An unusual distribution of the niacin receptor in cattle

Responses to pharmacological doses of niacin, an agonist for GPR109A (niacin receptor), were different in cattle than in humans and rodents. Thus, the tissue distribution of GPR109A was investigated in cattle. Samples of tail head fat, back fat, perirenal fat, longissimus muscle, and liver were analyzed for abundance of GPR109A mRNA by quantitative real-time reverse transcription-PCR and for abundance of GPR109A protein by Western blotting. Niacin receptor transcript and protein were detected in all tissues analyzed. The mRNA for GPR109A was more abundant in liver than in the other tissues sampled (GPR109A:RPS9 mRNA abundance = 0.56 in liver compared with 0.06 in longissimus muscle, 0.15 in kidney fat, 0.11 in back fat, 0.23 in tail head fat; standard error of the mean = 0.028). Additionally, mRNA for GPR109A was found (GPR109A:RPS9 mRNA abundance ≥0.004) in each of the 5 regions of bovine brain that were analyzed: cerebral cortex, cerebellum, thalamus, hypothalamus, and brain stem. Evaluation of liver tissue by immunofluorescence suggested that GPR109A was expressed in parenchymal cells and not localized exclusively to immune-system cells. Finally, analysis of the putative bovine GPR109A sequence verified that AA residues required for binding niacin in human GPR109A are conserved, suggesting that the bovine sequence identified encodes a functional niacin receptor. The identification of GPR109A in bovine liver, muscle, and brain is a novel finding.