Bisphenol TMC exhibits greater estrogenic activity than Bisphenol A and three other structural analogues exemplified by higher estrogen receptor α-mediated gene expression and breast cancer cell proliferation.

Bisphenol A (BPA) and its structural analogues are widely used in plastics production, raising concern due to endocrine-disrupting properties. While many analogues share structural similarities with BPA, their endocrine-disrupting effects remain insufficiently characterized. Cyclo-di-bisphenol A diglycidyl ether (cyclo-di-BADGE), tetrabromobisphenol S (TBBPS), bisphenol SIP (BPSIP), and bisphenol TMC (BPTMC) are particularly understudied. We assessed the estrogenic activity of these four BPA analogues compared to BPA. Transactivation assays in HEK-293 cells expressing estrogen receptor alpha (ERα) revealed that BPTMC was a more potent ERα agonist than BPA, with an EC of 87 ± 20 nM versus 400 ± 100 nM for BPA, while the other tested analogues showed no significant agonistic activity. In silico analysis attributed this higher affinity to greater hydrophobicity and a bulkier bridging group between its phenolic rings. None of the compounds inhibited 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) activity. However, BPTMC selectively inhibited 17β-HSD2 (IC = 4.8 ± 0.6 µM) but not BPA. Importantly, 24 h exposure of ERα-positive MCF-7 breast cancer cells to 1 µM BPTMC upregulated the expression of the ERα target genes GREB1, TFF1, and PGR, comparable to 10 nM E2, which was abolished by 100 nM of the ERα antagonist fulvestrant. Moreover, BPTMC stimulated MCF-7 cell proliferation at nanomolar concentrations over 72 h, and cell count analyses confirmed this effect. BPA also increased cell numbers, and both effects were reversed by fulvestrant. Collectively, we identified BPTMC as a potent ERα agonist capable of eliciting transcriptional and mitogenic responses at low concentrations, raising concerns about its endocrine-disrupting and breast cancer-promoting effects.