Show me your signaling– and I’ll tell you who you are
Authors: Jörn M. Schattenberg, Peter R. Galle
Journal: Journal of Hepatology (2009)

Ras pathway activation in hepatocellular carcinoma and anti-tumoral effect of combined sorafenib and rapamycin in vivoJournal of HepatologyVol. 51Issue 4PreviewThe success of sorafenib in the treatment of advanced hepatocellular carcinoma (HCC) has focused interest on the role of Ras signaling in this malignancy. We investigated the molecular alterations of the Ras pathway in HCC and the antineoplastic effects of sorafenib in combination with rapamycin, an inhibitor of mTOR pathway, in experimental models. Full-Text PDF Cancer research and therapy have come a long way: the field started out in search of a "magic bullet" according to Paul Ehrlich's theory, and was hoping to identify a target which was pivotal to signaling survival in transformed cells. Indeed, certain diseases with monocausal mutations were identified, and targeting of the mutational products has helped in the design of treatment strategies. In chronic myeloid leukemia (CML), the constitutive activation of the tyrosine kinase BCR-ABL is pathognomonic [[1]Daley G.Q. Van Etten R.A. Baltimore D. Induction of chronic myelogenous leukemia in mice by the P210bcr/abl gene of the Philadelphia chromosome.Science. 1990; 247: 824-830Crossref PubMed Scopus (1930) Google Scholar], and multiple BCR-ABL kinase inhibitors (e.g. imatinib mesylate, dasatinib, nilotinib) have been developed and successfully used in the treatment of CML offering near-normal life expectancy to patients under continuous therapy. The success of these compounds has inspired research in the field of specific kinase inhibitors. Subsequently, it was recognized that the developed kinase inhibitors did not exhibit a high degree of selectivity, but rather caused inhibition of several kinases [[2]Chaparro M. Gonzalez M.L. Trapero-Marugan M. Medina J. Moreno-Otero R. Review article: pharmacological therapy for hepatocellular carcinoma with sorafenib and other oral agents.Aliment Pharmacol Ther. 2008; 28: 1269-1277Crossref PubMed Scopus (57) Google Scholar]. Unfortunately, the success of tyrosine kinase inhibition in haematological malignancies could not be readily translated to solid tumors which exhibit a comparatively high degree of genomic alterations of signaling pathways. Nevertheless, based on the high expression levels of EGFR and HER-2 in solid tumors, EGFR inhibitors such as gefitinib and erlotinib were developed. Their implementation in various malignancies resulted in clinically meaningful, although moderate improvement of overall survival for different tumor entities and they were first introduced for non-small-cell lung cancer [[3]Ghoreschi K. Laurence A. O'Shea J.J. Selectivity and therapeutic inhibition of kinases: to be or not to be?.Nat Immunol. 2009; 10: 356-360Crossref PubMed Scopus (202) Google Scholar]. Other tumor entities followed and currently eight kinase inhibitors are FDA-approved. The indication for use includes hepatocellular carcinoma (HCC), where a major breakthrough was achieved in the SHARP trial in 2008 which demonstrated improved survival in patients with advanced HCC receiving sorafenib [[4]Llovet J.M. Ricci S. Mazzaferro V. Hilgard P. Gane E. Blanc J.F. et al.Sorafenib in advanced hepatocellular carcinoma.N Engl J Med. 2008; 359: 378-390Crossref PubMed Scopus (9429) Google Scholar]. Sorafenib was shown to inhibit roughly 15 kinases at nanomolar potency [[2]Chaparro M. Gonzalez M.L. Trapero-Marugan M. Medina J. Moreno-Otero R. Review article: pharmacological therapy for hepatocellular carcinoma with sorafenib and other oral agents.Aliment Pharmacol Ther. 2008; 28: 1269-1277Crossref PubMed Scopus (57) Google Scholar] supporting the concept that a "dirty inhibitory effect" on multiple kinases rather than on a single signaling molecule will lead to recognizable responses in the treatment of solid organ tumors including HCC [[4]Llovet J.M. Ricci S. Mazzaferro V. Hilgard P. Gane E. Blanc J.F. et al.Sorafenib in advanced hepatocellular carcinoma.N Engl J Med. 2008; 359: 378-390Crossref PubMed Scopus (9429) Google Scholar]. As a consequence, the precise mode of action of sorafenib needs further clarification. The work presented by Newell and colleagues in the current issue of the Journal of Hepatology[[5]Newell P. Toffanin S. Villanueva A. Chiang D.Y. Minguez B. Cabellos L. et al.Ras pathway activation in hepatocellular carcinoma and anti-tumoral effect of combined sorafenib and rapamycin in vivo.J Hepatol. 2009; 51: 725-733Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar] examines the antineoplastic potential of combining sorafenib with rapamycin, an inhibitor of the mTOR pathway, in experimental hepatocarcinogenesis. In agreement with an earlier study by Wang et al. [[6]Wang Z. Zhou J. Fan J. Qiu S.J. Yu Y. Huang X.W. et al.Effect of rapamycin alone and in combination with sorafenib in an orthotopic model of human hepatocellular carcinoma.Clin Cancer Res. 2008; 14: 5124-5130Crossref PubMed Scopus (130) Google Scholar], the combination of the two compounds lead to an additive effect with respect to reduced cell proliferation, increased cancer cell apoptosis and increased tumor necrosis in a xenograft model of HCC. The combination of these two agents is appealing since two independent signaling pathways which have been both implicated in the pathophysiology of HCC are targeted [[7]Villanueva A. Chiang D.Y. Newell P. Peix J. Thung S. Alsinet C. et al.Pivotal role of mTOR signaling in hepatocellular carcinoma.Gastroenterology. 