Abstract 236: Therapeutic targeting of the pentose phosphate pathway in colorectal cancer
Authors: Ghanem Noorhan , El Baba Chirine , Al Saleh Lara , Hayar Berthe , Aouad Patrick , Al Hassan Marwa , El-Khoury Riyad , Usta Julnar , Darwiche Nadine 
Published: September 2020

Abstract
Colorectal cancer is the third most common neoplasia and the second cause of cancer-related deaths worldwide. Unlike normal cells, tumor cells reprogram metabolic pathways to meet their bioenergetics, redox, biosynthetic demands and rely mostly on aerobic glycolysis (Warburg effect). Colorectal cancer cells upregulate the pentose phosphate pathway (PPP), and p53 is a crucial regulator. The PPP is a significant route for glucose catabolism and is required for DNA synthesis of rapidly-proliferating cells. Its oxidative phase (oxPPP), catalyzed by the rate-limiting enzyme glucose-6-phosphate dehydrogenase (G6PD), provides the cell with nicotinamide adenine dinucleotide phosphate (NADPH) that is needed for biosynthetic and detoxifying processes. For the past six decades, 5-Fluorouracil (5-FU) has been the treatment of choice in colorectal cancer. However, 5-FU exhibits high toxicity and drug resistance in patients. Therefore, we hypothesized that targeting the PPP might offer novel therapeutic opportunities in colorectal cancer and improve the response to 5-FU at lower and safer doses. Accordingly, we aim to investigate and characterize the anti-tumor effect of the G6PD inhibitor, 6-aminonicotinamide (6-AN), alone, or in combination with 5-FU on colorectal cancer cells. We explored the effects of oxPPP inhibition on colorectal cancer cell growth, reactive oxygen species (ROS) production, cell cycle regulation, and senescence. We have shown that 6-AN arrested cell growth in HCT116 and HCT116 p53−/− colorectal cancer cells while spared normal-like colon cells (NCM460D). Moreover, 6-AN induced S-phase arrest and early senescence in HCT116 cells as demonstrated by cell cycle analysis using flow cytometry and senescence-associated beta-galactosidase assay, respectively. We also tested the effects of 6-AN/5-FU combination treatments on cellular viability using MTT assay and validated our results using SRB and trypan blue exclusion assays. Interestingly, combination treatments of 6-AN with 5-FU resulted in synergistic effects as estimated using Compusyn software. We used HCT116 colorectal cancer cells resistant to 5-FU (HCT116 5FU-R) or lacking p53, as models for tumor resistance. Importantly, the combination treatment sensitized both HCT116 p53−/− and HCT116 5FU-R colorectal cancer cells to 5-FU and was accompanied by a reduction in G6PD activity and increased ROS using NBT reduction assay. Our findings indicate that combining 6-AN with 5-FU may decrease resistance and further sensitize colorectal cancer cells to 5-FU treatment independently of p53 and drug resistance status. Exploiting this metabolic vulnerability may offer a novel clinical approach and improve patient's therapy.
Citation Format: Noorhan Ghanem, Chirine El Baba, Lara Al Saleh, Berthe Hayar, Patrick Aouad, Marwa Al Hassan, Riyad El-Khoury, Julnar Usta, Nadine Darwiche. Therapeutic targeting of the pentose phosphate pathway in colorectal cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 236.