A dual role for interferon γ signalling in hepatocellular carcinoma
Authors: Robert Eferl
Journal: Journal of Hepatology (2012)

Deletion of IFNγ enhances hepatocarcinogenesis in FXR knockout miceJournal of HepatologyVol. 57Issue 5PreviewLiver tumor, especially hepatocellular carcinoma (HCC), is closely associated with chronic inflammation. We previously showed that farnesoid X receptor knockout (FXR−/−) mice displayed chronic inflammation and developed spontaneous liver tumors when they aged. However, the mechanism by which inflammation leads to HCC in the absence of FXR is unclear. Because IFNγ is one of the most upregulated pro-inflammatory cytokines in FXR−/− livers, we generated IFNγ−/−FXR−/− double knockout mice to determine IFNγ's roles in hepatocarcinogenesis. Full-Text PDF Hepatocellular carcinoma (HCC) is a malignant disease with heterogeneous aetiology. Despite this heterogeneity, HCCs have been classified into distinct subtypes based on candidate approaches or global screening methods [1Calvisi D.F. Ladu S. Gorden A. Farina M. Lee J.S. Conner E.A. et al.Mechanistic and prognostic significance of aberrant methylation in the molecular pathogenesis of human hepatocellular carcinoma.J Clin Invest. 2007; 117: 2713-2722Crossref PubMed Scopus (336) Google Scholar, 2Hou J. Lin L. Zhou W. Wang Z. Ding G. Dong Q. et al.Identification of miRNomes in human liver and hepatocellular carcinoma reveals miR-199a/b-3p as therapeutic target for hepatocellular carcinoma.Cancer Cell. 2011; 19: 232-243Abstract Full Text Full Text PDF PubMed Scopus (609) Google Scholar, 3Laurent-Puig P. Legoix P. Bluteau O. Belghiti J. Franco D. Binot F. et al.Genetic alterations associated with hepatocellular carcinomas define distinct pathways of hepatocarcinogenesis.Gastroenterology. 2001; 120: 1763-1773Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar, 4Lee J.S. Chu I.S. Heo J. Calvisi D.F. Sun Z. Roskams T. et al.Classification and prediction of survival in hepatocellular carcinoma by gene expression profiling.Hepatology. 2004; 40: 667-676Crossref PubMed Scopus (721) Google Scholar, 5Roessler S. Long E.L. Budhu A. Chen Y. Zhao X. Ji J. et al.Integrative genomic identification of genes on 8p associated with hepatocellular carcinoma progression and patient survival.Gastroenterology. 2012; 142 (e912): 957-966Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 6Shen J. Wang S. Zhang Y.J. Kappil M. Wu H.C. Kibriya M.G. et al.Genome-wide DNA methylation profiles in hepatocellular carcinoma.Hepatology. 2012; 55: 1799-1808Crossref PubMed Scopus (155) Google Scholar]. Moreover, whole genome sequencing of HCCs and putative secondary growths have defined genetic changes associated with intrahepatic metastasis [[7]Tao Y. Ruan J. Yeh S.H. Lu X. Wang Y. Zhai W. et al.Rapid growth of a hepatocellular carcinoma and the driving mutations revealed by cell-population genetic analysis of whole-genome data.Proc Natl Acad Sci USA. 2011; 108: 12042-12047Crossref PubMed Scopus (112) Google Scholar]. However, the identification of driver mutations in HCC is still a challenging task [[8]Sawey E.T. Chanrion M. Cai C. Wu G. Zhang J. Zender L. et al.Identification of a therapeutic strategy targeting amplified FGF19 in liver cancer by oncogenomic screening.Cancer Cell. 2011; 19: 347-358Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar]. Mouse models are valuable tools to identify the contribution of defined genetic changes to HCC formation. As recently reviewed by Gen-Sheng Feng, controversial results have been obtained in mice for several candidate genes that seemed to display both, tumour-promoting and tumour-suppressive activities [[9]Feng G.S. Conflicting roles of molecules in hepatocarcinogenesis: paradigm or paradox.Cancer Cell. 2012; 21: 150-154Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar]. These controversial