Chen W. Liaw

The role of the GPR91 ligand succinate in hematopoiesis

Regulation of cellular metabolism by the citric acid cycle occurs in the mitochondria. However, the citric acid cycle intermediate succinate was shown recently to be a ligand for the G‐protein‐coupled receptor GPR91. Here, we describe a role for succinate and its receptor in the stimulation of hematopoietic progenitor cell (HPC) growth. GPR91 mRNA and protein expression were detected in human bone marrow CD34+ progenitor cells, as well as in erythroid and megakaryocyte cultures and the erythroleukemic cell line TF‐1. Treatment of these cell cultures with succinate resulted in increased proliferation rates. The proliferation response of TF‐1 cells was pertussis toxin (PTX)‐sensitive, suggesting a role for Gi signaling. Proliferation was also blocked when TF‐1 cells were transfected with small interfering RNA specific for GPR91. Succinate stimulated activation of the Erk MAPK pathway and inositol phosphate accumulation in a PTX‐sensitive manner. Pretreatment of TF‐1 cells with the Erk1/2 kinase (MEK) inhibitor PD98059 blocked the proliferation response. Succinate treatment additionally protected TF‐1 cells from cell death induced by serum deprivation. Finally, in vivo administration of succinate was found to elevate the levels of hemoglobin, platelets, and neutrophils in a mouse model of chemotherapy‐induced myelosuppression. These results suggest that succinate‐GPR91 signaling is capable of promoting HPC development.

Differential tissue and ligand-dependent signaling of GPR109A receptor: Implications for anti-atherosclerotic therapeutic potential

Until recently, the anti-atherosclerotic effects of niacin were attributed primarily to its lipid modification properties mediated by adipocyte G-protein coupled receptor GPR109A, though recent studies have raised significant doubts about this mechanism. In fact, in rodents it has recently been demonstrated that niacin inhibits progression of atherosclerosis through actions on immune cells, particularly via macrophage-expressed GPR109A, independent of lipid-modifying properties. Here, we studied GPR109A signal transduction in human Langerhans cells, macrophages and adipocytes. We find that the consequences of receptor activation are profoundly influenced by cellular context and that ligand-biased signaling significantly impacts functionally relevant signaling. In Langerhans cells, niacin initiates GPR109A-mediated signaling pathways (Erk1/2 and Ca(2+)) responsible for the release of vasodilatory prostanoids, while the synthetic GPR109A agonist MK-0354 fails to elicit any signaling, providing a mechanistic basis for the latter compounds inability to cause flushing. While GPR109A mediates inhibition of cAMP in adipocytes, in macrophages GPR109A signaling via Gβγ subunits results in paradoxical augmentation of intracellular cAMP levels. Also, in macrophages niacin and GPR109A full agonists induce Erk1/2 and Ca(2+) signaling, release of prostanoids, upregulation of cholesterol transporters ABCA1 and ABCG1 and stimulation of reverse cholesterol transport in GPR109A dependent manner. A mechanism is presented in which signals from the autocrine action of released prostanoids and Gi protein mediated cAMP augmentation are integrated leading to modulation of reverse cholesterol transport regulatory components. These studies provide key insights into mechanisms by which GPR109A may influence cholesterol efflux in macrophages; a process that may be at least partially responsible for niacins anti-atherosclerotic activity. MK-0354 does not induce niacin-like GPR109A signaling in macrophages, suggesting that biased agonists devoid of the flushing side-effect may also lack properties required for macrophage-mediated anti-atherosclerotic effects.