Treatment of rats with glucagon or mannoheptulose increases mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase activity and decreases succinyl-CoA content in liver

Emerging Role of Hepatic Ketogenesis in Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD), the most common chronic liver diseases, arise from non-alcoholic fatty liver (NAFL) characterized by excessive fat accumulation as triglycerides. Although NAFL is benign, it could progress to non-alcoholic steatohepatitis (NASH) manifested with inflammation, hepatocyte damage and fibrosis. A subset of NASH patients develops end-stage liver diseases such as cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD is highly complex and strongly associated with perturbations in lipid and glucose metabolism. Lipid disposal pathways, in particular, impairment in condensation of acetyl-CoA derived from β-oxidation into ketogenic pathway strongly influence the hepatic lipid loads and glucose metabolism. Current evidence suggests that ketogenesis dispose up to two-thirds of the lipids entering the liver, and its dysregulation significantly contribute to the NAFLD pathogenesis. Moreover, ketone body administration in mice and humans shows a significant improvement in NAFLD. This review focuses on hepatic ketogenesis and its role in NAFLD pathogenesis. We review the possible mechanisms through which impaired hepatic ketogenesis may promote NAFLD progression. Finally, the review sheds light on the therapeutic implications of a ketogenic diet in NAFLD.

Peroxisome-generated succinate induces lipid accumulation and oxidative stress in the kidneys of diabetic mice

Diabetes normally causes lipid accumulation and oxidative stress in the kidneys, which plays a critical role in the onset of diabetic nephropathy; however, the mechanism by which dysregulated fatty acid metabolism increases lipid and reactive oxygen species (ROS) formation in the diabetic kidney is not clear. As succinate is remarkably increased in the diabetic kidney, and accumulation of succinate suppresses mitochondrial fatty acid oxidation and increases ROS formation, we hypothesized that succinate might play a role in inducing lipid and ROS accumulation in the diabetic kidney. Here we demonstrate a novel mechanism by which diabetes induces lipid and ROS accumulation in the kidney of diabetic animals. We show that enhanced oxidation of dicarboxylic acids by peroxisomes leads to lipid and ROS accumulation in the kidney of diabetic mice via the metabolite succinate. Furthermore, specific suppression of peroxisomal β-oxidation improved diabetes-induced nephropathy by reducing succinate generation and attenuating lipid and ROS accumulation in the kidneys of the diabetic mice. We suggest that peroxisome-generated succinate acts as a pathological molecule inducing lipid and ROS accumulation in kidney, and that specifically targeting peroxisomal β-oxidation might be an effective strategy in treating diabetic nephropathy and related metabolic disorders.

Fasting induces hepatic lipid accumulation by stimulating peroxisomal dicarboxylic acid oxidation

Fasting induces lipid accumulation in the liver, while the mechanisms by which fasting dysregulates liver fatty acid oxidation are not clear. Fatty acid ω-oxidation is induced in the fasting state, and administration of dicarboxylic acids to fasting animals decreases plasma ketone bodies. We hypothesized that endogenous dicarboxylic acids might play a role in controlling mitochondrial β-oxidation in fasting animals. A peroxisome proliferator-activated receptor-alpha agonist and an inhibitor for peroxisomal β-oxidation were administered to the fasting rats to investigate the role of dicarboxylic acids in liver fatty acid oxidation and lipid homeostasis. We observed that excessive β-oxidation of endogenous dicarboxylic acids by peroxisomes generated considerable levels of succinate in the liver. Excessive succinate oxidation subsequently increased the mitochondrial NADH/NAD⁺ ratio and led to an accumulation of 3-OH-CoA and 2-enoyl-CoA intermediates in the liver. This further induced feedback suppression of mitochondrial β-oxidation and promoted hepatic lipid deposition and steatosis. Specific inhibition of peroxisomal β-oxidation attenuated fasting-induced lipid deposition in the liver by reducing succinate production and enhancing mitochondrial fatty acid oxidation. We conclude that suppression of mitochondrial β-oxidation by oxidation of dicarboxylic acids serves as a mechanism for fasting-induced hepatic lipid accumulation and identifies cross talk between peroxisomal and mitochondrial fatty acid oxidation.

A Look into Liver Mitochondrial Dysfunction as a Hallmark in Progression of Brain Energy Crisis and Development of Neurologic Symptoms in Hepatic Encephalopathy

BACKGROUND

The relationship between liver disease and neuropathology in hepatic encephalopathy is well known, but the genesis of encephalopathy in liver failure is yet to be elucidated. Conceptually, the main cause of hepatic encephalopathy is the accumulation of brain ammonia due to impaired liver detoxification function or occurrence of portosystemic shunt. Yet, as well as taking up toxic ammonia, the liver also produces vital metabolites that ensure normal cerebral function. Given this, for insight into how perturbations in the metabolic capacity of the liver may be related to brain pathology, it is crucial to understand the extent of ammonia-related changes in the hepatic metabolism that provides respiratory fuel for the brain, a deficiency of which can give rise to encephalopathy.

METHODS

Hepatic encephalopathy was induced in starved rats by injection of ammonium acetate. Ammonia-induced toxicity was evaluated by plasma and freeze-clamped liver and brain energy metabolites, and mitochondrial, cytoplasmic, and microsomal gluconeogenic enzymes, including mitochondrial ketogenic enzymes. Parameters of oxidative phosphorylation were recorded polarographically with a Clark-type electrode, while other measures were determined with standard fluorometric enzymatic methods.

