Role of protein kinase-C in regulation of insulin-like growth factor-binding protein-1 production by HepG2 cells

Insulin-like growth factor binding protein-1 (IGFBP-1) is a liver-derived protein that modulates the mitogenic actions of the insulin-like growth factors (IGFs). IGFBP-1 production is potently inhibited by insulin both in vivo and in HepG2 human hepatoma cells. To further define the pathways of IGFBP-1 regulation, we studied the effects of modulators of protein kinase-C (PKC) on HepG2 cell IGFBP-1 production. Phorbol 12-myristate 13-acetate (PMA) stimulated IGFBP-1 production in a time- and dose-dependent manner, with maximal stimulation occurring at 10-100 nmol/L. The degree of stimulation was dependent on cell density, ranging from about 2-fold in confluent to more than 10-fold in sparse cultures. Preincubation with PMA abolished the inhibitory effect of insulin, while preincubation with insulin did not inhibit PMA stimulation. The transient PKC activator diC8 had no effect, while studies with the PKC inhibitors sphinganine and H-7 were limited by solvent vehicle cytotoxicity. Staurosporine (STS), a potent PKC inhibitor, stimulated IGFBP-1 production 2- to 4-fold and augmented the stimulatory effect of PMA. Concanavalin-A, an inhibitor of PMA-stimulated PKC translocation and down-regulation, inhibited the effects of PMA and STS. Our findings indicate that PKC is involved in the regulation of hepatic IGFBP-1 production. The effects of PMA, which causes rapid activation, followed by membrane translocation and down-regulation of PKC, are similar to those of STS and are countered by Concanavalin-A. These data suggest that PKC activity may mediate tonic inhibition of IGFBP-1 production, while PKC downregulation stimulates the production of this regulatory protein.