Glucagon activates mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase in vivo by decreasing the extent of succinylation of the enzyme

Protein succinylation associated with the progress of hepatocellular carcinoma

Although post‐translational modification is critical to tumorigenesis, how succinylation modification of lysine sites influences hepatocellular carcinoma (HCC) remains obscure. 90 tumours and paired adjacent normal tissue of liver cancer were enrolled for succinylation staining. 423 HCC samples with 20 genes related to succinylation modification from TCGA were downloaded for model construction. Statistical methods were employed to analyse the data, including the Non‐Negative Matrix Factorization (NMF) algorithm, t‐Distributed Stochastic Neighbour Embedding (t‐SNE) algorithm, and Cox regression analysis. The staining pan‐succinyllysine antibody staining indicated that tumour tissues had a higher succinyllysine level than adjacent tissues (p < 0.001), which could be associated with a worse prognosis (p = 0.02). The survival was associated with pathological stage, tumour recurrence status and succinyllysine intensity in the univariate or multivariable cox survival analysis model. The risk model from 20 succinyllysine‐related genes had the best prognosis prediction. The high expression of succinylation modification in HCC contributed to the worse patient survival prognosis. Model construction of 20 genes related to succinylation modification (MEAF6, OXCT1, SIRT2, CREBBP, KAT5, SIRT4, SIRT6, SIRT7, CPT1A, GLYATL1, SDHA, SDHB, SDHC, SDHD, SIRT1, SIRT3, SIRT5, SUCLA2, SUCLG1 and SUCLG2) could be reliable in predicting prognosis in HCC.

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.

Beta-Hydroxybutyrate: A Dual Function Molecular and Immunological Barrier Function Regulator

Ketone bodies are crucial intermediate metabolites widely associated with treating metabolic diseases. Accumulating evidence suggests that ketone bodies may act as immunoregulators in humans and animals to attenuate pathological inflammation through multiple strategies. Although the clues are scattered and untrimmed, the elevation of these ketone bodies in the circulation system and tissues induced by ketogenic diets was reported to affect the immunological barriers, an important part of innate immunity. Therefore, beta-hydroxybutyrate, a key ketone body, might also play a vital role in regulating the barrier immune systems. In this review, we retrospected the endogenous ketogenesis in animals and the dual roles of ketone bodies as energy carriers and signal molecules focusing on beta-hydroxybutyrate. In addition, the research regarding the effects of beta-hydroxybutyrate on the function of the immunological barrier, mainly on the microbiota, chemical, and physical barriers of the mucosa, were outlined and discussed. As an inducible endogenous metabolic small molecule, beta-hydroxybutyrate deserves delicate investigations focusing on its immunometabolic efficacy. Comprehending the connection between ketone bodies and the barrier immunological function and its underlining mechanisms may help exploit individualised approaches to treat various mucosa or skin-related diseases.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Succinylation Links Metabolism to Protein Functions

Post-translational modifications (PTMs) are important regulators of protein function, and integrate metabolism with physiological and pathological processes. Phosphorylation and acetylation are particularly well studied PTMs. A relatively recently discovered novel PTM is succinylation in which metabolically derived succinyl CoA modifies protein lysine groups. Succinylation causes a protein charge flip from positive to negative and a relatively large increase in mass compared to other PTMs. Hundreds of protein succinylation sites are present in proteins of multiple tissues and species, and the significance is being actively investigated. The few completed studies demonstrate that succinylation alters rates of enzymes and pathways, especially mitochondrial metabolic pathways. Thus, succinylation provides an elegant and efficient mechanism to coordinate metabolism and signaling by utilizing metabolic intermediates as sensors to regulate metabolism. Even though the brain is one of the most metabolically active organs, an understanding of the role succinylation in the nervous system is largely unknown. Data from other tissues and other PTMs suggest that succinylation provides a coupling between metabolism and protein function in the nervous system and in neurological diseases. This review provides a new insight into metabolism in neurological diseases and suggests that the drug development for these diseases requires a better understanding of succinylation and de-succinylation in the brain and other tissues.

Effect of Saccharomyces cerevisiae fermentation product on feed intake parameters, lactation performance, and metabolism of transition dairy cattle

