Simvastatin attenuates the additive effects of TNF-α and IL-18 on the connexin 43 up-regulation and over-proliferation of cultured aortic smooth muscle cells

Statin therapy is known to down-regulate inflammatory activities in atheromatous tissues of animals. The aims of this study were to examine the regulatory role of interleukin-18 (IL-18) in the connexin 43 (Cx43) and the proliferation of cultured aortic smooth muscle cells (SMCs) as well as to elucidate the underlying therapeutic mechanism of simvastatin. Vytorin therapy significantly alleviated high-cholesterol diet-induced hypercholesterolemia, suppressed neointimal hyperplasia, macrophage infiltration, and Cx43 and IL-18 expression in rabbit aortic walls. In vitro study using an aortic SMC line showed that IL-18 up-regulated constitutive Cx43 expression and potentiated tumor necrosis factor-alpha (TNF-alpha)-triggered Akt and MAPK signaling pathways. Simvastatin treatment alone reduced constitutive Cx43 levels and prevented the TNF-alpha-induced IL-18 up-regulation. Mechanistic investigation using kinase-specific inhibitors showed that simvastatin pretreatment attenuated TNF-alpha-elicited Akt and ERK1/2 phosphorylation, whereas PI3K and all MAPK activities were also implied in the additive effect of TNF-alpha and IL-18 on Cx43 up-regulation. Proliferation assay indicated that IL-18 stimulated SMC proliferation and synergized the TNF-alpha-stimulated cell proliferation. Likewise, simvastatin treatment suppressed the SMC over-proliferation induced not only by TNF-alpha alone, but also by simultaneous treatment with TNF-alpha and IL-18. The suppression of simvastatin in SMC proliferation was not mediated through mitochondrial related pro-apoptogenesis under both scenarios. In conclusion, simvastatin attenuates the additive effects of TNF-alpha and IL-18 on Cx43 up-regulation and over-proliferation of aortic SMCs, mainly through the blockade of Akt signaling pathway. These findings may fortify the rationale underlying the atheroprotective mechanism of statin therapy. (C) 2013 Elsevier Ltd. All rights reserved.