Nitrosation, Nitration, and Autoxidation of the Selective Estrogen Receptor Modulator Raloxifene by Nitric Oxide, Peroxynitrite, and Reactive Nitrogen/Oxygen Species

Catalytic Mechanism of Aromatic Nitration by Cytochrome P450 TxtE: Involvement of a Ferric-Peroxynitrite Intermediate

The cytochromes P450 are heme-dependent enzymes that catalyze many vital reaction processes in the human body related to biodegradation and biosynthesis. They typically act as mono-oxygenases; however, the recently discovered P450 subfamily TxtE utilizes O₂ and NO to nitrate aromatic substrates such as L-tryptophan. A direct and selective aromatic nitration reaction may be useful in biotechnology for the synthesis of drugs or small molecules. Details of the catalytic mechanism are unknown, and it has been suggested that the reaction should proceed through either an iron(III)-superoxo or an iron(II)-nitrosyl intermediate. To resolve this controversy, we used stopped-flow kinetics to provide evidence for a catalytic cycle where dioxygen binds prior to NO to generate an active iron(III)-peroxynitrite species that is able to nitrate l-Trp efficiently. We show that the rate of binding of O₂ is faster than that of NO and also leads to l-Trp nitration, while little evidence of product formation is observed from the iron(II)-nitrosyl complex. To support the experimental studies, we performed density functional theory studies on large active site cluster models. The studies suggest a mechanism involving an iron(III)-peroxynitrite that splits homolytically to form an iron(IV)-oxo heme (Compound II) and a free NO₂ radical via a small free energy of activation. The latter activates the substrate on the aromatic ring, while compound II picks up the ipso-hydrogen to form the product. The calculations give small reaction barriers for most steps in the catalytic cycle and, therefore, predict fast product formation from the iron(III)-peroxynitrite complex. These findings provide the first detailed insight into the mechanism of nitration by a member of the TxtE subfamily and highlight how the enzyme facilitates this novel reaction chemistry.

Selective Human Estrogen Receptor Partial Agonists (ShERPAs) for Tamoxifen-Resistant Breast Cancer

Almost 70% of breast cancers are estrogen receptor α (ERα) positive. Tamoxifen, a selective estrogen receptor modulator (SERM), represents the standard of care for many patients; however, 30-50% develop resistance, underlining the need for alternative therapeutics. Paradoxically, agonists at ERα such as estradiol (E2) have demonstrated clinical efficacy in patients with heavily treated breast cancer, although side effects in gynecological tissues are unacceptable. A drug that selectively mimics the actions of E2 in breast cancer therapy but minimizes estrogenic effects in other tissues is a novel, therapeutic alternative. We hypothesized that a selective human estrogen receptor partial agonist (ShERPA) at ERα would provide such an agent. Novel benzothiophene derivatives with nanomolar potency in breast cancer cell cultures were designed. Several showed partial agonist activity, with potency of 0.8-76 nM, mimicking E2 in inhibiting growth of tamoxifen-resistant breast cancer cell lines. Three ShERPAs were tested and validated in xenograft models of endocrine-independent and tamoxifen-resistant breast cancer, and in contrast to E2, ShERPAs did not cause significant uterine growth.

Formation of raloxifene homo-dimer in CYP3A4, evidence for multi-substrate binding in a single catalytically competent P450 active site

Raloxifene is a polyaromatic compound which has been reported to form radicals when incubated with horseradish peroxidase resulting in formation of a homo-dimer product. Polyaromatic phenols have also been reported to undergo oxidation by P450 enzymes to form reactive intermediates, presumably through the formation of phenoxy radical species. Recently, we observed that a raloxifene homo-dimer was formed in vitro when incubated with CYP3A4. In response to this finding, a series of experiments were designed to determine whether the observed raloxifene homo-dimer was formed via solution phase chemistry similar to that previously documented with horseradish peroxidase or if generation of the homo-dimer occurred within the P450 active site. To this end, a series of experiments were carried out to determine the structure of the CYP3A4 generated raloxifene homo-dimer using analytical techniques including: high resolution MS, NMR and H/D exchange. In addition, a variety of in vitro techniques were applied to characterize the mechanism responsible for formation of the raloxifene homo-dimer. Collectively, the results of these experiments suggest that unlike the homo-dimer formed by peroxidase enzymes, raloxifene homo-dimer formation mediated by CYP3A4 is a consequence of two raloxifene molecules binding simultaneously within the active site of a catalytically competent P450 enzyme.

