Abrogation of hyperosmotic impairment of insulin signaling by a novel class of 1,2-dithiole-3-thiones through the inhibition of S6K1 activation

A previous study from this laboratory showed that oltipraz and synthetic dithiolethiones prevent tumor necrosis factor-α-induced hepatic insulin resistance via AMP-activated protein kinase-dependent p70S6 kinase (S6K) 1 inhibitory pathway. This study investigated whether oltipraz and a novel class of 1,2-dithiole-3-thiones were capable of preventing insulin resistance induced by hyperosmotic stress, thereby enhancing insulin-dependent signals, and, if so, whether the restoration of insulin signal was mediated with the inhibition of S6K1 activity stimulated by hyperosmotic stress. In HepG2 cells, oltipraz treatment inhibited insulin receptor substrate (IRS) 1 serine phosphorylation, a marker of insulin resistance, induced by sorbitol-, mannitol-, or sodium chloride-induced hyperosmotic stress. Consequently, this allowed cells to restore insulin signals, which was evidenced by decrease in the ratio of serine to tyrosine phosphorylations of IRS1 and increase in the phosphorylations of Akt and glycogen synthase kinase (GSK) 3β. Hyperosmotic stress markedly activated S6K1; S6K1 activation was completely abolished by oltipraz pretreatment. An experiment using dominant-negative S6K1 supports the essential role of S6K1 in the hyperosmolarity-stimulated phosphorylation of IRS1. Transfection of constitutive active mutant S6K1 eliminated the protective effect of oltipraz on GSK3β phosphorylation, indicating that oltipraz restores insulin signaling by inhibiting S6K1 activation. A variety of synthetic 1,2-dithiole-3-thione derivatives also inhibited S6K1 activity and insulin resistance induced by hyperosmotic stress in HepG2 cells. The results of this study demonstrate that a novel class of 1,2-dithiole-3-thiones improve insulin sensitivity under the condition of hyperosmotic stress, which results from the inhibition of S6K1 activation.