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.

Platelet activation at sites of vascular injury is essential for primary hemostasis, but also underlies arterial thrombosis leading to myocardial infarction or stroke 1,2 . Platelet activators such as adenosine diphosphate, thrombin or thromboxane A 2 (TXA 2 ) activate receptors that are coupled to heterotrimeric G proteins 1,3 . Activation of platelets through these receptors involves signaling through G q , G i and G z (refs. 4-6). However, the role and relative importance of G 12 and G 13 , which are activated by various platelet stimuli 7-9 , are unclear. Here we show that lack of Gα 13 , but not Gα 12 , severely reduced the potency of thrombin, TXA 2 and collagen to induce platelet shape changes and aggregation in vitro. These defects were accompanied by reduced activation of RhoA and inability to form stable platelet thrombi under high shear stress ex vivo. Gα 13 deficiency in platelets resulted in a severe defect in primary hemostasis and complete protection against arterial thrombosis in vivo. We conclude that G 13 -mediated signaling processes are required for normal hemostasis and thrombosis and may serve as a new target for antiplatelet drugs.

Anaphylactic shock is a severe allergic reaction involving multiple organs including the bronchial and cardiovascular system. Most anaphylactic mediators, like platelet-activating factor (PAF), histamine, and others, act through G protein–coupled receptors, which are linked to the heterotrimeric G proteins Gq/G11, G12/G13, and Gi. The role of downstream signaling pathways activated by anaphylactic mediators in defined organs during anaphylactic reactions is largely unknown. Using genetic mouse models that allow for the conditional abrogation of Gq/G11- and G12/G13-mediated signaling pathways by inducible Cre/loxP-mediated mutagenesis in endothelial cells (ECs), we show that Gq/G11-mediated signaling in ECs is required for the opening of the endothelial barrier and the stimulation of nitric oxide formation by various inflammatory mediators as well as by local anaphylaxis. The systemic effects of anaphylactic mediators like histamine and PAF, but not of bacterial lipopolysaccharide (LPS), are blunted in mice with endothelial Gαq/Gα11 deficiency. Mice with endothelium-specific Gαq/Gα11 deficiency, but not with Gα12/Gα13 deficiency, are protected against the fatal consequences of passive and active systemic anaphylaxis. This identifies endothelial Gq/G11-mediated signaling as a critical mediator of fatal systemic anaphylaxis and, hence, as a potential new target to prevent or treat anaphylactic reactions.

Background— Platelet inhibition is a major strategy to prevent arterial thrombosis, but it is frequently associated with increased bleeding because of impaired primary hemostasis. The activating platelet collagen receptor, glycoprotein VI (GP VI), may serve as a powerful antithrombotic target because its inhibition or absence results in profound protection against arterial thrombosis but no major bleeding in mice. Methods and Results— Mice lacking (−/−) or expressing half-levels (+/−) of the other major platelet collagen receptor, integrin α 2 β 1 , were injected with the anti–GP VI antibody JAQ1 and analyzed on day 5. Anti–GP VI treatment resulted in a marked hemostatic defect in α 2 −/− or α 2 +/− mice, as shown by dramatically prolonged tail bleeding times. Platelet adhesion to collagen was studied in an ex vivo whole-blood perfusion system under high shear conditions. Weak integrin activation by thromboxane A 2 (TxA 2 ) receptor stimulation restored defective adhesion of anti–GP VI–treated wild-type but not α 2 −/− or α 2 +/− platelets to collagen. This process required the simultaneous activation of the G q and G 13 signaling pathways, as demonstrated by use of the respective knockout strains. Conversely, inhibition of TxA 2 production by aspirin severely compromised hemostasis in anti–GP VI–treated or GP VI/Fc receptor γ-chain–deficient but not control mice. Conclusions— Anti–GP VI therapy may result in defective hemostasis in patients with reduced α 2 β 1 levels or concomitant aspirin therapy. These observations may have important implications for a potential use of anti–GP VI–based therapeutics in the prevention of cardiovascular disease.