Effect of sub-toxic exposure to Malathion on glucose uptake and insulin signaling in L6 myoblast derived myotubes

Abstract

Biochemical basis of Malathion exposure-induced diabetes mellitus is not known. Hence, effects of its sub-toxic exposure on redox sensitive kinases (RSKs), insulin signaling and insulin-induced glucose uptake were assessed in rat muscle cell line. In this in vitro study, rat myoblast (L6) cells were differentiated to myotubes and were exposed to sub-toxic concentrations (10 mg/l and 20 mg/l) of Malathion for 18 hours. Total antioxidant level and insulin-stimulated glucose uptake by myotubes were assayed. Activation of JNK, NFκB, p38MAPK and insulin signaling from tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and serine phosphorylation of Akt were assessed in myotubes after Malathion exposure by western blot and was compared with those in controls. Paraoxonase (PON) activity was measured in cell lysate using p-nitrophenyl acetate as substrate. PON1 and PON2 expression in myotubes were assessed by PCR. The glucose uptake and total antioxidant level in L6-derived myotubes after sub-toxic exposure to Malathion were decreased in a dose-dependent manner. Phosphorylation levels of RSKs (JNK, p38MAPK and IκBα component of NFκB) were increased and that of IRS-1 and Akt on insulin stimulation was decreased following Malathion exposure as compared to those in controls. PON1 and PON2 genes were expressed in myotubes with and without Malathion exposure. Significant PON activity was present in cell lysate. We conclude that sub-toxic Malathion exposure induces oxidative stress in muscle cells activating RSKs that impairs insulin signaling and thereby insulin-stimulated glucose uptake in muscle cells. This probably explains the biochemical basis of Malathion-induced insulin resistance state and diabetes mellitus.