Endothelial Nitric Oxide Synthase-Dependent Mechanism of Hydroxyurea-Induced S-Phase Arrest in Erythroid Cells

Hydroxyurea (HU) is a ribonucleotide reductase inhibitor widely used for the treatment of sickle cell disease and myeloproliferative disorders, yet a precise nitric oxide (NO) synthase (NOS)-dependent mechanism remains incompletely defined. The role of NOS3 in HU-mediated proliferation, cell cycle, and apoptosis was analyzed in HEL92.1.7 erythroleukemic cells and primary mouse erythroid progenitors upon genetic knockdown/knockout and pharmacological NOS2/NOS3 inhibition. NOS3 expression, phosphorylation, NO and citrulline production, and protein nitrosylation were assessed via immunoblotting and biochemical assays. Computational docking and molecular dynamics simulations were performed to examine the interaction between HU and NOS3. HU enhanced NOS3 expression and phosphorylation, leading to increased NO and citrulline production. Computational analysis predicted HU binding within the NOS3 active site, whereas functional activation was AKT1-dependent. A biotin switch assay revealed cooperative NOS2-/NOS3-mediated protein nitrosylation under HU treatment. NOS3 depletion or inhibition abrogated HU-induced S-phase accumulation and restored cell proliferation. NOS3 protein depletion increased late apoptosis in erythroleukemic cells, while in murine erythroid cells, both Nos3 deficiency and inhibition decreased early and increased late apoptosis. NOS2 and NOS3 act as complementary mediators of proliferation and apoptosis, with NOS3 playing a distinct role in HU-induced proliferation arrest in erythroid cells. These findings highlight the therapeutic potential of NOS targeting to enhance the efficacy of HU and overcome resistance in hematologic malignancies.