ROLE OF HYPERINSULINEMIA ON HEPATIC INSULIN-RECEPTOR CONCENTRATION AND AUTOPHOSPHORYLATION IN THE PRESENCE OF HIGH GROWTH-HORMONE LEVELS INTRANSGENIC MICE OVEREXPRESSING GROWTH-HORMONE GENE

Overexpression oi bovine growth hormone (bGH) in transgenic (PEPCK-bGH ) mice induces resistance to insulin, which is compensated by a major increase in insulin levels, In these animals, hepatic insulin receptor s (InsRs) are downregulated while tyrosine kinase activity of wheat ge rm agglutinin (WCA)-purified InsRs towards exogenous substrates is une xpectedly increased. By normalizing insulinemia, we attempted to deter mine whether the alterations detected in the early steps of insulin si gnal transduction are due to exposure to chronically high GH levels or are secondary to hyperinsulinemia. Transgenic PEPCK-bGH animals were treated with a single intraperitoneal administration of streptozotocin (STZ) or were deprived of food for 48 h, to normalize insulin levels. Both fasting and STZ treatment were effective in reducing insulin blo od levels to control values or below, while GH levels remained unchang ed (STZ treatment) or increased (fasted animals). In the liver of untr eated transgenic mice, the number of InsRs as determined by I-125-insu lin binding was significantly diminished (65 +/- 5% and 60 +/- 6% of n ormal values in microsomes and solubilized membranes respectively; P < 0.01 vs control mice). In treated transgenic mice, the number oi InsR s increased to values similar to or slightly higher than those found i n normal control mice (STZ-treated: 139 +/- 26% and 126 +/- 8%; lasted : 128 +/- 5% (P < 0.05) and 102 +/- 1.5%, for microsomes and solubiliz ed membranes respectively). Neither treatment altered InsR affinity. I nsR concentration in liver as determined by immunoblotting using an an tibody against the P-subunit of the insulin receptor was found to de r educed in transgenic mice (69 +/- 3% of normal values, P < 0.001) and was normalized after both STZ treatment (105 +/- 4%) and fasting (109 +/- 4%). Insulin-stimulated autophosphorylation activity of InsRs in t ransgenic mice was increased (154 +/- 13%, P < 0.01 compared with the control group), essentially normalized by STZ treatment (96 +/- 14%), and reduced by fasting, to below the values measured in normal control mice (56 +/- 15%, P < 0.05). The potential influence of basal serine/ threonine (Ser/Thr) phosphorylation of the InsR beta-subunit on the re gulation of the InsRs from transgenic mice was also investigated.The a utophosphorylation activity of WGA-purified InsRs from all groups of m ice studied was essentially unchanged after dephosphorylation with alk aline phosphatase or mild trypsinization. Consequently, our results su ggest that the observed changes in InsR number and autophosphorylation activity in the liver of bGH transgenic mice are directly related to changes in insulin blood levels, and that Ser/Thr phosphorylation is a pparently not involved in the regulation of the InsR autophosphorylati on activity in this model oi insulin resistance.