Modulation of HDL metabolism by the niacin receptor GPR109A in mouse hepatocytes. Biochem Pharmacol. 2010;80:1450-1457. [PMID: 20655299 DOI: 10.1016/j.bcp.2010.07.023]

GPCRs involved in metabolic diseases: pharmacotherapeutic development updates

G protein-coupled receptors (GPCRs) are expressed in a variety of cell types and tissues, and activation of GPCRs is involved in enormous metabolic pathways, including nutrient synthesis, transportation, storage or insulin sensitivity, etc. This review intends to summarize the regulation of metabolic homeostasis and mechanisms by a series of GPCRs, such as GPR91, GPR55, GPR119, GPR109a, GPR142, GPR40, GPR41, GPR43 and GPR120. With deep understanding of GPCR's structure and signaling pathways, it is attempting to uncover the role of GPCRs in major metabolic diseases, including metabolic syndrome, diabetes, dyslipidemia and nonalcoholic steatohepatitis, for which the global prevalence has risen during last two decades. An extensive list of agonists and antagonists with their chemical structures in a nature of small molecular compounds for above-mentioned GPCRs is provided as pharmacologic candidates, and their preliminary data of preclinical studies are discussed. Moreover, their beneficial effects in correcting abnormalities of metabolic syndrome, diabetes and dyslipidemia are summarized when clinical trials have been undertaken. Thus, accumulating data suggest that these agonists or antagonists might become as new pharmacotherapeutic candidates for the treatment of metabolic diseases.

Balancing NAD+ deficits with nicotinamide riboside: therapeutic possibilities and limitations

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development. We also discuss what are the current limitations in NR-based therapies and potential ways to overcome them. Overall, this review will not only provide tools to understand NAD+ biology and assess its changes in disease situations, but also to decide which NAD+ precursor could have the best therapeutic potential.

The Role of GPR109a Signaling in Niacin Induced Effects on Fed and Fasted Hepatic Metabolism

Signaling through GPR109a, the putative receptor for the endogenous ligand β-OH butyrate, inhibits adipose tissue lipolysis. Niacin, an anti-atherosclerotic drug that can induce insulin resistance, activates GPR109a at nM concentrations. GPR109a is not essential for niacin to improve serum lipid profiles. To better understand the involvement of GPR109a signaling in regulating glucose and lipid metabolism, we treated GPR109a wild-type (+/+) and knockout (-/-) mice with repeated overnight injections of saline or niacin in physiological states characterized by low (ad libitum fed) or high (16 h fasted) concentrations of the endogenous ligand, β-OH butyrate. In the fed state, niacin increased expression of apolipoprotein-A1 mRNA and decreased sterol regulatory element-binding protein 1 mRNA independent of genotype, suggesting a possible GPR109a independent mechanism by which niacin increases high-density lipoprotein (HDL) production and limits transcriptional upregulation of lipogenic genes. Niacin decreased fasting serum non-esterified fatty acid concentrations in both GPR109a +/+ and -/- mice. Independent of GPR109a expression, niacin blunted fast-induced hepatic triglyceride accumulation and peroxisome proliferator-activated receptor α mRNA expression. Although unaffected by niacin treatment, fasting serum HDL concentrations were lower in GPR109a knockout mice. Surprisingly, GPR109a knockout did not affect glucose or lipid homeostasis or hepatic gene expression in either fed or fasted mice. In turn, GPR109a does not appear to be essential for the metabolic response to the fasting ketogenic state or the acute effects of niacin.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

G protein-coupled receptors as potential targets for nonalcoholic fatty liver disease treatment

Nonalcoholic fatty liver disease (NAFLD) is a broad-spectrum disease, ranging from simple hepatic steatosis to nonalcoholic steatohepatitis, which can progress to cirrhosis and liver cancer. Abnormal hepatic lipid accumulation is the major manifestation of this disease, and lipotoxicity promotes NAFLD progression. In addition, intermediate metabolites such as succinate can stimulate the activation of hepatic stellate cells to produce extracellular matrix proteins, resulting in progression of NAFLD to fibrosis and even cirrhosis. G protein-coupled receptors (GPCRs) have been shown to play essential roles in metabolic disorders, such as NAFLD and obesity, through their function as receptors for bile acids and free fatty acids. In addition, GPCRs link gut microbiota-mediated connections in a variety of diseases, such as intestinal diseases, hepatic steatosis, diabetes, and cardiovascular diseases. The latest findings show that gut microbiota-derived acetate contributes to liver lipogenesis by converting dietary fructose into hepatic acetyl-CoA and fatty acids. GPCR agonists, including peptides and natural products like docosahexaenoic acid, have been applied to investigate their role in liver diseases. Therapies such as probiotics and GPCR agonists may be applied to modulate GPCR function to ameliorate liver metabolism syndrome. This review summarizes the current findings regarding the role of GPCRs in the development and progression of NAFLD and describes some preclinical and clinical studies of GPCR-mediated treatment. Overall, understanding GPCR-mediated signaling in liver disease may provide new therapeutic options for NAFLD.

GPR109A mediates the effects of hippuric acid on regulating osteoclastogenesis and bone resorption in mice

The G protein-coupled receptor 109 A (GPR109A) is robustly expressed in osteoclastic precursor macrophages. Previous studies suggested that GPR109A mediates effects of diet-derived phenolic acids such as hippuric acid (HA) and 3-(3-hydroxyphenyl) propionic acid (3-3-PPA) on promoting bone formation. However, the role of GPR109A in metabolic bone homeostasis and osteoclast differentiation has not been investigated. Using densitometric, bone histologic and molecular signaling analytic methods, we uncovered that bone mass and strength were significantly higher in tibia and spine of standard rodent diet weaned 4-week-old and 6-month-old GPR109A gene deletion (GPR109A-/-) mice, compared to their wild type controls. Osteoclast numbers in bone and in ex vivo bone marrow cell cultures were significantly decreased in GPR109A-/- mice compared to wild type controls. In accordance with these data, CTX-1 in bone marrow plasma and gene expression of bone resorption markers (TNFα, TRAP, Cathepsin K) were significantly decreased in GPR109A-/- mice, while on the other hand, P1NP was increased in serum from both male and female GPR109A-/- mice compared to their respective controls. GPR109A deletion led to suppressed Wnt/β-catenin signaling in osteoclast precursors to inhibit osteoclast differentiation and activity. Indeed, HA and 3-3-PPA substantially inhibited RANKL-induced GPR109A expression and Wnt/β-catenin signaling in osteoclast precursors and osteoclast differentiation. Resultantly, HA significantly inhibited bone resorption and increased bone mass in wild type mice, but had no additional effects on bone in GPR109A-/- mice compared with their respective untreated control mice. These results suggest an important role for GPR109A during osteoclast differentiation and bone resorption mediating effects of HA and 3-3-PPA on inhibiting bone resorption during skeletal development.

