The insulin-like effect of growth hormone on insulin-like growth factor II receptors is opposed by cyclic AMP. Evidence for a common post-receptor pathway for growth hormone and insulin action

Cellular mechanism of the insulin-like effect of growth hormone in adipocytes. Rapid translocation of the HepG2-type and adipocyte/muscle glucose transporters

The cellular mechanism whereby growth hormone (GH) acutely stimulates adipocyte glucose uptake was studied in cultures of primary rat adipocytes differentiated in vitro. Preadipocytes were isolated by collagenase digestion of inguinal fat-pads from young rats and were differentiated in the presence of 3-isobutyl-1-methylxanthine, insulin and dexamethasone. The development of an adipocyte morphology (i.e. lipid inclusions) was observed over 6 days after initiation of differentiation. Coincident with this phenotypic change was an increase in glyceraldehyde-3-phosphate dehydrogenase (GPDH) activity and in cellular content of the HepG2-type (Glut1) and adipocyte/muscle (Glut4) glucose transporter isoforms as determined by Western immunoblotting of total cellular protein. Age-matched undifferentiated cells expressed the Glut1 transporter and low levels of GPDH, but neither accumulated lipid nor exhibited measurable expression of the Glut4 protein. On day 6 after the initiation of differentiation, GH and insulin stimulated 2-deoxy[14C]glucose uptake in a dose- and time-dependent fashion in adipocytes cultured under serum-free conditions for at least 15 h. Western-blot analysis of subcellular fractions revealed that both GH and insulin rapidly (within 20 min) stimulated translocation of the Glut1 and Glut4 proteins from a low-density microsomal fraction to the plasma membrane. Confirmatory evidence was provided in immunocytochemical experiments utilizing antisera directed against the C-terminal region of the Glut4 protein and a fluorescein isothiocyanate-labelled second antibody. Observation of the cells via confocal laser microscopic imaging was consistent with glucose transporter redistribution from an intracellular region to the plasma membrane after treatment with GH or insulin. On the basis of these data, we suggest that the insulin-like effect of GH on adipocyte glucose transport involves translocation of the Glut1 and Glut4 proteins to the plasma membrane. Furthermore, stimulation of glucose-transporter translocation by both GH and insulin may indicate a common cell signalling element between the adipocyte GH and insulin receptors or, alternatively, the existence of multiple cellular mechanisms for stimulating glucose-transporter translocation.

Amiloride inhibits insulin sensitivity and responsiveness in rat adipocytes through different mechanisms

To investigate the mechanisms by which amiloride inhibits insulin action rat adipocytes were treated with insulin and with amiloride added before or after energy depleting the cells with 2 mM KCN. Amiloride decreased the insulin response on 3-0-methylglucose transport, IGF-II- and insulin binding in both intact and energy depleted cells. In contrast, the sensitivity to insulin was inhibited by amiloride only when it was added before KCN. The effect of amiloride on insulin sensitivity was probably exerted through the impaired activation of the insulin receptor tyrosine kinase and the decreased insulin binding. However, insulin responsiveness was probably impaired through a direct effect on the plasma membrane proteins. In contrast to a recent report with pituitary cells, amiloride did not affect the activation of the inhibitory GTP-binding protein (Gi) in rat adipocytes.