The Insulin-Like Growth Factors (IGFs) and IGF Binding Proteins in Postnatal Development of Murine Mammary Glands

The insulin-like growth factors are mitogens and survival factors for normal mammary epithelialcells in vitro. Data reviewed here demonstrate that mRNAs for IGF-I and IGF-II, the IGFtype I receptor and the IGFBPs are expressed locally in mammary tissue during pubertal andpregnancy-induced growth and differentiation of murine mammary glands. IGF-I, IGF-II andthe IGF-IR were expressed in terminal end buds (TEBs) in virgin glands during ductal growth.In addition, IGF-II and IGF-IR mRNAs were expressed in ductal and alveolar epithelium inglands throughout postnatal development. Consistent with these results, IGF-I promoted ductalgrowth and proliferation in mouse mammary glands in organ culture. In addition to endogenousexpression of the IGFs and IGF-IR, the IGFBPs showed a varied pattern of expression inmammary tissue during postnatal development. For example, IGFBP-3 and -5 mRNAs wereexpressed in TEBs and ducts while IGFBP-2 and -4 mRNAs were expressed in stromal cellsimmediately surrounding the epithelium. These results support a role for the IGFs and IGFBPsas local mediators of postnatal mammary gland growth and differentiation.