br Discussion Variable estradiol effects on cell proliferati
Discussion Variable estradiol effects on cell proliferation have been reported according to animal species, tissues, and type of estrogen receptor [16,21,22,, , ]. In the kidney, 17βE was found to increase [3H]-thymidine incorporation in primary rabbit kidney proximal tubule Z-DEVD-AFC australia , and hamster tubular cells [21,22]. This proliferative effect of estradiol in the hamster kidney was considered to contribute to renal neoplastic growth [21,22]. Of note, patients from which primary cells were obtained had no renal tumors. Noteworthy, HRTEC cultures were used until passage five, because in later passages the cells began to be quiescent and did not respond to hormonal stimuli. It has been shown that estradiol exerts a renal protective effect in different animal models of renal injury [2,32,33]. However, the cellular and molecular mechanisms through which estrogens might produce protective effects in the kidney are poorly understood. Our studies in human epithelial cells, demonstrate that 17βE stimulated BrdU uptake also in tubular structures of 3D-HRTEC cultures, suggesting that this model may resemble the proliferative response of human renal tubular epithelia regeneration. Therefore, hormones such as estradiol could be involved in the regulation of cell proliferation, which could contribute to the normal regeneration of the human renal tubular epithelia after injury. Herein, we demonstrate that the proximal tubular epithelia of human pediatric kidney express the classical cytosolic estrogen receptors ERα and ERβ as well as the GPER-1 receptor. The effect of 17βE in increasing HRTEC proliferation seems to be associated with activation of GPER-1 and ERα, but not ERβ. It was reported that ERα and ERβ have opposite actions at the promoters of genes involved in cell proliferation, resulting of a balance between ERα and ERβ signaling [34,35].
Declaration of interest
Acknowledgments We are grateful to Dr. Horacio A. Repetto, and the Pediatric Surgery team at the “Sección de Cirugía Pediátrica, Hospital Nacional Prof. A. Posadas”, Buenos Aires, Argentina, for the provision of human kidney samples for research purposes. We thank Natalia Beltramone (IFIBIO-Houssay) for technical assistance, Fundings: This work was supported by grants to C. Silberstein and F.R. Ibarra from the Universidad de Buenos Aires (UBACYT 20020120200062 and 20120160100213BA).
Introduction Regulation of estrogenic signaling in neuronal cells is canonically viewed as a prototypical nuclear hormone receptor system. Estrogenic compounds are synthesized by a distant endocrine gland, the ovaries, and secreted into the bloodstream to perfuse estrogen-sensitive target tissues throughout the body including the brain (McEwen et al., 1982). As lipophilic molecules, estrogens can penetrate the lipid membrane of target cells and diffuse to the nucleus, where they bind to and activate their prototypical nuclear hormone receptors to regulate estrogen-dependent gene expression. Nuclear estrogen receptors exhibit both ligand-dependent and ligand-independent modes of activation that are mechanistically distinct (Tora et al., 1989). Ligand-dependent activation of estrogen receptor alpha (ERα) is accomplished through concerted function of the C, D, and E domains and denoted activation function-2 (AF-2) (Webster et al., 1988). Ligand-independent activation of ERα is initiated through concerted function of the A/B and C domains and denoted activation function-1 (AF-1) (Kumar et al., 1987). There is evidence to suggest that both that ligand-dependent and ligand-independent activation of estrogen receptors continue to occur in neuronal cells in absence of ovarian estrogens. The aromatase enzyme catalyzes the final step of estrogen synthesis, converting testosterones to estrogens (Thompson and Siiteri, 1974). This enzyme is expressed in discrete regions of the primate and rodent brain including the hippocampus (Wehrenberg et al., 2001). Aromatase enzyme inhibitor treatment is detrimental to cognitive function in both post-menopausal women (Bender et al., 2015; Collins et al., 2009; Bayer et al., 2015) and ovariectomized rodents (Vierk et al., 2012; Martin et al., 2003). Furthermore, significant aromatase enzyme activity continues to occur in primary cultures of hippocampal neurons for at least 12 days in vitro (Prange-Kiel et al., 2003), functioning to maintain synaptic spine density through activation of nuclear ERα (Kretz et al., 2004; Zhou et al., 2014). Therefore, local neuroestrogen synthesis may continue to activate nuclear estrogen receptors in absence of ovarian estrogens.