Mitogens regulate the production of insulin-like growth factor-binding protein by Swiss 3T3 cells

Quiescent Swiss 3T3 cells can be stimulated to reenter the cell cycle by various mitogens used in synergistic combinations with insulin-like growth factors (IGFs). The cells constitutively secrete an IGF-binding protein (IGFBP), which can modulate the interaction of IGFs with their receptors and could, therefore, alter cellular responsiveness to IGFs. We have now characterized the IGFBP secreted by Swiss 3T3 cells and tested whether its secretion is regulated by heterologous mitogens. Ligand blotting using [125I]IGF-I revealed a major IGFBP of 40,000 mol wt, and treatment of the cells with tunicamycin reduced the mol wt of this protein to about 32,000. mRNA from Swiss 3T3 cells hybridized to a 32P-labeled oligonucleotide (50-mer) complementary to rat IGFBP-3. Taken together, these results indicate that the principal IGFBP secreted by Swiss 3T3 cells is probably the N-glycosylated IGFBP-3. Production of this IGFBP by Swiss 3T3 cells was stimulated by 50-150% by the mitogens bombesin, vasopressin, platelet-derived growth factor, epidermal growth factor, and 12-O-tetradecanoylphorbol 13-acetate and also by IGF-I. The increased production of IGFBP was first detected after 4-6h of incubation and was then maintained for 48-72 h. Agents that elevate intracellular cAMP and the glucocorticoid dexamethasone reduced IGFBP output. In cells in which protein kinase-C had been down-modulated, the stimulation of IGFBP output by 12-O-tetradecanoylphorbol 13-acetate was abolished, but the stimulation induced by the other mitogens was not prevented. Thus, the production of IGFBP by Swiss 3T3 cells can be regulated by a number of different signalling pathways.