Stefan Offermanns

Activation of Gpr109a, receptor for niacin and the commensal metabolite butyrate, suppresses colonic inflammation and carcinogenesis

Commensal gut microflora and dietary fiber protect against colonic inflammation and colon cancer through unknown targets. Butyrate, a bacterial product from fermentation of dietary fiber in the colon, has been implicated in this process. GPR109A (encoded by Niacr1) is a receptor for butyrate in the colon. GPR109A is also a receptor for niacin, which is also produced by gut microbiota and suppresses intestinal inflammation. Here we showed that Gpr109a signaling promoted anti-inflammatory properties in colonic macrophages and dendritic cells and enabled them to induce differentiation of Treg cells and IL-10-producing T cells. Moreover, Gpr109a was essential for butyrate-mediated induction of IL-18 in colonic epithelium. Consequently, Niacr1(-/-) mice were susceptible to development of colonic inflammation and colon cancer. Niacin, a pharmacological Gpr109a agonist, suppressed colitis and colon cancer in a Gpr109a-dependent manner. Thus, Gpr10a has an essential role in mediating the beneficial effects of gut microbiota and dietary fiber in colon.

ArticleActivation of Gpr109a, Receptor for Niacin and the Commensal Metabolite Butyrate, Suppresses Colonic Inflammation and Carcinogenesis

Commensal gut microflora and dietary fiber protect against colonic inflammation and colon cancer through unknown targets. Butyrate, a bacterial product from fermentation of dietary fiber in the colon, has been implicated in this process. GPR109A (encoded by Niacr1) is a receptor for butyrate in the colon. GPR109A is also a receptor for niacin, which is also produced by gut microbiota and suppresses intestinal inflammation. Here we showed that Gpr109a signaling promoted anti-inflammatory properties in colonic macrophages and dendritic cells and enabled them to induce differentiation of Treg cells and IL-10-producing T cells. Moreover, Gpr109a was essential for butyrate-mediated induction of IL-18 in colonic epithelium. Consequently, Niacr1(-/-) mice were susceptible to development of colonic inflammation and colon cancer. Niacin, a pharmacological Gpr109a agonist, suppressed colitis and colon cancer in a Gpr109a-dependent manner. Thus, Gpr10a has an essential role in mediating the beneficial effects of gut microbiota and dietary fiber in colon.

Activation of Platelet Function Through G Protein–Coupled Receptors

Because of their ability to become rapidly activated at places of vascular injury, platelets are important players in primary hemostasis as well as in arterial thrombosis. In addition, they are also involved in chronic pathological processes including the atherosclerotic remodeling of the vascular system. Although primary adhesion of platelets to the vessel wall is largely independent of G protein–mediated signaling, the subsequent recruitment of additional platelets into a growing platelet thrombus requires mediators such as ADP, thromboxane A2, or thrombin, which act through G protein–coupled receptors. Platelet activation via G protein–coupled receptors involves 3 major G protein–mediated signaling pathways that are initiated by the activation of the G proteins Gq, G13, and Gi. This review summarizes recent progress in understanding the mechanisms underlying platelet activation and thrombus extension via G protein–mediated signaling pathways.

The microbial metabolite butyrate regulates intestinal macrophage function via histone deacetylase inhibition.

Significance The mammalian intestines contain an enormous number of microorganisms within the lumen. Given the constant exposure to these microbes, the intestinal immune system has the difficult task of maintaining tolerance to commensal bacteria while remaining responsive to potential pathogens. The mechanisms by which this balance is achieved are relatively unknown. Here, we identify a bacterial metabolite, n-butyrate, that exerts immunomodulatory effects on intestinal macrophages and renders them hyporesponsive to commensals that reside in the colon. Our studies elucidate a possible mechanism that contributes to immune homeostasis in the intestines. Given the trillions of microbes that inhabit the mammalian intestines, the host immune system must constantly maintain a balance between tolerance to commensals and immunity against pathogens to avoid unnecessary immune responses against otherwise harmless bacteria. Misregulated responses can lead to inflammatory bowel diseases such as ulcerative colitis or Crohns disease. The mechanisms by which the immune system maintains this critical balance remain largely undefined. Here, we demonstrate that the short-chain fatty acid n-butyrate, which is secreted in high amounts by commensal bacteria, can modulate the function of intestinal macrophages, the most abundant immune cell type in the lamina propria. Treatment of macrophages with n-butyrate led to the down-regulation of lipopolysaccharide-induced proinflammatory mediators, including nitric oxide, IL-6, and IL-12, but did not affect levels of TNF-α or MCP-1. These effects were independent of toll-like receptor signaling and activation of G-protein–coupled receptors, two pathways that could be affected by short-chain fatty acids. In this study, we provide several lines of evidence that suggest that these effects are due to the inhibition of histone deacetylases by n-butyrate. These findings elucidate a pathway in which the host may maintain tolerance to intestinal microbiota by rendering lamina propria macrophages hyporesponsive to commensal bacteria through the down-regulation of proinflammatory effectors.

