B56δ subunit of protein phosphatase 2A decreases phosphorylation of endothelial nitric oxide synthase at serine 116: Mechanism underlying aphidicolin-stimulated NO production

DNA damage is significant in endothelial cells (EC), particularly in anticancer chemotherapy. Here, we explored whether and how aphidicolin, a DNA-damaging chemical with a promising anticancer activity, alters NO production in bovine aortic endothelial cells (BAEC). In addition to increasing eNOS-Ser¹¹⁷⁹ phosphorylation, aphidicolin decreased eNOS-Ser¹¹⁶ phosphorylation with a concomitant increase in NO production in a time-dependent manner. The amino acid sequence around the eNOS-Ser¹¹⁶ residue was identified as the substrate site of the regulatory subunit B56δ of protein phosphatase 2A (PP2A). As expected, okadaic acid, a specific PP2A inhibitor, reversed aphidicolin-induced eNOS-Ser¹¹⁶ dephosphorylation in a dose-dependent manner. Aphidicolin also increased B56δ-Ser⁵⁶⁶ phosphorylation, although expression of neither the catalytic subunit Cα (PP2A Cα) nor B56δ was altered. Ectopic expression of dominant negative (dn)-B56δ reversed all of the observed effects of aphidicolin with respect to phosphorylation of eNOS-Ser¹¹⁶ and B56δ-Ser⁵⁶⁶. Lastly, aphidicolin-stimulated NO production was also partially attenuated by ectopic expression of dn-B56δ. Taken together, our results are the first to demonstrate that aphidicolin decreases phosphorylation of eNOS-Ser¹¹⁶ as well as eNOS-Ser¹¹⁷⁹, at least in part by activating PP2A B56δ, resulting in NO release in BAEC.