ROLE OF INTERLEUKIN-17 IN ENDOTHELIAL CELL ACTIVATION AND VASCULAR FUNCTION

Abstract

Endothelial cell (EC) activation is a change of the endothelium from a quiescent state to one that is involved in immune reactions. Activation of ECs is associated with the inception of atherosclerosis. Atherosclerosis is a chronic inflammatory disease that involves adaptive and innate immunity. There are many pro-inflammatory stimuli which activate the endothelium. The pro-inflammatory cytokine interleukin-17 (IL-17) has been shown to activate lung microvascular ECs. Enhanced expression of the IL-17 receptor by synovial ECs is associated with rheumatoid arthritis. These studies suggest that IL-17 plays an important role in EC biology. Nevertheless, the role of IL-17 in EC activation and endothelial dysfunction in the context of hyperlipidemia-induced atherosclerosis has not been studied. In the current study, we investigated the role of IL-17 in EC activation in vitro with mouse aortic ECs and human aortic ECs. In addition, we used the IL-17/ApoE double knock-out mouse to determine the role of IL-17 in vessel function and atherosclerosis development. First, we found that hyperlipidemia increased the number of IL-17-producing cells in the spleens from wild type mice and ApoE-/- mice that were fed a Western diet when compared to their respective normal chow diet controls. We also found that after treatment with the pro-atherogenic factor, oxidized LDL, there was an increase in the expression of IL-17 receptor by ECs. Using an EC specific array, we found that IL-17 induced significant up-regulation of four genes that are associated with EC activation in mouse aortic ECs. The four genes induced in IL-17-treated mouse aortic ECs were Cxcl1, Cxcl2, Il6, and Csf2. Moreover, we also found that IL-17 induced these four genes in human aortic ECs, and we showed that enhanced monocyte adhesion to ECs was dependent on these four genes. It was previously observed that a Western diet induced vessel dysfunction in the aortas of ApoE-/- mice. Thus, we sought to determine whether IL-17-deficiency rescues impaired endothelium-dependent relaxation in ApoE-/- mice that were fed a Western diet with the Wire Myograph System. We found that ApoE-/- mice on a 3-week Western diet had impaired endothelium-dependent relaxation when compared to IL-17-/-ApoE-/- mice. Endothelium-independent relaxation in response to sodium nitroprusside (SNP) and contraction responses induced by potassium chloride (KCl) and phenylephrine (PE) were not different in ApoE-/- mice and IL-17-/-ApoE-/- mice. Since our in vitro studies and vessel function assay pointed to a pro-atherogenic role for IL-17, we investigated lesion formation in ApoE-/- mice and IL-17-/-ApoE-/- mice. Lesion formation was assessed with Sudan IV staining of the whole aorta and Oil red O staining of aortic sinus cross sections. IL-17 deficiency in ApoE-/- mice did not affect atherosclerotic lesion formation in our study. Hyperlipidemia is a well-established risk factor for atherosclerosis so we investigated whether the pro-atherogenic role of IL-17 may have been compromised by lipid levels in vivo. The lipid profiles of mice which measured the levels of LDL, HDL, triglyceride, non-esterified free fatty acid, and total cholesterol were determined. The lipid profiles showed that IL-17 deficiency in ApoE-/- mice modulated the levels of the lipids in the plasma. Taken together, our data suggest that IL-17 is up-regulated in hyperlipidemia and IL-17 induces aortic EC activation. IL-17 also contributes to endothelial dysfunction in ApoE-/- mice induced by a Western diet. Moreover, IL-17 may modify lipid metabolism in mice. The effects of IL-17 on lipid levels may weaken its pro-atherogenic potential and contribute to the lack of an atherosclerotic phenotype in our atherosclerosis study. Our current work has shed light on the role of IL-17 on EC biology and has provided insights into the effects of IL-17 on EC activation, lipid modification, and vascular function. These important findings may serve as the stepping stone to the development of therapeutics that target vascular inflammation and its underlying mechanisms. The studies in this dissertation were supported by grants from the National Institutes of Health (NIH) and a fellowship from the American Heart Association (AHA).