Abstract Body: Disruption of ubiquitin-dependent proteolysis is increasingly recognized as a driver of cardiomyopathy and heart failure, yet the underlying mechanisms remain poorly understood. Cullin-RING E3 ligases (CRLs) are the largest family of E3 ligases that ubiquitinate target proteins, thereby targeting them for proteasomal degradation. The catalytic component of CRLs is either RBX1 or RBX2 (SAG). RBX1 is associated with different adaptor proteins to assemble diverse CRL complexes (CRL1-4). Emerging evidence indicates that RBX1-CRLs control transcriptional regulators and chromatin remodeling factors. However, how RBX1 function intersects with chromatin maintenance in postmitotic hearts is unknown.
Here we report that RBX1 is significantly downregulated in human failing hearts. Genetic studies revealed functional cooperation between RBX1 and RBX2 in the adult heart, as RBX1 haploinsufficiency accelerated the progression of cardiac dysfunction in RBX2 (SAG) cardiac-specific knockout mice. Cardiomyocyte-specific deletion of RBX1 in adult mice via MCM-Cre resulted in heart failure, demonstrating a critical cardiomyocyte-autonomous role for RBX1. Mechanistically, RBX1 was predominantly localized to cardiomyocyte nuclei, where it interacted with nuclear proteins. Bulk RNA-sequencing of RBX1-deficient hearts revealed broad dysregulation of chromatin regulatory pathways. Consistently, RBX1 deletion caused a marked reduction in perinuclear heterochromatin, as assessed by electron microscopy and analysis of heterochromatin-associated histone marks. Integrated proteomic and transcriptomic analyses identified the H3K9me2 demethylase KDM3A, a protein associated with cardiac dysfunction, as a candidate RBX1-CRL substrate. RBX1 loss led to accumulation of KDM3A protein, reduced H3K9me2 levels, and upregulation of profibrotic gene programs. Biochemical analyses further demonstrated that RBX1 promotes ubiquitin-dependent degradation of KDM3A in the heart.
Together, these findings uncover a previously unrecognized proteostasis–epigenetic axis in which RBX1 maintains chromatin organization and gene expression programs essential for adult cardiac function by regulating KDM3A stability.