Sorin Tunaru

Nicotinic acid– and monomethyl fumarate–induced flushing involves GPR109A expressed by keratinocytes and COX-2–dependent prostanoid formation in mice

The antidyslipidemic drug nicotinic acid and the antipsoriatic drug monomethyl fumarate induce cutaneous flushing through activation of G protein-coupled receptor 109A (GPR109A). Flushing is a troublesome side effect of nicotinic acid, but may be a direct reflection of the wanted effects of monomethyl fumarate. Here we analyzed the mechanisms underlying GPR109A-mediated flushing and show that both Langerhans cells and keratinocytes express GPR109A in mice. Using cell ablation approaches and transgenic cell type-specific GPR109A expression in Gpr109a-/- mice, we have provided evidence that the early phase of flushing depends on GPR109A expressed on Langerhans cells, whereas the late phase is mediated by GPR109A expressed on keratinocytes. Interestingly, the first phase of flushing was blocked by a selective cyclooxygenase-1 (COX-1) inhibitor, and the late phase was sensitive to a selective COX-2 inhibitor. Both monomethyl fumarate and nicotinic acid induced PGE2 formation in isolated keratinocytes through activation of GPR109A and COX-2. Thus, the early and late phases of the GPR109A-mediated cutaneous flushing reaction involve different epidermal cell types and prostanoid-forming enzymes. These data will help to guide new efficient approaches to mitigate nicotinic acid-induced flushing and may help to exploit the potential antipsoriatic effects of GPR109A agonists in the skin.

An Autocrine Lactate Loop Mediates Insulin-Dependent Inhibition of Lipolysis through GPR81

Lactate is an important metabolic intermediate released by skeletal muscle and other organs including the adipose tissue, which converts glucose into lactate under the influence of insulin. Here we show that lactate activates the G protein-coupled receptor GPR81, which is expressed in adipocytes and mediates antilipolytic effects through G(i)-dependent inhibition of adenylyl cyclase. Using GPR81-deficient mice, we demonstrate that the receptor is not involved in the regulation of lipolysis during intensive exercise. However, insulin-induced inhibition of lipolysis and insulin-induced decrease in adipocyte cAMP levels were strongly reduced in mice lacking GPR81, although insulin-dependent release of lactate by adipocytes was comparable between wild-type and GPR81-deficient mice. Thus, lactate and its receptor GPR81 unexpectedly function in an autocrine and paracrine loop to mediate insulin-induced antilipolytic effects. These data show that lactate can directly modulate metabolic processes in a hormone-like manner, and they reveal a new mechanism underlying the antilipolytic effects of insulin.

GPR109A, GPR109B and GPR81, a family of hydroxy-carboxylic acid receptors

G-protein-coupled receptors (GPCRs) are the most versatile receptor family as they have the ability to respond to chemically diverse ligands. Despite intensive efforts during the past two decades, there are still more than 100 orphan GPCRs for which endogenous ligands are unknown. Recently, GPR109A, GPR109B and GPR81, which form a GPCR subfamily, have been deorphanized. The physiological ligands of these receptors are the ketone body 3-hydroxy-butyrate, the metabolite 2-hydroxy-propanoate (lactate) as well as the beta-oxidation intermediate 3-hydroxy-octanoate. Thus, this receptor subfamily is activated by hydroxy-carboxylic acid ligands which are intermediates of energy metabolism. All three receptors are predominantly expressed in adipocytes and mediate antilipolytic effects. In this article, we propose that the hydroxy-carboxylic acid structure of their endogenous ligands is the defining property of this receptor subfamily and that hydroxy-carboxylic acid receptors function as metabolic sensors which fine-tune the regulation of metabolic pathways.

Loss of FFA2 and FFA3 increases insulin secretion and improves glucose tolerance in type 2 diabetes

Type 2 diabetes is a major health problem worldwide, and one of its key features is the inability of elevated glucose to stimulate the release of sufficient amounts of insulin from pancreatic beta cells to maintain normal blood glucose levels. New therapeutic strategies to improve beta cell function are therefore believed to be beneficial. Here we demonstrate that the short-chain fatty acid receptors FFA2 (encoded by FFAR2) and FFA3 (encoded by FFAR3) are expressed in mouse and human pancreatic beta cells and mediate an inhibition of insulin secretion by coupling to Gi-type G proteins. We also provide evidence that mice with dietary-induced obesity and type 2 diabetes, as compared to non-obese control mice, have increased local formation by pancreatic islets of acetate, an endogenous agonist of FFA2 and FFA3, as well as increased systemic levels. This elevation may contribute to the insufficient capacity of beta cells to respond to hyperglycemia in obese states. Indeed, we found that genetic deletion of both receptors, either on the whole-body level or specifically in pancreatic beta cells, leads to greater insulin secretion and a profound improvement of glucose tolerance when mice are on a high-fat diet compared to controls. On the other hand, deletion of Ffar2 and Ffar3 in intestinal cells did not alter glucose tolerance in diabetic animals, suggesting these receptors act in a cell-autonomous manner in beta cells to regulate insulin secretion. In summary, under diabetic conditions elevated acetate acts on FFA2 and FFA3 to inhibit proper glucose-stimulated insulin secretion, and we expect antagonists of FFA2 and FFA3 to improve insulin secretion in type 2 diabetes.

