Cellular oxygen sensing by the PHD-HIF pathway does not require vitamin C. (A) Proliferation of HeLa cells growing in FCS-free, chemically defined medium containing either no or 50μM ascorbate. (B) Stabilization of HIF-1α protein in HeLa cells maintained in FCS-free medium containing either no (−Asc.) or 50μM ascorbate. Cells were exposed to 21% and 0.2% oxygen for 6 hours, and then protein levels were analyzed by immunoblotting. (C) Induction of HIF-dependent luciferase activity (pH3SVL vector) in HeLa cells maintained in FCS-free medium containing either no or 50μM ascorbate and exposed to 0.2% to 21% oxygen for 24 hours.