Growth hormone (GH) modulates an insulin-sensitive phosphatidylinositol phospholipase C (PI-PLC) in adipose tissue of theob/ob mouse.

Abstract

The present study was conducted to determine (a) if insulin can activate a PI-PLC in adipose membranes of the ob/ob mouse and (b) if the activation of the enzyme of insulin can be modulated by hGH and reduced and S-carboxymethylated hGH (RCM-hGH). After exposure of isolated adipose segments, adipocytes or plasma membranes to insulin, oxytocin, vasopressin or GTP$\gamma$S, membrane PI-PLC activity was assayed for the ability to produce ($\sp3$H) inositol phosphate from exogenous phosphatidyl ($\sp3$H) inositol. Insulin activated a calcium-dependent membrane PI-PLC rapidly and transiently, which started at 2 min, peaked by 5 min and subsided to basal 10 min after start of incubation of adipocytes with insulin. The insulin-induced activation of PI-PLC was concentration-dependent, and was achieved by increasing the Vmax of the enzyme without affecting the Km. The activation of the enzyme by insulin was also demonstrated in a cell-free system in the presence of magnesium, manganese, ATP and GTP. The enzyme activation by insulin was blocked by pertussis toxin and by antibodies against G$\sb{\rm i}\alpha$ subunits suggesting that a G$\sb{\rm i}$ protein was involved. The activation of PI-PLC evoked by insulin was not additive to that evoked by oxytocin, indicating that both insulin and oxytocin may activate the same PI-PLC signaling pathway. RCM-hGH had no effect on insulin-stimulated PI-PLC activity 3 h after its injection into the ob/ob mice, but by 6 h activation of the enzyme by insulin, oxytocin or vasopressin was completely blocked, whereas basal activity of the enzyme was not affected. During this same period, a rise in blood glucose concentration occurred in response to RCM-hGH, presumably resulting from the onset of enhanced insulin resistance. Thus, the action of RCM-hGH on the insulin-sensitive PI-PLC occurred within the same time frame as its diabetogenic effects on glucose metabolism. Administration of hGH exerted a similar inhibitory effect on insulin-induced activation of PI-PLC. Furthermore, the ability of GTP$\gamma$S to activate PI-PLC was impaired by RCM-hGH, suggesting that GH inhibits the activation of PI-PLC by impairing the function of G proteins. This inhibitory effect of GH on PI-PLC may be one of the mechanisms by which GH produces insulin resistance.