Transforming Growth Factor-β Adaptor, β2-Spectrin, Modulates Cyclin Dependent Kinase 4 to Reduce Development of Hepatocellular Cancer

Transforming growth factor beta (TGF-beta) is an important regulator of cell growth, and loss of TGF-beta signaling is a hallmark of carcinogenesis. The Smad3/4 adaptor protein beta 2-spectrin (beta 2SP) is emerging as a potent regulator of tumorigenesis through its ability to modulate the tumor suppressor function of TGF-beta. However, to date the role of the TGF-beta signaling pathway at specific stages of the development of hepatocellular carcinoma (HCC), particularly in relation to the activation of other oncogenic pathways, remains poorly delineated. Here we identify a mechanism by which beta 2SP, a crucial Smad3 adaptor, modulates cyclin dependent kinase 4 (CDK4), cell cycle progression, and suppression of HCC. Increased expression of beta 2SP inhibits phosphorylation of the retinoblastoma gene product (Rb) and markedly reduces CDK4 expression to a far greater extent than other CDKs and cyclins. Furthermore, suppression of CDK4 by beta 2SP efficiently restores Rb hypophosphorylation and cell cycle arrest in G(1). We further demonstrate that beta 2SP interacts with CDK4 and Smad3 in a competitive and TGF-beta-dependent manner. In addition, haploinsufficiency of cdk4 in beta 2sp(+/-) mice results in a dramatic decline in HCC formation compared to that observed in beta 2sp(+/-) mice. Conclusion: beta 2SP deficiency leads to CDK4 activation and contributes to dysregulation of the cell cycle, cellular proliferation, oncogene overexpression, and the formation of HCCs. Our data highlight CDK4 as an attractive target for the pharmacologic inhibition of HCC and demonstrate the importance of beta 2sp(+/-) mice as a model of preclinical efficacy in the treatment of HCC. (HEPATOLOGY 2011;53:1676-1684)