Chronic viral infections in the liver can lead to organ dysfunction and ultimately to liver tumors in a progression invariably characterized by viruses that proliferate free of immune system restraints.
Although it has been known for decades that chronic viral infection of the liver can lead to cancer, medical investigators have only now begun to fully appreciate how the disruption of molecular signaling sets the stage for virus-induced liver cancer.
In an elegant series of cellular studies at the National Taiwan University’s College of Medicine in Taipei City, scientists have found that a transmembrane enzyme (a protein embedded in the cell with active portions above and below the cell surface) plays a powerful role in damaging liver cells.
That enzyme goes by the name of hepsin, and is produced by the host. It increases vulnerability to liver cancer because it’s a noteworthy turncoat – a biological traitor – when active in the milieu of a viral infection.
Although the team in Taiwan saw the damaging activity in the lab when two types of viruses, Sendai and herpes, were studied, the major global health crisis involving liver infections and cancer are centered squarely on hepatitis B and C.
Hepsin, as it turns out, doesn’t even mess with the viruses themselves to create havoc in the liver; it irrevocably damages a protective protein called STING. Once STING is crippled, viruses are free to run roughshod through the liver.
“Our study provides new insights,” declared Drs. Fu Hsin and Helene Minyi Liu, writing in the journal Science Signaling. “Chronic viral infections of the liver can lead to organ dysfunction and hepatocellular carcinoma.
“The transmembrane serine protease hepsin, suppresses type I interferon induction by cleaving STING,” Hsin and Liu wrote, referring to the tremendous biological damage heaped on the protein known as STING.
The Taiwanese research is demonstrating that hepsin suppresses the body’s natural antiviral responses that are launched by STING, which stands for Stimulator of Interferon Genes.
When STING is intact, its signaling activity is involved in the release of a massive flood of virus-fighting interferons, proteins that thwart viruses.
STING activation stimulates the expression of genes encoding type I interferons as part of the antiviral response. Type I interferons comprise a large subgroup of proteins that help regulate the activity of the immune system. In addition to its role activating interferons that control viruses, STING activation also figures prominently in antitumor immunity, making it critical in the overall innate immune response.
In terms of timing, STING is supposed to be activated in an early phase of the innate immune response when viruses are sensed and cascades of molecular assaults from a stimulated immune system are needed for a full-bore attack.
As it turns out – and as bad luck would have it – turncoat hepsin literally chops up STING, thereby disabling the innate immune response.
The immune system constituents sabotaged by hepsin are the type I interferons, proteins that not only kill viruses but are crucial to signaling activities that marshal additional components of the immune system. The word ‘traitor’ seems too kind when considering the degree of liver destruction hepsin inadvertently allows.
“Hepsin, which is predominantly present in hepatocytes, inhibited the induction of type I interferon during viral infections,” asserted Hsin and Liu, noting that without type I interferons, viral infection can progress largely unchecked.
Damage to STING wasn’t merely theorized; the Taiwanese team actually saw how hepsin assaulted STING in the lab, observing the damage in infected mouse embryonic fibroblasts and infected human hepatocytes.
“Hepsin co-localized with STING at the endoplasmic reticulum and cleaved STING,” the scientists asserted.
Aside from being a turncoat in the liver, hepsin goes by another name in other tissues, particularly the nasopharynx, which forms the upper respiratory tract and the lower respiratory tract, which includes the lungs. In these tissues, hepsin known as TMPRSS1 and, not surprisingly, exists in those cell populations as a transmembrane traitor, too.
In the case of SARS-CoV-2 infection, TMPRSS1 sends signals that can alert the virus to the presence of the ACE2 receptor. The ACE2 receptor is TMPRSS1’s next-door neighbor – and the vulnerable gateway that SARS-CoV-2 breaches to invade human cells, leading to the sometimes deadly pandemic disease, COVID-19. There’s no question among investigators that TMPRSS1’s signaling aids the virus.
To be clear, one of the major roles of the immune system is preventing viruses from commandeering cells. But when hepsin cripples STING, amid an ongoing liver infection, the stage is set for liver cancer. The Taipei City research not only opens a new window of understanding into how a signaling molecule can be damaged, it also demonstrates how a minuscule transmembrane enzyme plays a role in causing the destruction.