Metabolic Functions of G Protein-Coupled Receptors in Hepatocytes-Potential Applications for Diabetes and NAFLD

G protein-coupled receptors (GPCRs) are cell surface receptors that mediate the function of extracellular ligands. Understanding how GPCRs work at the molecular level has important therapeutic implications, as 30–40% of the drugs currently in clinical use mediate therapeutic effects by acting on GPCRs. Like many other cell types, liver function is regulated by GPCRs. More than 50 different GPCRs are predicted to be expressed in the mouse liver. However, knowledge of how GPCRs regulate liver metabolism is limited. A better understanding of the metabolic role of GPCRs in hepatocytes, the dominant constituent cells of the liver, could lead to the development of novel drugs that are clinically useful for the treatment of various metabolic diseases, including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). In this review, we describe the functions of multiple GPCRs expressed in hepatocytes and their role in metabolic processes.

Niacin improves maturation and cryo-tolerance of bovine in vitro matured oocytes: An experimental study

Background

Nicotinic acid (niacin) is a broad-spectrum lipid-modifying agent that has potent antioxidant properties and reduces the production of lipid peroxidation.

Objective

The purpose of the present study was to investigate the maturation, embryo development and cryo-tolerance merit, and levels of malondialdehyde (MDA), total oxidant status, and total antioxidant capacity following the supplementation of bovine oocytes maturation medium with different concentrations of niacin.

Materials and Methods

Immature cumulus-oocyte complexes were cultured in tissue culture medium-199 maturation media supplemented with 0, 100, 200, and 400 µM niacin under a standard in vitro culture condition. After 24 hr of culture, the nuclear maturation rate was assessed. Then, two groups of immature cumulus-oocyte complexes were cultured in TCM-199 either with or without 400 µM niacin and evaluated for embryo development. Also, matured cumulus-oocyte complexes in both groups were frozen using a standard vitrification procedure. After vitrification, oocytes were warmed in two steps and evaluated for embryo development. In addition, the level of total antioxidant capacity, total oxidant status, and MDA were measured.

Results

The results indicated that although the treatment with 400 µM niacin increased in vitro nuclear maturation (87.6±5.3), it did not improved the embryo development to the blastocyst stage. Higher cleavage and blastocyst rates were observed in vitrified oocytes that were cultured with supplemented 400 µM niacin compared to the control group (without niacin) (53.6±2.7 and 10.6±1.6 vs. 46.2±4.1 and 6.3±2.4, respectively). Also, the addition of 400 μM niacin to the maturation media could decrease MDA levels after maturation.

Conclusion

Niacin could improve the quality of in vitro embryo production (IVP) embryos and tolerance of bovine oocytes to vitrification.

Metabolite-Sensing G Protein-Coupled Receptors Connect the Diet-Microbiota-Metabolites Axis to Inflammatory Bowel Disease

Increasing evidence has indicated that diet and metabolites, including bacteria- and host-derived metabolites, orchestrate host pathophysiology by regulating metabolism, immune system and inflammation. Indeed, autoimmune diseases such as inflammatory bowel disease (IBD) are associated with the modulation of host response to diets. One crucial mechanism by which the microbiota affects the host is signaling through G protein-coupled receptors (GPCRs) termed metabolite-sensing GPCRs. In the gut, both immune and nonimmune cells express GPCRs and their activation generally provide anti-inflammatory signals through regulation of both the immune system functions and the epithelial integrity. Members of GPCR family serve as a link between microbiota, immune system and intestinal epithelium by which all these components crucially participate to maintain the gut homeostasis. Conversely, impaired GPCR signaling is associated with IBD and other diseases, including hepatic steatosis, diabetes, cardiovascular disease, and asthma. In this review, we first outline the signaling, function, expression and the physiological role of several groups of metabolite-sensing GPCRs. We then discuss recent findings on their role in the regulation of the inflammation, their existing endogenous and synthetic ligands and innovative approaches to therapeutically target inflammatory bowel disease.

Implications of altered NAD metabolism in metabolic disorders

Nicotinamide adenine dinucleotide (NAD) is an important coenzyme that participates in various energy metabolism pathways, including glycolysis, β-oxidation, and oxidative phosphorylation. Besides, it is a required cofactor for post-translational modifications such as ADP-ribosylation and deacetylation by poly (ADP-ribose) polymerases (PARPs) and sirtuins, respectively. Thus, NAD regulates energy metabolism, DNA damage repair, gene expression, and stress response through these enzymes. Numerous studies have shown that NAD levels decrease with aging and under disturbed nutrient conditions, such as obesity. Additionally, a decline in NAD levels is closely related to the development of various metabolic disorders, including diabetes and fatty liver disease. In addition, many studies have revealed that administration of NAD precursors, such as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR), efficiently increase NAD levels in various tissues and prevent such metabolic diseases. These NAD precursors are contained in natural foods, such as cow milk, vegetables, and meats. Therefore, altered NAD metabolism can be a practical target for nutritional intervention. Recently, several human clinical trials using NAD precursors have been conducted to investigate the safety, pharmacokinetics, and efficacy against metabolic disorders such as glucose intolerance. In this review, we summarize current knowledge on the implications of NAD metabolism in metabolic diseases and discuss the outcomes of recent human clinical trials.

Butyrate, a metabolite of intestinal bacteria, enhances sleep

Emerging evidence suggests that the intestinal microbiota is a source of sleep-promoting signals. Bacterial metabolites and components of the bacterial cell wall are likely to provide important links between the intestinal commensal flora and sleep-generating mechanisms in the brain. Butyrate is a short-chain fatty acid produced by the intestinal bacteria by the fermentation of nondigestible polysaccharides. We tested the hypothesis that butyrate may serve as a bacterial-derived sleep-promoting signal. Oral gavage administration of tributyrin, a butyrate pro-drug, elicited an almost 50% increase in non-rapid-eye movement sleep (NREMS) in mice for 4 hours after the treatment. Similarly, intraportal injection of butyrate led to prompt and robust increases in NREMS in rats. In the first 6 hours after the butyrate injection, NREMS increased by 70%. Both the oral and intraportal administration of butyrate led to a significant drop in body temperature. Systemic subcutaneous or intraperitoneal injection of butyrate did not have any significant effect on sleep or body temperature. The results suggest that the sleep-inducing effects of butyrate are mediated by a sensory mechanism located in the liver and/or in the portal vein wall. Hepatoportal butyrate-sensitive mechanisms may play a role in sleep modulation by the intestinal microbiota.

