For a long time the

For a long time, the mechanism for the TG-lowering effect of nicotinic PF 04418948 has been thought that the lowered NEFA substrate from adipose tissue limits TG production from the liver (so-called NEFA hypothesis) [[1], [2], [3], [4]]. However, recent emerging evidences have suggested the involvement of more than the NEFA hypothesis behind the TG-lowering efficacy. For example, nicotinic acid administration reduced liver Dgat2 expression levels in both dog [29] and rat [30]. Additionally, subjects with the DGAT2 rs3060 T > C variant had a lower TG reduction in the treatment of extended-release niacin [37]. This polymorphism is located in the 3′UTR region, and so transcriptional regulation of Dgat2 might be important in nicotinic acid-mediated TG lowering. Moreover, Apoc3 inhibition has been reported to be one of the mechanisms contributing to the effect on lipid levels [31]. In our study, we observed a trend towards Dgat2 reduction and a reduction in Apoc3 in Tg rats. Previous reports demonstrated that both Dgat2 and Apoc3 levels were not different when hepatocytes were directly treated with nicotinic acid [31,38]. Taken these data together, reduced NEFA flux might be the trigger for their reduction, as has previously been proposed for Apoc3 [31]. Furthermore, considering the very low expression level of GPR109A in the liver, these effects in Tg rats are likely to be caused by NEFA reductions rather than by direct liver GPR109A agonism. Although we cannot exclude the possibility of a contribution of hepatic GPR109A activity, it is worth mentioning that the inhibition of Dgat2 and Apoc3 expression arising from nicotinic acid treatment results partially from enhanced GPR109A signaling. Taking into account the fact that both Gk and Cideb levels were decreased following nicotinic acid treatment, and this effect was also mirrored in Tg rats, there are multiple mechanisms behind the TG-lowering effect of nicotinic acid, and some of them are mediated by GPR109A activation. Lipid accumulation in non-adipose tissues has been associated with insulin resistance, thus limiting NEFA release from adipose tissue has been thought to be one of the strategies for improving insulin sensitivity. Kroon et al. reported that nicotinic acid infusion reduced plasma insulin levels without affecting plasma glucose levels [34]. Here, we also observed a reduction in insulin levels in Tg rats compared to non-Tg rats when NEFA levels were reduced, along with maintenance of glucose levels. Additionally, it has been reported that when the NEFA reduction was maintained by nicotinic acid infusion, the glucose infusion rate during an insulin clamp increased [34], suggesting that the nicotinic acid-mediated NEFA reduction leads to increased insulin sensitivity. Similar results were reported in studies examining proteins that have anti-lipolytic or lipolytic activity, such as the adenosine A1 receptor [39,40], and HSL [41]. In our study, Tg rats exhibited low HOMA-IR values compared to non-Tg rats. These finding together, although further studies are required, the decreased NEFA levels mediated by GPR109A agonism might lead to increased insulin sensitivity. In a clinical study using a GPR109A agonist, treatment with GSK256073 for two days decreased HOMA-IR from 27 to 47% in diabetic patients [13]. However, this effect was not sustained in 12-week administration [14]. Since the plasma concentrations of GSK256073 increased, and the extent of NEFA reduction decreased at week 6 compared to the first dosing, tachyphylaxis of GPR109A receptor signaling may be one reason for this results, as discussed previously [14]. Thus, to elicit the beneficial effect of insulin sensitization from GPR109A agonism, an important goal would be to maintain reduced NEFA levels with little tachyphylaxis. In the present study, we observed a decrease in body weight gain in Tg rats. It is uncertain whether this effect is due to Denr, Gpr81, or Gpr109a. In this regard, Gpr81 mice have been reported to have reduced body weight gain [28]. Moreover, a small clinical study has reported that obese patients taking nicotinic acid had decreased body weight [42]. Thus, although further studies are required to investigate this mechanism, the body weight-lowering effect of nicotinic acid might be partially the result of inducing GPR109A signaling.