The new research arrives at a critical juncture as international health agencies, especially the World Health Organization, map strategies to reduce the burden of liver disease and liver cancer. Globally, liver cancer is a leading form of cancer-related death, and a primary cause of those malignancies is viral infection of the organ, predominantly by hepatitis B and C.
An estimated 1.1 million people die annually of liver cancer caused by viral infections, according to the WHO, which additionally defines the most vulnerable regions for hepatitis-linked liver cancer as resource poor countries.
Even though a vaccine, in use since the 1980s, is capable of preventing hepatitis B infection, and in more recent years, drug therapy effectively cures hepatitis C, these interventions still elude many parts of the world.
Grasping the underlying molecular mechanisms that lead to cancer in the first place, might pave the way toward newer methods of liver cancer prevention, such as developing pharmaceutical strategies that block hepsin, the team reporting said.
Hsin and Liu, who collaborated with scientists from the Liver Disease Prevention and Treatment Research Foundation, also in Taipei City, noted in their research that the release of interferons is critical during liver infection for more than one reason: It isn’t just a matter of interferons quelling viruses.
The infection-fighting proteins also stimulate antiviral pathways in nearby cells. And hepsin, they say, not only makes liver infections worse by preventing the release of interferons, aberrant hepsin activity also may be involved in non-infectious diseases, worsening them as well.
“Hepsin makes hepatocytes vulnerable to viral infection and may contribute to the poor response of prostate cancer cells to immunotherapies that rely on STING activation,” the scientists said.
Epidemiology of viral hepatitis associated hepatocellular carcinoma
Liver cancer is the third leading cause of cancer-associated mortality (781631 people/year), despite being ranked seventh on global incidence (841080 people/year). Approximately 12% of all cancer cases globally arise from chronic infections with bloodborne oncogenic viral pathogens including hepatitis B virus (HBV), hepatitis C virus (HCV), and hepatitis delta virus (HDV).
Although incidence in the majority of cancers has decreased, primary liver cancer incidence is the fastest-growing cancer with regards to incidence and mortality. Hepatocellular carcinoma (HCC) represents 90% of all liver cancer cases and the risk factors are well defined: Viral infection with HBV, (54% of all HCCs) and/or HCV (31% of all HCCs), cirrhosis (80% of all HCCs), high alcohol consumption, obesity, genetic disorders such as hemochromatosis, exposure to aflatoxins, sex (male) and older age (50+)[4-7].
Virus-induced HCC is present worldwide, however, there are considerable differences in populations that develop HBV or HDV induced HCC vs HCV induced HCC. HBV and HDV associated HCC is more common in low and middle-human development index countries, while HCV induced HCC is more common in high and very high-human development index.
Chronic hepatitis B (CHB) infection affects around 257 million people worldwide, of which 48-60 million people are co-infected with HDV and an estimated 2.6 million are co-infected with HCV[8-10]. Exposure to infected blood/bodily fluids is the primary mode of transmission for HBV and HBV/HDV, with majority of exposures occurring from mother to child during birth or early years of life.
Unvaccinated neonates and children who have been exposed to HBV have > 95% risk of developing chronic disease, while infection during adulthood results in < 2% chance of developing chronic disease.
HBV/HDV co-infection have the highest mortality rate (20%) associated with any viral hepatitis infection and most severe liver disease (i.e. acute liver failure, cirrhosis within 5 years, and HCC within 10 years)[10,12,13]. HCV has established chronic infection in 70 million people primarily through horizontal blood-borne transmission routes such as intravenous drug use, needle pricks, unscreened blood transfusions, and high-risk sexual practices.
In comparison to HBV or HCV mono-infection, individuals who are co-infected with HBV/HCV have increased rates of HCC development. Overall, viral etiologies represent approximately 80% of all HCC related cases, highlighting the importance of investigating the role of these viruses in the development of liver cancer.
Preventative measures against HBV and HDV induced liver cancer include birth-dose vaccinations, hepatitis B immunoglobulin treatment for children born to infected mothers as well as treatment of mothers with high HBV viral load with nucleos(t)ide inhibitors in the third trimester.
For those individuals who are already chronic carriers of HBV/HDV, there is no virological cure; however, treatment with nucleos(t)ide analogs can lower the risk of HCC development.
There is no protective vaccine available for HCV, but there are effective direct-acting antivirals that can cure > 90% of chronic carriers. Those who have a sustained virological response from direct-acting antiviral treatment have a significantly lower risk of HCC development if cirrhosis is absent.
