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-alpha, VDR and RXR-alpha mRNAs. RA or D3 alone or in combination did not affect RAR-alpha and VDR expression, while 9-cis RA and 9-cis RA plus all-trans RA significantly reduced RXR-alpha expression. Interestingly, D3 could restore the down-regulation of RXR-alpha 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.