Biomechanical stress induces IL-6 expression in smooth muscle cells via Ras/Rac1-p38 MAPK-NF-κB signaling pathways

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Abstract
IL-6, a proinflammatory cytokine, has been implicated in the development of vascular diseases. We previously demonstrated that mechanical stress can initiate signaling pathways leading to smooth muscle cell (SMC) proliferation and apoptosis, but little is known concerning cyclic stress-induced inflammatory response. To explore the role of stretch in the upregulation of cytokine expression in SMCs we performed RNase protection assay for a panel of cytokines and found that mechanical stress resulted in a time-dependent induction of IL-6 mRNA but not other cytokines, e.g., IL-1α, IL-1β, IL-6, IL-10, IL-12p35, IL-12p40, IL-18, IFN-γ, and macrophage migration inhibitory factor (MIF). This induction also correlated with elevated IL-6 protein levels in the supernatant. Pretreatment of the cells with NF-κB inhibitors inhibited NF-κB activity and resulted in marked inhibition (50%) of IL-6 protein. Moreover, SMC lines stably expressing dominant-negative Ras (RasN17) or Rac (RacN17) exhibited a remarkable decrease in p38 MAPK activity and IL-6 mRNA induction by mechanical stress. Furthermore, a significant inhibition of 30 and 40% in IL-6 protein was observed in SMCs pretreated with inhibitors of p38 MAPK and ERK1/2, respectively, but not JNK. Interestingly, SMCs isolated from PKC-δ-deficient mice exhibited higher levels of IL-6 compared with wild-type cells. Finally, high levels of IL-6 expression were observed in atherosclerotic lesions of vein bypass grafts, which are related to altered biomechanical stress. Our findings demonstrate that biomechanical stress-induced IL-6 expression occurs via a mechanism that involves Ras/Rac/p38 MAPK/NF-κB/NF-IL6 signaling pathways, which is downregulated by PKC-δ, and suggest that modulation of this event contributes to the pathogenesis of atherosclerosis.