Hepatocarcinogenesis is a slow process during which genomic changes progressively alter the hepatocellular phenotype to produce cellular intermediates that evolve into hepatocellular carcinoma. During the long preneoplastic stage, in which the liver is often the site of chronic hepatitis, cirrhosis, or both, hepatocyte cycling is accelerated by upregulation of mitogenic pathways, in part through epigenetic mechanisms. This leads to the production of monoclonal populations of aberrant and dysplastic hepatocytes that have telomere erosion and telomerase re-expression, sometimes microsatellite instability, and occasionally structural aberrations in genes and chromosomes. Development of dysplastic hepatocytes in foci and nodules and emergence of hepatocellular carcinoma are associated with the accumulation of irreversible structural alterations in genes and chromosomes, but the genomic basis of the malignant phenotype is heterogeneous. The malignant hepatocyte phenotype may be produced by the disruption of a number of genes that function in different regulatory pathways, producing several molecular variants of hepatocellular carcinoma. New strategies should enable these variants to be characterized.

Molecular pathogenesis of human hepatocellular carcinoma

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Hepatitis B Virus Biology

IGF-I receptor (IGF-IR) signaling and functions are mediated through the activities of a complex molecular network of positive (e.g., type I IGF) and negative (e.g., the type II IGF receptor, IGF-IIR) effectors. Under normal physiological conditions, the balance between the expression and activities of these molecules is tightly controlled. Changes in this delicate balance (e.g., overexpression of one effector) may trigger a cascade of molecular events that can ultimately lead to malignancy. In recent years, evidence has been mounting that the IGF axis may be involved in human cancer progression and can be targeted for therapeutic intervention. Here we review old and more recent evidence on the role the IGF system in malignancy and highlight experimental and clinical studies that provide novel insights into the complex mechanisms that contribute to its oncogenic potential. Controversies arising from conflicting evidence on the relevance of IGF-IR and its ligands to human cancer are discussed. Our review highlights the importance of viewing the IGF axis as a complex multifactorial system and shows that changes in the expression levels of any one component of the axis, in a given malignancy, should be interpreted with caution and viewed in a wider context that takes into account the expression levels, state of activation, accessibility, and functionality of other interacting components. Because IGF targeting for anticancer therapy is rapidly becoming a clinical reality, an understanding of this complexity is timely because it is likely to have an impact on the design, mode of action, and clinical outcomes of newly developed drugs.

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The role of the IGF system in cancer growth and metastasis: overview and recent insights.

Several decades of basic and clinical research have demonstrated that there is an association between the insulin-like growth factors (IGFs) and neoplasia. We begin with a brief discussion of the function and regulation of expression of the IGFs, their receptors and the IGF-binding proteins (IGFBPs). A number of investigational interventional strategies targeting the GH or IGFs are then reviewed. Finally, we have assembled the available scientific knowledge about this relationship for each of the major tumor types. The tumors have been grouped together by organ system and for each of the major tumors, various key elements of the relationship between IGFs and tumor growth are discussed. Specifically these include the presence or absence of autocrine IGF-I and IGF-II production; presence or absence of IGF-I and IGF-II receptor expression; the expression and functions of the IGFBPs; in vitro and in vivo experiments involving therapeutic interventions; and available results from clinical trials evaluating the effect of GH/IGF axis down-regulation in various malignancies.

The Effects of Insulin-Like Growth Factors on Tumorigenesis and Neoplastic Growth

Publisher Summary This chapter focuses on the role of the insulin-like growth factor system in human cancer. The normal process of growth and differentiation results from the genetically programmed action of a number of different cellular and extracellular factors. Derangement in the function of one or more of those agents can result in a pathologic phenotype, including neoplastic growth. A family of growth factors shown to be intimately involved in the regulation of cell growth as well as in cellular transformation is the insulin-like growth factor (IGF) family. IGF-I and IGF-II are mitogenic polypeptides produced in the largest amounts by the liver and secreted into the circulation, where they mediate the effects of growth hormone (GH) on longitudinal growth. In addition to ligands and receptors, the IGF system comprises a third category of molecules, which bind IGFs in the circulation and in extracellular compartments. Six IGF-binding proteins (IGFBPs) have been characterized to date. Binding of IGFs to the IGF-I receptor induces receptor autophosphorylation. The cell cycle consists of four major phases: (1) the presynthetic phase, G; (2) the phase of DNA synthesis, S; (3) the premitotic phase, G; and (4) mitosis, M. IGF-I, IGF-II, and insulin are chemotactic agents for the human melanoma cell line A2058, as assayed in a modified Boyden chamber. The chapter also gives selected examples of IGF involvement in human cancer. The wealth of information generated in the IGF field, as well as continued research efforts, both basic and clinic, promise to produce rational therapeutic approaches for those cancers in which the IGF system is involved.

The Role of the Insulin-like Growth Factor System in Human Cancer

Recent results obtained using molecular biology techniques have suggested a possible role for insulin-like growth factor II (IGF-II) in the pathogenesis of hepatocellular carcinoma (HCC). To investigate this phenomenon, a monoclonal antibody was used against IGF-II to study 54 patients with HCC. The presence of HBsAg was also tested both in serum and liver tissue. A positive immunoreaction was found in 9/15 (60%) of the HCC arising in cirrhotic livers of patients who had serum markers for HBV (HBV + positive patients). These results provide further evidence that HBV might play a role in the expression of IGF-II. In HCC of patients without any markers of HBV infection (HBV- negative patients), IGF-II was detected in 10/39 (25.6%) of the tumors, and in some benign neoplastic lesions. It was found not only in neoplastic cells but also in some dysplastic nodules. The speculation arises that IGF-II expression may play a role in some steps of hepato-carcinogenesis.

Expression of insulin-like growth factor II (IGF-II) in human hepatocellular carcinomas: an immunohistochemical study.

HBV genomes have been described even in HBsAg-negative patients with hepatocellular carcinoma (HCC), but the role of hepatitis B virus (HBV) in liver transformation is still unclear. We assessed the rate of persistence, the structure, and the RNA expression of HBV genomes in HBsAg-negative HCC using HBV- DNA PCR and HBV-RNA RT-PCR. HBV DNA genomes have been detected in 37 (59%) of 63 HBsAg-negative HCC. Defective genomes have been found more frequently in tumorous than nontumorous tissues (19/29 vs 1/9) and in seronegative than seroconverted patients (9/12 vs 1/7). A significant accumulation of X RNA was shown in 7/9 tumorous and 7/8 nontumorous tissues. By contrast, S and C RNAs were barely expressed in both tumorous and nontumorous tissues. These results suggest a role for the potentially transforming, transactivating X protein in the HBV-related liver carcinogenesis in HBsAg-negative patients.

Stella De Mitri

Papers

Expression of insulin-like growth factor II (IGF-II) in human hepatocellular carcinomas: an immunohistochemical study.

Rate of Persistence, Structure, and Expression of HBV Genome in HCC Developing in HBsAg-negative Patients