Calreticulin Regulation of Lung Endothelial NOS Activity

Increased synthesis of a multifunctional calcium binding protein calreticulin has been reported under diverse physiologic and pathophysiologic conditions in various tissues in eluding stimulation of vascular endothelium by angiotensin IV (Ang-IV), a metabolic product of Ang-II. Ang-IV-mediated early and sustained activation of lung endothelial cell nitric oxide synthase (eNOS) is mediated through increased mobilization of intracellular calcium and by increased expression of calreticulin. Immunoprecipitation and confocal imaging studies revealed that eNOS and calreticulin are co-localized in Ang-IV-stimulated lung endothelial cells. Catalytic activity of purified eNOS in the absence of calmodulin was increased in a concentration-dependent fashion by calreticulin. The studies monitoring the effect of calreticulin on the rate of electron transfer from the reductase to the oxygenase domain of eNOS revealed that the calreticulin/eNOS interaction promotes electron transfer and mimics eNOS activation in the absence of exogenous calmodulin and enhances electron transfer and the catalytic activity of eNOS in the presence of calmodulin. Thus, calreticulin/eNOS proteimprotein interaction enhances the rate of electron transfer, a critical event in the regulation of the catalytic activity of eNOS.