Effect of Subtoxic DDT Exposure on Glucose Uptake and Insulin Signaling in Rat L6 Myoblast-Derived Myotubes.

Exposure to persistent organic pollutants including dichlorodiphenyltrichloroethane (DDT) induces insulin resistance. But the mechanism is not clearly known. The present study was designed to explore the effect of subtoxic DDT exposure on (1) insulin-stimulated glucose uptake, (2) malondialdehyde (MDA) level and total antioxidant content, (3) activation of redox sensitive kinases (RSKs), and (4) insulin signaling in rat L6 myoblast-derived myotubes. Exposure to 30 mg/L and 60 mg/L of DDT for 18 hours dose dependently decreased glucose uptake and antioxidant content in myotubes and increased MDA levels. The exposures did not alter tumor necrosis factor α (TNF-α) level as determined by enzyme-linked immunosorbent assay, despite decreased messenger RNA expression following DDT exposures. Phosphorylation of c-Jun N-terminal kinases and IκBα, an inhibitory component of nuclear factor κB (NFκB), was increased, suggesting activation of RSKs. The level of tyrosine phosphorylation of insulin receptor substrate 1 and serine phosphorylation of protein kinase B (Akt) on insulin stimulation decreased in myotubes with exposure to subtoxic concentrations of DDT, but there was no change in tyrosine phosphorylation level of insulin receptors. We conclude that subtoxic DDT exposure impairs insulin signaling and thereby induces insulin resistance in muscle cells. Data show that oxidative stress-induced activation of RSKs is responsible for impairment of insulin signaling on DDT exposure.

Sub-toxic exposure to lindane activates redox sensitive kinases and impairs insulin signaling in muscle cell culture: The possible mechanism of lindane-induced insulin resistance.

Lindane exposure is claimed to be involved in pathogenesis of type 2 diabetes mellitus (T2DM) and insulin resistance state by an as yet unknown mechanism. The redox sensitive kinases (RSKs) and heat shock proteins (HSPs) interfere with insulin signaling and induce insulin resistance. The present study was designed to explore the mechanism of insulin resistance induced by sub-toxic lindane exposure. In an in vitro study, exposure to 60 mg/L and 120 mg/L of lindane for 18 h on rat L6 myoblasts derived myotubes significantly increased malondialdehyde level & superoxide dismutase activity, decreased total antioxidant level and insulin-induced glucose uptake in a dose dependent manner. The extent of activation of RSKs and HSP25 as measured by western blot from the extent of phosphorylation of IκBα, p38 MAPK, JNK & HSP25 in lindane-exposed myotubes was higher. HSP70 was induced and insulin signaling as measured from tyrosine phosphorylation of insulin receptor (IR) & insulin receptor substrate-1 (IRS-1) and serine phosphorylation of Akt was attenuated in comparison to those in untreated myotubes. We conclude that sub-toxic lindane exposure induces oxidative stress, activates RSKs & HSP25 and induces HSP25. These in turn, impair insulin signaling to impart insulin resistance in myotubes induced by sub-toxic lindane exposure.

Effect of sub-toxic chlorpyrifos on redox sensitive kinases and insulin signaling in rat L6 myotubes.

OBJECTIVES: Sub-chronic exposures to chlorpyrifos, an organophosphorus pesticide is associated with incidence of diabetes mellitus. Biochemical basis of chlorpyrifos-induced diabetes mellitus is not known. Hence, effect of its sub-toxic exposure on redox sensitive kinases, insulin signaling and insulin-induced glucose uptake were assessed in rat muscle cell line. METHODS: In an in vitro study, rat myoblasts (L6) cell line were differentiated to myotubes and then were exposed to sub-toxic concentrations (6 mg/L and 12 mg/L) of chlorpyrifos for 18 h. Then total anti-oxidant level in myotubes was measured and insulin-stimulated glucose uptake was assayed. Assessment of activation of NFκB & p38MAPK and insulin signaling following insulin stimulation from tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and serine phosphorylation of Akt were done in myotubes after chlorpyrifos exposure by western blot (WB) and compared with those in vehicle-treated controls. RESULTS: The glucose uptake and total antioxidant level in L6-derived myotubes after sub-toxic exposure to chlorpyrifos were decreased in a dose-dependent manner. As measured from band density of WB, phosphorylation levels increased for redo-sensitive kinases (p38MAPK and IκBα component of NFκB) and decreased for IRS-1 (at tyrosine 1222) and Akt (at serine 473) on insulin stimulation following chlorpyrifos exposure as compared to those in controls. CONCLUSION: We conclude that sub-toxic chlorpyrifos exposure induces oxidative stress in muscle cells activating redox sensitive kinases that impairs insulin signaling and thereby insulin-stimulated glucose uptake in muscle cells. This probably explains the biochemical basis of chlorpyrifos-induced insulin resistance state and diabetes mellitus.