Upstream eventActivation, Stellate cells
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Protein Alkylation leading to Liver Fibrosis||adjacent||High|
|Endocytic lysosomal uptake leading to liver fibrosis||adjacent||High|
|Rattus norvegicus||Rattus norvegicus||High||NCBI|
Life Stage Applicability
|All life stages|
Key Event Relationship Description
Up-regulation of collagen synthesis following hepatic stellate cell (HSC) activation is among the most striking molecular responses of HSCs to injury and is mediated by both transcriptional and post-transcriptional mechanisms. Activated HSCs do not only proliferate and increase cell number, but also increase collagen production per cell. Synthesis of type I collagen is initiated by expression of the col1a1 and col1a2 genes, giving rise to α 1(I) and α 2(I) procollagen mRNAs in a 2:1 ratio. Upon activation of HSCs and other myofibroblast precursors, there is a > 50-fold increase in α 1(I) procollagen mRNA levels. The half-life of collagen α1(I) mRNA increases 20-fold in activated HSCs compared with quiescent HSCs. Monocytes and macrophages are involved in inflammatory actions by producing large amounts of Nitric oxide (NO) and inflammatory cytokines such as TNF-α which have a direct stimulatory effect on HSC collagen synthesis. Synthesis of TGF-α and TGF-β promotes activation of neighbouring quiescent HSCs, whereas the release of HGF (hepatocyte growth factor) stimulates regeneration of adjacent hepatocytes.
The basement membrane-like matrix is normally comprised of collagens IV and VI, which is progressively replaced by collagens I and III and cellular fibronectin during fibrogenesis. Although multiple extracellular matrix (ECM) components are up-regulated, type I collagen is the most abundant protein. These changes in ECM composition initiate several positive feedback pathways that further amplify collagen production. Increasing matrix stiffness is a stimulus for HSC activation and matrix-provoked signals link to other growth factor receptors through integrin-linked kinase and transduce via membrane-bound guanosine triphosphate binding proteins, in particular Rho67 and Rac, signals to the actin cytoskeleton that promote migration and contraction.
The overall amount of collagen deposited by fibroblasts is a regulated balance between collagen synthesis and collagen catabolism. Down-regulated expression of degrading Matrix metalloproteinases (MMPs) and up-regulation of tissue inhibitors of metalloproteinases (TIMPs), MMP- inhibitors, lead to a net decrease in protease activity, and therefore, matrix accumulation. Chronic inflammation, hypoxia and oxidative stress reactivate epithelial-mesenchymal transition (EMT) developmental programmes that converge in the activation of NF-kB. Cells that may transdifferentiate into fibrogenic myofibroblasts are hepatocytes and cholangiocytes. Additional sources of ECM include bone marrow (which probably gives rise to circulating fibrocytes) and portal fibroblasts (Benyon and Arthur; 2001; Milani et al., 1994; Safadi and Friedman, 2002; Kolios et al.,2006; Bataller and Brenner, 2005; Lee und Friedman 2011; Guo and Friedman, 2007; Li, Jing-Ting et al., 2008; Kershenobich Stalnikowitz and Weisssbrod , 2003; López-Novoa and Nieto, 2009; Friedman, 2010; 2008; Dalton et al., 2009; Leung, et al., 2008; Nan et al., 2013; Hamdy and El-Demerdash, 2012;Li, Li et al., 2012; Natajaran et al., 2006; Luckey and Petersen, 2001; Chen and Raghunath, 2009;Thompson et al., 2011; Henderson and Iredale, 2007).
Evidence Supporting this KER
There is general acceptance that HSCs are collagen producing cells and key actors in fibrogenesis. The functional relationship between these KEs is consistent with biological knowledge (Benyon and Arthur; 2001; Milani et al., 1994; Safadi and Friedman, 2002; Kolios et al.,2006; Bataller and Brenner, 2005; Lee und Friedman 2011; Guo and Friedman, 2007; Li, Jing-Ting et al., 2008; Kershenobich Stalnikowitz and Weisssbrod , 2003; López-Novoa and Nieto, 2009).
It is difficult to stimulate sufficient collagen production and its subsequent incorporation into a pericellular matrix in vitro; therefore analytical methods have focused on measurement of pro-collagen secreted into culture medium or measurement of α-smooth muscle actin (α-SMA) expression, a marker of fibroblast activation. In primary culture, HSCs from normal liver begin to express α-SMA coincident with culture-induced activation ( Chen and Raghunath, 2009; Rockey et al.,1992).
Uncertainties and Inconsistencies
Quantitative Understanding of the Linkage
no quantitative data
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Human: Safadi and Friedman, 2002; Bataller and Brenner, 2005; Lee und Friedman 2011.
