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. H ere, we report on the autocrine mechanisms of action of IGF-I and horm onal regulation of expression of IGFBPs in bovine mammary epithelial M D-IGF-I cells which express recombinant IGF-I under the control of the glucocorticoid-inducible mouse mammary tumor virus-long terminal repe at (MMTV-LTR). Levels of IGFBP-3 mRNA and secretion of IGFBP-3 by MD-I GF-I cells were stimulated by IGF-I, insulin (INS), and IGF-I analogs but not prolactin (PRL). Conversely, parental MAC-T cells expressed li ttle IGF-I and secreted primarily IGFBP-2 (29-32 kDa) in response to s timulation 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 a ssociated 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 [H-3]thymidine incorporation into DNA of parental MAC-T cells, comp ared with uninduced cells. Moreover, inclusion of exogenous IGF-I with CM from MD-IGF-I cells triggered an additional 3.0-fold increase in l abel incorporation, but only a 1.6-fold increase in the presence of IG FBP-2-containing media conditioned by MAC-T cells. Des(1-3)IGF-I added to CM from both MAC-T and MD-IGF-I cells respectively, stimulated a 1 0.2 and 6.9-fold increase in [H-3]thymidine incorporation into DNA of MAC-T cells. We suggest that expression of IGF-I-induced IGFBP-3 is an important component of an autocrine loop which potentiates the local mitogenic actions of IGF-I in bovine mammary epithelial cells.