Endogenous retinoid X receptor ligands in mouse hematopoietic cells

The retinoid X receptor α (RXRA) has been implicated in diverse hematological processes. To identify natural ligands of RXRA that are present in hematopoietic cells, we adapted an upstream activation sequence-green fluorescent protein (UAS-GFP) reporter mouse to detect natural RXRA ligands in vivo. We observed reporter activity in diverse types of hematopoietic cells in vivo. Reporter activity increased during granulocyte colony-stimulating factor (G-CSF)-induced granulopoiesis and after phenylhydrazine (PHZ)-induced anemia, suggesting the presence of dynamically regulated natural RXRA ligands in hematopoietic cells. Mouse plasma activated Gal4-UAS reporter cells in vitro, and plasma from mice treated with G-CSF or PHZ recapitulated the patterns of reporter activation that we observed in vivo. Plasma from mice with dietary vitamin A deficiency only mildly reduced RXRA reporter activity, whereas plasma from mice on a fatty acid restriction diet reduced reporter activity, implicating fatty acids as plasma RXRA ligands. Through differential extraction coupled with mass spectrometry, we identified the long-chain fatty acid C24:5 as a natural RXRA ligand that was greatly increased in abundance in response to hematopoietic stress. Together, these data suggest that natural RXRA ligands are present and dynamically increased in abundance in mouse hematopoietic cells in vivo. We thank the Alvin J. Siteman Cancer Center at Washington University School of Medicine and Barnes-Jewish Hospital in St. Louis, MO. for the use of the Flow Cytometry Core. The Siteman Cancer Center is supported in part by an NCI Cancer Center Support Grant P30 CA91842. We thank High-Throughput Screening Center at Washington University School of Medicine in St. Louis, MO. We thank Deborah Laflamme for technical assistance and Feng Gao for statistical assistance. This work was supported by NIH R01 HL128447 (JS Welch), NIH P50 CA171963 (Project 1, JS Welch), and by grants from the Spanish Ministry of Economy and Competitiveness (SAF2015-64287R, SAF2015-71878-REDT) (M Ricote). The mass spectrometry facility at Washington University is supported by NIH P30 DK020579, Daniel Ory. J.S.W., H.N. and M.R. designed experiments, performed experiments, and wrote the manuscript. H.F., O.M., G.H., M.P.M, T.E.F., G.R.B. designed and performed experiments. Sí