Expression of gemcitabine metabolizing enzymes in different pancreatic cancer model systems

Abstract

Pancreatic cancer still exhibits a dismal five-year overall survival rate of 9 % and is foreseen to even increase in incidence and mortality rates in the future. This is due to unspecific symptoms leading to late diagnosis, and a lack of effective screening procedures and treat-ment strategies. The chemotherapeutic drug gemcitabine is frequently used as a backbone agent in the clinical setting, but has failed to be as effective as in preclinical trials. One rea-son for this could be the discrepancy in expression of gemcitabine metabolizing enzymes between patients and in vitro and in vivo models. Moreover, the tumor microenvironment includes different cellular and acellular components of the stroma that have been implicated in chemoresistance. Recently, various in vivo models for pancreatic cancer have been estab-lished in the preclinical research setting to investigate mechanisms of therapeutic resistance. In particular, the transgenic KPC mouse model, the orthotopically transplanted mouse and the patient-derived xenograft mouse model are of current use. This work investigated the expression of gemcitabine metabolizing enzymes and stromal components in different pancreatic cancer model systems. This was achieved by immuno-histochemistry for stromal components and gemcitabine metabolizing enzymes performed in in vivo models and human primary resected tissue. Another aim was to investigate wheth-er single soluble and solid components of the tumor microenvironment had an impact on the expression of gemcitabine metabolizing enzymes in tumor cells in vitro and in vivo. Hence, conditioned medium experiments, cultivation of tumor cells on type I collagen and evaluation of a KPC mouse model devoid of mature collagen, were performed. It was shown that gemcitabine metabolizing enzyme expression in KPC mice was signifi-cantly higher than in mice orthotopically transplanted with 2D-cultured KPC tumor cells. Notably, orthotopically transplanted tumors revealed reduced amounts of stromal compo-nents as compared to endogenous tumors from KPC mice. Moreover, gemcitabine metabo-lizing enzyme expression levels in patient-derived xenograft mice did not significantly differ from original human primary resected tissue although patient-derived xenograft mice exhib-ited a significantly lower number of stromal components. In line with the results from the in vivo models, subsequent in vitro experiments did not reveal a significant effect of fibroblast conditioned medium or type I collagen on gemcitabine metabolizing enzyme expression in human and murine tumor cells. The expression of gemcitabine metabolizing enzymes was not significantly altered in the collagen-depleted KPC mouse model compared to control mice. In conclusion, this work contributed to a deeper understanding of tumor-stroma crosstalk within pancreatic cancer. Whereas expression of gemcitabine metabolizing enzymes varies between KPC mice and orthotopically transplanted mice, the patient-derived xenograft mouse model displayed comparable levels of gemcitabine metabolizing enzymes despite different amounts of tumor stroma. In conjunction with the obtained in vitro experiments, our results suggest that the tumor stroma and gemcitabine metabolizing enzymes are likely two separate therapeutic targets in pancreatic cancer that could be targeted synergistically, e. g. by depletion of tumor stroma to increase drug delivery, and parallel epigenetic targeting of gemcitabine metabolizing enzymes to sensitize tumors to gemcitabine. Whereas the KPC model is likely the most appropriate model to experimentally probe stromal depletion strat-egies, patient-derived xenografts closely recapitulate the expression of gemcitabine metabo-lizing enzymes in corresponding human tumors.