G12-G13-LARG-mediated signaling in vascular smooth muscle is required for salt-induced hypertension.

The tone of vascular smooth muscle cells is a primary determinant of the total peripheral vascular resistance and hence the arterial blood pressure. Most forms of hypertension ultimately result from an increased vascular tone that leads to an elevated total peripheral resistance. Regulation of vascular resistance under normotensive and hypertensive conditions involves multiple mediators, many of which act through G protein–coupled receptors on vascular smooth muscle cells. Receptors that mediate vasoconstriction couple with the G-proteins Gq-G11 and G12-G13 to stimulate phosphorylation of myosin light chain (MLC) via the Ca2+/MLC kinase– and Rho/Rho kinase–mediated signaling pathways, respectively. Using genetically altered mouse models that allow for the acute abrogation of both signaling pathways by inducible Cre/loxP-mediated mutagenesis in smooth muscle cells, we show that Gq-G11–mediated signaling in smooth muscle cells is required for maintenance of basal blood pressure and for the development of salt-induced hypertension. In contrast, lack of G12-G13, as well as of their major effector, the leukemia-associated Rho guanine nucleotide exchange factor (LARG), did not alter normal blood pressure regulation but did block the development of salt-induced hypertension. This identifies the G12-G13–LARG–mediated signaling pathway as a new target for antihypertensive therapies that would be expected to leave normal blood pressure regulation unaffected.

Plexin-B1 Directly Interacts with PDZ-RhoGEF/LARG to Regulate RhoA and Growth Cone Morphology

Plexins are widely expressed transmembrane proteins that, in the nervous system, mediate repulsive signals of semaphorins. However, the molecular nature of plexin-mediated signal transduction remains poorly understood. Here, we demonstrate that plexin-B family members associate through their C termini with the Rho guanine nucleotide exchange factors PDZ-RhoGEF and LARG. Activation of plexin-B1 by semaphorin 4D regulates PDZ-RhoGEF/LARG activity leading to RhoA activation. In addition, a dominant-negative form of PDZ-RhoGEF blocks semaphorin 4D-induced growth cone collapse in primary hippocampal neurons. Our study indicates that the interaction of mammalian plexin-B family members with the multidomain proteins PDZ-RhoGEF and LARG represents an essential molecular link between plexin-B and localized, Rho-mediated downstream signaling events which underly various plexin-mediated cellular phenomena including axonal growth cone collapse.

Absence of pressure overload induced myocardial hypertrophy after conditional inactivation of Gα q /Gα 11 in cardiomyocytes

Myocardial hypertrophy is an adaptational response of the heart to increased work load, but it is also associated with a high risk of cardiac mortality due to its established role in the development of cardiac failure, one of the leading causes of death in developed countries. Multiple growth factors and various downstream signaling pathways involving, for example, ras, gp-130 (ref. 4), JNK/p38 (refs. 5,6) and calcineurin/NFAT/CaM-kinase have been implicated in the hypertrophic response. However, there is evidence that the initial phase in the development of myocardial hypertrophy involves the formation of cardiac para- and/or autocrine factors like endothelin-1, norepinephrine or angiotensin II (refs. 7,8), the receptors of which are coupled to G-proteins of the Gq/11-, G12/13- and Gi/o-families. Cardiomyocyte-specific transgenic overexpression of α1-adrenergic or angiotensin (AT1)-receptors as well as of the Gq α-subunit, Gαq, results in myocardial hypertrophy. These data demonstrate that chronic activation of the Gq/G11-family is sufficient to induce myocardial hypertrophy. In order to test whether Gq/G11 mediate the physiological hypertrophy response to pressure overload, we generated a mouse line lacking both Gαq and Gα11 in cardiomyocytes. These mice showed no detectable ventricular hypertrophy in response to pressure-overload induced by aortic constriction. The complete lack of a hypertrophic response proves that the Gq/G11-mediated pathway is essential for cardiac hypertrophy induced by pressure overload and makes this signaling process an interesting target for interventions to prevent myocardial hypertrophy.

