The cellular mechanism(s) by which GH produces
insulin resistance in peripheral tissues is poorly understood. Recent evidence suggests that
insulin exerts certain of its intracellular actions by rapidly activating
phosphatidylinositol-specific phospholipase C(s) (PI-PLC) in the plasma membranes of target cells. Therefore, the present study was conducted to determine whether
insulin can activate PI-PLC in adipose tissue of the genetically obese (ob/ob) mouse, an animal that responds markedly to GH with enhanced peripheral
insulin resistance. Also, experiments were performed to determine whether the activation of PI-PLC by
insulin could be blocked by S-carboxymethylated human GH (
RCM-hGH), a GH derivative possessing mainly diabetogenic activity. Isolated adipose segments were incubated for various periods with
insulin (10 mU/ml), homogenized and centrifuged to obtain a 150,000 x g pellet, and the latter was assayed for the ability to produce [3H]
inositol phosphate from phosphatidyl[3H]
inositol. PI-PLC activity was significantly stimulated 5 min after exposure of the segments to
insulin. By 10 min, the
insulin effect was no longer apparent, and after 30 min,
insulin reduced the activity of the
enzyme. One hour after exposure to
insulin, PI-PLC activity returned to the control level. When adipose segments of
RCM-hGH-treated mice (200 micrograms/day for 3 days sc) were incubated for 5 min with
insulin, the ability of
insulin to activate PI-PLC was abolished. However,
RCM-hGH did not alter basal PI-PLC, indicating that its action involves the mechanism by which the
enzyme is activated by
insulin. Also, studies utilizing acute
RCM-hGH treatment showed that its inhibitory effect on
insulin activation of PI-PLC occurs within the same time frame as the onset of enhanced
insulin resistance in the adipose tissue. Thus, the ability of GH to inhibit the activation of PI-PLC by
insulin in adipocytes may account, at least in part, for its ability to induce
insulin resistance in these cells.