Glycerol-3-phosphate Acyltransferase (GPAT)-1, but Not GPAT4, Incorporates Newly Synthesized Fatty Acids into Triacylglycerol and Diminishes Fatty Acid Oxidation

Angela A Wendel,Daniel E Cooper,Olga R Ilkayeva,Deborah M Muoio,Rosalind A Coleman

doi:10.1074/jbc.m113.485219

Abstract

Four glycerol-3-phosphate acyltransferase (GPAT) isoforms, each encoded by a separate gene, catalyze the initial step in glycerolipid synthesis; in liver, the major isoforms are GPAT1 and GPAT4. To determine whether each of these hepatic isoforms performs a unique function in the metabolism of fatty acid, we measured the incorporation of de novo synthesized fatty acid or exogenous fatty acid into complex lipids in primary mouse hepatocytes from control, Gpat1(-/-), and Gpat4(-/-) mice. Although hepatocytes from each genotype incorporated a similar amount of exogenous fatty acid into triacylglycerol (TAG), only control and Gpat4(-/-) hepatocytes were able to incorporate de novo synthesized fatty acid into TAG. When compared with controls, Gpat1(-/-) hepatocytes oxidized twice as much exogenous fatty acid. To confirm these findings and to assess hepatic β-oxidation metabolites, we measured acylcarnitines in liver from mice after a 24-h fast and after a 24-h fast followed by 48 h of refeeding with a high sucrose diet to promote lipogenesis. Confirming the in vitro findings, the hepatic content of long-chain acylcarnitine in fasted Gpat1(-/-) mice was 3-fold higher than in controls. When compared with control and Gpat4(-/-) mice, after the fasting-refeeding protocol, Gpat1(-/-) hepatic TAG was depleted, and long-chain acylcarnitine content was 3.5-fold higher. Taken together, these data demonstrate that GPAT1, but not GPAT4, is required to incorporate de novo synthesized fatty acids into TAG and to divert them away from oxidation.

Highlights

The independent functions of the glycerol-3-phosphate acyltransferase (GPAT) isoforms are unknown
To determine whether each of these hepatic isoforms performs a unique function in the metabolism of fatty acid, we measured the incorporation of de novo synthesized fatty acid or exogenous fatty acid into complex lipids in primary mouse hepatocytes from control, Gpat1؊/؊, and Gpat4؊/؊ mice
The residual NEM-sensitive GPAT activity is likely due to GPAT2 and -3. These results show that isolated and cultured primary hepatocytes retained GPAT activity and that the specific activity was similar in Gpat1Ϫ/Ϫ and Gpat4Ϫ/Ϫ hepatocytes

Summary

Background

The independent functions of the glycerol-3-phosphate acyltransferase (GPAT) isoforms are unknown. When compared with control and Gpat4؊/؊ mice, after the fasting-refeeding protocol, Gpat1؊/؊ hepatic TAG was depleted, and long-chain acylcarnitine content was 3.5-fold higher Taken together, these data demonstrate that GPAT1, but not GPAT4, is required to incorporate de novo synthesized fatty acids into TAG and to divert them away from oxidation. Because SREBP1c concomitantly up-regulates both GPAT1 and the enzymes that catalyze the de novo synthesis of fatty acids, including acetyl-CoA carboxylase and fatty acid synthase [14], we hypothesized that the function of GPAT1 and de novo lipogenesis might be intrinsically linked To test this hypothesis, we compared the ability of GPAT1 and GPAT4 to use exogenous versus de novo synthesized fatty acids in control and knock-out mice and in their hepatocytes

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION