The Vicious Cycle of Arrhythmia and Myocardial Fibrosis (2019)

This article refers to ‘Irregular pacing of ventricular cardiomyocytes induces pro-fibrotic signalling involving paracrine effects of transforming growth factor beta and connective tissue growth factor’ by J. Slawik et al., published in this issue on pages 482–491. Cardiac remodelling includes changes in the myocytic compartment (cardiomyocyte hypertrophy) as well as changes in the non-myocytic compartment (myocardial fibrosis).1 Various ‘drivers’ may underpin the remodelling process, and it has been postulated that some triggers have more effects on cardiomyocytes whilst others are more specific to changes outside the cardiomyocyte, but in fact, there is intense cross talk between the two, and several studies have demonstrated that remodelled cardiomyocytes cause fibroblast proliferation and production of extracellular matrix (ECM).2-4 When a certain threshold is reached, rhythm disturbances, like atrial fibrillation (AF), may develop, and not surprisingly, AF is an important risk factor of heart failure (HF) development. In both animal models and in humans, AF tends to show a strong correlation with cardiac fibrosis and, indeed, myocardial fibrosis is an important substrate for the development of AF.2, 4 Fibrosis creates re-entry circuits in the myocardium, enhances automaticity and hinders adequate action potential conduction in cardiomyocytes by interfering with ion channels, ion pumps and gap junction proteins.2, 4 Thereby, myocardial fibrosis disturbs normal electrophysiology, leading to various rhythm disturbances including AF. As discussed, cross talk between pathophysiological remodelled cardiomyocytes and myofibroblasts exists, but does this also account for irregular contracting cardiomyocytes, hereby creating their own substrate? Possibly, the stressed irregular contracting cardiomyocytes promote the development of myocardial fibrosis. Therewith, a vicious cycle of arrhythmia and fibrosis could exist, similar to the vicious cycle of fibrosis and cell death that has been suggested for the development of HF.4 In this issue of the Journal, Slawik and colleagues report on a cell culture model assessing the influence of irregular paced cardiomyocytes on fibrosis and oxidative stress.5 They describe that irregular pacing of neonatal rat ventricular cardiomyocytes increases paracrine signalling pathways, resulting in activation of fibroblasts and thereby increased myocardial fibrosis development. In these experiments, both intracellular and extracellular parameters of pro-fibrotic signalling and oxidative stress, and collagen deposition by fibroblasts, were investigated. The results support the hypothesis of the existence of a vicious cycle of arrhythmia and fibrosis. Firstly, the authors showed major differences in fibrotic paracrine profiles between irregularly and regularly paced cardiomyocytes in vitro, including increased extracellular levels of both transforming growth factor beta (TGF-β) and connective tissue growth factor (CTGF). Extracellular TGF-β produced by irregularly paced cardiomyocytes was able to increase production of collagens by cardiac fibroblasts. Both TGF-β and CTGF are well-known paracrine inducers of myocardial fibrosis and both play a role in fibroblast to myofibroblast transition (FMT) and fibroblast differentiation.1, 4, 6 After cleavage, TGF-β binds to its cellular receptor and SMAD transcription factors are activated, eventually resulting in increased expression of ECM proteins. CTGF is a regulator of TGF-β induced fibroblast proliferation and differentiation, and thereby plays an important role in myocardial fibrosis. In the current study, no changes in SMAD pathways were observed and the authors suggest non-canonical pathways could be involved. Still, the differences in paracrine signalling profiles of regularly vs. irregularly paced cardiomyocytes are striking. Also, these results corroborate several other studies, showing increased TGF-β1 and CTGF expression in human atrial tissue derived from patients with chronic AF7 and the induction of atrial fibrosis and AF in an overexpression animal model of TGF-β1.8 Secondly, the production of reactive oxygen species (ROS) was increased in irregularly paced cardiomyocytes, resulting in a major increase in ROS-mediated DNA damage. This was accompanied by increased gene expression levels of anti-oxidative proteins. The authors suggest a vicious cycle of arrhythmia and oxidative stress, which both promote each other. Oxidative stress can promote AF by its influence on cardiac ion channels and by promoting triggered activity and re-entry in the myocardium.9, 10 Moreover, oxidative stress also promotes cardiac fibrosis.1, 11 The results of the current study corroborate these previous observations, since increased fibrosis and higher levels of both ROS and anti-oxidative proteins were observed in association with irregular pacing of cardiomyocytes. Finally, in irregularly paced cardiomyocytes, the authors observed increased gene expression levels of both the purported HF plasma biomarker soluble suppression of tumorigenicity (sST2) and interleukin-33 (IL-33). On protein level this was also true for ST2, but IL-33 protein levels were not increased. Based on the observed changes, the authors suggest that the ST2/IL-33 