The potential role of resistin in atherogenesis

Abstract

Resistin, an adipocyte-derived cytokine linked to insulin resistance and obesity, has recently been shown to activate endothelial cells (ECs). Using microarrays, we found that along with numerous other pro-atherosclerotic genes, resistin expression levels are elevated in the aortas of C57BL/6J apoE−/− mice; these findings led us to further explore the relation between resistin and atherosclerosis. Using TaqMan PCR and immunohistochemistry, we found that ApoE−/− mice had significantly higher resistin mRNA and protein levels in their aortas, and elevated serum resistin levels, compared to C57BL/6J wild-type mice. Incubation of murine aortic ECs with recombinant resistin increased monocyte chemoattractant protein (MCP)-1 and soluble vascular cell adhesion molecule (sVCAM)-1 protein levels in the conditioned medium. Furthermore, human carotid endarterectomy samples stained positive for resistin protein, while internal mammary artery did not show strong staining. Patients diagnosed with premature coronary artery disease (PCAD) were found to have higher serum levels of resistin than normal controls. In summary, resistin protein is present in both murine and human atherosclerotic lesions, and mRNA levels progressively increase in the aortas of mice developing atherosclerosis. Resistin induces increases in MCP-1 and sVCAM-1 expression in murine vascular endothelial cells, suggesting a possible mechanism by which resistin might contribute to atherogenesis. Finally, PCAD patients exhibited increased serum levels of resistin when compared to controls. These findings suggest a possible role of resistin in cardiovascular disease.

Introduction

Insulin resistance, an important component of the metabolic syndrome, is a major risk factor for atherosclerotic cardiovascular disease. The proposed mechanisms by which insulin resistance and the metabolic syndrome predispose to atherosclerosis are many. Because obesity is a component of insulin resistance/metabolic syndrome, it is of interest that the adipocyte has been identified as a source of multiple cytokines (adipokines), and recent studies have implicated these molecules as not only playing a role in the genesis of the metabolic syndrome, but also in altering endothelial function. Since altered endothelial function is one of the earliest manifestations of atherosclerosis, such effects further support a role of adipokines in the development of atherosclerosis.

Of note, a novel adipocyte-derived hormone, resistin, displays characteristics that suggest it as a possible candidate linking insulin resistance with atherogenesis. Resistin was discovered in 2001 during a search for targets of the thiozolidinediones (TZDs), a class of insulin sensitizing drugs [1]. Elevated levels of circulating resistin were found in obese mice [2], i.p. injection of recombinant resistin in normal mice caused insulin resistance, and administration of an anti-resistin antibody improved insulin sensitivity of obese and insulin resistant rodents. Recently, Banerjee et al. [3] have reported that mice lacking the gene for resistin have low blood glucose levels following fasting, as a result of reduced glucose production by the liver.

Although some previous studies are not entirely consistent with these results, thereby emphasizing the complex biologic activity of resistin [4], [5], [6], [7], [8], there are other characteristics of this molecule that further suggest activities that could contribute to the development of atherosclerosis. Thus, resistin appears to have inflammatory activity; it is a member of a novel family of cysteine-rich secretory proteins called “resistin-like molecules” (RELM), or “found in inflammatory zone” (FIZZ). A related molecule, FIZZ1, has been associated with pulmonary inflammation [9]. Indeed, although resistin was originally found to be expressed in adipocytes, several reports have indicated that resistin is highly expressed in human peripheral blood mononuclear cells [10], [11], and even in leukemia cells [12], suggesting that resistin may be importantly involved in inflammation and/or immune modulation.

In addition, Verma et al. [13] recently found that resistin activates human endothelial cells (ECs) in vitro; when incubated with ECs, recombinant resistin increases endothelin-1 mRNA expression and protein secretion, increases vascular cell adhesion molecule-1 (VCAM-1) and monocyte chemoattractant protein-1 (MCP-1) expression, and impairs tumor necrosis factor receptor-associated factor-3 (TRAF-3) expression. These activities support the hypothesis that resistin is linked to cardiovascular disease, and exerts its effects, at least in part, through activation of the endothelium.

In the present investigation, we have determined that resistin mRNA levels progressively increase and resistin protein is expressed in developing aortic atherosclerotic lesions of ApoE−/− mice, and that resistin protein is present in human atherosclerotic lesions. Furthermore, we have provided in vivo/in vitro correlative support for the in vitro “pro-atherosclerotic” changes findings of Verma et al. [13] and Kawanami et al. [14] in human endothelial cells—we demonstrate in a mouse atherosclerotic model in which we show an association between resistin mRNA and protein levels and the development of atherosclerosis that resistin upregulates MCP-1 and sVCAM-1 expression in cultured murine aortic endothelial cells. Interestingly, we have also found that patients with premature coronary artery disease (CAD) have elevated serum levels of resistin compared to control individuals. Our data, therefore, further strengthen the concept that resistin is an effector molecule that mechanistically links the metabolic syndrome and insulin resistance to atherosclerotic cardiovascular disease, possibly through an inflammatory mechanism. These novel findings suggest that further study of resistin, in the context of cardiovascular disease, could yield interesting and important results.