All-trans and 9-cis retinoic acid enhance 1,25-dihydroxyvitamin D₃-induced monocytic differentiation of U937 cells

Retinoic acid (RA) and 1,25-dihydroxyvitamin D₃ (D₃) are well known for inducing differentiation in many leukemic cell lines. The nuclear signalling pathways of RA and D₃ are mediated through their cognate receptors, the retinoic acid receptor (RAR) and vitamin D₃ receptor (VDR), respectively. Retinoid X receptor (RXR) is an auxiliary factor that forms a heterodimer with RAR and VDR, enabling their efficient transcriptional activation, 9-cis RA, a high-affinity ligand for RXR, greatly enhanced D₃-induced CD14 expression in U937 cells, while RA alone did not induce CD14 expression. 9-cis RA also resulted in morphological changes of U937 cells to macrophage-like cells when combined with D₃ while RA alone resulted in granulocyte-like cells. RA and D₃ together enhanced c-fms expression, phagocytic activity, and acted synergistically to promote nitroblue tetrazolium reduction activity and inhibit proliferation. Northern analysis showed that U937 cells constitutively expressed RAR-α, VDR and RXR-α mRNAs. RA or D₃ alone or in combination did not affect RAR-α and VDR expression, while g-cis RA and 9-cis RA plus all-trans RA significantly reduced RXR-α expression. Interestingly, D₃ could restore the down-regulation of RXR-α mRNA by 9-cis RA. These findings suggest that there is crossover of the nuclear signalling pathways of RA and D₃ This may have clinical implications in that RA and D₃ may be used in combination for differentiation-inducing therapy in acute myelogenous leukemia and myelodysplastic syndrome.