Pathways of retinoid synthesis in mouse macrophages and bone marrow cells

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. Using a UAS-GFP reporter transgene, and retrovirally expressed Gal4-RARA in primary mouse bone marrow cells, we identified two distinct enzymatic pathways utilized by mouse myeloid cells ex vivo to synthesize RARA ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit+ bone marrow progenitor cells utilize diethylaminobenzaldehyde (DEAB)-sensitive enzymes, whereas bone marrow-derived macrophages use DEAB-insensitive enzymes to synthesize natural RARA-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the DEAB-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3, but instead express Aldh3b1, which we found is capable of DEAB-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular RARA-activating retinoids despite expression of these enzymes and a vitamin A sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate limiting step in the synthesis of intracellular RARA-activating retinoids in myeloid bone marrow cells and that that RARA remains in an un-liganded configuration during adult hematopoiesis. In order to identify additional enzymes that might contribute to DEAB-insensitive retinoid synthesis in bone marrow-derived macrophages, we compared gene expression in Kit+ progenitor cells vs bone marrow macrophages by Affymetrix array profiling. We analyzed 3 biological samples each of mouse bone marrow-derived macrophages and Kit+ progenitor cells