RESULTS

Progressive impairment of liver mitochondrial respiration in the initial stage of ammonia-induced hepatotoxicity and the subsequent energy crisis due to decreased ATP synthesis lead to cessation of gluconeogenesis and ketogenesis. Reduction in glucose and ketone body supply to the brain is a terminal event in liver toxicity, preceding the development of coma.

CONCLUSIONS

Our study provides a framework to further explore the relationship between hepatic dysfunction and progression of brain energy crisis in hepatic encephalopathy.

Pharmacologic activation of peroxisome proliferator-activating receptor-α accelerates hepatic fatty acid oxidation in neonatal pigs

Up-regulation of peroxisome proliferator-activating receptor-α (PPARα) and increasing fatty acid oxidation are important for reducing pre-weaning mortality of pigs. We examined the time-dependent regulatory effects of PPARα activation via oral postnatal clofibrate administration (75 mg/(kg-BW·d) for up to 7 days) on mitochondrial and peroxisomal fatty acid oxidation in pigs, a species with limited hepatic fatty acid oxidative capacity due to low ketogenesis. Hepatic oxidation was increased by 44-147% (depending on fatty acid chain-length) and was attained after only 4 days of clofibrate treatment. Acyl-CoA oxidase (ACO) and carnitine palmitoyltransferase I (CPTI) activities accelerated in parallel. The increase in CPTI activity was accompanied by a rapid reduction in the sensitivity of CPTI to malonyl-CoA inhibition. The mRNA abundance of CPTI and ACO, as well as peroxisomal keto-acyl-CoA thiolase (KetoACoA) and mitochondrial malonyl-CoA decarboxylase (MCD), also were augmented greatly. However, the increase in ACO activity and MCD expression were different from CPTI, and significant interactions were observed between postnatal age and clofibrate administration. Furthermore, the expression of acetyl-CoA carboxylase β (ACCβ) decreased with postnatal age and clofibrate had no effect on its expression. Collectively these results demonstrate that the expression of PPARα target genes and the increase in fatty acid oxidation induced by clofibrate are time- and age-dependent in the liver of neonatal pigs. Although the induction patterns of CPTI, MCD, ACO, KetoACoA, and ACCβ are different during the early postnatal period, 4 days of exposure to clofibrate were sufficient to robustly accelerate fatty acid oxidation.

The influence of Shc proteins and high-fat diet on energy metabolism of mice

The purpose of this study was to determine if Shc proteins influence the metabolic response to acute (7 days) feeding of a high-fat diet (HFD). To this end, whole animal energy expenditure (EE) and substrate oxidation were measured in the Shc knockout (ShcKO) and wild-type (WT) mice fed a control or HFD. The activities of enzymes of glycolysis, the citric acid cycle, electron transport chain (ETC), and β-oxidation were also investigated in liver and skeletal muscle of ShcKO and WT animals. The study showed that ShcKO increases (P < .05) EE adjusted for either total body weight or lean mass. This change in EE could contribute to decreases in weight gain in ShcKO versus WT mice fed an HFD. Thus, our results indicate that Shc proteins should be considered as potential targets for developing interventions to mitigate weight gain on HFD by stimulating EE. Although decreased levels of Shc proteins influenced the activity of some enzymes in response to high-fat feeding (eg, increasing the activity of acyl-CoA dehydrogenase), it did not produce concerted changes in enzymes of glycolysis, citric acid cycle, or the ETC. The physiological significance of observed changes in select enzyme activities remains to be determined.

SIGNIFICANCE OF THE STUDY

We report higher EE in ShcKO versus WT mice when consuming the HFD. Although decreased levels of Shc proteins influenced the activity of a central enzyme of β-oxidation in response to high-fat feeding, it did not produce concerted changes in enzymes of glycolysis, citric acid cycle, or the ETC. Thus, an increase in EE in response to consumption of an HFD may be a mechanism that leads to decreased weight gain previously reported in ShcKO mice with long-term consumption of an HFD.

β-Hydroxybutyrate: A Signaling Metabolite

Various mechanisms in the mammalian body provide resilience against food deprivation and dietary stress. The ketone body β-hydroxybutyrate (BHB) is synthesized in the liver from fatty acids and represents an essential carrier of energy from the liver to peripheral tissues when the supply of glucose is too low for the body's energetic needs, such as during periods of prolonged exercise, starvation, or absence of dietary carbohydrates. In addition to its activity as an energetic metabolite, BHB is increasingly understood to have cellular signaling functions. These signaling functions of BHB broadly link the outside environment to epigenetic gene regulation and cellular function, and their actions may be relevant to a variety of human diseases as well as human aging.

PAX6 maintains β cell identity by repressing genes of alternative islet cell types

Type 2 diabetes is thought to involve a compromised β cell differentiation state, but the mechanisms underlying this dysfunction remain unclear. Here, we report a key role for the TF PAX6 in the maintenance of adult β cell identity and function. PAX6 was downregulated in β cells of diabetic db/db mice and in WT mice treated with an insulin receptor antagonist, revealing metabolic control of expression. Deletion of Pax6 in β cells of adult mice led to lethal hyperglycemia and ketosis that were attributed to loss of β cell function and expansion of α cells. Lineage-tracing, transcriptome, and chromatin analyses showed that PAX6 is a direct activator of β cell genes, thus maintaining mature β cell function and identity. In parallel, we found that PAX6 binds promoters and enhancers to repress alternative islet cell genes including ghrelin, glucagon, and somatostatin. Chromatin analysis and shRNA-mediated gene suppression experiments indicated a similar function of PAX6 in human β cells. We conclude that reduced expression of PAX6 in metabolically stressed β cells may contribute to β cell failure and α cell dysfunction in diabetes.

