Upstream eventSustained proliferation
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Cyp2E1 Activation Leading to Liver Cancer||non-adjacent||Moderate||Not Specified|
Life Stage Applicability
Key Event Relationship Description
Every time a cell divides, there is a small chance that a mutation might occur. Because hepatocytes are polyploid, there is an increased rate of error-prone division due to multipolar mitotic spindles, which can result in aneuploidy in daughter cells (Stanger 2015). The risk for mutation is further increased when these cells are under stress (e.g., by a chemical exposure or increased oxidative stress). While it is generally understood that increased cellular proliferation is a predisposing factor to chemical carcinogenesis— ‘sustained proliferative signaling’ is one of the Hallmarks of Cancer (Hanahan and Weinberg 2000, Hanahan and Weinberg 2011) and IARC identifies altered cell proliferation as a key characteristic of a carcinogen (Smith, et al. 2015)—the exact mechanism for how one leads to the other is not altogether clear. There will be many steps in between observations of overt cellular proliferation leading to hepatocellular carcinoma. Thus, we describe this as an indirect KER with the hopes that additional empirical data to support the intervening steps will be available in the future, and that these additional KE(R)s can be developed at that time.
Evidence Supporting this KER
It is broadly accepted that pro-proliferative signaling is activated in an attempt to compensate for increases in cell death (Stanger 2015). Increased hepatocyte proliferation on a background of polyploidy, elevated cell death and oxidative DNA damage has the effect of increasing the likelihood of fixing harmful mutations, which are necessary for malignant transformation (Celton-Morizur and Desdouets 2010, Shi and Line 2014). However, the precise mechanistic processes defining this relationship have not been mapped out.
There are many examples of published studies demonstrating that cellular proliferation precedes tumour formation. For example, experiments exploring liver histopathology in rodents in parallel with the two-year rodent cancer bioassays for chloroform (NTP 1976), furan (Moser, et al. 2009, NTP 1993), carbon tetrachloride (NTP 1977) and ethanol (NTP 2004) all demonstrate an increase in cellular proliferation prior to tumour formation in mice and rats. A detailed example for the chemical furan, a rodent hepatocarcinogen proposed to operate through a mode of action associated with cytotoxicity and increased hepatocellular proliferation, is given below.
A three week exposure of female B6C3F1 mice to furan (0, 2, 4, 8 mkd, gavage) resulted in increased liver cell proliferation (by BrdU incorportation) at the highest dose (8 mkd). In the same study, a second set of animals was exposed for two years to furan in parallel. These animals developed hepatocellular adenoma beginning at 4 mkd and hepatocellular carcinoma at 8 mkd (Moser, et al. 2009). Thus, there is both temporal- and dose-concordance for liver cell proliferation and cancer in this experiment. F344 rats exposed for 13 weeks to furan (0-60 mkd, gavage) showed a dose dependent increase in hyperplasia of hepatocytes beginning at 15 mkd and 30 mkd in males and females, respectively (NTP 1993). A second group of rats (exposed to 0-8 mkd furan) was studied at nine months, fifteen months, and two years. Hyperplasia was observed in all animals of the 4 and 8 mkd groups by 9 months. After fifteen months, hyperplasia was significantly increased in all animals (male and female) of each dose group. After two years, rats developed hepatocellular adenomas and carcinomas beginning at 4 mkd in male and female rats. Kaplan-Meier survival curves showed a dose-dependent decrease in survivorship with increasing dose of furan (NTP 1993).
Promotion of hepatocellular carcinoma can be achieved in rats with exposure to chemical carcinogens (diethylnitrosamine, N-methyl-N-nitrosourea, 1,2-dimethylhydrazine, or benzo(a)pyrene), which is followed by two weeks of dietary 2-acetylaminofluorene (2-AAF) plus partial hepatectomy (PHx), or the administration of a necrogenic dose of carbon tetrachloride (CCl4) (Solt, et al. 1983). The 2-AAF+PH or CCl4 treatments were required for the cancer phenotype, which developed months later. These experiments demonstrate that liver injury, followed by organ repair by cellular proliferation occur before carcinogenesis.