Niacin fine-tunes energy homeostasis through canonical GPR109A signaling

The incidence of overweight and obesity has become a global public health problem, constituting a major risk factor for numerous comorbidities. Despite tremendous efforts, effective pharmacological agents for the treatment of obesity are still limited. Here, we showed that in contrast to lactate receptor GPR81, niacin receptor GPR109A-deficient mice had progressive weight gain and hepatic fat accumulation. Using high-fat diet-induced mouse model of obesity, we demonstrated that niacin treatment apparently protected against obesity without affecting food intake in wild-type mice but not in GPR109A-deficient mice. Further investigation showed that niacin treatment led to a remarkable inhibition of hepatic de novo lipogenesis. Additionally, we demonstrated that niacin treatment triggered brown adipose tissue and/or white adipose tissue thermogenic activity via activation of GPR109A. Moreover, we observed that mice exposed to niacin exhibited a dramatic decrease in intestinal absorption of sterols and fatty acids. Taken together, our findings demonstrate that acting on GPR109A, niacin shows the potential to maintain energy homeostasis through multipathways, representing a potential approach to the treatment of obesity, diabetes and cardiovascular disease.-Ye, L., Cao, Z., Lai, X., Wang, W., Guo, Z., Yan, L., Wang, Y., Shi, Y., Zhou, N. Niacin fine-tunes energy homeostasis through canonical GPR109A signaling.

The nutrition-gut microbiome-physiology axis and allergic diseases

Dietary and bacterial metabolites influence immune responses. This raises the question whether the increased incidence of allergies, asthma, some autoimmune diseases, cardiovascular disease, and others might relate to intake of unhealthy foods, and the decreased intake of dietary fiber. In recent years, new knowledge on the molecular mechanisms underpinning a 'diet-gut microbiota-physiology axis' has emerged to substantiate this idea. Fiber is fermented to short chain fatty acids (SCFAs), particularly acetate, butyrate, and propionate. These metabolites bind 'metabolite-sensing' G-protein-coupled receptors such as GPR43, GPR41, and GPR109A. These receptors play fundamental roles in the promotion of gut homeostasis and the regulation of inflammatory responses. For instance, these receptors and their metabolites influence Treg biology, epithelial integrity, gut homeostasis, DC biology, and IgA antibody responses. The SCFAs also influence gene transcription in many cells and tissues, through their inhibition of histone deacetylase expression or function. Contained in this mix is the gut microbiome, as commensal bacteria in the gut have the necessary enzymes to digest dietary fiber to SCFAs, and dysbiosis in the gut may affect the production of SCFAs and their distribution to tissues throughout the body. SCFAs can epigenetically modify DNA, and so may be one mechanism to account for diseases with a 'developmental origin', whereby in utero or post-natal exposure to environmental factors (such as nutrition of the mother) may account for disease later in life. If the nutrition-gut microbiome-physiology axis does underpin at least some of the Western lifestyle influence on asthma and allergies, then there is tremendous scope to correct this with healthy foodstuffs, probiotics, and prebiotics.

Dimethyl fumarate mediates Nrf2-dependent mitochondrial biogenesis in mice and humans

The induction of mitochondrial biogenesis could potentially alleviate mitochondrial and muscle disease. We show here that dimethyl fumarate (DMF) dose-dependently induces mitochondrial biogenesis and function dosed to cells in vitro, and also dosed in vivo to mice and humans. The induction of mitochondrial gene expression is more dependent on DMF's target Nrf2 than hydroxycarboxylic acid receptor 2 (HCAR2). Thus, DMF induces mitochondrial biogenesis primarily through its action on Nrf2, and is the first drug demonstrated to increase mitochondrial biogenesis with in vivo human dosing. This is the first demonstration that mitochondrial biogenesis is deficient in Multiple Sclerosis patients, which could have implications for MS pathophysiology and therapy. The observation that DMF stimulates mitochondrial biogenesis, gene expression and function suggests that it could be considered for mitochondrial disease therapy and/or therapy in muscle disease in which mitochondrial function is important.

GPCR-Mediated Signaling of Metabolites

In addition to their bioenergetic intracellular function, several classical metabolites act as extracellular signaling molecules activating cell-surface G-protein-coupled receptors (GPCRs), similar to hormones and neurotransmitters. "Signaling metabolites" generated from nutrients or by gut microbiota target primarily enteroendocrine, neuronal, and immune cells in the lamina propria of the gut mucosa and the liver and, through these tissues, the rest of the body. In contrast, metabolites from the intermediary metabolism act mainly as metabolic stress-induced autocrine and paracrine signals in adipose tissue, the liver, and the endocrine pancreas. Importantly, distinct metabolite GPCRs act as efficient pro- and anti-inflammatory regulators of key immune cells, and signaling metabolites may thus function as important drivers of the low-grade inflammation associated with insulin resistance and obesity. The concept of key metabolites as ligands for specific GPCRs has broadened our understanding of metabolic signaling significantly and provides a number of novel potential drug targets.

Dietary Niacin Supplementation Suppressed Hepatic Lipid Accumulation in Rabbits

An experiment was conducted to investigate the effect of niacin supplementation on hepatic lipid metabolism in rabbits. Rex Rabbits (90 d, n = 32) were allocated to two equal treatment groups: Fed basal diet (control) or fed basal diet with additional 200 mg/kg niacin supplementation (niacin). The results show that niacin significantly increased the levels of plasma adiponectin, hepatic apoprotein B and hepatic leptin receptors mRNA (p<0.05), but significantly decreased the hepatic fatty acid synthase activity and adiponectin receptor 2, insulin receptor and acetyl-CoA carboxylase mRNA levels (p<0.05). Plasma insulin had a decreasing tendency in the niacin treatment group compared with control (p = 0.067). Plasma very low density lipoproteins, leptin levels and the hepatic adiponectin receptor 1 and carnitine palmitoyl transferase 1 genes expression were not significantly altered with niacin addition to the diet (p>0.05). However, niacin treatment significantly inhibited the hepatocytes lipid accumulation compared with the control group (p<0.05). In conclusion, niacin treatment can decrease hepatic fatty acids synthesis, but does not alter fatty acids oxidation and triacylglycerol export. And this whole process attenuates lipid accumulation in liver. Besides, the hormones of insulin, leptin and adiponectin are associated with the regulation of niacin in hepatic lipid metabolism in rabbits.

