THE REGULATION OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-1 MESSENGER-RIBONUCLEIC-ACID IN CULTURED RAT HEPATOCYTES - THE ROLES OF GLUCAGON AND GROWTH-HORMONE

In previous studies it was shown that bovine GH (bGH) suppressed and g lucagon stimulated the level of 24- and 30- to 34-kilodalton insulin-l ike growth factor-binding proteins (IGFBPs) in the media of cultured r at hepatocytes. In the present study we have evaluated the regulation of IGFBP-1 gene expression in primary rat hepatocyte cultures. Glucago n produced a dose-dependent stimulation of hepatocyte IGFBP-1 messseng er RNA (mRNA), attaining levels 2- to 6-fold greater than control at a glucagon concentration of 100 ng/ml. GH inhibited the accumulation of IGFBP-1 mRNA in a dose-dependent manner producing, 40-70% inhibition at 50 ng/ml. The effect of glucagon was comparable to and additive wit h dexamethasone (1 mu M) The addition of 3-isobutyl-1-mehtylxanthine ( 100 mu M) and (Bu)(2)cAMP (100 mu M) augmented IGFBP-1 mRNA levels 5- to 6-fold. 4 beta-Phorbol 12 beta-myristate 13 alpha-acetate (300 nM) was found to inhibit IGFBP-1 mRNA levels by 40-50%. The inhibitory eff ect of bGH on IGFBP-1 mRNA levels was abolished after preincubation wi th 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (10 mu M) for 24 h, whereas glucagon's stimulatory effect was unaffected. The addition of staurosporine (500 nM) and H-7 (1 mM) abolished the inhibitory effe ct of GH but also significantly inhibited the stimulatory effect of gl ucagon, a result consistent with these agents acting on both protein k inase C (PKC) and PKA. In the presence of 10 mu g/ml cycloheximide, IG FBP-1 gene expression was superinduced by bGH, whereas the effect of g lucagon was uninfluenced. Thus the inhibitory action of GH involves, i n part, the activation of PKC. Glucagon's stimulatory effect seems to involve the activation of PKA. The inhibitory effect of bGH on IGFBP-1 gene expression may require the continuing synthesis of one or more l abile protein(s).