Endocrine Abstracts

Insulin-like growth factor (IGF-I) is an important signal for mammary gland development and epithelial cell survival. Following lactation, the mammary gland undergoes involution through extensive epithelial cell apoptosis and thus, the IGF-I survival signal must be inhibited. IGF actions are regulated by association with high affinity binding proteins (IGFBPs) and the aim of this study was to investigate the expression of IGFBPs during mouse mammary gland development to determine if IGFBP suppression of IGF survival signals is a mechanism for initiating apoptosis and involution.

Mouse mammary gland tissue was obtained during pregnancy, lactation and on days 1-5 of involution. Apoptosis, indicated by the presence of DNA ladders, was maximal at involution days 2 and 3. Western ligand blot analysis revealed the appearance of a 30 kDa IGF binding protein which also had maximal expression on involution day 2 and 3. Immunoprecipitation and ligand blot analysis identified the binding protein as IGFBP-5.

To further characterize the role of IGFBPs in the mouse mammary gland, the expression of IGFBPs and the effect of exogenous IGFBPs on primary cultures of mammary epithelial cells (MECs) was studied. MECs were found to secrete IGFBP-5 and IGFBP-3 from their basal surface, but only when cells were grown on laminin-rich basement membrane. IGF-I (1nM) induced phosphorylation of IRS-1 and PKB in MECs which was reversed by the addition of 1-10 nM IGFBP-3 or -5. Similarly, IGF-I mediated survival was inhibited in these cells since apoptosis was significantly increased with IGF-I in the presence of IGFBP-3 or -5 (p<0.02). Thus, IGFBP-3 and -5 inhibited both IGF signaling and function.

These studies indicate that IGFBPs play an important role in mammary apoptosis, providing a mechanism for the initiation of epithelial cell apoptosis during involution of the mouse mammary gland.