Anti-inflammatory effects of the hydroxycarboxylic acid receptor 2

The hydroxycarboxylic acid receptors (HCA1-3) are a family of G-protein-coupled receptors that are critical for sensing endogenous intermediates of metabolism. All three receptors are predominantly expressed on adipocytes and mediate anti-lipolytic effects. In addition to adipocytes, HCA2 is highly expressed on immune cells, including macrophages, monocytes, neutrophils and dermal dendritic cells, among other cell types. The endogenous ligand for HCA2 is beta-hydroxybutyrate (β-OHB), a ketone body produced by the liver through β-oxidation when an individual is in a negative energy balance. Recent studies demonstrate that HCA2 mediates profound anti-inflammatory effects in a variety of tissues, indicating that HCA2 may be an important therapeutic target for treating inflammatory disease processes. This review summarizes the roles of HCA2 on inflammation in a number of tissues and clinical states.

Characterization of the hydroxycarboxylic acid receptor 2 in cats

The hydroxycarboxylic acid receptor 2 (HCA2) belongs to a family of nutrient-sensing receptors that bind β-hydroxybutyrate, an alternative fuel source produced during a negative energy balance. The HCA2 receptor has not been identified or characterized in cats. Therefore, the following were the objectives of this study: (1) identify the feline HCA2 receptor protein sequence and compare against known human and rodent sequences, (2) determine tissue distribution and relative expression in lean, healthy cats, and (3) demonstrate in vitro functionality in feline adipose tissue. Tissues (n = 6) and primary adipocytes (n = 4) were collected from lean, healthy, female cats. The published genomic sequence for cats was used to design primers for polymerase chain reaction isolation of HCA2. Relative tissue distribution was evaluated using reverse transcriptase-polymerase chain reaction with RNA isolated from 9 different tissues (spleen, pancreas, lymph node, jejunum, kidney, liver, heart, and subcutaneous and abdominal adipose tissue). Receptor function was evaluated in primary feline adipocyte culture, and changes were compared with basal lipolysis. The in silico predicted feline HCA2 protein sequence exhibited 83.1% and 86.5% amino acid similarity to human and mouse sequences, respectively. The feline HCA2 receptor is predominantly expressed in adipose tissue and spleen. Exposure of feline adipocytes to niacin, a pharmacologic ligand of HCA2, inhibited lipolysis to a similar degree as insulin, a potent lipolytic inhibitor. In conclusion, the feline HCA2 receptor is similar to human and murine receptors in sequence, distribution, and functionality. By gaining a better understanding of the HCA2 receptor in cats, we will be able to better manage feline patients.

NAD(+) Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus

NAD(+) has emerged as a vital cofactor that can rewire metabolism, activate sirtuins, and maintain mitochondrial fitness through mechanisms such as the mitochondrial unfolded protein response. This improved understanding of NAD(+) metabolism revived interest in NAD(+)-boosting strategies to manage a wide spectrum of diseases, ranging from diabetes to cancer. In this review, we summarize how NAD(+) metabolism links energy status with adaptive cellular and organismal responses and how this knowledge can be therapeutically exploited.

Effects of formulation design on niacin therapeutics: mechanism of action, metabolism, and drug delivery

Niacin is a highly effective, lipid regulating drug associated with a number of metabolically induced side effects such as prostaglandin (PG) mediated flushing and hepatic toxicity. In an attempt to reduce the development of these adverse effects, scientists have investigated differing methods of niacin delivery designed to control drug release and alter metabolism. However, despite successful formulation of various orally based capsule and tablet delivery systems, patient adherence to niacin therapy is still compromised by adverse events such as PG-induced flushing. While the primary advantage of orally dosed formulations is ease of use, alternative delivery options such as transdermal delivery or polymeric micro/nanoparticle encapsulation for oral administration have shown promise in niacin reformulation. However, the effectiveness of these alternative delivery options in reducing inimical effects of niacin and maintaining drug efficacy is still largely unknown and requires more in-depth investigation. In this paper, we present an overview of niacin applications, its metabolic pathways, and current drug delivery formulations. Focus is placed on oral immediate, sustained, and extended release niacin delivery as well as combined statin and/or prostaglandin antagonist niacin formulation. We also examine and discuss current findings involving transdermal niacin formulations and polymeric micro/nanoparticle encapsulated niacin delivery.

Gut microbiota-derived short-chain Fatty acids, T cells, and inflammation

T cells are central players in the regulation of adaptive immunity and immune tolerance. In the periphery, T cell differentiation for maturation and effector function is regulated by a number of factors. Various factors such as antigens, co-stimulation signals, and cytokines regulate T cell differentiation into functionally specialized effector and regulatory T cells. Other factors such as nutrients, micronutrients, nuclear hormones and microbial products provide important environmental cues for T cell differentiation. A mounting body of evidence indicates that the microbial metabolites short-chain fatty acids (SCFAs) have profound effects on T cells and directly and indirectly regulate their differentiation. We review the current status of our understanding of SCFA functions in regulation of peripheral T cell activity and discuss their impact on tissue inflammation.

Niacin receptor activation improves human microvascular endothelial cell angiogenic function during lipotoxicity

OBJECTIVE

Niacin (nicotinic acid) as a monotherapy can reduce vascular disease risk, but its mechanism of action remains controversial, and may not be dependent on systemic lipid modifying effects. Niacin has recently been shown to improve endothelial function and vascular regeneration, independent of correcting dyslipidemia, in rodent models of vascular injury and metabolic disease. As a potential biosynthetic precursor for NAD(+), niacin could elicit these vascular benefits through NAD(+)-dependent, sirtuin (SIRT) mediated responses. Alternatively, niacin may act through its receptor, GPR109A, to promote endothelial function, though endothelial cells are not known to express this receptor. We hypothesized that niacin directly improves endothelial cell function during exposure to lipotoxic conditions and sought to determine the potential mechanism(s) involved.

