Transforming growth factor-β adaptor, β2-spectrin, modulates cyclin dependent kinase 4 to reduce development of hepatocellular cancer

• DOI: 10.1002/hep.24128
• Publication Date: 2010-12-13T16:47:25Z
• Authors: Hye Jung Baek; Michael J. Pishvaian; Yi Tang; Tae Hyun Kim; Shaoxian Yang; Majed El Zouhairi; Jon Mendelson; Kirti Shetty; Bhaskar Kallakury; Deborah L. Berry; Kyung Hwan Shin; Bibhuti Mishra; Premkumar E. Reddy; Sang Soo Kim; Lopa Mishra
• Keywords: 0301 basic medicine; Carcinoma, Hepatocellular; Liver Neoplasms; Cyclin-Dependent Kinase 4; Spectrin; Haploinsufficiency; 3. Good health; Mice; 03 medical and health sciences; Transforming Growth Factor beta; Animals; Smad3 Protein; Cell Proliferation

Abstract

Transforming growth factor beta (TGF-β) is an important regulator of cell growth, and loss of TGF-β signaling is a hallmark of carcinogenesis. The Smad3/4 adaptor protein β2-spectrin (β2SP) is emerging as a potent regulator of tumorigenesis through its ability to modulate the tumor suppressor function of TGF-β. However, to date the role of the TGF-β 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 β2SP, a crucial Smad3 adaptor, modulates cyclin dependent kinase 4 (CDK4), cell cycle progression, and suppression of HCC. Increased expression of β2SP 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 β2SP efficiently restores Rb hypophosphorylation and cell cycle arrest in G1. We further demonstrate that β2SP interacts with CDK4 and Smad3 in a competitive and TGF-β-dependent manner. In addition, haploinsufficiency of

cdk4

in

β2sp+/−

mice results in a dramatic decline in HCC formation compared to that observed in

β2spZ+/−

mice.

Conclusion

: β2SP 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

β2sp+/−

mice as a model of preclinical efficacy in the treatment of HCC. (Hepatology 2011;)