Key Event Title
Key Event Component
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|Chronic Cyp2E1 Activation Leading to Liver Cancer||KeyEvent|
Level of Biological Organization
How This Key Event Works
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).
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.
Evidence Supporting Taxonomic 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).
Alcorn, J.A., Feitelberg, S.P., Brenner, D.A., 1990. Transient induction of c-jun during hepatic regeneration. Hepatology 11, 909-915.
Behrens, A., Sibilia, M., David, J.P., Möhle-Steinlein, U., Tronche, F., Schütz, G., Wagner, E.F., 2002. Impaired postnatal hepatocyte proliferation and liver regeneration in mice lacking c-jun in the liver. EMBO J. 21, 1782-1790.
Cox, A.G., Goessling, W., 2015. The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration. Curr. Opin. Genet. Dev. 32, 153-161.
Cressman, D.E., Greenbaum, L.E., Haber, B.A., Taub, R., 1994. Rapid activation of post-hepatectomy factor/nuclear factor κB in hepatocytes, a primary response in the regenerating liver. J. Biol. Chem. 269, 30429-30435.
Eferl, R., Sibilia, M., Hilberg, F., Fuchsbichler, A., Kufferath, I., Guertl, B., Zenz, R., Wagner, E.F., Zatloukal, K., 1999. Functions of c-Jun in liver and heart development. J. Cell Biol. 145, 1049-1061.
FitzGerald, M.J., Webber, E.M., Donovan, J.R., Fausto, N., 1995. Rapid DNA binding by nuclear factor κB in hepatocytes at the start of liver regeneration. Cell Growth and Differentiation 6, 417-427.
Goessling, W., Sadler, K.C., 2015. Zebrafish: an important tool for liver disease research. Gastroenterology 149, 1361-1377.
Jochum, W., Passegué, E., Wagner, E.F., 2001. AP-1 in mouse development and tumorigenesis. Oncogene 20, 2401-2412.
Kwon, Y.J., Lee, K.G., Choi, D., 2015. Clinical implications of advances in liver regeneration. Clin. Mol. Hepatol. 21, 7-13.
Li, Q., Van Antwerp, D., Mercurio, F., Lee, K.F., Verma, I.M., 1999. Severe liver degeneration in mice lacking the IkappaB kinase 2 gene. Science 284, 321-325.
Mao, S.A., Glorioso, J.M., Nyberg, S.L., 2014. Liver regeneration. Transl. Res. 163, 352-362.
Mitchell, C., Willenbring, H., 2014. Addendum: A reproducible and well-tolerated method for 2/3 partial hepatectomy in mice. Nat. Protoc. 9, 10.1038/nprot.2014.122.
Mitchell, C., Willenbring, H., 2008. A reproducible and well-tolerated method for 2/3 partial hepatectomy in mice. Nat. Protoc. 3, 1167-1170.
Rudolph, D., Yeh, W.-., Wakeham, A., Rudolph, B., Nallainathan, D., Potter, J., Elia, A.J., Mak, T.W., 2000. Severe liver degeneration and lack of NF-κB activation in NEMO/IKK γ- deficient mice. Genes and Development 14, 854-862.
Schrum, L.W., Black, D., Iimuro, Y., Rippe, R.A., Brenner, D.A., Behrns, K.E., 2000. c-Jun does not mediate hepatocyte apoptosis following NFκB inhibition and partial hepatectomy. J. Surg. Res. 88, 142-149.
Stanger, B.Z., 2015. Cellular homeostasis and repair in the mammalian liver. Annu. Rev. Physiol. 77, 179-200.
Wood, C.E., Hukkanen, R.R., Sura, R., Jacobson-Kram, D., Nolte, T., Odin, M., Cohen, S.M., 2015. Scientific and Regulatory Policy Committee (SRPC) Review: Interpretation and Use of Cell Proliferation Data in Cancer Risk Assessment. Toxicol. Pathol. 43, 760-775.
Yanger, K., Stanger, B.Z., 2011. Facultative stem cells in liver and pancreas: fact and fancy. Dev. Dyn. 240, 521-529.