METHODS AND RESULTS

Angiogenic function in excess palmitate was assessed by tube formation following treatment of human microvascular endothelial cells (HMVEC) with either a relatively low concentration of niacin (10 μM), or nicotinamide mononucleotide (NMN) (1 μM), a direct NAD(+) precursor. Although both niacin and NMN improved HMVEC tube formation during palmitate overload, only NMN increased cellular NAD(+) and SIRT1 activity. We further observed that HMVEC express GRP109A. Activation of this receptor with either acifran or MK-1903 recapitulated niacin-induced improvements in HMVEC tube formation, while GPR109A siRNA diminished the effect of niacin.

CONCLUSION

Niacin, at a low concentration, improves HMVEC angiogenic function under lipotoxic conditions, likely independent of NAD(+) biosynthesis and SIRT1 activation, but rather through niacin receptor activation.

Energy and metabolic sensing G protein-coupled receptors during lactation-induced changes in energy balance

The free fatty acid receptor FFA1, FFA2, and FFA3 and hydroxy-carboxylic acid receptor (HCA2) are G protein-coupled receptors, acting as energy and metabolic sensors. Herein, we characterized the tissue-specific mRNA abundance of genes encoding for these receptors at different stages of lactation. In addition, potential effects of supplementation with or without conjugated linoleic acids (CLA) were tested. Tissues from pluriparous cows (subcutaneous adipose tissue [SAT] and liver) and from primiparous cows (3 SAT locations, 3 visceral adipose tissues, liver, mammary gland, and skeletal muscle) were used from 2 separate trials. In primiparous cows, the mRNA abundance of all receptors (FFA3 was not detectable by the applied protocol in muscle and udder) was lowest in muscle (P < 0.05). With the exception of FFA1, gene expression of the investigated receptors was higher in adipose tissue than in the non-adipose tissue. Expression of FFA1 in liver (P < 0.03), FFAR2 in SAT (P < 0.01), and HCA2 in SAT (P < 0.01) from pluriparous cows changed during the observation period (days 21 to 252 relative to parturition). The correlation between mRNA abundance of HCA2 and peroxisome proliferator-activated receptor gamma (PPARG) and likewise PPARG2 (P < 0.01) in SAT indicates a link between HCA2 and PPARG. Differences in receptor mRNA abundance between the CLA-fed and the control animals were scarce and limited to HCA2 and FFA1 in 1 and 2 time points, respectively (less hepatic HCA2mRNA in CLA-fed pluriparous cows and greater FFA1 mRNA abundance in 2 visceral adipose tissue depots in CLA-treated primiparous cows). In view of the metabolic changes occurring during the different phases of lactation, in particular, the altered concentrations of non-esterified fatty acids and β-hydroxybutyrate acting as receptor ligands, the longitudinal tissue-specific characterization provided herein allows for a first insight into the regulation of these receptors at the gene expression level.

The role of short-chain fatty acids in health and disease

There is now an abundance of evidence to show that short-chain fatty acids (SCFAs) play an important role in the maintenance of health and the development of disease. SCFAs are a subset of fatty acids that are produced by the gut microbiota during the fermentation of partially and nondigestible polysaccharides. The highest levels of SCFAs are found in the proximal colon, where they are used locally by enterocytes or transported across the gut epithelium into the bloodstream. Two major SCFA signaling mechanisms have been identified, inhibition of histone deacetylases (HDACs) and activation of G-protein-coupled receptors (GPCRs). Since HDACs regulate gene expression, inhibition of HDACs has a vast array of downstream consequences. Our understanding of SCFA-mediated inhibition of HDACs is still in its infancy. GPCRs, particularly GPR43, GPR41, and GPR109A, have been identified as receptors for SCFAs. Studies have implicated a major role for these GPCRs in the regulation of metabolism, inflammation, and disease. SCFAs have been shown to alter chemotaxis and phagocytosis; induce reactive oxygen species (ROS); change cell proliferation and function; have anti-inflammatory, antitumorigenic, and antimicrobial effects; and alter gut integrity. These findings highlight the role of SCFAs as a major player in maintenance of gut and immune homeostasis. Given the vast effects of SCFAs, and that their levels are regulated by diet, they provide a new basis to explain the increased prevalence of inflammatory disease in Westernized countries, as highlighted in this chapter.

L-2-oxothiazolidine-4-carboxylic acid attenuates oxidative stress and inflammation in retinal pigment epithelium

PURPOSE

Oxidant- and inflammation-induced damage to retinal pigment epithelial (RPE) cells is central to the pathogenesis of age-related macular degeneration (AMD). Thus, developing novel strategies to protect these cells is important. We reported previously on the robust antioxidant and therefore cell-protective effects of the cysteine pro-drug L-2-oxothiazolidine-4-carboxylic acid (OTC) in cultured human RPE cells. New reports citing a novel anti-inflammatory role for OTC in addition to the known glutathione-stimulating and antioxidant properties emerged recently; however, this role has not been evaluated in RPE cells or in intact retina. Given the crucial causative roles of oxidative stress and inflammation in AMD pathogenesis, knowing whether OTC might exhibit a similar benefit in this cell and tissue type has high clinical relevance; thus, we evaluated OTC in the present study.

METHODS

ARPE-19 and primary RPE cells isolated from wild-type, Gpr109a(-/-) , or Slc5a8(-/-) mouse eyes were exposed to TNF-α in the presence or absence of OTC, followed by analysis of IL-6 and Ccl2 expression with real-time quantitative polymerase chain reaction or enzyme-linked immunosorbent assay. Cellular and molecular markers of inflammation and oxidative stress (i.e., IL-1β, TGF-β, ABCG1, ABCA1, reduced glutathione, and dihydroethidium) were evaluated in Ccl2(-/-)/Cx3cr1(-/-) double knockout mice on rd8 background (DKO rd8) treated with OTC (10 mg/ml) in drinking water for a period of 5 months.

RESULTS

OTC treatment significantly inhibited the expression and secretion of IL-6 and Ccl2 in TNF-α-stimulated ARPE-19 cells. Studies conducted using DKO rd8 animals treated with OTC in drinking water confirmed these findings. Cellular and molecular markers of inflammation were significantly suppressed in the retinas of the OTC-treated DKO rd8 animals. Subsequent in vitro and in vivo studies of the possible mechanism(s) to explain these actions revealed that although OTC is an agonist of the anti-inflammatory G-protein coupled receptor GPR109A and a transportable substrate of the sodium-coupled monocarboxylate transporter SMCT1 (SLC5A8), these properties may play a role but do not explain entirely the anti-inflammatory effects this compound elicits in cultured RPE cells and the intact mouse retina.

