Alcoholic Hepatitis and HCV Interactions in Liver Disease
Alcoholic Hepatitis and HCV Interactions in Liver Disease
The pathogenesis of AH is based on inflammation and increased oxidative stress. In AH, elevated levels of endotoxin, a component of the cell wall of Gram-negative bacteria, are associated with upregulation of inflammatory cytokines. Endotoxemia is caused by increased intestinal permeability in the presence of chronic alcohol use, alcoholic cirrhosis, AH and even after binge drinking. Intestinal permeability allows intestinal bacteria to migrate to mesenteric lymph nodes and other extraintestinal sites, otherwise known as bacterial translocation. A short period of abstinence not only reduces gut permeability, but also the level of endotoxins detected in serum, and selective intestinal decontamination with antibiotics decreases endotoxin levels.
In AH and alcoholic cirrhosis, the translocation of the endotoxins from the gut into portal circulation leads to the activation of toll-like receptor -4 (TLR4) on Kupffer cells that produce TNF-α. Other pathways are also activated including the inflammasome that produces IL-1 β that amplifies liver inflammation. Anti-inflammatory pathways such as the signal transducer and activator transcription factor (STAT) pathway, which leads to production of IL-10 and stimulation of CD4+ and CD8+ T lymphocytes, are suboptimally activated. Together, this leads to activation of inflammatory cells, production of pro-inflammatory cytokines and apoptosis. Hepatic damage in AH is also caused by increased reactive oxygen species (ROS) generated during alcoholic metabolism and depleted antioxidants, such as glutathione. The increased susceptibility to infection is also related to neutrophil dysfunction with high resting oxidative burst and reduced phagocytic capacity. Additionally, alcohol use results in impaired antigen-presenting cell function affecting dendritic cells and antigen-specific T-cell activation. Together, these immune alternations contribute to impaired immune clearance of HCV infection.
HCV damages the liver by modulating hepatocytes and immune responses. Furthermore, HCV, like alcohol, increases reactive oxygen species (ROS) leading to hepatic injury. The HCV core protein binds to the mitochondria and facilitates the uptake of Ca2+ into the endoplasmic reticulum of a cell which also increases ROS production and glutathione depletion. Therefore, both alcohol and HCV worsen the balance between oxidative stress and antioxidants and impair antiviral immunity.
Immune modulation by alcohol during HCV infection has been shown to contribute to the higher incidence of hepatitis C virus (HCV) infection. HCV has a single-stranded RNA genome recognized by host cell pattern recognition receptors (PRRs) and induces type I IFNs and inflammatory mediators. Despite this host immune surveillance, HCV can escape host immune recognition systems and establish chronic infection during alcohol abuse. Additionally, alcohol and HCV infection, independently or synergistically, can attenuate the capacity of myeloid dendritic cells to fulfil their antigen-presenting and T-cell stimulatory function. Activation of primary human dendritic cells in the presence of alcohol and HCV significantly decreased IL-12 and increased IL-10 production. Chronic alcohol consumption also increases nonspecific inflammatory responses while decreasing type I IFN production that is important for antiviral immunity.
Alcohol and HCV are independent risk factors that cause liver inflammation, cirrhosis and HCC with similar and distinct molecular mechanisms. Studies suggest that increased HCV replication, increased hepatic oxidative stress, impaired host immune response and enhanced hepatocyte cell death contribute to the development of liver disease when alcohol abuse is associated with HCV infection (Figure 1).
(Enlarge Image)
Figure 1.
Summary of molecular interactions of HCV infection associated with alcohol abuse. The synergistic effect on how alcohol use associated with HCV infection exacerbates the progression of liver disease involves the modulation of HCV viral replication, hepatocyte apoptosis, hepatic oxidative stress, alcohol-induced ?leaky gut?, increased Lipogenesis and decrease fatty acid oxidation, miR-122 and host protein modulation and suppressed antiviral immune function. Schematics were prepared using some Motifolio templates (www.motifolio.com).
HCV infection induces the production of ROS and nitric oxide (NO) in the liver. Of the 10 viral proteins, HCV core protein increases oxidative stress in hepatocytes. Other HCV proteins, including NS3, or NS5A, E1, E2 and NS4B, are also involved in inducing oxidative stress. In addition to their other functions, HCV core, NS5A and NS3 proteins increase calcium uptake by mitochondria and cause oxidation of mitochondrial glutathione leading to increased ROS.
Chronic alcohol enhances inflammation, because HCV-infected patients who consume alcohol exhibit greater liver inflammation than patients who consume no alcohol. Upregulated programmed cell death pathways (apoptosis) play a crucial role in HCV-infected hepatocytes in the presence of alcohol. Damage-associated molecular patterns (DAMPs) from cell death induced by alcohol or HCV can exacerbate immune activation together with increased gut bacteria as a consequence of alcohol abuse.
