Evidence linking CD44-positive cells and gemcitabine resistance in pancreatic cancer cells: need for further substantiation

Abstract

With great interest, we read the recent publication “CD44-positive cells are responsible for gemcitabine resistance in pancreatic cancer cells” by Hong et al.1 In this study, the authors established gemcitabine-resistant pancreatic cancer cells in vitro and characterized them in various ways, in vitro and in vivo. The authors describe these gemcitabine-resistant cells as being more tumorigenic in vitro and in vivo as judged from colony formation assays and tumor growth in mice, respectively. In addition, the resistant cells had greater sphere-forming activity than the parental cells and developed enhanced CD44 expression during acquisition of gemcitabine resistance. Further, the authors also noted an enhanced expression of some ABC transporters and conclude from this that upregulation of ABC transporters is one of the mechanisms leading to multidrug resistance in gemcitabine-resistant pancreatic cancer cells and that CD44-positive cells might be responsible for multidrug resistance. However, although this article certainly contains some new and convincing data, we do not agree with some important details of this publication. First, the increased CD44 expression in gemcitabine-resistant cells is well demonstrated in this article and lead the authors to the conclusion implied in the title, namely, that the positive correlation of CD44 expression is causative for the gemcitabine resistance of these cells. We missed the experimental evidence to corroborate this causal link, which could have been provided, e.g., by testing the gemcitabine cytotoxicity in CD44-silenced cells. Further, the demonstration that CD44 knock-down by siRNA leads to decreased proliferation just shows this, namely a decrease in proliferation, but not an altered gemcitabine sensitivity of these cells. Also, the assumed causal relation between MDR1 expression and gemcitabine sensitivity lacks experimental evidence in this article. Again, silencing MDR1 would have been far more convincing to allow for a strong argument that it is this very ABC transporter that is upregulated in gemcitabine-resistant cells and thus influences gemcitabine sensitivity of the cells. The role of ABC transporters in the chemoresistant phenotype of pancreatic cancer cells certainly deserves great attention, as several of these export pumps have been demonstrated to confer multidrug resistance to chemotherapeutic drugs.2-6 The upregulation of ABCB1 (MDR1) in the here reported gemcitabine-resistant cells is impressive, but several aspects of ABC transporters are not convincing in this article. First, we do not agree with the exclusive statement in the abstract that “ABC transporters…are known as the mechanism of drug resistance”. In contrast, there exists a plethora of factors known to be involved in and contributing to drug resistance, some of which the authors correctly list in the introduction. Second, the authors investigated some ABC transporters, but none of the investigated transporters has yet been characterized as being directly or indirectly involved in gemcitabine transport. Particularly, MDR1, to our knowledge, has not been reported to transport gemcitabine or its metabolites, and from the known structural requirements for MDR1 substrates it is also not likely to do so.7 This aspect, however, is completely missing in this article. Third, verapamil certainly inhibits MDR1 but is not a general “inhibitor of ABC transporters,” as the authors state in the paragraph on therapeutic implications. Fourth, the authors write in the introduction of “the nucleoside transporter” as being among the responsible factors for gemcitabine resistance. This simplification is not correct; the authors ignore that there exists more than one nucleoside transporter, actually there exist two families (ENTs, CNTs) with at least five members, and not all of them transport gemcitabine nor are they all expressed in pancreatic cells.8, 9 Yours sincerely, Wolfgang Hagmann, Ralf Jesnowski, J. Matthias Löhr.