CASZ1b is a novel transcriptional corepressor of mineralocorticoid receptor

Aldosterone is a biological ligand for mineralocorticoid receptor (MR) that elevates blood pressure by promoting sodium reabsorption in the kidneys. However, the molecular mechanisms of aldosterone-MR-mediated transcription and the role of this transcription in hypertension remain largely unknown. In this study, we aimed to identify novel MR coregulators and elucidate one of the molecular mechanisms of hypertension. We purified MR-interacting factors from HEK293F cells stably expressing FLAG-MR through a biochemical approach and identified the zinc finger protein castor homolog 1 isoform b (CASZ1b) as a candidate novel MR coregulator via liquid chromatography—tandem mass spectrometry analysis. The CASZ1 gene has been implicated in hypertension in genome-wide single-nucleotide polymorphism studies, but its role in the development of hypertension remains unclear. We found that CASZ1b colocalized with MR in the kidneys and interacted with MR in an aldosterone-dependent manner. In luciferase assays using HEK293F cells, overexpression of CASZ1b reduced aldosterone-dependent MR transcriptional activity by ~50%. In contrast, knockdown of CASZ1b via RNA interference increased the expression levels of the aldosterone-induced MR target genes epithelial Na⁺ channel-α (ENaCα) and serum/glucocorticoid regulated kinase 1 (SGK1) by approximately twofold and 2.3-fold, respectively. Upon aldosterone-MR binding, CASZ1b interacted with MR and formed a protein complex with nucleosome remodeling deacetylase (Mi-2/NuRD), a corepressor complex with chromatin remodeling and histone deacetylation activity, which suppressed ENaCα and SGK1. These findings reveal a critical role of CASZ1b in regulating MR-mediated transcriptional activity and provide new insights into the pathophysiology of hypertension.