Reductive Stress Induces Unresolved ER Stress and Proteotoxic Cardiomyopathy

Chronic reductive stress (cRS), induced by constitutive activation of Nrf2 in transgenic (TG) mouse hearts leads to pathological cardiac remodeling and diastolic dysfunction. Transcriptomic analysis revealed that both pro-reductive (PR) and reductive stress (RS) conditions disrupt ER-associated gene expression in a dose-dependent manner, with pronounced dysregulation in high-expressing TG (TGH) mice. These shifts were associated with persistent activation of the unfolded protein response (UPR), impaired ER function, and redox imbalance marked by elevated glutathione and reduced ROS levels. Proteostasis disruption under cRS led to protein misfolding, ER dilation, and aggregation of mis/unfolded proteins. TGH mice showed increased ubiquitination and accumulation of aggregated proteins, alongside inadequate proteasome activity, indicating inadequate protein quality control (PQC) mechanisms. RNA-seq data revealed transcriptional upregulation of ubiquitin-proteasome genes and downregulation of key chaperones, suggesting a failed compensatory response. Speckle-tracking echocardiography (STE) detected myocardial dyssynchrony and progressive strain abnormalities in TGH mice, correlating with increased proteotoxic burden and impaired redox homeostasis. Elevated TEI index values confirmed systolic and diastolic dysfunction. Time- and dose-dependent upregulation of Nogo/Reticulon4 transcripts and proteins further supported maladaptive cardiac remodeling. Collectively, these findings highlight that chronic RS disrupts ER homeostasis, induces proteotoxicity, and impairs cardiac structure and function, particularly in high transgene-expressing hearts.