Myocardial iron dysregulation in type 4 cardiorenal syndrome

Background Chronic kidney disease (CKD) is a common and independent risk factor for the development of heart failure (HF), which is called cardiorenal syndrome type 4 (CRS4). In CRS4, the heart becomes hypertrophic and exhibits diastolic dysfunction while maintaining ejection fraction. The exact mechanisms driving the progression to CRS4 remain poorly understood, and therapeutic options are limited. Iron is an essential mineral for energy metabolism, and iron deficiency commonly occurs in both CKD and HF, conferring worse outcomes. Iron is tightly regulated in the body, but the interplay between iron homeostasis, cardiac function, and CRS4 progression is not well defined. We hypothesize that there is myocardial iron imbalance in CRS4, and that myocardial labile iron is associated with cardiac remodeling and dysfunction. Methods CRS4 was induced in Yorkshire pigs (20kg, n=8) via staged renal artery embolization. At 4 months, echocardiographic and pressure-volume loop measurements were taken, and tissues were harvested. Size-matched pigs (n=5) served as controls. Serum hepcidin was measured via ELISA. Intracellular labile iron and total iron were measured in cardiac tissue using ferene-based assays. Electron microscopy was used to assess for signs of ferroptosis. T test was used for intergroup analysis and values reported as mean ± SD. Correlation analysis was performed using Spearman’s test and values reported as r and 95% confidence intervals. Results Serum hepcidin, a key regulator of iron metabolism, was increased in CRS4 pigs vs controls (21±3 vs 16±2 ng/mL, p<0.01). This was accompanied by an increase in myocardial labile iron (124±3 vs 109±8 nmol/g, p<0.01) and decrease in bound iron (477±19 vs 534±15 nmol/g, p<0.01), increasing the labile:bound iron ratio to 0.260±0.005 vs 0.206± 0.018 (p<0.01). The labile:bound iron ratio correlated to aortic pressures (r= 0.83, CI: 0.44 to 0.96, p<0.01), wall thickness (r= 0.67, CI: 0.10 to 0.91, p<0.05), E/A ratio (r= -0.71, CI: -0.92 to -0.17, p<0.05), EDP (r= 0.76, CI: 0.28 to 0.94, p<0.01), tau (r= 0.87, CI: 0.56 to 0.97, p<0.01), and EDPVR (r= 0.0.75, CI: 0.26 to 0.94, p<0.01). Electron microscopy revealed mitochondria exhibiting signs of ferroptosis, with decreased size, loss of cristae, and membrane hyper-density. Conclusions There is cardiac iron dysregulation in CRS4. The labile:bound iron ratio in the heart was correlated with detrimental changes in both cardiac structure and function. Labile iron-triggered ferroptosis may be responsible for the progression of CKD to HF. Therapeutics targeting myocardial iron homeostasis may prevent development of CRS4.