β-hydroxybutyrate: much more than a metabolite

The ketone body β-hydroxybutyrate (βOHB) is a convenient carrier of energy from adipocytes to peripheral tissues during fasting or exercise. However, βOHB is more than just a metabolite, having important cellular signaling roles as well. βOHB is an endogenous inhibitor of histone deacetylases (HDACs) and a ligand for at least two cell surface receptors. In addition, the downstream products of βOHB metabolism including acetyl-CoA, succinyl-CoA, and NAD+ (nicotinamide adenine dinucleotide) themselves have signaling activities. These regulatory functions of βOHB serve to link the outside environment to cellular function and gene expression, and have important implications for the pathogenesis and treatment of metabolic diseases including type 2 diabetes.

Ketone bodies as signaling metabolites

Traditionally, the ketone body β-hydroxybutyrate (βOHB) has been looked upon as a carrier of energy from liver to peripheral tissues during fasting or exercise. However, βOHB also signals via extracellular receptors and acts as an endogenous inhibitor of histone deacetylases (HDACs). These recent findings support a model in which βOHB functions to link the environment, in this case the diet, and gene expression via chromatin modifications. We review the regulation and functions of ketone bodies, the relationship between ketone bodies and calorie restriction, and the implications of HDAC inhibition by the ketone body βOHB in the modulation of metabolism and in diseases of aging.

An update on lysine deacylases targeting the expanding "acylome"

Lysine ε-amino acetylation has long been recognized as an epigenetically relevant post-translational modification of multiple residues in histone proteins. However, it has become clear that lysine acetylation is not restricted to histones, and therefore, it may be involved in the regulation of a wide variety of proteins, some of which have been identified and studied in detail. More recently, post-translational modifications of lysine side chains by additional acyl groups have also been identified, and some of these appear to be regulated by histone deacetylases (HDACs) and/or sirtuins. In this Concept, new developments are discussed with emphasis on the enzymes that have been shown to catalyze the cleavage of these novel marks, including new assays and inhibitors. Ultimately, a deeper understand of these mechanisms should facilitate the development of ligands with therapeutic potential.

SIRT5 regulates the mitochondrial lysine succinylome and metabolic networks

Reversible posttranslational modifications are emerging as critical regulators of mitochondrial proteins and metabolism. Here, we use a label-free quantitative proteomic approach to characterize the lysine succinylome in liver mitochondria and its regulation by the desuccinylase SIRT5. A total of 1,190 unique sites were identified as succinylated, and 386 sites across 140 proteins representing several metabolic pathways including β-oxidation and ketogenesis were significantly hypersuccinylated in Sirt5(-/-) animals. Loss of SIRT5 leads to accumulation of medium- and long-chain acylcarnitines and decreased β-hydroxybutyrate production in vivo. In addition, we demonstrate that SIRT5 regulates succinylation of the rate-limiting ketogenic enzyme 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2) both in vivo and in vitro. Finally, mutation of hypersuccinylated residues K83 and K310 on HMGCS2 to glutamic acid strongly inhibits enzymatic activity. Taken together, these findings establish SIRT5 as a global regulator of lysine succinylation in mitochondria and present a mechanism for inhibition of ketogenesis through HMGCS2.

Old enzymes, new tricks: sirtuins are NAD(+)-dependent de-acylases

Seven mammalian sirtuins are nicotinamide adenine dinucleotide (NAD)(+)-dependent deacetylases and are important modulators of energy metabolism and stress resistance. Two new studies by Du et al. (2011) and Peng et al. (2011) identify a new enzymatic activity for SIRT5, expanding the cellular repertoire of posttranslational modifications targeted by the sirtuins.

Proteomics analysis reveals diabetic kidney as a ketogenic organ in type 2 diabetes

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease. To date, the molecular mechanisms of DN remain largely unclear. The present study aimed to identify and characterize novel proteins involved in the development of DN by a proteomic approach. Proteomic analysis revealed that 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase 2 (HMGCS2), the key enzyme in ketogenesis, was increased fourfold in the kidneys of type 2 diabetic db/db mice. Consistently, the activity of HMGCS2 in kidneys and 24-h urinary excretion of the ketone body β-hydroxybutyrate (β-HB) were significantly increased in db/db mice. Immunohistochemistry, immunofluorescence, and real-time PCR studies further demonstrated that HMGCS2 was highly expressed in renal glomeruli of db/db mice, with weak expression in the kidneys of control mice. Because filtered ketone bodies are mainly reabsorbed in the proximal tubules, we used RPTC cells, a rat proximal tubule cell line, to examine the effect of the increased level of ketone bodies. Treating cultured RPTC cells with 1 mM β-HB significantly induced transforming growth factor-β1 expression, with a marked increase in collagen I expression. β-HB treatment also resulted in a marked increase in vimentin protein expression and a significant reduction in E-cadherin protein levels, suggesting an enhanced epithelial-to-mesenchymal transition in RPTCs. Collectively, these findings demonstrate that diabetic kidneys exhibit excess ketogenic activity resulting from increased HMGCS2 expression. Enhanced ketone body production in the diabetic kidney may represent a novel mechanism involved in the pathogenesis of DN.