findings are reconciled by the fact that gene activities can influence HCC formation by two mechanisms: (1) a candidate gene is required for tumour cell proliferation or survival (oncogenic function); (2) a candidate gene is essential for survival of hepatocytes, thereby preventing chronic liver injury and regeneration (anti-oncogenic function). These mechanisms might operate in all tissues with high regenerative capacity. Consequently, chronic liver injury induced by ablation of an essential candidate gene could drive formation of HCCs with specific mutations that override tumour cell-autonomous requirements for the candidate gene. This dual mechanism exacerbates our interpretations of cell-autonomous gene functions in HCC formation when corresponding mouse models differ from control groups with respect to chronic liver injury. Moreover, many models do not reflect the human aetiology of HCC, which is associated with inflammation, chronic liver damage, and liver fibrosis. In this issue of the Journal of Hepatology, Meng et al. employed mice lacking the farnesoid X receptor (FXR−/−) to investigate oncogenic functions of type II interferon (IFNγ) signalling in HCC formation [[10]Meng Z. Wang X. Gan Y. Zhang Y. Zhou H. Van Ness C. et al.Deletion of IFNγ enhances hepatocarcinogenesis in FXR knockout mice.J Hepatol. 2012; 57: 1004-1012Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. FXR−/− mice represent a model of HCC induced by metabolic dysfunction and the authors have recently demonstrated that this model can recapitulate several aspects of human HCC [[11]Liu N. Meng Z. Lou G. Zhou W. Wang X. Zhang Y. et al.Hepatocarcinogenesis in FXR−/− mice mimics human HCC progression that operates through HNF1alpha regulation of FXR expression.Mol Endocrinol. 2011; 26: 775-785Crossref Scopus (86) Google Scholar]. They showed that FXR downregulation in human HCCs was mediated by cytokines that interfered with HNF1α (hepatocyte nuclear factor 1α) chromatin binding and proposed multiple mechanisms on how loss of FXR could support hepatocarcinogenesis [[11]Liu N. Meng Z. Lou G. Zhou W. Wang X. Zhang Y. et al.Hepatocarcinogenesis in FXR−/− mice mimics human HCC progression that operates through HNF1alpha regulation of FXR expression.Mol Endocrinol. 2011; 26: 775-785Crossref Scopus (86) Google Scholar]. FXR is required for hepatocyte survival thereby preventing chronic liver injury but also regulates expression of target genes, such as SHP (small heterodimer partner), that are implicated in hepatocarcinogenesis. Moreover, chronic liver injury in FXR−/− mice led to infiltration of macrophages and elevated hepatic expression of inflammatory cytokines which induced abnormal hepatocyte proliferation, a driving force for expansion of transformed cells [[11]Liu N. Meng Z. Lou G. Zhou W. Wang X. Zhang Y. et al.Hepatocarcinogenesis in FXR−/− mice mimics human HCC progression that operates through HNF1alpha regulation of FXR expression.Mol Endocrinol. 2011; 26: 775-785Crossref Scopus (86) Google Scholar]. One of the most prominently upregulated cytokines in FXR−/− mice was IFNγ [[12]Yang F. Huang X. Yi T. Yen Y. Moore D.D. Huang W. Spontaneous development of liver tumors in the absence of the bile acid receptor farnesoid X receptor.Cancer Res. 2007; 67: 863-867Crossref PubMed Scopus (389) Google Scholar]. This cytokine, however, suppresses formation of various tumour types by cell-autonomous and non-cell-autonomous mechanisms. IFNγ stimulates immune surveillance functions of the innate immune system and can induce expression of immunomodulatory factors such as MHC (major histocompatibility complex) molecules in tumour cells. Moreover, IFNγ signalling interferes with tumour cell proliferation and promotes apoptosis. Many of these functions are mediated by the transcription factor STAT1 (signal transducer and activator