Two human ACAT2 mRNA variants produced by alternative splicing and coding for novel isoenzymes

Acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2) plays an important role in cholesterol absorption.Human ACAT2 is highly expressed in small intestine and fetal liver,but its expression is greatly diminished in adult liver.The full-length human ACAT2 mRNA encodes a protein,designated ACAT2a,with 522 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 mRNA variants 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 from ACAT2a 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 and intestine cells.Furthermore,phosphorylation and enzymatic activity analyses demonstrated that the novel isoenzymes ACAT2b and ACAT2c lacked the phosphorylatable site SLLD,and their enzymatic activities reduced to 25%-35% of that of ACAT2a.These evidences indicate that alternative splicing produces two human 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.