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Increased pro-inflammatory mediators leads to Activation, Stellate cells
Key Event Relationship Overview
AOPs Referencing Relationship
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
HSC Initiation is associated with rapid gene induction resulting from paracrine stimulation by inflammatory cells and injured hepatocytes . Also Kupffer cell infiltration and activation play a prominent role in HSC activation.[Li et al., 2008]
Lymphocytes, especially CD4 T-helper (Th) lymphocytes, help orchestrate the host response via cytokine production and can differentiate into Th1 and Th2 subsets. In general, Th1 cells produce cytokines promoting cell-mediated immunity, including interferon (IFN)-γ, TNF, and interleukin (IL)-2. Th2 cells produce IL-4, IL-5, IL-6, and IL-13 and promote humoral immunity. Results from previous experimental models imply that Th2 lymphocytes favor fibrogenesis in liver injury over Th1 lymphocytes. [Shi et al., 1997] However, recent studies of Wynn [Wynn,2004] suggest that more than two T-cell subsets underlying a highly complex, orchestrated response are involved, and they also provide us a more important paradigm for how these intersecting pathways may regulate fibrosis. In animal models, IL-13 has emerged as a key mediator because it increases TGF-β1 and MMP expression by macrophages, whereas IL-4 has a limited role. One study examined the activity of IL-13 in cultured HSCs and suggested that IL-4 and IL-13 directly affect HSCs by increasing collagen production and suppressing HSC proliferation. [Sugimoto et al., 2005]
Leukocytes recruited to the liver during injury join with Kupffer cells in producing compounds that modulate HSC behavior
Transforming growth factor beta 1 (TGF-β1) is the most potent fibrogenic factor for hepatic stellate cells (HSCs). In response to TGF-β1, HSCs activate into myofibroblast-like cells, producing type I, III and IV collagen, proteoglycans like biglycan and decorin, glycoproteins like laminin, fibronectin, tenascin and glycosaminoglycan. [Kisseleva and Brenner, 2007] In the further course of events activated HSCs themselves express TGF-β1. TGF-β1 induces its own mRNA to sustain high levels in local sites of liver injury. The effects of TGF-β1 are mediated by intracellular signalling via Smad proteins. Smads 2 and 3 are stimulatory whereas Smad 7 is inhibitory. Smad1/5/8, MAP kinase and PI3 kinase are further signalling pathways in different cell types for TGF-β1 effects. [Parsons et al., 2007] Concomitant with increased TGF-β production, HSC increase production of collagen. Connective tissue growth factor (CTGF) is a profibrogenic peptide induced by TGF-β, that stimulates the synthesis of collagen type I and fibronectin and may mediate some of the downstream effects of TGF-β. It is upregulated during activation of HSC, suggesting that its expression is another determinant of a fibrogenic response to TGF-β. [Williams et al.,2000] During fibrogenesis, tissue and blood levels of active TGF-β are elevated and overexpression of TGF-β1 in transgenic mice can induce fibrosis. Additionally, experimental fibrosis can be inhibited by anti-TGF-β treatments with neutralizing antibodies or soluble TbRs (TGF-β receptors). [Qi et al., 1999]
Evidence Supporting this KER
There is good understanding and broad acceptance of this KER. [Kisseleva and Brenner, 2007; Williams et al., 2000; Qi et al., 1999; Gressner et al., 2002; Kolios et al., 2006; Bataller and Brenner, 2005; Guo and Friedman, 2007; Brenner, 2009; Kaimori et al., 2007; Kershenobich Stalnikowitz and Weissbrod, 2003; Li et al., 2008; Matsuoka and Tsukamoto, 1990; Kisseleva and Brenner, 2008; Poli, 2000; Parsons et al., 2007; Friedman, 2008; Liu et al., 2006]
Uncertainties and Inconsistencies
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Human [Kolios et al., 2006; Guo and Friedman, 2007]
Rat [Dooley et al., 2000]
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- Sugimoto R, Enjoji M, Nakamuta M, Ohta S, Kohjima M, Fukushima M, et al. Effect of IL-4 and IL-13 on collagen production in cultured LI90 human hepatic stellate cells. Liver Int 2005;25:420–8.
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- Williams, E.J. et al. (2000), Increased expression of connective tissue growth factor in fibrotic human liver and in activated hepatic stellate cells, J Hepatol, vol. 32, no. 5, pp. 754-761.
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