*2.1. Common Mediators in Inflammation and Fibrogenesis*

Both inflammation and fibrogenesis are complex processes in which numerous pro-inflammatory and pro-fibrogenic mediators can be involved. On one hand, there are chemokines considered to regulate immune cell entry into the inflamed tissue. On the other hand, there is a convolute of cytokines that modulate important functions in the inflammatory response and in the progression of inflammation to fibrosis. In addition, enhanced production of non-peptidic factors, such as reactive oxygen species (ROS), oxidized lipid mediators, and acetaldehyde, contribute to endothelial dysfunction and tissue injury during inflammation [60].

Chemokines are a group of small, mostly basic, structurally-related molecules that modulate the trafficking of leukocytes and have critical immunological functions [61]. In the last few decades, different chemokines have emerged as important molecules whose importance extends far beyond their most famous function as inflammatory mediators [61]. For several chemokines, the exact functions in different organs were identified, while others can act in an organ-independent manner [55]. Prototypically, the C-C motif chemokine 2 (CCL2), also known as monocyte chemoattractant protein-1 (MCP-1), can activate tissue macrophages and fibroblasts during the inflammatory response in many organs. Chemokines are produced by a broad range of cells and can act in an autocrine and paracrine manner. They bind to surface-exposed receptors and transmit their signals via specific intracellular signaling pathways that modulate the expression or activity of downstream targets. Besides TGF-β and PDGF that were already discussed above, different interleukins (ILs) possessing pleiotropic activities in the innate and adaptive immune response critically contribute to the onset and progression of inflammatory responses. However, their biological activity and impact in different disease settings is more variable than the effects mediated by TGF-β and PDGF. In addition, individual members of the IL family can have overlapping, but also distinct, biological activities and may exert pro- or anti-inflammatory activities [62]. Another important cytokine in the process of inflammation and fibrosis is the tumor necrosis factor-α (TNF-α), which belongs to the TNF family composed of about 20 different proteins and is produced by macrophages, amongst others [63]. The activation of its cognate receptors (TNFR1 and TNFR2) stimulates two different signaling pathways. While TNFR1 activates NF-κB and is associated with apoptosis, TNFR2 mainly triggers cell survival pathways [64]. Therefore,

it is obvious that this dual activity makes TNF-α one of the key switches that determines the outcome of an inflammatory response.

Some of the biological activities of ROS are directly linked to its potential to induce TGF-β expression and activity [65]. Under physiological conditions, NADPH oxidase-derived ROS are essential modulators of signal transduction pathways that control cell growth, proliferation, migration, differentiation, apoptosis, diverse biochemical pathways, and immune responses [66]. However, elevated ROS quantities cause direct irreversible oxidative damage of all kinds of biomolecules, thereby contributing to various pathological alterations, including inflammation [66]. In particular, ROS is known to induce parenchymal cell necrosis and apoptosis and stimulate the production and release of pro-fibrogenic signaling molecules [55].

Similarly, oxidized (phospho-)lipids, such as oxidized phosphatidylcholine, can be formed and accumulate in macrophages during fibrogenesis [67]. The oxidized products promote M2 polarization of macrophages and the enhanced production of TGF-β, thereby critically contributing to fibrogenic signaling cascades. A direct effect of acetaldehyde on type I collagen expression in HSCs is mediated through acetaldehyde-responsive elements (AcRE), which are co-localized with the TGF-β-responsive element [68]. Although the mechanisms leading to an increase of collagen expression by TGF-β and acetaldehyde rely on the formation of H2O2, the kinetics of these mediators in triggering collagen expression are different. Therefore, it was assumed that early acetaldehyde-dependent events induce TGF-β expression and create an H2O2-dependent autocrine loop that amplifies the fibrogenic process [68].

More recently, it was realized that parenchymal cells under inflammatory conditions can form extracellular membranous vesicles (exosomes) that are generated by inward budding of the plasma membrane into early endosomes and multivesicular endosomes [69]. The cargo of these particles can contain diverse molecules (proteins, mRNA, microRNA, DNA, lipids), which can be transferred to distant recipient cells. Although their precise function in the transmission of signals between the different cells is not fully understood, first reports have demonstrated that in alcoholic hepatitis, hepatocyte-derived exosomes contain different microRNAs that induce a hyperinflammatory phenotype in monocytes/macrophages [70]. Similarly, a high-fat diet in rats increased the number of circulating extracellular vesicles that promote inflammation [71]. These pilot studies confirm the assumption that exosomes can mediate the communication between donor and target cells and reprogram the cells involved in inflammation. Intriguingly, recent data by Metcalfe et al. [72] showed the generation of extracellular vesicles (EVs) with an MC signature in patients with SM, which could transfer KIT to an HSC line eliciting proliferation, cytokine production, and differentiation.
