Hydroxyurea up-Regulates Voltage-Dependent Anion Channels in Human Sickle Cell Erythrocytes.

Hydroxyurea (HU) is clinically effective in reducing the frequency of pain crises in adults and children with sickle cell disease (SCD). In addition to increased fetal hemoglobin production, HU is known to induce macrocytosis in sickle cell erythrocytes and prolong red cell survival. However, the precise mechanism by which HU produces its varied effects is unknown. Isoelectric focusing has been applied to characterize changes in the red cell membrane following exposure to HU, but this technique has limitations in its ability to identify differences in specific membrane proteins. Mass spectrometry (MS) provides another proteomic approach in analyzing red cell proteins in SCD.

Red blood cells were obtained from patients with SCD on HU therapy. After multiple wash and lysis steps, ghost membranes were obtained following ultracentrifugation. The membrane samples were then analyzed via tandem mass spectrometry and western immunoblot assay.

In comparison to a normal control, patients with SCD on HU therapy were found to have significantly elevated levels of voltage-dependent anion channels (VDAC) by MS, using a false discovery rate of less than five percent. The application of a stringent threshold initially identified 346 protein candidates. Out of these 346 proteins, 125 contained multiple peptides which all passed quantitation data filters according to quality parameters. Within the set of 125 proteins, 4 proteins were significantly up-regulated (up to 50-fold) in patients with SCD on HU therapy. VDAC2 and VDAC3 are members of the eukaryotic mitochondrial porin family which form channels through the mitochondrial outer membrane allowing diffusion of small hydrophilic molecules. VDAC1 can be found in both the mitochondrial outer membrane and the plasma membrane, where it is involved in cell volume regulation and apoptosis.

Up-regulation of VDAC proteins may play an important role in altering the intracellular osmotic composition of the sickle cell erythrocyte resulting in decreased sickling and improved red cell survival in patients with SCD on HU therapy.

No relevant conflicts of interest to declare.