Hydroxycarboxylic acid receptors are essential for breast cancer cells to control their lipid/fatty acid metabolism

Cancer cells exhibit characteristic changes in their metabolism with efforts being made to address them therapeutically. However, targeting metabolic enzymes as such is a major challenge due to their essentiality for normal proliferating cells. The most successful pharmaceutical targets are G protein-coupled receptors (GPCRs), with more than 40% of all currently available drugs acting through them.We show that, a family of metabolite-sensing GPCRs, the Hydroxycarboxylic acid receptor family (HCAs), is crucial for breast cancer cells to control their metabolism and proliferation.We found HCA1 and HCA3 mRNA expression were significantly increased in breast cancer patient samples and detectable in primary human breast cancer patient cells. Furthermore, siRNA mediated knock-down of HCA3 induced considerable breast cancer cell death as did knock-down of HCA1, although to a lesser extent. Liquid Chromatography Mass Spectrometry based analyses of breast cancer cell medium revealed a role for HCA3 in controlling intracellular lipid/fatty acid metabolism. The presence of etomoxir or perhexiline, both inhibitors of fatty acid β-oxidation rescues breast cancer cells with knocked-down HCA3 from cell death.Our data encourages the development of drugs acting on cancer-specific metabolite-sensing GPCRs as novel anti-proliferative agents for cancer therapy.

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.

International Union of Basic and Clinical Pharmacology. LXXXII: Nomenclature and Classification of Hydroxy-carboxylic Acid Receptors (GPR81, GPR109A, and GPR109B)

The G-protein-coupled receptors GPR81, GPR109A, and GPR109B share significant sequence homology and form a small group of receptors, each of which is encoded by clustered genes. In recent years, endogenous ligands for all three receptors have been described. These endogenous ligands have in common that they are hydroxy-carboxylic acid metabolites, and we therefore have proposed that this receptor family be named hydroxy-carboxylic acid (HCA) receptors. The HCA(1) receptor (GPR81) is activated by 2-hydroxy-propanoic acid (lactate), the HCA(2) receptor (GPR109A) is a receptor for the ketone body 3-hydroxy-butyric acid, and the HCA(3) receptor (GPR109B) is activated by the β-oxidation intermediate 3-hydroxy-octanoic acid. HCA(1) and HCA(2) receptors are found in most mammalian species, whereas the HCA(3) receptor is present only in higher primates. The three receptors have in common that they are expressed in adipocytes and are coupled to G(i)-type G-proteins mediating antilipolytic effects in fat cells. HCA(2) and HCA(3) receptors are also expressed in a variety of immune cells. HCA(2) is a receptor for the antidyslipidemic drug nicotinic acid (niacin) and related compounds, and there is an increasing number of synthetic ligands mainly targeted at HCA(2) and HCA(3) receptors. The aim of this article is to give an overview on the discovery and pharmacological characterization of HCAs, and to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature. We will also discuss open questions regarding this receptor family as well as their physiological role and therapeutic potential.

Biological and pharmacological roles of HCA receptors

The hydroxy-carboxylic acid (HCA) receptors HCA(1), HCA(2), and HCA(3) were previously known as GPR81, GPR109A, and GPR109B, respectively, or as the nicotinic acid receptor family. They form a cluster of G protein-coupled receptors with high sequence homology. Recently, intermediates of energy metabolism, all HCAs, have been reported as endogenous ligands for each of these receptors. The HCA receptors are predominantly expressed on adipocytes and mediate the inhibition of lipolysis by coupling to G(i)-type proteins. HCA(1) is activated by lactate, HCA(2) by the ketone body 3-hydroxy-butyrate, and HCA(3) by hydroxylated β-oxidation intermediates, especially 3-hydroxy-octanoic acid. Both HCA(2) and HCA(3) are part of a negative feedback loop which keeps the release of fat stores in check under starvation conditions, whereas HCA(1) plays a role in the antilipolytic (fat-conserving) effect of insulin. HCA(2) was first discovered as the molecular target of the antidyslipidemic drug nicotinic acid (or niacin). Many synthetic agonists have since been designed for HCA(2) and HCA(3), but the development of a new, improved HCA-targeted drug has not been successful so far, despite a number of clinical studies. Recently, it has been shown that the major side effect of nicotinic acid, skin flushing, is mediated by HCA(2) receptors on keratinocytes, as well as on Langerhans cells in the skin. In this chapter, we summarize the latest developments in the field of HCA receptor research, with emphasis on (patho)physiology, receptor pharmacology, major ligand classes, and the therapeutic potential of HCA ligands.

