Author(s): Raphaël Maréchal 1 , Jean-Luc Van Laethem [[dagger]] 2
deoxycytidine kinase; gemcitabine; Hu antigen R; pancreatic adenocarcinoma
Gemcitabine, a 2´2´-difluoro-2´-deoxycytidine analog, is the standard of care for pancreatic ductal adenocarcinoma (PDA) patients, in both the adjuvant and metastatic settings [1,2] . Cellular resistance to gemcitabine treatment is common and may be an initial property (intrinsic resistance), while it can also be acquired during gemcitabine treatment. Our understanding of intracellular uptake and activation of gemcitabine has provided important insights for the identification of the mechanisms confering resistance to gemcitabine. Permeation of gemcitabine through the plasma membrane by diffusion is low. Prior to its intracellular 'activation', intracellular uptake of gemcitabine depends on specialized membrane nucleoside transporter proteins, of which the predominant one is the human equilibrative nucleoside transporter (hENT)1 and, to a lesser degree, the human concentrative nucleoside transporter (hCNT)3. After intracellular uptake, gemcitabine is phosphorylated by the deoxycytidine kinase (dCK) to monophosphate and subsequently by nucleotide kinases to difluorodeoxycytidine diphosphate and triphosphate. The monophosphate of gemcitabine can also be deamined by deoxycytidine monophosphate (dCMP) deaminase to difluorodeoxyuridine monophosphate (dFdUMP), an inhibitor of thymidilate synthase. Deficiency in dCK activity has been associated with intrinsic resistance to gemcitabine  and is also common in acquired gemcitabine resistance [4,5] .
The stress response protein Hu antigen R (HuR) is a RNA-binding protein that binds U- and AU-rich sequences in the 3´ untranslated regions of target mRNAs, stabilizing them and/or modulating their expression. HuR has been proposed to contribute to the tumorigenesis process, and cytoplasmic accumulation serves as a prognostic factor of poor clinical outcome in some cancers [6,7] . The study of Costantino and colleagues evaluates the role of the consequence of modulating HuR levels in PDA and whether HuR expression can predict response to gemcitabine  .
Methods & results
Hu antigen R-transfected PDA cell lines were used to evaluate the impact of HuR in the sensitization to nucleoside (gemcitabine and AraC) and non-nucleoside analogues. Intracellular HuR localization was analyzed, as well as the association with dCK mRNA upon gemcitabine treatment exposure. Ultimately, HuR immunostaining was performed on 40 resected PDA specimens that were treated with an adjuvant gemcitabine-based regimen. The level of protein expression was then correlated with the overall survival (OS).
In vitro , the authors observed that HuR mRNA overexpression prefentially sensitized pancreatic cancer cell lines to gemcitabine and AraC, another nucleoside analogue that uses dCK for metabolization. This sensitization is in part due to the association (i.e., complex formation) of HuR with dCK mRNA. HuR overexpression elevates, whereas HuR silencing reduces, dCK protein expression in pancreatic cancer cells. Gemcitabine exposure to pancreatic cancer cells enriches the association between HuR mRNA and dCK, and increases the cytoplasmic concentration levels of HuR mRNA through the translocation of HuR mRNA from the nuclear to the cytoplasmic compartment.
Owing to the relationship between HuR and dCK expression, the authors evaluated the prognostic value of HuR in a cohort of 40 resected pancreatic adenocarcinoma patients treated with adjuvant gemcitabine-based radiochemotherapy or chemotherapy. Patients with low cytoplasmic HuR levels had...