Characterization of the chimeric retinoic acid receptor RARα/VDR

Abstract

The chimeric receptor, RARα/VDR, contains the DNA-binding domain of the retinoic acid receptor (RARα) and the ligand-binding domain of the vitamin D receptor (VDR). The ligand-binding properties of RARα/VDR are equivalent to that of VDR, with an observed K_d for 1α,25 dihydroxy-vitamin D₃ (D₃) of 0.5 nM. In CV-1 cells, both RARα and RARα/VDR induce comparable levels of ligand-mediated transcriptional activity from the retinoic acid responsive reporter gene, β(RARE)₃-TK-luciferase, in the presence of the ligand predicted from the receptor ligand-binding domain. Two chimeric RAR receptors were constructed which contained the ligand-binding domain of the estrogen receptor (ER): RARα/ER and ER/RARα/ER. Both RARα/ER and ER/RARα/ER bind β-estradiol with high affinity, and are transcriptionally active only from palindromic RAREs (TREpal and/or (TRE3)₃). Only RARα/VDR matched in kind and degree the functional characteristics of RARα: (1) maximally active from the β(RARE); (2) moderately active from the TREs; (3) inactive from the retinoic X receptor response elements (RXREs) ApoA1 and CRBP II; (4) forms heterodimers with RXRα; and (5) binds to the βRARE. F9 embryonal carcinoma cell lines were generated which express RARα/VDR mRNA (F9-RARα/VDR cells) and compared with F9 wild-type (F9-Wt) cells, which do not express VDR mRNA. Treatment with all-trans retinoic acid (tRA) inhibits cell growth and induces the differentiation morphology in both F9-Wt and F9-RARα/VDR cells; whereas, treatment with D₃ is similarly effective only for F9-RARα/VDR cells. It is concluded RARα/VDR is an useful ‘tool’ to pinpoint, or to augment transcription from RAREs in gene pathways controlled by RAR without inhibiting the retinoid responsiveness of endogenous RARs.