D-Mannose Slows Glioma Growth by Modulating Myeloperoxidase Activity

Abstract

Host immune response in the tumor microenvironment plays key roles in tumorigenesis. We hypothesized that D-mannose, a simple sugar with anti-inflammatory properties, could decrease oxidative stress and slow glioma progression. Using a glioma stem cell model in immunocompetent mice, we induced gliomas in the brain and tracked MPO activity in vivo with and without D-mannose treatment. As expected, we found that D-mannose treatment decreased the number of MPO⁺ cells and slowed glioma progression compared to PBS-treated control animals with gliomas. Unexpectedly, instead of decreasing MPO activity, D-mannose increased MPO activity in vivo, revealing that D-mannose boosted the MPO activity per MPO⁺ cell. On the other hand, D-glucose had no effect on MPO activity. To better understand this effect, we examined the effect of D-mannose on bone marrow-derived myeloid cells. We found that D-mannose modulated MPO activity via two mechanisms: directly via N-glycosylation of MPO, which boosted the MPO activity of each molecule, and indirectly by increasing H₂O₂ production, the main substrate for MPO. This increased host immune response acted to reduce tumor size, suggesting that increasing MPO activity such as through D-mannose administration may be a potential new therapeutic direction for glioma treatment.

Keywords: D-mannose; MPO activity; glioma; leukocytes.

Figure 1

Tumor size characterized by bioluminescence (BLI) and T2-weighted MR imaging. (A) Representative BLI images performed at the third week after glioma induction for the D-mannose-treated and PBS-treated groups. (B) Quantification of tumor sizes on BLI (n = 8–9 per group). (C) Representative T2-weighted MRI images performed at the fourth week after glioma induction for the two groups. Arrows indicate the tumors. (D) Quantification of tumor sizes (n = 8–9 per group). Data are shown as mean ± standard errors of measurements.

Figure 2

MPO-Gd MR imaging, and MPO protein level and activity in glioma mice. (A) Representative MRI of glioma mice performed before and at 75 min after MPO-Gd administration (n = 3 per group). (B) Comparison of contrast-to-noise ratios (CNR) in animals with D-mannose treatment and control group. (C) Ex vivo measurements of MPO protein level and MPO activity from brain tissues from glioma-bearing mice (n = 7 per group). Data are shown as mean ± SEM.

Figure 3

Decrease in MPO⁺ cells in brains of D-mannose-treated mice. (A) Representative flow cytometry histograms comparing the number of MPO⁺ cells between D-mannose-treated and PBS-control group (light blue: negative control (unstained), dark blue: D-mannose-treated, and red: PBS-treated). (B) Quantification of the number of MPO⁺ cells as a percentage of CD11b⁺ leukocytes (n = 9–10 mice per group). Data are shown as mean ± SEM.

Figure 4

Biochemical analysis of PMA-stimulated mouse bone marrow-derived leukocytes. (A) Relative MPO activities were normalized to untreated leukocytes samples (+ = 1 mg/mL, ++ = 2 mg/mL, ++++ = 4 mg/mL). (B) Comparison of MPO activity with and without D-mannose and D-glucose (++ = 2 mg/mL). ns = not significant. (C) The amount of H₂O₂ produced by leukocytes was measured under similar conditions as in (A). (D) Comparison of MPO activity produced by leukocytes with and without D-mannose and 2-deoxy-D-glucose (2DG). All data are in mean ± SEM.