THE B-ISOFORM OF THE INSULIN-RECEPTOR SIGNALS MORE EFFICIENTLY THAN THE A-ISOFORM IN HEPG2 CELLS

We have demonstrated previously that dexamethasone treatment of HepG2 cells caused an enhancement of insulin's metabolic effects (Kosaki, A, and Webster, N. J. (1993) J. Biol. Chem. 268, 21990-21996). This correlated with increased expression of the mRNA encoding the B isoform of the insulin receptor (IR). In the present study, we have demonstrated that dexamethasone treatment caused in addition an enhancement in insulin-stimulated immediate-early gene expression (c-fos and egr-1). Dexamethasone treatment caused an increase in in vivo IR autophosphorylation and insulin receptor substrate-1 (IRS-1) phosphorylation both early events in the insulin signaling pathway. Furthermore, the IRS-1 phosphorylation was distinctly left shifted, although the level of IRS-1 protein was unchanged. Total cellular tyrosine phosphatase activity was unaltered when assayed with P-32-labeled IR and IRS-1. Studies in vitro on wheat-germ agglutinin-purified receptors showed that the B isoform of the IR had increased kinase activity both toward itself and the exogenous substrates poly-glu(4):tyr(1) and recombinant IRS-1 protein. In addition, two dimensional tryptic phosphopeptide maps indicated that the B isoform has an additional phosphopeptide that is not seen for the A isoform. In conclusion, it appears that the B isoform of the IR signals more efficiently than the A isoform in HepG2 cells.