A new link between cancer and inflammation?☆
Article Outline
Frequent in-frame somatic deletions activate gp130 in inflammatory hepatocellular tumours. Rebouissou S, Amessou M, Couchy G, Poussin K, Imbeaud S, Pilati C, Izard T, Balabaud C, Bioulac-Sage P, Zucman-Rossi J.
Inflammatory hepatocellular adenomas are benign liver tumours defined by the presence of inflammatory infiltrates and by the increased expression of inflammatory proteins in tumour hepatocytes. Here we show a marked activation of the interleukin (IL)-6 signalling pathway in this tumour type; sequencing candidate genes pinpointed this response to somatic gain-of-function mutations in the IL6ST gene, which encodes the signalling co-receptor gp130. Indeed, 60% of inflammatory hepatocellular adenomas harbour small in-frame deletions that target the binding site of gp130 for IL-6, and expression of four different gp130 mutants in hepatocellular cells activates signal transducer and activator of transcription 3 (STAT3) in the absence of ligand. Furthermore, analysis of hepatocellular carcinomas revealed that rare gp130 alterations are always accompanied by beta-catenin-activating mutations, suggesting a cooperative effect of these signalling pathways in the malignant conversion of hepatocytes. The recurrent gain-of-function gp130 mutations in these human hepatocellular adenomas fully explains activation of the acute inflammatory phase observed in tumourous hepatocytes, and suggests that similar alterations may occur in other inflammatory epithelial tumours with STAT3 activation.
[Abstract reproduced by permission of Nature 2009;457:200–204.]
Hepatocellular adenoma (HCA) is a rare benign liver tumor mostly affecting younger females and it poses relatively few clinico-therapeutic problems, once its diagnosis has been established. Recently, HCA has become a rewarding topic for molecular studies, which were mainly driven by Jessica Zucman-Rossi and her group. Their recent paper [1] is the latest in a series of publications that step by step have unravelled the molecular pathogenesis and classification of HCA [1], [2], [3], [4], [5]. A definite prerequisite for this task was the persistent and prospective collection of a sufficient number of representative and relevant cases and their precise morphological characterization peered by Paulette Bioulac-Sage. Morphology can subtype HCA into cases with extensive fatty change (∼35–45%), those with prominent inflammation and telangiectasia (∼30–35%), a minor group of atypical/questionable/‘borderline’ cases (with higher risk of malignant transformation) (10–15%), and a waste basket of cases not belonging to any of the other groups (10–15%). In contrast to the extremely complex molecular pathology of hepatocellular carcinoma (HCC) different subtypes of HCA can be assigned to defined molecular changes now. Zucman-Rossi and co-workers demonstrated that HCAs with prominent fatty change show inactivating mutations of HNF1α, which leads to reduced or even lost expression of Liver-Fatty Acid Binding Protein (L-FABP) [4], [5]. Atypical HCAs occur more frequently in males and carry activating β-catenin mutations, which can be identified immunohistologically by overexpression of glutamine synthetase. This subtype frequently shows cytological abnormalities and pseudo-glandular formation, thus sharing features with highly differentiated HCCs. Previously, the group obtained evidence that the rare ‘telangiectatic focal nodular hyperplasia’ is a clonal lesion belonging to the inflammatory HCA subtype (IHCA) [2]. Now, the paper of Rebouissou et al. [1] addresses IHCAs, which have escaped molecular definition so far. They are characterized by a polymorphic inflammatory cell infiltrate and activation of acute phase proteins, such as C-reactive protein and serum amyloid A, and predominantely occur in women with obesity and/or drinking history.
