207. Effect of TGFB1 on mammary gland development is dependent on cellular source of gene expression

The cytokine TGFB1 is implicated in development of the mammary gland through regulation of epithelial cell proliferation and differentiation during puberty and pregnancy. We have compared mammary gland morphogenesis in virgin Tgfb1+/+, Tgfb1+/− and Tgfb1−/− mice, and transplanted Tgfb1+/+ and Tgfb1−/− epithelium to determine the impact of TGFB1 deficiency on development. When mammary gland tissue was evaluated respective to timing of puberty, invasion through the mammary fat pad of the ductal epithelium progressed similarly irrespective of genotype, albeit fewer terminal end buds were observed in mammary glands from Tgfb1−/− mice. The terminal end buds appeared morphologically normal, and a comparable amount of epithelial proliferation was evident. However, when transplanted into wildtype recipients, Tgfb1−/− epithelium showed accelerated invasion compared with Tgfb1+/+ epithelium. This suggests that the normal rate of ductal extension in Tgfb1−/− null mutant mice is the net result of impaired endocrine or paracrine support acting to limit the consequences of unrestrained epithelial growth. By adulthood, mammary glands in cycling virgin Tgfb1−/− mice were morphologically similar to those in Tgfb1+/+ and Tgfb1+/− animals, a normal branching pattern was observed, and the tissue differentiated into early alveolar structures in the diestrus phase of the ovarian cycle. Transplanted mammary gland epithelium showed a similar extent of ductal branching and evidence of secretory differentiation of luminal cells in pregnancy. These results reveal two opposing actions of TGFB1 during pubertal mammary gland morphogenesis, dependent on the cellular source of gene expression. When expressed in the epithelium, TGFB1 inhibits epithelial ductal growth, when expressed systemically, TGFB1 promotes epithelial ductal growth.