CONCLUSIONS

This study represents, to our knowledge, the first report of the suppressive effects of OTC on inflammation in cultured RPE cells and on inflammation and oxidative stress in the retina in vivo.

Screening of Different Organs of Rats for HCA2 Receptor mRNA

Interest in hydroxy - carboxylic acid 2 (HCA2) receptor has been raised since it is the target of antidyslipidemic drug nicotinic acid. The present study aimed to evaluate the presence of mRNA of this receptor in different organs of laboratory rat. Twenty two different organs of rats including mesenteric fat, epididymis (head, body and tail), testis, ovary, xiphoid process, liver, adrenal gland, femoral head, proximal epiphyseal and metaphyseal bone marrow of femur, esophagus, glandular stomach, forestomach, intestines, colons, heart, spleen, kidney, trachea, lung, skeletal muscle (quadriceps), cerebrum and cerebellum were removed and examined for HCA2 mRNA by RT- PCR method. The mRNA for HCA2 receptor was detected in all analyzed tissues. In conclusion, the different organs of rat express HCA2 receptor mRNA which makes a proper animal model for future studies on the physiological and pharmacological roles of this receptor in vivo.

Abundance of adiponectin system and G-protein coupled receptor GPR109A mRNA in adipose tissue and liver of F2 offspring cows of Charolais × German Holstein crosses that differ in body fat accumulation

In addition to its role in energy storage, adipose tissue (AT) is an important endocrine organ and it secretes adipokines. The adipokine adiponectin improves insulin sensitivity by activation of its receptors AdipoR1 and AdipoR2. Lipolysis in AT is downregulated by the G-protein coupled receptor (GPR109A), which binds the endogenous ligand β-hydroxybutyrate (BHBA). Insulin sensitivity is reduced during the transition from late pregnancy to early lactation in dairy cattle and BHBA is increased postpartum, implying the involvement of the adiponectin system and GPR109A in this process. The aim of the current investigation was to study the effect of the genetic background of cows on the mRNA abundance of the adiponectin system, as well as GPR109A, in an F(2) population of 2 Charolais × German Holstein families. These families were deduced from full- and half-sibs sharing identical but reciprocal paternal and maternal Charolais grandfathers. The animals of the 2 families showed significant differences in fat accretion and milk secretion and were designated fat-type (high fat accretion but low milk production) and lean-type (low fat accretion but high milk production). The mRNA of the adiponectin system and GPR109A were quantified by real-time PCR in different fat depots (subcutaneous from back, mesenteric, kidney) and liver. The mRNA data were correlated with AT masses (intermuscular topside border fat, kidney, mesenteric, omental, total inner fat mass, total subcutaneous fat mass, and total fat mass) and blood parameters (glucose, nonesterified fatty acids, BHBA, urea, insulin, and glucagon). The abundance of adiponectin system mRNA was higher in discrete AT depots of fat-type cows [adiponectin mRNA in mesenteric fat (trend), AdipoR1 in kidney and mesenteric AT, and AdipoR2 in subcutaneous fat (trend)] than in lean-type cows. More GPR109A mRNA was found in kidney fat of the lean-type family than in that of the fat-type family. In liver, the abundance of AdipoR2 and GPR109A (trend) mRNA was higher in lean-type than in fat-type cows. Correlation analyses disclosed clear differences between the groups. In total, the results revealed obvious disparities for the mRNA targets between the 2 families with common but reciprocal paternal and maternal genetic backgrounds. Visceral AT mass of both families showed most correlations with the mRNA abundance of the target genes in different AT depots. The effect of adiponectin secretion, especially by visceral AT depots, on liver metabolism should be clarified in further studies.

Nicotinic acid inhibits hepatic APOA gene expression: studies in humans and in transgenic mice

Elevated plasma lipoprotein(a) (LPA) levels are recognized as an independent risk factor for cardiovascular diseases. Our knowledge on LPA metabolism is incomplete, which makes it difficult to develop LPA-lowering medications. Nicotinic acid (NA) is the main drug recommended for the treatment of patients with increased plasma LPA concentrations. The mechanism of NA in lowering LPA is virtually unknown. To study this mechanism, we treated transgenic (tg) APOA mice with NA and measured plasma APOA and hepatic mRNA levels. In addition, mouse and human primary hepatocytes were incubated with NA, and the expression of APOA was followed. Feeding 1% NA reduced plasma APOA and hepatic expression of APOA in tg-APOA mice. Experiments with cultured human and mouse primary hepatocytes in addition to reporter assays performed in HepG2 cells revealed that NA suppresses APOA transcription. The region between -1446 and -857 of the human APOA promoter harboring several cAMP response element binding sites conferred the negative effect of NA. In accordance, cAMP stimulated APOA transcription, and NA reduced hepatic cAMP levels. It is suggested that cAMP signaling might be involved in reducing APOA transcription, which leads to the lowering of plasma LPA.

G protein-coupled receptors for energy metabolites as new therapeutic targets

Several G protein-coupled receptors (GPCRs) that are activated by intermediates of energy metabolism - such as fatty acids, saccharides, lactate and ketone bodies - have recently been discovered. These receptors are able to sense metabolic activity or levels of energy substrates and use this information to control the secretion of metabolic hormones or to regulate the metabolic activity of particular cells. Moreover, most of these receptors appear to be involved in the pathophysiology of metabolic diseases such as diabetes, dyslipidaemia and obesity. This Review summarizes the functions of these metabolite-sensing GPCRs in physiology and disease, and discusses the emerging pharmacological agents that are being developed to target these GPCRs for the treatment of metabolic disorders.

Niacin reduces plasma CETP levels by diminishing liver macrophage content in CETP transgenic mice

The anti-dyslipidemic drug niacin has recently been shown to reduce the hepatic expression and plasma levels of CETP. Since liver macrophages contribute to hepatic CETP expression, we investigated the role of macrophages in the CETP-lowering effect of niacin in mice. In vitro studies showed that niacin does not directly attenuate CETP expression in macrophages. Treatment of normolipidemic human CETP transgenic mice, fed a Western-type diet with niacin for 4 weeks, significantly reduced the hepatic cholesterol concentration (-20%), hepatic CETP gene expression (-20%), and plasma CETP mass (-30%). Concomitantly, niacin decreased the hepatic expression of CD68 (-44%) and ABCG1 (-32%), both of which are specific markers for the hepatic macrophage content. The decrease in hepatic CETP expression was significantly correlated with the reduction of hepatic macrophage markers. Furthermore, niacin attenuated atherogenic diet-induced inflammation in liver, as evident from decreased expression of TNF-alpha (-43%). Niacin similarly decreased the macrophage markers and absolute macrophage content in hyperlipidemic APOE*3-Leiden.CETP transgenic mice on a Western-type diet. In conclusion, niacin decreases hepatic CETP expression and plasma CETP mass by attenuating liver inflammation and macrophage content in response to its primary lipid-lowering effect, rather than by attenuating the macrophage CETP expression level.

