The role of TGF-beta in patterning and growth of the mammary ductal tree.

Evidence that transforming growth factor beta (TGF-beta) influences pattern formation in the developing mammary gland and negatively regulates ductal growth is reviewed. In the mouse, overexpression of TGF-beta transgenes during puberty reduces the rate of growth of the ductal tree and simplifies the pattern of arborization, while expression during pregnancy also interferes with lactation. Expression studies in the normal mouse gland indicate that TGF-beta is synthesized in the mammary epithelium, with the three isoforms showing somewhat different spatial and temporal distributions. Exogenous TGF-beta applied directly to the gland in situ inhibits epithelial cell division within hours, and strongly stimulates extracellular matrix synthesis over a longer time course. Normal human breast cells as well as certain breast cancer cell lines also secrete TGF-beta and are themselves inhibited by it, suggesting an autoregulatory feedback circuit, that in some cases appears to be modulated by estradiol. Taken together, the evidence suggests a model in which growth and patterning of the mammary ductal tree are regulated, at least in part, by TGF-beta operating through an autocrine feedback mechanism and by paracrine circuits associated with epithelial-stromal interactions.