It takes a village: channeling fatty acid metabolism and triacylglycerol formation via protein interactomes

Abstract

Diet, hormones, gene transcription, and posttranslational modifications control the hepatic metabolism of FAs; metabolic dysregulation causes chronic diseases, including cardiovascular disease, and warrants exploration into the mechanisms directing FA and triacylglycerol (TAG) synthesis and degradation. Long-chain FA metabolism begins by formation of an acyl-CoA by a member of the acyl-CoA synthetase (ACSL) family. Subsequently, TAG synthesis begins with acyl-CoA esterification to glycerol-3-phosphate by a member of the glycerol-3-phosphate acyltransferase (GPAT) family. Our studies of the isoforms ACSL1 and GPAT1 strongly suggest that these proteins are members of larger protein assemblies (interactomes). ACSL1 targeted to the ER interacts with peroxisomal, lipid droplet, and tethering proteins, uncovering a dynamic role for ACSL1 in organelle and lipid droplet interactions. On the outer mitochondrial membrane (OMM), PPARα upregulates ACSL1, which interacts with proteins believed to tether lipid droplets to the OMM. In contrast, GPAT1 is upregulated nutritionally by carbohydrate and insulin in a coordinated sequence of enzyme reactions, from saturated FA formation via de novo lipogenesis to FA esterification by GPAT1 and entry into the TAG biosynthesis pathway. We propose that involved enzymes form a dynamic protein interactome that facilitates esterification and that other lipid-metabolizing pathways will exist in similar physiologically regulated interactomes.

Keywords: acyl-coenzyme A; beta oxidation; de novo lipogenesis; lipid droplets.

Graphical abstract

Fig. 1.

Synthesis of glycerolipids from long-chain FAs. Members of the family of long-chain ACSLs (ACSL, ACSVL, ACSBg) thioesterify long-chain FAs to form acyl-CoAs. These may be esterified to the sn-1 position of glycerol-3-phosphate by one of four GPATs to form LPA. One of several 1-acylglycerol-3-phosphate acyltransferases (AGPATs; also known as LPA acyltransferases) uses an acyl-CoA to form PA. After one of three PA phosphohydrolases (PAPases/lipins) cleaves the phosphate, the remaining DAG product is esterified by one of two DGATs to form TAG. The TAG may remain in the cytosol within a lipid droplet or, in liver, be secreted as part of a VLDL. The LPA, PA, and DAG intermediates may initiate signaling cascades, and PA and DAG are also precursors of all the glycerophospholipids: phosphatidylinositol (PI), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and cardiolipin (CL). Acyl-CoAs can also be converted to acyl-carnitines by carnitine palmitoyltransferase (CPT1) to enter the mitochondria for β-oxidation.

Fig. 2.

Altered ACSL1 interactome at the OMM in the absence or presence of glucose. Primary hepatocytes from male and female mice were incubated with or without 25 mM of glucose plus 1 mM of pyruvate for 16 h before immunoprecipitation of Ad-ACSL1-Flag (12). Carnitine palmitoyltransferase-1 (CPT) coimmunoprecipitated with ACSL1 under both conditions, but the OMM-lipid droplet tethering proteins, VAMP4 and SNAP23, coimmunoprecipitated only when glucose was absent.

Fig. 3.

Suggested interaction of enzymes involved in DNL and TAG synthesis via GPAT1. The citrate carrier (CiC) transports citrate out of the mitochondria and into the cytosol where it is cleaved by ATP citrate lyase to form oxaloacetate (OAA) and acetyl-CoA. Malate dehydrogenase (DH) and malic enzyme convert the OAA successively to malate and pyruvate together with the production of NADPH. Acetyl-CoA carboxylase (ACC) converts the acetyl-CoA to malonyl-CoA, which is subsequently converted primarily to palmitate by FAS. After activation to palmitoyl-CoA by an ACSL, GPAT1 esterifies it to glycerol-3-phosphate to form LPA. Subsequent steps described in Fig. 1 convert the LPA to TAG or glycerophospholipids (PL) in the ER. Some of these steps may occur on structures variously termed mitochondria-associated membranes (MAM) and mitochondria-associated vesicles (MAV) (48).

Fig. 4.

Potential organization of a protein assembly that links enzymes of DNL and GPAT1. ATP citrate lyase (ATP CL), acetyl-coA carboxylase (ACC), FAS, and ACSL ultimately produce the palmitoyl-CoA that GPAT1 esterifies to glycerol-3-phosphate. Although thought of as cytosolic, the enzymes of DNL may interact with membrane and organellar proteins, and both GPAT1 and its associated ACSL are membrane-bound proteins. Interactions at specific sites could help to organize the metabolon. The substrates and products, oxaloacetate, acetyl-CoA, malonyl-CoA, and palmitate, are depicted as colored ovals integrated within the hypothesized protein assembly.