Use of Nonhuman Sera as a Highly Cost-Effective Internal Standard for Quantitation of Multiple Human Proteins Using Species-Specific Tryptic Peptides: Applicability in Clinical LC-MS Analyses

Quantitation of proteins using liquid chromatography-tandem mass spectrometry (LC-MS/MS) is complex, with a multiplicity of options ranging from label-free techniques to chemically and metabolically labeling proteins. Increasingly, for clinically relevant analyses, stable isotope-labeled (SIL) internal standards (ISs) represent the "gold standard" for quantitation due to their similar physiochemical properties to the analyte, wide availability, and ability to multiplex to several peptides. However, the purchase of SIL-ISs is a resource-intensive step in terms of cost and time, particularly for screening putative biomarker panels of hundreds of proteins. We demonstrate an alternative strategy utilizing nonhuman sera as the IS for quantitation of multiple human proteins. We demonstrate the effectiveness of this strategy using two high abundance clinically relevant analytes, vitamin D binding protein [Gc globulin] (DBP) and albumin (ALB). We extend this to three putative risk markers for cardiovascular disease: plasma protease C1 inhibitor (SERPING1), annexin A1 (ANXA1), and protein kinase, DNA-activated catalytic subunit (PRKDC). The results show highly specific, reproducible, and linear measurement of the proteins of interest with comparable precision and accuracy to the gold standard SIL-IS technique. This approach may not be applicable to every protein, but for many proteins it can offer a cost-effective solution to LC-MS/MS protein quantitation.

The Edge Effect in High-Throughput Proteomics: A Cautionary Tale

In order for mass spectrometry to continue to grow as a platform for high-throughput clinical and translational research, careful consideration must be given to quality control by ensuring that the assay performs reproducibly and accurately and precisely. In particular, the throughput required for large cohort clinical validation in biomarker discovery and diagnostic screening has driven the growth of multiplexed targeted liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) assays paired with sample preparation and analysis in multiwell plates. However, large scale MS-based proteomics studies are often plagued by batch effects: sources of technical variation in the data, which can arise from a diverse array of sources such as sample preparation batches, different reagent lots, or indeed MS signal drift. These batch effects can confound the detection of true signal differences, resulting in incorrect conclusions being drawn about significant biological effects or lack thereof. Here, we present an intraplate batch effect termed the edge effect arising from temperature gradients in multiwell plates, commonly reported in preclinical cell culture studies but not yet reported in a clinical proteomics setting. We present methods herein to ameliorate the phenomenon including proper assessment of heating techniques for multiwell plates and incorporation of surrogate standards, which can normalize for intraplate variation.

Severe acute respiratory syndrome coronavirus 2 may be an underappreciated pathogen of the central nervous system

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) causes a highly contagious respiratory disease referred to as COVID‐19. However, emerging evidence indicates that a small but growing number of COVID‐19 patients also manifest neurological symptoms, suggesting that SARS‐CoV‐2 may infect the nervous system under some circumstances. SARS‐CoV‐2 primarily enters the body through the epithelial lining of the respiratory and gastrointestinal tracts, but under certain conditions this pleiotropic virus may also infect peripheral nerves and gain entry into the central nervous system (CNS). The brain is shielded by various anatomical and physiological barriers, most notably the blood–brain barrier (BBB) which functions to prevent harmful substances, including pathogens and pro‐inflammatory mediators, from entering the brain. The BBB is composed of highly specialized endothelial cells, pericytes, mast cells and astrocytes that form the neurovascular unit, which regulates BBB permeability and maintains the integrity of the CNS. In this review, potential routes of viral entry and the possible mechanisms utilized by SARS‐CoV‐2 to penetrate the CNS, either by disrupting the BBB or infecting the peripheral nerves and using the neuronal network to initiate neuroinflammation, are briefly discussed. Furthermore, the long‐term effects of SARS‐CoV‐2 infection on the brain and in the progression of neurodegenerative diseases known to be associated with other human coronaviruses are considered. Although the mechanisms of SARS‐CoV‐2 entry into the CNS and neurovirulence are currently unknown, the potential pathways described here might pave the way for future research in this area and enable the development of better therapeutic strategies.

