Inhibitory effect on protein kinase Ctheta by Crocetin attenuates palmitate-induced insulin insensitivity in 3T3-L1 adipocytes.

Epidemiologic and experimental studies have pointed to an etiologic role of elevated plasma free fatty acids in insulin resistance, which is frequently associated with a state of low-grade inflammation. In this study, we investigated the effects of Crocetin, a unique carotenoid, on insulin resistance induced by palmitate in 3T3-L1 adipocytes. Exposure of palmitate led to an increase in insulin receptor substrate-1 (IRS-1) serine307 phosphorylation as well as activation of c-Jun NH2-terminal kinase (JNK) and inhibitor κB kinase β (IKKβ), concomitantly with reductions of IRS-1 function and glucose metabolism. Interestingly, pretreatment with Crocetin almost reversed all of these abnormalities in a dose-dependent manner. IRS-1 serine307 phosphorylation was significantly reduced by JNK or IKKβ inhibitor, especially by combination of these two inhibitors. Moreover, palmitate treatment induced activation of protein kinase Cθ (PKCθ) while blocking PKCθ significantly inhibited JNK and IKKβ activation induced by palmitate or phorbol 12-myristate 13-acetate (PKC activator, PMA), and attenuated the palmitate-induced defects in insulin action. Crocetin demonstrated an impressive suppression in the activation of PKCθ induced not only by palmitate but also by PMA in a dose-dependent manner. Taken together, Crocetin inhibited JNK and IKKβ activation via suppression of PKCθ phosphorylation, attenuating insulin insensitivity induced by palmitate in 3T3-L1 adipocytes.