Role of Smad proteins in cell growth regulation

细胞周期控制与癌症密切相关。从G1到S期的细胞周期进程由G1细胞周期蛋白依赖性激酶（CDK）控制，其包括CDK 4（和同源CDK 6）以及CDK 2。 TGF-β可通过使细胞周期停滞在G1期而有效地抑制细胞周期进程。 Smad蛋白可通过调节几个细胞周期相关基因的表达来介导TGF-β生长抑制作用。 例如，Smad蛋白已显示或涉及上调CDK抑制剂p21和p15的表达，并下调c-myc原癌基因的表达。我们最近发现，转染或重组Smad 3可以被G1期CDK，特别是CDK 4/6有效磷酸化。 CDK磷酸化可以抑制Smad 3的基础以及TGF-β诱导的转录活性。 Smad 2与Smad 3同源，具有与Smad 3重叠和不同的活性，也被G1 CDK磷酸化，尽管不如Smad 3广泛。 我们最近的研究也表明Smad 3和Smad 2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，并且CDK磷酸化可以抑制它们的活性，从而促进细胞周期进展和致癌转化。 本提案的具体目的包括证明内源性Smad 3和Smad 2被G1期CDK磷酸化，以确定CDK磷酸化如何影响其活性，并研究Smad 3和Smad 2在细胞生长调节中的作用。 我们预计，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.