TGF-β/smad signaling in prostate cancer

Adenocarcinoma of the prostate is the most common type of cancer, excluding skin cancer, and the second leading cause of cancer death in adult men in the United States. The lifetime risk for developing symptomatic prostate cancer is one in five for an American man. A pivotal step in carcinogenesis is a shift in the balance between proliferation, differentiation, and apoptosis that favors cell proliferation. Transforming growth factor-β (TGF-β) is a key negative growth regulator in the normal prostate. Although TGF-β inhibits the proliferation of normal prostate cells and functions as a tumor suppressor in early tumorigenesis, it acts as a tumor promoter in later stages of tumor progression. Elevated expression of TGF-β in prostate cancer cells is associated with poor clinical outcome. Over-expression of TGF-β aids tumorigenesis by not only stimulating angiogenesis and suppressing the immune system, but also by acting directly on the prostate tumor cells. While prostate cancer cells become resistant to TGF-β-induced growth inhibition and apoptosis, they retain other TGF-β-induced responses that enhance tumorgenicity, such as induction of extracellular matrix proteins, cell adhesion proteins and proteases. These direct tumor effects are mediated primarily through Smad signaling. This review addresses the mechanisms by which prostate cancer cells may acquire TGF-β resistance and promote tumorgenicity. Understanding the mechanisms underlying TGF-β resistance is important for the identification and development of better diagnostic markers and more effective strategies for treating prostate cancer.