Lymphotoxins: new targets for hepatocellular carcinoma

Abstract

In this issue of Cancer Cell, Haybaeck et al unravel the role of lymphotoxin pathway in the development of hepatocellular carcinoma (HCC). Aberrant activation of this cascade in mice livers recapitulates the stages of fibrosis and inflammation that precedes human liver cancer, providing a novel family of potential therapeutic targets.

Hepatocellular carcinoma (HCC) is, worldwide, the third cause of cancer-related death, and one of the fastest growing malignancies in terms of incidence in Western populations (Llovet et al., 2003). Despite recent progresses, HCC is still considered as a disease with poor prognosis since less than 30% of cases will be eligible for potential curative treatments (e.g. surgical resection, liver transplantation or percutaneous ablation). Recent results of a phase III trial showed that sorafenib, a multikinase inhibitor against BRAF, PDGFR and VEGFR, significantly improved survival in patients with advanced tumors (Llovet et al., 2008). This pivotal study established a new standard of care for these patients, and cleared the path for the development of novel molecular therapies for this malignancy.

Molecular basis that links inflammation and cancer are progressively been uncovered. Robust epidemiological data support the role of inflammation induced by chronic hepatitis B or C viral infections and alcohol abuse as a key player for HCC development. However, the exact molecular mechanisms and gatekeepers accounting for cellular transformation remain elusive. In this issue of Cancer Cell, Haybaeck et al describe the role of sustained lymphotoxin signaling in HCC development (Haybaeck et al., 2009). The authors found aberrant expression of lymphotoxins α, β and LTβ receptor in human samples, and by applying numerous animal models, they tease out some relevant molecular mechanisms of hepatitis-induced HCC. Transgenic mice over-expressing lymphotoxins α and β in hepatocytes developed chronic hepatitis at 9 months, and HCC at 12 months, which ultimately disseminate. To investigate the mechanisms by which hepatitis and HCC were induced, the authors crossed these mice with 4 different know-outs (TNF receptor 1, TNF receptor 2, hepatocyte IKK kinase β, and mature B and T lymphocytes depletion), showing that NF-κB pathway is important for liver inflammation and HCC development.

The identification of lymphotoxin activated signaling as key in HCC development was confirmed by the fact that the administration of lymphotoxin antagonists during two-months in mice with chronic hepatitis reduced inflammatory activity and prevented the development of HCC as compared with control animals. Overall, these data introduces lymphotoxins as a novel family of potential targets for HCC chemoprevention, aids to clarify the role of NF-κB signaling in HCC formation, and suggests anti-fibrotic activity by blocking lymphotoxin signaling. In addition, it complements previous knowledge on the role of NF-κB in HCC (Karin, 2006), and supports efforts to evaluate pharmacologic modulation of this pathway in the clinical arena.

Despite several attempts, there is no efficacious drug for prevention of HCC development in cirrhotic patients. Chronic administration of interferon failed to show significant benefits in preventing cancer development in patients with advanced fibrosis or cirrhosis (Di Bisceglie, NEJM 2008). This disappointing scenario results from the elementary understanding of the key gatekeepers and early molecular events leading to human hepatocarcinogenesis and also reflects the lack of an accurate marker to identify patients at high risk for HCC development. The potential role of growth factors in this process has been highlighted by recent studies with human and experimental data. A case-control study found a 4-fold increased risk of HCC development in cirrhotic patients with a specific single nucleotide polymorphism located in the epidermal growth factor (EGF) gene (Tanabe et al., 2008). Subsequent experiments showed that this polymorphism favors EGF stability, and thus, enhances signaling through the EGF receptor in vitro. Furthermore, gefitinib, a selective EGF receptor tyrosine kinase inhibitor, was able to prevent tumor development in a chemical-induced rat model of HCC (Schiffer et al., 2005). Another growth factor cascade, the platelet-derived growth factor (PDGF) pathway, has also been implicated in the transition from liver inflammation to HCC. Transgenic mice over-expressing PDGFR-C adequately recapitulate the sequence of histological events that precede HCC in humans (i.e. esteatosis, fibrosis, and dysplasia), ultimately developing HCC at 9 months of age (Campbell et al., 2005). (Figure 1)

Figure 1. Potential targets of prevention and treatment of hepatocellular carcinoma (HCC).