Alcohol can modulate HCV replication by diverse mechanisms including modulation of host proteins, micro-RNAs and suppressing cellular immune signalling. While there are conflicting findings as to the direct role of alcohol in modulating HCV replication in vitro, most studies found that alcohol can increase HCV replication through alcohol metabolites, increased GW812, increased HSP90 and decreasing cyclin G1 expression. Studies using Huh7.5-CYP2E1 cells or HCV replicon cell lines found that alcohol metabolites can increase HCV replication in vitro. Importantly, alcohol exposure can increase HCV replication by increasing cellular host protein expression including HSP90 and GW182 while decreasing cyclin G1 expression.
Previous studies indicated that alcohol use in HCV-infected patients can significantly increase serum HCV RNA levels. In recent reports, we demonstrated that alcohol can increase HCV replication in vitro and identified a critical role of microRNA-122 (miRNA-122) in the process. While numerous clinical studies have advanced the role of alcohol, in enhancing HCV replication, a meta-analysis showed that alcoholics with HCV infection showed no significant differences in HCV expression. miRNAs are noncoding RNAs that modulate gene expression and regulate the numerous pathological processes. In hepatocytes, miRNA-122 is highly expressed compared to other miRNAs. Recently, we found that alcohol exposure could significantly increase miRNA-122 levels in Huh7.5 hepatoma cells and through this mechanism, increases HCV replication. Additionally, miRNA-122 regulation of HCV is enhanced by the RISC (RNA-induced silencing complex) complex molecules Argonaute 2(Ago2), HSP90 and GW182 which also increased during alcohol exposure in the cultured human hepatoma cell line Huh7.5. Independent of HCV, increased serum miRNA-122 appears to positively correlate with the severity of liver damage in alcoholic hepatitis and virologic response to pegylated interferon therapy against HCV infection. Given that alcohol and HCV infection increase miR-122 and RISC complex proteins that can increase HCV replication, their synergism in this molecular process might account for the reason that alcoholics show faster progression towards advanced liver disease.
Regardless of aetiology, decompensated cirrhotics have a higher prevalence of bacterial translocation, likely related to increased intestinal permeability, compared to compensated cirrhotics and controls. Endotoxin concentrations, although higher in alcoholic liver disease, are also elevated in chronic HCV and HCV cirrhosis and are associated with increased hepatic venous pressure gradients. Mouse models have demonstrated increased cytokines and nitric oxide production in the presence of bacterial translocation and cirrhosis.
In patients with HCV cirrhosis, AH precipitates acute decompensation, causes acute on chronic liver injury, or if organ failure develops, acute on chronic liver failure (ACLF). The definition of ACLF is still developing but is primarily based on the presence of acute decompensation (e.g. ascites, encephalopathy, GI bleed) and organ failure, from a precipitating factor or superimposed liver insult. ACLF is also characterized by rapid progression and high mortality, based primarily on the severity of the organ failure. One-month mortality is estimated at 30–53% in patients who develop ACLF. The precipitating factor may be due to liver injury including viral or AH or nonhepatic stress such as sepsis or surgery. One retrospective study on aetiologies of ACLF found that 47% of cases were triggered by extrahepatic causes and AH caused 29% of the hepatic causes.
The underlying mechanisms of ACLF injury may be related to the underlying cirrhosis and the triggering event that induces the inflammatory response. Elevated inflammatory cytokines including TNF-α, IFN-γ, IL 2, IL2R, IL-6, IL-8 and IL-10, many of which are key factors in hepatic damage induced by alcohol and HCV, are found in cases of ACLF. Infection may not only precipitate ACLF, but is also associated with increased mortality and morbidity in other triggers of ACLF. Immune dysfunction in ACLF is multifactorial. Decreased activity by phagocytic cells and the reticuloendothelial system leads to accumulation of microbes including damaging endotoxins. Endotoxins and other bacteria that may be present trigger the release of inflammatory cytokines and ultimately a systemic response leading to systemic inflammatory response syndrome (SIRS). These are characteristics of acute alcoholic hepatitis. ACLF develops from a cycle in which sepsis or SIRS continues the pro-inflammatory response further depressing the immune system and increasing susceptibility to further infections.
The mechanism and alcoholic hepatitis superimposed on chronic HCV liver disease is a prime example of the pathophysiology of ACLF. The inflammatory response and cytokine release in the chronic liver disease is likely exacerbated by the additional inflammation caused by AH. Increased gut permeability, endotoxemia, cytokine release and decreased immune response lead to susceptibility to infection which is the most common cause of mortality in both ACLF and AH.