The transition period in dairy cattle is characterized by many stressors, including an abrupt diet change, but yeast product supplementation can alter the rumen environment to increase dairy cattle productivity. Saccharomyces cerevisiae fermentation product (SCFP) was fed from -29 ± 5 to 42 d relative to calving (RTC) to evaluate the effects on feed intake, milk production, and metabolism. Treatments were control (n = 30) or SCFP (n = 34) incorporated into a total mixed ration. Cows were individually fed 3×/d prepartum and 2×/d postpartum. Blood samples were collected once during each of the following time points RTC: d -28 to -24 (wk -4), d -14 to -10 (wk -2), d 3 to 7 (wk 1), d 12 to 16 (wk 2), and d 31 to 35 (wk 5). Liver biopsies were taken once between d -19 and d -12 (wk -3) and at 14 d in milk. Cows were milked 2×/d, and samples were taken 2 d/wk for composition analysis. Dry matter intake did not differ by treatment, but SCFP increased meals per day and decreased time between meals. Body weight (measured at enrollment, d 0, and d 42 RTC) and body condition score (scored weekly) were not affected by treatment. Milk, energy-corrected milk, and fat-corrected milk yields did not differ by treatment. Milk fat concentration was greater for SCFP, with significant differences in wk 4 and 5. Milk lactose concentration tended to be greater for the control and milk urea nitrogen tended to be lesser for the control, but there were no treatment effects on milk protein concentration or somatic cell count. Assuming equal digestibility, energy balance deficit was greater for SCFP than for the control (-6.15 vs. -4.34 ± 0.74 Mcal/d), with significant differences in wk 4 and 5. Plasma concentrations of free fatty acids, β-hydroxybutyrate, glucose, and insulin did not differ with treatment, but cholesterol was greater for SCFP. Liver triglyceride increased and liver cholesterol decreased with time. Liver triglyceride did not differ by treatment, but liver cholesterol tended to be lesser in SCFP. Relative mRNA abundance of cholesterol-related genes (SREBF2, HMGCS1, HMGCR, MTTP, SPOB100, APOA1), FGF21, and CPT1A did not differ by treatment, but PCK1 tended to be greater for SCFP. The ketogenic transcript HMGCS2 was greater for SCFP, which aligns with SCFP increasing incidence of subclinical ketosis; however, BDH did not differ between treatments. In conclusion, SCFP supplementation increased meals per day with less time between meals, increased milk fat concentration, altered cholesterol metabolism, and increased incidence of subclinical ketosis, but early-lactation milk yield and metabolism were generally unaffected.

Team Players or Opponents: Coadministration of Selective Glucagon and GLP-1 Receptor Agonists in Obese Diabetic Monkeys

We assessed the therapeutic contribution of the individual components of glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR) agonists alone and in combination upon energy homeostasis and glycemic control in diet-induced obese, diabetic nonhuman primates. The pharmacological active dose ranges of selective agonists were established through a dose-finding study, followed by a 6-week chronic study. Repeated subcutaneous administration of a selective GCGR agonist (30 µg/kg once daily) did not affect food intake or body weight, whereas the selective GLP-1R agonist (3 µg/kg once daily) alone decreased energy intake by 18% and body weight by 3.8% ± 0.9%. Combination of both agonists reduced significantly cumulative food intake by 27% and body weight by 6.6% ± 0.9%. Fasting plasma glucose (FPG) was improved by GLP-1R agonist (baseline vs end of study, 176.7 ± 34.0 vs 115.9 ± 16.1 mg/dL). In contrast, groups exposed to GCGR agonist experienced nonsignificant elevations of FPG. More accurate assessment of therapeutic interventions on glucose homeostasis was tested by an IV glucose tolerance test. Glucose excursion was significantly elevated by chronic GCGR agonist administration, whereas it was significantly decreased in GLP-1R agonist-treated monkeys. In the combination group, a nonsignificant increase of glucose excursion was seen, concomitantly with significantly increased insulin secretion. We conclude that chronic glucagon agonism does not affect energy homeostasis in nonhuman primates. In combination with GLP-1R agonism, glucagon agonism synergistically enhances negative energy balance with resulting larger body weight loss. However, adding GCGR to GLP-1R agonism diminishes glycemic control in diabetic monkeys. Therefore, long-term therapeutic implications of using GLP-1R/GCGR coagonists for weight management in diabetes warrants further scrutiny.

Genome Wide Association Study Identifies the HMGCS2 Locus to be Associated With Chlorthalidone Induced Glucose Increase in Hypertensive Patients

Background

Thiazide and thiazide‐like diuretics are first‐line medications for treating uncomplicated hypertension. However, their use has been associated with adverse metabolic events, including hyperglycemia and incident diabetes mellitus, with incompletely understood mechanisms. Our goal was to identify genomic variants associated with thiazide‐like diuretic/chlorthalidone‐induced glucose change.

Methods and Results

Genome‐wide analysis of glucose change after treatment with chlorthalidone was performed by race among the white (n=175) and black (n=135) participants from the PEAR‐2 (Pharmacogenomic Evaluation of Antihypertensive Responses‐2). Single‐nucleotide polymorphisms with P<5×10⁻⁸ were further prioritized using in silico analysis based on their expression quantitative trait loci function. Among blacks, an intronic single‐nucleotide polymorphism (rs9943291) in the HMGCS2 was associated with increase in glucose levels following chlorthalidone treatment (ß=12.5; P=4.17×10⁻⁸). G‐allele carriers of HMGCS2 had higher glucose levels (glucose change=+16.29 mg/dL) post chlorthalidone treatment compared with noncarriers of G allele (glucose change=+2.80 mg/dL). This association was successfully replicated in an independent replication cohort of hydrochlorothiazide‐treated participants from the PEAR study (ß=5.54; P=0.023). A meta‐analysis of the 2 studies was performed by race in Meta‐Analysis Helper, where this single‐nucleotide polymorphism, rs9943291, was genome‐wide significant with a meta‐analysis P value of 3.71×10⁻⁸. HMGCS2, a part of the HMG‐CoA synthase family, is important for ketogenesis and cholesterol synthesis pathways that are essential in glucose homeostasis.

