Targeting FGFR4 for Therapy of Colorectal Cancer

Colorectal Cancer (CRC) is a common cancer in western industrialized countries and in spite of recent advances in therapy it is still highly resistant to most treatment options. Consequently, identification of new therapeutic targets is essential to improve therapeutic options for CRC. Over the past years FGF/FGFR signaling has increasingly drawn interest in this respect, because most malignancies investigated have presented with deregulation of some aspect of FGF-signaling. In CRC progressive up-regulation of FGF18 during tumor development has been shown to establish an autocrine growth and survival loop. Among the receptors capable of mediating this effect FGFR4 that has been shown to be up-regulated in about 50% of CRC tissue specimens by our own recent analysis. Consequently, FGFR4 may be a suitable target for CRC therapy. However, the situation is complicated by the presence of a polymorphic allele producing a receptor variant with a gly388arg amino acid substitution in the transmembrane domain. Divergent activities have been reported for the polymorphic variants with FGFR4 arg being associated with either higher cancer risk or more aggressive disease. We have therefore, initiated an investigation of the oncogenic impact and cellular mechanisms of action of FGFR4 in CRC with the ultimate aim to characterize the receptor as a therapeutic target. The first stage of this project has demonstrated clear oncogenic effects for both polymorphic forms of FGFR4. Results also indicated that standard tissue culture conditions may not be sufficient to assess the role of FGFR4 in colorectal cancer. Consequently, it is the objective of this follow-up project to evaluate the impact of FGFR4 in 3-dimensional (3-D) culture models as well as in SCID-mouse xenotransplants. In detail, we plan to (1) investigate the cellular mechanisms of FGFR4- mediated oncogenic effects in a 3-D setting and (2) develop tools to block or knock-down FGFR4 and assess the consequences of this blockade on tumor cell growth and invasion in vitro as well as in vivo. Lastly (3) we will evaluate available small molecule inhibitors of FGFRs as to their activity towards FGFR4. The expected results will contribute to development of therapeutic strategies that may improve outcome and prognosis for CRC patients.

Project P23693 studied the role of fibroblast growth factor 4 (FGFR4) in colorectal cancer. There is a polymorphic variant of the FGFR4 gene coding for an amino acid exchange from glycine (FGFR4gly) to arginine (FGFR4arg) in the transmembrane domain of the receptor molecule. 55% of the population carry at least one copy of the polymorphic gene, which is associated with more aggressive tumors at several tumor sites including the colon and rectum.Results of an earlier project (P19920) had shown that fibroblast growth factor 18 (FGF18), an FGFR4 ligand, was up-regulated in the majority of colorectal cancers. FGFR4 itself was upregulated in about 25% of colorectal tumors. The presence of an FGFR4arg allele was actually associated with more advanced, aggressive disease.Results of this project showed, that both forms of FGFR4 had oncogenic impact in colorectal cancer cells: the FGFR4gly form stimulated malignant growth in vitro and in vivo, while the FGFR4arg form enhanced cell migration and metastasis. Genetic blockade by gene knock- down or dominant-negative receptor constructs inhibited both malignant growth and cell migration in vitro and in vivo. Chemical blockade of the receptor kinase activity had no impact on tumor cell growth however. This suggests that the cellular mechanisms underlying FGFR4- effects are not exclusively regulated by the receptor kinase activity. This observation complicates development of FGFR4-targeting therapies.One important cellular target of FGFR4 we could identify was RAD51. The protein is a key regulator of homologous-recombination repair that removes radiation-induced DNA-double strand breaks in an error-free manner. Cells with high FGFR4 expression more efficiently induce RAD51 after irradiation and RAD51 protein levels are maintained longer than in control cells. Consequently, repair of therapy-induced strand-breaks is more efficient and radiation- induced apoptosis is hampered. Our results suggest that FGFR4 expression can be used as a predictive marker for radiation therapy response in rectal cancer patients. Using in vitro CRC models, we found that inhibition of FGFR-kinase enhanced radiation response indicating that in this context combination with an FGFR4-targeting therapy may be an option.In summary, results of the project demonstrated that FGFR4 stimulates tumor growth, metastasis and resistance to radiation therapy. Synergistic impact of FGFR4 targeting and radiation needs to be further explored in future projects.