Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Cyp2E1 Activation Leading to Liver Cancer||adjacent||High||Not Specified|
Life Stage Applicability
Key Event Relationship Description
Hepatocytes are typically quiescent, with only about 1-2% turnover. However, following surgical resection or chemically induced injury, the liver is able to activate cell division and regenerate itself.
On a cellular level, 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).
On a molecular level, how liver regeneration occurs is less completely understood. Molecular signals that are released from dying cells trigger regenerative proliferation of existing cells. Important players include the transcription factors AP-1 (particularly the c-Jun monomer) and NF-kappaB, both of which are not normally expressed in adult liver, but are up-regulated following partial hepatectomy and are required for hepatic regeneration.
Liver regeneration has been well studied in mice, rats, 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). 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).
Evidence Supporting this KER
Strong. The liver is well known to regenerate itself following chemical or surgical injury. It is widely accepted that significant cytotoxicity to the liver leads to regenerative proliferation (Forbes and Newsome 2016).
Strong. That hepatotoxicity leads to regenerative proliferation has been widely reported for a number of liver toxicants (as well as surgical resection of the liver). This is a very data-rich field. Below we summarize a few examples of the empirical data supporting this relationship.
Furan is a rodent hepatocarcinogenic chemical that is proposed to operate through a mode of action involving cytotoxicity followed by regenerative proliferation (Fransson-Steen, et al. 1997, Moser, et al. 2009). In support of the relationship between hepatotoxicity and regenerative proliferation, mice and rats exposed to dose ranges of furan present with cytotoxicity at lower doses than the doses at which they present regenerative proliferation. Mice exposed to a dose range of 0, 0.5, 1, 2, 4, 8 mkd had significant levels of liver cytotoxicity (measured by serum ALT) beginning at 1 mkd, and significant levels of cellular proliferation (measured by BrdU incorporation) at 8 mkd (Moser, et al. 2009). F344 rats exposed to 0-16 mkd furan showed significant increases in cell death starting at 2-4 mkd, and proliferation at 8-16 mkd (Ding, et al. 2012). Cytotoxicity also begins at earlier time points than regenerative proliferation. Cell death and proliferation were also measured in male Sprague-Dawley rats exposed to 30 mkd furan over a three month time course. Apoptosis was detected after one day, whereas proliferation began to occur after three days (Hickling, et al. 2010).
Male Wistar rats treated with a single, necrogenic dose of thioacetamide had serum AST levels of approximately: 500, 2250, 1900 and 500 at 12, 24 (peak), 48 and 72 hours post-exposure; levels were restored to normal after 96 hours. Levels of regenerative cellular proliferation followed closely after, and peaked at 48 hours; thereby demonstrating a temporal concordance between cytotoxicity and regenerative proliferation (Bautista, et al. 2010). Rats exposed to thioacetamide presented with hepatotoxicity (increased ALT) beginning at 24 hours post-exposure followed by increased regenerative proliferation at 36 hours post-exposure (samples were taken over the following time-course: 6, 12, 24, 36, 48, 72 and 96 hours); thereby demonstrating a temporal concordance between cytotoxicity and regenerative proliferation (Mangipudy, et al. 1995).
B6C3F1 mice were exposed to 0, 34, 90, 138, or 277 mg/kg/day of chloroform for 4 days or for 5 days/week for 3 weeks. Hepatic necrosis was observed to be elevated above control in all dose groups at both time points. Cellular proliferation (by labelling index, BrdU incorporation) in the liver increased in a dose-dependent manner at both time points (4 days and 3 weeks), but were first significantly increased above control levels at 34 mg/kg bw (4 day group) or 138 mg/kg bw (3 week group). These data demonstrate the temporal concordance between cytotoxicity and regenerative proliferation of hepatocytes; the trend is particularly clear at the 3 week time-point (Larson, et al. 1994).