Mechanisms
Acute Alcoholic Hepatitis
The pathogenesis of AH is based on inflammation and increased oxidative stress. In AH, elevated levels of endotoxin, a component of the cell wall of Gram-negative bacteria, are associated with upregulation of inflammatory cytokines. Endotoxemia is caused by increased intestinal permeability in the presence of chronic alcohol use, alcoholic cirrhosis, AH and even after binge drinking. Intestinal permeability allows intestinal bacteria to migrate to mesenteric lymph nodes and other extraintestinal sites, otherwise known as bacterial translocation. A short period of abstinence not only reduces gut permeability, but also the level of endotoxins detected in serum, and selective intestinal decontamination with antibiotics decreases endotoxin levels.
In AH and alcoholic cirrhosis, the translocation of the endotoxins from the gut into portal circulation leads to the activation of toll-like receptor -4 (TLR4) on Kupffer cells that produce TNF-α. Other pathways are also activated including the inflammasome that produces IL-1 β that amplifies liver inflammation. Anti-inflammatory pathways such as the signal transducer and activator transcription factor (STAT) pathway, which leads to production of IL-10 and stimulation of CD4+ and CD8+ T lymphocytes, are suboptimally activated. Together, this leads to activation of inflammatory cells, production of pro-inflammatory cytokines and apoptosis. Hepatic damage in AH is also caused by increased reactive oxygen species (ROS) generated during alcoholic metabolism and depleted antioxidants, such as glutathione. The increased susceptibility to infection is also related to neutrophil dysfunction with high resting oxidative burst and reduced phagocytic capacity. Additionally, alcohol use results in impaired antigen-presenting cell function affecting dendritic cells and antigen-specific T-cell activation. Together, these immune alternations contribute to impaired immune clearance of HCV infection.
Hepatitis C Infection
HCV damages the liver by modulating hepatocytes and immune responses. Furthermore, HCV, like alcohol, increases reactive oxygen species (ROS) leading to hepatic injury. The HCV core protein binds to the mitochondria and facilitates the uptake of Ca2+ into the endoplasmic reticulum of a cell which also increases ROS production and glutathione depletion. Therefore, both alcohol and HCV worsen the balance between oxidative stress and antioxidants and impair antiviral immunity.
HCV, Alcohol and Host Response
Immune modulation by alcohol during HCV infection has been shown to contribute to the higher incidence of hepatitis C virus (HCV) infection. HCV has a single-stranded RNA genome recognized by host cell pattern recognition receptors (PRRs) and induces type I IFNs and inflammatory mediators. Despite this host immune surveillance, HCV can escape host immune recognition systems and establish chronic infection during alcohol abuse. Additionally, alcohol and HCV infection, independently or synergistically, can attenuate the capacity of myeloid dendritic cells to fulfil their antigen-presenting and T-cell stimulatory function. Activation of primary human dendritic cells in the presence of alcohol and HCV significantly decreased IL-12 and increased IL-10 production. Chronic alcohol consumption also increases nonspecific inflammatory responses while decreasing type I IFN production that is important for antiviral immunity.
Alcohol and HCV Interactions at the Cellular and Molecular Levels
Alcohol and HCV are independent risk factors that cause liver inflammation, cirrhosis and HCC with similar and distinct molecular mechanisms. Studies suggest that increased HCV replication, increased hepatic oxidative stress, impaired host immune response and enhanced hepatocyte cell death contribute to the development of liver disease when alcohol abuse is associated with HCV infection (Figure 1).
(Enlarge Image)
Figure 1.
Summary of molecular interactions of HCV infection associated with alcohol abuse. The synergistic effect on how alcohol use associated with HCV infection exacerbates the progression of liver disease involves the modulation of HCV viral replication, hepatocyte apoptosis, hepatic oxidative stress, alcohol-induced ?leaky gut?, increased Lipogenesis and decrease fatty acid oxidation, miR-122 and host protein modulation and suppressed antiviral immune function. Schematics were prepared using some Motifolio templates (www.motifolio.com).
HCV infection induces the production of ROS and nitric oxide (NO) in the liver. Of the 10 viral proteins, HCV core protein increases oxidative stress in hepatocytes. Other HCV proteins, including NS3, or NS5A, E1, E2 and NS4B, are also involved in inducing oxidative stress. In addition to their other functions, HCV core, NS5A and NS3 proteins increase calcium uptake by mitochondria and cause oxidation of mitochondrial glutathione leading to increased ROS.
Chronic alcohol enhances inflammation, because HCV-infected patients who consume alcohol exhibit greater liver inflammation than patients who consume no alcohol. Upregulated programmed cell death pathways (apoptosis) play a crucial role in HCV-infected hepatocytes in the presence of alcohol. Damage-associated molecular patterns (DAMPs) from cell death induced by alcohol or HCV can exacerbate immune activation together with increased gut bacteria as a consequence of alcohol abuse.
