Insulin can enhance GLUT4 gene expression in 3T3-F442A cells and this effect is mimicked by vanadate but counteracted by cAMP and high glucose - potential implications for insulin resistance

It is well-established that high levels of cAMP or glucose can produce insu lin resistance, The aim of this study was to characterize the interaction b etween these agents and insulin with respect to adipose tissue/muscle gluco se transporter isoform (glucose transporter 4, GLUT4) gene regulation in cu ltured 3T3-F442A adipocytes and to further elucidate the GLUT4 related mech anisms in insulin resistance. Insulin (10(4) muU/ml) treatment for 16 h cle arly increased GLUT4 mRNA level in cells cultured in medium containing 5.6 mM glucose but not in cells cultured in medium with high glucose (25 mM), 8 -Bromo-cAMP (1 or 4 mM) or N-6-monobutyryl cAMP, a hydrolyzable and a non-h ydrolyzable cAMP analog, respectively, markedly decreased the GLUT4 mRNA le vel irrespective of glucose concentrations. In addition, these cAMP analogs also inhibited the upregulating effect of insulin on GLUT4 mRNA level. Int erestingly, the tyrosine phosphatase inhibitor vanadate (1-50 muM) clearly increased GLUT4 mRNA level in a time- and concentration-dependent manner. F urthermore, cAMP-induced inhibition of the insulin effect was also prevente d by vanadate. In parallel to the effects on GLUT4, gene expression, both i nsulin, vanadate and cAMP produced similar changes in cellular GLUT4 protei n content and cAMP impaired the effect of insulin to stimulate C-14-deoxygl ucose uptake. In contrast, insulin, vanadate or cAMP did not alter insulin receptor (IR) mRNA or the cellular content of IR protein. In conclusion: (1 ) Both insulin and vanadate elicit a stimulating effect on GLUT4 gene expre ssion in 3T3-F442A cells, but a prerequisite is that the surrounding glucos e concentration is low. (2) Cyclic AMP impairs the insulin effect on GLUT 1 gene expression, but this is prevented by vanadate, probably by enhancing the tyrosine phosphorylation of signalling peptides and/or transcription fa ctors. (3) IR gene and protein expression is not altered by insulin, vanada te or cAMP in this cell type, (4) The changes in GLUT4 gene expression prod uced by cAMP or vanadate are accompanied by similar alterations in GLUT4 pr otein expression and glucose uptake, suggesting a role of GLUT4 gene expres sion for the long-term regulation of cellular insulin action on glucose tra nsport. (C) 2001 Elsevier Science B.V. All rights reserved.