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

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Abstract
Evidence that transforming growth factor beta (TGF-β)4 influences pattern formation in the developing mammary gland and negatively regulates ductal growth is reviewed. In the mouse, overexpression of TGF-β 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-β is synthesized in the mammary epithelium, with the three isoforms showing somewhat different spatial and temporal distributions. Exogenous TGF-β applied directly to the glandin 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-β 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-β operating through an autocrine feedback mechanism and by paracrine circuits associated with epithelial-stromal interactions.