Tyrosine Nitration Contributes to Nitric Oxide-Stimulated Degradation of CYP2B6

Low levels of nitric oxide promotes heme maturation into several hemeproteins and is also therapeutic

Nitric oxide (NO) is a signal molecule and plays a critical role in the regulation of vascular tone, displays anti-platelet and anti-inflammatory properties. While our earlier and current studies found that low NO doses trigger a rapid heme insertion into immature heme-free soluble guanylyl cyclase β subunit (apo-sGCβ), resulting in a mature sGC-αβ heterodimer, more recent evidence suggests that low NO doses can also trigger heme-maturation of hemoglobin and myoglobin. This low NO phenomena was not only limited to sGC and the globins, but was also found to occur in all three nitric oxide synthases (iNOS, nNOS and eNOS) and Myeloperoxidase (MPO). Interestingly high NO doses were inhibitory to heme-insertion for these hemeproteins, suggesting that NO has a dose-dependent dual effect as it can act both ways to induce or inhibit heme-maturation of key hemeproteins. While low NO stimulated heme-insertion of globins required the presence of the NO-sGC-cGMP signal pathway, iNOS heme-maturation also required the presence of an active sGC. These effects of low NO were significantly diminished in the tissues of double (n/eNOS^−/−) and triple (n/i/eNOS^−/−) NOS knock out mice where lung sGC was found be heme-free and the myoglobin or hemoglobin from the heart/lungs were found be low in heme, suggesting that loss of endogenous NO globally impacts the whole animal and that this impact of low NO is both essential and physiologically relevant for hemeprotein maturation. Effects of low NO were also found to be protective against ischemia reperfusion injury on an ex vivo lung perfusion (EVLP) system prior to lung transplant, which further suggests that low NO levels are also therapeutic.

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Low levels of NO enable heme-maturation of the globins by a process that required an NO triggered heme-insertion into sGCβ.

•This effect of low NO was also found to occur for all three nitric oxide synthases (NOSs) and Myeloperoxidase (MPO).

•Tissues from n/eNOS^−/− and n/i/eNOS^−/− knock out mice had low heme levels in the globins, while sGC was largely heme-free.

•Low NO at ppm levels also manifests itself as a therapy during ischemic reperfusion injury of lungs on the EVLP.

New roles for GAPDH, Hsp90, and NO in regulating heme allocation and hemeprotein function in mammals

The intracellular trafficking of mitochondrial heme presents a fundamental challenge to animal cells. This article provides some background on heme allocation, discusses some of the concepts, and then reviews research done over the last decade, much in the author's laboratory, that is uncovering unexpected and important roles for glyceraldehyde 3-phosphate dehydrogenase (GAPDH), heat shock protein 90 (hsp90), and nitric oxide (NO) in enabling and regulating the allocation of mitochondrial heme to hemeproteins that mature and function outside of the mitochondria. A model for how hemeprotein functions can be regulated in cells through the coordinate participation of GAPDH, hsp90, and NO in allocating cellular heme is presented.

High-Throughput Production of Diverse Xenobiotic Metabolites with Cytochrome P450-Transduced Huh7 Hepatoma Cell Lines

Precision medicine and exposomics require methods to assess xenobiotic metabolism in human metabolomic analyses, including the identification of known and undocumented drug and chemical exposures as well as their metabolites. Recent work demonstrated the use of high-throughput generation of xenobiotic metabolites with human liver S-9 fractions for their detection in human plasma and urine. Here, we tested whether a panel of lentivirally transduced human hepatoma cell lines (Huh7) that express individual cytochrome P450 (P450) enzymes could be used to generate P450-specific metabolites in a high-throughput manner, while simultaneously identifying the enzymes responsible. Cell-line activities were verified using P450-specific probe substrates. To increase analytical throughput, we used a pooling strategy where 36 chemicals were grouped into 12 unique mixtures, each mixture containing 6 randomly selected compounds, and each compound being present in two separate mixtures. Each mixture was incubated with 8 different P450 cell lines for 0 and 2 hours and extracts were analyzed using liquid chromatography-high-resolution mass spectrometry. Cell lines selectively metabolized test substrates, e.g., pazopanib, bupropion, and β-naphthoflavone with expected substrate-enzyme specificities. Predicted metabolites from the remaining 33 compounds as well as many unidentified m/z features were detected. We also showed that a specific bupropion metabolite generated by CYP2B6 cells, but not detected in the S9 system, was identified in human samples. Our data show that the chemical mixtures approach accelerated characterization of xenobiotic chemical space, while simultaneously identifying enzyme sources that can be used for scalable generation of metabolites for their identification in human metabolomic analyses. SIGNIFICANCE STATEMENT: High-resolution mass spectrometry (HRMS) enables the detection of exposures to drugs and other xenobiotics in human samples, but chemical identification can be difficult for several reasons. This paper demonstrates the utility of a panel of engineered cytochrome P450-expressing hepatoma cells in a scalable workflow for production of xenobiotic metabolites, which will facilitate their use as surrogate standards to validate xenobiotic detection by HRMS in human metabolomic studies.

