SPHINGOMYELINASE AND CERAMIDE SUPPRESS INSULIN-INDUCED TYROSINE PHOSPHORYLATION OF THE INSULIN-RECEPTOR SUBSTRATE-1

The sphingomyelin pathway is a newly described signal transduction pat hway mediating the action of several cytokines including tumor necrosi s factor-alpha (TNF), TNF was recently shown to interfere with insulin -induced tyrosine phosphorylation of the insulin receptor substrate-1 (IRS-1). In this work we examined the possible effect of direct activa tion of the sphingomyelin pathway on insulin-induced tyrosine phosphor ylation of IRS-1. Incubation of the insulin-sensitive rat hepatoma Fao cells with bacterial sphingomyelinase (SMase) that causes membrane hy drolysis of sphingomyelin led to a time- and dose-dependent decrease i n insulin-induced tyrosine phosphorylation of IRS-1, The effect was ap parent after 10 min of incubation and with a dose of 10 milliunits/ml SMase, It was not associated with a decrease in insulin receptor autop hosphorylation, In addition, SMase treatment interrupted the associati on of the 85-kDa catalytic subunit of phosphatidylinositol 3-kinase wi th IRS-1. A similar impact on IRS-1 tyrosine phosphorylation was obser ved after addition of cell-permeable ceramide analogs (C2 and C6). Com parable changes in IRS-1 tyrosine phosphorylation and electrophoretic mobility were found after exposure of cells to either TNF, SMase, or c eramide. Our findings suggest that TNF may utilize the sphingomyelin p athway in its effect on the insulin-stimulated tyrosine phosphorylatio n of IRS-1.