Hypocaloric high-protein diet improves fatty liver and hypertriglyceridemia in sucrose-fed obese rats via two pathways

The mechanism by which replacement of some dietary carbohydrates with protein during weight loss favors lipid metabolism remains obscure. In this study, we investigated the effect of an energy-restricted, high-protein/low-carbohydrate diet on lipid metabolism in obese rats. High-sucrose-induced obese rats were assigned randomly to one of two energy-restricted dietary interventions: a carbohydrate-based control diet (CD) or a high-protein diet (HPD). Lean rats of the same age were assigned as normal control. There was significantly greater improvement in fatty liver and hypertriglyceridemia with the HPD diet relative to the CD diet. Expression of genes regulated by fibroblast growth factor-21 (FGF21) and involved in liver lipolysis and lipid utilitization, such as lipase and acyl-CoA oxidase, increased in obese rats fed the HPD. Furthermore, there was an inverse correlation between levels of FGF21 gene expression (regulated by glucagon/insulin balance) and increased triglyceride concentrations in liver from obese rats. Expression of hepatic stearoyl-CoA desaturase-1 (SCD1), regulated primarily by the dietary carbohydrate, was also markedly reduced in the HPD group (similar to plasma triglyceride levels in fasting animals) relative to the CD group. In conclusion, a hypocaloric high-protein diet improves fatty liver and hypertriglyceridemia effectively relative to a carbohydrate diet. The two cellular pathways at work behind these benefits include stimulation of hepatic lipolysis and lipid utilization mediated by FGF21 and reduction of hepatic VLDL-TG production by SCD1 regulation.

Comparative studies of early liver dysfunction in senescence-accelerated mouse using mitochondrial proteomics approaches

The liver is a complex and unique organ responsible for a breadth of functions crucial to sustaining life, especially for various metabolic processes in its mitochondria. Senescence-accelerated mouse prone/8 (SAMP8), a widely used aging model, exhibits an oxidative stress-induced aging phenotype and severe mitochondria-related liver pathology that are not seen in senescence-accelerated mouse resistant/1 (SAMR1). Here we used both two-dimensional electrophoresis- and ICAT-based mitochondrial proteomics analysis to view the liver mitochondrial protein alterations between SAMP8 and SAMR1. Compared with SAMR1, decreased expression and activity of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase were detected in SAMP8 at 6 months old (SAMP8-6m). As the key enzyme of ketogenesis, 3-hydroxy-3-methylglutaryl-CoA synthase is well known to be transcriptionally regulated by peroxisome proliferator-activated receptor alpha, which was also expressed at lower levels in SAMP8-6m livers. In addition, down-regulation of two peroxisome proliferator-activated receptor alpha target gene products (acyl-CoA oxidase and enoyl-CoA hydratase), elevation of triglyceride, and reduction of acetyl-CoA were observed, indicating abnormal fatty acid metabolism in SAMP8-6m livers. In addition eight proteins (NDUAA, NDUBA, NDUB7, NDUS1, NDUS3, NDUV1, ETFA, and UCRI) of mitochondrial complexes were down-regulated in SAMP8-6m, resulting in mitochondria-related liver dysfunction characterized by enhanced oxidative stress-induced molecular damage (lipid peroxide and oxidized protein) and depressed energy production (ATP). Glutamine synthetase and ornithine aminotransferase involved in glutamine synthesis were up-regulated in SAMP8 livers at both 1 and 6 months old that may be related to the accumulation of glutamate and glutamine. Our work provided useful clues to understanding the molecular mechanism underlying liver dysfunction in senescence-accelerated mouse.

Physical training reverses the increased activity of the hepatic ketone body synthesis pathway in chronically diabetic rats

This study was designed to examine whether the training-induced improvement in the plasma concentration of ketone bodies in experimental diabetes mellitus could be explained by changes in the activity of the hepatic ketone body synthesis pathway and/or the plasma free fatty acid levels. Diabetes mellitus was induced by an intravenous injection of streptozotocin (50 mg/kg), and training was carried out on a treadmill. The plasma concentration of beta-hydroxybutyric acid was increased (P < 0.001) in sedentary diabetic rats, and this was partly reversed by training (P < 0.001). The plasma concentration of free fatty acids was increased (P < 0.001) in sedentary diabetic rats, and this was reversed to normal by training (P < 0.001). Diabetes was also associated with an increased activity of the hepatic ketone body synthesis pathway. When the data are expressed as per total liver, physical training decreased the activity of the hepatic ketone body synthesis pathway by 18% in nondiabetic rats (P < 0.05) and by 22% in diabetic rats (P < 0.01), the activity present in trained diabetic rats being not statistically different from that of sedentary control rats. These data suggest that the beneficial effects of physical training on the plasma beta-hydroxybutyric acid levels in the diabetic state are probably explained in part by a decrease in the activity of the hepatic ketone body synthesis pathway and in part by a decrease in plasma free fatty acid levels.

Heme, iron, and the mitochondrial decay of ageing

Heme, the major functional form of iron, is synthesized in the mitochondria. Although disturbed heme metabolism causes mitochondrial decay, oxidative stress, and iron accumulation, all of which are hallmarks of ageing, heme has been little studied in nutritional deficiency, in ageing, or age-related disorders such as Alzheimer's disease (AD). Biosynthesis of heme requires Vitamin B(6), riboflavin, biotin, pantothenic acid, and lipoic acid and the minerals zinc, iron, and copper, micronutrients are essential for the production of succinyl-CoA, the precursor for porphyrins, by the TCA (Krebs) cycle. Only a small fraction of the porphyrins synthesized from succinyl-CoA are converted to heme, the rest are excreted out of the body together with the degradation products of heme (e.g. bilirubin). Therefore, the heme biosynthetic pathway causes a net loss of succinyl-CoA from the TCA cycle. The mitochondrial pool of succinyl-CoA may limit heme biosynthesis in deficiencies for micronutrients (e.g. iron or biotin deficiency). Ageing and AD are also associated with hypometabolism, increase in heme oxygenase-1, loss of complex IV, and iron accumulation. Heme is a common denominator for all these changes, suggesting that heme metabolism maybe altered in age-related disorders. Heme can also be a prooxidant: it converts less reactive oxidants to highly reactive free radicals. Free heme has high affinity for different cell structures (protein, membranes, and DNA), triggering site-directed oxidative damage. This review discusses heme metabolism as related to metabolic changes seen in ageing and age-related disorders and highlights the possible role in iron deficiency.

