Insights into Myocardial Fibrosis in Advanced Chronic Kidney Disease Using Human Tissue

People who suffer from advanced CKD, especially those who require dialysis (CKD-G5D), often experience heart disease, which can manifest as heart failure, cardiac arrhythmias, or sudden cardiac death. Underlying these risks is the uremic cardiomyopathy phenotype, characterized by hypertrophy, diffuse fibrosis, and impaired ventricular function. Myocardial fibrosis is a key feature of uremic cardiomyopathy, impeding electrical conduction, ventricular relaxation and contraction, and microcirculatory efficiency. Therefore, understanding the mechanisms of fibrogenesis in CKD is important to improving cardiovascular health.¹

Myocardial fibrosis in CKD is characterized by the deposition of collagen in the interstitium, termed myocardial interstitial fibrosis. The buildup of collagen fibers reduces the elasticity of the ventricles, hindering diastolic filling and the contraction of cardiac myocytes.² The severity of these mechanical disturbances depends on the quantity, types, and cross-linking of interstitial fibrosis, which varies by the cause of heart disease.³ For example, the total and relative amounts of the two most common collagen types (type I and type III) vary, with the ratio of type I to type III collagen increased in hypertensive heart disease and decreased in diabetic cardiomyopathy.³ Type I collagen is thicker and stiffer than the relatively pliable type III collagen, and thus, differing proportions of these types may affect function. In addition, increased cross-linking of collagen fibers results in stiff, insoluble collagen fibers that are associated with more severe impairment and worse clinical outcomes.²

The abundance of collagen in myocardial fibrosis is caused by imbalanced deposition and degradation, with the activation of cardiac fibroblasts and stable or reduced collagen degradation. Mechanical and humoral stimuli can activate fibroblasts to differentiate into myofibroblasts, with the expression of contractile proteins and collagen. In pressure overload, such as with hypertension, tensile forces are transmitted to fibroblasts through their cytoskeleton, inducing a profibrotic gene program. Humoral stressors known to induce myocardial fibrosis include neurohumoral factors (e.g., angiotensin II, aldosterone), the TGF-β family, and other cytokines.⁴ Although these general mechanical and humoral factors are undoubtedly important in advanced CKD, less is known about CKD-specific factors, such as mineral and bone derangements and uremic solutes. Abnormalities of phosphate, parathyroid hormone, vitamin D metabolism, and fibroblast growth factor 23 have varying degrees of evidence for causing fibrosis. Epidemiologic associations between these factors and left ventricular hypertrophy have been observed, but causal relationships and treatment effects remain uncertain, as does much else about myocardial fibrosis in CKD.

In this issue of Kidney360, Narayanan et al. present a two-part investigation of myocardial fibrosis in advanced CKD. First, they analyzed left ventricle tissue from deceased persons, and second, they conducted experiments on human cardiac fibroblast cell line. Left ventricle tissue samples were obtained from a biorepository from persons with CKD G5 treated by dialysis (CKD-G5D) on hemodialysis at the time of death (n=18), with hypertension but no evidence of kidney dysfunction at the time of death (n=8), and without cardiovascular, kidney, or metabolic disease at the time of death (n=17). These groups were similar across most relevant variables, with the notable exception of diabetes. As expected, the hearts from CKD-G5D donors and hypertensive donors were larger and thicker than the normal controls, and fibrosis increased from the control to the hypertensive to the CKD-G5D group. CKD-G5D hearts and hypertensive hearts had increased levels of type I collagen compared with the control group, and the CKD-G5D group had less type III collagen than either the hypertensive group or control group, implying stiff, inelastic ventricles. Furthermore, there were lower levels of the collagenases MMP1 and MMP2 in CKD-G5D hearts compared with control hearts, suggesting decreased collagen degradation. In a subset of hearts, bulk transcriptome profiling was performed, but no differences in gene expression for the fibrosis-related genes of interest were found.