Proteomic Characterization of Circulating Molecular Perturbations Associated With Pancreatic Adenocarcinoma Following Intravenous ω-3 Fatty Acid and Gemcitabine Administration: A Pilot Study

BACKGROUND

Administration of intravenous ω-3 fatty acid (ω-3FA) in advanced pancreatic adenocarcinoma patients receiving gemcitabine chemotherapy shows disease stabilization and improved progression-free survival. Using high-definition plasma proteomics, the underlying biological mechanisms responsible for these clinical effects are investigated.

METHODS AND RESULTS

A pilot study involving plasma that was collected at baseline from 13 patients with histologically confirmed, unresectable pancreatic adenocarcinoma (baseline group) after 1-month treatment with intravenous gemcitabine and ω-3FA (treatment group) and intravenous gemcitabine only (control group) and was prepared for proteomic analysis. A 2-arm study comparing baseline vs treatment and treatment vs control was performed. Proteins were isolated from plasma with extensive immunodepletion, then digested and labeled with isobaric tandem mass tag peptide tags. Samples were then combined, fractionated, and injected into a QExactive-Orbitrap Mass-Spectrometer and analyzed on Proteome Discoverer and Scaffold with ensuing bioinformatics analysis. Selective reaction monitoring analysis was performed for verification. In total, 3476 proteins were identified. Anti-inflammatory markers (C-reactive protein, haptoglobin, and serum amyloid-A1) were reduced in the treatment group. Enrichment analysis showed angiogenesis downregulation, complement immune systems upregulation, and epigenetic modifications on histones. Pathway analysis identified direct action via the Pi3K-AKT pathway. Serum amyloid-A1 significantly reduced (P < .001) as a potential biomarker of efficacy for ω-3FA.

CONCLUSIONS

This pilot study demonstrates administration of ω-3FA has potential anti-inflammatory, antiangiogenic, and proapoptotic effects via direct interaction with cancer-signaling pathways in patients with advanced pancreatic adenocarcinoma. Further studies in a larger sample size is required to validate the clinical correlation found in this preliminary study.

Plasma proteomic approach in patients with heart failure: insights into pathogenesis of disease progression and potential novel treatment targets

AIMS

To provide insights into pathogenesis of disease progression and potential novel treatment targets for patients with heart failure by investigation of the plasma proteome using network analysis.

METHODS AND RESULTS

The plasma proteome of 50 patients with heart failure who died or were rehospitalised were compared with 50 patients with heart failure, matched for age and sex, who did not have an event. Peptides were analysed on two-dimensional liquid chromatography coupled to tandem mass spectrometry (2D LC ESI-MS/MS) in high definition mode (HDMSE). We identified and quantified 3001 proteins, of which 51 were significantly up-regulated and 46 down-regulated with more than two-fold expression changes in those who experienced death or rehospitalisation. Gene ontology enrichment analysis and protein-protein interaction networks of significant differentially expressed proteins discovered the central role of metabolic processes in clinical outcomes of patients with heart failure. The findings revealed that a cluster of proteins related to glutathione metabolism, arginine and proline metabolism, and pyruvate metabolism in the pathogenesis of poor outcome in patients with heart failure who died or were rehospitalised.

CONCLUSIONS

Our findings show that in patients with heart failure who died or were rehospitalised, the glutathione, arginine and proline, and pyruvate pathways were activated. These pathways might be potential targets for therapies to improve poor outcomes in patients with heart failure.

