RESUMO
UNLABELLED: Transforming growth factor beta (TGF-ß) is an important regulator of cell growth, and loss of TGF-ß signaling is a hallmark of carcinogenesis. The Smad3/4 adaptor protein ß2-spectrin (ß2SP) is emerging as a potent regulator of tumorigenesis through its ability to modulate the tumor suppressor function of TGF-ß. However, to date the role of the TGF-ß 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 ß2SP, a crucial Smad3 adaptor, modulates cyclin dependent kinase 4 (CDK4), cell cycle progression, and suppression of HCC. Increased expression of ß2SP 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 ß2SP efficiently restores Rb hypophosphorylation and cell cycle arrest in G(1) . We further demonstrate that ß2SP interacts with CDK4 and Smad3 in a competitive and TGF-ß-dependent manner. In addition, haploinsufficiency of cdk4 in ß2sp(+/-) mice results in a dramatic decline in HCC formation compared to that observed in ß2sp(+/-) mice. CONCLUSION: ß2SP 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 ß2sp(+/-) mice as a model of preclinical efficacy in the treatment of HCC.