Conclusions

These results suggest that HMGCS2 is a promising candidate gene involved in chlorthalidone and Hydrochlorothiazide (HCTZ)‐induced glucose change. This may provide insights into the mechanisms involved in thiazide‐induced hyperglycemia that may ultimately facilitate personalized approaches to antihypertensive selection for hypertension treatment.

Clinical Trial Registration

URL: http://www.clinicaltrials.gov. Unique identifiers: NCT00246519 and NCT01203852.

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.

Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones

Fatty liver can be diet, endocrine, drug, virus or genetically induced. Independent of cause, hepatic lipid accumulation promotes systemic metabolic dysfunction. By acting as peroxisome proliferator-activated receptor (PPAR) ligands, hepatic non-esterified fatty acids upregulate expression of gluconeogenic, beta-oxidative, lipogenic and ketogenic genes, promoting hyperglycemia, hyperlipidemia and ketosis. The typical hormonal environment in fatty liver disease consists of hyperinsulinemia, hyperglucagonemia, hypercortisolemia, growth hormone deficiency and elevated sympathetic tone. These endocrine and metabolic changes further encourage hepatic steatosis by regulating adipose tissue lipolysis, liver lipid uptake, de novo lipogenesis (DNL), beta-oxidation, ketogenesis and lipid export. Hepatic lipid accumulation may be induced by 4 separate mechanisms: (1) increased hepatic uptake of circulating fatty acids, (2) increased hepatic de novo fatty acid synthesis, (3) decreased hepatic beta-oxidation and (4) decreased hepatic lipid export. This review will discuss the hormonal regulation of each mechanism comparing multiple physiological models of hepatic lipid accumulation. Nonalcoholic fatty liver disease (NAFLD) is typified by increased hepatic lipid uptake, synthesis, oxidation and export. Chronic hepatic lipid signaling through PPARgamma results in gene expression changes that allow concurrent activity of DNL and beta-oxidation. The importance of hepatic steatosis in driving systemic metabolic dysfunction is highlighted by the common endocrine and metabolic disturbances across many conditions that result in fatty liver. Understanding the mechanisms underlying the metabolic dysfunction that develops as a consequence of hepatic lipid accumulation is critical to identifying points of intervention in this increasingly prevalent disease state.

Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics

Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations.

Regulation of Ketone Body Metabolism and the Role of PPARα

Ketogenesis and ketolysis are central metabolic processes activated during the response to fasting. Ketogenesis is regulated in multiple stages, and a nuclear receptor peroxisome proliferator activated receptor α (PPARα) is one of the key transcription factors taking part in this regulation. PPARα is an important element in the metabolic network, where it participates in signaling driven by the main nutrient sensors, such as AMP-activated protein kinase (AMPK), PPARγ coactivator 1α (PGC-1α), and mammalian (mechanistic) target of rapamycin (mTOR) and induces hormonal mediators, such as fibroblast growth factor 21 (FGF21). This work describes the regulation of ketogenesis and ketolysis in normal and malignant cells and briefly summarizes the positive effects of ketone bodies in various neuropathologic conditions.

Evidence That in Uncontrolled Diabetes, Hyperglucagonemia Is Required for Ketosis but Not for Increased Hepatic Glucose Production or Hyperglycemia

Several lines of evidence implicate excess glucagon secretion in the elevated rates of hepatic glucose production (HGP), hyperglycemia, and ketosis characteristic of uncontrolled insulin-deficient diabetes (uDM), but whether hyperglucagonemia is required for hyperglycemia in this setting is unknown. To address this question, adult male Wistar rats received either streptozotocin (STZ) to induce uDM (STZ-DM) or vehicle and remained nondiabetic. Four days later, animals received daily subcutaneous injections of either the synthetic GLP-1 receptor agonist liraglutide in a dose-escalating regimen to reverse hyperglucagonemia or its vehicle for 10 days. As expected, plasma glucagon levels were elevated in STZ-DM rats, and although liraglutide treatment lowered glucagon levels to those of nondiabetic controls, it failed to attenuate diabetic hyperglycemia, elevated rates of glucose appearance (Ra), or increased hepatic gluconeogenic gene expression. In contrast, it markedly reduced levels of both plasma ketone bodies and hepatic expression of the rate-limiting enzyme involved in ketone body production. To independently confirm this finding, in a separate study, treatment of STZ-DM rats with a glucagon-neutralizing antibody was sufficient to potently lower plasma ketone bodies but failed to normalize elevated levels of either blood glucose or Ra. These data suggest that in rats with uDM, hyperglucagonemia is required for ketosis but not for increased HGP or hyperglycemia.

