TUMOR-NECROSIS-FACTOR-ALPHA ACUTELY INHIBITS INSULIN SIGNALING IN HUMAN ADIPOCYTES - IMPLICATION OF THE P80 TUMOR-NECROSIS-FACTOR RECEPTOR

Tumor necrosis factor (TNF)-alpha is postulated to play a major role i n the pathogenesis of obesity-linked insulin resistance, probably resu lting from an interaction with insulin signaling pathways. This cross talk has now been investigated in human adipocytes at the level of pho sphatidylinositol (PI) 3-kinase, and the TNF receptors (TNFRs) mediati ng these processes have been identified. Equilibrium binding studies u sing human adipocytes from mammary tissue indicated the presence of tw o populations of TNFR with apparent affinity constants of 13 pmol/l an d 1.6 nmol/l, respectively. Interaction of TNF-alpha with insulin sign aling was determined by quantification of insulin receptor substrate ( IRS)-1-associated PI 3-kinase activity. Under control conditions, PI 3 -kinase was activated about 10-fold in response to insulin (10(-7) mol /l, 5 min). Preincubation of adipocytes with 5 nmol/l TNF-alpha for 15 min resulted in a 60-70% reduction of insulin action, reaching a stab le inhibition (40%) after longer incubation with the cytokine. The inh ibitory action of TNF-alpha was dose-dependent, already detectable at 10 pmol/l, and was correlated to inhibition of tyrosine phosphorylatio n of IRS-1 with an unaltered autophosphorylation of the insulin recept or beta-subunit. The modulation of insulin signaling by TNF-alpha was found to be paralleled by a comparable inhibition of insulin-stimulate d glucose transport. An agonistic TNFR1 antibody completely mimicked t he inhibitory action of TNF-alpha on insulin signaling, whereas at 100 pmol/l TNF-alpha, a nonagonistic p80 TNFR antibody, was shown to amel iorate the inhibitory action of the cytokine. These findings indicate that in human adipocytes, low concentrations of TNF-alpha induce a rap id inhibition of insulin signaling at the level of PI 3-kinase, We sug gest that under these conditions, the p80 TNFR is essential for initia ting the intracellular cross talk that involves signaling by the p60 T NFR.