Upstream eventN/A, Cell injury/death
Increased, Activation and Recruitment of Hepatic macrophages (Kupffer Cells)
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|Protein Alkylation leading to Liver Fibrosis||directly leads to||Strong|
|Rattus norvegicus||Rattus norvegicus||Strong||NCBI|
Life Stage Applicability
How Does This Key Event Relationship Work
Damaged hepatocytes release reactive oxygen species (ROS), cytokines such as TGF-β1 and TNF-α, and chemokines which lead to oxidative stress, inflammatory signalling and finally activation of Kupffer cells (KCs). ROS generation in hepatocytes results from oxidative metabolism by NADH oxidase (NOX) and cytochrome 2E1 activation as well as through lipid peroxidation. Damaged liver cells trigger a sterile inflammatory response with activation of innate immune cells through release of damage-associated molecular patterns (DAMPs), which activate KCs through toll-like receptors and recruit activated neutrophils and monocytes into the liver. Central to this inflammatory response is the promotion of ROS formation by these phagocytes. Upon initiation of apoptosis hepatocytes undergo genomic DNA fragmentation and formation of apoptotic bodies; these apoptotic bodies are consecutively engulfed by KCs and cause their activation. This increased phagocytic activity strongly up-regulates NOX expression in KCs, a superoxide producing enzyme of phagocytes with profibrogenic activity, as well as nitric oxide synthase (iNOS) mRNA transcriptional levels with consequent harmful reaction between ROS and nitricoxide (NO), like the generation of cytotoxic peroxinitrite (N2O3). ROS and/or diffusible aldehydes also derive from liver sinusoidal endothelial cells (LSECs) which are additional initial triggers of KC activation.           
Weight of Evidence
There is a functional relationship between cell injury/death and KC activation, consistent with established biological knowledge.
Empirical Support for Linkage
There is convincing theoretical evidence that hepatocyte injury and apoptosis causes KC activation, as well as inflammation and oxidative stress. But there are only limited experimental studies which could show that there is a direct relationship between these two events with temporal concordance. Specific markers for activated KCs have not been identified yet. KC activation cannot be detected morphologically by staining techniques since cell morphology does not change, but cytokines release can be measured (with the caveat that KCs activate spontaneously in vitro) and used as marker for KC activation. Tukov et al. examined the effects of KCs cultured in contact with rat hepatocytes. They found that by adding KCs to the cultures they could mimic in vivo drug-induced inflammatory responses. Experiments on cells of the macrophage lineage showed significant aldehyde-induced stimulation of the activity of protein kinase C, an enzyme involved in several signal transduction pathways. Further, 4-Hydroxynonenal (HNE) was demonstrated to up-regulate TGF-β1 expression and synthesis in isolated rat KCs. Canbay et al could prove that engulfment of hepatocyte apoptotic bodies stimulated KC generation of cytokines. 
Uncertainties or Inconsistencies
The detailed mechanisms of the KC - hepatocyte interaction and its consequences for both normal and toxicant-driven liver responses remain to be determined. KC activation followed by cytokine release is associated in some cases with evident liver damage, whereas in others this event is unrelated to liver damage or may be even protective; apparently this impact is dependent on the quantity of KC activation; excessive or prolonged release of KC mediators can switch an initially protective mechanism to a damaging inflammatory response. Evidence suggests that low levels of cytokine release from KCs constitute a survival signal that protects hepatocytes from cell death and in some cases, stimulates proliferation. 
Quantitative Understanding of the Linkage
no quantitative data
Evidence Supporting Taxonomic Applicability
- Winwood, P.J., and M.J. Arthur (1993), Kupffer cells: their activation and role in animal models of liver injury and human liver disease, Semin Liver Dis, vol. 13, no. 1, pp. 50-59.
- Luckey, S.W., and D.R. Petersen (2001), Activation of Kupffer cells during the course of carbon tetrachloride-induced liver injury and fibrosis in rats, Exp Mol Pathol, vol. 71, no. 3, pp. 226-240.
- Roberts, R.A. et al. (2007), Role of the Kupffer cell in mediating hepatic toxicity and carcinogenesis, Toxicol Sci, vol. 96, no. 1, pp. 2-15.
- Malhi, H. et al. (2010), Hepatocyte death: a clear and present danger, Physiol Rev, vol. 90, no. 3, pp. 1165-1194.
- Canbay, A., S.L. Friedman and G.J. Gores (2004), Apoptosis: the nexus of liver injury and fibrosis, Hepatology, vol. 39, no. 2, pp. 273-278.
- Orrenius, S., P. Nicotera and B. Zhivotovsky (2011), Cell death mechanisms and their implications in toxicology, Toxicol. Sci, vol. 119, no. 1, pp. 3-19.
- Kolios, G., V. Valatas and E. Kouroumalis (2006), Role of Kupffer cells in the pathogenesis of liver disease, World J.Gastroenterol, vol. 12, no. 46, pp. 7413-7420.
- Kisseleva T and Brenner DA, (2008), Mechanisms of Fibrogenesis, Exp Biol Med, vol. 233, no. 2, pp. 109-122.
- Jaeschke, H. (2011), Reactive oxygen and mechanisms of inflammatory liver injury: Present concepts, J Gastroenterol Hepatol. vol. 26, suppl. 1, pp. 173-179.
- Li, Jing-Ting et al. (2008), Molecular mechanism of hepatic stellate cell activation and antifibrotic therapeutic strategies, J Gastroenterol, vol. 43, no. 6, pp. 419–428.
- Poli, G. (2000), Pathogenesis of liver fibrosis: role of oxidative stress, Mol Aspects Med, vol. 21, no. 3, pp. 49 – 98.
- Canbay, A. et al. (2003), Kupffer cell engulfment of apoptotic bodies stimulates death ligand and cytokine expression, Hepatology, vol. 38, no. 5, pp. 1188-1198.
- Soldatow, V.Y. et al. (2013), In vitro models for liver toxicity testing, Toxicol Res, vol. 2, no.1, pp. 23–39.
- Tukov, F.F. et al. (2006), Modeling inflammation-drug interactions in vitro: a rat Kupffer cell-hepatocyte co-culture system, Toxicol In Vitro, vol. 20, no. 8, pp. 1488-1499.
- LeCluyse, E.L. et al. (2012), Organotypic liver culture models: meeting current challenges in toxicity testing, Crit Rev Toxicol, vol. 42, no. 6, 501-548.