POTENTIATION OF VD-INDUCED MONOCYTIC LEUKEMIA-CELL DIFFERENTIATION BYRETINOIDS INVOLVES BOTH RAR AND RXR SIGNALING PATHWAYS

Retinoids and vitamin D (VD) cooperate to induce the differentiation a nd inhibit the proliferation of human myelomonocytic leukemia cells, T wo classes of retinoids receptors, the RARs and RXRs, respectively, ca n mediate these effects, RXR forms heterodimers with a variety of nucl ear receptors, including RAR and the VD receptor, We have previously f ound that VD treatment increases RXR alpha levels in myelomonocytic le ukemia cells. By immunoanalysis, we observed in the present work that the RAR alpha protein is expressed in proliferating U937, HL-60 and TH P-1 human leukemia cells and that VD treatment induces alterations of its electrophoretic pattern, although with large differences between c ell lines, In the three cell lines, g-cis RA, an agonist of both RARs and RXRs, cooperated with VD more efficiently than all-trans RA and RA R-specific synthetic ligands, thus suggesting an involvement of both R AR and RXR pathways in cell differentiation. Using U937 cells as a mod el, we delineated the relative contributions of RAR and RXR by assessi ng the effects of receptor-selective synthetic retinoids. The synergy between VD and all-trans RA or RAR-specific agonists (TTNPB and Ho 40- 6055) was abrogated by a RAR alpha-specific antagonist (Ro 41-5253), c onfirming an involvement of RAR alpha. However, the cooperation betwee n VD and 9-cis RA, although reduced, was not suppressed by the antagon ist, suggesting also an involvement of the RXR pathway, The role of RX R as a ligand-activated receptor was confirmed using RXR-specific agon ists (CD2608 and LGD1069), which also proved able to cooperate with VD , Finally, while each synthetic agonist alone was significantly less p otent than 9-cis RA, combinations of the RAR and RXR selective agonist s TTNPB and LGD1069 appeared to be as effective as the pan agonist 9-c is-RA. These results confirm that various retinoids can cooperate with VD and demonstrate that, at a whole cell level, optimal effects requi re the activation of both RAR and RXR receptors.