High-Dose Ascorbate Administration Increases Tumor Ascorbate Levels and Decreases Hypoxia-Inducible Factor-1 Activity and Tumor Growth in Mice

The activation of the transcription factor hypoxia-inducible factor (HIF)-1 is associated with an aggressive tumor phenotype. HIF-1 is regulated by hydroxylase enzymes that require vitamin C (ascorbate) as cofactor and analyses of clinical tumor samples and dietary supplementation studies in mice have demonstrated an inverse correlation between tumor ascorbate levels and HIF-1 activity. In this study we investigated whether pharmacological doses of ascorbate can affect tumor ascorbate levels and exert an anti-tumor effect in tumor-bearing Gulo -/- mice, a model of human ascorbate dependency. Ascorbate concentrations peaked within an hour after a single intra-peritoneal injection (1g/kg), with millimolar levels measureable in plasma, and increased levels in liver and tumor tissues. Plasma and liver ascorbate returned to baseline within 16h, but tumor levels remained elevated for 48h. Modelling ascorbate uptake and stability in tumors suggests that increased stability under hypoxia may account for its prolonged presence within the tumor tissue. The expression of HIF-1 and its target proteins was down-regulated during the first 4h post-injection, and increased again after 16–48h. Increased tumor ascorbate levels could be maintained with repeated daily or alternate day administration. HIF-1 and vascular endothelial growth factor proteins were inversely correlated with tumor ascorbate and were significantly down-regulated in tumors when ascorbate was administered daily. Increased tumor ascorbate was associated with reduced microvessel density, tumor hypoxia and tumor growth. These results support the suppression of the hypoxic response as being a possible anti-tumor activity of ascorbate that may be useful in a clinical setting.