Expression of ACAT-1 Protein in Human Atherosclerotic Lesions and Cultured Human Monocytes-Macrophages

From the Department of Biochemistry, Dartmouth Medical School, Hanover, NH (A.M., O.L., C.C.Y.C., T.-Y.C.); the Second Department of Pathology (N.S., K.T.) and the Department of Biochemistry (A.M., S.H., H.H.), Kumamoto University School of Medicine, Kumamoto, Japan; and the Department of Microbiology, Dartmouth Medical School and Veteran's Administration Hospital, White River Junction, Vt (P.M.M.).

Correspondence to Ta-Yuan Chang, PhD, Professor of Biochemistry, Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755-3844. E-mail Ta.Yuan.Chang{at}dartmouth.edu

Abstract—The acyl coenzyme A:cholesterol acyltransferase (ACAT) gene was first cloned in 1993 (Chang et al, J Biol Chem. 1993;268:20747–20755; designated ACAT-1). Using affinity-purified antibodies raised against the N-terminal portion of human ACAT-1 protein, we performed immunohistochemical localization studies and showed that the ACAT-1 protein was highly expressed in atherosclerotic lesions of the human aorta. We also performed 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 vitro to monitor the ACAT-1 expression in differentiating monocytes-macrophages. The ACAT-1 protein content increased by up to 10-fold when monocytes spontaneously differentiated into macrophages. This increase occurred within the first 2 days of culturing the monocytes and reached a plateau level within 4 days of culturing, indicating that the increase in ACAT-1 protein content is an early event during the monocyte differentiation process. The ACAT-1 protein expressed in the differentiating monocytes-macrophages was shown to be active by enzyme assay in vitro. The high levels of ACAT-1 present in macrophages maintained in culture can explain the high ACAT-1 contents found in atherosclerotic lesions. Our results thus support the idea that ACAT-1 plays an important role in differentiating monocytes and in forming macrophage foam cells during the development of human atherosclerosis.

Key Words: ACAT • atherosclerosis • immunohistochemistry • differentiation • monocytes

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