Recently we have shown that PKC beta1 and beta2 are able to inhibit the tyrosine kinase activity of the human insulin receptor (HIR). Now we have investigated whether a distinct PKC isoform might be involved in the inhibitory effect of TNF alpha on insulin signaling in HEK293 cells. TNF alpha induces a rapid translocation of the PKC isoform epsilon (TNF alpha 10(-9) M, maximal effect within 5-10 min) in rat-1 fibroblasts, while no effect occurred on other isoforms. Cotransfection of HIR with PKC epsilon did not significantly reduce the insulin stimulated receptor kinase activity; however, when cells were incubated with TNF alpha for 10 min (10(-9) M) a 62 +/- 17% (n = 5) inhibition of the insulin receptor kinase activity was observed which was significantly (P<0.01) higher than that observed in cells which were not transfected with PKC (32 +/- 11.5%, n = 5). The data suggest that translocation of PKC epsilon induced by TNF alpha enables this PKC isoform to interact with insulin signaling and to inhibit the insulin receptor kinase activity.