Identification of peroxisome proliferator-responsive human genes by elevated expression of the peroxisome proliferator-activated receptor alpha in HepG2 cells

In mice and other sensitive species, PPARalpha mediates the induction of mitochondrial, microsomal, and peroxisomal fatty acid oxidation, peroxisome proliferation, liver enlargement, and tumors by peroxisome proliferators. In order to identify PPARalpha-responsive human genes, HepG2 cells were engineered to express PPARalpha at concentrations similar to mouse liver. This resulted in the dramatic induction of mRNAs encoding the mitochondrial HMG-CoA synthase and increases in fatty acyl-CoA synthetase (3-8-fold) and carnitine palmitoyl-CoA transferase IA (2-4-fold) mRNAs that were dependent on PPARalpha expression and enhanced by exposure to the PPARalpha agonist Wy14643. A PPAR response element was identified in the proximal promoter of the human HMG-CoA synthase gene that is functional in its native context. These data suggest that humans retain a capacity for PPARalpha regulation of mitochondrial fatty acid oxidation and ketogenesis. Human liver is refractory to peroxisome proliferation, and increased expression of mRNAs for the peroxisomal fatty acyl-CoA oxidase, bifunctional enzyme, or thiolase, which accompanies peroxisome proliferation in responsive species, was not evident following Wy14643 treatment of cells expressing elevated levels of PPARalpha. Additionally, no significant differences were seen for the expression of apolipoprotein AI, AII, or CIII; medium chain acyl-CoA dehydrogenase; or stearoyl-CoA desaturase mRNAs.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase: a control enzyme in ketogenesis

Cytosolic and mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthases were first recognized as different chemical entities in 1975, when they were purified and characterized by Lane's group. Since then, the two enzymes have been studied extensively, one as a control site of the cholesterol biosynthetic pathway and the other as an important control site of ketogenesis. This review describes some key developments over the last 25 years that have led to our current understanding of the physiology of mitochondrial HMG-CoA synthase in the HMG-CoA pathway and in ketogenesis in the liver and small intestine of suckling animals. The enzyme is regulated by two systems: succinylation and desuccinylation in the short term, and transcriptional regulation in the long term. Both control mechanisms are influenced by nutritional and hormonal factors, which explains the incidence of ketogenesis in diabetes and starvation, during intense lipolysis, and in the foetal-neonatal and suckling-weaning transitions. The DNA-binding properties of the peroxisome-proliferator-activated receptor and other transcription factors on the nuclear-receptor-responsive element of the mitochondrial HMG-CoA synthase promoter have revealed how ketogenesis can be regulated by fatty acids. Finally, the expression of mitochondrial HMG-CoA synthase in the gonads and the correction of auxotrophy for mevalonate in cells deficient in cytosolic HMG-CoA synthase suggest that the mitochondrial enzyme may play a role in cholesterogenesis in gonadal and other tissues.

Hepatic mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme a synthase deficiency

There are at least two isoenzymes of 3-hydroxy-3-methylglutaryl (HMG)-CoA synthase (EC 4.1.3.5) located in the mitochondrial matrix and the cytoplasm of hepatocytes, respectively. The mitochondrial enzyme is necessary for the synthesis of ketone bodies, which are important fuels during fasting. We report a child with a deficiency of this isoenzyme. He presented at 16 mo with hypoglycemia. There was no rise in ketone bodies during fasting or after a long chain fat load but there was a small rise after a leucine load. Measurement of beta-oxidation flux in fibroblasts was normal. Using antibodies specific for mitochondrial HMG-CoA synthase, no immunoreactive material could be detected on Western blotting. Total HMG-CoA synthase activity in liver homogenate was only slightly lower than in control samples. Presumably, as there was no mitochondrial HMG-CoA synthase enzyme protein, this activity arose from the cytoplasmic or other (e.g. peroxisomal) isoenzymes. With avoidance of fasting, our patient has had no problems since presentation and is developing normally at 4 y of age.

Evidence for the interaction of avian 3-hydroxy-3-methylglutaryl-CoA synthase histidine 264 with acetoacetyl-CoA

