Short-chain fatty acids stimulate leptin production in adipocytes through the G protein-coupled receptor GPR41

Leptin is an adipose-derived hormone that regulates a wide variety of physiological processes, including feeding behavior, metabolic rate, sympathetic nerve activity, reproduction, and immune response. Circulating leptin levels are tightly regulated according to energy homeostasis in vivo. Although mechanisms for the regulation of leptin production in adipocytes are not well understood, G protein-coupled receptors may play an important role in this adipocyte function. Here we report that C2-C6 short-chain fatty acids, ligands of an orphan G protein-coupled receptor GPR41, stimulate leptin expression in both a mouse adipocyte cell line and mouse adipose tissue in primary culture. Acute oral administration of propionate increases circulating leptin levels in mice. The concentrations of short-chain fatty acids required to stimulate leptin production are within physiological ranges, suggesting the relevance of this pathway in vivo.     

The development of metabolic endotoxemia is dependent on the type of sweetener and the presence of saturated fat in the diet

ABSTRACT

Fat and sweeteners contribute to obesity. However, it is unknown whether specific bacteria are selectively modified by different caloric and noncaloric sweeteners with or without a high-fat diet (HFD). Here, we combined extensive host phenotyping and shotgun metagenomics of the gut microbiota to investigate this question. We found that the type of sweetener and its combination with an HFD selectively modified the gut microbiota. Sucralose and steviol glycosides led to the lowest α-diversity of the gut microbiota. Sucralose increased the abundance of B. fragilis in particular, resulting in a decrease in the abundance of occludin and an increase in proinflammatory cytokines, glucose intolerance, fatty acid oxidation and ketone bodies. Sucrose+HFD showed the highest metabolic endotoxemia, weight gain, body fat, total short chain fatty acids (SCFAs), serum TNFα concentration and glucose intolerance. Consumption of sucralose or sucrose resulted in enrichment of the bacterial genes involved in the synthesis of LPS and SCFAs. Notably, brown sugar and honey were associated with the absence of metabolic endotoxemia, increases in bacterial gene diversity and anti-inflammatory markers such as IL-10 and sIgA, the maintenance of glucose tolerance and energy expenditure, similar to the control group, despite the consumption of an HFD. These findings indicate that the type of sweetener and an HFD selectively modify the gut microbiota, bacterial gene enrichment of metabolic pathways involved in LPS and SCFA synthesis, and metabolic endotoxemia associated with different metabolic profiles.     

Metabolic fuel utilization and pyruvate oxidation during the postnatal period

Summary The transplacental supply of nutrients is interrupted at birth, which diverts maternal metabolism to lactation. After birth, energy homeostasis is rapidly regained through milk nutrients which supply the newborn with the fatty acids and ketone bodies required for neonatal development. However, immediately after birth and before the onset of suckling there is a time lapse in which the newborn undergoes a unique kind of starvation. During this period glucose is scarce and ketone bodies are not available owing to the delay in ketogenesis. Under these circumstances, the newborn is supplied with another metabolic fuel, lactate, which is utilized as a source of energy and carbon skeletons. Neonatal rat lung, heart, liver and brain utilize lactate for energy production and lipogenesis. Lactate is also utilized by the brain of human babies with type I glycogenosis. Both rat neurons and astrocytes in primary culture actively use lactate as an oxidizable substrate and as a precursor of phospholipids and sterols. Lactate oxidation is enhanced by dichloroacetate, an inhibitor of the pyruvate dehydrogenase kinase in neurons but not in astrocytes, suggesting that the pyruvate dehydrogenase is regulated differently in each type of cell. Despite the low activity of this enzyme in newborn brain, pyruvate decarboxylation is the main fate of glucose in both neurons and astrocytes. The occurrence of a yeast-like pyruvate decarboxylase activity in neonatal brain may explain these results.     

