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S12-03 IMPACT OF IMP-GMP SPECIFIC 5'-NUCLEOTIDASE ON METABOLIC REGULATION AND DRUG RESISTANCE IN TUMORS Pesi R 1 , Allegrini S. 1 , Petrotto E. 1 , Garcia-Gil M. 1 , Camici M. 1 , Jordheim L.P. 2 , Tozzi M.G. 1 1 University of Pisa, Italy; 2 University of Lyon, France. Cytosolic 5 ′ -nucleotidase II (cN-II) is an ubiquitous enzyme, highly expressed in replicative tissues and tumours. It catalyzes the dephosphorylation of purine nucleoside monophosphates into their corresponding nucleosides and inorganic phosphate. cN-II has a preference for IMP and GMP, followed by AMP, and it is highly regulated through the interaction of its two effector sites with adenosine derivatives or several phosphorylated compounds(1,2) It has been shown to be involved in the regulation of intracellular purine nucleotides, and the secretion of nucleosides is increased in cN-II- overexpressing cells (3). The role of cN-II in cancer and sensitivity to cancer treatments has been repeatedly demonstrated with a worse outcome of nucleoside- analogue-treated AML patients with high cN-II expression in their cancer cells as compared to those with low cN-II expression. Nucleoside analogues are dependent on intracellular activation by phosphorylation, and it was first hypothesized that cN-II could decrease the accumulation of active phosphorylated metabolites by dephosphorylating monophosphorylated cytarabine. However, as this metabolite is not a substrate for cN-II, the mechanism of cN-II-mediated resistance to nucleoside analogues remains unknown. The more recent observation of resistance to thiopurines in relapsed ALL patients with hyperactive mutants of cN-II suggests that a disturbance of nucleotide pools can be responsible of the mechanism of drug resistance (4). The direct role of cN-II in the sensitivity of cancer cells to nucleoside analogues was recently demonstrated by the sensitization of hematological malignant cell lines with shRNA-mediated decreased cN-II expression. A growing amount of results also indicate a direct or indirect role of cN-II in cell growth. First, yeasts expressing human cN-II grow faster than control strains, second, human neuroblastoma cells with decreased cN-II expression grow slower than control cells, whereas the same cells overexpressing wild-type cN-II or the hyperactive mutant R367Q grow faster than control cells (5). This is, however, not consistently observed as malignant hematological cell lines with decreased cN-II grow similarly to control cells. This might be explained by complex differences between cancer cells and normal cells or by a threshold effect. Based on the above mentioned evidences, cN-II has emerged as a potential target for cancer treatment. Furthermore, cN-II can interact with a number of other proteins, such as NRLC4 (IPAF) a protein involved in inflammosome formation, caspase.-1 activation and interleukines production when stimulated by intracellular or extracellular damage signals. Nevertless the involvement of cN-II in inflammation is still to be unravelled. We finally, recently, demonstrated that a partial cN-II silencing resulted in a change in both metabolic and proliferative capacity in tumoral cancer cells. In fact, the decrease of cN-II activity caused an imbalance in purine and pyrimidine intracellular nucleotides, inhibition of AKT phosphorylation, a decrease of proliferation and cell motility and a more oxidative metabolism, associated to an increase of p53 phosphorylation (6). References : Tzoneva G, Dieck CL, et al (2018 ) Nature. 553, 511-514. doi: 10.1038/nature25186. Cividini F, Cros-Perrial E, et al (2015) Int J Biochem Cell Biol 65:222–229. Pesi R., Petrotto E. et al (2018) Int J Mol Sci. 2018 Jul 20;19(7). pii: E2115. doi: 10.3390/ijms19072115.