Acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2) plays an important role in cholesterolabsorption.Human ACAT2 is highly expressed in small intestine and fetal liver,but its expression is greatlydiminished in adult liver.The full-length human ACAT2 mRNA encodes a protein,designated ACAT2a,with522 amino acids.We have previously reported the organization of the human ACAT2 gene and the differen-tiation-dependent promoter activity in intestinal Caco-2 cells.In the current work,two human ACAT2 mRNAvariants produced by alternative splicing are cloned and predicted to encode two novel ACAT2 isoforms,named ACAT2b and ACAT2c,with 502 and 379 amino acids,respectively.These mRNA variants differ fromACAT2a mRNA by lack of the exon 4 (ACAT2b mRNA) and exons 4-5 plus 8-9-10 (ACAT2c mRNA).Significantly,comparable amounts of the alternatively spliced ACAT2 mRNA variants were detected by RT-PCR,and Western blot analysis confirmed the presence of their corresponding proteins in human liver andintestine cells.Furthermore,phosphorylation and enzymatic activity analyses demonstrated that the novelisoenzymes ACAT2b and ACAT2c lacked the phosphorylatable site SLLD,and their enzymatic activitiesreduced to 25%-35% of that of ACAT2a.These evidences indicate that alternative splicing produces twohuman ACAT2 mRNA variants that encode the novel ACAT2 isoenzymes.Our findings might help to under-stand the regulation of the ACAT2 gene expression under certain physiological and pathological conditions.