Rat: Li, Li et al., 2012; Luckey and Petersen, 2001; Rockey et al., 1992
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- Milani, S. et al. (1994), Differential expression of matrix-metalloproteinase-1 and -2 genes in normal and fibrotic human liver, Am J Pathol, vol. 144, no. 3, pp. 528-537.
- ↑Safadi, R. and S.L. Friedman (2002), Hepatic fibrosis--role of hepatic stellate cell activation, MedGenMed, vol 4, no. 3, p. 27.
- Kolios, G., V. Valatas and E. Kouroumalis (2006), Role of Kupffer cells in the pathogenesis of liver disease, World J.Gastroenterol, vol. 12, no. 46, pp. 7413-7420.
- Bataller, R. and D.A. Brenner (2005), Liver Fibrosis, J.Clin. Invest, vol. 115, no. 2, pp. 209-218.
- Lee, U.E. and S.L. Friedman (2011), Mechanisms of Hepatic Fibrogenesis, Best Pract Res Clin Gastroenterol, vol. 25, no. 2, pp. 195-206.
- Guo, J. and S. L. Friedman (2007), Hepatic fibrogenesis, Semin Liver Dis, vol. 27, no. 4, pp. 413-426.
- Brenner, D.A. (2009), Molecular Pathogenesis of Liver Fibrosis, Trans Am Clin Climatol Assoc, vol. 120, pp. 361–368.
- Li, Jing-Ting et al. (2008), Molecular mechanism of hepatic stellate cell activation and antifibrotic therapeutic strategies, J Gastroenterol, vol. 43, no. 6, pp. 419–428.
- Kershenobich Stalnikowitz, D. and A.B. Weisssbrod (2003), Liver Fibrosis and Inflammation. A Review, Annals of Hepatology, vol. 2, no. 4, pp.159-163.
- López-Novoa, J.M. and M.A. Nieto (2009), Inflammation and EMT: an alliance towards organ fibrosis and cancer progression, EMBO Mol Med, vol. 1. no. 6-7, pp. 303–314.
- Friedman, S.L (2010), Evolving challenges in hepatic fibrosis, Nat. Rev. Gastroenterol. Hepatol, vol. 7, no. 8, pp. 425–436.
- Friedman, S.L. (2008), Mechanisms of Hepatic Fibrogenesis, Gastroenterology, vol. 134, no. 6, pp. 1655–1669.
- Dalton, S.R. et al. (2009), Carbon tetrachloride-induced liver damage in asialoglycoprotein receptor-deficient mice, Biochem Pharmacol, vol. 77, no. 7, pp. 1283-1290.
- Leung, T.M. et al. (2008), Endothelial nitric oxide synthase is a critical factor in experimental liver fibrosis, Int J Exp Pathol, vol. 89, no. 4, pp. 241-250.
- Nan, Y.M. et al. (2013), Activation of peroxisome proliferator activated receptor alpha ameliorates ethanol mediated liver fibrosis in mice, Lipids in Health and Disease, vol. 12, p. 11.
- Hamdy, N. and E. El-Demerdash. (2012), New therapeutic aspect for carvedilol: antifibrotic effects of carvedilol in chronic carbon tetrachloride-induced liver damage, Toxicol Appl Pharmacol, vol. 261, no. 3, pp. 292-299.
- Li, Li et al. (2012), Establishment of a standardized liver fibrosis model with different pathological stages in rats, Gastroenterol Res Pract; vol. 2012, Article ID 560345.
- Natajaran, S.K. et al. (2006), Oxidative stress in the development of liver cirrhosis: a comparison of two different experimental models, J Gastroenterol Hepatol, vol. 21, no. 6, pp. 947-957.
- Luckey, S.W., and D.R. Petersen (2001), Activation of Kupffer cells during the course of carbon tetrachloride-induced liver injury and fibrosis in rats, Exp Mol Pathol, vol. 71, no. 3, pp. 226-240.
- Chen, C. and M. Raghunath (2009), Focus on collagen: in vitro systems to study fibrogenesis and antifibrosis state of the art, Fibrogenesis Tissue Repair, vol. 15, no. 2, p. 7.
- Thompson, K.J., I.H. McKillop and L.W. Schrum (2011), Targeting collagen expression in alcoholic liver disease, World J Gastroenterol, vol. 17, no. 20, pp. 2473-2481.
- Henderson, N.C. and J.P. Iredale (2007), Liver fibrosis: cellular mechanisms of progression and resolution, Clin Sci (Lond), vol. 112, no. 5, pp. 265-280.
- Rockey, D.C. et al. (1992), Rat hepatic lipocytes express smooth muscle actin upon activation in vivo and in culture, J Submicrosc Cytol Pathol, vol. 24, no. 2, pp. 193-203.