br Experimental Procedures br Acknowledgments br Bisphenol A
Bisphenol A (BPA) has been recognized as one of the most potent endocrine disruptors, functioning even at very low doses. Although it is anticipated that S63845 receptor (ER) would mediate these effects of low-dose BPA, studies revealed that BPA bound to ER very weakly , , , , . On the other hand, we have recently discovered that a human nuclear receptor, estrogen-related receptor γ (ERRγ), acts as a specific receptor for BPA (=ca. 5.5nM) , . ERRγ elicits a high basal constitutive activity with no ligand, and BPA was found to fully retain this activity. These findings necessitate that we reevaluate the low-dose effects of BPA in relation to the high-binding ability of BPA to ERRγ. Recently, we successfully performed an X-ray analysis of the crystal structure of the complex formed between BPA and the ligand-binding domain (LBD) of ERRγ. BPA, 2,2-bis(4-hydroxyphenyl)propane, was found in the binding pocket of ERRγ-LBD just like a tightly bound natural ligand. BPA-bound ERRγ maintains the activation conformation of authentic ERRγ, which helps to explain why the BPA-ERRγ complex retains a high basal constitutive activity. In this study, we preliminarily examined the binding ability of 4-α-cumylphenol, which lacks one of the two phenol-hydroxyl groups of BPA (). Surprisingly, 4-α-cumylphenol was found to bind to ERRγ as potently as BPA. Since diphenylpropane with no phenol-hydroxyl group was completely inactive, the phenol-hydroxyl group of 4-α-cumylphenol is essential for the binding to ERRγ. This means that any compounds having the phenol group are potential candidates for strong binders of ERRγ.
The nuclear receptor superfamily (NRs) is composed of receptors for small lipophilic ligands such as steroid hormones, thyroid hormone, retinoids, and vitamin D. In addition to the ligand-regulated receptors, the family includes a large group of receptors whose physiological ligands remain to be identified. These receptors are referred to as orphan NRs , , . NRs share a modular structure with a centrally located conserved DNA-binding domain (DBD), a carboxyl-terminal ligand-binding domain (LBD), and a variable amino-terminal domain (NTD). Most NRs regulate gene transcription as dimers but certain NRs are able to regulate transcription also as monomers . NRs contain in general two dimerization interfaces, one in the DBD and the other in the LBD . The dimerization interface in the LBD has been mapped to a conserved region in the carboxyl-terminal part of the domain (I-box) , , , , , . Estrogen-related receptors (ERRα, ERRβ, and ERRγ) belong to the category of orphan nuclear receptors , . No physiological ERR ligands have been so far identified and the recently solved crystal structure of the ERRγ LBD demonstrates that the LBD can adopt an active conformation and interact with coactivator proteins in the absence of any bound ligand . ERRγ also activates reporter gene expression constitutively when expressed in cultured cells , . Thus, ERRγ seems to be a true orphan receptor lacking the requirement for agonist-dependent activation. However, ERRγ binds certain pharmacologically active compounds such as 4-hydroxytamoxifen (4OHT) and diethylstilbestrol (DES) , , which inhibit its activity. ERRs have been shown to bind DNA as monomers and to recognize both estrogen response elements (EREs) and extended NR DNA response elements referred to as ERREs , , , . ERRα has also been shown to form homodimers and to heterodimerize with estrogen receptor α (ERα) , , , . ERRγ homodimerization in vitro has also been reported , .
Introduction Podocytes are glomerular visceral epithelial cells covering the urinary side of the glomerular basement membrane (GBM) and play an important role in maintaining normal glomerular structure and function (Mathieson, 2012). Podocytes are highly specialized and terminally differentiated, thereby having a limited mitotic capability (Lal et al., 2015). A growing body of evidence demonstrated that decreased podocyte number resulted from apoptosis predicts progressive decline of renal function and is a crucial determinant of the development of glomerulosclerosis in glomerular diseases (Wiggins, 2007, Anil Kumar et al., 2014, Wada et al., 2005). Many insults can cause podocyte apoptosis such as puromycin aminonucleoside (PAN) (Ding et al., 2002, Okamura et al., 2013, Durvasula et al., 2004). Recent studies consistently indicated that PAN treatment resulted in podocyte apoptosis in both animals and in vitro cells (Kim et al., 2001, Zheng et al., 2008, Wang et al., 2014). However, the detailed molecular mechanisms leading to podocyte apoptosis are still elusive.