RXR-dependent and RXR-independent transactivation by retinoic acid receptors

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• Retinoic acid receptors (RARs) heterodimerize with retinoid X receptors (RXRs) to enhance DNA binding affinity.
• This heterodimerization is often considered essential for RAR-mediated transcriptional responses.

Purpose of the Study:

• To investigate the role of retinoid X receptors (RXRs) in retinoic acid receptor (RAR)-mediated gene transcription.
• To determine if RARs can mediate transcriptional responses independently of RXRs.

Main Methods:

• Transfection of Drosophila SL-3 cells (lacking endogenous RARs and RXRs) with RAR and RXR.
• Stimulation with all-trans retinoic acid (RA) and 9-cis RA.
• Point mutations in the RAR beta 2 gene promoter RA response element (RARE).
• Analysis of transcriptional activity in both Drosophila and mammalian cells.

Main Results:

• RAR alone conferred a response to all-trans RA in RXR-deficient cells.
• Co-transfection of RAR and RXR with their respective ligands resulted in a synergistic response.
• Distinct differences in transcriptional activity were observed based on the presence or absence of RXR and 9-cis RA.
• Both RXR-independent and RXR-dependent pathways for all-trans RA action were identified in both cell types.

Conclusions:

• Retinoid X receptor (RXR) is not always a prerequisite for retinoic acid receptor (RAR)-mediated transcription.
• All-trans retinoic acid can activate transcription via both RXR-dependent and RXR-independent pathways.
• The availability of 9-cis retinoic acid dictates which pathway is utilized for all-trans retinoic acid signaling.