Insulin Resistance,Energy Balance and Sympathetic Nervous System Activity

Insulin resistance and hyperinsulinemia are commonly associated with hypertension in the obese. The nature of this association is obscure. An hypothesis is developed that attributes obesity-related hypertension to sympathetic stimulation. The relationship between insulin and the sympathetic nervous system (SNS) has its origins in the mediation of dietary thermogenesis. Fasting suppresses while carbohydrate and fat feeding stimulate sympathetic activity. Insulin-mediated glucose metabolism within critical central neurons links dietary intake and central sympathetic outflow. The sympathetic nervous system, in turn, contributes to changes in metabolic rate that accompany alterations in dietary intake. It is hypothesized that insulin resistance is a mechanism recruited in the obese to limit further weight gain and stabilize body mass. Insulin-mediated sympathetic stimulation is one mechanism that may restore energy balance in the obese since the obese are not resistant to the stimulatory effect of insulin on the SNS. Sympathetically mediated stimulation of the heart, vasculature and kidney contributes, in genetically predisposed individuals, to the development of hypertension. Viewed in this light, obesity-related hypertension is the unfortunate by-product of an adaptive mechanism (insulin resistance) recruited to restore energy balance in the obese. Possible implications of this formulation are discussed.     

Changes of hepatic fatty acid metabolism produced by chronic thioacetamide administration in rats.

Hepatic mitochondrial functions related to fatty acid metabolism, including the respiratory control ratio, fatty acid oxidative capacity and carnitine palmitoyltransferase I activity, were studied in vitro with mitochondria isolated from rats treated with thioacetamide for up to 12 wk. The levels of ketone bodies, carnitine, carnitine esters and malonyl-coenzyme A were also determined in liver extracts. Polarography of mitochondrial respiration from succinate or glutamate plus malate showed a lower respiratory control ratio in thioacetamide-treated rats, whereas uncoupled oxygen consumption was not altered. This suggests that the mitochondrial respiratory chain capacity remained intact in the thioacetamide-treated rats. The oxygen consumption associated with palmitoyl-coenzyme A and palmitoyl-L-carnitine oxidation by isolated liver mitochondria was increased by thioacetamide treatment on both a per-mitochondrial protein and a per-total liver basis. The carnitine palmitoyl-transferase I activity; the tissue levels of ketone bodies, carnitine and carnitine esters; and the beta-hydroxybutyrate/acetoacetate ratio were all higher in the livers of thioacetamide-treated animals than in control livers, whereas the hepatic malonyl-coenzyme A level was decreased by thioacetamide. These results indicate the increased diversion of cytosolic long-chain acyl-coenzyme As into the mitochondria for beta-oxidation rather than their esterification and use in lipogenesis. These intrahepatic metabolic changes induced by chronic thioacetamide administration may reflect the whole-body catabolic state and can be seen as adaptive for maintaining energy homeostasis under conditions of impaired glucose tolerance.     

Inborn errors of ketogenesis and ketone body utilization

Ketone bodies acetoacetate and 3-hydroxy-n-butyric acid are metabolites derived from fatty acids and ketogenic amino acids such as leucine. They are mainly produced in the liver via reactions catalyzed by the ketogenic enzymes mitochondrial 3-hydroxy-3-methylglutary-coenzyme A synthase and 3-hydroxy-3-methylglutary-coenzyme A lyase. After prolonged starvation, ketone bodies can provide up to two-thirds of the brain’s energy requirements. The rate-limiting enzyme of ketone body utilization (ketolysis) is succinyl-coenzyme A:3-oxoacid coenzyme A transferase. The subsequent step of ketolysis is catalyzed by 2-methylactoacetyl-coenzyme A thiolase, which is also involved in isoleucine catabolism. Inborn errors of metabolism affecting those four enzymes are presented and discussed in the context of differential diagnoses. While disorders of ketogenesis can present with hypoketotic hypoglycemia, inborn errors of ketolysis are characterized by metabolic decompensations with ketoacidosis. If those diseases are considered early and appropriate treatment is initiated without delay, patients with inborn errors of ketone body metabolism often have a good clinical outcome.     

