The TGF-beta paradox in human cancer: an update.

TGF-beta plays an essential role in maintaining tissue homeostasis through its ability to induce cell cycle arrest, differentiation and apoptosis, and to preserve genomic stability. Thus, TGF-beta is a potent anticancer agent that prohibits the uncontrolled proliferation of epithelial, endothelial and hematopoietic cells. Interestingly, tumorigenesis typically elicits aberrations in the TGF-beta signaling pathway that engenders resistance to the cytostatic activities of TGF-beta, thereby enhancing the development and progression of human malignancies. Moreover, these genetic and epigenetic events conspire to convert TGF-beta from a suppressor of tumor formation to a promoter of their growth, invasion and metastasis. The dichotomous nature of TGF-beta during tumorigenesis is known as the 'TGF-beta paradox', which remains the most critical and mysterious question concerning the physiopathological role of this multifunctional cytokine. Here we review recent findings that directly impact our understanding of the TGF-beta paradox and discuss their importance to targeting the oncogenic activities of TGF-beta in developing and progressing neoplasms.