3D models of MBP, a biologically active metabolite of bisphenol A, in human estrogen receptor α and estrogen receptor β

Abstract

Bisphenol A [BPA] is a widely dispersed environmental chemical that is of much concern because the BPA monomer is a weak transcriptional activator of human estrogen receptor α [ERα] and ERβ in cell culture. A BPA metabolite, 4-methyl-2,4-bis(4-hydroxyphenyl)pent-1-ene [MBP], has transcriptional activity at nM concentrations, which is 1000-fold lower than the concentration for estrogenic activity of BPA, suggesting that MBP may be an environmental estrogen. To investigate the structural basis for the activity of MBP at nM concentrations and the lower activity of BPA for human ERα and ERβ, we constructed 3D models of human ERα and ERβ with MBP and BPA for comparison with estradiol in these ERs. These 3D models suggest that MBP, but not BPA, has key contacts with amino acids in human ERα and ERβ that are important in binding of estradiol by these receptors. Metabolism of BPA to MBP increases the spacing between two phenolic rings, resulting in contacts between MBP and ERα and ERβ that mimic those of estradiol with these ERs. Mutagenesis of residues on these ERs that contact the phenolic hydroxyls will provide a test for our 3D models. Other environmental chemicals containing two appropriately spaced phenolic rings and an aliphatic spacer instead of an estrogenic B and C ring also may bind to ERα or ERβ and interfere with normal estrogen physiology. This analysis also may be useful in designing novel chemicals for regulating the actions of human ERα and ERβ.

Figure 1. Structures of MBP, BPA, E2 and DES.

MBP, BPA and DES have a phenolic ring that can mimic the A ring on E2 in binding to ERα and ERβ. The spacing between the first and second phenolic hydroxyls on MBP and DES is similar to that between C3 hydroxyl and the 17β-hydroxyl on E2. In contrast, the distance between the two phenolic hydroxyls in BPA is shorter than that in E2.

Figure 2. Analysis of two 3D models of MBP in human ERα. A.

3D model of MBP in orientation 1 in human ERα. The first phenolic ring on MBP contacts Glu-353, Arg-394 and Phe-404 on ERα and the second phenolic ring contacts Gly-521, His-524 and Leu-525. Favorable van der Waals contacts have a distance of 4.25 Å or less between MBP and amino acids on ERα. B. 3D model of MBP in orientation 2 in human ERα. The first phenolic ring on MBP contacts Glu-353, Arg-394 and Phe-404 on ERα, and the second phenolic ring contacts Gly-521, His-524 and Leu-525. However, in contrast to Orientation 1, the backbone oxygen on Leu-387 does not contact the phenolic hydroxyl on MBP. Phe-404 and Met-421 do not have van der Waals contacts with the linker between the two phenolic rings on MBP.

Figure 3. Interaction of E2 with amino acids in human ERα and ERβ. A.

Interaction of E2 with human ERα , , , , , , . The phenolic hydroxyl of E2 contacts Glu-353, Arg-394 and Leu-387. The 17β-hydroxyl contacts His524 and Leu-525. The D ring contacts Met343, Met421, Gly-521 and Ile-424. Favorable van der Waals contacts have a distance of 4.25 Å or less between E2 and amino acids on ERα. B. Interaction of E2 with human ERβ . The phenolic hydroxyl of E2 contacts Glu-305, Arg-346 and Leu-339. The 17β-hydroxyl contacts Gly-472, His473 and Leu-476. The D ring contacts Met-336 and Ile-373. Favorable van der Waals contacts have a distance of 4.25 Å or less between E2 and amino acids on ERβ.

Figure 4. Analysis of two 3D models of MBP in human ERβ. A.

3D model of MBP in orientation 1 in human ERβ. The first phenolic ring on MBP contacts Glu-305, Arg-346, Leu-339, Leu-343 and Phe-356. The second phenolic ring contacts Gly-472, His-475 and Leu-476, which are important in the interaction of the D ring of E2 with ERβ. B. 3D model of MBP in orientation 2 in human ERβ. The first phenolic ring on MBP contacts the backbone oxygen on Leu-339, Cβ on Ala-302 and Leu-343. These contacts are absent between MBP in Orientation 2 in ERα [Figure 2B].

Figure 5. Analysis of two 3D models of BPA in human ERα. A.

3D model of BPA in orientation 1 in human ERα. The first phenolic ring on BPA contacts Glu-353, Arg-394 and Phe-404 on ERα, but does not contact either Leu-387 or Leu-391. Moreover, the second phenolic ring does not contact either Gly-521, His-524 or Leu-525. Instead, the second phenolic ring contacts Phe-404, Met-421 and Ile-424. B. 3D model of BPA in orientation 2 in human ERα. The first phenolic ring on BPA contacts Glu-353 and Arg-394 on ERα, but does not contact Leu-387 or Phe-404. The second phenolic ring does not contact either Gly-521 or His-524. Instead, the second phenolic ring has novel contacts with Thr-347 and Leu-384.

Figure 6. Analysis of two 3D models of BPA in human ERβ. A.

3D model of BPA in orientation 1 in human ERβ. The first phenolic ring on BPA contacts Glu-305, Arg-346, and Phe-356, but does not contact either the backbone oxygen on Leu-339 or Cδ2 on Leu-343. The second phenolic ring contacts Gly-472, His-475 and Leu-476. B. 3D model of BPA in orientation 2 in human ERβ. The first phenolic ring on BPA contacts Glu-305, Arg-346, Phe-356, the backbone oxygen on Leu-339 and Cδ2 on Leu-343. The second phenolic ring does not contact either Gly-472, His-475 or Leu-476.

Figure 7. Structures of bisphenols that are potent synthetic estrogens.

Bisphenols, linked with one, two or three carbons, can have high affinity for ERs. 3-(3-fluoropropyl)cyclofenil, hexestrol and benzestrol have a higher affinity for ERα that does E2 , , .