Roles of 3-nitrotyrosine- and 4-hydroxynonenal-modified brain proteins in the progression and pathogenesis of Alzheimer's disease

Proteins play an important role in normal structure and function of the cells. Oxidative modification of proteins may greatly alter the structure and may subsequently lead to loss of normal physiological cell functions and may lead to abnormal function of cell and eventually to cell death. These modifications may be reversible or irreversible. Reversible protein modifications, such as phosphorylation, can be overcome by specific enzymes that cause a protein to 'revert' back to its original protein structure, while irreversible protein modifications cannot. Several important irreversible protein modifications include protein nitration and HNE modification, both which have been extensively investigated in research on the progression of Alzheimer's disease (AD). From the earliest stage of AD throughout the advancement of the disorder there is evidence of increased protein nitration and HNE modification. These protein modifications lead to decreased enzymatic activity, which correlates directly to protein efficacy and provides support for several common themes in AD pathology, namely altered energy metabolism, mitochondrial dysfunction and reduced cholinergic neurotransmission. The current review summarized some of the findings on protein oxidation related to different stages of Alzheimer's disease (AD) that will be helpful in understanding the role of protein oxidation in the progression and pathogenesis of AD.

Uterine peroxidase-catalyzed formation of diquinone methides from the selective estrogen receptor modulators raloxifene and desmethylated arzoxifene

Long-term usage of the selective estrogen receptor modulator (SERM) tamoxifen has been associated with an increased risk of endometrial cancer. One potential mechanism of tamoxifen-induced carcinogenesis involves metabolism to reactive intermediates, such as an o-quinone, quinone methide, and carbocations. We have previously shown that the benzothiophene SERMs, raloxifene and desmethylated arzoxifene (DMA), can also be bioactivated to electrophilic quinoids by rat/human liver microsomes and rat hepatocytes [(2006) Chem. Res. Toxicol. 19, 1125-1137]. Because the uterus is a major target tissue of estrogens and antiestrogens, it was of interest to determine if quinoids could be formed from SERMs in uterine tissue potentially producing cytotoxic effects. Incubations with rat uterine microsomes showed that both raloxifene and DMA could be oxidized to electrophilic diquinone methides that were trapped as the corresponding GSH conjugates. A new raloxifene GSH-dependent conjugate was identified as raloxifene Cys-Gly that was formed from the hydrolysis of 7-glutathinyl raloxifene by gamma-glutamyl transpeptidase. Interestingly, the metabolism of raloxifene and DMA in rat uterine microsomes was not NADPH-dependent and could be inhibited by cyanide and NADPH or enhanced by H2O2. In addition, coincubations with the peroxidase substrates guaiacol or o-phenlyenediamine inhibited diquinone methide GSH conjugate formation from both SERMs. Incubations of raloxifene and DMA with horseradish peroxidase (HRP) were studied as models of the interaction between benzothiophene SERMs and peroxidase. The results showed that HRP could directly oxidize raloxifene and DMA to the corresponding dimers via the formation of phenoxyl radicals in the absence of exogenous hydrogen peroxide. In addition, GSH appears to be involved in multiple peroxidase-catalyzed oxidative metabolic pathways of benzothiophene SERMs. Finally, COATag (covert oxidatively activated tag) methodology, which involves the utilization of biotin-conjugated raloxifene and DMA, was used to identify target proteins by affinity chromatography. Incubations of raloxifene and DMA COATags with rat uterine microsomes showed several modified proteins by Western blot analysis. The protein modification could be enhanced by the addition of H2O2 and decreased by the addition of NADPH, suggesting that unlike liver metabolism the formation of quinoids in the uterus could be mediated by uterine peroxidases.