Alpha-ketoglutarate dehydrogenase complex-dependent succinylation of proteins in neurons and neuronal cell lines

Reversible post-translation modifications of proteins are common in all cells and appear to regulate many processes. Nevertheless, the enzyme(s) responsible for the alterations and the significance of the modification are largely unknown. Succinylation of proteins occurs and causes large changes in the structure of proteins; however, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remain unknown. These studies focused on succinylation of mitochondrial proteins. The results demonstrate that the α-ketoglutarate dehydrogenase complex (KGDHC) can serve as a trans-succinylase that mediates succinylation in an α-ketoglutarate-dependent manner. Inhibition of KGDHC reduced succinylation of both cytosolic and mitochondrial proteins in cultured neurons and in a neuronal cell line. Purified KGDHC can succinylate multiple proteins including other enzymes of the tricarboxylic acid cycle leading to modification of their activity. Inhibition of KGDHC also modifies acetylation by modifying the pyruvate dehydrogenase complex. The much greater effectiveness of KGDHC than succinyl-CoA suggests that the catalysis owing to the E2k succinyltransferase is important. Succinylation appears to be a major signaling system and it can be mediated by KGDHC. Reversible post-translation modifications of proteins are common and may regulate many processes. Succinylation of proteins occurs and causes large changes in the structure of proteins. However, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remains unknown. The results demonstrate that the mitochondrial α-ketoglutarate dehydrogenase complex (KGDHC) can succinylate multiple mitochondrial proteins and alter their function. Succinylation appears to be a major signaling system and it can be mediated by KGDHC.

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.

Ketogenesis prevents diet-induced fatty liver injury and hyperglycemia

Nonalcoholic fatty liver disease (NAFLD) spectrum disorders affect approximately 1 billion individuals worldwide. However, the drivers of progressive steatohepatitis remain incompletely defined. Ketogenesis can dispose of much of the fat that enters the liver, and dysfunction in this pathway could promote the development of NAFLD. Here, we evaluated mice lacking mitochondrial 3-hydroxymethylglutaryl CoA synthase (HMGCS2) to determine the role of ketogenesis in preventing diet-induced steatohepatitis. Antisense oligonucleotide-induced loss of HMGCS2 in chow-fed adult mice caused mild hyperglycemia, increased hepatic gluconeogenesis from pyruvate, and augmented production of hundreds of hepatic metabolites, a suite of which indicated activation of the de novo lipogenesis pathway. High-fat diet feeding of mice with insufficient ketogenesis resulted in extensive hepatocyte injury and inflammation, decreased glycemia, deranged hepatic TCA cycle intermediate concentrations, and impaired hepatic gluconeogenesis due to sequestration of free coenzyme A (CoASH). Supplementation of the CoASH precursors pantothenic acid and cysteine normalized TCA intermediates and gluconeogenesis in the livers of ketogenesis-insufficient animals. Together, these findings indicate that ketogenesis is a critical regulator of hepatic acyl-CoA metabolism, glucose metabolism, and TCA cycle function in the absorptive state and suggest that ketogenesis may modulate fatty liver disease.

Nonenzymatic protein acylation as a carbon stress regulated by sirtuin deacylases

Cellular proteins are decorated with a wide range of acetyl and other acyl modifications. Many studies have demonstrated regulation of site-specific acetylation by acetyltransferases and deacetylases. Acylation is emerging as a new type of lysine modification, but less is known about its overall regulatory role. Furthermore, the mechanisms of lysine acylation, its overlap with protein acetylation, and how it influences cellular function are major unanswered questions in the field. In this review, we discuss the known roles of acetyltransferases and deacetylases and the sirtuins as a conserved family of a nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacylases that are important for response to cellular stress and homeostasis. We also consider the evidence for an emerging idea of nonenzymatic protein acylation. Finally, we put forward the hypothesis that protein acylation is a form of protein "carbon stress" that the deacylases evolved to remove as a part of a global protein quality-control network.

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.

Ketone body metabolism and cardiovascular disease

Ketone bodies are metabolized through evolutionarily conserved pathways that support bioenergetic homeostasis, particularly in brain, heart, and skeletal muscle when carbohydrates are in short supply. The metabolism of ketone bodies interfaces with the tricarboxylic acid cycle, β-oxidation of fatty acids, de novo lipogenesis, sterol biosynthesis, glucose metabolism, the mitochondrial electron transport chain, hormonal signaling, intracellular signal transduction pathways, and the microbiome. Here we review the mechanisms through which ketone bodies are metabolized and how their signals are transmitted. We focus on the roles this metabolic pathway may play in cardiovascular disease states, the bioenergetic benefits of myocardial ketone body oxidation, and prospective interactions among ketone body metabolism, obesity, metabolic syndrome, and atherosclerosis. Ketone body metabolism is noninvasively quantifiable in humans and is responsive to nutritional interventions. Therefore, further investigation of this pathway in disease models and in humans may ultimately yield tailored diagnostic strategies and therapies for specific pathological states.

