The liver isoform of carnitine palmitoyltransferase 1 is not targeted to the endoplasmic reticulum

Substrate and product complexes reveal mechanisms of Hedgehog acylation by HHAT

Hedgehog proteins govern crucial developmental steps in animals and drive certain human cancers. Before they can function as signaling molecules, Hedgehog precursor proteins must undergo amino-terminal palmitoylation by Hedgehog acyltransferase (HHAT). We present cryo-electron microscopy structures of human HHAT in complex with its palmitoyl-coenzyme A substrate and of a product complex with a palmitoylated Hedgehog peptide at resolutions of 2.7 and 3.2 angstroms, respectively. The structures reveal how HHAT overcomes the challenges of bringing together substrates that have different physiochemical properties from opposite sides of the endoplasmic reticulum membrane within a membrane-embedded active site for catalysis. These principles are relevant to related enzymes that catalyze the acylation of Wnt and of the appetite-stimulating hormone ghrelin. The structural and mechanistic insights may advance the development of inhibitors for cancer.

Hedgehog Acyltransferase Promotes Uptake of Palmitoyl-CoA across the Endoplasmic Reticulum Membrane

Attachment of palmitate to the N terminus of Sonic hedgehog (Shh) is essential for Shh signaling. Shh palmitoylation is catalyzed on the luminal side of the endoplasmic reticulum (ER) by Hedgehog acyltransferase (Hhat), an ER-resident enzyme. Palmitoyl-coenzyme A (CoA), the palmitate donor, is produced in the cytosol and is not permeable across membrane bilayers. It is not known how palmitoyl-CoA crosses the ER membrane to access the active site of Hhat. Here, we use fluorescent and radiolabeled palmitoyl-CoA probes to demonstrate that Hhat promotes the uptake of palmitoyl-CoA across the ER membrane in microsomes and semi-intact cells. Reconstitution of purified Hhat into liposomes provided further evidence that palmitoyl-CoA uptake activity is an intrinsic property of Hhat. Palmitoyl-CoA uptake was regulated by and could be uncoupled from Hhat enzymatic activity, implying that Hhat serves a dual function as a palmitoyl acyltransferase and a conduit to supply palmitoyl-CoA to the luminal side of the ER.

Palmitoylation of hedgehog proteins by Hedgehog acyltransferase (Hhat) occurs on the luminal side of the ER. However, the palmitoyl-CoA donor for the reaction is membrane impermeable. Asciolla and Resh show that Hhat serves a dual function as both an acyltransferase and a transporter that promotes palmitoyl-CoA uptake across the ER membrane.

Acyl-Coenzyme A Thioesterase 9 Traffics Mitochondrial Short-Chain Fatty Acids Toward De Novo Lipogenesis and Glucose Production in the Liver

BACKGROUND AND AIMS

Obesity-induced pathogenesis of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) is associated with increased de novo lipogenesis (DNL) and hepatic glucose production (HGP) that is due to excess fatty acids. Acyl-coenzyme A (CoA) thioesterase (Acot) family members control the cellular utilization of fatty acids by hydrolyzing (deactivating) acyl-CoA into nonesterified fatty acids and CoASH.

APPROACH AND RESULTS

Using Caenorhabditis elegans, we identified Acot9 as the strongest regulator of lipid accumulation within the Acot family. Indicative of a maladaptive function, hepatic Acot9 expression was higher in patients with obesity who had NAFLD and NASH compared with healthy controls with obesity. In the setting of excessive nutrition, global ablation of Acot9 protected mice against increases in weight gain, HGP, steatosis, and steatohepatitis. Supportive of a hepatic function, the liver-specific deletion of Acot9 inhibited HGP and steatosis in mice without affecting diet-induced weight gain. By contrast, the rescue of Acot9 expression only in the livers of Acot9 knockout mice was sufficient to promote HGP and steatosis. Mechanistically, hepatic Acot9 localized to the inner mitochondrial membrane, where it deactivated short-chain but not long-chain fatty acyl-CoA. This unique localization and activity of Acot9 directed acetyl-CoA away from protein lysine acetylation and toward the citric acid (TCA) cycle. Acot9-mediated exacerbation of triglyceride and glucose biosynthesis was attributable at least in part to increased TCA cycle activity, which provided substrates for HGP and DNL. β-oxidation and ketone body production, which depend on long-chain fatty acyl-CoA, were not regulated by Acot9.