3-Hydroxy-fatty acid analysis by gas chromatography-mass spectrometry

The mitochondrial fatty acid beta-oxidation is integral to normal cellular metabolism and maintenance of cellular energy supplies. Disorders of this pathway interrupt the body's ability to deal with fasting states, as well as compromising the functioning of organs and systems whose high-energy requirements utilize fats for a continuous energy source, such as heart and skeletal muscle. This method quantitatively measures intermediate metabolites of fatty acid beta-oxidation, specifically the 3-hydroxy-fatty acids produced by the third step in the pathway. The method is useful for helping to diagnose disorders of the pathway, especially defects in the L-3-hydroxyacyl CoA dehydrogenases. Serum or plasma samples are used for routine clinical evaluation; however, measurement of 3-hydroxy-fatty acid intermediates in fibroblast cell culture media and in samples from mice also allows the method to be used for research into fatty acid oxidation and interconnected pathways. The method is a stable isotope dilution, electron impact ionization gas chromatography/mass spectrometry (GC/MS) procedure.

Deorphanization of GPR109B as a receptor for the beta-oxidation intermediate 3-OH-octanoic acid and its role in the regulation of lipolysis

The orphan G-protein-coupled receptor GPR109B is the result of a recent gene duplication of the nicotinic acid and ketone body receptor GPR109A being found in humans but not in rodents. Like GPR109A, GPR109B is predominantly expressed in adipocytes and is supposed to mediate antilipolytic effects. Here we show that GPR109B serves as a receptor for the beta-oxidation intermediate 3-OH-octanoic acid, which has antilipolytic activity on human but not on murine adipocytes. GPR109B is coupled to Gi-type G-proteins and is activated by 2- and 3-OH-octanoic acid with EC50 values of about 4 and 8 microM, respectively. Interestingly, 3-OH-octanoic acid plasma concentrations reach micromolar concentrations under conditions of increased beta-oxidation rates, like in diabetic ketoacidosis or under a ketogenic diet. These data suggest that the ligand receptor pair 3-OH-octanoic acid/GPR109B mediates in humans a negative feedback regulation of adipocyte lipolysis to counteract prolipolytic influences under conditions of physiological or pathological increases in beta-oxidation rates.

Analysis of isomeric long-chain hydroxy fatty acids by tandem mass spectrometry: application to the diagnosis of long-chain 3-hydroxyacyl CoA dehydrogenase deficiency

Acetyl trimethylaminoethyl ester iodide derivatives have been used to selectively analyze isomeric long-chain hydroxy fatty acids by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The binary derivatives of 2-, 3-, 12- and 16-hydroxypalmitic acids afford remarkably different product ion spectra. Further discrimination between isomers is possible by acylating with pivaloyl chloride. 2- and omega-Hydroxy long-chain fatty acids form pivaloyl esters in quantitative yield whereas other secondary alcohols only partially react. Cotton-based filter paper used for blood collection contains substantial amounts of esterified long-chain hydroxy fatty acids. From the product ion spectra of the acetyl trimethylaminoethyl esters the hydroxydocosanoic and -tetracosanoic acids are >90% omega-hydroxy. All remaining saturated and unsaturated hydroxy acids are >90% 2-hydroxy acids. A method for the quantification of free 3-hydroxypalmitic acid in plasma by ESI-MS/MS for the diagnosis of long-chain 3-hydroxyacyl CoA dehydrogenase deficiency (LCHAD) is described. Median plasma concentrations of 0.43 micromol/L (control, n = 22) and 12.2 micromol/L (LCHAD, n = 3) were obtained from 5 microL plasma samples.