Synthesis of novel nitro-substituted triaryl pyrazole derivatives as potential estrogen receptor ligands

Novel tetrasubstituted pyrazole derivatives bearing a nitro substituent on their A-phenol ring were synthesized and their binding affinity towards the estrogen receptor (ER) subtypes ERalpha and ERbeta was determined. Among compounds tested, the 2-nitrophenol derivative 5c was found to bind satisfactorily to both estrogen receptor subtypes (RBAalpha=5.17 and RBAbeta=3.27). In general, the introduction of a nitro group into the A ring of these compounds was found to benefit their ERbeta binding abilities.

Chemical modification modulates estrogenic activity, oxidative reactivity, and metabolic stability in 4'F-DMA, a new benzothiophene selective estrogen receptor modulator

The benzothiophene selective estrogen receptor modulators (SERMs), raloxifene and arzoxifene, in the clinic or clinical trials for treatment of breast cancer and postmenopausal symptoms, are highly susceptible to oxidative metabolism and formation of electrophilic metabolites. 4'F-DMA, fluoro-substituted desmethyl arzoxifene (DMA), showed attenuated oxidation to quinoids in incubation with rat hepatocytes as well as in rat and human liver microsomes. Incubations of 4'F-DMA with hepatocytes yielded only one glucuronide conjugate and no GSH conjugates, whereas DMA underwent greater metabolism giving two glucuronide conjugates, one sulfate conjugate, and two GSH conjugates. Phase I and phase II metabolism were further evaluated in human small intestine microsomes and in human intestinal Caco-2 cells. In comparison to DMA, 4'F-DMA formed significantly less glucuronide and sulfate conjugates. The formation of quinoids was further explored in hepatocytes in which DMA was observed to give concentration- and time-dependent depletion of GSH accompanied by damage to DNA, which showed inverse dependence on GSH; in contrast, GSH depletion and DNA damage were almost completely abrogated in incubations with 4'F-DMA. 4'F-DMA shows ligand binding affinity to estrogen receptor (ER)alpha and ERbeta with similarity to both raloxifene and to DMA. ER-mediated biological activity was measured with the ERE-luciferase reporter system in transfected MCF-7 cells and Ishikawa cells, and in MCF-7 cells, proliferation was measured. In all systems, 4'F-DMA exhibited anitestrogenic activity of comparable potency to raloxifene but did not manifest estrogenic properties, mirroring previous results on inhibition of estradiol-mediated induction of alkaline phosphatase activity in Ishikawa cells. These results suggest that 4'F-DMA might be an improved benzothiophene SERM with similar antiestrogenic activity to raloxifene but improved metabolic stability and attenuated toxicity, showing that simple chemical modification can abrogate oxidative bioactivation to potentially toxic metabolites without loss of activity.

Estrogen attenuates gp120- and tat1-72-induced oxidative stress and prevents loss of dopamine transporter function

Postmenopausal women who are infected with HIV are at risk for experiencing dementia and Parkinson's-like symptoms associated with low levels of estrogen. Neurotoxic damage leading to these symptoms may involve HIV-associated proteins gp120 and/or tat(1-72) (tat). Our hypothesis is that 17beta-Estradiol (E(2)) is an effective agent for protection against gp120/tat-induced damage associated with increased oxidative stress, with particular focus on peroxynitrite-induced oxidative stress. We used SK-N-SH cells and striatal synaptosomes from Sprague-Dawley rats as model systems to assess neuroprotection by E(2). Cells coincubated with SIN-1(3-morpholinosydnonimine) or tat and gp120, together or separately, significantly increased oxidative stress on the SK-N-SH cells, as indicated by the increase in the levels of dichlorofluorescein (DCFH) fluorescence. These data suggest that a component of tat and gp120 neurotoxicity may be due to increased oxidative stress. Coincubation with E(2) attenuated tat- and gp120-induced increase in fluorescence. Coincubation with progesterone had no effect on tat-induced fluorescence, whereas coincubation with the E(2) antagonist ICI 182,780 and E(2) completely prevented the effects observed with E(2) alone. Both gp120 and tat decreased [(3)H] dopamine uptake into striatal synaptosomes by decreasing the V(max) of the dopamine transporter (DAT). Pretreatment of synaptosomes with E(2) (100 nM) partially reversed this reduction. In conclusion, E(2) appears to be effective for preventing the oxidative stress and loss of DAT function associated with gp120/tat neurotoxicity.