Hormonal and metabolic changes in starving obese patients during exhaustive physical exercise

Exhaustive graded exercise leads to changes of hormones, carbohydrate, and lipid metabolism in normal controls and obese patients after prolonged starvation. Concomitant with a large increase of plasma catecholamines, insulin concentration is reduced and blood glucose levels slowly increase. More glucose is made available by glycogenolysis and gluconeogenesis than can be oxidized in the mitochondria. Lactate associated metabolic acidosis appears. Starving obese patients in the basal state have reduced blood glucose concentrations, but their initial values for free glycerol, free fatty acids, and ketone bodies are much higher than in normal controls. This is caused by the starvation induced lipolysis. With exhaustive exercise adrenaline, noradrenaline, and free glycerol increase. In contrast, free fatty acids and ketone bodies decrease, because they are consumed as fuel. Prolonged starvation changes basal values of hormones and metabolites, but it does not change the quality of exercise-induced shifts in these values when compared with those of the normal controls.     

The starved colon—Diminished mucosal nutrition,diminished absorption,and colitis

Nutrition of colonic epithelial cells is mainly from short chain fatty acids (SCFAs) produced by bacterial fermentation in the colonic lumen. n-Butyrate contributes more carbon of oxidation to epithelial cells than glucose or glutamine from the vasculature. Incomplete starvation of colonic epithelial cells through lack of luminal SCFAs leads, in the short term, to mucosal hypoplasia with either diminished absorption or diarrhea. A chronic lack of SCFAs or complete organ starvation in conjunction with other factors leads to nutritional colitis, either "diversion colitis" or "starvation colitis." Whether predominantly diarrhea or colitis develops in mucosal malnutrition appears to depend upon the severity and duration of starvation. Ulcerative colitis may be classified as a nutritional colitis in that colonic epithelial cells are unable to utilize SCFAs reflecting epithelial starvation despite abundant SCFAs.     

Thermic effect of feeding in man: Increased plasma norepinephrine levels following glucose but not protein or fat consumption

In seven healthy male subjects, intake of 100 g protein hydrolysate produced significantly greater increments in energy expenditure than intake of 100 g glucose, 44 g fat, or a noncaloric control solution during the first 4 hr postcibum. Glucose and fat intake produced similar increments in energy expenditure. In contrast to the effects on thermogenesis, protein and fat intake did not alter sympathetic nervous system (SNS) activity, as estimated by plasma norepinephrine (NE) levels, whereas glucose intake significantly increased NE levels. Plasma levels of immunoreactive insulin were stimulated by glucose intake to a much greater level than by protein intake, and were unaffected by ingestion of the fat and control solutions. Pulse rate significantly increased following ingestion of all nutrients compared to pulse rate changes during the control test. These data support the traditional concept of a greater thermic effect of protein than of carbohydrate or fat, but the possibility of SNS involvement in the thermic effect of protein and fat is not supported.     

Regulation of ketonaemia in hyperthyroidism: study of the role of free fatty acids

We have indirectly assessed the role of free fatty acids (FFA) and of hepatic ketogenesis in the regulation of ketone body levels in euthyroid and hyperthyroid subjects. All studies were performed under controlled dietary conditions (35 and 45 kcal/kg body weight/day respectively for euthyroid and hyperthyroid subjects). In the post-absorptive state hyperthyroid patients had normal, glucagon levels and, in spite of increased insulinemia (p less than 0.02), elevated concentrations of blood glucose (p less than 0.01), glycerol (p less than 0.001) and ketone bodies (p less than 0.05). In the face of this hyperketonaemia, there was no significant increase of plasma FFA, suggesting possible increased hepatic conversion of FFA to ketone bodies. However, when FFA were acutely raised to high levels, the induced rise of ketone bodies was similar in hyperthyroid and euthyroid subjects. Oral propranolol administration to hyperthyroid patients in the post-absorptive state decreased the concentrations of glycerol (p less than 0.05) and ketone bodies (p less than 0.05) without altering insulin concentrations. Ketone bodies fell without any significant decrease of FFA suggesting a possible direct effect of propranolol administration on hepatic ketogenesis. However, the ketone body response to raised FFA levels was unaffected by propranolol. We have evidence in hyperthyroid patients of increased lipolysis and ketogenesis in spite of increased daily caloric intake. These results suggest that these metabolic abnormalities are not merely due to relative starvation. We have tentative evidence for modification of the intra-hepatic conversion of FFA to ketone bodies in the post-absorptive state but no supporting evidence when FFA levels were experimentally raised.     