Although there are treatment options to lower the risk of HCC in those who have chronic viral hepatitis infection, globally many individuals are unaware of their status, lack access to testing, and effective treatment.
In this review article, we discuss the molecular biology of HBV, HCV, and HDV, common features associated with virus-induced cancers, viral oncogenic mechanisms leading to HCC relating to the hallmarks of cancer, common molecular pathways deregulated in HCC, and current as well as emerging treatments for HCC.
CHRONIC INFLAMMATION-MEDIATED BY VIRAL HEPATITIS
Non-resolving inflammation is a hallmark of cancer that significantly contributes to the development and progression of HCC. Approximately 80% of HCC cases arise from hepatocyte injury and chronic inflammation resulting in cirrhosis[6,58]. HCC in chronic hepatitis B, C, or HBV/HDV co-infection patients occurs in the presence of cirrhosis[59,60]. In contrast, 10%-20% of HBV-related HCC can occur in the absence of cirrhosis and liver inflammation.
Under normal circumstances, the innate and adaptive immune responses are activated during an infection or tissue injury and immune cells are recruited to fight against the pathogen and induce wound healing. Following the elimination of the pathogen via cytolytic and non-cytolytic mechanisms, the damaged tissue is repaired through the wound-healing process.
However, the persistence of the inflammatory stimuli (e.g. chronic viral infection) or dysregulation of the immune regulatory mechanisms prevents complete wound-healing and causes non-resolving inflammation that may lead to liver complications resulting in autoimmunity, fibrosis, cirrhosis, metaplasia and/or tumor growth.
There are five clinical phases of chronic hepatitis B infection (Figure (Figure6A)6A) from the 2019 AASLD guidelines: HBeAg+ chronic infection, HBeAg+ chronic hepatitis, HBeAg- chronic infection, HBeAg- chronic hepatitis, and a functional cure (HBsAg-). Each clinical phase is defined by a host immune response with respect to HBV viral activity.
During the initial HBeAg+ chronic infection phase the host is immune response has a poorly activated HBV-specific CD8+ T-cell response. Transition to the chronic hepatitis phase is characterized by increased activation of the adaptive immune response (e.g., HBV-specific CD8+ T-cells, pro-inflammatory cytokines) which causes decreased HBV DNA levels, liver inflammation, and variable/progressive liver fibrosis.
Failure to completely clear HBV in the HBeAg+ chronic infection phase results in prolonged over-active immune cell-mediated damage that leads to rapid liver disease progression. Immune-mediated liver damage is facilitated by natural killer cells and T-cells through the release of ROS and proinflammatory cytokines which causes bouts of necroinflammation, hepatocyte regeneration/healing and remodeling of the liver microenvironment[65,66].
Constant necroinflammation and failed wound healing responses lead to prolonged oxidative stress exposure which can promote the rapid development of fibrosis, cirrhosis, and cell transformation (epigenetic alterations, oncogenic mutations, telomere shortening, and genomic instability)[67,68]. The 5-year cumulative HCC risk for CHB patients with cirrhosis ranges from 9.7%-15.5%. However, 20% of HCC caused by HBV does not require liver cirrhosis, indicating there are other intrinsic viral associated factors that are responsible for transforming hepatocytes.
HDV infection occurs either in a co-infection model with HBV or as a superinfection from horizontal transmission in individuals with CHB (Figure (Figure6B).6B). The mechanisms used by HDV to modulate the immune system are different from that expressed by HBV and HCV due to the consistent presence of co-infection.
The natural history of chronic HDV infection is also dynamic and can be characterized as: (1) Suppressed HBV replication and active HDV replication with high ALT; (2) Slightly decreased HDV replication and HBV reactivation with moderate ALT; and (3) Late-stage disease where cirrhosis and HCC are caused by either HBV/HDV or remission resulting in a reduction of both HBV and HDV viral load. During initial infection, HDV evades IFN-α-mediated innate immune responses to promote cell survival and viral persistence.
Under normal cellular conditions, double-stranded RNA induces expression IFN-α which binds to the IFN receptor-associated JAK kinase tyrosine kinase-2(tyr-2). Dimerization of the tyr-2 receptor activates a JAK/STAT signaling cascade to produce innate antiviral proteins: myxovirus resistance A, 2’,5’-oligoadenylate synthase, and dsRNA-activated protein kinase. HDV blocks phosphorylation of tyr-2 to prevent downstream signaling and impairs phosphorylation activity and nuclear accumulation of STAT1/STAT2.
