IGF-I-induced IGFBP-3 potentiates the mitogenic actions of IGF-I in mammary epithelial MD-IGF-I cells.

Limited information is available concerning the molecular and cellular mechanisms that regulate expression of insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs) in bovine mammary epithelial cells. Here, we report on the autocrine mechanisms of action of IGF-I and hormonal regulation of expression of IGFBPs in bovine mammary epithelial MD-IGF-I cells which express recombinant IGF-I under the control of the glucocorticoid-inducible mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Levels of IGFBP-3 mRNA and secretion of IGFBP-3 by MD-IGF-I cells were stimulated by IGF-I, insulin (INS), and IGF-I analogs but not prolactin (PRL). Conversely, parental MAC-T cells expressed little IGF-I and secreted primarily IGFBP-2 (29-32 kDa) in response to stimulation with INS, dexamethasone (DEX), or IGF-I analogs. Secretion of recombinant IGF-I caused a 26.5-fold increase in secretion of IGFBP-3, as measured by densitometric analysis of ligand blots, which was associated with a 1.7-fold increase in total DNA. Conditioned media (CM) from MD-IGF-I cells induced with DEX stimulated a 2.8-fold increase in [3H]thymidine incorporation into DNA of parental MAC-T cells, compared with uninduced cells. Moreover, inclusion of exogenous IGF-I with CM from MD-IGF-I cells triggered an additional 3.0-fold increase in label incorporation, but only a 1.6-fold increase in the presence of IGFBP-2-containing media conditions by MAC-T cells.(ABSTRACT TRUNCATED AT 250 WORDS)