2008; 135: 1972-1983Abstract Full Text Full Text PDF PubMed Scopus (611) Google Scholar]. While sorafenib inhibits the Ras/Raf/MEK/Erk pathway [[8]Wilhelm S.M. Adnane L. Newell P. Villanueva A. Llovet J.M. Lynch M. Preclinical overview of sorafenib, a multikinase inhibitor that targets both Raf and VEGF and PDGF receptor tyrosine kinase signaling.Mol Cancer Ther. 2008; 7: 3129-3140Crossref PubMed Scopus (1158) Google Scholar], rapamycin inhibits activation of the mTOR complex downstream of the phosphoinositide 3-kinase (PI3K) and Akt. Both pathways promote proliferation in HCC [[9]Pang R.W. Poon R.T. From molecular biology to targeted therapies for hepatocellular carcinoma: the future is now.Oncology. 2007; 72: 30-44Crossref PubMed Scopus (159) Google Scholar]. Additional evidence supporting the rationale of simultaneous inhibition of more than one signaling pathway comes from studies showing a compensatory overactivation of one parallel growth promoting pathway following signal inhibition of another. In this sense mTOR inhibition can result in compensatory activation of the Ras signaling pathway leading to cancer cell resistance [[10]Carracedo A. Ma L. Teruya-Feldstein J. Rojo F. Salmena L. Alimonti A. et al.Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer.J Clin Invest. 2008; 118: 3065-3074PubMed Google Scholar] and thus, additional Ras inhibition potentially counteracts such a compensatory mechanism. Apart from the biological response in the xenograft model, the authors found changes in methylation patterns of oncogens of the Ras pathway in advanced tumor stages. It is difficult to assess the biological meaning of such observations and it is likely that many changes that are seen in dedifferentiated tumor tissue occur without significantly contributing to tumor biology and thus the relevance of those in vitro findings in predicting potential targets or markers that can be useful in clinical trials is questionable. In order to separate signal from noise, to identify predominant signaling pathways in solid tumors, defining tumor biology, the concept of oncogenic addiction has been developed [[11]Weinstein I.B. Joe A.K. Mechanisms of disease: oncogene addiction – a rationale for molecular targeting in cancer therapy.Nat Clin Pract Oncol. 2006; 3: 448-457Crossref PubMed Scopus (567) Google Scholar]. This concept was derived from observations revealing tumorous tissue to be dependent on only one or a few genes for cell survival and maintaining the malignant phenotype despite a high degree of genetic and epigenetic abnormalities [[11]Weinstein I.B. Joe A.K. Mechanisms of disease: oncogene addiction – a rationale for molecular targeting in cancer therapy.Nat Clin Pract Oncol. 2006; 3: 448-457Crossref PubMed Scopus (567) Google Scholar]. Identification of these pivotal pathways which are crucial to tumor cell survival will advance the success in the treatment of HCC rapidly. In addition to the above mentioned pathways, important targets to be assessed for combination therapy include the Wnt/β-catenin pathway, cell cycle regulators (CDK), angiogenic factors (VEGFR and PDGFR) and proteases such as MMP-14, MMP-9 or topoisomerase [[12]Llovet J.M. Bruix J. Molecular targeted therapies in hepatocellular carcinoma.Hepatology. 2008; 48: 1312-1327Crossref PubMed Scopus (905) Google Scholar]. As long as tumor defining signaling pathways can not be defined precisely enough to develop highly selective inhibitors of downstream targets aimed to inhibit the oncogenes, which are crucial to tumor cell growth, low selectivity as seen with sorafenib will help to achieve an anti-tumor effects despite the lack of knowledge of the exact mechanisms. Moreover, the combined inhibition through unspecificity or the addition of a second pathway inhibitor might help to overcome resistance attributable to activation of multiple cell growth pathways. The challenge of future research in this field will be to engineer reliable systems that predict targets which will lead to inhibition of the pathway the tumor "is addicted to". In the age past the "Human Genome Project" tumor biology will have to develop treatment strategies detached from a deterministic, genetic view and produce "pathway fingerprints" of a tumor which can predict the individualized response towards inhibition of kinases or combinations thereof. These concepts will rely on system biology which will help to predict the response of tumors to pathway inhibition depending on the predominant tumor biology and to define future targets of therapy. However the biggest challenge will remain, regardless how good the rationale for testing drugs has been elaborated in vitro: clinical trials still have to be used in the end to define the benefit for the patient and here we have seen many surprises of success and failure previously not predicted on theoretical grounds.