A quantitative map of the liver mitochondrial phosphoproteome reveals posttranslational control of ketogenesis

Mitochondria are dynamic organelles that play a central role in a diverse array of metabolic processes. Elucidating mitochondrial adaptations to changing metabolic demands and the pathogenic alterations that underlie metabolic disorders represent principal challenges in cell biology. Here, we performed multiplexed quantitative mass spectrometry-based proteomics to chart the remodeling of the mouse liver mitochondrial proteome and phosphoproteome during both acute and chronic physiological transformations in more than 50 mice. Our analyses reveal that reversible phosphorylation is widespread in mitochondria, and is a key mechanism for regulating ketogenesis during the onset of obesity and type 2 diabetes. Specifically, we have demonstrated that phosphorylation of a conserved serine on Hmgcs2 (S456) significantly enhances its catalytic activity in response to increased ketogenic demand. Collectively, our work describes the plasticity of this organelle at high resolution and provides a framework for investigating the roles of proteome restructuring and reversible phosphorylation in mitochondrial adaptation.

Mechanistic insights into the regulation of metabolic enzymes by acetylation

The activity of metabolic enzymes is controlled by three principle levels: the amount of enzyme, the catalytic activity, and the accessibility of substrates. Reversible lysine acetylation is emerging as a major regulatory mechanism in metabolism that is involved in all three levels of controlling metabolic enzymes and is altered frequently in human diseases. Acetylation rivals other common posttranslational modifications in cell regulation not only in the number of substrates it modifies, but also the variety of regulatory mechanisms it facilitates.

Rumen epithelial adaptation to high-grain diets involves the coordinated regulation of genes involved in cholesterol homeostasis

The molecular mechanisms underlying rumen epithelial adaption to high-grain (HG) diets are unknown. To gain insight into the metabolic mechanisms governing epithelial adaptation, mature nonlactating dairy cattle (n = 4) were transitioned from a high-forage diet (HF, 0% grain) to an HG diet (65% grain). After the cattle were fed the HG diet for 3 wk, they returned to the original HF diet, which they were fed for an additional 3 wk. Continuous ruminal pH, ruminal short chain fatty acids, and plasma β-hydroxybutyrate were measured on a weekly basis, and rumen papillae were biopsied from the ventral sac to assess alterations in mRNA expression profiles. The subacute form of ruminal acidosis was diagnosed during the first week of the HG period (4.6 ± 1.6 h/day <pH 5.6), but not during weeks 2 and 3, thereby indicating ruminal adaption to the HG diet. Changes in the mRNA expression profile of rumen papillae were initially examined using Bovine Affymetrix microarrays; a total of 521 differentially expressed genes (false discovery rate P < 0.08) were uncovered from the first to third week of the HG period. Ingenuity Pathway Analysis of microarray results revealed that enzymes involved in cholesterol synthesis were coordinately downregulated from the first to third week of the HG period. In addition, the LXR/RXR activation pathway was significant and included several genes involved in intracellular cholesterol homeostasis. The differential expression signature of eight genes representing the key regulatory points of cholesterol homeostasis was confirmed by quantitative real-time PCR. Based upon our pathway and network results we propose a model to explain cellular events during rumen epithelial adaptation to HG diets and thus provide molecular targets that may be useful in the treatment and prevention of ruminal acidosis.

SIRT3 deacetylates mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase 2 and regulates ketone body production

The mitochondrial sirtuin SIRT3 regulates metabolic homeostasis during fasting and calorie restriction. We identified mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase 2 (HMGCS2) as an acetylated protein and a possible target of SIRT3 in a proteomics survey in hepatic mitochondria from Sirt3(-/-) (SIRT3KO) mice. HMGCS2 is the rate-limiting step in β-hydroxybutyrate synthesis and is hyperacetylated at lysines 310, 447, and 473 in the absence of SIRT3. HMGCS2 is deacetylated by SIRT3 in response to fasting in wild-type mice, but not in SIRT3KO mice. HMGCS2 is deacetylated in vitro when incubated with SIRT3 and in vivo by overexpression of SIRT3. Deacetylation of HMGCS2 lysines 310, 447, and 473 by incubation with wild-type SIRT3 or by mutation to arginine enhances its enzymatic activity. Molecular dynamics simulations show that in silico deacetylation of these three lysines causes conformational changes of HMGCS2 near the active site. Mice lacking SIRT3 show decreased β-hydroxybutyrate levels during fasting. Our findings show SIRT3 regulates ketone body production during fasting and provide molecular insight into how protein acetylation can regulate enzymatic activity.

