TNFalpha inhibits insulin's antiapoptotic signaling in vascular smooth muscle cells

Tumor necrosis factor alpha (TNFalpha) 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 TNFalpha 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 TNFalpha 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. TNFalpha had no effect on insulin-induced tyrosine-phosphorylation 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 TNFalpha in a similar pattern. We next examined the effect of TNFalpha on insulin's protective actions on H(2)O(2)-induced apoptosis. Insulin alone prevented 72.8% of H(2)O(2)-induced apoptosis, which was significantly inhibited by TNFalpha. TNFalpha alone did not induce apoptosis. In contrast, TNFalpha had no effect on PDGF-induced antiapoptotic signal transduction via Akt. Thus, TNFalpha selectively interferes with insulin's antiapoptotic signaling in VSMC by inhibiting the association of IRS-1/PI3K and the downstream activation of Akt.