TUMOR-NECROSIS-FACTOR (TNF)-ALPHA INHIBITS INSULIN SIGNALING THROUGH STIMULATION OF THE P55 TNF RECEPTOR AND ACTIVATION OF SPHINGOMYELINASE

Tumor necrosis factor (TNF)alpha plays a central role in the state of insulin resistance associated with obesity. It has previously been sho wn that one important mechanism by which TNF-alpha interferes with ins ulin signaling is through the serine phosphorylation of insulin recept or substrate-1 (IRS-1), which can then function as an inhibitor of the tyrosine kinase activity of the insulin receptor (IR), However, the r eceptors and the signaling pathway used by TNF-alpha that mediate the inhibition of IR activity are unknown. We show here that human TNF-alp ha, which binds only to the murine p55 TNF receptor (TNFR), is as effe ctive at inhibiting insulin-dependent tyrosine phosphorylation of IR a nd IRS-1 in adipocytes and myeloid 32D cells as murine TNF-alpha, whic h binds to both p55 TNFR and p75 TNFR. Likewise, antibodies that are s pecific agonists for p55 TNFR or p75 TNFR demonstrate that stimulation of p55 TNFR is sufficient to inhibit insulin signaling, though a smal l effect can also be seen with antibodies to p75 TNFR. Exogenous sphin gomyelinase and ceramides, known to be formed by activat- ion of p55 T NFR, inhibit IR and IRS-1 tyrosine phosphorylation and convert IRS-1 i nto an inhibitor of IR tyrosine kinase in vitro. Myeloid 32D cells exp ressing LR and IRS-1 are sensitive to this inhibition, but cells expre ssing IR and IRS-2 are resistant, pointing to an important difference in the biological function between IRS-1 and IRS-2. These data strongl y suggest that TNF-alpha inhibits insulin signaling via stimulation of p55 TNFR and sphingomyelinase activity, which results in the producti on of an inhibitory form of IRS-1.