An insulin-related peptide expressed in 3T3L1 adipocytes is localized in GLUT4 vesicles and secreted in response to exogenous insulin, which augments the insulin-stimulated glucose uptake

Gliclazide protects 3T3L1 adipocytes against insulin resistance induced by hydrogen peroxide with restoration of GLUT4 translocation

Increased oxidative stress under hyperglycemia may contribute to progressive deterioration of peripheral insulin sensitivity. In this study, we investigated whether gliclazide, a second-generation sulfonylurea, can protect 3T3L1 adipocytes from insulin resistance induced by oxidative stress, and whether gliclazide can restore insulin-stimulated glucose transporter 4 (GLUT4) translocation under oxidative stress. We incubated 3T3L1 adipocytes in hydrogen peroxide to produce oxidative stress, then administered various concentrations of gliclazide, N-acetylcystein (NAC), or glibenclamide. Cells treated with these drugs were next exposed to insulin, subsequent glucose uptake was measured, and the insulin-stimulated GLUT4 translocation was monitored in living cells. We found that hydrogen peroxide treatment alone suppressed glucose uptake by insulin stimulation to 65.9%+/-7.8% of the corresponding controls (P<.01). However, addition of 0.1 to 10 micromol/L gliclazide to hydrogen peroxide-treated cells dose-dependently restored glucose uptake, with 5 micromol/L gliclazide significantly restoring glucose uptake to 93.3+/-6.6% (P<.01) even under hydrogen peroxide. Treatment with the known anti-oxidant NAC also dose-dependently (0.1-10 mmol/L) restored insulin-induced glucose uptake in the presence of hydrogen peroxide. However, glibenclamide (0.1-10 micromol/L), another second-generation sulfonylurea, failed to improve glucose uptake. Similarly, treatment with 5 micromol/L gliclazide or 10 mmol/L NAC significantly overcome the reduction in insulin-stimulated GLUT4 translocation by hydrogen peroxide (P<.01), whereas 5 micromol/L glibenclamide did not. Therefore our data regarding gliclazide further characterize its mechanism of hypoglycemic effect: the observed improvements in insulin sensitivity and in GLUT4 translocation indicate that gliclazide counters the hydrogen peroxide-induced insulin resistance in 3T3L1 adipocytes and also would further augment the hypoglycemic effect of this drug as insulinotropic sulfonylurea.

Activators of AMP-activated protein kinase enhance GLUT4 translocation and its glucose transport activity in 3T3-L1 adipocytes

To determine whether the increase in glucose uptake following AMP-activated protein kinase (AMPK) activation in adipocytes is mediated by accelerated GLUT4 translocation into plasma membrane, we constructed a chimera between GLUT4 and enhanced green fluorescent protein (GLUT4-eGFP) and transferred its cDNA into the nucleus of 3T3-L1 adipocytes. Then, the dynamics of GLUT4-eGFP translocation were visualized in living cells by means of laser scanning confocal microscopy. It was revealed that the stimulation with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and 2,4-dinitrophenol (DNP), known activators of AMPK, promptly accelerates its translocation within 4 min, as was found in the case of insulin stimulation. The insulin-induced GLUT4 translocation was markedly inhibited after addition of wortmannin (P < 0.01). However, the GLUT4 translocation through AMPK activators AICAR and DNP was not affected by wortmannin. Insulin- and AMPK-activated translocation of GLUT4 was not inhibited by SB-203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Glucose uptake was significantly increased after addition of AMPK activators AICAR and DNP (P < 0.05). AMPK- and insulin-stimulated glucose uptake were similarly suppressed by wortmannin (P < 0.05-0.01). In addition, SB-203580 also significantly prevented the enhancement of glucose uptake induced by AMPK and insulin (P < 0.05). These results suggest that AMPK-activated GLUT4 translocation in 3T3-L1 adipocytes is mediated through the insulin-signaling pathway distal to the site of activated phosphatidylinositol 3-kinase or through a signaling system distinct from that activated by insulin. On the other hand, the increase of glucose uptake dependent on AMPK activators AICAR and DNP would be additionally due to enhancement of the intrinsic activity in translocated GLUT4 protein, possibly through a p38 MAPK-dependent mechanism.