Role of Smad proteins in cell growth regulation

细胞周期控制与癌症密切相关。细胞周期从G1期到S期的进展是由G1周期蛋白依赖性激酶（CDKs）控制的，包括CDK4（和同源CDK6）以及CDK2。tgf - β可以通过在G1期引起细胞周期阻滞而有效地抑制细胞周期进程。Smad蛋白可以通过调节几种细胞周期相关基因的表达来介导tgf - β的生长抑制作用。例如，Smad蛋白已被证明或涉及上调CDK抑制剂p21和p15的表达，下调c-myc原癌基因的表达。我们最近发现转染的或重组的Smad3可以被G1 CDKs，特别是CDK4/6有效磷酸化。CDK磷酸化可以抑制Smad3的基础转录活性以及tgf - β诱导的转录活性。Smad2与Smad3同源，与Smad3具有重叠和不同的活性，也被G1 CDKs磷酸化，尽管不像Smad3那样广泛。我们最近的研究也表明Smad3和Smad2可能可以抑制细胞生长。基于这些发现，我们假设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程和致癌转化。该提案的具体目的包括证明内源性Smad3和Smad2被G1 CDKs磷酸化，确定CDK磷酸化如何影响其活性，并研究Smad3和Smad2在细胞生长调节中的作用。我们期待这一应用的发现将为肿瘤发生机制提供新的见解。

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.