The CTNNB1-TRIM28 complex governs hormone-induced RNA polymerase II dynamics in kidney epithelial cells.

Arginine vasopressin maintains water homeostasis by regulating epithelial water permeability through complex transcriptional mechanisms in kidney collecting duct cells. Although CTNNB1 (β-catenin) functions as a transcriptional coregulator in vasopressin-responsive gene transcription, its role remains poorly understood. To identify CTNNB1-dependent components mediating the vasopressin-responsive transcription, we profiled transcriptomic changes following Ctnnb1 knockdown in mouse kidney collecting duct cells using RNA sequencing (RNA-Seq). RNA-Seq and promoter enrichment analyses identified bromodomain-containing proteins (TRIM28, TRIM33, BRD4, CREBBP, and EP300) as components of a CTNNB1-dependent complex regulating RNA Polymerase II (Pol II) activity. Biochemical analyses revealed physical interactions between TRIM28, CTNNB1, Pol II, and CDK9. Functionally, Trim28 knockdown blunted vasopressin-induced expression of the Aqp2 gene. Quantitative genomic binding assays demonstrated that TRIM28 is required for robust genomic occupancy and stabilization of Pol II at the transcription start site of Aqp2. Additionally, dynamic formation of phase-separated nuclear TRIM28 condensates in response to vasopressin suggests that TRIM28-associated machinery functions at specialized chromatin hubs. These findings reveal that TRIM28 facilitates Pol II recruitment, pause release, and elongation upon vasopressin stimulation. Our study establishes the CTNNB1-TRIM28 machinery as a critical transcriptional scaffold that controls Pol II dynamics and chromatin structure, thereby driving osmotic water reabsorption and urine concentration.