Hepatic Scavenger Receptor Class B Type 1 Knockdown Reduces Atherosclerosis and Enhances the Antiatherosclerotic Effect of Brown Fat Activation in APOE∗3-Leiden.CETP Mice

Objective: Brown fat activation attenuates atherosclerosis development by accelerating triglyceride-rich lipoprotein turnover and/or stimulation of reverse cholesterol transport via the SRB1 (scavenger receptor class B type 1). The aim of this study was to investigate the specific role of hepatic SRB1 in the atheroprotective properties of brown fat activation. Approach and Results: APOE∗3-Leiden.CETP mice, a well-established model of human-like lipoprotein metabolism and atherosclerosis, were treated with vehicle or adenoassociated virus serotype 8-short hairpin RNA, which decreased hepatic SRB1 protein levels by 40% to 55%. After 2 weeks, mice without or with hepatic SRB1 knockdown were treated with vehicle or the β3-adrenergic receptor agonist CL316 243 to activate brown fat for 4 weeks to determine HDL (high-density lipoprotein) catabolism and for 9 weeks to evaluate atherosclerosis. Surprisingly, hepatic SRB1 knockdown additively improved the beneficial effects of β3-adrenergic receptor agonism on atherosclerosis development. In fact, hepatic SRB1 knockdown per se not only increased HDL-cholesterol levels but also reduced plasma triglyceride and non-HDL-cholesterol levels, thus explaining the reduction in atherosclerosis development. Mechanistic studies indicated that this is due to increased lipolytic processing and hepatic uptake of VLDL (very low density lipoprotein) by facilitating VLDL-surface transfer to HDL. Conclusions: Hepatic SRB1 knockdown in a mouse model with an intact ApoE (apolipoprotein E)-LDLR (low density lipoprotein receptor) clearance pathway, relevant to human lipoprotein metabolism, reduced atherosclerosis and improved the beneficial effect of brown fat activation on atherosclerosis development, explained by pleiotropic effects of hepatic SRB1 knockdown on lipolytic processing and hepatic uptake of VLDL. Brown fat activation could thus be an effective strategy to treat cardiovascular disease also in subjects with impaired SRB1 function.