A molecular basis for insulin resistance. Elevated serine/threonine phosphorylation of IRS-1 and IRS-2 inhibits their binding to the juxtamembrane region of the insulin receptor and impairs their ability to undergo insulin-induced tyrosine phosphorylation

Tumor necrosis factor α (TNFα) or chronic hyperinsulinemia that induce insulin resistance trigger increased Ser/Thr phosphorylation of the insulin receptor (IR) and of its major insulin receptor substrates, IRS-1 and IRS-2. To unravel the molecular basis for this uncoupling in insulin signaling, we undertook to study the interaction of Ser/Thr-phosphorylated IRS-1 and IRS-2 with the insulin receptor. We could demonstrate that, similar to IRS-1, IRS- 2 also interacts with the juxtamembrane (JM) domain (amino acids 943-984) but not with the carboxyl-terminal region (amine acids 1245-1331) of IR expressed in bacteria as His₆ fusion peptides. Moreover, incubation of rat hepatoma Fao cells with TNFα, bacterial sphingomyelinase, or other Ser(P)/Thr(P)- elevating agents reduced insulin-induced Tyr phosphorylation of IRS-1 and IRS-2, markedly elevated their Ser(P)/Thr(P) levels, and significantly reduced their ability to interact with the JM region of IR. Withdrawal of TNFα for periods as short as 30 min reversed its inhibitory effects on IR- IRS interactions. Similar inhibitory effects were obtained when Fao cells were subjected to prolonged (20-60 min) pretreatment with insulin. Incubation of the cell extracts with alkaline phosphatase reversed the inhibitory effects of insulin. These findings suggest that insulin resistance is associated with enhanced Ser/Thr phosphorylation of IRS-1 and IRS-2, which impairs their interaction with the JM region of IR. Such impaired interactions abolish the ability of IRS-1 and IRS-2 to undergo insulin- induced Tyr phosphorylation and further propagate the insulin receptor signal. Moreover, the reversibility of the TNFα effects and the ability to mimic its action by exogenously added sphingomyelinase argue against the involvement of a proteolytic cascade in mediating the acute inhibitory effects of TNFα on insulin action.