Using matrix assisted laser desorption ionisation mass spectrometry (MALDI-MS) profiling in order to predict clinical outcomes of patients with heart failure

BACKGROUND

Current risk prediction models in heart failure (HF) including clinical characteristics and biomarkers only have moderate predictive value. The aim of this study was to use matrix assisted laser desorption ionisation mass spectrometry (MALDI-MS) profiling to determine if a combination of peptides identified with MALDI-MS will better predict clinical outcomes of patients with HF.

METHODS

A cohort of 100 patients with HF were recruited in the biomarker discovery phase (50 patients who died or had a HF hospital admission vs. 50 patients who did not have an event). The peptide extraction from plasma samples was performed using reversed phase C18. Then samples were analysed using MALDI-MS. A multiple peptide biomarker model was discovered that was able to predict clinical outcomes for patients with HF. Finally, this model was validated in an independent cohort with 100 patients with HF.

RESULTS

After normalisation and alignment of all the processed spectra, a total of 11,389 peptides (m/z) were detected using MALDI-MS. A multiple biomarker model was developed from 14 plasma peptides that was able to predict clinical outcomes in HF patients with an area under the receiver operating characteristic curve (AUC) of 1.000 (p = 0.0005). This model was validated in an independent cohort with 100 HF patients that yielded an AUC of 0.817 (p = 0.0005) in the biomarker validation phase. Addition of this model to the BIOSTAT risk prediction model increased the predictive probability for clinical outcomes of HF from an AUC value of 0.643 to an AUC of 0.823 (p = 0.0021). Moreover, using the prediction model of fourteen peptides and the composite model of the multiple biomarker of fourteen peptides with the BIOSTAT risk prediction model achieved a better predictive probability of time-to-event in prediction of clinical events in patients with HF (p = 0.0005).

CONCLUSIONS

The results obtained in this study suggest that a cluster of plasma peptides using MALDI-MS can reliably predict clinical outcomes in HF that may help enable precision medicine in HF.

Growth hormone for risk stratification and effects of therapy in acute myocardial infarction

CONTEXT

Excess growth hormone (GH) is associated with early mortality.

OBJECTIVES

We assessed the association of GH with prognosis after acute myocardial infarction (AMI), and the effects of secondary prevention therapies.

METHODS

GH was measured using a high-sensitivity assay in 953 AMI patients (687 males, mean age 66.1 ± 12.8 years).

RESULTS

During 2 years follow-up, there were 281 major adverse cardiac events (MACE). Patients with MACE had higher GH levels (median [range], 0.91 [0.04-26.28] μg/L) compared to event-free survivors (0.59 [0.02-21.6], p < 0.0005). In multivariate Cox survival analysis, GH was a significant predictor of MACE (hazard ratios 1.43, p = 0.026 and 1.49, p = 0.01, respectively) with significant interactions with beta blocker therapy (p = 0.047) and angiotensin converting enzyme inhibitor or angiotensin receptor blocker (ACE/ARB) therapy (p = 0.016).

CONCLUSIONS

GH levels post-AMI are prognostic for MACE and may indicate those patients who benefit from beta blocker and ACE/ARB therapy.

Identification of novel biomarkers in plasma for prediction of treatment response in patients with heart failure

BACKGROUND

Heart failure is a complex clinical syndrome that occurs at the end stage of heart disease. Despite advances in therapy for heart failure, improvement of clinical outcomes remains a challenge for physicians. The identification of treatment response early in the course of disease would be useful to improve management of these patients. The aim of this study was to identify novel biomarkers in plasma that could predict treatment response in patients with heart failure.

METHODS

Patients with heart failure who met inclusion and exclusion criteria according to the guidelines of the European Society of Cardiology were recruited. Uptitration of angiotensin-converting enzyme inhibitors and β blockers was performed over 6 months. Patients were followed up for clinical events within the next 24 months. Plasma proteins in patients who responded to standard treatment (responders) were compared with patients who died or were re-admitted for heart failure (non-responders). Plasma samples were depleted of 14 high abundance proteins with a multiple affinity removal system column (MARS). Then plasma samples were analysed on two-dimensional liquid chromatography coupled to a tandem mass spectrometry (2D LC-ESI-MS/MS) in high definition mode (HDMS(E)) to identify and quantify the different expression of proteins in plasma. Finally, ELISA was used to verify candidate biomarkers.

