Increased serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) is associated with cellular insulin resistance. We have recently identified serine 318 (Ser318) as a novel protein kinase C-zeta (PKC-zeta) -dependent phosphorylation site within IRS-1. As other kinases may phosphorylate at this serine residue as well, we aimed to identify such kinases in the present study. In C2C12 myotubes, exposure to insulin or phorbol ester markedly increased Ser318 phosphorylation. In contrast, high glucose, tumor necrosis factor-alpha, and free fatty acids did not provoke Ser318 phosphorylation. JNK and the PI 3-kinase/mTOR pathway were found to be implicated in insulin-induced Ser318 phosphorylation, but not in TPA-stimulated phosphorylation that was, at least partly, mediated by classical or novel PKC. In conclusion, with JNK and the PI 3-kinase/mTOR pathway as mediators of insulin-induced Ser318 phosphorylation, we have identified kinases that have previously been reported to play key roles in phosphorylation of other serine residues in IRS-1. (C) 2005 Elsevier Inc. All rights reserved.