Cancer and Stem Cell Biology Signature Research Program, Duke-NUS Medical School, Singapore, Singapore.
Cancer Science Institute of Singapore, National University of Singapore, Singapore, Singapore.
PLoS One. 2018 Oct 11;13(10):e0205254. doi: 10.1371/journal.pone.0205254. eCollection 2018.
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.
癌细胞，包括慢性髓性白血病（CML）中的癌细胞，依赖于低氧反应在宿主体内持续存在并逃避治疗。因此，人们对靶向癌症特异性低氧反应的药物治疗产生了浓厚的兴趣。然而，白血病面临的一个主要挑战是识别白血病细胞和正常细胞之间的差异和可靶向的低氧反应。之前，我们发现精氨酸酶 2（ARG2），尿素循环中的一种酶，在 CML 中过表达，但在正常祖细胞中不表达。ARG2 是缺氧诱导因子（HIF1-α 和 HIF2-α）的靶标，是多胺生成所必需的，多胺是细胞生长所必需的。因此，我们探讨了在低氧条件下，临床上测试的精氨酸酶抑制剂 Nω-羟基-nor-精氨酸（nor-NOHA）是否对白血病细胞有效。值得注意的是，nor-NOHA 在低氧条件下可有效诱导 ARG2 表达细胞凋亡，但在常氧条件下则不然。nor-NOHA 与 nor-NOHA 联合治疗可克服低氧介导的对 BCR-ABL1 激酶抑制剂的耐药性。虽然 nor-NOHA 本身在靶向白血病低氧反应方面很有前景，但我们意外地发现其抗白血病活性与 ARG2 抑制无关。使用 CRISPR/Cas9 对 ARG2 进行基因敲除对白血病细胞的活力及其对 nor-NOHA 的敏感性没有影响。ARG2 敲除和 nor-NOHA 对细胞呼吸的不同影响进一步证明了这一差异。总之，我们表明 nor-NOHA 在表达 ARG2 的低氧细胞中具有显著但非靶向的抗白血病活性。由于 nor-NOHA 已在临床试验中使用，并广泛用于内皮功能障碍、免疫抑制和代谢研究，在将其活性归因于 ARG 抑制之前，必须谨慎评估 nor-NOHA 的多种生物学效应。
