Analysis of the Effects of CRABP I Expression on the RA-Induced Transcription Mediated by Retinoid Receptors

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Abstract
The involvement of cellular retinoic acid-binding protein I (CRABP I) in the RA signaling was investigated by examining its effects on the interaction of retinoid receptors (RARs and RXRs) with RA-response elements (RAREs) as well as on the RA-induced transcription mediated by retinoid receptors. Analysis of the expression of mouse CRABP I from a cDNA expression plasmid in COS-1 cells revealed that this protein was about 5-fold more abundant in cytosol than in nuclei. The identity and the localization of CRABP I in the cytoplasm as well as the nuclei were also confirmed by the immunoperoxidase staining of the transfected COS-1 cells with CRABP I-specific antibody. When the nuclear extract containing a 10-fold molar excess of CRABP I was incubated with RAR α extract in the presence of [3H]RA and resolved on an FPLC size-exclusion column, a 20% decrease in the bound radioactivity in the RAR α fraction was accompanied by a proportional increase in the CRABP I fraction. In contrast, the addition of CRABP I did not significantly affect the interaction of RAR α or RAR α−RXR α heterodimers with RAREs. Moreover, the coexpression of CRABP I in CV-1 cells did not markedly inhibit or enhance the transcription activated by RARs and RAR α−RXR α heterodimers under RA concentrations ranging from 10-10 to 10-6 M. These results demonstrate that CRABP I, while it might be important for RA homeostasis, is not directly involved in the retinoid receptor-mediated RA-signaling pathway.

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