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Liver cell damage (also known as hepatocellular injury) refers to biological or physical damage to the liver cells or hepatocytes as a result of different pathological changes. Liver cell damage involves apoptosis or necrosis of liver cells or hepatocytes. Common signs and symptoms can be fever, jaundice, right upper quadrant abdominal pain, fatigue and nausea. Causes of liver cell damage involve oxidative stress from free radicals and antioxidant consumption, immune responses against foreign substances and virus infections or nonspecific causes which can be environmental or genetic factors. Diagnosis of liver cell damage encompasses detections of biomarkers which are enzymes in the liver that reflect inflammation or hepatocyte damage condition. Therapeutic interventions of liver cell damage can be specific for liver cell damage originated from different causes. Examples of therapeutic interventions are liver transplantation, stem cell therapy, antioxidants, vaccinations and immunosuppressants.

Types of liver cell damage
Liver cell damage can be in the form of necrosis, apoptosis or both. Necrosis can be resulting from deteriorated apoptosis, particularly mitochondrial dysfunctioning process in apoptosis, which directs to ATP depletion and oxidative stress.

Apoptosis
Apoptosis (programmed cell death), characterized by chromatin condensation, cytoplasm shrinkage and DNA fragmentation, is more predominantly observed in all types of liver cell injuries. It can be stimulated under pathological conditions such as hepatotoxicity, radiation exposure and infections with hepatotropic viruses. Apoptosis occurs in two pathways, namely extrinsic and intrinsic pathways.

In extrinsic pathways, death receptors bind with ligands(e.g. TNF-α, Fas, and tumor necrosis factor–related apoptosis-inducing ligand),forming the death-inducing signaling complex. This is by activation of caspase 8 which is the initiator caspases that activate executioner caspases that are responsible for the protein cleavage in apoptosis.

In intrinsic pathways, in the presence of apoptotic inducing signal, BH3 family protein(Bid/Bim) activates Bax/Bak which are pro-apoptotic proteins that insert into the mitochondria membrane, causing membrane outer membrane permeabilization(MOMP) and release of cytochrome c from mitochondria which further activating caspases that cleaves proteins in apoptosis. MOMP increases membrane permeability, thereby promoting leakage of mitochondrial material into the cytosol.

Necrosis
Necrosis is an irreversible cellular process, often driven by the increase in oxidative stress level in cells, particularly hepatocytes. Properties of necrotic cells involve oncosis which is cell swelling due to inability of maintaining cell ion potentials, karyolysis due to enzymatic degradation of chromatin and breaking of cell membrane.

Oxidative stress
Oxidative stress indicates disequilibrium in the levels of oxidants and antioxidants where there is excess level of oxidants due to overproduction of ROS or reduced removal of ROS and low levels of antioxidants due to high consumptions of antioxidants. Free radicals are molecules containing unpaired electrons that are derived from chemicals and can be either nitrogen or oxygen based. Examples of free radicals contain superoxide, peroxyl, oxygen based molecules are also named reactive oxygen species(ROS). Some free radical metabolites are generated from chemicals or drugs that induce hepatotoxicity which results in liver cell damage(necrosis and apoptosis). Liver acts as the main site of drug metabolism. The metabolites are formed from the metabolism system in liver, involving drug oxidation phase by p450 enzymes. Cytochrome P4502E1 or CYP540 in particular participates in biotransformations of substances including ethanol, CCl4, Acetaminophen(APAP) and N-nitrosodimethylamine(NDMA) which all contribute to oxidative stress formation. Oxidative stress activates MAPKs and JNKs which in turn accumulates the oxidative stress. Ultimately, lipid peroxidation increases followed by activation of membrane permeability transition and migration of mitochondrial proteins(e.g. apoptosis inducing factor AIF, endonuclease G) trigger the DNA fragmentation, liver cell apoptosis and necrosis. In alcohol consumption, oxidative stress originates from ethanol conversion to acetaldehyde by either aldehyde dehydrogenase, which converts acetaldehyde into acetyl radical and hydrogen or CYP2E1-catalyzed oxidation, which deprives the reducing agent NADH(nicotinamide adenine dinucleotide). ALD(alcoholic liver disease) is the liver disease caused by alcohol consumption. Oxidative stress as a result of alcohol consumption leads to increase in expressions of pro-inflammatory cytokines such as TNF-ɑ that facilitates death receptors to enhance apoptosis of liver cells. Some studies suggest alcohol consumption increases iron concentration in hepatocytes which amplifies oxidative stress and lipid peroxidation while others do not. Animals with ALD manifests impaired function in liver regeneration.

