Endothelial Nitric-oxide Synthase (Type III) Is Activated and Becomes Calcium Independent upon Phosphorylation by Cyclic Nucleotide-dependent Protein Kinases*

Endothelial nitric-oxide synthase (NOS-III) is defined as being strictly dependent on Ca2+/calmodulin (CaM) for activity, although NO release from endothelial cells has been reported to also occur at intracellular free Ca2+levels that are substimulatory for the purified enzyme. We demonstrate here that NOS-III, but neither NOS-I nor -II, is rapidly and strongly activated and phosphorylated on both Ser and Thr in the presence of cGMP-dependent protein kinase II (cGK II) and the catalytic subunit of cAMP-dependent protein kinase (cAK) in vitro. Phosphopeptide analysis by mass spectrometry identified Ser1177, as well as Ser633which is situated in a recently defined CaM autoinhibitory domain within the flavin-binding region of human NOS-III. Phosphoamino acid analysis identified a putative phosphorylation site at Thr495in the CaM-binding domain. Importantly, both cAK and cGK phosphorylation of NOS-IIIin vitrocaused a highly reproducible partial (10–20%) NOS-III activation which was independent of Ca2+/CaM, and as much as a 4-fold increase in Vmaxin the presence of Ca2+/CaM. cAK stimulation in intact endothelial cells also increased both Ca2+/CaM-independent and -dependent activation of NOS-III. These data collectively provide new evidence for cAK and cGK stimulation of both Ca2+/CaM-independent and -dependent NOS-III activity, and suggest possible cross-talk between the NO and prostaglandin I2pathways and a positive feedback mechanism for NO/cGMP signaling.