Apolipoprotein E interrupts interleukin-1β signaling in vascular smooth muscle cells

Abstract

OBJECTIVES - Apolipoprotein E (apoE) exerts antiatherogenic effects but precise mechanisms remain unclear. We here investigated the effect of apoE on intracellular signaling by interleukin-1β (IL-1β), a proinflammatory cytokine present in atherosclerotic lesions. METHODS AND RESULTS - IL-1β-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-κB and activating protein-1 (AP-1) transactivation, suggesting that the interruption of IL-1β signaling occurs upstream of transcription factors. Studies in VSMCs overexpressing IL-1β signaling intermediates revealed that NF-κB 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-1β 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-1β 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-α or polyinosinic:polycytidylic acid. CONCLUSION - ApoE targets IRAK-1 activation and thereby interrupts IL-1β and IL-18 signaling in VSMCs. This antiinflammatory effect represents a novel antiatherogenic activity of apoE. © 2007 American Heart Association, Inc.