Potential nuances in renoprotective properties of estrogen in females

Abstract

A current study by Kitai et al. found that ovariectomy before estrogen/female sex hormone sensitization at puberty provided protection against kidney ischemia reperfusion injury, challenging the general consensus within the field that estrogen provides renoprotective function. These results are intriguing and could have important clinical implications, while requiring some clarification and substantiation of the conclusions reported.

The role of sex hormones in kidney health and function has been a major area of focus in medical research for decades, with sexual dimorphism in kidney injury recognized in animal models for many years. Over time, some prominent findings have been elucidated. Male sex hormones exacerbate kidney injury, whereas female sex hormones mitigate kidney injury; the sex hormones involved having undergone the most scrutiny are testosterone and estrogen (i.e., 17β-estradiol), respectively. Overwhelmingly, data show that estrogen plays a protective role in kidney ischemia-reperfusion injury (IRI). Sources of these data range from clinical disease and outcome analyses to basic and translational studies in rats and mice. Although the general consensus is that estrogen is renoprotective, there exists evidence to the contrary. Maenosono et al.¹ report the increased likelihood of graft rejection in young (15–34 years) as compared with middle-aged (35–54 years) and older women (55–74 years). Others have reported enhanced deterioration of kidney function during puberty for girls with pre-existing kidney disease.² In an attempt to address the described phenomenon of worsening kidney injury during puberty for girls, Kitai et al.³ report that puberty-induced estrogen production increased kidney susceptibility to IRI. These findings are of interest as they challenge the prevailing paradigm, yet there are discrepancies and points of concern to be addressed.

An extensive body of research supports that estrogen is renoprotective. Clinically, sex affects the outcome of kidney disease,⁴ primary glomerulonephritis,⁵ and kidney IRI,⁶ where females suffer less severe injury than males. In the case of human kidney transplantation, both female recipient and female donor kidneys had lower rates of delayed graft function than their male counterparts,⁶ controlled for donor and recipient body size match. Experimental models of IRI in rats and mice have not only found the same delineation between sexes, but also found that estrogen specifically contributes a protective effect in female kidneys. This has been consistently demonstrated by a loss of IRI protection in ovariectomized females, which can then be reversed with 17β-estradiol treatment.^6–8 Treatment with 17β-estradiol was also renoprotective in neutered males, though not in intact males.⁶ The role of estrogen receptors remains unclear, as there are conflicting reports. In the case of estrogen receptor alpha depletion, IRI protection in female mice was lost,⁶ suggesting a role in estrogen-mediated kidney protection. Furthermore, selective estrogen receptor modulators such as raloxifene have been shown to be protective against kidney IRI in female mice.⁹ Together these studies represent the foundation for which estrogen has been established as renoprotective. Although these studies were not aimed at the role of estrogen in prepuberty and puberty, at least one study performed ovariectomy (ovx) and orchiectomy at prepuberty in females and males, respectively, and showed a median kidney IRI tolerance state similar in both neutered groups, which was still sensitive to kidney protection with 17β-estradiol in both sexes.⁶