FINDINGS

Participants were 100 patients with heart failure matched for sex and age (50 responders [25 women], 50 non-responders [25 women], mean age 76·6 years [SD 8·1]). Of the non-responders, 18 died and 32 were re-admitted to hospital. 2D LC-ESI-MS/MS showed that the expression of neurotrimin (NTM) was highly upregulated, by 26·5 times (p<0·0001), in the responder group compared with the non-responder group. ELISA in the verification phase showed that the concentrations of NTM in plasma were significantly higher in the responders and lower in the non-responders (mean 4·73 log10 relative light units [SD 0·07] vs 4·70 [0·08], p=0·036). When ANOVA with Bonferroni post-hoc comparisons was used in three outcome subgroups (responders, patients re-admitted to hospital, and deaths), NTM concentrations were significantly different between death and the other groups (higher in responder vs death group, p<0·0001; higher in re-admission vs death group, p=0·001).

INTERPRETATION

Our findings suggest that NTM as a novel biomarker in heart failure will not only add information to understand the pathophysiological mechanisms of heart failure better, but also might provide a more accurate prediction of treatment response to guide medical therapy. In addition, a novel therapeutic target could be identified for design of drugs to improve outcomes. Futher work is required in larger populations to confirm this biomarker.

FUNDING

European Union's Seventh Framework Programme (BIOSTAT-CHF), John and Lucille van Geest Foundation.

Proenkephalin and prognosis after acute myocardial infarction

OBJECTIVES

The goal of this research was to assess the prognostic value of proenkephalin (PENK) levels in acute myocardial infarction (AMI) by using N-terminal pro-B-type natriuretic peptide and Global Registry of Acute Coronary Events (GRACE) scores as comparators and to identify levels that might be valuable in clinical decision making.

BACKGROUND

PENK is a stable analyte of labile enkephalins. Few biomarkers predict recurrent AMI.

METHODS

We measured PENK in 1,141 patients (820 male subjects; mean age 66.2 ± 12.8 years) with AMI. Endpoints were major adverse events (composite of death, myocardial infarction [MI], and heart failure [HF] hospitalization) and recurrent MI at 2 years. GRACE scoring was used for comparisons with PENK for the death and/or MI endpoint at 6 months.

RESULTS

During follow-up, 139 patients died, and there were 112 HF hospitalizations and 149 recurrent AMIs. PENK levels were highest on admission and were related to estimated glomerular filtration rate, left ventricular wall motion index, sex, blood pressure, and age. Multivariable Cox regression models found that the PENK level was a predictor of major adverse events (hazard ratio [HR]: 1.52 [95% confidence interval (CI): 1.19 to 1.94]), death and/or AMI (HR: 1.76 [95% CI: 1.34 to 2.30]), and death and/or HF (HR: 1.67 [95% CI: 1.24 to 2.25]) (all comparisons p < 0.001), as well as recurrent AMI (HR: 1.43 [95% CI: 1.07 to 1.91]; p < 0.01). PENK levels were independent predictors of 6-month death and/or MI compared with GRACE scores. PENK-adjusted GRACE scores reclassified patients significantly (overall category-free net reclassification improvement [>0] of 21.9 [95% CI: 4.5 to 39.4]; p < 0.014). PENK levels <48.3 pmol/l and >91 pmol/l detected low- and high-risk patients, respectively.

CONCLUSIONS

PENK levels reflect cardiorenal status post-AMI and are prognostic for death, recurrent AMI, or HF. Cutoff values define low- and high-risk groups and improve risk prediction of GRACE scores.

