Tumor necrosis factor alpha (TNF alpha) interferes with insulin signaling in adipose tissue and may promote insulin resistance. Insulin resistance is associated with vascular injury, but little is known about the interaction of TNF alpha and insulin in the vasculature. By activating the Insulin receptor (IR) --> IRS-1 --> phosphatidylinositol-3-kinase (PI3K) --> Akt-pathway, insulin protects vascular smooth muscle cells (VSMC) from undergoing apoptosis. We therefore investigated the effect of TNF alpha on insulin's antiapoptotic signaling in rat aortic VSMC. Insulin induced rapid tyrosine-phosphorylation of the IR and IRS-1 and caused a 2.8-fold increase of IRS-1-bound PI3K. TNF alpha had no effect on insulin-induced tyrosinephosphorylation of IR or IRS-1, but inhibited insulin-stimulated IRS-1/PI3K-association by 84%. Insulin-induced phosphorylation of Akt downstream of PI3K was inhibited by TNF alpha in a similar pattern. We next examined the effect of TNF alpha on insulin's protective actions on H2O2-induced apoptosis. Insulin alone prevented 72.8% of H2O2-induced apoptosis, which was significantly inhibited by TNF alpha. TNF alpha alone did not induce apoptosis. In contrast, TNF alpha had no effect on PDGF-induced antiapoptotic signal transduction via Akt. Thus, TNF alpha selectively interferes with insulin's antiapoptotic signaling in VSMC by inhibiting the association of IRS-1/PI3K and the downstream activation of Akt.