Absence of ABCA1 mutations in individuals with low serum HDL-cholesterol

New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.

LXR-mediated activation of macrophage stearoyl-CoA desaturase generates unsaturated fatty acids that destabilize ABCA1

Abnormal HDL metabolism among patients with diabetes and insulin resistance may contribute to their increased risk of atherosclerosis. ATP binding cassette transporter A1 (ABCA1) mediates the transport of cholesterol and phospholipids from cells to HDL apolipoproteins and thus modulates HDL levels and atherogenesis. Because fatty acids are increased in diabetes, we examined their effects on ABCA1 activity in cultured macrophages. cAMP analogs and ligands for the liver X receptor/retinoid X receptor (LXR/RXR) system can induce Abca1 transcription in murine macrophages. When induced by cAMP, unsaturated but not saturated long-chain fatty acids inhibit apolipoprotein-mediated lipid efflux by destabilizing ABCA1 protein. Here, we show that the saturated fatty acids palmitate and stearate also destabilize ABCA1 when Abca1 is induced by LXR/RXR ligands instead of cAMP. This was associated with increased palmitate and stearate desaturation by stearoyl-CoA desaturase (SCD), another gene product induced by LXR/RXR ligands. The SCD inhibitors conjugated linoleic acid and troglitazone nearly abolished ABCA1 destabilization by palmitate and stearate but not by linoleate. These results suggest that LXR/RXR ligands generate ABCA1-destabilizing monounsaturated fatty acids from their saturated precursors by activating SCD. Thus, with cholesterol-loaded macrophages exposed to saturated fatty acids, activated LXR/RXR may counteract the enhanced ABCA1 transcription by reducing the ABCA1 protein content.