CONCLUSIONS

Taken together, our findings indicate that Acot9 channels hepatic acyl-CoAs toward increased HGP and DNL under the pathophysiology of obesity. Therefore, Acot9 represents a target for the management of NAFLD.

Mechanisms of CPT1C-Dependent AMPAR Trafficking Enhancement

In neurons, AMPA receptor (AMPAR) function depends essentially on their constituent components:the ion channel forming subunits and ion channel associated proteins. On the other hand, AMPAR trafficking is tightly regulated by a vast number of intracellular neuronal proteins that bind to AMPAR subunits. It has been recently shown that the interaction between the GluA1 subunit of AMPARs and carnitine palmitoyltransferase 1C (CPT1C), a novel protein partner of AMPARs, is important in modulating surface expression of these ionotropic glutamate receptors. Indeed, synaptic transmission in CPT1C knockout (KO) mice is diminished supporting a positive trafficking role for that protein. However, the molecular mechanisms of such modulation remain unknown although a putative role of CPT1C in depalmitoylating GluA1 has been hypothesized. Here, we explore that possibility and show that CPT1C effect on AMPARs is likely due to changes in the palmitoylation state of GluA1. Based on in silico analysis, Ser 252, His 470 and Asp 474 are predicted to be the catalytic triad responsible for CPT1C palmitoyl thioesterase (PTE) activity. When these residues are mutated or when PTE activity is inhibited, the CPT1C effect on AMPAR trafficking is abolished, validating the CPT1C catalytic triad as being responsible for PTE activity on AMPAR. Moreover, the histidine residue (His 470) of CPT1C is crucial for the increase in GluA1 surface expression in neurons and the H470A mutation impairs the depalmitoylating catalytic activity of CPT1C. Finally, we show that CPT1C effect seems to be specific for this CPT1 isoform and it takes place solely at endoplasmic reticulum (ER). This work adds another facet to the impressive degree of molecular mechanisms regulating AMPAR physiology.

Malonyl-CoA, a key signaling molecule in mammalian cells

Malonyl-CoA can be formed within the mitochondria, peroxisomes, and cytosol of mammalian cells. Besides being an intermediate in the pathways of de novo fatty acid biosynthesis and fatty acid elongation, malonyl-CoA has an important signaling function through its allosteric inhibition of carnitine palmitoyltransferase 1, the enzyme that normally exerts flux control over mitochondrial beta-oxidation. Malonyl-CoA is rapidly turned over in mammalian cells, and the activities of acetyl-CoA carboxylase and malonyl-CoA decarboxylase are important determinants of its cytosolic concentration. It is now recognized that malonyl-CoA participates in a diverse range of physiological or pathological responses and systems. These include the ketogenic response of the liver to fasting and diabetes, carbohydrate versus fat fuel selection in muscle tissues, metabolic changes in muscle during contracture, alterations in fatty acid metabolism during cardiac ischemia and postischemic reperfusion, stimulation of B cell insulin secretion by glucose, and the hypothalamic control of appetite.

Mitochondrial glycerol-3-P acyltransferase 1 is most active in outer mitochondrial membrane but not in mitochondrial associated vesicles (MAV)

