EXPRESSION OF ACAT-1 PROTEIN IN HUMAN ATHEROSCLEROTIC LESIONS AND CULTURED HUMAN MONOCYTES-MACROPHAGES

The acyl coenzyme A:cholesterol acyltransferase (ACAT) gene was first cloned in 1993 (Chang et al, J Biol Chem. 1993;268:20747-20755; design ated ACAT-1). Using affinity-purified antibodies raised against the N- terminal portion of human ACAT-1 protein, we performed immunohistochem ical localization studies and showed that the ACAT-1 protein was highl y expressed in atherosclerotic lesions of the human aorta. We also per formed cell-specific localization studies using double immunostaining and showed that ACAT-1 was predominantly expressed in macrophages but not in smooth muscle cells. We then used a cell culture system in vitr o to monitor the ACAT-1 expression in differentiating monocytes-macrop hages. The ACAT-1 protein content increased by up to 10-fold when mono cytes spontaneously differentiated into macrophages. This increase occ urred within the first 2 days of culturing the monocytes and reached a plateau level within 3 days of culturing, indicating that the increas e in ACAT-1 protein content is an early event during the monocyte diff erentiation process, The ACAT-1 protein expressed in the differentiati ng monocytes-macrophages was shown to be active by enzyme assay in vit ro. The high levels of ACAT-1 present in macrophages maintained in cul ture can explain the high ACAT-1 contents found in atherosclerotic les ions. Our results thus support the idea that ACAT-1 plays an important role in differentiating monocytes and in forming macrophage foam cell s during the development of human atherosclerosis.