GPR A a G protein coupled

GPR109A, a G-protein-coupled receptor located mainly on adipocyte cell membranes, has been identified as the molecular target for nicotinic RG2833 [[5], [6], [7]] and mediator of NEFA reduction [5]. As a mechanism of NEFA-lowering, it is recognized that activation of GPR109A leads to Gi-mediated inhibition of adenylyl cyclase resulting in the reduction of activated hormone-sensitive lipase (HSL) and decrease in hydrolysis of TG to NEFA in adipose tissue [8]. GPR109A is also expressed on Langerhans cells, and stimulation of this receptor in epidermal leads to flushing symptom, well-known side effect in nicotinic acid therapy, through the production of prostaglandin D2 and prostaglandin E2 [9,10]. The contribution of GPR109A to flushing is widely accepted, however the results whether GPR109A activation leads to plasma lipid changes is controversial. To date, besides nicotinic acid analogs, acifran and acipimox, four selective and chemically designed GPR109A agonists (MK-0354, MK-1903, SCH900271 and GSK256073), have been clinically tested [[11], [12], [13], [14]]. Even though all these agonists reduced plasma NEFA levels, there seems to be variations in the plasma lipid changes [15]. Indeed, 50 mg once-daily GSK256073 treatment exhibited almost 36% reduction in plasma TG levels at 12 weeks. On the other hand, MK-0354, MK-1903 and SCH9 00271 did not show clinically meaningful reduction in TG levels. The reason for this remains unclear, but the magnitude of the pharmacodynamic effects of these GPR109A agonists may be reflective of differences in receptor potency, pharmacokinetic profiles, and/or dosing regimens used in the clinical studies. In addition, a recent study has demonstrated that the TG-lowering effect of nicotinic acid was not diminished in Gpr109a mice [12], suggesting that there was a GPR109A independent mechanism of nicotinic acid efficacy, which was different from the previous finding [5]. Thus, although there is general agreement that GPR109A has anti-lipolytic activity, the issue of whether GPR109A signaling leads to the plasma lipid changes remains unclear. The half-life of nicotinic acid is relatively short, 20–60 min [4,16], and its rapid elimination from circulating blood induces a rebound of NEFA levels [17]. In addition, the NEFA-lowering effect is lost by sustained nicotinic acid exposure [18], thereby, parameters of pharmacodynamics are largely influenced depending on the administration regimen, and the timing of blood sampling. Thus, it is difficult to correctly interpret the results of the study that investigates the in vivo physiological function of GPR109A by use of nicotinic acid as a tool of GPR109A agonist. Moreover, since the phenotype of a knock-out animal does not always show the opposite result of agonist treatment, using knock-out animals might be unsuitable for investigating the hypothesis that GPR109A activation leads to lipid changes. Instead, using a genetically overexpressing GPR109A animal is one effective way to investigate the physiological function of GPR109A in lipid metabolism in vivo, and to estimate the effects of agonist treatment of a receptor. Therefore, we created a bacterial artificial chromosome (BAC) transgenic rat expressing human GPR109A (Tg rat). Rodents are deficient in cholesteryl ester transfer protein (CETP), which has been thought to be an important mediator of LDL-C reduction and HDL-C elevation following TG reduction in nicotinic acid treatment [[19], [20], [21]]. Thus, we focused on investigating whether GPR109A signaling leads to TG lowering. Additionally, since nicotinic acid treatment decreases plasma insulin levels, and dosing of GSK256073 for 2 days decreased plasma NEFA levels and HOMA-IR scores (27–47%) [13], we examined insulin and glucose levels and the HOMA-IR index.
Materials and methods
Results
Discussion Although there is general agreement that GPR109A has anti-lipolytic activity and that the NEFA reduction in response to nicotinic acid is mediated by GPR109A, whether GPR109A activation has any impact on plasma TG levels is unclear. Since the phenotype of knock-out animals does not always reflect the effect of agonist treatment, the use of an overexpressing animal is an effective way to gain insight into this point. Here, we addressed this issue by using hGPR109A BAC transgenic rats, which express hGPR109A under native gene regulation. We demonstrated that plasma NEFA and TG levels under fasting conditions were lower in Tg rats than those in non-Tg rats, suggesting that GPR109A activation leads to plasma TG lowering in vivo. Our observations are generally consistent with recently published results from in vivo studies of lipolytic genes. For example, mice with a knock-out of adipocyte specific adipose triglyceride lipase, which is the rate limiting enzyme for TG hydrolysis in adipose tissue, had lower serum NEFA and TG levels [35]. It has also been reported that an adipose tissue specific Gs-α knock-out mice, caused a reduction in NEFA release in response to the β3 agonist CL31624, and exhibited reduced serum NEFA and TG levels [36]. The impact of GPR109A signaling on LDL-C and HDL-C levels is not clear, but GPR109A signaling leads to pronounced reduction in plasma TG levels.