Previous work on HMG-CoA synthase has implied the presence of a reactive active site histidine, prompting our examination of the possible function of invariant histidine residues by site-directed mutagenesis. Mutations encoding H197N, H264N/A, and H436N HMG-CoA synthases were constructed, and the mutant enzymes were overexpressed in Escherichia coli BL21(DE3). Kinetic characterization of the isolated synthase variants indicates that, while H197N and H436N enzymes behave similarly to wild-type synthase, H264N and H264A synthases exhibit significant differences. Although the k(m) for acetyl-CoA is not substantially altered, H264N/A synthases catalyze production of HMG-CoA at a diminished (approximately 25-fold slower) rate. In contrast, H264N/A synthases can efficiently catalyze the acetyl-CoA hydrolysis partial reaction exhibiting a k(m) for acetyl-CoA that, again, approximates the value obtained with the wild-type enzyme. These mutants also retain the ability to form significant levels of the acetyl-S-enzyme reaction intermediate. The functional catalysis of partial reactions argues that the H264 mutant proteins retain substantial structural integrity. In this context, it appears significant that the H264N/A synthases exhibit a approximately 100-fold increase in the k(m) for acetoacetyl-CoA. In order to test whether the two orders of magnitude effect may be largely attributed to a decreased affinity of acetoacetyl-CoA for these enzymes and, more specifically, whether H264 interacts with the carbonyl oxygen of acetoacetyl-CoA's thioester, turnover of S-(3-oxobutyl)-CoA, a thioether analog of acetoacetyl-CoA, was investigated. This alternative substrate, in which a methylene group replaces the thioester carbonyl, is utilized by wild-type synthase with an apparent Vmax that is approximately 100-fold lower and an apparent k(m) that is 25-fold higher than the values obtained using the physiological substrate, acetoacetyl-CoA. H264A synthase also catalyzes the turnover of S-(3-oxobutyl)-CoA; the diminution in rate supported by the alternative substrate is comparable in magnitude to the effect observed for wild-type enzyme. In contrast, H264A exhibits comparable apparent k(m) values for S-(3-oxobutyl)-CoA and acetoacetyl-CoA. Thus, unlike wild-type synthase, there is no penalty in terms of efficiency of H264A saturation when the alternative thioether substrate replaces the physiological substrate. These data suggest that the imidazole of H264 in avian enzyme may play a role in anchoring the second substrate, acetoacetyl-CoA, by interacting with the carbonyl oxygen of the thioester functionality.

Immunolocalization of mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase in rat liver

We report the preparation of specific polyclonal antibodies raised against two synthetic peptides deduced from the cDNA sequence for the rat liver mitochondrial 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) synthase gene. Immunoelectron microscopy using these antibodies on hepatic cryoultrathin sections confirms the mitochondrial localization of this protein in hepatocytes. Immunofluorescence microscopy on frozen sections of adult rat liver revealed fluorescence inside all hepatocytes, with no evidence of zonation, indicating that ketogenesis may not be limited to specific regions of rat liver but is extended to all hepatocytes.

Gene expression of enzymes regulating ketogenesis and fatty acid metabolism in regenerating rat liver

Levels of mRNA for mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, carnitine palmitoyltransferase I (CPT I) and carnitine palmitoyltransferase II (CPT II), fatty acid synthase (FAS) and actin were analysed during liver regeneration. mRNA levels for mitochondrial HMG-CoA synthase decreased rapidly, reaching a minimum 12 h after partial hepatectomy and returning to normal at 24-36 h. In contrast, CPT I, CPT II and FAS mRNAs increased throughout the period examined. Expression of actin increased significantly during regeneration. Levels of mRNA for mitochondrial HMG-CoA synthase also decreased as a result of surgical stress, although the effect of hepatectomy was much greater. We determined the levels of mitochondrial HMG-CoA synthase using specific antibodies. The amount of protein rapidly decreased, although less markedly than the corresponding mRNA levels. These results show that the decrease described in ketogenesis in partially hepatectomized rats correlated with the decrease in the expression of mitochondrial HMG-CoA synthase, suggesting that this enzyme may also be a control point in ketogenesis in the regenerating liver, as it is in normal and diabetic rats.

Hepatic ketogenesis in newborn pigs is limited by low mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase activity

In newborn-pig hepatocytes, the rate of oleate oxidation is extremely low, despite a very low malonyl-CoA concentration. By contrast, the sensitivity of carnitine palmitoyltransferase (CPT) I to malonyl-CoA inhibition is high, as suggested by the very low concentration of malonyl-CoA required for 50% inhibition of CPT I (IC50). The rates of oleate oxidation and ketogenesis are respectively 70 and 80% lower in mitochondria isolated from newborn-pig liver than from starved-adult-rat liver mitochondria. Using polarographic measurements, we showed that the oxidation of oleoyl-CoA and palmitoyl-L-carnitine is very low when the acetyl-CoA produced is channelled into the hydroxymethylglutaryl-CoA (HMG-CoA) pathway by addition of malonate. In contrast, the oxidation of the same substrates is high when the acetyl-CoA produced is directed towards the citric acid cycle by addition of malate. We demonstrate that the limitation of ketogenesis in newborn-pig liver is due to a very low amount and activity of mitochondrial HMG-CoA synthase as compared with rat liver mitochondria, and suggest that this could promote the accumulation of acetyl-CoA and/or beta-oxidation products that in turn would decrease the overall rate of fatty acid oxidation in newborn- and adult-pig livers.

Methylation of the regulatory region of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene leads to its transcriptional inactivation

The mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene is expressed in a limited set of tissues in the adult rat. Methylation of the 5' flanking region of the gene in vitro leads to its transcriptional inactivation when transfected in hepatoma-derived cell lines. In liver and kidney, expression of the gene correlates inversely with its degree of methylation, indicating that the methylation of the 5' flanking region and the first exon of the gene may be one of the factors responsible for the repression of its transcription. During the fetal/neonatal transition, a process of selective undermethylation of specific sites takes place in the 5' flanking region of the mitochondrial HMG-CoA synthase gene. Moreover, treatment with the hypomethylating agent 5-azacytidine of a hepatoma-derived cell line that presents barely detectable levels of mitochondrial HMG-CoA synthase mRNA leads to a significant increase in the mRNA levels. These results point to methylation as one of the regulatory mechanisms that operate on the mitochondrial HMG-CoA synthase gene.

