CHARACTERIZATION OF THE CHIMERIC RETINOIC ACID RECEPTOR RAR-ALPHA VDR/

The chimeric receptor, RAR alpha/VDR, contains the DNA-binding domain of the retinoic acid receptor (RAR alpha) and the ligand-binding domai n of the vitamin D receptor (VDR), The ligand-binding properties of RA R alpha/VDR are equivalent to that of VDR, with an observed K-d for 1 alpha,25 dihydroxy-vitamin D-3 (D-3) of 0.5 nM. In CV-1 cells. both RA R alpha and RAR alpha/VDR induce comparable levels of ligand-mediated transcriptional activity from the retinoic acid responsive reporter ge ne, beta(RARE)(3)-TK-luciferase, in the presence of the ligand predict ed from the receptor ligand-binding domain. Two chimeric RAR receptors were constructed which contained the ligand-binding domain of the est rogen receptor (ER): RAR alpha/ER and ER/RAR alpha/ER. Both RAR alpha/ ER and ER/RAR alpha/ER bind beta-estradiol with high affinity, and are transcriptionally active only from palindromic RAREs (TREpal and/or ( TRE3)(3)). Only RAR alpha/VDR matched in kind and degree the functiona l characteristics of RAR alpha: (1) maximally active from the beta(RAR E); (2) moderately active from the TREs; (3) inactive from the retinoi c X receptor response elements (RXREs) ApoA1 and CRBP II; (4) forms he terodimers with RXR alpha; and (5) binds to the beta RARE. F9 embryona l carcinoma cell lines were generated which express RAR alpha/VDR mRNA (F9-RAR alpha/VDR cells) and compared with F9 wild-type (FS-Wt) cells , which do not express VDR mRNA. Treatment with all-trans retinoic aci d (tRA) inhibits cell growth and induces the differentiation morpholog y in both FS-Wt and F9-RAR alpha/VDR cells; whereas, treatment with D- 3 is similarly effective only for F9-RAR alpha/VDR cells. It is conclu ded RAR alpha/VDR is an useful 'tool' to pinpoint, or to augment trans cription from RAREs in gene pathways controlled by RAR without inhibit ing the retinoid responsiveness of endogenous RARs.