In Paracetamol metabolism, with the depletion of GSH reserve in the body, NAPQI forms protein-NAPQI adduct with hepatic cytosolic proteins, especially mitochondrial proteins. Since GSH serves as the antioxidant agents, depletion of GSH causes oxidative stress. NAPQI can also be the source of oxidative stress. Binding of NAPQI to mitochondrial antioxidant proteins such as glutathione peroxidase renders the protein inactive and excessive oxidative stress is introduced in the mitochondria. Binding of NAPQI to CaATPase(Calcium dependent ATPase) inactivates CaATPase, resulting in accumulation of calcium in cells, activation of calcium dependent degradative enzymes and liver cell necrosis.

Signs and symptoms
Signs and symptoms can be different according to various types of liver cell damage manifestations in disease conditions.

Immune system responses
Cell damage may arise from immune system activation in liver against foreign substances.

In alcohol consumption, acetaldehyde binds with macromolecules such as proteins in hepatocytes. The acetaldehyde-protein adduct can be recognized by infiltrated neutrophils as antigens, triggering cellular immune response of phagocytosis. The acetaldehyde can also form highly adduct with malondialdehyde, a metabolite from lipid peroxidation, to elicit both cellular immune response or humoral immune responses and consequently leading to hepatic cell deaths.

Viral infections
Hepatitis A, hepatitis B and hepatitis C are the most common types of inflammation seen in infected cases. Patients can be infected through different modes of transmission, including viral contact with food, water or contaminated objects, genetical transmission, sexual transmission and blood transmission.

Hepatitis C protein produced from RNA genome of virus hepatitis C possess both antiapoptotic and apoptotic abilities. Upon binding to death domain of FADD(fas associated death domain), it promotes expressions of FADD. Both TNF-α and FAS induced apoptosis get up-regulated. Hepatitis B virus contains both apoptotic and anti-apoptotic properties. Lymphocytes infiltration such as macrophage kupffer cells in hepatocyte apoptotic sites presents activities of host lymphocytes against antigens. Innate immune responses occur after internalization of virus protein HBsAg by Kupffer cells, synthesis of IL-6, TNF, and CXCL8 and NK(natural killer) cells activation. NK cells kill infected hepatocytes non-specifically which accounts for most of liver cell damage.

Anti-HBs antibodies work by binding with HBsAg, neutralizing it and thereby halting the further spread of virus infections to other hepatocytes through internalization of HBV via sodium taurocholate co-transporting polypeptide (NTCP) receptors. However, HBsAg transforms into spherical filamentous subviral particles (SVPs) which in turn neutralize anti-HBs antibodies. As a result, virus gets uncontrolled and spreads out to other hepatocytes. HBsAg also turns off adaptive immune response by down-regulating IL-12 production through inhibiting of c-Jun using phosphorylation. C-Jun is a transcription factor for interleukin 12. This reduces production of a cytokine essential for activation of T cells and proliferations of T cells.

Other non specific causes
Autoimmune Hepatitis (AIH) is chronic disease of progressive inflammation of the liver from unknown causes. Despite the unknown nature of the specific cause of the liver cell damage, it is most likely related to genetic factors, environmental causation, and failure of the immune system.

Non-alcoholic fatty liver disease (NAFLD) is a disease which is caused by fat accumulation in the liver of who consume little to no alcohol. NAFLD includes of a spectrum of liver diseases ranging from simple steatosis to non-alcoholic steatohepatitis (NASH), which brings inflammation and liver cell damage. NAFLD results from a combination of genetic and environmental factors. NAFLD have symptoms such as fatigue, abdominal discomfort and an enlarged liver.

