Inhibitory effect of ceramide on insulin-induced protein kinase Cζ translocation in rat adipocytes

Ceramide has been confirmed to be a signal mediator of apoptosis that is induced by tumor necrosis factor-alpha (TNF-alpha). it has also been reported that ceramide may induce insulin resistance as well as TNF-alpha. We investigated the effect of ceramide on insulin signaling pathways, such as insulin receptor (IR) beta-subunit, insulin receptor substrate 1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and protein kinase C (PKC) in rat adipocytes. We examined insulin-stimulated [H-3]2-deoxyglucose (2-DOG) uptake in rat adipocytes pretreated with N-hexanoyisphingosine (C-6-ceramide, 10 to 30 mumol/L). Insulin-induced 2-DOG uptake was significantly reduced by C-6-ceramide pretreatment. We also examined the effect of various concentrations of C-6-ceramide pretreatment on insulin-induced autophosphorylation of the IR beta-subunit, tyrosine phosphorylation of IRS-1, enzyme activity of PI3K, and membrane-associated PKCzeta immunoreactivity. Pretreatment with C-6-ceramide significantly reduced autophosphorylation of the IR beta-subunit, tyrosine phosphorylation of IRS-1, and enzyme activity of PI3K. Moreover, membrane-associated PKCzeta immunoreactivity and immunoprecipitable PKCzeta enzyme activity, downstream of PI3K, were significantly suppressed by C-6-ceramide pretreatment. These results suggest that ceramide may induce insulin resistance via the suppression of IRS-1-PI3K signaling, and subsequent activation of PKCzeta. Copyright 2003, Elsevier Science (USA). All rights reserved.