Downregulation of nitric oxide formation by cytosolic phospholipase A2-released arachidonic acid.
Exposure of PC12 cells to A23187 or thapsigargin caused a concentration-dependent release of arachidonic acid (AA) mediated by cytosolic phospholipase A2 (PLA2). Under the same conditions, however, analysis of nitric oxide (NO) formation revealed that activation of NO synthase (NOS) is best described by a bell-shaped curve. Reduced detection of NO observed at increasing A23187 or thapsigargin concentrations was not due to formation of peroxynitrite or to activation of NO-consuming processes, but rather to AA-dependent inhibition of NOS activity. Furthermore, NO formation observed under optimal conditions for NOS activity was suppressed by AA as well as by the PLA2 activator melittin. Finally, the effects of AA were not the consequence of direct enzyme inhibition, because this lipid messenger failed to inhibit formation of NO by purified neuronal NOS, but were mediated by an AA-dependent signaling and not by downstream products of the cyclooxygenase and lipoxygenase pathways. In conclusion, the present study underscores a novel mechanism whereby endogenous, or exogenous, AA promotes inhibition of NOS activity. Because AA is generated in response to various agonists acting on membrane receptors and extensively released in inflammatory conditions, these findings have important physiopathological implications.
Palomba
Letizia
L
Istituto di Farmacologia e Farmacognosia, Università di Urbino Carlo Bo, Via S. Chiara 27, 61029 Urbino (PU), Italy.
Bianchi
Marzia
M
Persichini
Tiziana
T
Magnani
Mauro
M
Colasanti
Marco
M
Cantoni
Orazio
O
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism*; D045744:Cell Line, Tumor; D003600:Cytosol / Q000201:enzymology / Q000378:metabolism*; D015536:Down-Regulation / Q000502:physiology*; D007150:Immunohistochemistry; D008084:Lipoxygenase / Q000378:metabolism; D051379:Mice; D009569:Nitric Oxide / Q000096:biosynthesis / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000378:metabolism; D016716:PC12 Cells; D010741:Phospholipases A / Q000378:metabolism*; D054467:Phospholipases A2; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism; D051381:Rats; D013481:Superoxides / Q000032:analysis; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000378:metabolism
D013481:Superoxides; D016718:Arachidonic Acid; D009569:Nitric Oxide; C002744:3-nitrotyrosine; D014443:Tyrosine; D008084:Lipoxygenase; D019001:Nitric Oxide Synthase; D011451:Prostaglandin-Endoperoxide Synthases; D010741:Phospholipases A; D054467:Phospholipases A2