As a treatment for dyslipidemia, oral doses of 1-3 grams of nicotinic acid per day lower serum triglycerides, raise high density lipoprotein cholesterol, and reduce mortality from coronary heart disease ( Tavintharan

Nicotinic acid, used for atherosclerosis treatment, has an adverse effect of skin flushing. The flushing mechanism, thought to be caused by the release of prostaglandin D(2) (PGD(2)), is not well understood. We aimed to identify which cells mediate the flushing effect. Nicotinic acid receptor (GPR109A) gene expression was assessed in various tissues and cell lines. Cells expressing GPR109A mRNA were further assayed for PGD(2) release in response to nicotinic acid. Of all samples, only skin was able to release PGD(2) upon stimulation with nicotinic acid. The responsive cells were localized to the epidermis, and immunocytochemical studies revealed the presence of GPR109A on epidermal Langerhans cells. CD34+ cells isolated from human blood and differentiated into Langerhans cells (hLC-L) also showed GPR109A expression. IFNgamma treatment increased both mRNA and plasma membrane expression of GPR109A. IFNgamma-stimulated hLC-Ls released PGD(2) in response to nicotinic acid in a dose-dependant manner (effector concentration for half-maximum response=1.2 mM+/-0.7). Acifran, a structurally distinct GPR109A ligand, also increased PGD(2) release, whereas isonicotinic acid, a nicotinic acid analog with low affinity for GPR109A, had no effect. These results suggest that nicotinic acid mediates its flushing side effect by interacting with GPR109A on skin Langerhans cells, resulting in release of PGD(2).

The three ␣ 2 -adrenergic receptor (␣ 2 AR) 1 subtypes are members of the type II, biogenic amine-binding, G protein-coupled receptor family. These receptor subtypes all couple via the G i /G o family of GTP-binding proteins to the inhibition of adenylyl cyclase, inhibition of voltage-dependent calcium channels, potentiation of potassium currents via G protein-coupled, inwardly rectifying potassium channels, activation of phospholipase D, and activation of MAP kinase in native cells (1-4). In heterologous cell systems, these receptors also couple to the activation of a variety of signaling molecules, including Ras (5-7), p70 S6 kinase (8), MAP kinase (9, 10), and phospholipase D (11).Although all three ␣ 2 ARs appear to activate similar signaling pathways, differences in the cellular trafficking of these subtypes have been reported, both in naive cells and following agonist activation. Subtype-selective differences in agonistelicited ␣ 2 AR redistribution have been noted in several experimental systems (12-18). The ␣ 2B AR subtype is readily internalized following agonist activation, whereas the ␣ 2A AR subtype typically is not (14, 18). The ␣ 2C AR subtype has not been explored in as much detail with regard to agonist-elicited redistribution because of its considerable accumulation intracellularly (14). The ␣ 2 AR subtypes also manifest different trafficking itineraries in polarized Madin-Darby canine kidney II (MDCKII) cells, even in the absence of agonist treatment. The ␣ 2A AR subtype is targeted directly to the basolateral surface (19), whereas the ␣ 2B AR subtype is delivered randomly to both the apical and basolateral surfaces but is selectively retained on the basolateral surface (t1 ⁄2 ϭ 10 -12 h) in contrast to its rapid loss from the apical surface (t1 ⁄2 ϭ 5-15 min) (20). These findings suggest that there is a molecular mechanism responsible for the selective retention of the ␣ 2B AR on the basolateral sub-domain of MDCK cells, probably a retention mechanism shared by the basolaterally targeted ␣ 2A -and ␣ 2C AR subtypes (20). Although ␣ 2C ARs, like ␣ 2A ARs, are directly targeted to and retained on the basolateral subdomain, a significant proportion of these receptors is identifiable in an intracellular pool at steady state (14,18,20); the functional relevance of this intracellular ␣ 2C AR pool has yet to be clarified.Receptor retention on the lateral subdomain of MDCKII cells likely involves the third intracellular loop of the ␣ 2 AR subtypes. For example, deletion of this loop in the ␣ 2A AR subtype (⌬3i ␣ 2A AR) results in accelerated basolateral receptor turnover (t1 ⁄2 Х 4.5 h) when compared with that for the wild-type receptor or with ␣ 2A AR structures that have been mutated in the N terminus or the C-terminal tail (all possessing a t1 ⁄2 of 10 -12 h) (21). Similarly, the ⌬3i ␣ 2B AR is not enriched at the basolateral surface of MDCKII cells at steady state (22).Based on our findings that the ␣ 2B AR is rapidly removed from the apical surface following random delivery and that removal of the...

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Nicotinic acid remains the most effective therapeutic agent for the treatment and prevention of atherosclerosis resulting from low high density lipoprotein cholesterol. The therapeutic actions of nicotinic acid are mediated by GPR109A, a G i proteincoupled receptor, expressed primarily on adipocytes, Langerhans cells, and macrophage. Unfortunately, a severe, cutaneous flushing side effect limits its use and patient compliance. The mechanism of high density lipoprotein elevation is not clearly established but assumed to be influenced by an inhibition of lipolysis in the adipose. The flushing side effect appears to be mediated by the release of prostaglandin D2 from Langerhans cells in the skin. We hypothesized that the signal transduction pathways mediating the anti-lipolytic and prostaglandin D2/flushing pathways are distinct and that agonists may be identified that are capable of selectively eliciting the therapeutic, anti-lipolytic pathway while avoiding the activation of the parallel flush-inducing pathway. We have identified a number of GPR109A pyrazole agonists that are capable of fully inhibiting lipolysis in vitro and in vivo and not only fail to elicit a flushing response but can antagonize the ability of nicotinic acid to elicit a flush response in vivo. In contrast to flushing agonists, exposure of cells expressing GPR109A to the non-flushing agonists fails to induce internalization of the receptor or to activate ERK 1/2 mitogen-activated protein kinase phosphorylation.Nicotinic acid (niacin, vitamin B3, pyridine-3-carboxylic acid) is the most effective therapeutic agent to date for raising high density lipoprotein (HDL) 2 levels. It also offers protection against other cardiovascular risk factors by lowering very low density lipoprotein (VLDL), low density lipoprotein (LDL), and lipoprotein(a) plasma concentrations (1, 2). Although the mechanism by which nicotinic acid raises HDL is not clear, one hypothesis is that it is the ability of nicotinic acid to inhibit lipolysis in adipocytes that results in a decrease in the concentration of free fatty acids available for the liver to use for triglyceride synthesis and VLDL production. The attenuated synthesis of the triglyceride-rich VLDL particles in the liver leads to a decreased rate of HDL metabolism via limiting the cholesterol ester transfer protein (CETP)-mediated exchange of cholesterol from HDL to VLDL, and triglyceride from VLDL to HDL (3-6). Another hypothesis is that nicotinic acid inhibits the uptake and subsequent catabolism of Apo-AI-containing HDL particles in hepatocytes (7,8).Identification of a high affinity nicotinic acid binding site that was localized to adipose, macrophage, and spleen tissues and appeared to function in a G i protein-coupled manner (9) led to the molecular identification of the high affinity nicotinic acid receptor GPR109A (HM74A in humans and PUMA-G in mice) (10 -12). In the adipose, GPR109A mediates an anti-lipolytic response that can attenuate cAMP-stimulated lipolysis (11). A low affinity nicotinic acid receptor ...