To investigate the distribution of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) in various human tissues , we examined tissues of autopsy cases immunohistochemically. ACAT-1 was demonstrated in macrophages , antigen-presenting cells , steroid hormone-producing cells , neurons , cardiomyocytes, smooth muscle cells , mesothelial cells , epithelial cells of the urinary tracts , thyroid follicles , renal tubules , pituitary , prostatic , and bronchial glands, alveolar and intestinal epithelial cells , pancreatic acinar cells , and hepatocytes. These findings showed that ACAT-1 is present in a variety of human tissues examined. The immunoreactivities are particularly prominent in the macrophages , steroid hormoneproducing cells , followed by hepatocytes , and intestinal epithelia. In cultured human macrophages, immunoelectron microscopy revealed that ACAT-1 was located mainly in the tubular rough endoplasmic reticulum; immunoblot analysis showed that the ACAT-1 protein content did not change with or without cholesterol loading; however , on cholesterol loading , about 30 to 40% of the total immunoreactivity appeared in small-sized vesicles. These vesicles were also enriched in 78-kd glucose-regulated protein (GRP 78), a specific marker for the endoplasmic reticulum. Immunofluorescent microscopy demon- Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is a key enzyme involved in cellular cholesterol metabolism. It catalyzes the formation of cholesteryl esters from cholesterol and long-chain fatty acyl-coenzyme A. 1 ACAT activities are present in various tissues such as liver, intestines, adrenal glands, and aorta and are involved in intracellular cholesterol storage, lipoprotein assembly, steroid hormone production, and dietary cholesterol absorption. 2 Previous studies showed that ACAT is a membrane-bound enzyme; its activity is found only in the membrane fractions of intracellular organelles, especially in the rough endoplasmic reticulum. 3,4 More recently, molecular probes of ACAT have become available. In 1993, the first cDNA of ACAT, designated as ACAT-1, was cloned from a human THP-1 cell cDNA library by using a somatic cell and molecular genetic approach. 5 The sequence of human ACAT-1 cDNA led to the clonings of its homologues from various other species, including mice. 6 ACAT-1 gene knockout mice were produced. 7,8 Analyses of these mice showed that ACAT activities were significantly decreased in selected tissues examined but not in the livers, strongly suggesting that one or more additional ACAT genes, distinct from the ACAT-1 gene, probably exist in mice. More recently, a different ACAT cDNA, designated ACAT-2, was cloned. 9 -11 The sequence of ACAT-2 is homologous but different from that of ACAT-1. Earlier, two different ACAT-like genes were found in the simple eukaryote Sachromycetes saravesea. 12,13 The exact roles of ACAT-1 and ACAT-2 in different species remain unknown. Their physiological functions are currently under intensive investigation in several laboratories.Recently, specific polycl...