Non-invasive assessment of oxidatively damaged DNA: liquid chromatography-tandem mass spectrometry analysis of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine

The ability to non-invasively assess DNA oxidation and its repair, has significant utility in large-scale, population-based studies. Such studies could include the assessments of: the efficacy of antioxidant intervention strategies, pathological roles of DNA oxidation in various disease states and population or interindividual differences in antioxidant defence and DNA repair. The most popular method, to non-invasively assess oxidative insult to the genome is by the analysis of urine for 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), using chromatographic techniques or immunoassay procedures. The provenance of extracellular 8-oxodG remains a subject for debate. However, previous studies have shown that factors, such as diet and cell death, do not appear to contribute to extracellular 8-oxodG, leaving processes, such as the repair of DNA and/or the 2'-deoxyribonucleotide pool, as the sole source of endogenous 8-oxodG. The method in this chapter describes a non-invasive approach for assessing oxidative stress, via the efficient extraction of urinary 8-oxodG using a validated solid-phase extraction procedure. Subsequent analysis by liquid chromatography-tandem mass spectrometry provides the advantages of sensitivity, internal standardisation, and robust peak identification, and is widely considered to be the "gold standard".

Development of a novel site-specific mutagenesis assay using MALDI-ToF MS (SSMA-MS)

We have developed and validated a novel site-specific mutagenesis assay, termed SSMA-MS, which incorporates MALDI-ToF mass spectrometry (MALDI-MS) analysis as a means of determining the mutations induced by a single DNA adduct. The assay involves ligating an adducted deoxyoligonucleotide into supF containing pSP189 plasmid. The plasmid is transfected into human Ad293 kidney cells allowing replication and therefore repair or a mutagenic event to occur. Escherichia coli indicator bacteria are transformed with recovered plasmid and plasmids containing the insert are identified colormetrically, as they behave as frameshift mutations. The plasmid is then amplified and digested using a restriction cocktail of Mbo11 and Mnl1 to yield 12 bp deoxyoligonucleotides, which are characterized by MALDI-MS. MALDI-MS takes advantage of the difference in molecular weight between bases to identify any induced mutations. This analysis method therefore provides qualitative and quantitative information regarding the type and frequency of mutations induced. This assay was developed and validated using an O⁶-methyl-2′-deoxyguanosine adduct, which induced the expected GC→AT substitutions, when replicated in human or bacterial cells. This approach can be applied to the study of any DNA adduct in any biologically relevant gene sequence (e.g. p53) in human cells and would be particularly amenable to high-throughput analysis.

Constitutive expression of interleukin-8 by Mutatect cells markedly affects their tumor biology

Interleukin-8 (IL-8) is a chemokine for neutrophils and an angiogenic factor. Human tumors that express IL-8 may exhibit intense neutrophil infiltration and increased vascularization. Mutatect cells are a murine fibrosarcoma that can be grown as subcutaneous tumors in syngeneic C57BL/6 mice. Since neutrophils are a source of cytotoxic and genotoxic species, we constructed Mutatect cell lines that constitutively express human IL-8 to explore the involvement of neutrophils in tumor biology and genetic instability. An IL-8/neo expression plasmid was stably transfected into Mutatect MC17-51 cells and clone MIL-4 was isolated. Tumors initiated with 5x10(5) MIL-4 cells grew very slowly compared with tumors from pure MC17-51 cells or from 0.5 to 4x10(5) MIL-4 cells mixed with 5x10(5) MC17-51 cells. Over 95% of cells recovered from slow-growing pure MIL-4 tumors lost the transgene as measured by loss of (i) resistance to G418, (ii) expression of IL-8 protein and (iii) IL-8-specific DNA sequences. When tumors from mixed cell types were examined, loss of the transgene did not occur; rather, IL-8 producing cells appeared to have some growth advantage. The neutrophil content of tumors (as measured by myeloperoxidase) was directly proportional to the level of IL-8 expressed at the time tumors were excised. As reported earlier, the frequency of mutations at the hypoxanthine phosphoribosyltransferase locus was also directly proportional to neutrophil content. To explain some of these biological findings, we postulate that early in development of pure MIL-4 tumors, genotoxic/cytotoxic neutrophils are attracted by IL-8, which in turn leads to loss of the transgene and to localized cytotoxicity of IL-8 producing cells. In mixed tumors, where the initial IL-8 concentration may be lower, tumors might become established more readily because fewer neutrophils may be attracted. This relatively simple experimental paradigm has revealed some of the complex biological changes that can occur as a result of IL-8 in tumors.

