Key Event Title
|Level of Biological Organization|
Key Event Components
|cellular response to oxidative stress||increased|
|macrophage activation involved in immune response||macrophage||increased|
|hypoxia||hypoxia-inducible factor 1-alpha||decreased|
|hypoxia||von Hippel-Lindau disease tumor suppressor||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|Cyp2E1 Activation Leading to Liver Cancer||KeyEvent|
Key Event Description
The liver has two modes of regeneration: (1) the liver can regenerate via cellular hypertrophy and division of existing cells; or (2) the liver can regenerate via proliferation of a population of facultative stem cells, called biliary epithelial cells (BECs), located at the Canals of Hering (in zone 1 where canaliculi join and drain into the main bile duct). Facultative stem cells are functional, differentiated cells that will dedifferentiate in response to tissue damage, thereby becoming a population of progenitor cells that can redifferentiate to replace multiple lost cell types.In a process known as ductal expansion, BECs dedifferentiate into oval cells, which then redifferentiate into hepatocytes or BECs in order to regenerate damaged liver tissue. Liver regeneration has been reviewed (Mao, et al. 2014, Stanger 2015, Yanger and Stanger 2011).
At the molecular level, two dimeric transcription factors, AP-1 (particularly the c-Jun monomer) and NF-kappaB, are key players during liver regeneration. While neither is expressed in normal liver tissue, they are upregulated during normal hepatic regeneration, and are required for regeneration (Alcorn, et al. 1990, Cressman, et al. 1994, FitzGerald, et al. 1995). Indeed, rodents lacking AP-1 or NF-kappaB display impaired liver regeneration, often leading to death (Behrens, et al. 2002, Schrum, et al. 2000). Both NF-kappaB and c-Jun (AP-1) are required for embryonic liver development, and a loss of either one is embryonic lethal due to widespread cell death and liver degeneration (Behrens, et al. 2002, Eferl, et al. 1999, Jochum, et al. 2001, Li, et al. 1999, Rudolph, et al. 2000).
A causal network for regerative proliferation in liver can also occur via WNT signaling and the following pathways: the network begins with oxidative stress or other mechanisms causing liver tissue injury which in turn causes (2) activation of macrophages and wound repair (Boulter et al., 2012), (3) increased hypoxia through diminished blood supply or activity of reactive oxygen species (Ju et al., 2016, Gonzalez et al., 2018) and (4) increased expression of Wnt ligands (Okabe et al., 2016). The activation of macrophages causes (5) activation of Wnt proteins and Wnt signaling (Boulter et al., 2012, Vanella and Wynn 2017). The activation and/or increased expression of Wnt signaling ligands causes (6) binding of the Wnt ligand to the co-receptors Frizzled (FZD family) and (7) Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) which then (8) recruit and phosphorylate Dishevelled (DVL1) and the scaffold protein Axin (Takigawa and Brown 2008).
The phosphorylation and recruitment of Axin (AXIN1, AXIN2), (33) inhibits formation of the beta-catenin destruction complex, composed of AXIN1 or AXIN2, adenomatosis polyposis coli (APC), beta-catenin (CTNNB1) and glycogen synthase kinase 3 (GSK3), which (10) targets beta-catenin for degradation. Inhibiting formation of the destruction complex increases the amount of available beta-catenin to (11) interact and complex with the transcription factor 7 and lymphoid enhancer-binding factor (TCF/LEF) family of transcription factors (TF7, TCF7L1, TCF7L2, LEF1; Takigawa and Brown 2008). The TCF/LEF:beta-catenin complex then (12) activates transcription of MYC proto-oncogene (MYC) and (13) cyclin D1 or CCND1 (Schuijers et al 2014;Katoh 2017). Activation of Wnt signaling (14) inhibits GSK3 phosphorylation activity which then (15) represses forkhead box M1 (FOXM1) activity, (34) causes increased turnover of CCND1 and (35) increases proteolysis of MYC (Katoh 2017). Activation of Wnt signaling (14) inhibits GSK3 phosphorylation activity which (15) represses forkhead box M1 (FOXM1), (34) causes increased turnover of CCND1 and (35) increased proteolysis of MYC (Gregory et al., 2003).
