Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|Chronic Cyp2E1 Activation Leading to Liver Cancer||indirectly leads to||Moderate||Not Specified|
Life Stage Applicability
How Does This Key Event Relationship Work
Cytotoxicity caused by necrosis, apoptosis and/or necroptosis leads to regenerative proliferation. However, in addition, cytotoxicity caused by necrosis/necroptosis results in the release of damage associated molecular patterns (DAMPs), which trigger inflammation (Brenner, et al. 2013, Kaczmarek, et al. 2013, Luedde, et al. 2014). Chronic inflammation is considered to be a risk factor for cancer (although the mechanism is not clear). The inflammatory response is part of the innate immune response and includes vasodilation (causing heat and redness) and increased vasopermeability (causing edema and infiltration of neutrophils, macrophages, and plasma proteins). Acute inflammation is a helpful and necessary response to tissue damage or infection; whereas, chronic inflammation exacerbates (and could even cause) a large number of diseases. During chronic inflammation, bouts of healing and cytotoxicity occur simultaneously, typically leading to inflammation-dependent tissue damage, fibrosis, and production of oxidative stress.
Chronic inflammation is responsible for facilitating carcinogenesis in the liver through what is known as the Inflammation-Fibrosis-Cancer (IFC) axis, which is thought to accompany ~90% of hepatocellular carcinoma cases. Liver inflammation is largely mediated by Kupffer cells (resident liver macrophages) that release cytokines that bind hepatocyte plasma membrane receptors to activate intra-cellular signaling cascades. One outcome of this complex signaling is the activation of the NF-kappaB transcription factor, which is a master regulator of the IFC axis (Elsharkawy and Mann 2007, Karin 2006, Karin 2009, Luedde and Schwabe 2011). Increased NF-kappaB expression is important for the transition from liver injury and inflammation to tumour promotion and hepatocellular cancer (Pikarsky, et al. 2004, Wang, et al. 2003). However, NF-kappaB is a dimeric, redox sensitive molecule that is controlled through double inhibition (see below) (Vallabhapurapu and Karin 2009). Therefore, its actions are very much context dependent and it has a dynamic range of molecular outcomes and effects. The contradictory nature of NF-kappaB’s role in carcinogenesis has been well documented (Finkin and Pikarsky 2011, Pikarsky and Ben-Neriah 2006, Vainer, et al. 2008).
NF-kappaB is a dimeric transcription factor that has at least six potential monomers and is thought to bind regulatory sequences of ~200 target genes. Transcriptional activity of NF-kappaB is controlled through double inhibition: the inhibitor of NF-kappaB (IkappaB) sequesters NF-kappaB in the cytoplasm. IkappaB, in turn, is inhibited by the IkappaB kinase (IKK). Mdr2-knockout mice spontaneously develop cholestatic hepatitis and hepatocellular carcinoma and therefore represent a good model system for this disease. When NF-kappaB is lost (using a hepatocyte-specific inducible IkappaB-super-repressor transgene) during early stages of the disease, there is no effect on development of hepatitis or malignant transformation; however, blocking NF-kappaB at later stages of the disease (>7 months) results in apoptosis and failure to develop hepatocellular carcinoma (Pikarsky, et al. 2004). NF-kappaB activity and regulation is perplexingly complex. Certainly, it is clear that the context of NF-kappaB activation is crucial to its outcome.
Weight of Evidence
Cell death by necrosis and necroptosis produces DAMPs that trigger inflammation. Inflammation is widely considered to be an important risk factor that sets the stage for malignant transformation; however, mechanistically, it is unclear how it does so.
Empirical Support for Linkage
Empirical evidence broadly supports the notion that cytotoxicity occurs at doses lower than those that cause liver cancer, and at early time points. A few examples are shown below.
