Phosphatidylinositol 3′-Kinase and p70s6k Are Required for Insulin but Not Bisperoxovanadium 1,10-Phenanthroline (bpV(phen)) Inhibition of Insulin-like Growth Factor Binding Protein Gene Expression

doi:10.1074/jbc.272.1.138

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)*

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

^§ To whom correspondence should be addressed:
Polypeptide Hormone Laboratory, Strathcona Anatomy Building, 3640 University St., Room W315, Montreal, Quebec, Canada H3A 2B2.
Tel.: 514-398-4101; Fax: 514-398-3923, E-mail: mc85{at}musica.mcgill.ca

Abstract

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.

Footnotes

↵^‡ Supported by a joint scholarship from les Fonds de Recherche de Santé du Québec and les Fonds pour la Formation de Chercheurs et l'Aide a la Recherche.
↵* This work was supported by a grant from the Medical Research Council of Canada (to B. I. P.). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

IRK

insulin receptor kinase

IRS-1

insulin receptor substrate-1

p70^s6k

p70/p85 ribosomal S6 protein kinase

PI3-kinase

phosphotidylinositol 3′-kinase

MAP

mitogen-activated protein

MEK

MAP kinase kinase

ERK

extracellular signal-regulated kinase

IGF

insulin-like growth factor

IGFBP

IGF binding protein

IR

insulin receptor

Gluc

glucagon

Dex

dexamethasone

Dex/Gluc

dexamethasone/glucagon combined

MBP

myelin basic protein

GAPDH

glyceraldehyde-3-phosphate dehydrogenase

bpV(phen)

bisperoxovanadium 1, 10-phenanthroline.
↵²J. O. Contreres, unpublished data.
↵³C. J. Band, unpublished data.
- Received August 22, 1996.
- Revision received October 24, 1996.