Activation of endothelial nitric oxide synthase by the pro-apoptotic drug embelin: Striking discrepancy between nitric oxide-mediated cyclic GMP accumulation and L-citrulline formation.
The benzoquinone derivative embelin is a multifunctional drug that not only induces apoptosis by inhibiting XIAP, the X chromosome-linked inhibitor of apoptosis protein, but also blocks nuclear factor-kappaB signaling pathways, thereby leading to down-regulation of a variety of gene products involved in tumor cell survival, proliferation, invasion, angiogenesis, and inflammation. Here, we report that embelin activates and modulates l-arginine/nitric oxide/cyclic GMP signaling in cultured endothelial cells. Embelin elicited a rapid increase of intracellular free Ca(2+), leading to activation of endothelial nitric oxide synthase (eNOS) and NO-induced cGMP accumulation. While the cGMP response was comparable to that caused by other Ca(2+)-mobilizing agents, the stimulatory effect of embelin on l-citrulline formation (approximately 4-fold) was substantially lower than that observed upon activation of eNOS with the Ca(2+) ionophore A23187 (approximately 18-fold), the receptor agonist ATP (approximately 16-fold) or the sarco-endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin (approximately 14-fold). The apparent discrepancy between NO/cGMP and l-citrulline formation in embelin-treated cells was not due to enhanced metabolism and/or efflux of l-citrulline, increased NO bioavailability, inhibition of cGMP hydrolysis, sensitization of soluble guanylate cyclase (sGC) to NO, or enhanced formation of a sGC/eNOS complex. Our puzzling observations suggest that embelin improves coupling of endothelial NO synthesis to sGC activation through mobilization of an as yet unrecognized signaling pathway.
Department of Pharmacology and Toxicology, Karl-Franzens-Universität Graz, Austria. kurt.schmidt@uni-graz.at
Kurt
K
Schmidt
Jens
J
Martens-Lobenhoffer
Andreas
A
Meinitzer
Wolfgang F
WF
Graier
Christina M
CM
Torres
Richard C
RC
Venema
Bernd
B
Mayer
D000818:Animals; D017209:Apoptosis; D016227:Benzoquinones / Q000494:pharmacology*; D002956:Citrulline / Q000096:biosynthesis*; D006152:Cyclic GMP / Q000378:metabolism*; D042783:Endothelial Cells / Q000201:enzymology*; D009569:Nitric Oxide / Q000378:metabolism; D052250:Nitric Oxide Synthase Type III / Q000378:metabolism*; D015398:Signal Transduction
D016227:Benzoquinones; D002956:Citrulline; D009569:Nitric Oxide; D052250:Nitric Oxide Synthase Type III; D006152:Cyclic GMP; C010945:embelin