Gut microbiota and GLP-1

A large body of evidence suggests that the regulation of energy balance and glucose homeostasis by fermentable carbohydrates induces specific changes in the gut microbiota. Among the mechanisms, our research group and others have demonstrated that the gut microbiota fermentation (i.e., bacterial digestion of specific compounds) of specific prebiotics or other non-digestible carbohydrates is associated with the secretion of enteroendocrine peptides, such as the glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), produced by L-cells. In this review, we highlight past and recent results describing how dietary manipulation of the gut microbiota, using nutrients or specific microbes, can stimulate GLP-1 secretion in rodents and humans. Furthermore, the purpose of this review is to discuss the putative mechanisms by which specific bacterial metabolites, such as short chain fatty acids, trigger GLP-1 secretion through GPR41/43-dependent mechanisms. Moreover, we conclude by discussing the molecular advance showing that the endocannabinoid system or related bioactive lipids modulated by the gut microbiota may contribute to the regulation of glucose, lipid and energy homeostasis.     

Sympathoadrenal activity in human obesity: heterogeneity of findings since 1980.

Alterations in sympathetic nervous system (SNS) activity are widely believed to contribute to the pathophysiology of the obese state. Disagreement, however, exists as to whether the predominant sympathetic abnormality is a decrease in neuronal activity (leading to diminished sympathetically-mediated energy expenditure and weight gain) or an increase (leading to hypertension). Findings summarized from over 40 separate studies support both hypotheses as well as the alternative thesis that SNS activity does not differ in obese humans compared to lean controls. Another abnormality being noted with increasing frequency in human obesity is reduced adrenaline (Ad) levels in plasma, both at rest or in response to a stimulus such as physical activity. Whether diminished adrenal medullary function is a cause or consequence of the obese state and whether the adrenal medulla plays any role in the regulation of energy metabolism on a daily basis are not known at the present time. Thus, while depressed SNS activity may be a sufficient explanation for the development of obesity, it is not a necessary condition. Suppressed adrenal medullary function may also contribute to this disorder.     

The role of the liver in metabolic homeostasis: Implications for inborn errors of metabolism

Summary The mechanisms by which the liver maintains a constant supply of oxidizable substrates, which provide energy to the body as a whole, are reviewed. During feeding, the liver builds up energy stores in the form of glycogen and triglyceride, the latter being exported to adipose tissue. During fasting, it releases glucose and ketone bodies. Glucose is formed by degradation of glycogen and by gluconeogenesis from gluconeogenic amino acids provided by muscle. Ketone bodies are produced from fatty acids, released by adipose tissue, and from ketogenic amino acids. The major signals which control the transition between the fed and the fasted state are glucose, insulin and glucagon. These influence directly or indirectly the enzymes which regulate liver carbohydrate and fatty acid metabolism and thereby orient metabolic fluxes towards either energy storage or substrate release. In the fed state, the liver utilizes the energy generated by glucose oxidation to synthesize triglycerides. In the fasted state it utilizes that produced by-oxidation of fatty acids to synthesize glucose. The mechanisms whereby a number of inborn errors of glycogen metabolism, of gluconeogenesis and of ketogenesis cause hypoglycaemia are also briefly overviewed.     

Targeted disruption of GPR7, the endogenous receptor for neuropeptides B and W,leads to metabolic defects and adult-onset obesity

Gold-thioglucose (GTG) induces lesions in the ventromedial nucleus of the hypothalamus, resulting in hyperphagia and obesity. To identify genes involved in the hypothalamic regulation of energy homeostasis, we used a screen for genes that are dysregulated in GTG-induced obese mice. We found that GPR7, the endogenous G protein-coupled receptor for the recently identified ligands neuropeptide B and neuropeptide W, was down-regulated in hypothalamus after GTG treatment. Here we show that male GPR7-/- mice develop an adult-onset obese phenotype that progressively worsens with age and was greatly exacerbated when animals are fed a high-fat diet. GPR7-/- male mice were hyperphagic and had decreased energy expenditure and locomotor activity. Plasma levels of glucose, leptin, and insulin were also elevated in these mice. GPR7-/- male mice had decreased hypothalamic neuropeptide Y RNA levels and increased proopiomelanocortin RNA levels, a set of effects opposite to those evident in ob/ob mice. Furthermore, ob/ob GPR7-/- and Ay/a GPR7-/- double mutant male mice had an increased body weight compared with normal ob/ob or Ay/a male mice, suggesting that the obesity of GPR7-/- mice is independent of leptin and melanocortin signaling. Female mice did not show any significant weight increase or associated metabolic defects. These data suggest a potential role for GPR7 and its endogenous ligands, neuropeptide B and neuropeptide W, in regulating energy homeostasis independent of leptin and melanocortin signaling in a sexually dimorphic manner.     