Alcohol can modulate HCV replication by diverse mechanisms including modulation of host proteins, micro-RNAs and suppressing cellular immune signalling. While there are conflicting findings as to the direct role of alcohol in modulating HCV replication in vitro, most studies found that alcohol can increase HCV replication through alcohol metabolites, increased GW812, increased HSP90 and decreasing cyclin G1 expression. Studies using Huh7.5-CYP2E1 cells or HCV replicon cell lines found that alcohol metabolites can increase HCV replication in vitro. Importantly, alcohol exposure can increase HCV replication by increasing cellular host protein expression including HSP90 and GW182 while decreasing cyclin G1 expression.
MiR-122, HCV and Alcohol
Previous studies indicated that alcohol use in HCV-infected patients can significantly increase serum HCV RNA levels. In recent reports, we demonstrated that alcohol can increase HCV replication in vitro and identified a critical role of microRNA-122 (miRNA-122) in the process. While numerous clinical studies have advanced the role of alcohol, in enhancing HCV replication, a meta-analysis showed that alcoholics with HCV infection showed no significant differences in HCV expression. miRNAs are noncoding RNAs that modulate gene expression and regulate the numerous pathological processes. In hepatocytes, miRNA-122 is highly expressed compared to other miRNAs. Recently, we found that alcohol exposure could significantly increase miRNA-122 levels in Huh7.5 hepatoma cells and through this mechanism, increases HCV replication. Additionally, miRNA-122 regulation of HCV is enhanced by the RISC (RNA-induced silencing complex) complex molecules Argonaute 2(Ago2), HSP90 and GW182 which also increased during alcohol exposure in the cultured human hepatoma cell line Huh7.5. Independent of HCV, increased serum miRNA-122 appears to positively correlate with the severity of liver damage in alcoholic hepatitis and virologic response to pegylated interferon therapy against HCV infection. Given that alcohol and HCV infection increase miR-122 and RISC complex proteins that can increase HCV replication, their synergism in this molecular process might account for the reason that alcoholics show faster progression towards advanced liver disease.
Acute on Chronic Liver Damage
Regardless of aetiology, decompensated cirrhotics have a higher prevalence of bacterial translocation, likely related to increased intestinal permeability, compared to compensated cirrhotics and controls. Endotoxin concentrations, although higher in alcoholic liver disease, are also elevated in chronic HCV and HCV cirrhosis and are associated with increased hepatic venous pressure gradients. Mouse models have demonstrated increased cytokines and nitric oxide production in the presence of bacterial translocation and cirrhosis.
In patients with HCV cirrhosis, AH precipitates acute decompensation, causes acute on chronic liver injury, or if organ failure develops, acute on chronic liver failure (ACLF). The definition of ACLF is still developing but is primarily based on the presence of acute decompensation (e.g. ascites, encephalopathy, GI bleed) and organ failure, from a precipitating factor or superimposed liver insult. ACLF is also characterized by rapid progression and high mortality, based primarily on the severity of the organ failure. One-month mortality is estimated at 30–53% in patients who develop ACLF. The precipitating factor may be due to liver injury including viral or AH or nonhepatic stress such as sepsis or surgery. One retrospective study on aetiologies of ACLF found that 47% of cases were triggered by extrahepatic causes and AH caused 29% of the hepatic causes.
The underlying mechanisms of ACLF injury may be related to the underlying cirrhosis and the triggering event that induces the inflammatory response. Elevated inflammatory cytokines including TNF-α, IFN-γ, IL 2, IL2R, IL-6, IL-8 and IL-10, many of which are key factors in hepatic damage induced by alcohol and HCV, are found in cases of ACLF. Infection may not only precipitate ACLF, but is also associated with increased mortality and morbidity in other triggers of ACLF. Immune dysfunction in ACLF is multifactorial. Decreased activity by phagocytic cells and the reticuloendothelial system leads to accumulation of microbes including damaging endotoxins. Endotoxins and other bacteria that may be present trigger the release of inflammatory cytokines and ultimately a systemic response leading to systemic inflammatory response syndrome (SIRS). These are characteristics of acute alcoholic hepatitis. ACLF develops from a cycle in which sepsis or SIRS continues the pro-inflammatory response further depressing the immune system and increasing susceptibility to further infections.
The mechanism and alcoholic hepatitis superimposed on chronic HCV liver disease is a prime example of the pathophysiology of ACLF. The inflammatory response and cytokine release in the chronic liver disease is likely exacerbated by the additional inflammation caused by AH. Increased gut permeability, endotoxemia, cytokine release and decreased immune response lead to susceptibility to infection which is the most common cause of mortality in both ACLF and AH.