β1 Integrin Cytoplasmic Variants Differentially Regulate Expression of the Antiangiogenic Extracellular Matrix Protein Thrombospondin 1

beta(1) integrins play an important role in regulating cell proliferation and survival. Using small interfering RNA or an inhibitory antibody to beta(1), we show here that, in vivo, beta(1) integrins are essential for prostate cancer growth. Among the five known beta(1) integrin cytoplasmic variants, two have been shown to differentially affect prostate cell functions. The beta(1A) variant promotes normal and cancer cell proliferation, whereas the beta(1C) variant, which is down-regulated in prostate cancer, inhibits tumor growth and appears to have a dominant effect On beta(1A). To investigate the mechanism by which beta(1C) inhibits the tumorigenic potential of beta(1A), we analyzed changes in gene expression in cells transfected with either beta(1C) or beta(1A). The results show that beta(1C) expression increases the levels of an extracellular matrix protein, thrombospondin 1 (TSP1), an angiogenesis inhibitor. TSPI protein levels are increased upon beta(1C) expression in prostate cancer cells as well as in beta(1)-null GD25 cells. We show that TSPI does not affect proliferation, apoptosis, or anchorage-independent growth of prostate cancer cells. In contrast, the newly synthesized TSPI, secreted by prostate cancer cells expressing beta(1C), prevents proliferation of endothelial cells. In conclusion, our novel findings indicate that expression of the beta(1C) integrin variant in prostate glands prevents cancer progression by up-regulation of TSP1 levels and inhibition of angiogenesis. [Cancer Res 2009;69(13):5374-82]