A novel SCFA receptor,the microbiota,and blood pressure regulation

The maintenance of blood pressure homeostasis is a complex process which is carefully regulated by a variety of inputs. We recently identified two sensory receptors (Olfactory receptor 78 and G protein couple receptor 41) as novel regulators of blood pressure. Both Olfr78 and Gpr41 are receptors for short chain fatty acids (SCFAs), and we showed that propionate (a SCFA) modifies blood pressure in a manner which is differentially modulated by the absence of either Olfr78 or Gpr41. In addition, propionate modifies renin release in an Olfr78-dependent manner. Our study also demonstrated that antibiotic treatment modulates blood pressure in Olfr78 null mice, indicating that SCFAs produced by the gut microbiota likely influence blood pressure regulation. In this addendum, we summarize the findings of our recent study and provide a perspective on the implications of the interactions between the gut microbiota and blood pressure control.     

A novel SCFA receptor,the microbiota,and blood pressure regulation

The maintenance of blood pressure homeostasis is a complex process which is carefully regulated by a variety of inputs. We recently identified two sensory receptors (Olfactory receptor 78 and G protein couple receptor 41) as novel regulators of blood pressure. Both Olfr78 and Gpr41 are receptors for short chain fatty acids (SCFAs), and we showed that propionate (a SCFA) modifies blood pressure in a manner which is differentially modulated by the absence of either Olfr78 or Gpr41. In addition, propionate modifies renin release in an Olfr78-dependent manner. Our study also demonstrated that antibiotic treatment modulates blood pressure in Olfr78 null mice, indicating that SCFAs produced by the gut microbiota likely influence blood pressure regulation. In this addendum, we summarize the findings of our recent study and provide a perspective on the implications of the interactions between the gut microbiota and blood pressure control.     

Chronic sympathetic attenuation and energy metabolism in autonomic failure

The sympathetic nervous system regulates thermogenesis and energy homeostasis in humans. When activated it increases energy expenditure, particularly resting energy expenditure. Most human studies used acute infusion of β-blockers as a model to eliminate sympathetic stimulation and to examine the contribution of the sympathetic nervous system to energy metabolism and balance. Clinically, however, it is also important to assess the effect of chronic sympathetic attenuation on energy metabolism. In this context, we hypothesized that resting energy expenditure is decreased in patients with autonomic failure who, by definition, have low sympathetic tone. We measured 24-hour energy expenditure using whole-room indirect calorimeter in 10 adults with chronic autonomic failure (6 women; age, 64.9±9.1 years; body mass index, 25.2±4.4 kg/m(2)) and 15 sedentary healthy controls of similar age and body composition (8 women; age, 63.1±4.0 years; body mass index, 24.4±3.9 kg/m(2)). In 4 patients, we eliminated residual sympathetic activity with the ganglionic blocker trimethaphan. We found that, after adjusting for body composition, resting energy expenditure did not differ between patients with autonomic failure and healthy controls. However, resting energy expenditure significantly decreased when residual sympathetic activity was eliminated. Our findings suggest that sympathetic tonic support of resting energy expenditure is preserved, at least in part, in pathophysiological models of chronic sympathetic attenuation.     