Glycerol 3-phosphate acyltransferase-1 (GPAT1), catalyzes the committed step in phospholipid and triacylglycerol synthesis. Because both GPAT1 and carnitine-palmitoyltransferase 1 are located on the outer mitochondrial membrane (OMM) it has been suggested that their reciprocal regulation controls acyl-CoA metabolism at the OMM. To determine whether GPAT1, like carnitine-palmitoyltransferase 1, is enriched in both mitochondrial contact sites and OMM, and to correlate protein location and enzymatic function, we used Percoll and sucrose gradient fractionation of rat liver to obtain submitochondrial fractions. Most GPAT1 protein was present in a vesicular membrane fraction associated with mitochondria (MAV) but GPAT specific activity in this fraction was low. In contrast, highest GPAT1 specific activity was present in purified mitochondria. Contact sites from crude mitochondria, which contained markers for both endoplasmic reticulum (ER) and mitochondria, also showed high expression of GPAT1 protein but low specific activity, whereas contact sites isolated from purified mitochondria lacked ER markers and expressed highly active GPAT1. To determine how GPAT1 is targeted to mitochondria, recombinant protein was synthesized in vitro and its incorporation into crude and purified mitochondria was assayed. GPAT1 was rapidly incorporated into mitochondria, but not into microsomes. Incorporation was ATP-driven, and lack of GPAT1 removal by alkali and a chaotropic agent showed that GPAT1 had become an integral membrane protein after incorporation. These results demonstrate that two pools of GPAT1 are present in rat liver mitochondria: an active one, located in OMM and a less active one, located in membranes (ER-contact sites and mitochondrial associated vesicles) associated with both mitochondria and ER.

The Human Hydroxyacylglutathione Hydrolase (HAGH) Gene Encodes Both Cytosolic and Mitochondrial Forms of Glyoxalase II

In yeast and higher plants, separate genes encode the cytosolic and mitochondrial forms of glyoxalase II. In contrast, although glyoxalase II activity has been detected both in the cytosol and mitochondria of mammals, only a single gene encoding glyoxalase II has been identified. Previously it was thought that this gene (the hydroxyacylglutathione hydrolase gene), comprised 8 exons that are transcribed into mRNA and that the resulting mRNA species encoded a single cytosolic form of glyoxalase II. Here we show that this gene gives rise to two distinct mRNA species transcribed from 9 and 10 exons, respectively. The 9-exon-derived transcript encodes two protein species: mitochondrially targeted glyoxylase II, which is initiated from an AUG codon in a previously uncharacterized part of the mRNA sequence, and cytosolic glyoxalase II, which is initiated by internal ribosome entry at a downstream AUG codon. The transcript deriving from 10 exons has an in-frame termination codon between the two initiating AUG codons and hence only encodes the cytosolic form of the protein. Confocal fluorescence microscopy indicates that the mitochondrially targeted form of glyoxalase II is directed to the mitochondrial matrix. Analysis of glyoxalase II mRNA sequences from a number of species indicates that dual initiation from alternative AUG codons is conserved throughout vertebrates.

Membrane transport of fatty acylcarnitine and free L-carnitine by rat liver microsomes

Recent studies have suggested that parts of the hepatic activities of diacylglycerol acyltransferase and acyl cholesterol acyltransferase are expressed in the lumen of the endoplasmic reticulum (ER). However the ER membrane is impermeable to the long-chain fatty acyl-CoA substrates of these enzymes. Liver microsomal vesicles that were shown to be at least 95% impermeable to palmitoyl-CoA were used to demonstrate the membrane transport of palmitoylcarnitine and free L-carnitine - processes that are necessary for an indirect route of provision of ER luminal fatty acyl-CoA through a luminal carnitine acyltransferase (CAT). Experimental conditions and precautions were established to permit measurement of the transport of [14C]palmitoylcarnitine into microsomes through the use of the luminal CAT and acyl-CoA:ethanol acyltransferase as a reporter system to detect formation of luminal [14C]palmitoyl-CoA. Rapid, unidirectional transport of free L-[3H]carnitine by microsomes was measured directly. This process, mediated either by a channel or a carrier, was inhibited by mersalyl but not by N-ethylmaleimide or sulfobetaine - properties that differentiate it from the mitochondrial inner membrane carnitine/acylcarnitine exchange carrier. These findings are relevant to the understanding of processes for the reassembly of triacylglycerols that lipidate very low density lipoprotein particles as part of a hepatic triacylglycerol lipolysis/re-esterification cycle.