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-α (TNF-α)-triggered Akt and MAPK signaling pathways. Simvastatin treatment alone reduced constitutive Cx43 levels and prevented the TNF-α-induced IL-18 up-regulation. Mechanistic investigation using kinase-specific inhibitors showed that simvastatin pretreatment attenuated TNF-α-elicited Akt and ERK1/2 phosphorylation, whereas PI3K and all MAPK activities were also implied in the additive effect of TNF-α and IL-18 on Cx43 up-regulation. Proliferation assay indicated that IL-18 stimulated SMC proliferation and synergized the TNF-α-stimulated cell proliferation. Likewise, simvastatin treatment suppressed the SMC over-proliferation induced not only by TNF-α alone, but also by simultaneous treatment with TNF-α 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-α 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.
