Objectives--Apolipoprotein E (apoE) exerts antiatherogenic effects but precise mechanisms remain unclear. We here investigated the effect of apoE on intracellular signaling by interleukin-1 (IL-1), a proinflammatory cytokine present in atherosclerotic lesions.

Methods and Results--IL-1-induced expression and activation of inducible nitric oxide synthase and cyclooxygenase-2 were inhibited by apoE in vascular smooth muscle cells (VSMCs). These inhibitory effects were linked to the suppression of both NF-B and activating protein-1 (AP-1) transactivation, suggesting that the interruption of IL-1 signaling occurs upstream of transcription factors. Studies in VSMCs overexpressing IL-1 signaling intermediates revealed that NF-B transactivation was inhibited by apoE in MyD88- and IRAK1- but not in TRAF6-transfected cells. Furthermore, apoE prevented IRAK1 phosphorylation and IRAK1-TRAF6 but not MyD88-IRAK1 complex formation. Inhibitory effects of apoE on IL-1 signaling were abolished after silencing LDL receptor-related protein-1 (LRP1) expression with siRNA. In addition, inhibitors of adenylyl cyclase and protein kinase A (PKA) restored IL-1 signaling in apoE-treated VSMCs, whereas apoE stimulated PKA activity. ApoE inhibited VSMC activation in response to IL-18 but not to tumor necrosis factor- or polyinosinic:polycytidylic acid.

Conclusion--ApoE targets IRAK-1 activation and thereby interrupts IL-1 and IL-18 signaling in VSMCs. This antiinflammatory effect represents a novel antiatherogenic activity of apoE.