Structure, Function and Regulation of the ABC1 Gene Product

The role of the ATP-binding cassette transporter 1 (ABCA1) in cellular lipid efflux and high density lipoprotein metabolism has been recently documented by mutations in genetic HDL deficiency syndromes such as classical Tangier disease. Analysis of ABCA1 knockout mice and overexpression studies have established the importance of ABCA1 as a major determinant of HDL cholesterol in plasma. These studies also indicate that ABCA1 is critically involved in cellular trafficking of cholesterol and choline-phospholipids and in total body lipid homeostasis, such as intestinal cholesterol and fat-soluble vitamin absorption and in the modulation of steroidogenesis. First insights into the upregulation of ABCA1 gene expression by cellular cholesterol and cAMP have identified critical ABCA1 promoter elements, which bind the transcription factors liver X receptor, retinoid X receptor, Sp1 and E-box proteins. The finding that a lipid sensitive subgroup of ABC transporters is able to translocate cholesterol and phospholipids supports the concept that in ABCA1 deficiency, compensatory mechanisms possibly involving MDR1, MDR3 and MRP-family members could be active. This suggests that a network of ABC transporters involved in cellular lipid transport exists, which is under the tight control of energy pathways directly linked to high density lipoprotein metabolism and atherogenesis.