Viral hepatitis and alcohol abuse are the main risk factors for developing chronic liver inflammation leading to advanced fibrosis and cirrhosis. Once advanced fibrosis has been established, evidence suggests the implication of inflammatory signals and growth factor cascades in the development of HCC. In this setting, chemoprevention strategies are expected to target key pro-oncogenic molecules. In established HCC, the molecular classification of the tumors would allow targeting specific pathways and oncogenic loops in a more personalized manner.

The advent of high-throughput genomic technologies has allowed pinpointing potential gatekeepers and defining at-risk populations for tumor recurrence. Two recent translational studies have underscored the role of liver inflammation in the initiation and dissemination of HCC. In the first study conducted in more than 300 HCC patients, downstream targets of interleukin-6 were strongly enriched in a gene signature able to identify patients with poor survival after surgical resection (Hoshida et al., 2008). Prognosis of these patients was mainly determined by the occurrence of de novo HCC, suggesting that the signature was capturing molecular features related to new primary tumors arising in an already damaged organ (“field effect”). The second study, conducted mainly on HBV cirrhotic patients, identified a gene signature correlated to the risk of developing intra-hepatic metastasis (Budhu et al., 2006). This signature showed a marked increase in Th2 cytokines implying that an anti-inflammatory status precedes patients with metastatic HCC (Figure 1). In the current study, lymphotoxin transgenic mice developed multifocal tumors with identical chromosomal aberrations, indicating clonal spread from the primary tumor. The implication of this pathway in intra-hepatic dissemination also suggests that lymphotoxin antagonists could play a role in the treatment of overt HCC. Unfortunately, lymphotoxin pathway inhibitors, such as baminercept (Biogen Idec, Cambrigde, MA), have so far only been tested in inflammatory diseases (e.g. rheumatoid arthritis).

Different investigators have identified subgroups of HCC patients based on homologies in their tumor gene expression profiles. Among them, samples with activation of WNT canonical pathway, and those enriched in genes related to cell cycling and proliferation are common among different studies. In addition, genomic data suggest that a subset of tumors (~15–20%) may derive from a progenitor origin. Interestingly, Haybaeck et al. observed proliferation of A6+ oval cells in chronically inflamed lymphotoxin transgenic livers. Whether this pathway is relevant in driving cancer stem cells proliferation remains to be explored.

From the chemopreventive standpoint, it is plausible that modulation of the inflammatory response, (e.g. targeting IL-6, NF-ΚB and lymphotoxin pathways) along with abrogation of different growth factor pathways (e.g. EGFR and PDGFR inhibitors) will be required to counteract some of the numerous oncogenic signals present in cirrhotic tissue. In this sense, an adequate monitoring of side effects related to new molecular therapies will be essential. Selection of the appropriate chemoprevention strategy will depend on the predominant molecular mechanism present in a given patient. Once the tumor has been developed, integration of the genomic information from the tumor and the adjacent cirrhotic tissue with other clinical variables will determine patient prognosis and guide therapeutic decision-making. In this sense, data presented by Haybaeck et al. regarding lymphotoxin pathway provides scientists with a novel molecular bridge between hepatic inflammation and oncogenesis, opening new opportunities for selective therapies in the setting of prevention and treatment of primary liver cancers.

Abbreviations

HCV

hepatitis C virus

HBV

hepatitis B virus

NASH

non-alcoholic steatohepatitis

Il-6

interleukin-6

EGF

epidermal growth factor

PDGF

platelet-derived growth factor