Fibroblast growth factor 21 regulates lipolysis in white adipose tissue but is not required for ketogenesis and triglyceride clearance in liver

Fibroblast growth factors (Fgfs) are polypeptide growth factors with diverse functions. Fgf21, a unique member of the Fgf family, is expected to function as a metabolic regulator in an endocrine manner. Hepatic Fgf21 expression was increased by fasting. The phenotypes of hepatic Fgf21 transgenic or knockdown mice and high-fat, low-carbohydrate ketogenic diet-fed mice suggests that Fgf21 stimulates lipolysis in the white adipose tissue during normal feeding and is required for ketogenesis and triglyceride clearance in the liver during fasting. However, the physiological roles of Fgf21 remain unclear. To elucidate the physiological roles of Fgf21, we generated Fgf21 knockout (KO) mice by targeted disruption. Fgf21 KO mice were viable, fertile, and seemingly normal. Food intake, oxygen consumption, and energy expenditure were also essentially unchanged in Fgf21 KO mice. However, hypertrophy of adipocytes, decreased lipolysis in adipocytes, and decreased blood nonesterified fatty acid levels were observed when Fgf21 KO mice were fed normally. In contrast, increased lipolysis in adipocytes and increased blood nonesterified fatty acid levels were observed in Fgf21 KO mice by fasting for 24 h, indicating that Fgf21 stimulates lipolysis in the white adipose tissue during feeding but inhibits it during fasting. In contrast, unexpectedly, hepatic triglyceride levels were essentially unchanged in Fgf21 KO mice. In addition, ketogenesis in Fgf21 KO mice was not impaired by fasting for 24 h. The present results indicate that Fgf21 regulates lipolysis in adipocytes in response to the metabolic state but is not required for ketogenesis and triglyceride clearance in the liver.

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.

Hepatic fibroblast growth factor 21 is regulated by PPARalpha and is a key mediator of hepatic lipid metabolism in ketotic states

Mice fed a high-fat, low-carbohydrate ketogenic diet (KD) exhibit marked changes in hepatic metabolism and energy homeostasis. Here, we identify liver-derived fibroblast growth factor 21 (FGF21) as an endocrine regulator of the ketotic state. Hepatic expression and circulating levels of FGF21 are induced by both KD and fasting, are rapidly suppressed by refeeding, and are in large part downstream of PPARalpha. Importantly, adenoviral knockdown of hepatic FGF21 in KD-fed mice causes fatty liver, lipemia, and reduced serum ketones, due at least in part to altered expression of key genes governing lipid and ketone metabolism. Hence, induction of FGF21 in liver is required for the normal activation of hepatic lipid oxidation, triglyceride clearance, and ketogenesis induced by KD. These findings identify hepatic FGF21 as a critical regulator of lipid homeostasis and identify a physiological role for this hepatic hormone.

KetogenicHMGCS2Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

HMGCS2, the gene that regulates ketone body production, is expressed in liver and several extrahepatic tissues, such as the colon. In CaCo-2 colonic epithelial cells, the expression of this gene increases with cell differentiation. Accordingly, immunohistochemistry with specific antibodies shows that HMGCS2 is expressed mainly in differentiated cells of human colonic epithelium. Here, we used a chromatin immunoprecipitation assay to study the molecular mechanism responsible for this expression pattern. The assay revealed that HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity. c-Myc transrepression is mediated by blockade of the transactivating activity of Miz-1, which occurs mainly through a Sp1-binding site in the proximal promoter of the gene. Accordingly, the expression of human HMGCS2 is down-regulated in 90% of Myc-dependent colon and rectum tumors. HMGCS2 protein expression is down-regulated preferentially in moderately and poorly differentiated carcinomas. In addition, it is also down-regulated in 80% of small intestine Myc-independent tumors. Based on these findings, we propose that ketogenesis is an undesirable metabolic characteristic of the proliferating cell, which is down-regulated through c-Myc-mediated repression of the key metabolic gene HMGCS2.

Down-regulation of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene by insulin: the role of the forkhead transcription factor FKHRL1

Normal physiological responses to carbohydrate shortages cause the liver to increase the production of ketone bodies from the acetyl-CoA generated from fatty acid oxidation. This allows the use of ketone bodies for energy, thereby preserving the limited glucose for use by the brain. This adaptative response is switched off by insulin rapidly inhibiting the expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (HMGCS2) gene, which is a key control site of ketogenesis. We decided to investigate the molecular mechanism of this inhibition. In the present study, we show that FKHRL1, a member of the forkhead in rhabdosarcoma (FKHR) subclass of the Fox family of transcription factors, stimulates transcription from transfected 3-hydroxy-3-methylglutaryl-CoA synthase promoter-luciferase reporter constructs, and that this stimulation is repressed by insulin. An FKHRL1-responsive sequence AAAAATA, located 211 bp upstream of the HMGCS2 gene transcription start site, was identified by deletion analysis. It binds FKHRL1 in vivo and in vitro and confers FKHRL1 responsiveness on homologous and heterologous promoters. If it is mutated, it partially blocks the effect of insulin in HepG2 cells, both in the absence and presence of overexpressed FKHRL1. These results suggest that FKHRL1 contributes to the regulation of HMGCS2 gene expression by insulin.

Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes

Ketone bodies are produced by the liver and used peripherally as an energy source when glucose is not readily available. The two main ketone bodies are acetoacetate (AcAc) and 3-beta-hydroxybutyrate (3HB), while acetone is the third, and least abundant, ketone body. Ketones are always present in the blood and their levels increase during fasting and prolonged exercise. They are also found in the blood of neonates and pregnant women. Diabetes is the most common pathological cause of elevated blood ketones. In diabetic ketoacidosis (DKA), high levels of ketones are produced in response to low insulin levels and high levels of counterregulatory hormones. In acute DKA, the ketone body ratio (3HB:AcAc) rises from normal (1:1) to as high as 10:1. In response to insulin therapy, 3HB levels commonly decrease long before AcAc levels. The frequently employed nitroprusside test only detects AcAc in blood and urine. This test is inconvenient, does not assess the best indicator of ketone body levels (3HB), provides only a semiquantitative assessment of ketone levels and is associated with false-positive results. Recently, inexpensive quantitative tests of 3HB levels have become available for use with small blood samples (5-25 microl). These tests offer new options for monitoring and treating diabetes and other states characterized by the abnormal metabolism of ketone bodies.

The liver carnitine pool reflects alterations in hepatic fatty acid metabolism in rats with bile duct ligation before and after biliodigestive anastomosis

BACKGROUND/AIMS

Rats with long-term bile duct ligation (BDL rats) have impaired hepatic fatty acid metabolism and alterations in carnitine homeostasis. Analysis of the carnitine tissue and body fluid pools was used as a tool to study hepatic fatty acid metabolism in BDL rats and after reversal of bile duct ligation by Roux-en-Y anastomosis for 5 (RY5) or 14 days (RY14)

METHODS

Control rats were pair-fed to treated rats, and all rats were studied after starvation for 24 h. Carnitine was analyzed by a radioenzymatic method and by high performance liquid chromatography.

RESULTS

Both BDL and RY rats had decreased plasma beta-hydroxybutyrate concentrations, whereas free fatty acid plasma concentrations were not different from control rats. Free carnitine plasma concentrations were not different between BDL or RY and control rats, whereas acetylcarnitine concentrations were decreased in BDL and RY rats, and showed a positive correlation with the plasma beta-hydroxybutyrate concentrations. In comparison to control rats, the total hepatic carnitine content was increased in BDL and RY rats, both when expressed per g tissue and per total liver. This rise in the hepatic carnitine content was due to increases in both free and acylcarnitines, including acetylcarnitine. In comparison to control rats, the hepatic concentration of beta-hydroxybutyrate was decreased in BDL and RY rats, findings compatible with impaired formation of ketone bodies from acetyl-CoA. Urinary excretion of total carnitine was not different between treated and control rats.

CONCLUSIONS

Hepatic metabolism of fatty acids is impaired in BDL rats and does not recover during the 14 days after Roux-en-Y anastomosis. The increased hepatic carnitine content in BDL and RY rats can best be explained by decreased export of carnitine from the hepatocytes. The alterations in the hepatic carnitine pool and impaired hepatic fatty acid metabolism in BDL and RY rats are compatible with impaired ketogenesis.

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.

The effect of dexamethasone treatment on the expression of the regulatory genes of ketogenesis in intestine and liver of suckling rats

The influence of the injection of dexamethasone on ketogenesis in 12 day old suckling rats was studied in intestine and liver by determining mRNA levels and enzyme activity of the two genes responsible for regulation of ketogenesis: carnitine palmitoyl transferase I (CPT I) and mitochondrial HMG-CoA synthase. Dexamethasone produced a 2 fold increase in mRNA and activity of CPT I in intestine, but led to a decrease in mit. HMG-CoA synthase. In liver the mRNA levels and activity of both CPT I and mit. HMG-CoA synthase decreased. Comparison of these values with the ketogenic rate of both tissues following dexamethasone treatment suggests that mit. HMG-CoA synthase could be the main gene responsible for the regulation of ketogenesis in suckling rats. The changes produced in serum ketone bodies by dexamethasone, with a profile that is more similar to the ketogenic rate in the liver than that in the intestine, indicate that liver contributes more to ketone body synthesis in suckling rats. Two day treatment with dexamethasone produced no change in mRNA or activity levels for CPT I in liver or intestine. While mRNA levels for mit. HMG-CoA synthase changed little, the enzyme activity is decreased in both tissues.

Cloning and characterization of the human mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase gene

We report the characterization of lambda and P1 phage clones containing the entire human mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase (mHS) gene. The human mHS locus (HMGCS2) on chromosome 1p12-13 spans 25 kb and contains 10 exons. Exon 1 contains most of the mitochondrial leader, consistent with a recent hypothesis of the evolution of the ketogenic pathway. By primer extension and cDNA amplification (RACE-PCR) we localized the transcription start point (tsp) to 60 bp upstream of the initiation codon. Nine blocks of conserved sequence were identified by comparing the 5' flanking regions of the mHS genes of human and rat. The 5' flanking region contains potential binding sites for TATA-binding protein, Sp1, nuclear factor 1 (NF1), CAAT-box binding protein (C/EBP), hepatocyte nuclear factors 1 and 5 (HNF1, HNF5) and activator proteins 1 and 2 (AP1, AP2).

