Autocrine insulin-like growth factor II inhibits beta-casein mRNA expression in a mammary cell line

A hallmark of mammary cell differentiation is the induction of beta-casein mRNA expression. A mouse mammary epithelial cell line (COMMA-1D) was treated with insulin, hydrocortisone (HC), and prolactin (Prl) at concentrations (50, 500, and 20 ng/ml, respectively) that resulted in less than half-maximal beta-casein mRNA expression. The cells secreted insulin-like growth factor (IGF)-II (106 pg/ml per 24 h) in the condition media under these conditions. Replacement of insulin with rhIGF-II (150 ng/ml) resulted in significantly less beta-casein mRNA expression. Long-Arg IGF-I (50 ng/ml) was similar to insulin in terms of its ability to induce differentiation, but its activity differed from that of insulin in that it also induced cell proliferation. When the two receptor-specific IGF-II analogs, Arg54,55IGF-II and Leu27IGF-II, were used in studies, only at high concentrations (150 ng/ml) was either analog capable of stimulating any beta-casein mRNA expression. When autocrine IGF-II was immuno-neutralized or bound by the addition of rhIGF binding protein-3 (IGFBP-3)beta-casein mRNA expression was enhanced seven-fold and three-fold, respectively. Exogenous application of IGF-II to counteract the IGF-II mAb stimulation resulted in increased cellular growth and reduced differentiation. We conclude that autocrine IGF-II inhibits mammary cell differentiation and that the blockage of autocrine IGF-II benefits mammary cell differentiation.