The vasodilator-stimulated phosphoprotein (VASP) is involved in cGMP- and cAMP-mediated inhibition of agonist-induced platelet aggregation, but is dispensable for smooth muscle function

The vasodilator‐stimulated phosphoprotein (VASP) is associated with actin filaments and focal adhesions, which form the interface between the cytoskeleton and the extracellular matrix. VASP is phosphorylated by both the cAMP‐ and cGMP‐dependent protein kinases in a variety of cells, including platelets and smooth muscle cells. Since both the cAMP and cGMP signalling cascades relax smooth muscle and inhibit platelet activation, it was speculated that VASP mediates these effects by modulating actin filament dynamics and integrin activation. To study the physiological relevance of VASP in these processes, we inactivated the VASP gene in mice. Adult VASP‐deficient mice had normal agonist‐induced contraction, and normal cAMP‐ and cGMP‐dependent relaxation of intestinal and vascular smooth muscle. In contrast, cAMP‐ and cGMP‐mediated inhibition of platelet aggregation was significantly reduced in the absence of VASP. Other cAMP‐ and cGMP‐dependent effects in platelets, such as inhibition of agonist‐induced increases in cytosolic calcium concentrations and granule secretion, were not dependent on the presence of VASP. Our data show that two different cyclic, nucleotide‐dependent mechanisms are operating during platelet activation: a VASP‐independent mechanism for inhibition of calcium mobilization and granule release and a VASP‐dependent mechanism for inhibition of platelet aggregation which may involve regulation of integrin function.

Rho/Rho-kinase mediated signaling in physiology and pathophysiology

Abstract. The small GTPase Rho is implicated in many cellular functions such as cell adhesion, cell motility and migration, growth control, cell contraction, and cytokinesis. One of its main effectors, Rho-kinase, appears to play a key role in the regulation of force and velocity of actomyosin crossbridging in smooth muscle and nonmuscle cells by inhibiting myosin phosphatase-mediated dephosphorylation of the regulatory chain of myosin II. Abnormal activation of the Rho/Rho-kinase pathway has been shown to play a role in diseases such as hypertension and bronchial asthma. This review summarizes the current knowledge on the physiological and pathophysiological function of the Rho/Rho-kinase mediated pathway in various tissues with a focus on its possible role as a target for therapeutic interventions.

GPR109A (PUMA-G/HM74A) mediates nicotinic acid–induced flushing

Nicotinic acid (niacin) has long been used as an antidyslipidemic drug. Its special profile of actions, especially the rise in HDL-cholesterol levels induced by nicotinic acid, is unique among the currently available pharmacological tools to treat lipid disorders. Recently, a G-protein-coupled receptor, termed GPR109A (HM74A in humans, PUMA-G in mice), was described and shown to mediate the nicotinic acid-induced antilipolytic effects in adipocytes. One of the major problems of the pharmacotherapeutical use of nicotinic acid is a strong flushing response. This side effect, although harmless, strongly affects patient compliance. In the present study, we show that mice lacking PUMA-G did not show nicotinic acid-induced flushing. In addition, flushing in response to nicotinic acid was also abrogated in the absence of cyclooxygenase type 1, and mice lacking prostaglandin D(2) (PGD(2)) and prostaglandin E(2) (PGE(2)) receptors had reduced flushing responses. The mouse orthologue of GPR109A, PUMA-G, is highly expressed in macrophages and other immune cells, and transplantation of wild-type bone marrow into irradiated PUMA-G-deficient mice restored the nicotinic acid-induced flushing response. Our data clearly indicate that GPR109A mediates nicotinic acid-induced flushing and that this effect involves release of PGE(2) and PGD(2), most likely from immune cells of the skin.