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Cross-Talk between PPARs and the Partners of RXR: A Molecular Perspective.

Chan LS, Wells RA - PPAR Res (2009)

Bottom Line: As a result, many heterodimers share the same DR element and must complete with each other for DNA binding.As a result, individual nuclear receptors must complete with each other for RXR to form functional heterodimers.These tools will enable future studies to dissect specific RXR heterodimers and their signaling pathways.

View Article: PubMed Central - PubMed

Affiliation: Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada M5G 2M9.

ABSTRACT
The PPARs are integral parts of the RXR-dependent signaling networks. Many other nuclear receptor subfamily 1 members also require RXR as their obligatory heterodimerization partner and they are often co-expressed in any given tissue. Therefore, the PPARs often complete with other RXR-dependent nuclear receptors and this competition has important biological implications. Thorough understanding of this cross-talk at the molecular level is crucial to determine the detailed functional roles of the PPARs. At the level of DNA binding, most RXR heterodimers bind selectively to the well-known "DR1 to 5" DNA response elements. As a result, many heterodimers share the same DR element and must complete with each other for DNA binding. At the level of heterodimerization, the partners of RXR share the same RXR dimerization interface. As a result, individual nuclear receptors must complete with each other for RXR to form functional heterodimers. Cross-talk through DNA binding and RXR heterodimerization present challenges to the study of these nuclear receptors that cannot be adequately addressed by current experimental approaches. Novel tools, such as engineered nuclear receptors with altered dimerization properties, are currently being developed. These tools will enable future studies to dissect specific RXR heterodimers and their signaling pathways.

No MeSH data available.


Related in: MedlinePlus

(a) Ribbon drawing of hRXRα/hPPARγ LBD heterodimer (PDB 3E00, also see Figure 2). (b) The detail view of the equivalent amino acid sidechains of mouse RXRα K422 and mouse PPARγ E405. The shortest distance between the negative (side chain carboxyl group of E405) and positive (side chain amino group of K422) charges is also displayed.
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fig4: (a) Ribbon drawing of hRXRα/hPPARγ LBD heterodimer (PDB 3E00, also see Figure 2). (b) The detail view of the equivalent amino acid sidechains of mouse RXRα K422 and mouse PPARγ E405. The shortest distance between the negative (side chain carboxyl group of E405) and positive (side chain amino group of K422) charges is also displayed.

Mentions: The research of RXR dependent signaling networks requires the precise characterization of individual signaling pathways. However, current experimental approaches are insufficient to achieve this objective due to the unique properties of the signaling network. A novel experimental system consisting of engineered RXR and partners with controllable heterodimerization specificity would be helpful to complement the existing approaches and to circumvent their limitations. Our laboratory has created and tested an engineered RXR/PPAR heterodimer consist of mouse RXRα K422E and mouse PPARγ E405K mutants [82]. The creation of these mutants is based on the proposed salt-bridge between RXRα K422 and PPARγ E405 (Figure 4). In addition, PPARγ E405 and RXRα K422 correspond to human RXRα E390 and K417, which are highly conserved among RXR and its partners in mammalian species (Figure 3). Since this salt-bridge is located within the heterodimer interface, we postulate that this salt-bridge may have a role in heterodimerization. We also hypothesized that reversing the polarity of the side chains of these amino acids may alter heterodimerization specificity but the salt-bridge will be kept intact. Indeed, our observations suggest that the mutant pair is able to form a heterodimer. Although PPARγ E405K can form a heterodimer with wild-type RXRα, RXRα K422E is not able to heterodimerize with wild-type PPARγ (Table 2). In addition, ligand response of the PPARγ E405K mutant is comparable to that of the wild-type PPARγ, suggesting that the general structure of this mutant is preserved. The restricted heterodimerization capacity of the RXRα K422E mutant is especially exciting, since the salt-bridge between RXRα K422 and PPARγ E405 (Figure 4(b)) is predicted to exist in other RXR's partners based on the crystal structures of the LBD heterodimers (RXR/RAR [39, 40], RXR/CAR [41, 42], RXR/LXR [43, 44], and RXR/PPAR [45–47]). We are currently conducting experiments to determine if the restriction on heterodimerization of RXRα K422E is also applicable to other NR partners, and if mutants of these partners (equivalent to PPARγ E405K) can restore heterodimerization with RXRα K422E. If true, our approach will permit rescue, in the context of RXR knockout, of specific NR pathway either by knock-in or by in vitro delivery of dimerization restricted NR pairs. Expression of RXRα K422E and PPARγ E405K mutants in RXRα −/− cells could, for instance, be used to restore specifically the functions mediated by RXRα/PPARγ heterodimer, thus identifying the contribution of this particular heterodimer to the whole phenotype. Hence, it maybe possible to dissect the RXR-dependent signaling pathway in a precise manner using dimerization-restricted NR pairs.


