Abstract 315: Inflamed HDL Loses Its Anti-inflammatory Properties on Adipocytes: Role of Interaction of Serum Amyloid A with the Extracellular Matrix

Abstract

Obesity induces adipocyte hypertrophy and accumulation of macrophages in adipose tissue, which associates with insulin resistance and cardiovascular disease risk. HDL and apoA-I have anti-inflammatory properties in addition to facilitating reverse cholesterol transport. We recently showed that HDL from healthy humans and lean mice inhibits palmitate-induced monocyte chemotactic factor expression by decreasing lipid raft (LR) content in adipocytes. We also have shown that adipocytes produced extracellular matrix (ECM) in vivo and in vitro when they became hypertrophic. Moreover, inflamed HDL loses its anti-inflammatory effect and ability to stimulate cholesterol efflux from adipocytes. From gain of function and loss of function experiments, it appears that serum amyloid A (SAA) is partially responsible for these effects. However, the mechanism by which SAA impairs HDL’s anti-inflammatory function is unknown. To generate HDL under conditions of sterile inflammation, HDL was purified from plasma of AgNO₃-injected C57BL/6 mice. Also, to generate SAA-enriched HDL, HDL was purified after HDL from healthy humans was exposed to 293 HEK cells with lentiviral overexpression of SAA2. Control, inflamed and SAA-enriched HDL were labelled using DiI dye and the ECM of adipocytes was stained by Alexa 488 conjugated wheat germ agglutinin. After exposure of control HDL to adipocytes, there was no co-localization of DiI and Alexa 488 staining. However, we found that inflamed and SAA-enriched HDL were co-localized with the ECM of adipocytes, suggesting that inflamed and SAA-enriched HDL are being trapped at the ECM and therefore are blocked from accessing the plasma membrane. Enzymatic digestion of the ECM restored inflamed and SAA-enriched HDL’s abilities to facilitate cholesterol efflux from adipocytes and to inhibit palmitate-induced chemotactic factor expression. Taken together, our results suggest that the presence of SAA on HDL makes the lipoprotein lose its anti-inflammatory effect on adipocytes by blocking accessibility of HDL to the plasma membrane. The interaction between SAA in HDL and the ECM of adipocytes should be considered as a new therapeutic target for improvement of HDL’s anti-inflammatory properties.