by 24h for cyclin B1 and cdc2 and by 48h for cdc25c. The inhibition was not a result of G1 arrest but a direct effect of TGFbeta which downregulates their expression at mRNA level. In proliferating cells, there was a significant formation of cdc2-pRb complexes, which was decreased to 30% of control levels by 48h after initiating TGFbeta treatment. Cdc2 showed a marked kinase activity on GST-Rb protein in proliferating cells detected by in vitro kinase assay, which was downregulated in response to TGFbeta. In addition, TGFbeta caused a rapid and transient dephosphorylation of cdc2 (Tyr15) and cdc25c (Ser216) for about 2-3h before a dramatic decrease of both molecules by 48h. Taken together, our data suggest that TGFbeta has a direct inhibitory effect on G2 checkpoint kinases, which is regulated at mRNA level. The transient activation of cdc2 and cdc25c and subsequent inhibition of cdc2, cyclin B1, and cdc25c could amplify TGFbeta-induced G1 arrest and growth inhibition.
转化生长因子-β（TGFβ）已知是G1细胞周期蛋白/细胞周期蛋白依赖性激酶（cdk）活性的负调节因子。目前尚不清楚TGFβ对G2期检查点激酶是否有任何影响。我们发现，在两个人类白血病细胞系中，TGFβ下调了包括cdc2、细胞周期蛋白B1和cdc25c在内的几种G2期检查点激酶的表达，但未导致细胞在G2/M期积累。这种抑制是时间依赖性的，细胞周期蛋白B1和cdc2在24小时时观察到最大抑制，cdc25c在48小时时观察到最大抑制。这种抑制不是G1期停滞的结果，而是TGFβ的直接作用，它在mRNA水平下调它们的表达。在增殖细胞中，有大量cdc2-pRb复合物形成，在开始TGFβ处理后48小时，该复合物减少至对照水平的30%。通过体外激酶测定检测到，在增殖细胞中cdc2对GST-Rb蛋白显示出显著的激酶活性，该活性在TGFβ作用下下调。此外，在这两种分子在48小时急剧减少之前，TGFβ导致cdc2（Tyr15）和cdc25c（Ser216）快速且短暂的去磷酸化约2 - 3小时。综上所述，我们的数据表明TGFβ对G2期检查点激酶有直接抑制作用，这种作用在mRNA水平受到调节。cdc2和cdc25c的短暂激活以及随后对cdc2、细胞周期蛋白B1和cdc25c的抑制可能会放大TGFβ诱导的G1期停滞和生长抑制。
