ATP-binding cassette protein A1 (ABCA1) mediates transfer of cellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I), an extracellular acceptor in plasma, to form highdensity lipoprotein (HDL). It is currently unknown to what extent ABCA1 endocytosis and recycling contribute to the HDL formation. To address this issue, we expressed human ABCA1 constructs with either an extracellular HA tag or an intracellular GFP tag in cells, and used this system to characterize endocytosis and recycling of ABCA1 and apoA-I. Under basal conditions, ABCA1 and apoA-I are endocytosed via a clathrin-and Rab5-mediated pathway and recycled rapidly back to the cell surface, at least in part via a Rab4-mediated route; approximately 30% of the endocytosed ABCA1 is recycled back to the cell surface. When receptor-mediated endocytosis is inhibited, the level of ABCA1 at the cell surface increases and apoA-I internalization is blocked. Under these conditions, apoA-I mediated cholesterol efflux from cells that have accumulated lipoprotein-derived cholesterol is decreased, whereas efflux from cells without excess cholesterol is increased. These results suggest that the retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation when excess lipoprotein-derived cholesterol has accumulated in cells.

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ABCA7 is a member of the subfamily A of adenosine triphosphate-binding cassette (ABC) proteins, and highly homologous to ABCA1, which mediates the release of cellular cholesterol and phospholipid to form high-density lipoprotein. ABCA1 and ABCA7 contain two large extracellular domains, ECD1 and 2, which are thought to be important for their functions. Interestingly, part of ECD1 of ABCA7 is deposited as an autoantigen of Sjögren's syndrome. To determine the relationship between ABCA7 and Sjögren's syndrome, an immunohistochemical study was conducted with salivary gland biopsy samples from patients with Sjögren's syndrome. ECD1 of human ABCA7 (amino acids 45-549) was expressed in Escherichia coli as a protein fused with glutathione-S-transferase and a monoclonal antibody, KM3095, was generated. KM3095-immunoreactive cells were observed in salivary glands from 10 of 18 patients with Sjögren's syndrome. Immunostaining of serial sections with the plasma cell marker NCL-PC indicated that most of the plasma cells infiltrating salivary glands of patients with Sjögren's syndrome were KM3095-immunoreactive. Although the pathological or biological significance is not clear, it will be intriguing to further examine the relationship between ABCA7 and Sjögren's syndrome.

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