F344 rats exposed for 13 weeks to furan (0-60 mkd, gavage) showed a dose dependent increase in degeneration and necrosis of hepatocytes beginning at 8 mkd and 15 mkd, in males and females, respectively. A second group of rats (exposed to 0-8 mkd furan) was studied at nine months, fifteen months, and two years. After nine months, degeneration and necrosis were observed in the hepatocytes of all animals in the 4 and 8 mkd groups. After fifteen months, these endpoints were 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 (NTP 1993). Thus, cytotoxicity precedes cancer and occurs at similar doses and the empirical evidence is concordant with a relationship between cytotoxicity and cancer.
In a study conducted by the NTP (1993), B6C3F1 mice (male and female) were exposed for 13 weeks to the hepatocarcinogen furan (0, 4, 8, 15, 30,60 mkd, gavage; n=10 per group) (nine male and four female mice exposed to 60 mkd died before the end of the study). Toxic lesions were seen in the liver at all doses and severity increased with dose, bile duct hyperplasia and cholangiofibrosis were observed at 30 and 60 mkd (NTP 1993). A two year study was conducted using a lower dose range (0, 4, 8, 15 mkd). Kaplan Meier survival curves showed a dose-dependent decrease in survivorship with increasing dose of furan. There was a dose dependent increase in hepatocellular adenoma and carcinoma in male and female mice (NTP 1993).
Empirical evidence supporting an underlying mechanism relating to inflammation as a mitigating factor in this relationship is less clear. Inflammation in response to furan exposure occurs in mice at around the same dose as cytotoxicity and increases in a dose and time dependent manner (Fransson-Steen, et al. 1997, Moser, et al. 2009, NTP 1993). Inflammation is accompanied by an increase of circulating inflammatory markers (Wang, et al. 2014), and changes in inflammation-associated gene expression (Jackson, et al. 2014). In Moser et al. (2009) mild inflammation arose at very low doses (0.5 mkd), was moderate at 1 mkd, and was marked at 4mkd; whereas hepatocellular adenomas and carcinomas were not observed until 4 and 8 mkd, respectively.
Most recently, the relationship between chemically induced hepatocellular carcinoma and inflammation was outlined in an award-winning article that was published in the journal Carcinogenesis: The 2016 Carcinogenesis Award Winners “Liver carcinogenesis: from naughty chemicals to soothing fat and the surprising role of NRF2” (Karin and Dhar 2016). Studies in which diethylnitrosamine (DEN, a liver carcinogen) was given to male mice demonstrated that induction of hepatocellular carcinoma is dependent upon induction of inflammation. A liver myeloid cell-specific ablation of IKK-beta was sufficient to inhibit DEN-dependent carcinogenesis, whereas its deletion in hepatocytes enhanced carcinogenesis (Maeda, et al. 2005). The latter was due to an increase in cell death, which caused a subsequent increase of the cytokine and tumour protomtor interleukin 6 (IL-6) (Maeda, et al. 2009). IL-6 is known to be inhibited by estrogen, which could account for the higher prevalence of hepatocellular carcinoma in males (Naugler, et al. 2007). IL-6 has been described as a “wolf in sheep’s clothing” and is thought to be an important link between inflammation and cancer (Naugler and Karin 2008). IL1-alpha released from dying hepatocytes is another important promoter of hepatocellular carcinoma (Sakurai, et al. 2008).
Uncertainties or Inconsistencies
This relationship appears to be valid for toxicants that produce moderate levels of cytotoxicity. Acetaminophen is a Cyp2E1 substrate that produces extremely high levels of hepatotoxicity. Acetaminophen does not cause liver cancer because death by liver failure occurs before cancer can develop.
Quantitative Understanding of the Linkage
Unable to determine.
Evidence Supporting Taxonomic Applicability
Brenner, C., Galluzzi, L., Kepp, O., Kroemer, G., 2013. Decoding cell death signals in liver inflammation. J. Hepatol. 59, 583-594.
Elsharkawy, A.M., Mann, D.A., 2007. Nuclear factor-kappaB and the hepatic inflammation-fibrosis-cancer axis. Hepatology 46, 590-597.
Finkin, S., Pikarsky, E., 2011. NF-kappaB in liver cancer: the plot thickens. Curr. Top. Microbiol. Immunol. 349, 185-196.
