ROLE OF IGF-I IN NORMAL MAMMARY DEVELOPMENT

Growth hormone (GH) is now believed to be the pituitary factor that is responsible for mammary ductal morphogenesis. Mammary development at puberty occurs because of synergy between GH and estrogen on formation of terminal end buds (TEBs). TEBs extend into the substance of the ma mmary gland fat pad, resulting in ductal morphogenesis. Ultimately, th e whole mammary fat pad accommodates a complex network of ducts. IGF-I or des(1-3) IGF-I mimic the actions of GH on TEB formation in hypophy sectomized, gonadectomized rats. Since GH stimulates IGF-I mRNA within the mammary gland synergistically, we hypothesize that IGF-I partiall y mediates actions of GH in mammary gland development. Studies in tran sgenic mice overexpressing IGF-I, des(1-3) IGF-I, or IGFBP-3 show that IGF-I causes ductal hypertrophy in the lactating mouse and prevention of post-lactational mammary gland involution. One of the mechanisms f or this effect involves apoptosis. The potential role of GH or IGF-I i n mammary carcinogenesis, and the applicability of animal studies to h umans, are discussed.