Growth hormone (GH) excess is associated with insulin resistance, but the molecular mechanisms of this association are poorly understood. In the current work, we have examined the consequences of exposure to high GH levels on the early steps of the insulin-signaling system in the muscle of bovine (b) GH-transgenic mice. The protein content and the tyrosine phosphorylation state of the insulin receptor (IR), the IR substrate-1 (IRS-1), the association between IRS-1 and the p85 subunit of phosphatidylinositol (PI) 3-kinase, and the phosphotyrosine-derived PI 3-kinase activity in this tissue were studied. We found that in skeletal muscle of bGH-transgenic mice, exposure to high circulating GH levels results in 1) reduced IR abundance, 2) reduced IR tyrosine phosphorylation, 3) reduced efficiency of IRS-1 tyrosine phosphorylation, and 4) defective activation of PI 3-kinase by insulin. These alterations may be related to the insulin resistance exhibited by these animals.

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the β-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the β-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. interestingly, the modulation of Insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those States.

The levels and characteristics of growth hormone (GH)-binding protein (GHBP) and the distribution of GH in peripheral circulation between the free and the bound fractions were studied in three lines of transgenic mice with various degrees of overexpression of bovine (b) GH gene. Two serum fractions bound GH specifically: one with low affinity and high capacity (GHBPI) and one with high affinity and low capacity (GHBPII). The GHBP binding capacity in normal mice (both sexes), transgenic male mice that express the metallothionein-I-hybrid bGH genes, transgenic female mice that express phosphoenolpyruvate carboxykinase (PEPCK)-bGH hybrid genes (PEPCK-bGH-1), and transgenic PEPCK-bGH-5 animals was 1.1 +/- 0.2, 2.0 +/- 0.1, 3.0 +/- 0.1, and 3.9 +/- 0.6 pmol/ml serum, respectively. The amount of GH bound to GHBP in transgenic animals vs. normal siblings was increased 1.8-, 2.5-, and 3.9-fold in these three lines. Consequently, the levels of GH-GHBP complexes in the circulation of PEPCK-bGH-1 transgenic mice were increased approximately 10-fold. Specific GHBP radioimmunoassay confirmed a threefold increase in GHBP in PEPCK-bGH-1 transgenic animals. The levels of GHBP were not significantly correlated to serum GH within or between lines, perhaps due to elevation of serum GH in PEPCK-bGH mice above the level producing maximal response. From these and previous studies, we conclude that life-long exposure to supranormal GH levels leads to major shifts in GH binding in the circulation and in the GH target organs.