The arginase inhibitor Nω−hydroxy−nor−arginine (nor−NOHA) induces apoptosis in leukemic cells specifically under hypoxic conditions but CRISPR/Cas9 excludes arginase 2 (ARG2) as the functional target

: Cancer cells, including in chronic myeloid leukemia (CML), depend on the hypoxic response to persist in hosts and evade therapy. Accordingly, there is significant interest in drugging cancer-specific hypoxic responses. However, a major challenge in leukemia is identifying differential and druggable hypoxic responses between leukemic and normal cells. Previously, we found that arginase 2 (ARG2), an enzyme of the urea cycle, is overexpressed in CML but not normal progenitors. ARG2 is a target of the hypoxia inducible factors (HIF1-α and HIF2-α), and is required for the generation of polyamines which are required for cell growth. We therefore explored if the clinically-tested arginase inhibitor Nω-hydroxy-nor-arginine (nor-NOHA) would be effective against leukemic cells under hypoxic conditions. Remarkably, nor-NOHA effectively induced apoptosis in ARG2-expressing cells under hypoxia but not normoxia. Co-treatment with nor-NOHA overcame hypoxia-mediated resistance towards BCR-ABL1 kinase inhibitors. While nor-NOHA itself is promising in targeting the leukemia hypoxic response, we unexpectedly found that its anti-leukemic activity was independent of ARG2 inhibition. Genetic ablation of ARG2 using CRISPR/Cas9 had no effect on the viability of leukemic cells and their sensitivity towards nor-NOHA. This discrepancy was further evidenced by the distinct effects of ARG2 knockouts and nor-NOHA on cellular respiration. In conclusion, we show that nor-NOHA has significant but off-target anti-leukemic activity among ARG2-expressing hypoxic cells. Since nor-NOHA has been employed in clinical trials, and is widely used in studies on endothelial dysfunction, immunosuppression and metabolism, the diverse biological effects of nor-NOHA must be cautiously evaluated before attributing its activity to ARG inhibition.