Dimerization with retinoid X receptors promotes nuclear localization and subnuclear targeting of vitamin D receptors

Area of Science:

• Cell biology
• Molecular endocrinology
• Gene regulation

Background:

• Vitamin D receptor (VDR) forms heterodimers with Retinoid X Receptor alpha (RXR) to regulate gene transcription.
• Understanding the impact of heterodimerization on VDR and RXR subcellular distribution is crucial for deciphering their regulatory mechanisms.

Purpose of the Study:

• To investigate how heterodimerization influences the subcellular localization of VDR and RXR in living cells.
• To explore the functional consequences of RXR-VDR heterodimerization on VDR transcriptional activity.

Main Methods:

• Development of fluorescent-protein chimeras for VDR and RXR.
• Confocal microscopy to assess receptor distribution.
• Fluorescence resonance energy transfer (FRET) microscopy to detect heterodimer formation and nuclear foci.
• Reporter gene assays to measure transcriptional activity.

Main Results:

• Unliganded VDR predominantly localized in the cytoplasm, while RXR showed greater nuclear distribution.
• Co-expression of RXR promoted VDR's nuclear import and retention, increasing nuclear heterodimer content and basal transcriptional activity.
• Calcitriol induced the formation of multiple nuclear foci of VDR-RXR heterodimers, correlating with DNA binding.
• RXR rescued the nuclear localization and transcriptional activity of a nuclear localization signal-mutant VDR.

Conclusions:

• RXR significantly influences VDR subcellular distribution, promoting its nuclear accumulation and intranuclear targeting.
• RXR-mediated VDR nuclear localization and foci formation are critical for regulating VDR transcriptional activity.
• Heterodimerization is a key mechanism for controlling VDR function, potentially impacting other nuclear receptor partners.