The superinfection of HDV in patients with CHB has the most severe liver disease outcome, partially due to the pre-existing liver damage caused by HBV infection. Moreover, superinfection with HDV leads to HBV viral load suppression through mechanisms that are not thoroughly understood.
Recognition of MHC-1 HDV antigens on infected cells by CD8+ T-cells mediates cellular killing. Released viral antigens are endocytosed by Kupffer cells (liver resident macrophages), B-lymphocytes, and dendritic cells and presented to CD4+ helper T-cells via MHC-II receptors. Clonal expansion of CD4+ T-cells releases IL-2, IL-10, and IFN-γ cytokines which stimulate immune-mediated killing of HDV infected cells, severe liver necrosis and progressive liver disease.
Infection from the HCV is usually acquired from horizontal transmission in adulthood, where 75%-80% of people develop a chronic infection from viral persistence. Chronic hepatitis C (CHC) infected individuals have mild liver inflammation (Figure (Figure6C),6C), stable HCV RNA titers, and liver disease that progresses especially in the presence of other risk factors (age, male, obesity, diabetes alcohol, HIV or HBV co-infection).
HCV infection activates intrinsic type I and III IFN responses which induces transcription of innate-antiviral IFN stimulated genes. Adaptive viral-specific CD8+/CD4+ T-cells and natural killer cells facilitate the release of pro-inflammatory cytokines and growth factors while destroying of HCV infected hepatocytes by promoting the inflammation-necrosis-proliferation cycle.
Immune-mediated damage produces large amounts of ROS mediated DNA damage, lipid peroxidation, epigenetic modifications, mitochondrial alteration, senescence, and chromosomal translocation that lead to hepatocyte transformation. Immune failure to remove all HCV infected cells causes the selection of viral escape mutants within a carrier population. These escape mutants prevent stimulation of CD4+/CD8+ T-cell responses, and aid in viral immune evasion, chronic infection, loss of immune regulation, and promotion of HCV-mediated HCC[80,81].
Moreover, persistent liver inflammation caused by immune cells over decades of infection can lead to the development of fibrosis, cirrhosis, and HCC. Approximately 10-20% of CHC patients develop cirrhosis in 20-30 years in the absence of treatment for hepatitis C, indicating the high risk for HCC development.
Cirrhosis is a major risk factor for HCC development
The liver is made up of approximately 80% parenchymal cells (i.e., hepatocytes) and 20% non-parenchymal cells (e.g., sinusoidal endothelial cells, hepatic stellate cells, and Kupffer cells). Infection with viral hepatitis primarily targets the large population of hepatocytes, leading to production of ROS. Release of ROS and pro-inflammatory cytokines by Kupffer cells/hepatocytes activate neighboring stellate cells and liver sinusoidal endothelial cells which are key players in the development of fibrosis[84,85] (Figure (Figure7).7). Stellate cells and fibroblasts enhance collagen synthesis and alter the extracellular matrix which lead to remodeling of the liver microenvironment.
Progressive inflammation and fibrosis pave the way for disease progression to cirrhosis which is the largest risk factor for HCC development (Figure (Figure7).7). Cirrhosis is irreversible and often individuals are asymptomatic, which makes diagnosis and treatment difficult. Those who develop severe symptoms of cirrhosis, are likely to have advanced liver disease and HCC.
This is especially problematic for the populations who are unaware of their infection status with HBV, HCV, and/or HDV because they are unable to seek treatment intervention to lower the risk of developing cirrhosis. During cirrhosis, altered blood flow can lead to a hypoxic environment for hepatocytes leading to altered molecular signaling and increased oxidative damage.
Cells within the context of cirrhosis have experienced a multitude of changes from inflammation mediated damage, repair, and regeneration. The hypoxic environment in the liver during cirrhosis can select for altered oncogenic cells and promote angiogenesis. For a comprehensive review of molecular mechanisms of host factors driving the progression of liver cirrhosis to HCC see (Fridland et al and Kanda et al).
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7579760/
More information: Fu Hsin et al, The transmembrane serine protease hepsin suppresses type I interferon induction by cleaving STING, Science Signaling (2021). DOI: 10.1126/scisignal.abb4752