Gene expression of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase in a poorly ketogenic mammal: effect of starvation during the neonatal period of the piglet

The low ketogenic capacity of pigs correlates with a low activity of mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase. To identify the molecular mechanism controlling such activity, we isolated the pig cDNA encoding this enzyme and analysed changes in mRNA levels and mitochondrial specific activity induced during development and starvation. Pig mitochondrial synthase showed a tissue-specific expression pattern. As with rat and human, the gene is expressed in liver and large intestine; however, the pig differs in that mRNA was not detected in testis, kidney or small intestine. During development, pig mitochondrial HMG-CoA synthase gene expression showed interesting differences from that in the rat: (1) there was a 2-3 week lag in the postnatal induction; (2) the mRNA levels remained relatively abundant through the suckling-weaning transition and at maturity, in contrast with the fall observed in rats at similar stages of development; and (3) the gene expression was highly induced by fasting during the suckling, whereas no such change in mitochondrial HMG-CoA synthase mRNA levels has been observed in rat. The enzyme activity of mitochondrial HMG-CoA synthase increased 27-fold during starvation in piglets, but remained one order of magnitude lower than rats. These results indicate that post-transcriptional mechanism(s) and/or intrinsic differences in the encoded enzyme are responsible for the low activity of pig HMG-CoA synthase observed throughout development or after fasting.

The effect of fasting/refeeding and insulin treatment on the expression of the regulatory genes of ketogenesis in intestine and liver of suckling rats

The influence of fasting/refeeding and insulin treatment on ketogenesis in 12-day-old suckling rats was studied in intestine and liver by determining mRNA levels and enzyme activity of the two genes responsible for regulation of ketogenesis: carnitine palmitoyl transferase I (CPT I) and mitochondrial HMG-CoA synthase. Fasting produced hardly any change in mRNA or activity of CPT 1 in intestine, but led to a decrease in mitochondrial (mit.) HMG-CoA synthase. In liver, while mRNA levels and activity for CPT I increased, neither parameter was changed in HMG-CoA synthase. The comparison of these values with the ketogenic rate of both tissues under the fasting/refeeding treatment shows that HMG-CoA synthase could be the main gene responsible for regulation of ketogenesis in suckling rats. The small changes produced in serum ketone bodies in fasting/refeeding, with a profile similar to the ketogenic rate of the liver, indicate that liver contributes most to ketone body synthesis in suckling rats under these experimental conditions. Short-term insulin treatment produced increases in mRNA levels and activity in CPT I in intestine, but it also decreased both parameters in mit. HMG-CoA synthase. In liver, graphs of mRNA and activity were nearly identical in both genes. There was a marked decrease in mRNA levels and activity, resembling those values observed in adult rats. As in fasting/refeeding, the ketogenic rate correlated better to mit. HMG-CoA synthase than CPT I, and liver was the main organ regulating ketogenesis after insulin treatment. Serum ketone body concentrations were decreased by insulin but recovered after the second hour. Long-term insulin treatment had little effect on the mRNA levels for CPT I or mit. HMG-CoA synthase, but both the expressed and total activities of mit. HMG-CoA synthase were reduced by half in both intestine and liver. The ketogenic rate of both organs was decreased to 40% by long-term insulin treatment. The different effects of refeeding and insulin treatment on the expression of both genes, on the ketogenic rate, and on ketone body concentrations are discussed.

Flexibility of zonation of fatty acid oxidation in rat liver

Periportal and perivenous hepatocytes were isolated from rats subjected to different treatments that induce (starvation, cold exposure) or depress (refeeding after starvation) hepatic fatty acid oxidation. These experiments were designed to determine factors that may be involved in creating and maintaining the asymmetrical distribution of this metabolic pathway in the acinus of the liver. The uneven distribution of mitochondrial [14C]-palmitate oxidation within the acinus (i) was very flexible and changed markedly with the physiological status of the animal (periportal/perivenous ratio: 1.5, 2.0, 1.0 and 0.4 for fed, starved, refed and cold-exposed animals respectively), (ii) coincided with a similar zonation of carnitine palmitoyltransferase I activity in fed as well as in cold-exposed animals, (iii) was paralleled by a comparable zonation of mitochondrial 3-hydroxy-3-methyl-glutaryl-CoA synthase activity in starved animals, and (iv) was not determined by zonal differences in any of the following parameters: sensitivity of carnitine palmitoyltransferase I to malonyl-CoA, intracellular concentration of malonyl-CoA, fatty acid synthesizing capacity, acetyl-CoA carboxylase activity, fatty acid synthase activity or relative content of the two hepatic acetyl-CoA carboxylase isoforms. Unlike mitochondrial oxidation, peroxisomal [14C]palmitate oxidation was always zonated towards the perivenous zone of the liver irrespective of the physiological status of the animal. The data presented show that changes in the acinar distribution of mitochondrial long-chain fatty acid oxidation involve specific long-term mechanisms under different physiological conditions.

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.