system might be the link between oxidative stress and fibrosis. Stressed cardiomyocytes and fibroblasts produce IL-33 and interaction of IL-33 with ST2L appears to be cardioprotective by, amongst others, reducing myocardial fibrosis.12, 13 Meanwhile, stressed cardiomyocytes and fibroblasts also produce sST2, which competitively binds to ST2L, thereby preventing the cardioprotective effects of the IL-33/ST2 interaction. Also in HF patients the ST2/IL-33 axis was up-regulated and gene expression of sST2 was increased, both in association with the amount of myocardial fibrosis, though in these patients circulating levels of sST2 were not associated with degree of fibrosis.14 Although a clear link to oxidative stress is not shown by the current study, the authors show that the ST2/IL-33 system is affected by the stressed cardiomyocytes and this could explain, at least partly, the increase in myocardial fibrosis observed in association with the cardiomyocyte stress caused by irregular pacing. To increase clinical relevance of these findings, the next question is how to prevent the development of myocardial fibrosis as a consequence of arrhythmia-induced fibrotic signalling. Probably the most logical approach is to achieve conversion to sinus rhythm using ablation techniques or by pharmacological rhythm control. This should reduce cardiomyocyte stress and thereby paracrine fibrotic signalling. Also, in theory, TGF-β inhibition could reduce FMT and thereby fibrosis. Indeed, in pressure overload models, TGF-β receptor inhibition resulted in attenuated cardiac fibrosis and preserved cardiac function, but unfortunately also resulted in valve lesions and aortic rupture.4 Therefore, clinical utility is probably limited. Moreover, CTGF inhibition could also reduce the development of myocardial fibrosis and, as observed in a cardiac pressure overload model, CTGF blockade by monoclonal antibodies resulted in reduced cardiac remodelling, including myocardial fibrosis, and improved cardiac function.15 Finally, interfering with the ST2/IL-33 axis, for example by promoting IL-33 release, or by administering either a ST2L agonist or a sST2 antagonist, also could exert cardioprotective effects, though both have not been extensively studied yet.13 Some limitations should also be discussed. In the current study, only TGF-β and CTGF, both well-known pro-fibrotic products, were investigated. However, likely several other pathways of fibrotic signalling are also affected by irregular pacing of cardiomyocytes. This knowledge is needed to explain in more detail the effect of irregular contraction on fibrosis development, and could provide additional targets for therapy. Moreover, only in vitro models were included and this warrants replication studies in more clinical relevant in vivo models of cardiac arrhythmia, also providing the chance to further study the mechanisms involved. Besides, the current results also trigger more questions. As no changes in SMAD pathways were observed, the full mechanism by which irregular contracting cardiomyocytes promote fibrosis development remains unclear. The authors suggest non-canonical pathways might be involved, but, possibly, this could also be true for other pathways of stress. It would be interesting to investigate effects on foetal gene programming. For example, does irregular pacing result in increased expression of natriuretic peptides, including natriuretic peptide precursor A (NPPA) and natriuretic peptide precursor B (NPPB)? In the current study, increased gene expression of the HF biomarker sST2 was observed. Possibly, sST2, NPPA and NPPB could serve as plasma biomarkers of the involved pathophysiological processes. It would be interesting to investigate expression levels of other purported HF biomarkers as well. Finally, the question remains how the cardiomyocyte can detect irregularity. Does irregular pacing has an effect on mitochondrial function? Do changes in ROS levels within cardiomyocytes serve as a trigger for the observed effects in this study? Additional studies will be needed to answer these questions and fully elucidate the mechanisms involved. In conclusion, the study of Slawik and colleagues shows that, as compared to regularly paced cardiomyocytes, irregularly paced cardiomyocytes show increased oxidative stress and changes in paracrine pro-fibrotic signalling, including increased levels of TGF-β and CTGF. Moreover, the authors showed that this results in increased myocardial fibrosis. Thereby, irregular contracting cardiomyocytes themselves create a substrate that promotes irregular contraction, suggesting the existence of a vicious cycle of arrhythmia and fibrosis, as schematically depicted in Figure 1. The speed of this cycle could be reduced by either interfering with fibrotic pathways or by converting AF to sinus rhythm, eventually slowing down or preventing HF development. Conflict of interest: All authors work for the University Medical Center Groningen (UMCG), Groningen, The Netherlands. The UMCG has received research grants and/or fees from AstraZeneca, Abbott, Bristol-Myers Squibb, Novartis, Roche, Trevena, and ThermoFisher GmbH. R.A.d.B. is a minority shareholder of scPharmaceuticals, Inc., and received personal fees from MadalMed Inc, Novartis and Servier. H.H.W.S. received research grants from AstraZeneca.