Volume 272, Number 1, Issue of January 3, 1997 pp. 138-145
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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Phosphatidylinositol 3-Kinase and p70^s6k Are Required for Insulin but Not Bisperoxovanadium 1,10-Phenanthroline (bpV(phen)) Inhibition of Insulin-like Growth Factor Binding Protein Gene Expression
EVIDENCE FOR MEK-INDEPENDENT ACTIVATION OF MITOGEN-ACTIVATED PROTEIN KINASE BY bpV(phen)

(Received for publication, August 22, 1996, and in revised form, October 24, 1996)

From the Polypeptide Hormone Laboratory and the Departments of Medicine and Physiology, McGill University, Montreal, Quebec, Canada H3A 2B2

The hormonal regulation of insulin-like growth factor binding protein (IGFBP)-1 and -4 mRNA was compared in serum-free primary rat hepatocyte cultures. The combination of dexamethasone and glucagon (Dex/Gluc) strongly increased IGFBP-1 and IGFBP-4 mRNA levels. Insulin suppressed Dex/Gluc-stimulated IGFBP-1 but not IGFBP-4 mRNA levels. In contrast, the peroxovanadium compound, bisperoxovanadium 1,10-phenanthroline (bpV(phen)), completely abrogated Dex/Gluc induction of both IGFBP mRNA species. Wortmannin and rapamycin blocked the inhibitory effect of insulin but not that of bpV(phen) on Dex/Gluc-stimulated IGFBP mRNA. Thus, although phosphatidylinositol 3-kinase and p70^s6k are necessary for insulin-mediated transcriptional inhibition of the IGFBP-1 gene, a signaling pathway, independent of phosphatidyloinositol 3-kinase and p70^s6k, is activated by bpV(phen) and mediates IGFBP-1 as well as IGFBP-4 mRNA inhibition. Mitogen-activated protein (MAP) kinase activity induced by insulin was suppressed to below basal levels in the presence of Dex/Gluc, whereas in response to bpV(phen), MAP kinase activity was high and unaffected by Dex/Gluc, consistent with a role of MAP kinases in bpV(phen)-mediated inhibition of IGFBP mRNA. The specific MAP kinase kinase (MEK) inhibitor, PD98059, inhibited insulin but not bpV(phen)-stimulated MAP kinase activity, suggesting that MAP kinases can be activated in a MEK-independent fashion. Peroxovanadium compounds are strong inhibitors of tyrosine phosphatases, which may inhibit specific tyrosine/threonine phosphatases involved in the negative regulation of MAP kinases.

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