title: The B isoform of the insulin receptor signals more efficiently than the A isoform in HepG2 cells.
abstract: 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 32P-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.glu4:tyr1 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.
authors.affiliation: Department of Medicine, University of California, San Diego, La Jolla 92093, USA.
authors.forename: A
authors.initials: A
authors.lastname: Kosaki
authors.forename: T S
authors.initials: TS
authors.lastname: Pillay
authors.forename: L
authors.initials: L
authors.lastname: Xu
authors.forename: N J
authors.initials: NJ
authors.lastname: Webster
mesh_terms: D001483:Base Sequence; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000096:biosynthesis; D003907:Dexamethasone / Q000494:pharmacology*; D051766:Early Growth Response Protein 1; D015870:Gene Expression / Q000187:drug effects*; D017781:Genes, Immediate-Early / Q000187:drug effects*; D016762:Genes, fos; D018197:Hepatoblastoma; D006801:Humans; D017874:Immediate-Early Proteins; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D007700:Kinetics; D008113:Liver Neoplasms; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000096:biosynthesis; D011972:Receptor, Insulin / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction; D014157:Transcription Factors / Q000096:biosynthesis; D014407:Tumor Cells, Cultured; D016335:Zinc Fingers
chemical_list: D017931:DNA Primers; D004268:DNA-Binding Proteins; C497325:EGR1 protein, human; D051766:Early Growth Response Protein 1; C526292:IRS1 protein, human; D017874:Immediate-Early Proteins; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D010750:Phosphoproteins; D016760:Proto-Oncogene Proteins c-fos; D014157:Transcription Factors; D003907:Dexamethasone; D011972:Receptor, Insulin; D017027:Protein Tyrosine Phosphatases