br Methods br Results br Discussion br Conclusion The in

Methods
Results
Discussion
Conclusion The in vivo pEC50 for NEFA release in the rat was derived for 12 GPR81/GPR109A agonists, including the historically well-known nicotinic acid. By integrating in vitro cell assay potency data for both receptors, these in vivo pEC50 values could be reasonably well predicted by a simple mathematical model. The prediction precision was higher compared to an alternative set of predictions based on data from rat primary adipocytes, which are comparatively more demanding to generate. Moreover, the model also enabled predictions of the in vivo selectivity between the receptors. These findings suggest that PD 169316 screening should be supported by predictive in vitro-in vivo models and that this approach is expected to reduce animal use and experimental cost, and therefore allows faster and leaner drug discovery. The following are the supplementary data related to this article.
Acknowledgment
Plasma levels of high density lipoprotein (HDL) cholesterol (C) and its major apolipoprotein, apoA-I, show a strong inverse association with the risk of atherosclerotic cardiovascular disease , . HDL-C and apoA-I levels are reduced in acute and chronic inflammatory states , . Niacin (nicotinic acid) is the most effective pharmacologic approach currently available for raising HDL-C levels, but many questions remain regarding the molecular mechanisms responsible for such an effect , , . Studies in cultured hepatocytes have suggested that niacin reduces the uptake of HDL while kinetic studies in humans have mixed results, with reports that niacin reduces HDL clearance from plasma or increases production of the major HDL protein apoA-I. GPR109A (also referred to as HM74A in humans and PUMA-G in mice) and GPR109B (also known as HM74 in human) are highly homologous G-protein-coupled receptors (GPCRs) and were identified as receptors for niacin , , in 2003. Both GPR109A and GPR109B are highly expressed in adipocytes and activated immune cells , , , . GPR109B has only been found in humans, not in rodents , , . β-Hydroxybutyrate, produced during fasting and starvation, was found to act as endogenous ligand of GPR109A . Activation of GPR109A in adipose tissue leads to inhibition of adenylate cyclase, reduced cyclic adenosine monophosphate (cAMP), reduced activity of protein kinase A, reduced adipocyte triglyceride lipolysis, and reduced release of free fatty acids. Studies in GPR109A knockout mice confirmed that GPR109A was responsible for the antilipolytic (and cutaneous flushing) effects of niacin . Reduced flux of free fatty acids from adipose to liver is believed to potentially underlie the triglyceride and LDL lowering effects of niacin. Whether the HDL-raising effects of niacin are related to activation of GPR109A is unknown. Interestingly, niacin treatment of wild-type mice lacking cholesteryl ester transfer protein (CETP) consistently results in reduction of HDL-C, whereas in mice expressing CETP niacin has a modest HDL-raising effect , , . The molecular mechanism(s) by which niacin reduces HDL-C in wild-type mice lacking CETP and whether this involves activation of GPR109A in the liver are unknown. The aim of the present study is to establish whether GPR109A activation in liver influences HDL metabolism and the HDL response to niacin. By using GPR109A knockout mice, we demonstrate that GPR109A mediates niacin's effect of reducing HDL-C in the mouse. We established that GPR109A is expressed in primary murine hepatocytes and human-derived hepatocytes at low basal levels, and that expression is markedly inducible after exposure to inflammatory stimuli. Hepatic-specific overexpression of GPR109A in mice reduced plasma HDL-C levels accompanied by reduction in hepatic cAMP, ABCA1 protein, cholesterol efflux to apoA-I, and a reduced HDL cholesterol production rate. These data show that the reduction of HDL-C seen in mice in response to niacin is mediated by hepatic GPR109A.