The temporal concordance between cytotoxicity and cellular proliferation (regeneration) has also been well documented following exposure to carbon tetrachloride (Benson and Springer 1999, Doolittle, et al. 1987, Eschenbrenner and Miller 1946, Lee, et al. 1998, Nakata, et al. 1985).
Uncertainties and Inconsistencies
We are not aware of any instance in which significant amounts of hepatotoxicity (in genetically normal livers) would not lead to regenerative cellular proliferation.
Quantitative Understanding of the Linkage
Unable to determine.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Bautista, M., Andres, D., Cascales, M., Morales-Gonzalez, J.A., Sanchez-Reus, M.I., 2010. Effect of gadolinium chloride on liver regeneration following thioacetamide-induced necrosis in rats. Int. J. Mol. Sci. 11, 4426-4440.
Benson, J., Springer, D., 1999. Improved risk estimates for carbon tetrachloride. Final Report. Project No., 54940. DE-FC04-96AL76406.
Ding, W., Petibone, D.M., Latendresse, J.R., Pearce, M.G., Muskhelishvili, L., White, G.A., Chang, C.-., Mittelstaedt, R.A., Shaddock, J.G., McDaniel, L.P., Doerge, D.R., Morris, S.M., Bishop, M.E., Manjanatha, M.G., Aidoo, A., Heflich, R.H., 2012. In vivo genotoxicity of furan in F344 rats at cancer bioassay doses. Toxicol. Appl. Pharmacol. 261, 164-171.
Doolittle, D.J., Muller, G., Scribner, H.E., 1987. Relationship between hepatotoxicity and induction of replicative DNA synthesis following single or multiple doses of carbon tetrachloride. J. Toxicol. Environ. Health 22, 63-78.
Eschenbrenner, A.B., Miller, E., 1946. Liver necrosis and the induction of carbon tetrachloride hepatomas in strain A mice. J. Natl. Cancer Inst. 6, 325-341.
Forbes, S.J., Newsome, P.N., 2016. Liver regeneration - mechanisms and models to clinical application. Nat. Rev. Gastroenterol. Hepatol. 13, 473-485.
Fransson-Steen, R., Goldsworthy, T.L., Kedderis, G.L., Maronpot, R.R., 1997. Furan-induced liver cell proliferation and apoptosis in female B6C3F1 mice. Toxicology 118, 195-204.
Hickling, K.C., Hitchcock, J.M., Oreffo, V., Mally, A., Hammond, T.G., Evans, J.G., Chipman, J.K., 2010. Evidence of oxidative stress and associated DNA damage, increased proliferative drive, and altered gene expression in rat liver produced by the cholangiocarcinogenic agent Furan. Toxicol. Pathol. 38, 230-243.
Larson, J.L., Wolf, D.C., Butterworth, B.E., 1994. Induced cytolethality and regenerative cell proliferation in the livers and kidneys of male B6C3F1 mice given chloroform by gavage. Fundamental and applied toxicology : official journal of the Society of Toxicology 23, 537-43.
Lee, V.M., Cameron, R.G., Archer, M.C., 1998. Zonal location of compensatory hepatocyte proliferation following chemically induced hepatotoxicity in rats and humans. Toxicol. Pathol. 26, 621-627.
Mangipudy, R.S., Chanda, S., Mehendale, H.M., 1995. Tissue repair response as a function of dose in thioacetamide hepatotoxicity. Environ. Health Perspect. 103, 260-267.
Moser, G.J., Foley, J., Burnett, M., Goldsworthy, T.L., Maronpot, R., 2009. Furan-induced dose–response relationships for liver cytotoxicity, cell proliferation, and tumorigenicity (furan-induced liver tumorigenicity). Experimental and Toxicologic Pathology 61, 101-111.
Nakata, R., Tsukamoto, I., Miyoshi, M., Kojo, S., 1985. Liver regeneration after carbon tetrachloride intoxication in the rat. Biochem. Pharmacol. 34, 586-588.
Stanger, B.Z., 2015. Cellular homeostasis and repair in the mammalian liver. Annu. Rev. Physiol. 77, 179-200.