Leukocyte recruitment/activation leads to Activation, Stellate cells

During hepatic injury quiescent hepatic stellate cells (HSCs) undergo activation which is associated with proliferation, increased contractile activity, fibrogenesis, changes in matrix protease activity, loss of intracellular retinoid storage, production of cytokines, and phenotypic transformation (Friedmann, 2000).

Different inflammatory cells activate HSCs to secrete collagen (Casini et al., 1997). Factors that promote activation of HSC include the cytokines, TNF-α and TGF-β (Bachem et al., 1993), endothelin-l (ET-1) (Rockey and Chung, 1996) and oxidative stress (Lee et al., 1995).

TGF-β is considered the most powerful mediator of HSC activation in vitro and in vivo (Friedmann, 2000; Bataller and Brenner, 2005). TGF-β triggers phenotypical HSC activation by paracrine and autocrine action, and induces collagen I expression and α-smooth muscle actin (α-SMA) stress fiber organization (Gressner et al., 2002; Dooley et al., 2003). TGF-β binds to heteromeric transmembrane receptors, including TβRI and TβRII (Heldin et al., 1997). Binding to TβRII triggers heteromerization with and transphosphorylation of TβRI. The signal is then propagated through phosphorylation of receptor associated Smad2 and Smad3 and their oligomerization with the common mediator Smad4. Complexes of phosphorylated Smad2 or 3 and Smad4 translocate into the nucleus, where they modulate the transcription of target genes, including those encoding extracellular matrix components (ECM) components (Piek et al., 1999; Miyazono et al., 2000; Moustakas et al., 2001). IFN-γ suppresses TGF-β and PDGF-dependent signalling pathways (Fujita et al., 2006).

IL-33 is released by stressed hepatocytes and attracts type 2 innate lymphoid cells (ILC2), which trigger the profibrogenic activation of HSCs via mediators such as IL-13 (McHedlidze et al., 2013; Heymann and Tacke, 2016). The binding of IL-33 to ST2 receptor activates NF-kB and mitogen activated protein kinases (MAPKs) and drives the production of pro-inflammatory and Th2 associated cytokines (Schmitz et al., 2005), which resulted in the stimulation of α-SMA and collagen expression in HSCs (Tan et al., 2018).

IL-13 indirectly activates TGF-β by upregulating the expression of matrix metalloproteinases (MMPs) that cleave the LAP–TGF-β1 complex (Lee et al., 2001; Lanone et al., 2002). Also, IL-13 has been reported to directly induce production of TGF-β1 in HSCs during liver fibrosis (Shimamura et al., 2008). When treating HSCs with IL-13 and performing a time-course analysis, a time-dependent activation of Smad proteins was observed (Liu et al., 2011).

TNF-α stimulated both p38 MAPK and JNK activity in a time-dependent manner, same as ET-1. However, TGF-β had no significant stimulatory effect on either of these MAPKs. The addition of p38 MAPK inhibitor pyridinyl imidazole derivative SB202190 resulted in reduction of α-SMA, indicator of activated HSC (Reeves et al., 2000)

Oxidative stress enhanced activation of HSCs and collagen synthesis in them, whereas antioxidants stopped the stimulatory effect of free radicals (Lee et al., 1995; Svegliati-Baroni et al., 2001). Oxidative stress molecules, such as superoxide, hydrogen peroxide, hydroxyl radicals, may be derived from hepatocytes, activated KCs, other inflammatory cells and HSCs (Natarajan et al., 2006; Kisseleva and Brenner, 2007; Lee and Friedman, 2011).

Liver-infiltrating CD14+ CD16+ monocytes secrete high levels of chemokines (such as CCL1, CCL2, CCL3, CCL5), cytokines (IL-1α, IL-1β, IL-6, IL-13, IL-16, TNF-α and macrophage migration inhibitory factor), growth factors (granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor) and can efficiently activate primary HSC in vitro (Liaskou et al., 2013; Zimmermann et al., 2010).

Neutrophils may activate HSCs through matrix degradation, secreting compounds like elastase, which then degrades laminin, an extracellular matrix protein in normal liver that is critical for keeping stellate cells in a quiescent state (Friedman et al., 1989). Neutrophil-derived reactive oxygen species (ROS) significantly stimulated procollagen type I accumulation in the HSC culture medium, while the addition of vitamin E or SOD impaired the ROS stimulated stimulation of procollagen I (Casini et al., 1997).

Macrophages are primary source of TGF-β1 in the fibrotic liver (Bataller and Brenner, 2009). Macrophages release several pro-inflammatory cytokines like TNF-α or IL-1 (Tacke and Zimmermann, 2014), which activate the transcription factor NF-kB in HSC and promote the survival of activated HSC (Pradere et al., 2013).

Kupffer cells, liver resident macrophages, after their activation, activate HSC via mechanisms that involve the potent profibrotic cytokines like TGF-β and platelet derived growth factor (PDGF), and ROS (Karlmark et al., 2008; Bataller and Brenner, 2009; Bataller and Lemon, 2012). Apart from directly stimulating matrix-secreting HSC, hepatic macrophages may aggravate scarring by promoting HSC survival via IL-1 and TNF-α induced NF-kB activation (Pradere et al., 2013).  Beside resident macrophages, infiltration of macrophages from blood is essential for liver fibrogenesis (Duffield et al., 2005; Imamura et al., 2005).

Th2 cell– derived cytokines, IL-4, IL-5, IL-13, can enhance fibrosis progression by stimulating TGF-β production in macrophages and by direct effects on HSCs (Wynn, 2004).

DCs have a minor contribution to NF-kB activation (Pradere et al., 2013).

NKT cells were also found to promote liver fibrogenesis in vivo, likely by releasing pro-inflammatory cytokines and activating HSCs (Wehr et al., 2013; Syn et al., 2012). However, there are also studies demonstrating that NKT cells may exert antifibrotic actions, because they can, under certain conditions, also kill HSC and produce IFN-γ, like NK cells (Gao et al., 2013; Park et al., 2009).

Signalling pathways for HSC activation include, for example, NF-kB that is involved in HSC activation upon lipopolysaccharide (LPS) or TLR4 stimulation or ATP induced cytosolic Ca²⁺ influx via purinergic signalling receptors (Dranoff et al., 2004). The activation of TLR4 receptor in HSC downregulates TGF-β pseudoreceptor BAMBI and sensitizes these cells for TGF-β, resulting in increased ECM production by HSCs and fibrosis (Seki et al., 2007).

Activated HSCs secrete inflammatory chemokines, express cell adhesion molecules, and modulate the activation of lymphocytes (Vinas et al., 2003). Therefore, a vicious circle in which inflammatory and fibrogenic cells stimulate each other is likely to occur (Maher, 2001).

Even though many studies claim that TGF-β is the major activating factor for HSCs, one study showed that TGF-β is not an activating factor for HSC, but that more likely it is required for the activated HSCs to survive (Imamura et al., 2005).

Human (Zimmermann et al., 2010; Liaskou et al., 2013)

Mouse (Seki et al., 2007; Gäbele et al., 2009; Pradere et al., 2013; McHedlidze et al., 2014)

Rat (Reeves et al., 2000;  Duffield et al., 2005; Imamura et al., 2005)