Using transcript profiling they found (not surprisingly) activation of many acute phase type 1 and type 2 genes including a JAK-STAT signature pinpointing to a crucial role of IL-6 and interferon signalling. Interestingly, this constellation matches data obtained in HCCs showing an interferon signature in a significant number of cases, which seems to be associated with features of better prognosis [6]. Since IL-6 was not overexpressed they reasoned that a crucial upstream member of the IL-6-induced signalling pathway might have been mutated. They concentrated on the gp130 co-receptor and – bingo – found the bonanza. Of the analysed IHCA, 60% contained somatic mutations of gp130, frequently small in-frame deletions. All mutations were monoallelic and coexpressed along with the wild-type allele suggesting a dominant (gain of function) effect of the mutations. Modelling predicted that the mutations may disrupt the gp130/IL-6 interaction and transfection of a respectively mutated gp130 construct into Hep3B cells resulted in the activation of acute phase response genes without the need for exogenous IL-6. The structural correlate was the ability of mutated gp130 to spontaneously homodimerize or heterodimerize with wt gp130, while wt gp130 alone essentially required binding to IL-6 for this step. Among the downstream mediators of gp130, which seems to act via activated nuclear pSTAT3, are HIF1A, which probably induces the telangiectatic phenotype via VEGF, and CCL20, which appears to be a broad spectrum chemoattractant for a variety of inflammatory cells. In the remaining 40% of IHCA extensive searches failed to identify further responsible mutations or chromosomal imbalances, but overexpression of gp130 and nuclear activation of pSTAT3 were present in a comparable manner. IHCA have a low transformation potential (⩽5%), which can be attributed to the coexistence of both, gp130-mutations and activated ß-catenin.
What do we learn from this elegant paper?
Firstly, it offers an interesting connection between a defined molecular tumor cell alteration and tumor-associated inflammatory response involving the constitutive activation of a tumor cell-derived cytokine signalling pathway. Other comparable ‘inside-out’ mechanisms have previously been demonstrated especially for mutations in oncogenic factors simultaneously generating tumor-specific, highly antigenic molecules, e.g. in malignant melanoma [7], or as a frequent event occurring in microsatellite instable cancer, driven by the underlying repair deficiency [8], both attracting a mainly T-cell derived inflammatory response.
Secondly, the story appears to have an additional facet, since constitutive activation of gp130 may not only trigger an intratumoral inflammation, but appears to show its janus face by activation of a cell autonomous protumorigenic response. So far it is too early to call gp130 an oncogene, as suggested by the authors, since the connection to malignant transformation is weak and indirect, but it certainly deserves attention; interestingly, ß-catenin mutations may act complementary, an aspect to be tested further.
The third aspect is the extension of a wonderful genotype–phenotype–correlation: specific molecular changes obviously correlate with typical morphological alterations of HCA [9] and the affected genes offer plausible explanations for the resulting phenotypes.
Should we separate hepatocellular adenoma into different entities now? Although both phenotypic and genotypic features allow HCA subtyping in a robust manner, it has little impact on clinical management and prognosis (excluding those rare HCAs with ß-catenin activation), therefore leaving a sufficiently coherent entity. Thus we better keep the entity HCA as a whole, but at the same time be aware of its molecular diversity.
Do diagnostic consequences arise from this work as a whole? Yes, they do. By understanding the molecular pathogenesis we have reliable confirmation that morphological diagnosis is a solid and rational primary measure to diagnose and subtype HCA. Using the defined molecular alterations we have a panel of immunohistological markers in hand, which support the difficult differential diagnosis of highly differentiated hepatocellular tumors and define those rare HCAs with an increased risk of malignant transformation.
The paper is a significant leap forward, but naturally open questions remain: the genetic and/or epigenetic alterations in the remaining 40% of IHCA without gp130 mutations and also the ‘waste basket’ of about <10% of HCA not belonging to any of the so far defined subtypes have to be yet identified. Furthermore, other tumor entities, especially those displaying a polymorphic inflammatory infiltrate, should be checked for molecular alterations in the IL-6 signalling pathway, especially gp130. We may not be exempt from further surprises.
References
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☆ The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.
PII: S0168-8278(09)00165-2
doi:10.1016/j.jhep.2009.03.004
© 2009 Published by Elsevier Inc.