Cross-Talk between PPARs and the Partners of RXR: A Molecular Perspective.

Chan LS, Wells RA - PPAR Res (2009)

(a) Ribbon drawing of hRXRα/hPPARγ LBD heterodimer (PDB 3E00, also see Figure 2). (b) The detail view of the equivalent amino acid sidechains of mouse RXRα K422 and mouse PPARγ E405. The shortest distance between the negative (side chain carboxyl group of E405) and positive (side chain amino group of K422) charges is also displayed.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2801013&req=5

fig4: (a) Ribbon drawing of hRXRα/hPPARγ LBD heterodimer (PDB 3E00, also see Figure 2). (b) The detail view of the equivalent amino acid sidechains of mouse RXRα K422 and mouse PPARγ E405. The shortest distance between the negative (side chain carboxyl group of E405) and positive (side chain amino group of K422) charges is also displayed.
Mentions: The research of RXR dependent signaling networks requires the precise characterization of individual signaling pathways. However, current experimental approaches are insufficient to achieve this objective due to the unique properties of the signaling network. A novel experimental system consisting of engineered RXR and partners with controllable heterodimerization specificity would be helpful to complement the existing approaches and to circumvent their limitations. Our laboratory has created and tested an engineered RXR/PPAR heterodimer consist of mouse RXRα K422E and mouse PPARγ E405K mutants [82]. The creation of these mutants is based on the proposed salt-bridge between RXRα K422 and PPARγ E405 (Figure 4). In addition, PPARγ E405 and RXRα K422 correspond to human RXRα E390 and K417, which are highly conserved among RXR and its partners in mammalian species (Figure 3). Since this salt-bridge is located within the heterodimer interface, we postulate that this salt-bridge may have a role in heterodimerization. We also hypothesized that reversing the polarity of the side chains of these amino acids may alter heterodimerization specificity but the salt-bridge will be kept intact. Indeed, our observations suggest that the mutant pair is able to form a heterodimer. Although PPARγ E405K can form a heterodimer with wild-type RXRα, RXRα K422E is not able to heterodimerize with wild-type PPARγ (Table 2). In addition, ligand response of the PPARγ E405K mutant is comparable to that of the wild-type PPARγ, suggesting that the general structure of this mutant is preserved. The restricted heterodimerization capacity of the RXRα K422E mutant is especially exciting, since the salt-bridge between RXRα K422 and PPARγ E405 (Figure 4(b)) is predicted to exist in other RXR's partners based on the crystal structures of the LBD heterodimers (RXR/RAR [39, 40], RXR/CAR [41, 42], RXR/LXR [43, 44], and RXR/PPAR [45–47]). We are currently conducting experiments to determine if the restriction on heterodimerization of RXRα K422E is also applicable to other NR partners, and if mutants of these partners (equivalent to PPARγ E405K) can restore heterodimerization with RXRα K422E. If true, our approach will permit rescue, in the context of RXR knockout, of specific NR pathway either by knock-in or by in vitro delivery of dimerization restricted NR pairs. Expression of RXRα K422E and PPARγ E405K mutants in RXRα −/− cells could, for instance, be used to restore specifically the functions mediated by RXRα/PPARγ heterodimer, thus identifying the contribution of this particular heterodimer to the whole phenotype. Hence, it maybe possible to dissect the RXR-dependent signaling pathway in a precise manner using dimerization-restricted NR pairs.

Bottom Line: As a result, many heterodimers share the same DR element and must complete with each other for DNA binding.As a result, individual nuclear receptors must complete with each other for RXR to form functional heterodimers.These tools will enable future studies to dissect specific RXR heterodimers and their signaling pathways.

View Article: PubMed Central - PubMed

Affiliation: Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada M5G 2M9.

ABSTRACT
The PPARs are integral parts of the RXR-dependent signaling networks. Many other nuclear receptor subfamily 1 members also require RXR as their obligatory heterodimerization partner and they are often co-expressed in any given tissue. Therefore, the PPARs often complete with other RXR-dependent nuclear receptors and this competition has important biological implications. Thorough understanding of this cross-talk at the molecular level is crucial to determine the detailed functional roles of the PPARs. At the level of DNA binding, most RXR heterodimers bind selectively to the well-known "DR1 to 5" DNA response elements. As a result, many heterodimers share the same DR element and must complete with each other for DNA binding. At the level of heterodimerization, the partners of RXR share the same RXR dimerization interface. As a result, individual nuclear receptors must complete with each other for RXR to form functional heterodimers. Cross-talk through DNA binding and RXR heterodimerization present challenges to the study of these nuclear receptors that cannot be adequately addressed by current experimental approaches. Novel tools, such as engineered nuclear receptors with altered dimerization properties, are currently being developed. These tools will enable future studies to dissect specific RXR heterodimers and their signaling pathways.

No MeSH data available.


Related in: MedlinePlus