The rat mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme-A-synthase gene contains elements that mediate its multihormonal regulation and tissue specificity

Mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme-A (HMG-CoA) synthase, a liver-specific enzyme, is a constituent of the HMG-CoA cycle responsible for ketone-body synthesis. We report the isolation and characterization of genomic clones that encompass the gene for rat mitochondrial HMG-CoA synthase. The gene spans at least 24 kbp and contains ten exons and nine introns. The 5' flanking region of the gene has also been cloned and characterized. Exon 1 contains the untranslated sequence of the transcript, extending downstream to enclose the coding region for the putative mitochondrial-targeting signal (35 amino acids). The 1149-bp proximal region of the transcription start point permits transcription of a reporter gene in transfected hepatoma cells but not in an extrahepatic cell line, confirming the function of the promoter. A truncated construct of 142 bp is still able to promote transcription in hepatoma cells, suggesting the presence of liver-specific enhancer elements in the proximal promoter region. The 5' flanking region contains typical promoter elements, including a TATA box and several putative recognition sequences for transcription factors involved in controlling both basal-level and hormone-modulated transcription rates. Furthermore, the presence in the mitochondrial HMG-CoA-synthase promoter of cis-elements, responsible for the multihormonal regulation of transcription, is supported by transient transfection experiments.

A top-down control analysis in isolated rat liver mitochondria: can the 3-hydroxy-3-methylglutaryl-CoA pathway be rate-controlling for ketogenesis?

We incubated isolated liver mitochondria with palmitoyl-CoA, 2,4-dinitrophenol and malonate. Under these conditions all the flux of carbon from palmitoyl-CoA was directed towards acetoacetate synthesis. We measured the rate of acetyl-CoA formation from palmitoyl-CoA (by measuring the rate of oxygen consumption) and the rate of acetoacetate production from acetyl-CoA at three different acetyl-CoA/CoA ratios. Using the top-down approach of metabolic control analysis we calculated the control over ketogenesis exerted by (a) the conversion of extramitochondrial palmitoyl-CoA to intramitochondrial acetyl-CoA and by (b) the conversion of acetyl-CoA to acetoacetate (the 'HMG-CoA pathway'). The overall flux control coefficients of the groups of enzymes involved in (a) and (b) over ketogenesis were 0.28 and 0.72, respectively. Our results show that it is possible for significant control to be exerted over ketogenesis by the enzymes of the HMG-CoA pathway.

Structural characterization of the 3' noncoding region of the gene encoding rat mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase

We report the structural analysis of the 3' noncoding region of the rat mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase-encoding gene. This region, encoded by a single exon of the gene, spans 410 nucleotides. By using S1-nuclease protection analysis, we have mapped two major alternative polyadenylation signals, in close proximity. Computer analysis of the 3' noncoding region has detected the existence of four inverted repeats that could form stem-and-loop structures, with the thermodynamic free energy of formation ranging from -12.2 kcal/mol to -20.4 kcal/mol. The putative regulatory role of these structural features in mRNA stability is discussed.

Monitoring of changes in hepatic fatty acid and glycerolipid metabolism during the starved-to-fed transition in vivo. Studies on awake, unrestrained rats

1. The technique of selective labelling of hepatic fatty acids in vivo [Moir and Zammit (1992) Biochem. J. 283, 145-149] has been used to monitor non-invasively the metabolism of fatty acids in the livers of awake unrestrained rats during the starved-to-refed transition. Values for the incorporation of labelled fatty acid into liver and plasma glycerolipids and into exhaled carbon dioxide after injection of labelled lipoprotein and Triton WR 1339 into rats with chronically cannulated jugular veins were obtained for successive 1 h periods from the start of refeeding of 24 h-starved rats. 2. Starvation for 24 h resulted in marked and reciprocal changes in the incorporation of label into glycerolipids and exhaled 14CO2, such that a 4-fold higher value was obtained for the oxidation/esterification ratio in livers of starved rats compared with fed animals. 3. Refeeding of starved rats did not return this ratio to the value observed for fed animals for at least 7 h; during the first 3 h of refeeding the ratio was at least as high as that for starved rats. Between 4 h and 6 h of refeeding the ratio was still approx. 70% of that in starved animals, and 2.5-fold higher than in fed rats. 4. These data support the hypothesis that the capacity of the liver to oxidize fatty acids is maintained at a high level during the initial stages of refeeding [Grantham and Zammit (1986) Biochem. J. 239, 485-488] and that control of the flux of hepatic fatty acids into the oxidative pathway is largely lost from the reaction catalysed by mitochondrial overt carnitine palmitoyltransferase (CPT I) during this phase of recovery from the starved state. 5. Refeeding also resulted in a rapid (< 1 h) increase in hepatic malonyl-CoA concentrations to values intermediate between those in livers of fed and starved animals. The sensitivity of CPT I to malonyl-CoA inhibition in isolated liver mitochondria was only partially reversed even after 5 h of refeeding. 6. Refeeding resulted in an acute 35% inhibition of the fraction of synthesized triacylglycerol that was secreted into the plasma; the maximal effect occurred 2-3 h after the start of refeeding. The inhibition of the fractional secretion rate was fully reversed after 5 h of refeeding. 7. The amount of 14C label that was incorporated into phospholipids as a fraction of total glycerolipid synthesis was doubled within 2 h of the start of refeeding.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of the expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene. Its role in the control of ketogenesis

We have explored the role of mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase in regulating ketogenesis. We had previously cloned the cDNA for mitochondrial HMG-CoA synthase and have now studied the regulation in vivo of the expression of this gene in rat liver. The amount of processed mitochondrial HMG-CoA synthase mRNA is rapidly changed in response to cyclic AMP, insulin, dexamethasone and refeeding, and is greatly increased by starvation, fat feeding and diabetes. We conclude that one point of ketogenic control is exercised at the level of genetic expression of mitochondrial HMG-CoA synthase.

