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.
Schmidt
Kurt
K
Department of Pharmacology and Toxicology, Karl-Franzens-Universität Graz, Austria. kurt.schmidt@uni-graz.at
Martens-Lobenhoffer
Jens
J
Meinitzer
Andreas
A
Graier
Wolfgang F
WF
Torres
Christina M
CM
Venema
Richard C
RC
Mayer
Bernd
B
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