Diagnosis
Diagnosis of liver cell damages consists of a combination of medical history, physical examination, and laboratory tests. The medical history examines alcohol and drug use, and family history. The physical examination looks for signs of liver damage, such as jaundice and an enlarged liver. Blood tests are used to assess liver function and detect liver damage, such as elevated levels of liver enzymes: alkaline phosphatase (ALP), alanine transaminase (ALT), aspartate aminotransferase (AST) and gamma-glutamyl transferase (GGT), bilirubin, prothrombin time (PT), and the international normalized ratio (INR), total protein and albumin. The different level of enzymes indicate the area of damage. Disproportionate increase in ALT and AST levels to alkaline phosphatase and bilirubin level indicate hepatocellular disease. A disproportionate increase in alkaline phosphatase and bilirubin to ALT and AST level indicate a cholestatic liver disease. Albumin level indicate liver functioning activity rate. Elevation of cytokeratin 18 neoantigen (M30) is indicative for continuous hepatocyte apoptosis in HCV patients.

Vaccination
Hepatitis B vaccine is in the form of hepatitis b surface antigen and is able to sensitize adaptive immune system of the recipients.

Cell death pathway inhibitors
Cell death pathways consist of many molecules which can be targets of therapeutic interventions. Inhibition of cell apoptotic pathway cytokines or proteins prevent the stream cell death signaling. Pan-caspase inhibitor, IDN 6556 is a caspase inhibitor involved in inhibiting hepatocyte apoptosis. Anti-TNF α agents such as etanercept, which is a TNF receptor protein that binds with serum TNF α, neutralizing and inactivating it. Infliximab is the monoclonal antibody that binds to TNF receptors and interfere with the interactions of TNF with its receptor and also cause cell lysis of kupffer cells that secrete TNF. N-methyl-4-isoleucine cyclosporine (NIM811) hinders the membrane permeability transition and leads to reduction in liver cell injury. JNK (c-Jun N-terminal kinases) inhibitors may be applicable to paracetamol-induced liver injury and non-alcoholic liver disease.

Antioxidant
Current therapeutic interventions of antioxidants focus on prevention of liver cell damage rather than treatment. Antioxidants such as carnosic acid have shown positive impacts of hepatoprotection and antioxidant in mices with paracetamol induced liver injury. N-acetylcysteine, the precursor molecule of the antioxidants GSH (glutathione) and vitamin C have demonstrated hepatoprotective effects in ALD mices by improving function of mitochondria and further exposure to excess oxidative stress. However, many studies proved effectiveness in animals but not in humans and more exploratory and confirmatory studies remain to be conducted to examine underlying mechanisms of various antioxidants in prevention and treatment of liver diseases with liver cell damage.

Immunosuppressant
Immunosuppressive drugs are medications used to suppress the immune system and prevent further damage to the liver. A widely used immunosuppressant is azathioprine. Azathioprine inhibits DNA and RNA replication of immune cells, and therefore controls the number of immune cells that attack the liver. When azathioprine enters the liver, it is converted into 6-mercaptopurine, which displays immunosuppressive effects. Dosage of immunosuppressants can be effective with a mixture of steroids as well, such as prednisolone. However, long-term use of these medications may bring various side effects such as weight gain, mood changes, increased risk of infection, and bone loss.

Liver Transplantation
Liver transplantation refers to a therapeutic surgery that replaces a damaged liver with a healthy liver from a donor. This treatment is applied when liver cell damage is severe, such as patients with chronic and acute end-stage liver disease. It can be applied to various causes of liver cell damage, which are not curable through medications. Successful liver transplantation can restore liver functions and increase probability of survival of patients with severe liver damage. However, after liver transplantation, several complications can follow including, infections, acute cell rejection and non-functioning of the liver. Also, dosage of immunosuppressive drugs is required to prevent organ rejection.

Stem Cell Therapy
Stem cell therapy is a therapeutic intervention for treatment of liver cell damage. There are two types of stem cells used in this therapy, mesenchymal stem cells (MSCs) and induced pluripotent stem cells (iPSCs).

MSCs are adult stem cells, easily found in various tissues, including the bone marrow, umbilical cord, and fat. MSCs regulate the immune system as well as the innate and adaptive differentiation of cells, such as macrophages, helper T cells, and B cells. This leads to a reduction in inflammation and recovery of damaged cells.

iPSCs are stem cells that have infinite potential to differentiate to any cell. They can be directly collected from the patients, such as from skin cells. IPSCs can be differentiated to liver cells that can be transplanted into the patient. This method can be beneficial, as it uses personalized cells and reduces the chances of cell rejection. However, stem cell therapy is still a developing method for liver cell damage, and still needs further studies to evaluate it's safety and efficacy.