Reciprocal regulation of the nitric oxide and cyclooxygenase pathway in pathophysiology: relevance and clinical implications

Abstract

The nitric oxide (NO) and cyclooxygenase (COX) pathways share a number of similarities. Nitric oxide is the mediator generated from the NO synthase (NOS) pathway, and COX converts arachidonic acid to prostaglandins, prostacyclin, and thromboxane A(2). Two major forms of NOS and COX have been identified to date. The constitutive isoforms critically regulate several physiological states. The inducible isoforms are overexpressed during inflammation in a variety of cells, producing large amounts of NO and prostaglandins, which may underlie pathological processes. The cross-talk between the COX and NOS pathways was initially reported by Salvemini and colleagues in 1993, when they demonstrated in a series of in vitro and in vivo studies that NO activates the COX enzymes to produce increased amounts of prostaglandins. Those studies led to the concept that COX enzymes represent important endogenous "receptor" targets for amplifying or modulating the multifaceted roles of NO in physiology and pathology. Since then, numerous studies have furthered our mechanistic understanding of these interactions in pathophysiological settings and delineated potential clinical outcomes. In addition, emerging evidence suggests that the canonical nitroxidative species (NO, superoxide, and/or peroxynitrite) modulate biosynthesis of prostaglandins through non-COX-related pathways. This article provides a comprehensive state-of-the art overview in this area.

Fig. 1.

Biosynthesis of nitric oxide and products of arachidonic acid metabolism. COX, cyclooxygenase; NO, nitric oxide; PLA, phospholipase A; PGG₂, prostaglandin G₂; PGH₂, prostaglandin H; TxA, thromboxane; ASS, argininosuccinate synthase; ASL, argininosuccinate lyase.

Fig. 2.

The scheme of COXs enzyme reaction mechanism. COX enzymes produce PGH₂ by two-step chemical reactions: cyclooxygenase and peroxidase. PGH₂ is further converted to other prostanoid by tissue-specific enzymes. Tyr, tyrosine; AA, amino acid; AH₂, nonspecific oxidizing agent that can donate 2 hydrogens.

Fig. 3.

Similarities between the nitric oxide synthase (NOS) and COX pathway. WBC, white blood cell; CNS, central nervous system.

Fig. 4.

Activation of COX-1 by NO. Sodium nitroprusside increase COX-1 activity in sheep seminal vesicles, as evidenced by a dose-dependent increase in PGE₂ release from arachidonic acid; hemoglobin (Hb) attenuates the effects of sodium nitroprusside (SNP) consistent with an NO-mediated activation of COX-1. Results are expressed as the means ± SE of seven experiments and analyzed by Student's unpaired t-test. *P < 0.01 compared with results obtained in the absence of SNP and †P < 0.01 compared with results obtained in the presence of SNP but absence of Hb. Experimental design is detailed in Refs. and 128.

Fig. 5.

Activation of COX-2 by NO. Coincubation of NO, SNP, or glyceryl trinitrate (GTN) with human fetal fibroblasts expressing COX-2 increased the production of PGE₂ by arachidonic acid. Results are the mean ± SE of seven experiments and analyzed by Student's unpaired t-test. *P < 0.01 compared with results obtained in the absence of the NO or the NO donor. Experimental design is detailed in Refs. and 128.

Fig. 6.

Reciprocal regulation of PG biosynthesis by nitroxidative stress. NO can directly modify/S-nitrosylate cysteine(s) in COX by direct protein-protein interaction between NOS and COX and activate it. COX also produces superoxide, which is speculated to be responsible for the autoinactivation of the COX enzymes. Rapid interaction between superoxide and NO reduces the levels of superoxide and produces PN, which can target either heme or tyrosine residue(s) in COX. PN may interact with Fe in the heme group of COX and forms a radical intermediate product, which can accelerate the enzyme reaction while PN-mediated modification of Tyr-385 in COX can inactivate the activity of the enzyme. In those conditions, in which both the NOS and COX systems coexist and are coactivated, there can be a nitroxidative-mediated modulation in the production of PG resulting in beneficial or detrimental effects, such as increased inflammation and tissue injury.

Fig. 7.

An interaction between inducible NOS (iNOS) and COX-2 is required to mediate NO-induced PGE₂ production. A: COX-2 and iNOS are simultaneously in activated macrophages by LPS treatment. COX-2 and iNOS are also coimmunoprecipitated by COX-2 antibody. B: dominant negative (DN; 484–604 amino acid based on COX-2 sequence) peptide, which is a minimum region required to bind to iNOS and inhibit endogenous protein-protein interaction significantly reduced PGE₂ production.