James K. Gierse

Nitric Oxide Trapping of the Tyrosyl Radical of Prostaglandin H Synthase-2 Leads to Tyrosine Iminoxyl Radical and Nitrotyrosine Formation

The determination of protein nitrotyrosine content has become a frequently used technique for the detection of oxidative tissue damage. Protein nitration has been suggested to be a final product of the production of highly reactive nitrogen oxide intermediates (e.g. peroxynitrite) formed in reactions between nitric oxide (NO⋅) and oxygen-derived species such as superoxide. The enzyme prostaglandin H synthase-2 (PHS-2) forms one or more tyrosyl radicals during its enzymatic catalysis of prostaglandin formation. In the presence of the NO⋅-generator diethylamine nonoate, the electron spin resonance spectrum of the PHS-2-derived tyrosyl radical is replaced by the spectrum of another free radical containing a nitrogen atom. The magnitude of the nitrogen hyperfine coupling constant in the latter species unambiguously identifies it as an iminoxyl radical, which is likely formed by the oxidation of nitrosotyrosine, a stable product of the addition of NO⋅ to tyrosyl radical. Addition of superoxide dismutase did not alter the spectra, indicating that peroxynitrite was not involved. Western blot analysis of PHS-2 after exposure to the NO⋅-generator revealed nitrotyrosine formation. The results provide a mechanism for nitric oxide-dependent tyrosine nitration that does not require formation of more highly reactive nitrogen oxide intermediates such as peroxynitrite or nitrogen dioxide.

A Novel Autotaxin Inhibitor Reduces Lysophosphatidic Acid Levels in Plasma and the Site of Inflammation

Autotaxin is the enzyme responsible for the production of lysophosphatidic acid (LPA) from lysophosphatidyl choline (LPC), and it is up-regulated in many inflammatory conditions, including but not limited to cancer, arthritis, and multiple sclerosis. LPA signaling causes angiogenesis, mitosis, cell proliferation, and cytokine secretion. Inhibition of autotaxin may have anti-inflammatory properties in a variety of diseases; however, this hypothesis has not been tested pharmacologically because of the lack of potent inhibitors. Here, we report the development of a potent autotaxin inhibitor, PF-8380 [6-(3-(piperazin-1-yl)propanoyl)benzo[d]oxazol-2(3H)-one] with an IC50 of 2.8 nM in isolated enzyme assay and 101 nM in human whole blood. PF-8380 has adequate oral bioavailability and exposures required for in vivo testing of autotaxin inhibition. Autotaxins role in producing LPA in plasma and at the site of inflammation was tested in a rat air pouch model. The specific inhibitor PF-8380, dosed orally at 30 mg/kg, provided >95% reduction in both plasma and air pouch LPA within 3 h, indicating autotaxin is a major source of LPA during inflammation. At 30 mg/kg PF-8380 reduced inflammatory hyperalgesia with the same efficacy as 30 mg/kg naproxen. Inhibition of plasma autotaxin activity correlated with inhibition of autotaxin at the site of inflammation and in ex vivo whole blood. Furthermore, a close pharmacokinetic/pharmacodynamic relationship was observed, which suggests that LPA is rapidly formed and degraded in vivo. PF-8380 can serve as a tool compound for elucidating LPAs role in inflammation.

Mutational Analysis of the Role of the Distal Histidine and Glutamine Residues of Prostaglandin-Endoperoxide Synthase-2 in Peroxidase Catalysis, Hydroperoxide Reduction, and Cyclooxygenase Activation

Site-directed mutants of prostaglandin-endoperoxide synthase-2 (PGHS-2) with changes in the peroxidase active site were prepared by mutagenesis, expressed in Sf-9 cells, and purified to homogeneity. The distal histidine, His193, was mutated to alanine and the distal glutamine, Gln189, was changed to asparagine, valine, and arginine. The guaiacol peroxidase activities of H193A, Q189V, and Q189R were drastically reduced to levels observed in the absence of protein; only Q189N retained wild-type PGHS-2 (wtPGHS-2) activity. The mechanism of hydroperoxide reduction by the PGHS-2 mutants was investigated using 15-hydroperoxyeicosatetraenoic acid (15-HPETE), a diagnostic probe of hydroperoxide reduction pathways. The hydroperoxide reduction activity of Q189V and Q189R was reduced to that of free Fe(III) protoporphyrin IX levels, whereas Q189N catalyzed more reduction events than wtPGHS-2. The percentage of two-electron reduction events was identical for wtPGHS-2 and Q189N. The number of hydroperoxide reductions catalyzed by H193A was reduced to ∼60% of wtPGHS-2 activity, but the majority of products were the one-electron reduction products, 15-KETE and epoxyalcohols. Thus, mutation of the distal histidine to alanine leads to a change in the mechanism of hydroperoxide reduction. Reaction of wtPGHS-2, Q189N, and H193A with varying concentrations of 15-HPETE revealed a change in product profile that suggests that 15-HPETE can compete with the reducing substrate for oxidation by the peroxidase higher oxidation state, compound I. The ability of the PGHS-2 proteins to catalyze two-electron hydroperoxide reduction correlated with the activation of cyclooxygenase activity. The reduced ability of H193A to catalyze two-electron hydroperoxide reduction resulted in a substantial lag phase in the cyclooxygenase assay. The addition of 2-methylimidazole chemically reconstituted the two-electron hydroperoxide reduction activity of H193A and abolished the cyclooxygenase lag phase. These observations are consistent with the involvement of the two-electron oxidized peroxidase intermediate, compound I, as the mediator of the activation of the cyclooxygenase of PGHS.

Mechanism of inhibition of novel COX-2 inhibitors.

Rome and Lands (1975) demonstrated that certain nonsteroidal anti-inflammatory drugs (NSAIDs), exemplified by indomethacin, displayed time dependent inhibition of cyclooxygenase (COX) and that this type of time dependent inhibition is consistent with a two step model. The first step in this model represents the association of enzyme and inhibitor to form a rapidly reversible complex. The second step represents formation of an extremely tight, non-covalent complex that is only slowly reversible. With the recognition of distinct isoforms, it has also been established that COX-2 selective inhibitors are time dependent inhibitors of COX-2, but not of COX-1. This difference in mechanism of inhibition of COX-1 and COX-2 is the basis for their selectivity (Copeland et al., 1994). Time dependence of COX-2 selective inhibitors has been correlated to the presence of a side pocket present in the active site of COX-2 but not COX-1 (Gierse et al., 1996).

The Development of Drugs That Target Cyclooxygenase-2

Cyclooxygenase (COX) catalyzes the committed step in the enzymatic conversion of arachidonic acid to prostaglandins (PGs). Although this enzyme was known to catalytically limit this pathway, other factors, including upstream production of arachidonic acid by phospholipase A2, were thought to regulate cellular PG production (1). In the early 1990s, laboratory observations which showed that both PG synthesis and COX protein are concomitantly upregulated by inflammatory cytokines and modulated by anti-inflammatory steroids like dexamethasone spawned a hypothesis for the existence of an inducible COX isozyme [reviewed in (2,3)]. This hypothesis would radically alter the fields of COX biology and anti-inflammatory therapy.