Monoacylglycerol O-acyltransferase (MGAT) is an enzyme that is involved in triglyceride synthesis by catalyzing the formation of diacylglycerol from monoacylglycerol and fatty acyl CoAs. Recently, we reported that MGAT1 has a critical role in hepatic TG accumulation and that its suppression ameliorates hepatic steatosis in a mouse model. However, the function of MGAT enzymes in hepatic lipid accumulation has not been investigated in humans. Unlike in rodents, MGAT3 as well as MGAT1 and MGAT2 are present in humans. In this study, we evaluated the differences between MGAT subtypes and their association with peroxisome proliferator-activated receptor gamma (PPAR gamma), a regulator of mouse MGAT1 expression. In human primary hepatocytes, basal expression of MGATI was lower than that of MGAT2 or MGAT3, but was strongly induced by PPAR gamma overexpression. A luciferase assay as well as an electro-mobility shift assay revealed that human MGATI promoter activity is driven by PPAR gamma by direct binding to at least two regions of the promoter in 2931 and HepG2 cells. Moreover, siRNA-mediated suppression of MGAT1 expression significantly attenuated lipid accumulation by PPAR gamma overexpression in HepG2 cells, as evidenced by oil-red-O staining. These results suggest that human MGAT1 has an important role in fatty liver formation as a target gene of PPAR gamma, and blocking MGAT1 activity could be an efficient therapeutic way to reduce nonalcoholic fatty liver diseases in humans. (C) 2015 Elsevier Inc. All rights reserved.