Aldosterone-dependent trans-activation and epigenetic derepression of ENaC: where is the balance?

the critical role of the kidney collecting duct and the epithelial Na+ channel (ENaC) in the regulation of arterial blood pressure has been recognized for many years (5). However, the factors that alter renal function and the pathways involved remain to be determined. The mineralocorticoid hormone aldosterone increases tubular sodium absorption in large part by increasing α-ENaC transcription. The pioneering demonstration of epigenetic regulation of ENaC was published by the Kone's laboratory several years ago, when it was reported that a complex containing the histone H3K79 methyltransferase disruptor of telomeric silencing-1 (Dot1) associates with and represses the α-ENaC promoter and that aldosterone and serum- and glucocorticoid-induced kinase-1 (Sgk1) act to disrupt this complex and its inhibitory effects (8, 9). Following studies indicated that either Af9 (in humans, encoded by MLLT3) or Sirt1 (encoded by SIRT1) directly binds to +78/+92 of α-ENaC and recruits Dot1a to repress basal and aldosterone-sensitive α-ENaC transcription (7, 10). Despite this epigenetic repression, basal α-ENaC transcription is still evident and physiologically necessary, indicating basal operation of positive regulators. In an issue of the American Journal of Physiology-Renal Physiology, Kone and associates (6) provide evidence that begins to clarify this gap in knowledge and show that Sp1 is one of such positive regulators. Sp1, a promoter-specific factor required for transcription of the SV40 early and late promoters, was the first cloned mammalian transcription factor (2). Sp1 clearly has functional importance, since gene disruption in mice is lethal (3). Sp1, a candidate transcription factor, chosen based on sequence analysis of the α-ENaC promoter, binds to the α-ENaC promoter at a single, specific transcription factor binding site (a cis-element at +222/+229). Thus, Sp1 represents the first described constitutive driver of α-ENaC transcription, which also contributes to maximal aldosterone trans-activation of α-ENaC. Importantly, presented data complement the previous observations of this group. Additional experiments were done to study the role of Sp1 into the context of Dot1a-mediated repression of transcription or aldosterone-mediated induction of α-ENaC transcription. The authors reported that Sp1 acts independently from and overcomes the Dot1a-Af9 repressor complex (Fig. 1). Thus, the model and studies reflect the balance of genetic and epigenetic factors: Sp1 acting on the promoter DNA being the former, and Dot1 acting on histones being the latter. Surprisingly, the authors did not try to place in the global picture regulation of the transcription of ENaC genes by Af17, which competes with Af9 for binding to Dot1a to upregulate transcription of α-ENaC (4) and regulate ENaC-mediated sodium reabsorption and blood pressure (1). Fig. 1. Simplified model for the aldosterone-sensitive regulation of α-epithelial Na+ channel (ENaC) expression. Af9 directly binds +78/+92 of the α-ENaC promoter and recruits Dot1a to this position to basally repress α-ENaC transcription. This manuscript presents direct evidence of regulation of α-ENaC by Sp1 and highlights the need for further investigation of mechanisms that initiate or repress transcription of genes encoding ENaC subunits. A few questions are raised from the current study. First, this study is focused on Sp1, but it cannot exclude the possibility that other related factors also interact with the GAGGGCGT boxes of the α-ENaC promoter. Sp1 is part of a large family of proteins, including other Sp factors and the extensive set of Kruppel-like factors. Some of these factors are expressed and functionally important in the cardiorenal system. These factors might have a greater influence than Sp1, either as coactivators, or dominant-negative competitors with Sp1, like Af17, which competes with AF9 for binding to Dot1a (4). Similarly, it is not clear whether Sp1 plays a unique role in regulation of maximal aldosterone-dependent trans-activation of α-ENaC and how other factors like SGK1 modulate this regulation. Furthermore, current studies are performed in cultured mouse inner medullary collecting duct 3 cells in vitro. It would be nice if these findings will be further confirmed in vivo, similar to a recent study where the mice with connecting tubule/cortical collecting duct-specific inactivation of Dot1a revealed greater α-ENaC mRNA levels compared with control (10). Moreover, the functional correlation would improve the physiological significance of these molecular studies. Additional experiments are required to determine whether the Sp1 is functionally important in regulating ENaC or not by overexpressing or knocking down Sp1 and measuring a channel's activity. In conclusion, this study by Yu et al. (6) implicates a key role for the transcription factor Sp1 in regulation of ENaC. The authors propose a mechanism whereby endogenous Sp1 factor is functionally involved in conferring constitutive expression and aldosterone regulation of α-ENaC gene. Sp1 has the potential to be a critical point for understanding the physiological control of ENaC gene expression in the kidney. Importantly, this study is one of the first steps in understanding of genetic and epigenetic regulation of ENaC and emphasizes the need for further investigation of this important area of research.