β1 integrins play an important role in regulating cell proliferation and survival. Using small interfering RNA or an inhibitory antibody to β1, we show here that, in vivo, β1 integrins are essential for prostate cancer growth. Among the five known β1 integrin cytoplasmic variants, two have been shown to differentially affect prostate cell functions. The β1A variant promotes normal and cancer cell proliferation, whereas the β1C variant, which is down-regulated in prostate cancer, inhibits tumor growth and appears to have a dominant effect on β1A. To investigate the mechanism by which β1C inhibits the tumorigenic potential of β1A, we analyzed changes in gene expression in cells transfected with either β1C or β1A. The results show that β1C expression increases the levels of an extracellular matrix protein, thrombospondin 1 (TSP1), an angiogenesis inhibitor. TSP1 protein levels are increased upon β1C expression in prostate cancer cells as well as in β1-null GD25 cells. We show that TSP1 does not affect proliferation, apoptosis, or anchorage-independent growth of prostate cancer cells. In contrast, the newly synthesized TSP1, secreted by prostate cancer cells expressing β1C, prevents proliferation of endothelial cells. In conclusion, our novel findings indicate that expression of the β1C integrin variant in prostate glands prevents cancer progression by up-regulation of TSP1 levels and inhibition of angiogenesis. ©2009 American Association for Cancer Research.