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.