Therapeutic Targeting of the Pentose Phosphate Pathway in Colorectal Cancer Cells Using Mannose

Abstract

Introduction: Colorectal cancer (CRC) is the third most commonly diagnosed cancer and the second leading cause of cancer-related death worldwide. 5-Fluorouracil (5-FU) remains the first-line treatment for CRC although its antineoplastic activity is limited in drug-resistant cancer cells. The pentose phosphate pathway (PPP) plays a pivotal role in assisting glycolytic cancer cells to meet their anabolic demands and combat oxidative stress. The PPP is a major glucose metabolic pathway in cancer cells. It is required for the synthesis of ribonucleotides and reactive oxygen species (ROS) scavenging through nicotinamide adenine dinucleotide phosphate (NADPH) generation. The oxidative phase of the PPP is regulated by the rate-limiting enzyme glucose-6-phosphate dehydrogenase (G6PD). D-mannose is a monosaccharide that competes with glucose for its transporter. However, the fate of mannose inside the cell depends on the levels of phosphomannose isomerase (PMI) enzyme. Recent studies demonstrated that mannose inhibits tumor growth by impeding glucose metabolism. Therefore, we aim to investigate the anti-tumor effect of mannose alone or in combination with 5-FU in CRC cells and its effect on the PPP.

Methods: To assess the effect of mannose and 5-FU alone or in combination, cell viability assays (MTT, SRB) on CRC cell lines of different p53 and 5-FU resistance status were used. Synergistic effects were estimated using the Compusyn software. To study the mechanism of action of the combination treatment on cell cycle and cell death, propidium iodide and immunoblotting assays were used. The PPP enzymes (G6PD, transketolase) product levels and/or activities were measured and ROS levels were evaluated.

Results: In silico analysis revealed elevated PMI transcripts in CRC versus normal tissue and high PMI protein levels in CRC patients compared to patients with other types of tumors. Mannose treatment reduced CRC cell viability. Moreover, mannose synergized with 5-FU treatment in HCT116, HCT116 p53-/-, and HCT116 5-FU resistant (5FUR) cells. The mannose/5-FU combination treatment induced an S-phase arrest and accumulated treated HCT116 and HCT116 p53-/- cells in the sub-G1 phase of the cell cycle. The mannose/5-FU combination reduced the activity of G6PD compared to 5-FU treatment while G6PD and transketolase protein levels remained unaltered. ROS levels increased upon treatment with mannose and or 5-FU combination.

Conclusion: These findings reveal that combining mannose with 5-FU decreased resistance and further sensitized CRC cells to 5-FU treatment independently of p53 and 5-FU drug resistance status. Furthermore, mannose inhibited G6PD and induced oxidative stress in CRC cells. Exploiting the metabolic vulnerability of cancer cells offers a novel clinical approach to CRC management.