Involution of the lactating mammary gland is inhibited by the IGF system in a transgenic mouse model

Development of the mammary gland during puberty, preg-nancy, and lactation is controlled by steroid and peptide hormones and growth factors. To determine the role of the insulin-like growth factors (IGFs) in this process we devel-oped a transgenic model using the whey acidic protein (WAP) gene to direct expression of rat IGF-I and human IGF binding protein-3 (IGFBP-3) to mammary tissue dur-ing late pregnancy and throughout lactation. High levels of expression of transgenic IGF-I and IGFBP-3 were seen in lobular-alveolar cells by in situ hybridization. There was no obvious effect on mammary development dur-ing pregnancy and lactation; indeed, mothers were capable of nursing their pups normally and the only structural dif-ference seen in the mammary glands at peak lactation wa