The study's second part involved the exposure of human cardiac fibroblast cell cultures to varying concentrations of calcium, phosphate, TGF-β, and TNF-α. Increased levels of calcium and phosphate resulted in elevated mRNA expression of periostin, LH2, alpha-smooth muscle actin, and TGF-β1 but did not affect collagen mRNA expression. Elevated calcium or phosphate led to increased production of type I collagen and decreased production of type III collagen and fibronectin. Finally, treatment of the cell cultures with TGF-β or TNF-α resulted in higher production of type I collagen.

The authors conclude that myocardial fibrosis in advanced CKD is characterized by an increase in the ratio of type I to type III collagen (through both an increase in type I collagen and a decrease in type III collagen). The absence of increased collagen mRNA expression in advanced CKD, along with the decrease in collagenase expression, suggests that decreased collagen degradation rather than increased collagen production may be the primary mechanism. They further conclude that elevated calcium and phosphate are likely the primary stimuli for highly cross-linked collagen deposition in advanced CKD, more so than TGF-β and TNF-α.

Narayanan et al. made an important contribution to understanding myocardial fibrosis in advanced CKD. Their molecular analysis of ventricular tissue samples from persons who were receiving dialysis is unique. In addition, the investigation of the effects of calcium and phosphate in addition to cytokines in human cardiac fibroblasts provides additional insight into the harms of CKD–mineral and bone disorder (CKD-MBD). This investigation tackled the complexities of advanced CKD, given one of the perennial challenges of nephrology is that kidney disease seldom exists in isolation. Narayanan et al. dealt with this in part through the use of a hypertensive control group, which should have been exposed to fibrosis-inducing elevated afterload (although likely not to the same degree as experienced in kidney failure). An added complication is that most ventricular samples from advanced CKD were from diabetic individuals (none of the hypertensive or control group had diabetes). This is a possible confounder, as diabetes is itself a cause of myocardial fibrosis.⁵ Narayanan et al. dealt with several important kidney disease–related factors with experiments varying calcium, phosphorus, TGF-β, and TNF-α. We hope they will be able to continue to grow their investigations to further reflect the complexity of CKD, incorporating additional stimuli, such as mechanical stress, parathyroid hormone, and fibroblast growth factor 23.

This study joins a body of evidence elucidating the central role of fibrosis in uremic cardiomyopathy. Previous indirect evidence has suggested that collagen cross-linking is elevated in CKD, and epidemiologic evidence of the links with CKD-MBD is strong.^2,6 This provides further support for current phosphate control goals. An additional lesson that can be taken away from the study is the importance of preventing the development of myocardial fibrosis. The demonstration of highly cross-linked collagen, combined with previous studies showing the rarity of improvement in myocardial fibrosis in other clinical scenarios, suggests the likely irreversibility of this disease process. Along with the control of CKD-MBD, hypertension, and diabetes, what else can be done to prevent the development of irreversible myocardial fibrosis? Sodium–glucose cotransporter 2 inhibitors may exert some of their salutary effects through the prevention of myocardial fibrosis,⁷ as may mineralocorticoid receptor antagonists.⁸ Obesity, highly prevalent in the US CKD population, is a cause of myocardial fibrosis, and treatment with glucagon-like peptide-1 receptor agonists seems to improve cardiac function,⁹ possibly in part through preventing fibrosis.¹⁰

The improvement of cardiac health in advanced CKD is a critical goal, and understanding the complex mechanisms causing myocardial fibrosis is crucial to preventing its development.

Disclosures

All authors have nothing to disclose.

Funding

C.P. Walther: National Institute of Diabetes and Digestive and Kidney Diseases (K23DK122131).

Author Contributions

Conceptualization: Nandan K. Mondal, Carl P. Walther.

Writing – original draft: Nandan K. Mondal, Carl P. Walther.

Writing – review & editing: Nandan K. Mondal, Carl P. Walther.