of transcription 1). Consistent with the anti-proliferative and pro-apoptotic activities of IFNγ/STAT1 signals, tumour cells try to escape from this pathway and acquire resistance by different means (downregulation of IFNγ receptors; deletion of the genomic IFNγ locus; promoter methylation of STAT1 or other factors implicated in IFNγ signalling which makes tumour cells refractory to IFNγ signals) [[13]Horras C.J. Lamb C.L. Mitchell K.A. Regulation of hepatocyte fate by interferon-gamma.Cytokine Growth Factor Rev. 2011; 22: 35-43Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar]. The tumour-suppressive activity of IFNγ/STAT1 is partly due to mutually interfering interactions between STAT1 and the closely related transcription factor STAT3 (signal transducer and activator of transcription 3). In contrast to STAT1, STAT3 is considered to promote formation of many tumours including HCC. However, administration of the carcinogen DEN (diethylnitrosamine) in mice with conditional inactivation of STAT3 in the liver has provided controversial results with two studies suggesting an oncogenic activity [14He G. Yu G.Y. Temkin V. Ogata H. Kuntzen C. Sakurai T. et al.Hepatocyte IKKbeta/NF-kappaB inhibits tumor promotion and progression by preventing oxidative stress-driven STAT3 activation.Cancer Cell. 2010; 17: 286-297Abstract Full Text Full Text PDF PubMed Scopus (359) Google Scholar, 15Wang H. Lafdil F. Wang L. Park O. Yin S. Niu J. et al.Hepatoprotective versus oncogenic functions of STAT3 in liver tumorigenesis.Am J Pathol. 2011; 179: 714-724Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar] and one study suggesting an anti-oncogenic activity of STAT3 [[16]Bard-Chapeau E.A. Li S. Ding J. Zhang S.S. Zhu H.H. Princen F. et al.Ptpn11/Shp2 acts as a tumor suppressor in hepatocellular carcinogenesis.Cancer Cell. 2011; 19: 629-639Abstract Full Text Full Text PDF PubMed Scopus (266) Google Scholar]. We have performed a similar experiment and found significant reduction of DEN-induced HCC formation in the absence of STAT3 (unpublished) suggesting an oncogenic function. The mutual interaction between STAT1 and STAT3 became particularly apparent when STAT1 or STAT3 activities were selectively ablated. Deletion of STAT1 switched the pro-apoptotic and anti-proliferative activities of IFNs to survival and proliferation signals in various cell types. This functional switch was at least in part due to aberrant activation of STAT3 by IFN signals that occurred only in the absence of STAT1. In contrast, STAT1 was activated by glycoprotein gp130-inducing signals when STAT3 was ablated. Consequently, STAT3-deficient fibroblasts responded to IL-6 (interleukin-6) with substantial STAT1 activation that resulted in unusual IL-6-mediated activation of IFN responsive genes [[17]Mair M. Blaas L. Osterreicher C.H. Casanova E. Eferl R. JAK–STAT signaling in hepatic fibrosis.Front Biosci. 2011; 17: 2794-2811Crossref Scopus (48) Google Scholar]. The molecular mechanisms that underlie the reciprocal influence of STAT1 and STAT3 activation include competition for common cytokine receptors, the implication of SOCS (suppressor of cytokine) proteins and the mutual inactivation of STATs by sequestration into STAT1:STAT3 heterodimer complexes [[18]Murray P.J. The JAK–STAT signaling pathway: input and output integration.J Immunol. 2007; 178: 2623-2629Crossref PubMed Scopus (913) Google Scholar]. The report by Meng et al. shed new light on the role of IFNγ and the mutual interplay of STAT transcription factors in hepatoprotection and HCC formation [[10]Meng Z. Wang X. Gan Y. Zhang Y. Zhou H. Van Ness C. et al.Deletion of IFNγ enhances hepatocarcinogenesis in FXR knockout mice.J Hepatol. 