Mini-reviewMechanisms of HCV-induced liver cancer: What did we learn from in vitro and animal studies?
Introduction
A recent re-analysis of the worldwide global burden of cancer [1] places liver as the 5th most prevalent target organ in terms of the estimated new cases in man, and 7th in women. Most importantly, liver cancer is one of the most deadly cancers, as it is the 2nd leading cause of cancer death in man and 6th in women worldwide. An estimated 748,000 new liver cancer cases and 696,000 cancer deaths occurred worldwide in 2008 with the highest liver cancer rates are reported in East and South-East Asia, and in Middle and Western Africa. Among primary liver cancers, hepatocellular carcinoma (HCC) represents the major subtype, accounting for 70–85% of the total liver cancer burden worldwide.
Among many potential etiological factors that have been causally linked to human cancers, including HCC, infectious agents represent an important sub-group of agents that have been classified as “carcinogenic to humans” (Group 1) by the International Agency for Research on Cancer Monographs Program [2]. Liver is a major cancer site associated with four Group 1 infectious agents: hepatitis B and C viruses, O. viverrini and C. sinensis. With regards to hepatitis C virus (HCV), the relative risk estimate for developing liver cancer in patients with serologically-confirmed HCV infection is estimated to be 17, as compared to 2.5 for HCV-associated non-Hodgkin lymphomas [3]. Importantly, the age-adjusted incidence of HCC is increasing in many countries, including the United States [4], and has been widely attributed to the spread of HCV infection in many industrialized countries [5].
About 2–3% of the world’s population is living with HCV infection, with country-specific prevalence rates ranging from <1% to over 10% [6]. Each year, it is estimated that over 350,000 people die worldwide of HCV-related diseases, predominantly liver cirrhosis and liver cancer [6]. In the United States alone, HCV and associated diseases carry a very high economic burden and it was estimated in 2012 that there are over 50 new drugs in development to treat hepatitis C [7], [8]. Given the societal, economic and other pressures, the field of HCV studies is very vibrant and spans the spectrum of investigations in infectious disease and virology from mechanistic and clinical research, to drug development and epidemiology. However, relatively few studies have focused on the mechanisms underlying the association of liver cancer with HCV, and it is still largely unresolved whether the virus is directly carcinogenic (e.g. causes mutations, genomic instability, or transformation of liver cells), or whether other pathological conditions in the liver (steatosis, inflammation, oxidative stress, and fibrogenesis) that are associated with the chronic viral infection are to blame [5].
Since the discovery of HCV in 1989 [9], our knowledge has expanded exponentially and a plethora of model systems is being utilized by researchers who are interested in HCV itself, or the diseases that HCV has been associated with. These include human subjects, non-human primates, genetically engineered mice, as well as both human- and animal-derived cells. The types of research questions that are being investigated using one or more of these model systems include: (i) the mechanisms of infection, viral life cycle and persistence; (ii) types, pathogenesis and mechanisms of HCV-associated liver diseases, including HCC; (iii) the role of co-morbidity and environmental co-exposure factors; (iv) pharmacotherapy options and treatment strategies; and (v) individual susceptibility factors. While there are a number of animal models for the study of HCV infection and related liver diseases (see [10] for a recent comprehensive review), few models have been applied to study the etiology and mechanisms of liver cancer.
Section snippets
Mechanisms of HCV-associated liver carcinogenesis
Evidence exists to suggest that HCV may be both directly and indirectly involved in the development and progression of HCC [5]. The evidence for the direct carcinogenic action of HCV is less prominent than that for other carcinogenic viruses (e.g. papillomaviruses, herpesviruses, Epstein-Barr virus) which integrate into cellular DNA and/or impair normal controls of proliferation and cell death. HCV is a positive-strand RNA virus that replicates outside of the nucleus and does not have any
Human genome-wide association studies (GWAS)
Current human research in HCV field is primarily focused on understanding liver disease susceptibility and progression and development of new treatments and patient management strategies. There are major differences in how people respond to HCV infection and its treatment. In addition to the virus serotype-specific reasons, host-specific factors play a clear role in whether HCV will lead to chronic infection, as about 30% of persons who acquire HCV infection resolve viremia, leaving only the
Non-human primates
Chimpanzees are the most relevant animal model for studies of HCV infection and related immune and other effects [30]. They are considered a “complete” model with replication, infection and virus production steps of the viral cycle. While the viremia levels are generally high and the human-like host response comprises both innate and adaptive immunity, the pathogenicity of HCV is relatively low in chimpanzees, making them a poor model for chronic liver disease and HCV-associated HCC [10]. Only
Mouse models
The restricted host range of HCV has hampered the development of a suitable small animal model of HCV infection; however, a number of research strategies have been proposed to take advantage of the genetic engineering tools available in the mouse [10]. Of many mouse models that have been developed in the past decade, only HCV transgenic mouse strains have been employed in chronic studies designed to detect liver cancer as an endpoint.
Primary human hepatocytes
Among many in vitro models that have been used in HCV research, adult primary human hepatocytes are considered to be the most human-relevant cell-based liver model [53]. It has been demonstrated more than a decade ago that human serum-derived HCV may infect these cells in culture [54]. Primary adult human hepatocytes are permissive to viral genome replication, although the level of replication is typically very restricted in magnitude [55], [56]. Primary adult human hepatocytes are closest to
Use of mouse model systems to understand factors that facilitate cancer development in HCV-infected liver
Spontaneous development of liver tumors have been observed only in a couple of HCV transgenic mouse strains, usually between 13 and 24 months of age [40], [41], [42]. Most of the HCV transgenic mouse models exhibit a limited overt liver phenotype, even late in life [42], yet are susceptible to a number of additional hepatotoxic challenges such as iron overload [80], carbon tetrachloride [81], alcohol [49], acetaminophen [82], or aflatoxin B1 [83].
Studies in HCV transgenic mice have demonstrated
Conflict of Interest
The authors have nothing to disclose.
Acknowledgements
The authors were supported, in part, by grants from the National Institutes of Health: P42-ES005948, R01-ES015241, R01-CA164029 and R01-AI095690.
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