Comparison between starvation and consumption of a high protein diet in rats: hepatic metabolites and amino acid levels during the first 24 hours

Changes in hepatic levels of lactate, pyruvate, phosphoenolpyruvate, alpha-ketoglutarate, malate, oxaloacetate, adenine nucleotides, inorganic phosphate, ketone bodies, alanine, serine, glycine, aspartate, glutamate, valine and urea were examined in adult rats during the first 24 h of either starvation or consumption of a high protein (HP) diet. No differences were found between these two conditions in the concentration of metabolites studied or the cytosolic redox state. Under both conditions, the cytosolic phosphorylation state decreased to a low 15 h into the experiment but the changes were more pronounced on the HP diet. Hepatic ketone bodies rose sharply after 12 h, with the increase 2.5 times greater for starved rats. In starvation, hepatic aspartate, valine, and urea were low and glycine was high, whereas the opposite was seen for the HP diet. In both groups, alanine fell within 9 h and remained low thereafter. These findings suggest that, in the first 24 h of starvation, the energy necessary for gluconeogenesis is obtained from fatty acid oxidation, while during HP feeding the energy for both gluconeogenesis and ureagenesis are derived from fatty acid oxidation and amino acid oxidation.     

Downregulation of GPR83 in the hypothalamic preoptic area reduces core body temperature and elevates circulating levels of adiponectin

The G protein-coupled receptor 83 (GPR83) was recently demonstrated in warm sensitive neurons (WSN) of the hypothalamic preoptic area (POA) that participate in temperature homeostasis. Thus, we investigated whether GPR83 may have a role in regulating core body temperature (CBT) by reducing its expression in the POA. Dissipation of energy in the form of heat is the primary mode of energy expenditure in mammals and can ultimately affect energy homeostasis. Thus, we also measured the level of important regulators of metabolism. Downregulation of GPR83 was obtained by lentiviral short-hairpin RNAs (shGPR83) vectors designed and selected for their ability to reduce GPR83 levels in vitro. Mice received POA injection of shGPR83 or non-silencing vectors and were monitored for CBT, motor activity, food intake body weight and circulating levels of IGF-1, insulin, leptin and adiponectin. Down-regulation of GPR83 in the POA resulted in a small (0.15°C) but significant reduction of CBT during the dark/active cycle of the day. Temperature reduction was followed by increased body weight gain independent of caloric intake. shGPR83 mice also had increased level of circulating adiponectin (31916±952pg/mL vs. 23474±1507pg/mL, P<.01) while no change was observed for insulin, IGF-1 or leptin. GPR83 may participate in central thermoregulation and the central control of circulating adiponectin. Further work is required to determine how GPR83 can affect POA WSN and what are the long term metabolic consequences of its down-regulation.     

Reciprocal relation of food intake and sympathetic activity: experimental observations and clinical implications

The neuropeptides, monoamines and many drugs involved with modulating food intake and fat stores have reciprocal effects on sympathetic activity and thermogenesis. Both serotonin, acting through 5HT1B/2C receptors, and norepinephrine acting through beta2 and/or beta3 receptors reduce food intake and augment sympathetic activity. Neuropeptide Y, beta-endorphin, orexin, galanin and melanin concentrating hormone all increase food intake and, where tested, reduce sympathetic activity. In contrast, a larger number of peptides including cholecystokinin, corticotrophin-releasing hormone/urocortin, enterostatin, leptin, CART and alpha-MSH reduce food intake and increase sympathetic activity. Nicotine, prostaglandin, dexfenfluramine and sibutramine also have this reciprocal effect on feeding and sympathetic nervous system (SNS) activity. Chronic administration of neuropeptide Y (NPY) can produce chronically increased food intake and obesity. This syndrome is similar to the ventromedial hypothalamus (VMH) syndrome and suggests that NPY must be acting as an inhibitory signal to stimulate a feeding system and inhibit sympathetic activity. The melanocortin receptor system may be particularly important in modulating food intake, because a transgenic mouse which does not express melanocortin-4 receptors is massively overweight. Adrenal glucocorticoids are important in obesity since adrenalectomy will reverse or prevent the development of all forms of obesity. The clinical importance of the sympathetic nervous system and food intake is emphasized by the inverse relation of sympathetic activity and body fat. The inhibition of food intake, lower body fat stores and higher energy expenditure in smokers also support this hypothesis. The reciprocal relationship between food intake and sympathetic activity is robust, suggesting that beta receptors in the periphery and brain may be involved in the control of feeding and a reduction in food intake in humans accounts for most of the weight loss with ephedrine and caffeine. We conclude that the inhibition of feeding by activating the SNS is an important satiety system which helps regulate body fat stores.