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Murine RARbeta4 displays reduced transactivation activity, lower affinity for retinoic acid, and no anti-AP1 activity
D R Soprano1, E Scanlon, M Shukri
1Department of Biochemistry, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Abstract:
The biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARalpha, beta, and gamma) and retinoid X receptors (RXR alpha, beta, and gamma). Each of the RARs is expressed as four to seven different isoforms. Four isoforms of RAR beta (beta1, beta2, beta3, and beta4), which differ only in their N-terminal sequence (A domain) have been described. These RARbeta isoforms display a specific pattern of expression in developing and adult animals and are highly evolutionarily conserved suggesting that they mediate distinct cellular effects of vitamin A. Experiments were performed to examine directly the RA-binding activity, transactivation activity, and anti-AP1 activity of each of these four RARbeta isoforms. The results demonstrate that RARbeta1, beta2, and beta3 bind RA with a similar K(d) value, have a similar EC(50) value in RA-dependent transactivation assays and inhibit AP1 activity to a similar level. By contrast, RARbeta4 has an elevated K(d) for RA, an increased EC(50) value in RA-dependent transactivation assays and does not display the ability to inhibit AP1 activity. This provides additional evidence that at least one RAR isoform, RARbeta4, may mediate distinct activities within a cell. Furthermore, these data suggest that the presence of an A domain in RARbeta is important for modulating these activities of RARs.
Insights
Retinoic acid receptors (RARs) mediate vitamin A
Area of Science:
- Molecular Biology
- Cellular Biology
- Endocrinology
Background:
- Biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs).
- RARs exist as multiple isoforms, including four variants of RAR beta (RARbeta1-4), which differ in their N-terminal A domain.
- These RARbeta isoforms exhibit distinct expression patterns and are evolutionarily conserved, suggesting unique cellular functions.
Purpose of the Study:
- To directly compare the RA-binding, transactivation, and anti-AP1 activities of the four RARbeta isoforms.
- To investigate the functional significance of the N-terminal A domain in RARbeta isoform activity.
- To determine if specific RARbeta isoforms mediate distinct cellular effects of vitamin A.
Main Methods:
- In vitro assays were used to measure RA-binding affinity (K(d)) for each RARbeta isoform.
- RA-dependent transactivation assays determined the EC(50) values for each isoform.
- AP1 activity inhibition assays assessed the anti-AP1 capabilities of the RARbeta isoforms.
Main Results:
- RARbeta1, beta2, and beta3 isoforms exhibited similar RA-binding affinities, transactivation efficiencies, and AP1 inhibitory activities.
- RARbeta4 demonstrated a reduced affinity for RA, higher EC(50) in transactivation assays, and lacked AP1 inhibitory activity.
- These findings indicate functional divergence among RARbeta isoforms.
Conclusions:
- RARbeta4 possesses distinct biological activities compared to RARbeta1, beta2, and beta3, supporting the hypothesis of isoform-specific functions.
- The N-terminal A domain of RARbeta plays a crucial role in modulating receptor activity and mediating distinct cellular effects of vitamin A.
- These results highlight the importance of RARbeta isoforms in the diverse biological actions of retinoic acid.