Analysis of protein covalent modification by xenobiotics using a covert oxidatively activated tag: raloxifene proof-of-principle study

Numerous xenobiotics, including therapeutics agents, are substrates for bioactivation to electrophilic reactive intermediates that may covalently modify biomolecules. Selective estrogen receptor modulators (SERMs) are in clinical use for long-term therapy of postmenopausal syndromes and chemoprevention and provide a potential alternative for hormone replacement therapy (HRT). Raloxifene, in common with many SERMs and other xenobiotics, is a polyaromatic phenol that has been shown to be metabolically bioactivated to electrophilic and redox active quinoids. Nucleic acid and glutathione adduct formation have been reported, but little is known about protein covalent modification. A novel COATag (covert oxidatively activated tag) was synthesized in which raloxifene was linked to biotin. The COATag was reactive toward a model protein, human glutathione-S-transferase P1-1, in the presence but not the absence of monooxygenase. The covalent modification of proteins in rat liver microsomal incubations was NADPH-dependent implicating cytochrome P450 oxidase. The COATag facilitated isolation and identification of covalently modified microsomal proteins: cytosolic glucose regulated protein (GRP78/BiP), three protein disulfide isomerases, and microsomal glutathione S-transferase 1. Oxidative metabolism of raloxifene produces reactive intermediates of sufficient lifetimes to covalently modify proteins in tissue microsomes, behavior anticipated for other polyaromatic phenol xenobiotics that can be tested by the COATag methodology. The combined use of a COATag with a simple biotin-linked electrophile (such as an iodoacetamide tag) is a new technique that allows quantification of protein covalent modification via alkylation vs oxidation in response to xenobiotic reactive intermediates. The identification of modified proteins is important for defining pathways that might lead alternatively to either cytotoxicity or cytoprotection.

17β-Estradiol nitration by peroxidase/H2O2/NO2−: a chemical assessment

Nitration of 17beta-estradiol by H(2)O(2) and nitrite in the presence of various peroxidases, viz. horseradish peroxidase, lactoperoxidase, and peroxidase-containing homogenates from bovine uteri, was systematically investigated to assess on a chemical basis its potential relevance to the mechanisms of impairment of estrogen functions under oxidative/nitrosative stress conditions. In the presence of excess nitrite 17beta-estradiol reacted smoothly to give 2-nitroestradiol (1), 4-nitroestradiol (2), and 2,4-dinitroestradiol (3). With 10-300 microM estradiol, formation yields of 1-3 were 12-55%, but dropped to 1% or less at lower estrogen concentration, for example, 1 microM, or in plasma as the reaction medium. Time course analysis showed that 2 is the prevalent nitration product under conditions of slow generation of nitrating species, suggesting some regioselectivity for estradiol nitration at C-4, whereas 1 prevails with bolus addition of reagents, due to faster degradation of 2. Competition experiments carried out with (15)NO(2)- showed that 2 is about twice more susceptible to nitration than 1 as determined by (15)N NMR analysis of the resulting 3. The biological effects of 1 and 2 were preliminarily tested on in vitro bovine embryo cultures. When 1 and 2 were substituted to the standard 17beta-estradiol in the oocyte maturation, a significant decrease in both cleavage and blastocyst efficiency was observed in the case of 1 but not 2. Overall, these results suggest that estradiol nitration is a potential pathway of hormonal dysfunction and toxicity but would require elevated estrogen levels of questionable physiological relevance.