Nicotinic acid: Do we know how it works after 55 years of clinical experience

Nicotinic acid (NA) comprises the oldest hypolipidemic drug, in use since 1955. Despite its established broad spectrum effect on lipid profile and the associated reduction in cardiovascular morbidity and mortality, the mechanisms by which NA achieves its beneficial effects remain elusive. Regarding the NA-associated reduction in triglyceride and low density lipoprotein cholesterol levels, data are controversial. The prevailing view which suggested that NA inhibits lipolysis and decreases free fatty acid (FFA) release both via activation of adipose tissue G-protein receptor-109A (GPR109A) and via inhibition of hepatic triglyceride synthesis is currently debated by the observation that the initially decreased FFA levels rebound during long-term NA treatment even though the beneficial NA effects on lipid metabolism are preserved, while other mechanisms involving modulation of transcription and translation pathways are emerging. In addition, NA has been demonstrated to affect high density lipoprotein (HDL) particles remodeling in a number of ways, including reducing cholesterol ester transfer protein levels and activity, increasing apolipoprotein A-I levels, eliminating HDL hepatic uptake, increasing cholesterol efflux via ATP-binding cassette A1, inhibiting hepatic lipase, thereby overall increasing the plasma residence time of HDL and apoA-I with retention of cholesterol esters in HDL. Focus of this article is to present the mechanisms by which NA exerts its broad spectrum hypolipidemic actions.

Extracellular nucleotides inhibit insulin receptor signaling, stimulate autophagy and control lipoprotein secretion

Hyperglycemia is associated with abnormal plasma lipoprotein metabolism and with an elevation in circulating nucleotide levels. We evaluated how extracellular nucleotides may act to perturb hepatic lipoprotein secretion. Adenosine diphosphate (ADP) (>10 µM) acts like a proteasomal inhibitor to stimulate apoB100 secretion and inhibit apoA-I secretion from human liver cells at 4 h and 24 h. ADP blocks apoA-I secretion by stimulating autophagy. The nucleotide increases cellular levels of the autophagosome marker, LC3-II, and increases co-localization of LC3 with apoA-I in punctate autophagosomes. ADP affects autophagy and apoA-I secretion through P2Y₁₃. Overexpression of P2Y₁₃ increases cellular LC3-II levels by ∼50% and blocks induction of apoA-I secretion. Conversely, a siRNA-induced reduction in P2Y₁₃ protein expression of 50% causes a similar reduction in cellular LC3-II levels and a 3-fold stimulation in apoA-I secretion. P2Y₁₃ gene silencing blocks the effects of ADP on autophagy and apoA-I secretion. A reduction in P2Y₁₃ expression suppresses ERK1/2 phosphorylation, increases the phosphorylation of IR-β and protein kinase B (Akt) >3-fold, and blocks the inhibition of Akt phosphorylation by TNFα and ADP. Conversely, increasing P2Y₁₃ expression significantly inhibits insulin-induced phosphorylation of insulin receptor (IR-β) and Akt, similar to that observed after treatment with ADP. Nucleotides therefore act through P2Y₁₃, ERK1/2 and insulin receptor signaling to stimulate autophagy and affect hepatic lipoprotein secretion.

GPR109A as an anti-inflammatory receptor in retinal pigment epithelial cells and its relevance to diabetic retinopathy

PURPOSE

Retinal pigment epithelium (RPE) expresses GPR109A, a receptor for the vitamin niacin and the ketone body β-hydroxybutyrate (β-HB). Because diabetes results in elevated levels of β-HB, here we studied expression of the receptor in diabetic retina. We also investigated its functional relevance in RPE.

METHODS

Retinal expression of GPR109A in diabetic mice and postmortem human eyes was evaluated by quantitative PCR (qPCR). ARPE-19 cells and primary wild-type and Gpr109a(-/-) mouse RPE cells were exposed to TNF-α in the presence or absence of niacin or β-HB, followed by analysis of IL-6 and Ccl2 expression via real-time qPCR and ELISA.

RESULTS

GPR109A expression was increased in diabetic mouse and human retina. TNF-α increased the expression and secretion of IL-6 and Ccl2 in ARPE-19 cells. Niacin and β-HB suppressed these effects, implicating GPR109A as the target responsible for mediation of the observed effects. Primary RPE cells from wild-type mice behaved similarly. In contrast, GPR109A ligands failed to suppress TNF-α-induced expression and secretion of IL-6 and Ccl2 in primary RPE cells from Gpr109a(-/-) mice, confirming that the observed anti-inflammatory effects were mediated specifically by Gpr109a.

CONCLUSIONS

GPR109A plays an anti-inflammatory role in RPE and its expression is upregulated in diabetes. Inflammation is a key causative factor in the pathogenesis of diabetic retinopathy. We speculate that the increased expression of GPR109A and elevation of its ligand β-HB in diabetes are mechanisms by which the tissue attempts to fight inflammation in this disease. Pharmacological activation of GPR109A may therefore have therapeutic potential in clinical management of diabetic retinopathy.

An unusual distribution of the niacin receptor in cattle

Responses to pharmacological doses of niacin, an agonist for GPR109A (niacin receptor), were different in cattle than in humans and rodents. Thus, the tissue distribution of GPR109A was investigated in cattle. Samples of tail head fat, back fat, perirenal fat, longissimus muscle, and liver were analyzed for abundance of GPR109A mRNA by quantitative real-time reverse transcription-PCR and for abundance of GPR109A protein by Western blotting. Niacin receptor transcript and protein were detected in all tissues analyzed. The mRNA for GPR109A was more abundant in liver than in the other tissues sampled (GPR109A:RPS9 mRNA abundance = 0.56 in liver compared with 0.06 in longissimus muscle, 0.15 in kidney fat, 0.11 in back fat, 0.23 in tail head fat; standard error of the mean = 0.028). Additionally, mRNA for GPR109A was found (GPR109A:RPS9 mRNA abundance ≥ 0.004) in each of the 5 regions of bovine brain that were analyzed: cerebral cortex, cerebellum, thalamus, hypothalamus, and brain stem. Evaluation of liver tissue by immunofluorescence suggested that GPR109A was expressed in parenchymal cells and not localized exclusively to immune-system cells. Finally, analysis of the putative bovine GPR109A sequence verified that AA residues required for binding niacin in human GPR109A are conserved, suggesting that the bovine sequence identified encodes a functional niacin receptor. The identification of GPR109A in bovine liver, muscle, and brain is a novel finding.

