The JAK/STAT signalling pathway in cardiovascular diseases

Several animal models have been developed to investigate the role of various signalling pathways in cardiovascular function under normal or pathologically conditions, including the JAK/STAT signalling pathway, which is a key regulator of several cardiovascular pathologies. Studies have been performed on its function in atherosclerosis, hypertension, myocardial infarction, hypertrophy, myocarditis, and ischemia-reperfusion-induced cardiac injury (El-Adawi et al., 2003; Mascareno et al., 2001; Ortiz-Muñoz et al., 2009; Satou and Gonzalez-Villalobos, 2012; Zhang et al., 2013). In the myocardium, STATs regulate the expression of inflammation- and extracellular matrix-related genes as well as genes regulating apoptosis, angiogenesis and proliferation (Figure 3).

Figure 3: STAT1 and STAT3 regulate transcriptional processes in the ischemic heart. The balance between the activation state of both members of the STAT protein family in the cardiomyocytes as well as in the immune infiltrating cells mediates the remodeling process after myocardial infarction.

STAT1 STAT3

anti-proliferative pro-apoptotic pro-inflammatory

anti-angiogenic

pro-proliferative pro-survival immunosupressive pro-angiogenic & pro-metastatic

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1.2.1 Myocardial infarction and leukocytes responses

Myocardial infarction (MI) triggers a cascade of events which recruit different types of immune cells such as neutrophils, macrophages, lymphoid cells, and mast cells to orchestrate several inflammatory responses. Following a cardiac insult, leukocytes move out of the circulation towards the site of injury, guided by a gradient of chemoattractant peptides, known as chemokines (Altara et al., 2016; Frangogiannis, 2014; Gomez et al., 2018; Meng et al., 2016; Nahrendorf and Swirski, 2013; Pinto et al., 2012). Recruited monocytes and neutrophils are initially mobilized from their original niche in the bone marrow to the spleen, generating myeloid cells that take part in multiple repair processes (Lambert et al., 2008; Nahrendorf et al., 2007). Heart-infiltrating immune cells perform a complex role clearing debris and stabilizing the heart wall, through a tuned balance between residents and recruited/differentiated hematopoietic progenitors (Heidt et al., 2014; Massa et al., 2005;

Nahrendorf and Swirski, 2016). Hence, trafficking of immune cells shapes the outcome following myocardial infarction by profoundly influencing cardiac repair, fibrosis, regeneration and scar formation, and exerting either pro-inflammatory or anti-inflammatory actions (Forte et al., 2018; Ruparelia et al., 2015). Activated chemokines signal through G-protein-coupled receptors, which are expressed on various immune cells. Dissociation of the

- and --subunits of G-proteins leads to downstream signalling cascades which ultimately result in changes in cell polarity and motility through small GTPases (Zweemer et al., 2014).

For example, highly coordinated migration and velocity of neutrophils towards sites of injury are established and maintained by cell adhesion molecules (CAMs). Notably, CAMs underpin a crucial cross-talk between innate and adaptive immune cells. They can either anchor the cell to the substratum or transduce signals between adjacent cells to reshape their migratory responses and dynamically remodel the organization of their actin cytoskeleton.

The migration of neutrophils into extravascular tissue can be viewed as a series of interactions which is mediated by (1) the integrin family, (2) the immunoglobulin superfamily, (3) selectins, and (4) cadherins. CAMs can also be classified according to the role they play to (1) recognize antigens, (2) adhere to each other and to the extracellular matrix, and (3) carry co-stimulation signal (Cavallaro and Dejana, 2011). Additionally, activated chemokine receptors stimulate a rapid elevation of diacylglycerol and cytosolic calcium levels to induce effector functions and coordinate microvascular remodeling, including nitric oxide production and release of reactive oxygen species (ROS) (Cavalera and Frangogiannis, 2014; Saparov et al., 2017). The ensuing removal of dead cells sustains a local environment that supports cardiomyocyte repair. Platelet activation is another hallmark of acute myocardial infarction,

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which can be stimulated by collagen, von Willebrand factor (VWF), thromboxane A2 (TXA2), adenosine diphosphate (ADP), and thrombin. The proceeding thrombus formation within the ventricle increases the myocardium stiffness (Dutta et al., 2012). Furthermore, myocardial ischemia reprograms catabolic and anabolic pathways in the heart to adjust for new requirements of energy acquisition and substrate utilization, and mediate repair processes, cell survival, and growth (Meyer and Voigt, 2017; Wende et al., 2017). Nearly a century ago, Otto Warburg found that cancer cells favor the metabolization of glucose via aerobic glycolysis. Similarly, deregulated metabolism with increased glycolysis has emerged as a significant hallmark of ischemic injury in the heart (Chen et al., 2018). The goal in near future would be to enhance our understanding of the metabolism mediated through the JAK/STAT signalling and propose a rational basis to reprogram metabolic pathways for an improved cardiac repair and regeneration (Doenst et al., 2013).

1.2.2 Regulation of leucocyte transendothelial migration by the JAK/STAT pathway Cell migration is an intricate, synchronized process in which numerous parts of the cell are involved, including surface receptors, intracellular signalling proteins, and the cytoskeleton.

Cumulating evidence has highlighted the role of inflammatory cytokines and transcription factors as crucial mediators of cell migration and polarization (Dustin and Chan, 2000; Nieto et al., 1997; Randolph, 2001). One of the best studied examples in cellular polarity is the epithelial-to-mesenchymal transition, in which cells lose epithelial polarity and attachment to adjacent cells (Lamouille et al., 2014). The inverse happens when migrating cells arrive at their target location, build an epithelium and/or integrate into a previous epithelial tissue (Muller, 2015). Examples of such transitions taking place are demonstrated by neural crest development in vertebrates (Bronner and LeDouarin, 2012). Similarly, heart regeneration depends on overlooked temporal and spatial roles for macrophages and neutrophils, where they mediate extracellular matrix regulation. Consequently, improper migration and polarization can potentially affect repair processes and thus contribute to the development of heart failure. STAT1 was found to be essential for IL-6 expression and the increased adhesion and migration of monocytes across the blood-brain barrier, using different in-vitro models in human immunodeficiency virus 1 (HIV-1) infection (Yang et al., 2009). Another study has shown that depletion of STAT1 in a fibroblast cell line resulted in a reduction of cell migration (Xie et al., 2001). Whereas, IFN- treatment arrested monocyte chemotaxis by modulating the organization of the cytoskeleton via RAC/CDC42 pathways (Hu et al., 2008).

Recent studies investigating the loss of STAT3 expression had revealed an elevation of Rac1