New mechanisms for transcriptional repression of ENaC And iNOS

Abstract

Gene transcription is highly regulated to ensure that specific genes are expressed at the appropriate times, places and levels in response to various genetic and environmental stimuli. Activation of some genes occurs by relief of basal repression controls, whereas termination of active transcription can involve feedback inhibition. We describe our characterization of aldosterone-triggered de-repression of the epithelial Na(+) channel-alpha subunit (ENaCalpha) gene in renal collecting duct cells in a process that involves a novel nuclear repressor complex, consisting of a histone H3 K79 methyltransferase and the putative transcription factor AF9, that regulates targeted histone H3 K79 methylation at the ENaCalpha promoter. As an example of feedback inhibition, we describe our work characterizing how the end product, nitric oxide, feedback inhibits inducible nitric oxide synthase (iNOS) gene transcription by S-nitrosylating its transactivator poly(ADP-ribose) polymerase (PARP-1) and, thereby, decreasing its ability to act at the iNOS promoter.

Fig. 1

Model of aldosterone-induced de-repression of ENaCα transcription. Under basal conditions (upper panel), AF9 and Dot1a form a nuclear repressor complex that associates with the promoter region of ENaCα, leading to hypermethylation (indicated by the “M” in large font) of histone H3 K79 and repression of ENaCα transcription. Aldosterone stimulates ENaCα transcription not only through the classical nuclear hormone receptors binding to hormone response elements of the ENaCα promoter (not shown) but also through relieving Dot1a-AF9-mediated repression (lower panel). Aldosterone downregulates the amount of the Dot1a-AF9 complex by limiting Dot1a and AF9 protein expression and by decreasing the affinity of AF9 for Dot1a, leading to histone H3 K79 hypomethylation (indicated by the “M” in smaller font) at the specific subregions and release of repression of ENaCα. Similar mechanisms appear to be applicable to other aldosterone-regulated genes, including preproendothelin, period homolog, and connective tissue growth factor.

Fig. 2

Model of feedback inhibition of iNOS transcription via S-nitrosylation of PARP-1. In IL-1β-treated mesangial cells, PARP-1 binds to a specific sequence in the iNOS promoter to transactivate it. Excessive NO production by iNOS may be directly cytotoxic and lead to peroxynitrite-induced DNA strand breaks, which could activate PARP-1 and lead to cellular suicide. To limit this adverse outcome, iNOS-generated NO S-nitrosylates (indicated by “SNO”) PARP-1 and thereby inhibits its binding to and transactivation of the iNOS promoter. The termination of iNOS transcription and NO generation by end-product (NO) feedback limits host cell damage.