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.
Jackson, A.F., Williams, A., Recio, L., Waters, M.D., Lambert, I.B., Yauk, C.L., 2014. Case study on the utility of hepatic global gene expression profiling in the risk assessment of the carcinogen furan. Toxicol. Appl. Pharmacol. 274, 63-77.
Kaczmarek, A., Vandenabeele, P., Krysko, D.V., 2013. Necroptosis: the release of damage-associated molecular patterns and its physiological relevance. Immunity 38, 209-223.
Karin, M., 2009. NF-kappaB as a critical link between inflammation and cancer. Cold Spring Harb Perspect. Biol. 1.
Karin, M., 2006. Nuclear factor-kappaB in cancer development and progression. Nature 441, 431-436.
Karin, M., Dhar, D., 2016. Liver carcinogenesis: from naughty chemicals to soothing fat and the surprising role of NRF2. Carcinogenesis 37, 541-546.
Luedde, T., Kaplowitz, N., Schwabe, R.F., 2014. Cell death and cell death responses in liver disease: mechanisms and clinical relevance. Gastroenterology 147, 765-783.e4.
Luedde, T., Schwabe, R.F., 2011. NF-κB in the liver-linking injury, fibrosis and hepatocellular carcinoma. Nature Reviews Gastroenterology and Hepatology 8, 108-118.
Maeda, S., Hikiba, Y., Sakamoto, K., Nakagawa, H., Hirata, Y., Hayakawa, Y., Yanai, A., Ogura, K., Karin, M., Omata, M., 2009. Ikappa B kinasebeta/nuclear factor-kappaB activation controls the development of liver metastasis by way of interleukin-6 expression. Hepatology 50, 1851-1860.
Maeda, S., Kamata, H., Luo, J.L., Leffert, H., Karin, M., 2005. IKKbeta couples hepatocyte death to cytokine-driven compensatory proliferation that promotes chemical hepatocarcinogenesis. Cell 121, 977-990.
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.
Naugler, W.E., Karin, M., 2008. The wolf in sheep's clothing: the role of interleukin-6 in immunity, inflammation and cancer. Trends Mol. Med. 14, 109-119.
Naugler, W.E., Sakurai, T., Kim, S., Maeda, S., Kim, K., Elsharkawy, A.M., Karin, M., 2007. Gender disparity in liver cancer due to sex differences in MyD88-dependent IL-6 production. Science 317, 121-124.
Pikarsky, E., Ben-Neriah, Y., 2006. NF-kappaB inhibition: a double-edged sword in cancer? Eur. J. Cancer 42, 779-784.
Pikarsky, E., Porat, R.M., Stein, I., Abramovitch, R., Amit, S., Kasem, S., Gutkovich-Pyest, E., Urieli-Shoval, S., Galun, E., Ben-Neriah, Y., 2004. NF-kappaB functions as a tumour promoter in inflammation-associated cancer. Nature 431, 461-466.
Sakurai, T., He, G., Matsuzawa, A., Yu, G.Y., Maeda, S., Hardiman, G., Karin, M., 2008. Hepatocyte necrosis induced by oxidative stress and IL-1 alpha release mediate carcinogen-induced compensatory proliferation and liver tumorigenesis. Cancer. Cell. 14, 156-165.
Vainer, G.W., Pikarsky, E., Ben-Neriah, Y., 2008. Contradictory functions of NF-kappaB in liver physiology and cancer. Cancer Lett. 267, 182-188.
Vallabhapurapu, S., Karin, M., 2009. Regulation and function of NF-kappaB transcription factors in the immune system. Annu. Rev. Immunol. 27, 693-733.
Wang, E., Chen, F., Hu, X., Yuan, Y., 2014. Protective effects of apigenin against furan-induced toxicity in mice. Food Funct. 5, 1804-1812.
Wang, J., Huang, Q., Chen, M., 2003. The role of NF-kappaB in hepatocellular carcinoma cell. Chin. Med. J. (Engl) 116, 747-752.