2012; 57: 1004-1012Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. Abolishing IFNγ signalling led to increased hepatic damage and tumour load in livers of IFNγ−/−, IFNγ−/− FXR−/− double-deficient and IFNγ−/− mice treated with DEN [[10]Meng Z. Wang X. Gan Y. Zhang Y. Zhou H. Van Ness C. et al.Deletion of IFNγ enhances hepatocarcinogenesis in FXR knockout mice.J Hepatol. 2012; 57: 1004-1012Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. These experiments provide new insights into tumour-suppressive activities of IFNγ signals that seem to operate not only in tumour cells but also through hepatoprotective mechanisms (Fig. 1). The hepatoprotective activity of IFNγ was age-dependent since old IFNγ−/− mice displayed hepatic inflammation and elevated levels of liver damage parameters that were not obvious in young mice. Interestingly, levels of IFNγ increased in humans and mice during ageing indicating that an age-dependent physiologic mechanism co-ordinately regulates IFNγ levels and IFNγ responsiveness of hepatocytes. Increased tumour load in double-deficient IFNγ−/− FXR−/− mice and IFNγ−/− mice treated with DEN might result from aggravated chronic liver damage and regeneration. Moreover, aggravated liver damage was accompanied by several molecular changes that promote HCC formation including STAT3 activation, reduction of NFκB activity, decreased p53 expression and elevated activation of JNK (Jun aminoterminal kinase) [[10]Meng Z. Wang X. Gan Y. Zhang Y. Zhou H. Van Ness C. et al.Deletion of IFNγ enhances hepatocarcinogenesis in FXR knockout mice.J Hepatol. 2012; 57: 1004-1012Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. STAT3 activation might be due to the aforementioned mutual interaction between IFNγ/STAT1 and IL-6/STAT3 signalling pathways, which was further demonstrated in vitro. Activation of JNK, mediated by reactive oxygen species (ROS) and blunted NFκB activation, could contribute to p53 downregulation via its target protein c-Jun, a transcription factor known to repress expression of p53 in hepatocarcinogenesis [[19]Eferl R. Ricci R. Kenner L. Zenz R. David J.P. Rath M. et al.Liver tumor development. c-Jun antagonizes the proapoptotic activity of p53.Cell. 2003; 112: 181-192Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar]. In this regard, JNK could act synergistically with known IFNγ/STAT1-dependent mechanisms for p53 regulation (Fig. 1). However, these mechanisms seemed to operate predominantly during HCC initiation because p53 expression was not affected in established tumours of IFNγ−/− FXR−/− mice. Recently, Katz et al. have investigated p53 functions in HCC. They demonstrated formation of HCCs with stem cell-like characteristics (Alb+ and K19+) in aged mice with conditional ablation of p53 in hepatocytes [[20]Katz S.F. Lechel A. Obenauf A.C. Begus-Nahrmann Y. Kraus J.M. Hoffmann E.M. et al.Disruption of Trp53 in livers of mice induces formation of carcinomas with bilineal differentiation.Gastroenterology. 2012; 142 (e1223): 1229-1239Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar]. It would be interesting to investigate whether the short window of p53 loss during tumour initiation is sufficient to promote formation of bilineal HCCs with stem cell-like characteristics in IFNγ−/− FXR−/− mice. In summary, the study by Meng et al. has proposed an important IFNγ-dependent cellular and molecular network between innate immune cells and hepatocytes. This network could provide protection from liver damage and HCC formation during viral hepatitis or autoimmune hepatitis that are associated with attraction of several hematopoietic producers of IFNγ. Paradoxically, gain and loss of IFNγ signalling lead to hepatocyte death but only loss of IFNγ promotes HCC formation. It remains to be investigated how the concerted action with additional IFNγ responses such as cell cycle arrest modulates anti-oncogenic activities of IFNγ-induced apoptosis. The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.