Effect of insulin on the properties of liver carnitine palmitoyltransferase in the starved rat: assessment by the euglycemic hyperinsulinemic clamp

The effect of insulin on the properties of liver carnitine palmitoyltransferase I (CPT I) was assessed in conscious starved rats with the euglycemic hyperinsulinemic clamp. A 24-hour clamp was necessary to fully reverse the effect of starvation on liver malonyl-CoA concentration, CPT I maximal activity, and apparent km and Ki for malonyl-CoA. Since glucagon was not decreased during the clamp, insulin is the major factor involved in the regulation of CPT I.

Relationship between the stimulation of citric acid cycle oxidation and the stimulation of fatty acid esterification and inhibition of ketogenesis by lactate in isolated rat hepatocytes

Isolated hepatocytes from fasted rats were used to study the effects of lactate on palmitate metabolism. Lactate was found to stimulate fatty acid esterification and citric acid cycle oxidation and to inhibit ketone body synthesis. These effects of lactate were largely maintained when gluconeogenesis was inhibited with either quinolinate or perfluorosuccinate, but were overcome by alpha-cyano-4-hydroxycinnamate. However, the responses of hepatocytes to lactate could be restored in the presence of alpha-cyano-4-hydroxycinnamate by the further addition of propionate. The stimulation of triacylglycerol synthesis by lactate was not associated with an increase in the concentration of glycerol 3-phosphate. Rather, there was a correlation between flux through the citric acid cycle and the rate of triacylglycerol synthesis. In all instances reduction of ketone body formation in the presence of lactate was accompanied by a stimulation of citric acid cycle oxidation.

Control of hepatic mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase during the foetal/neonatal transition, suckling and weaning in the rat

(1) We assayed active and total (i.e. active plus succinylated) 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase in mitochondria isolated from foetal, neonatal, suckling or weaned rats. (2) HMG-CoA synthase was substantially succinylated and inactivated in mitochondria isolated from term-foetal, (1-h-old, 6-h-old, 1-day-old) neonatal, suckling and high carbohydrate/low-fat (hc)-weaned rats. Succinylation of HMG-CoA synthase was very low in mitochondria isolated from the livers of foetal, 30-min-old neonatal and high-fat/carbohydrate-free (hf)-weaned rats. (3) There was a negative correlation between active HMG-CoA synthase and succinyl-CoA content in mitochondria isolated from term-foetal, suckling and hc-weaned rats. (4) Differences in active enzyme could not be entirely accounted for by differences in succinylation and inactivation of the synthase. Immunoassay confirmed that the absolute amounts of mitochondrial HMG-CoA synthase increased during the foetal/neonatal transition and decreased with hc weaning. The levels remained elevated with hf weaning. (5) From these data we propose that mitochondrial HMG-CoA synthase is controlled by two different mechanisms in young rats. Regulation by succinylation provides a mechanism for rapid modification of existing enzyme in response to changing metabolic states. Changes in the absolute amounts of HMG-CoA synthase provide a more long-term control in response to nutritional changes.

Expression of hepatic mitochondrial carbonic anhydrase V

We have raised specific (rabbit anti-rat) polyclonal antibodies to hepatic mitochondrial carbonic anhydrase V (CA V) and used them to assay the amounts of protein expressed in liver mitochondria isolated from term-foetal, control or diabetic adult rats and in perivenous and periportal rat hepatocytes. The levels of CA V expressed in mitochondria isolated from the livers of adult male and female rats are similar and increase (about 2-fold) in mitochondria from adult diabetic rats when compared to those isolated from the livers of control rats. The level of enzyme in adult liver was higher than in the livers of term-foetal rats. CA V is expressed in both perivenous and periportal hepatocytes, but the level of expression is greater (approx. 40%) in perivenous cells. The implications and significance of these findings are discussed with reference to the roles and properties of the other carbonic anhydrase isoenzymes and the metabolic function of the mitochondrial isoenzyme.

Mapping of reactive sulfhydryls in avian liver 3-hydroxy-3-methylglutaryl coenzyme A synthase

Avian liver mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase contains seven sulfhydryls per 53 kDa subunit. Peptides that harbor these sulfhydryls can be mapped by reverse-phase HPLC separation of tryptic digests of denatured 14C-carboxymethylated enzyme. Native enzyme is inactivated by a variety of reagents that target cysteine residues. Of particular interest is the enzyme's sensitivity to reagents (e.g., CdCl2, copper phenanthroline) that target vicinal thiols. The identity of the cysteines which are modified by these reagents can be determined by peptide mapping after denaturation. 14C-carboxymethylation and trypsin digestion of the sample. While the extent of reaction of any particular cysteinyl sulfhydryl depends on the identity of the reagent employed, three of the protein's seven cysteinyl sulfhydryls are frequently modified upon inactivation of the enzyme. The peptides which contain these reactive sulfhydryls have been isolated and their sequences have been determined by Edman degradation techniques. Comparison of these sequences with the deduced primary structure of the rodent cytosolic enzyme (Gil et al. (1986) J. Biol. Chem. 261, 3710) indicates strong homologies. These homologies allow assignment of the reactive residues as Cys-129, Cys-224 and Cys-268. The sensitivity of these residues to reagents that target vicinal thiols, coupled with the fact that cys-129 is the residue involved in formation of the acyl-S-enzyme intermediate (Vollmer et al. (1988) Biochemistry 27, 4288), suggests that these three residues may be closely juxtaposed within the enzyme's catalytic domain.