Effects of nicotinic acid on gene expression: potential mechanisms and implications for wanted and unwanted effects of the lipid-lowering drug

CONTEXT

Nicotinic acid (NA), or niacin, lowers circulating levels of lipids, including triglycerides, very low-density lipoprotein-cholesterol, and low-density lipoprotein-cholesterol. The lipid-lowering effects have been attributed to its effect to inhibit lipolysis in adipocytes and thus lower plasma free fatty acid (FFA) level. However, evidence accumulates that the FFA-lowering effect may account for only a fraction of NA effects on plasma lipids, and other mechanisms may be involved. Recent studies have reported NA effects on gene expression in various tissues in vivo and in cultured cells in vitro.

EVIDENCE ACQUISITION

We reviewed articles reporting NA effects on gene expression, identified by searching PubMed, focusing on potential underlying mechanisms and implications for unexplained NA effects.

CONCLUSION

The effects of NA on gene expression may be mediated directly via the NA receptor in the affected cells, indirectly via changes in circulating FFA or hormone levels induced by NA, or by activating the transcription factor FOXO1 in insulin-sensitive tissues. NA effects on gene expression provide new insights into previously unexplained NA effects, such as FFA-independent lipid-lowering effects, FFA rebound, and insulin resistance observed in clinics during NA treatment.

Pharmacologic suppression of hepatic ATP-binding cassette transporter 1 activity in mice reduces high-density lipoprotein cholesterol levels but promotes reverse cholesterol transport

BACKGROUND

The role of hepatic ATP-binding cassette transporter 1 (ABCA1) in maintaining plasma high density lipoprotein cholesterol (HDL-C) levels is well established, but its role in reverse cholesterol transport (RCT) is unclear. Probucol is a compound that reduces HDL-C levels but also reduces atherosclerosis in animal models and xanthomas in humans. The aim of the present study was to test the hypothesis that probucol inhibits hepatic ABCA1 activity, thereby reducing HDL-C levels but promoting RCT from macrophages.

METHODS AND RESULTS

Wild-type (WT) C57BL/6 mice and scavenger receptor class B type I (SR-BI) knockout mice were fed a chow diet containing 0.5% probucol or normal chow for 2 weeks. In WT mice, probucol, despite decreasing HDL-C by >80%, effectively maintained macrophage RCT. In SR-BI knockout mice, probucol also substantially reduced HDL-C but significantly increased macrophage RCT. Furthermore, probucol significantly enhanced the excretion of HDL-derived cholesterol into feces in both WT and SR-BI knockout mice. Probucol inhibited ABCA1-dependent cholesterol efflux from mouse primary hepatocytes, and this effect was shown to be responsible for the effect of probucol on increasing the fecal excretion of HDL-derived cholesterol in vivo.

CONCLUSIONS

We demonstrate that pharmacological inhibition of hepatic ABCA1 activity with probucol reduced HDL-C levels but promoted RCT through diversion of HDL-derived cholesterol from efflux back into plasma instead to excretion in the bile. These results explain the beneficial effects of probucol on atherosclerosis and xanthomas despite its HDL-lowering effects and suggest that inactivation of hepatic ABCA1 leads to increased RCT despite reducing plasma HDL-C levels.

Future of GPR109A agonists in the treatment of dyslipidaemia

Abnormal blood lipids are the major modifiable risk factor underlying the development of cardiovascular disease. Niacin has a profound ability to reduce low-density lipoprotein-C, very low-density lipoprotein-C and triglycerides and is the most effective pharmacological agent to increase high-density lipoprotein-C. Recently, the receptor for niacin, GPR109A, was discovered. GPR109A in the adipocyte mediates a niacin-induced inhibition of lipolysis, which could play a crucial part in its role as a lipid-modifying drug. GPR109A in epidermal Langerhans cells mediates flushing, an unwanted side effect of niacin therapy. For the past decade, the functions of GPR109A have been studied and full or partial agonists have been developed in an attempt to achieve the beneficial effects of niacin while avoiding the unwanted flushing side effect. This review presents what is known to date about GPR109A biology and function and the future of GPR109A as a pharmacological target.

Biased Ligands for Better Cardiovascular Drugs

Drug discovery efforts targeting G-protein-coupled receptors (GPCR) have been immensely successful in creating new cardiovascular medicines. Currently marketed GPCR drugs are broadly classified as either agonists that activate receptors or antagonists that prevent receptor activation by endogenous stimuli. However, GPCR couple to a multitude of intracellular signaling pathways beyond classical G-protein signals, and these signals can be independently activated by biased ligands to vastly expand the potential for new drugs at these classic targets. By selectively engaging only a subset of a receptor's potential intracellular partners, biased ligands may deliver more precise therapeutic benefit with fewer side effects than current GPCR-targeted drugs. In this review, we discuss the history of biased ligand research, the current understanding of how biased ligands exert their unique pharmacology, and how research into GPCR signaling has uncovered previously unappreciated capabilities of receptor pharmacology. We focus on several receptors to illustrate the approaches taken and discoveries made, and how these are steadily illuminating the intricacies of GPCR pharmacology. Discoveries of biased ligands targeting the angiotensin II type 1 receptor and of separable pharmacology suggesting the potential value of biased ligands targeting the β-adrenergic receptors and nicotinic acid receptor GPR109a highlight the powerful clinical promise of this new category of potential therapeutics.

NAD+ as a signaling molecule modulating metabolism

The ability of NAD(+) to act as a metabolic cofactor and as a rate-limiting cosubstrate for many enzymes, particularly the sirtuins, has led to the identification of a pivotal role of NAD(+) levels in the control of whole-body metabolic homeostasis. Bioavailability and compartmentalization of NAD(+) have become highly relevant issues that we need to understand in order to elucidate how NAD(+) acts both as a readout of the metabolic milieu and as an effector triggering appropriate cellular adaptations.