Antitumor Effects of Mannose on Human Colorectal Cancer Cells Under Normal and Diabetic Conditions

Abstract

Introduction: Colorectal cancer (CRC) is the third most diagnosed malignancy and the second leading cause of cancer-related deaths worldwide. In Lebanon, CRC ranks third in both incidence and mortality among males and females. The tumor suppressor gene TP53 is mutated in about 60% of CRC cases, such alterations lead to both gain- and loss-of-function activities that promote tumorigenesis. Despite advances in CRC management, innate or acquired drug resistance remains a major clinical challenge, with up to 90% of patients developing resistance to 5-fluorouracil, the cornerstone of CRC chemotherapy. Therefore, there is an urgent need to identify novel therapeutic strategies with improved efficacy and safety. The natural monosaccharide D-mannose (mannose), which is a C2 epimer of glucose, has been reported to exert anticancer effects in several preclinical models, including inhibition of cell proliferation, induction of DNA damage and oxidative stress, promotion of cancer cell death, and reduction of tumor growth, with minimal effects on normal cells. In parallel, type 2 diabetes mellitus (T2DM) is a critical global health issue with increasing incidence worldwide. Epidemiological evidence revealed a bidirectional link between T2DM and cancer, with T2DM-associated characteristics such as hyperinsulinemia, hyperglycemia, and chronic inflammation have been implicated in CRC development and poorer clinical outcomes. Interestingly, the antidiabetic drug metformin has demonstrated anticancer properties against several types of cancer, including CRC. Accordingly, this study aimed to evaluate the effects of mannose and metformin, alone and in combination, on human CRC cells cultured under normal- or high-glucose conditions, mimicking non-diabetic and diabetic environments, respectively. Methods: CRC cells were adapted to normal-glucose conditions (5 mM) through a sequential, gradual reduction in glucose concentration to maintain cell survival and stability. The effects of mannose and metformin, as single agents or in combinations, were assessed in two human CRC cell lines: the HCT116 (wild-type p53) and HT-29 (mutant p53), cultured under normal and high glucose conditions. Cell viability was evaluated using the Sulforhodamine B (SRB) assay and confirmed by the trypan blue exclusion assay. Mannose IC₅₀ values were determined and used as working concentrations for mechanistic experiments. Cell cycle distribution was analyzed by propidium iodide (PI)-based flow cytometry. Drug interaction between mannose and metformin was evaluated using Compusyn software. Results: Mannose and metformin monotherapies significantly reduced CRC cell viability in a time- and -dose dependent manner. HT-29 cells exhibited greater resistance to metformin compared with HCT116 cells. In addition, HT-29 cells cultured under high glucose were more resistant to metformin than those maintained under normal glucose conditions. PI-based flow cytometric analysis revealed that mannose did not induce significant alterations in cell cycle distribution. Importantly, Compusyn analysis revealed significant synergistic interactions between mannose and metformin in both cell lines at 48 and 72 hours, leading to enhanced inhibition of CRC cell growth and viability. Conclusion: Given the high prevalence of drug resistance in CRC and its association with T2DM, combining the natural, low-cost compound mannose with the clinically approved and widely used drug metformin may represent a safer and more effective anticancer strategy than either agent alone.