Thyrocyte-specific Gq/G11 deficiency impairs thyroid function and prevents goiter development

The function of the adult thyroid is regulated by thyroid-stimulating hormone (TSH), which acts through a G protein-coupled receptor. Overactivation of the TSH receptor results in hyperthyroidism and goiter. The Gs-mediated stimulation of adenylyl cyclase-dependent cAMP formation has been regarded as the principal intracellular signaling mechanism mediating the action of TSH. Here we show that the Gq/G11-mediated signaling pathway plays an unexpected and essential role in the regulation of thyroid function. Mice lacking the alpha subunits of Gq and G11 specifically in thyroid epithelial cells showed severely reduced iodine organification and thyroid hormone secretion in response to TSH, and many developed hypothyroidism within months after birth. In addition, thyrocyte-specific Galphaq/Galpha11-deficient mice lacked the normal proliferative thyroid response to TSH or goitrogenic diet, indicating an essential role of this pathway in the adaptive growth of the thyroid gland. Our data suggest that Gq/G11 and their downstream effectors are promising targets to interfere with increased thyroid function and growth.

Nicotinic acid (niacin): new lipid-independent mechanisms of action and therapeutic potentials

Nicotinic acid (niacin) has been used for decades to prevent and treat atherosclerosis. The well-documented antiatherogenic activity is believed to result from its antidyslipidemic effects, which are accompanied by unwanted effects, especially a flush. There has been renewed interest in nicotinic acid owing to the need for improved prevention of atherosclerosis in patients already taking statins. In addition, the identification of a nicotinic acid receptor expressed in adipocytes and immune cells has helped to elucidate the mechanisms underlying the antiatherosclerotic as well as the unwanted effects of this drug. Nicotinic acid exerts its antiatherosclerotic effects at least in part independently of its antidyslipidemic effects through mechanisms involving its receptor on immune cells as well as through direct and indirect effects on the vascular endothelium. Here, we review recent data on the pharmacological effects of nicotinic acid and discuss how they might be harnessed to treat other inflammatory diseases such as multiple sclerosis or psoriasis.

Sphingosine 1-phosphate and dioleoylphosphatidic acid are low affinity agonists for the orphan receptor GPR63

Five high affinity G-protein-coupled receptors for sphingosine 1-phosphate (S1P) have been characterised so far (S1P(1,2,3,4,5) formerly referred to as edg1,5,3,6,8). In this study, we show that S1P, dihydro-sphingosine 1-phosphate (dihydro-S1P) and dioleoylphosphatidic acid (doPA) are agonists for the orphan receptor GPR63. All three phospholipids mobilise intracellular calcium in CHO cells transiently transfected with GPR63. Calcium signals required cotransfection of a chimeric Galpha(q/i) protein in a fluorometric imaging plate reader (FLIPR) assay but did not require overexpressed G proteins in an aequorin assay, using a green fluorescent protein (GFP)-aequorin fusion protein as a bioluminescent Ca(2+) reporter. GPR63 expression in CHO cells confers proliferative responses to S1P in a pertussis toxin (PTX)-insensitive manner. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicated highest expression in brain, especially in the thalamus and the nucleus caudatus. In peripheral tissues, highest expression was observed in thymus, stomach and small intestine; lower abundance of transcripts was detected in kidney, spleen, pancreas and heart. The discovery that S1P, dihydro-S1P and dioleoylphosphatidic acid activate GPR63 will facilitate the identification of agonists and antagonists, and help to unravel the biological function of this receptor.

Castor oil induces laxation and uterus contraction via ricinoleic acid activating prostaglandin EP3 receptors

Castor oil is one of the oldest drugs. When given orally, it has a laxative effect and induces labor in pregnant females. The effects of castor oil are mediated by ricinoleic acid, a hydroxylated fatty acid released from castor oil by intestinal lipases. Despite the wide-spread use of castor oil in conventional and folk medicine, the molecular mechanism by which ricinoleic acid acts remains unknown. Here we show that the EP3 prostanoid receptor is specifically activated by ricinoleic acid and that it mediates the pharmacological effects of castor oil. In mice lacking EP3 receptors, the laxative effect and the uterus contraction induced via ricinoleic acid are absent. Although a conditional deletion of the EP3 receptor gene in intestinal epithelial cells did not affect castor oil-induced diarrhea, mice lacking EP3 receptors only in smooth-muscle cells were unresponsive to this drug. Thus, the castor oil metabolite ricinoleic acid activates intestinal and uterine smooth-muscle cells via EP3 prostanoid receptors. These findings identify the cellular and molecular mechanism underlying the pharmacological effects of castor oil and indicate a role of the EP3 receptor as a target to induce laxative effects.

Conserved MIP receptor–ligand pair regulates Platynereis larval settlement

Life-cycle transitions connecting larval and juvenile stages in metazoans are orchestrated by neuroendocrine signals including neuropeptides and hormones. In marine invertebrate life cycles, which often consist of planktonic larval and benthic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition. Settlement is regulated by a specialized sensory–neurosecretory system, the larval apical organ. The neuroendocrine mechanisms through which the apical organ transduces environmental cues into behavioral responses during settlement are not yet understood. Here we show that myoinhibitory peptide (MIP)/allatostatin-B, a pleiotropic neuropeptide widespread among protostomes, regulates larval settlement in the marine annelid Platynereis dumerilii. MIP is expressed in chemosensory–neurosecretory cells in the annelid larval apical organ and signals to its receptor, an orthologue of the Drosophila sex peptide receptor, expressed in neighboring apical organ cells. We demonstrate by morpholino-mediated knockdown that MIP signals via this receptor to trigger settlement. These results reveal a role for a conserved MIP receptor–ligand pair in regulating marine annelid settlement.

Deorphanization of GPR109B as a receptor for the β-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 β-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 μm, respectively. Interestingly, 3-OH-octanoic acid plasma concentrations reach micromolar concentrations under conditions of increased β-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 β-oxidation rates.