Diacylglycerol acyltransferase 2 acts upstream of diacylglycerol acyltransferase 1 and utilizes nascent diglycerides and de novo synthesized fatty acids in HepG2 cells

The two diacylglycerol acyltransferases, DGAT1 and DGAT2, are known to have non-redundant functions, in spite of catalysing the same reaction and being present in the same cell types. The basis for this distinctiveness, which is reflected in the very different phenotypes of Dgat1(-/-) and Dgat2(-/-) mice, has not been resolved. Using selective inhibitors of human DGAT1 and DGAT2 on HepG2 cells and gene silencing, we show that, although DGAT2 activity accounts for a modest fraction (< 20%) of overall cellular DGAT activity, inhibition of DGAT2 activity specifically inhibits (and is rate-limiting for) the incorporation of de novo synthesized fatty acids and of glycerol into cellular and secreted triglyceride to a much greater extent than it affects the incorporation of exogenously added oleate. By contrast, inhibition of DGAT1 affects equally the incorporation of glycerol and exogenous (preformed) oleate into cellular and secreted triacylglycerol (TAG). These data indicate that DGAT2 acts upstream of DGAT1, largely determines the rate of de novo synthesis of triglyceride, and uses nascent diacylglycerol and de novo synthesized fatty acids as substrates. By contrast, the data suggest that DGAT1 functions in the re-esterification of partial glycerides generated by intracellular lipolysis, using preformed (exogenous) fatty acids. Therefore, we describe distinct but synergistic roles of the two DGATs in an integrated pathway of TAG synthesis and secretion, with DGAT2 acting upstream of DGAT1.