Mitochondrial Fragmentation in a High Homocysteine Environment in Diabetic Retinopathy

Diabetic patients routinely have elevated homocysteine levels, and due to increase in oxidative stress, hyperhomocysteinemia is associated with increased mitochondrial damage. Mitochondrial homeostasis is directly related to the balance between their fission and fusion, and in diabetes this balance is disturbed. The aim of this study was to investigate the role of homocysteine in mitochondrial fission in diabetic retinopathy. Human retinal endothelial cells, either untransfected or transfected with siRNA of a fission protein (dynamin-related protein 1, Drp1) and incubated in the presence of 100 μM homocysteine, were analyzed for mitochondrial fragmentation by live-cell microscopy and GTPase activity of Drp1. Protective nucleoids and mtDNA damage were evaluated by SYBR DNA stain and by transcripts of mtDNA-encoded ND6 and cytochrome b. The role of nitrosylation of Drp1 in homocysteine-mediated exacerbation of mitochondrial fragmentation was determined by supplementing incubation medium with nitric-oxide inhibitor. Homocysteine exacerbated glucose-induced Drp1 activation and its nitrosylation, mitochondrial fragmentation and cell apoptosis, and further decreased nucleoids and mtDNA transcription. Drp1-siRNA or nitric-oxide inhibitor prevented glucose- and homocysteine-induced mitochondrial fission, damage and cell apoptosis. Thus, elevated homocysteine in a hyperglycemic environment increases Drp1 activity via increasing its nitrosylation, and this further fragments the mitochondria and increases apoptosis, ultimately leading to the development of diabetic retinopathy.

Protein Tyrosine Nitration in Plant Nitric Oxide Signaling

Nitric oxide (NO), which is ubiquitously present in living organisms, regulates many developmental and stress-activated processes in plants. Regulatory effects exerted by NO lies mostly in its chemical reactivity as a free radical. Proteins are main targets of NO action as several amino acids can undergo NO-related post-translational modifications (PTMs) that include mainly S-nitrosylation of cysteine, and nitration of tyrosine and tryptophan. This review is focused on the role of protein tyrosine nitration on NO signaling, making emphasis on the production of NO and peroxynitrite, which is the main physiological nitrating agent; the main metabolic and signaling pathways targeted by protein nitration; and the past, present, and future of methodological and strategic approaches to study this PTM. Available information on identification of nitrated plant proteins, the corresponding nitration sites, and the functional effects on the modified proteins will be summarized. However, due to the low proportion of in vivo nitrated peptides and their inherent instability, the identification of nitration sites by proteomic analyses is a difficult task. Artificial nitration procedures are likely not the best strategy for nitration site identification due to the lack of specificity. An alternative to get artificial site-specific nitration comes from the application of genetic code expansion technologies based on the use of orthogonal aminoacyl-tRNA synthetase/tRNA pairs engineered for specific noncanonical amino acids. This strategy permits the programmable site-specific installation of genetically encoded 3-nitrotyrosine sites in proteins expressed in Escherichia coli, thus allowing the study of the effects of specific site nitration on protein structure and function.

Regulation of cytochrome P450 enzyme activity and expression by nitric oxide in the context of inflammatory disease

Many hepatic cytochrome P450 enzymes and their associated drug metabolizing activities are down-regulated in disease states, and much of this has been associated with inflammatory cytokines and their signaling pathways. One such pathway is the induction of inducible nitric oxide synthase (NOS2) and generation of nitric oxide (NO) in many tissues and cells including the liver and hepatocytes. Experiments in the 1990s demonstrated that NO could bind to and inhibit P450 enzymes, and suggested that inhibition of NOS could attenuate, and NO generation could mimic, the down-regulation by inflammatory stimuli of not only P450 catalytic activities but also of mRNA expression and protein levels of certain P450 enzymes. This review will summarize and examine the evidence that NO functionally inhibits and down-regulates P450 enzymes in vivo and in vitro, with a particular focus on the mechanisms by which these effects are achieved.