Role of Smad proteins in cell growth regulation

细胞周期调控与癌症密切相关。细胞周期从G1期进入S期是由细胞周期蛋白依赖性蛋白激酶(CDKs)调控的，CDKs包括CDK4(及其同源的CDK6)和CDK2。转化生长因子-β可通过使细胞周期停滞于G1期而有效地抑制细胞周期进程。Smad蛋白可通过调节多种细胞周期相关基因的表达来介导转化生长因子-β的生长抑制作用。例如，Smad蛋白已被证明或涉及上调CDK抑制物p21和p15的表达，并下调c-myc原癌基因的表达。我们最近发现，转基因或重组的Smad3可以被G1CDKs特别是CDK4/6有效地磷酸化。CDK的磷酸化可以抑制Smad3的基础转录活性和转化生长因子-β诱导的转录活性。Smad2与Smad3同源，与Smad3有重叠和不同的活性，也被G1CDK磷酸化，尽管不像Smad3那样广泛。我们最近的研究还表明，Smad3和Smad2可能具有抑制细胞生长的作用。基于这些发现，我们推测Smads可以抑制细胞生长，CDK磷酸化可以抑制其活性，从而促进细胞周期进展和致癌转化。这一建议的具体目的包括证明内源性Smad3和Smad2被G1CDKs磷酸化，确定CDK磷酸化如何影响其活性，以及研究Smad3和Smad2在细胞生长调节中的作用。我们预计，这项应用的发现将为肿瘤发生的机制提供新的见解。

Cell cycle control is closely associated with cancer. Cell cycle progression from the G1 to S phase is governed by G1 cyclin-dependent kinases (CDKs), which include CDK4 (and the homologous CDK6) as well as CDK2. TGF-beta can potently inhibit cell cycle progression by causing cell cycle arrest at the G1 phase. Smad proteins can mediate the TGF-beta growth-inhibitory effects by regulating the expression of several cell cycle- related genes. For example, Smad proteins have been shown or implicated to upregulate the expression of CDK inhibitors p21 and p15, and downregulate the expression of the c-myc protooncogene. We have recently discovered that transfected or recombinant Smad3 can be potently phosphorylated by G1 CDKs, in particular, by CDK4/6. CDK phosphorylation can inhibit the basal as well as TGF-beta-induced transcriptional activity of Smad3. Smad2, which is homologous to Smad3 and has overlapping as well as distinct activities from Smad3, is also phosphorylated by G1 CDKs albeit not as extensively as Smad3. Our recent studies also suggest that Smad3 and Smad2 probably can inhibit cell growth. Based on these findings, we hypothesize that Smads can inhibit cell growth, and that CDK phosphorylation can suppress their activities, thus facilitating cell cycle progression and oncogenic transformation. The specific aims of this proposal include to demonstrate that endogenous Smad3 and Smad2 are phosphorylated by G1 CDKs, to determine how CDK phosphorylation affects their activities, and to investigate the role of Smad3 and Smad2 in cell growth regulation. We anticipate that the findings of this application will shed new insights into the mechanisms of tumorigenesis.