Polyunsaturated Fatty Acids and Acetoacetate Downregulate the Expression of the ATP-Binding Cassette Transporter A 1

Low HDL cholesterol is a frequent cardiovascular risk factor in diabetes. Because of its pivotal role for the regulation of HDL plasma levels, we investigated in vivo and in vitro regulation of the ATP-binding cassette transporter A1 (ABCA1) by insulin and metabolites accumulating in diabetes. Compared with euglycemic control mice, ABCA1 gene expression was severely decreased in the liver and peritoneal macrophages of diabetic mice. Treatment with insulin restored this deficit. Incubation of cultivated HepG2 hepatocytes and RAW264.7 macrophages with unsaturated fatty acids or acetoacetate, but not with insulin, glucose, saturated fatty acids, or hydroxybutyrate, downregulated ABCA1 mRNA and protein. The suppressive effect of unsaturated fatty acids and acetoacetate became most obvious in cells stimulated with oxysterols or retinoic acid but was independent of the expression of the thereby regulated transcription factors liver-X-receptor (LXR ) and retinoid-X-receptor (RXR ), respectively. Unsaturated fatty acids and acetoacetate also reduced ABCA1 promotor activity in RAW264.7 macrophages that were transfected with a 968-bp ABCA1 promotor/luciferase gene construct. As the functional consequence, unsaturated fatty acids and acetoacetate inhibited cholesterol efflux from macrophages. Downregulation of ABCA1 by unsaturated fatty acids and acetoacetate may contribute to low HDL cholesterol and increased cardiovascular risk of diabetic patients. Diabetes 51: 2922–2928, 2002 Numerous clinical and epidemiological studies have demonstrated the inverse association between HDL cholesterol and the risk of coronary heart disease events (1). It is controversially discussed whether this relationship is causal or only an epiphenomenon of a more general atherogenic disorder. On the one hand, HDL exerts various potentially antiatherogenic properties. For example, HDL particles transport cholesterol from cells of the arterial wall to the liver and to steroidogenic organs, in which cholesterol is used for the synthesis of bile acids, lipoproteins, vitamin D, and steroid hormones (1). On the other hand, low HDL cholesterol is frequently found as a component of the metabolic syndrome in many populations, i.e., together with overweight or obesity, glucose intolerance or overt diabetes, hypertriglyceridemia, and hypertension, which by themselves contribute to the pathogenesis of atherosclerosis (2). Moreover, many individuals with low HDL cholesterol have elevated fasting plasma levels of insulin, are resistant to exogenous insulin in euglycemic clamp studies, and bear an increased risk for future type 2 diabetes (3–5). The pathogenesis of low HDL cholesterol in insulin resistance is not well understood but may provide an important key to answer the question of whether low HDL cholesterol is a causal factor in the pathogenesis of atherosclerosis in patients with insulin resistance or diabetes. ATP-binding cassette transporter A1 (ABCA1) has previously been identified as a pivotal gene in the regulation of both HDL cholesterol plasma levels and the cellular cholesterol homeostasis, which is defective in patients with Tangier disease (1,6–9). In these patients and their heterozygous relatives, mutations in the ABCA1 gene cause gene dosage–dependent decreases in plasma levels of HDL cholesterol and in the capacity of skin fibroblasts and monocyte-derived macrophages to release cholesterol in the extracellular presence of apolipoproteins (1,6–9). As the clinical result, homozygous patients with Tangier disease accumulate macrophage-derived foam cells in various tissues and develop tonsil anomalies, hepatosplenomegaly, premature atherosclerosis, and peripheral neuropathy (10). Cyclic AMP and ligands of the nuclear transcription factors liver-X-receptor (LXR ) and retinoid-X-receptor (RXR ), i.e., oxysterols and retinoids, respectively, have From the Institute of Clinical Chemistry and Laboratory Medicine, Central Laboratory, Westphalian Wilhelms-University, Münster, Germany; the Institute of Arteriosclerosis Research, Münster, Germany; the University Hospital Zürich, Institute of Clinical Chemistry, Zürich, Switzerland; and the Department of Biochemistry, University of Münster, Münster, Germany. Address correspondence and reprint requests to Dr. Arnold von Eckardstein, University Hospital Zürich, Institute of Clinical Chemistry, Rämistrasse 100, CH 8091 Zürich, Switzerland. E-mail: arnold.voneckardstein@ikc.usz. ch. Received for publication 30 April 2002 and accepted in revised form12 July 2002. 9-cis-RA, 9-cis-retinoic acid; 22R-HC, 22(R)-hydroxycholesterol; ABCA1, ATP-binding cassette transporter A1; DMEM, Dulbecco’s modified Eagle’s medium; EPA, eicosapentaenoic acid; LXR , liver-X-receptor ; IFN , interferon; PPAR, peroxisome proliferator–activated receptor; PUFA, polyunsaturated fatty acid; RXR , retinoid-X-receptor .