Vitamin D3 and retinoic acid induced monocytic differentiation : Interactions between the endogenous vitamin D3, retinoic acid and retinoid X receptors in U-937 cells

• Retinoic acid (RA) and 1,25 alpha-dihydroxycholecalciferol (VitD3) are potent regulators of hematopoletic differentiation. Yet, little is known as to how the RA and VitD3 receptor network operates in hematopoietic cells, and whether receptor interactions can explain the interplay between the RA- and VitD3-signaling pathways during differentiation. Therefore, we analyzed the expression, DNA binding, and transcriptional activity of the endogenous RA and VitD3 receptors [retinoic acid receptors (RARs), retinoid X receptors (RXRs), and VitD3 receptor (VDR)] in the U-937 cell line, in which RA and VitD3 induce distinct monocytic differentiation pathways. VitD3 induction resulted in the formation of VDR/RXR DNA-binding complexes on both VitD3 response elements and RA response elements (RAREs). However, transcriptional activation was only observed from a VitD3 response element-driven reporter construct. Several DNA-binding complexes were detected on RAREs in undifferentiated cells. Stimulation by RA resulted in increased RAR beta/RXR DNA binding, activated RARE-dependent transcription, and increased expression of RAR-beta. Concomitant stimulation by VitD3 inhibited the RA-stimulated formation of RAR beta/RXR heterodimers, favoring VDR/RXR binding to the RARE. Also, VitD3 inhibited the expression of CD23 and CD49f, characteristic markers of retinoid-induced U-937 cell differentiation. In contrast, neither the RA-stimulated, RARE-mediated transcription nor the induced RAR-beta expression was suppressed by VitD3, suggesting that VitD3 selectively inhibited the retinoid-induced differentiation program but not the RARE-mediated signal. These results demonstrate a complex role for VitD3 in modifying the retinoid differentiation pathway and may have implications for differentiation-inducing therapy of hematopoietic tumors.

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