Abstract 142: Rac2 Modulates Macrophage IL-1β Production, Atherosclerotic Plaque Inflammation, and Calcific Atherosclerosis

Abstract

Ischemic heart disease caused by atherosclerosis is the leading cause of morbidity and mortality in the world. Calcification of atherosclerotic plaques is a well-described pathologic finding that has predictive value in terms of atherosclerotic burden, cardiovascular event risk, and mortality. Inflammation is critical to atherogenesis, but inflammatory mechanisms that drive atherosclerotic calcification are minimally understood. The Rho/Rac family of low molecular weight GTPases regulates the activation, recruitment, and differentiation of monocytes. Racs compete for similar regulatory elements, indicating that targeted modulation of specific family members may influence the role of other members in biologic processes. The hypothesis is that Rac2 can modulate atherosclerotic plaque inflammation and calcium composition via consequent effects on the activation of Rac1. Rac2 gene deletion results in elevated activation (GTP-binding) of Rac1 in macrophages. Increased activation of Rac1 is associated with higher macrophage IL-1β production in response to LPS-coupled cholesterol crystal exposure. In fact, macrophage IL-1β production is dependent on Rac1. When subjected to a high fat diet for 14 weeks, Rac2 -/-ApoE -/- mice demonstrate similar weight gain, serum cholesterol levels, circulating inflammatory monocyte levels, and aortic atherosclerotic lipid burden relative to Rac2 +/+ApoE -/- mice. Atherosclerotic plaques from Rac2 -/-ApoE -/- mice contain increased inflammatory cellular infiltration by histology and relative RNA expression. Ex vivo microCT imaging reveals calcified aortic plaques, and quantification of calcification by near-infrared imaging demonstrates increased calcium composition in plaques from Rac2 -/-ApoE -/- mice relative to Rac2 +/+ApoE -/- mice. Calcified aortic plaques are associated with an increase in IL-1β mRNA expression in the aortic tissue. In summary, Rac2 is a critical modulator of signaling mechanisms that regulate the activation state of Rac1, resulting in enhanced macrophage IL-1β expression in vitro and inflammatory calcification of atherosclerotic plaques in vivo. Targeting the balance of Rac regulation has therapeutic potential in modulation of atherosclerotic plaque composition.