FOXM1 activates (16) transcription of MYC and (17) transcription of MAPK8, the mitogen-activated protein kinase (also known as JNK1; Wierstra and Alves 2007; Wang et al., 2008). Transcriptional activation of MAPK8 then leads to (36) transcriptional activation of CCND1 (Wang et al., 2008). Transcriptional activation of MYC causes (18) transcription of cyclin-dependent kinase 4 (CDK4) which leads to (19, 20) formation of a CDK4 and CCND1 complex (Wang et al 2011). The cyclin-CDK complex then (21) inhibits activity of the retinoblastoma (RB1) transcriptional corepressor 1 which (22) negatively regulates the cell cycle (Burkhart and Sage 2008). Dysregulation of G1/S transition by inhibition of RB1 and/or FOXM1 (23) leads to cell proliferation (Wierstra and Alves 2007; Burkhart and Sage 2008).
MYC can also be activated via hypoxia signaling where an increase in hypoxia (24) decreases the activity of oxygen sensor hypoxia-inducible factor 1 alpha inhibitor (HIF1AN) thereby reducing the ability of HIF1AN to (25) hydroxylate and inhibit hypoxia-inducible factor 1 alpha (HIF1A) activity (Whyte et al., 2012; Mahon et al., 2001). Hypoxia also can (26) inhibit activity of the von Hippel-Lindau (VHL) tumor suppressor protein which has been shown to (27) hydroxylate HIF1A in an O2 dependent manner marking HIF1A for degradation and inactivation in addition to inhibiting expression of HIF1A (Mahon et al., 2001). In stem cells, activated HIF1A (28) increases expression of TCF/LEF leading to increased expression of genes including MYC (Whyte et al., 2012; Tiburcio et al., 2014).
The long noncoding RNA WSPAR is often highly expressed in hepatocellular carcinoma cells and has been found to (29) activate expression of members of the TCF/LEF family (Zhan et al 2017). TCF/LEF transcription factors (30) increase transcription of AXIN2 and increase destruction of beta-catenin in a Wnt signaling negative feedback loop (Jho et al., 2002). TCF/LEF transcription factors form a negative feedback loop that inhibits Wnt signaling by (31) activating transcription of the dickkopf Wnt signaling pathway inhibitor 1 (DKK1) which then (32) binds to the LRP co-receptor (Takigawa and Brown 2008). Finally, cellular G1/S transition can also be dysregulated by (35) phosphorylation of RB1 by the 26S proteasome non-ATPase regulatory subunit 10 (PSMD10) which results in an increase in proteosomal degradation of RB1 (Higashitsuji et al., 2005).
How It Is Measured or Detected
- Proliferation. In vivo or in vitro cellular proliferation can be measured following a multiday 5-bromo-2'-deoxyuridine (BrdU) exposure and quantification of BrdU incorporation in DNA by immunohistochemistry. Alternatively, cells or tissue sections may be stained for Ki-67 or proliferating cell nuclear antigen (PCNA) for a snapshot of cellular proliferation. Use of BrdU, Ki-67, and PCNA in risk assessment has been described in detail (Wood, et al. 2015). A variety of commercial kits exist for this assay.
- Regeneration. Liver regeneration can be observed following partial hepatectomy. Method for 2/3 partial hepatectomy have been described (Mitchell and Willenbring 2008, Mitchell and Willenbring 2014)
- Gene expression analysis can be conducted to demonstrate increased expression of AP-1 or NF-kappaB monomers, or decreased expression of negative regulators, which can be used as an indicator that there is increased cellular proliferation in the liver.
Domain of Applicability
Liver regeneration has been well studied in mice, rats (Taub 2004), and zebrafish (Cox and Goessling 2015, Goessling and Sadler 2015), which are all systems that are thought to work in a similar way to human liver regeneration (Kwon, et al. 2015).
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