Tumor necrosis factor alpha inhibits insulin-induced mitogenic signaling in vascular smooth muscle cells

Tumor necrosis factor alpha (TNF alpha) interferes with insulin signaling i n adipose tissue and may promote insulin resistance. Insulin binding to the insulin receptor (IR) triggers its autophosphorylation, resulting in phosp horylation of Shc and the downstream activation of p42/p44 extracellular si gnal-regulated kinase 1/2 mitogen-activated protein kinase (ERK1/2), which mediates insulin-induced proliferation in vascular smooth muscle cells (VSM C). Since insulin resistance is a risk factor for vascular disease, we exam ined the effects of TNF alpha on mitogenic signaling by insulin. In rat aor tic VSMC, insulin induced rapid phosphorylation of the IR and Shc and cause d a 5.3-fold increase in activated, phosphorylated ERK1/2 at 10 min. Insuli n induced a biphasic ERK1/2 activation with a transient peak at 10 min and a sustained late phase after 2 h, Preincubation (30-120 min) with TNF alpha had no effect on insulin-induced IR phosphorylation, In contrast, TNF alph a transiently suppressed insulin-induced ERK1/2 activation. Insulin-induced phosphorylation of Shc was inhibited by TNF alpha in a similar pattern. Si nce mitogenic signaling by insulin in VSMC requires ERK1/2 activation, we e xamined the effect of TNF alpha on insulin-induced proliferation. Insulin a lone induced a 3.4-fold increase in DNA synthesis, which TNF alpha inhibite d by 48%. TNF alpha alone was not mitogenic, Inhibition of ERK1/2 activatio n with PD98059 also inhibited insulin-stimulated DNA synthesis by 57%, TNF alpha did not inhibit platelet-derived growth factor induced ERK1/2 activat ion or DNA synthesis in VSMC, Thus, TNF alpha selectively interferes with i nsulin-induced mitogenic signaling by inhibiting the phosphorylation of Shc and the downstream activation of ERK1/2.