The current study by Kitai et al.³ examined the role of estrogen during puberty on kidney IRI. Female mice were ovariectomized either on postnatal day 21 or week 8, and then at week 10, they underwent warm unilateral ischemia with subsequent contralateral nephrectomy. They found that unlike those ovariectomized at 8 weeks, the mice ovariectomized at day 21 were protected from IRI as compared with intact (sham) female mice, as illustrated in Figure 1. Subsequently, with 17β-estradiol treatment after ovx, IRI protection was reversed. The authors conclude that ovx before estrogen/female sex hormone sensitization is protective in kidney IRI. These results deviate from previously published results from methodologically similar IRI and contralateral nephrectomy experiments with mice ovariectomized at 3 weeks of age.⁶ The authors claim that the reported results do not challenge the current paradigm of estrogen renoprotection in females, but rather they are consistent with this paradigm with the exception of females lacking pubertal hormone surge/sensitization. They explain that postnatal day 21 precedes the onset of female puberty and the hormone “surge” associated therein, whereas between days 22 and 28, puberty-associated genes induce expression at which point the protective effects of ovx are lost (Figure 1). They report that genes associated with the onset of female puberty (Slc22a7, Slc22a19, Akr1c14, and Akr1c18) were expressed in the kidneys at postnatal day 28, having not yet been induced at day 21 (Figure 1). Other studies have found that female mice undergo a surge of luteinizing hormone and estradiol at postnatal days 12–14 (Figure 1).¹⁰ The effects of this surge were not reflected in the expression of puberty-associated kidney hormones tested by Kitai et al.;³ however, this again highlights the necessity of a more thorough analysis of the mechanism(s) potentially involved with the reported 21-day ovx-IRI results. There needs to be further delineation of puberty hormone onset between 21 and 28 days coupled with evidence of the direct influence on kidney protection. A relevant experiment would be to compare kidney injury in mice having received IRI with contralateral nephrectomy at postnatal days 21 and 28, while acknowledging that this may not be feasible due to the size of mice to undergo operation. As the results from this study oppose those previously published, in addition to further delineation of puberty-associated hormone induction, more rigorous and comprehensive analysis of the described IRI protection in puberty-deprived females will be necessary to shift the conventional understanding in the field. The previously published findings in neutered and intact male mice do further inform the impact of pubertal hormone deprivation on renal IRI, and we suggest that Kitai et al. may wish to test the impact of their model on male mice.⁶

Figure 1 |. Diagram of methods and outcomes from ischemic injury with respect to estrogen renoprotection.

The horizontal timeline represents the postnatal age (days) of mice with the timing of ovariectomy (ovx) below and the timing of puberty-associated hormone expression above. At the age of ovx, illustrations of mouse kidneys after warm ischemia and subsequent contralateral nephrectomy (IRI + cNx) represent renal injury (red) or tolerance (blue). Above the timeline, the ages in which mice express puberty-associated hormones, illustrations of mice depict the location of hormone/gene expression. At 12–14 days, expression of estrogen (E2), luteinizing hormone (LH), and follicle stimulating hormone (FSH) surge in the ovaries. At 28 days, estrogen-responsive genes express in the kidneys, unlike at 21 days and prior. Between 22 and 27 days, expression levels of such genes are unknown.

Other concerns with respect to the current Kitai et al.³ study involve the methodology of warm IRI experiments. In the warm unilateral ischemia with the subsequent contralateral nephrectomy model, the time of ischemia used by Kitai et al.³ of 37 minutes is a very long time even for female B6 mice, which are more resilient than male mice to injury in this model. Other studies using this model in female mice use 21–30 minutes of warm ischemia.^6,8,9,11 Such a long period of ischemia raises concerns about the degree of injury reported. For example, the serum blood urea nitrogen levels reported at 3 days after IRI (37 minutes) were directly comparable to those reported by Aufhauser et al.⁶ at the same time point after 28-minute IRI. In addition, the period of time between ovx and IRI was inconsistent between mice ovariectomized at 21 days versus 8 weeks. In doing this, the authors kept the age at which IRI was performed consistently between the groups; however, the difference in time after ovx could have had an effect on IRI results. It is unclear whether or not these methodological concerns had a relevant impact on the results, hence requiring further investigation and substantiation of the conclusions.

At present, within the context of the field and the current study in mind, estrogen will remain to be considered renoprotective, though with a potential exception impacted by pubertal hormone exposure in females. Although the aforementioned concerns need to be addressed in order to substantiate the conclusions made by Kitai et al. with respect to the lack of estrogen-associated renoprotection at puberty, this study has presented additional valuable insights into factors and mechanisms involved in renal injury. They found that elevated levels of insulin-like growth factor 1 receptor (IGF1R) and IGF/IGF1R signaling in young female kidneys were associated with worsened IRI. They also describe a role of IGF/IGF1R signaling in kidney growth and cell cycle regulation within proximal renal tubules. Importantly, this study highlights an area within the field that deserves more attention with important clinical and therapeutic implications. We eagerly anticipate downstream studies that expand on this topic.