REGULATION OF EXPRESSION OF IGF-I-INDUCED IGFBP-3 AND IGF-I-RECEPTOR BY CONSTITUTIVE VS REGULATED EXPRESSION OF RECOMBINANT IGF-I IN TRANSFECTED MAMMARY EPITHELIAL-CELLS

This study was undertaken to investigate the effects of constitutive v s acute expression of IGF-I-induced 1GFBP-3 (INDUCED-BP3) on cellular responsiveness to IGF-I and regulation of expression of the type 1 IGF -receptor (IGF-IR) in transfected bovine mammary epithelial cells. Clo nal MAC-T cells were transfected with expression vectors containing an ovine 0.7-Kb exon-2 containing IGF-I cDNA under the control of the co nstitutively-active herpes simplex thymidine kinase (TK) and early sim ian virus 40 (SV40) promoters, and glucocorticoid-inducible mouse mamm ary tumor virus-long terminal repeat (MD-IGF-I) (Romagnolo et al. 1992 ). Constitutive expression of IGF-I in TK- and SV40-IGF-I cells sustai ned chronic secretion of both IGF-I and INDUCED-BP3. The chronic secre tion of IGF-I triggered loss of cell responsiveness to IGF-I and insul in in SV40-IGF-I cells and corresponded to downregulation of the IGF-I R at the cell surface. Loss of binding capacity by these cells was not prevented by secretion of IGF-I-induced IGFBP-3. In contrast, inducti on of MD-IGF-I cells with the glucocorticoid dexamethasone was require d to enhance secretion of INDUCED-BP3 whose secretion was associated w ith autonomous proliferation. The presence of IGFBP-3 in conditioned m edia from MD-, TK-, and SV40-IGF-I cells, as compared to IGFBP-2 in co nditioned media from parental MAC-T cells, was associated with enhance d cellular responsiveness of test MAC-T cells to IGF-I, but not Des(1- 3)IGF-I. We propose that cell derived INDUCED-BP3 is an important comp onent of an autocrine loop to modulate the mitogenic effects of IGF-I in bovine mammary epithelial cells expressing IGF-I.