Mdr2-/- mice have higher background levels of inflammation and genomic instability than wild type mice, which may lead to hepatocellular carcinoma in some cases. However, when these mice undergo PHx, which is followed by high levels of cellular replication, they have accelerated and increased tumorigenesis due to replication of cells with damaged DNA (Barash, et al. 2010). This study demonstrates that cellular proliferation occurs before carcinogenesis, and that carcinogenesis is facilitated by elevated inflammation and genomic instability.
Uncertainties and Inconsistencies
Not all cases where there is sustained cellular proliferation produce tumours (some simply regenerate the liver to its healthy form). For instance Barash et al. (2010) demonstrate that increased background levels of inflammation and genomic instability are required for the progression from sustained cellular proliferation following PHx to tumourigenesis. Therefore, it is clear that malignant transformation must be accompanied by some sort of abnormal cellular signaling or impaired homeostasis. It is well understood that ‘context’ plays an important, albeit poorly understood, role in malignant transformation (Bissell and Hines 2011). More work is needed in this field to determine the additional modifying factors that predict whether a chemical that induces hepatocellular proliferation will cause cancer.
Quantitative Understanding of the Linkage
Unable to determine. However, studies with chloroform have argued that there is a threshold amount of cytotoxicity and resulting increases in cellular proliferation required for malignant transformation to occur (Golden, et al. 1997, Templin, et al. 1996).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Any species that has a liver.
Barash, H., Gross, E.R., Edrei, Y., Ella, E., Israel, A., Cohen, I., Corchia, N., Ben-Moshe, T., Pappo, O., Pikarsky, E., Goldenberg, D., Shiloh, Y., Galun, E., Abramovitch, R., 2010. Accelerated carcinogenesis following liver regeneration is associated with chronic inflammation-induced double-strand DNA breaks. Proc. Natl. Acad. Sci. U. S. A. 107, 2207-2212.
Bissell, M.J., Hines, W.C., 2011. Why don't we get more cancer? A proposed role of the microenvironment in restraining cancer progression. Nat. Med. 17, 320-329.
Celton-Morizur, S., Desdouets, C., 2010. Polyploidization of liver cells. Adv. Exp. Med. Biol. 675, 123-135.
Golden, R.J., Holm, S.E., Robinson, D.E., Julkunen, P.H., Reese, E.A., 1997. Chloroform mode of action: implications for cancer risk assessment. Regul. Toxicol. Pharmacol. 26, 142-155.
Hanahan, D., Weinberg, R.A., 2000. The hallmarks of cancer. Cell 100, 57-70.
Hanahan, D., Weinberg, R.A., 2011. Hallmarks of cancer: the next generation. Cell 144, 646-674.
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.
NTP, 2004. NTP Technichal Report on the Toxicology and Carinogenesis Studies of Urethane, ethanol, and urethane/ethanol in B6C3F1 Mice (drinking water studies). NTP-TR-510.
NTP, 1977. Bioassay of 1,1,1-trichloroethane for possible carcinogenicity. NTP-TR-3.
NTP, 1976. Report of the Carcinogenesis Bioassay of Chloroform (CAS No. 67-66-3). TR-001.
Shi, J.-., Line, P.-., 2014. Effect of liver regeneration on malignant hepatic tumors. World J. Gastroenterol. 20, 16167-16177.
Smith, M.T., Guyton, K.Z., Gibbons, C.F., Fritz, J.M., Portier, C.J., Rusyn, I., DeMarini, D.M., Caldwell, J.C., Kavlock, R.J., Lambert, P., Hecht, S.S., Bucher, J.R., Stewart, B.W., Baan, R., Cogliano, V.J., Straif, K., 2015. Key Characteristics of Carcinogens as a Basis for Organizing Data on Mechanisms of Carcinogenesis. Environ. Health Perspect.
Solt, D.B., Cayama, E., Tsuda, H., Enomoto, K., Lee, G., Farber, E., 1983. Promotion of liver cancer development by brief exposure to dietary 2-acetylaminofluorene plus partial hepatectomy or carbon tetrachloride. Cancer Res. 43, 188-191.
Stanger, B.Z., 2015. Cellular homeostasis and repair in the mammalian liver. Annu. Rev. Physiol. 77, 179-200.
Templin, M.V., Jamison, K.C., Sprankle, C.S., Wolf, D.C., Wong, B.A., Butterworth, B.E., 1996. Chloroform-induced cytotoxicity and regenerative cell proliferation in the kidneys and liver of BDF1 mice. Cancer letters 108, 225-231.