RECIPROCAL REGULATION OF ENDOTHELIAL NITRIC-OXIDE SYNTHASE BY CA2-CALMODULIN AND CAVEOLIN()

The endothelial nitric-oxide synthase (eNOS) is a key determinant of v ascular homeostasis. Like all known nitric-oxide synthases, eNOS enzym e activity is diependent on Ca2+-calmodulin. eNOS is dynamically targe ted to specialized cell surface signal transducing domains termed plas malemmal caveolae and interacts with caveolin, an integral membrane pr otein that comprises a key structural component of caveolae, We have p reviously reported that the association between eNOS and caveolin is q uantitative and tissue-specific (Feron, O., Belhassen, L., Kobzick, L. , Smith, T. W., Kelly, R. A., and Michel, T. (1996) J. Biol. Chem. 271 , 22810-22814), We now report that in endothelial cells the interactio n between eNOS and caveolin is importantly regulated by Ca2+-calmoduli n, Addition of calmodulin disrupts the heteromeric complex formed betw een eNOS and caveolin in a Ca2+-dependent fashion. In addition, overex pression of caveolin markedly attenuates eNOS enzyme activity, but thi s inhibition is reversed by purified calmodulin, Caveolin overexpressi on does not affect the activity of the other NOS isoforms, suggesting eNOS-specific inhibition of NO synthase by caveolin, We propose a mode l of reciprocal regulation of eNOS in endothelial cells wherein the in hibitory eNOS-caveolin complex is disrupted by binding of Ca2+-calmodu lin to eNOS, leading to enzyme activation, These findings may have bro ad implications for the regulation of Ca2+-dependent signal transducti on in plasmalemmal caveolae.