Expression of interleukin-8 promotes neutrophil infiltration and genetic instability in mutatect tumors

Neutrophils represent a potential source of genotoxic reactive oxygen and nitrogen species in the tumor microenvironment. Using Mutatect cell lines, which can form subcutaneous tumors in syngeneic C57BL/6 mice, we have previously established that the number of spontaneously infiltrating neutrophils correlates with the number of mutations at the hypoxanthine phosphoribosyltransferase (Hprt) locus. We now describe the properties of four lines that express different levels of the neutrophil chemokine, interleukin-8 (IL-8), from a tetracycline (TET)-responsive promoter. In a series involving 45 animals, IL-8-expressing lines produced tumors with a higher neutrophil content than the control line. Analysis of the 45 tumors revealed that the neutrophil level again strongly correlated with hprt mutant frequency (MF) (P<.0001, r=0.88). Administration of TET was effective in lowering the neutrophil content of low IL-8-expressing tumors, but not high IL-8-expressing tumors. Although the IL-8 transgene was stable in all lines in vitro, high IL-8-expressing lines completely lost the transgene in vivo whereas low IL-8-expressing lines showed no evidence of transgene instability. These results provide further evidence, based on the study of an endogenous gene (hprt) and an IL-8 transgene, that neutrophils may contribute to genetic instability in tumors.

Effect of dietary vitamin E on spontaneous or nitric oxide donor-induced mutations in a mouse tumor model

BACKGROUND

Vitamin E, an antioxidant, has been investigated for its effect on cancer incidence in humans, but no firm conclusions about a protective effect can be drawn from these studies. Recently, we reported a statistically significant correlation in the Mutatect mouse tumor model between the number of neutrophils and the frequency of mutation at the hypoxanthine phosphoribosyltransferase (hprt) locus. We have now used this model to investigate vitamin E's effect on the hprt mutation rate.

METHODS

Mutatect cells were grown in mice as subcutaneous tumors for 2-3 weeks, the tumor cells were recovered, and 6-thioguanine-resistant (i.e., hprt mutant) colonies were scored. Myeloperoxidase activity was used as a measure of neutrophil infiltration. Vitamin E (2 IU/kg body weight) was provided in the diet for 3-4 weeks. In some experiments, glyceryl trinitrate (100 mg/kg body weight) was also administered as a source of nitric oxide. All statistical tests were two-sided.

RESULTS

Mouse tumors from the Mutatect MN-11 cell line exhibited a 3.2-fold higher median mutation frequency than the same cells in culture (P:<. 0001); vitamin E reduced this frequency by 24.9% (P: =.01). Mutatect TM-28-derived tumors (which secrete interleukin 8) were heavily infiltrated with neutrophils and had a correspondingly high mutation frequency; in two separate experiments, vitamin E reduced the median mutation frequency by 68.9% (P: =.0019) and 84.1% (P: =.011) and myeloperoxidase levels by 75.3% (P: =.0002) and 75.5% (P: =.026), respectively. Glyceryl trinitrate increased the mutation frequency in MN-11 tumors, and vitamin E reduced the median frequency by 61.4% (P: =.058).

CONCLUSIONS

Dietary vitamin E afforded strong protection against both spontaneously arising and nitric oxide-induced mutations. Two separate protective mechanisms by vitamin E may be operating: scavenging of a nitric oxide-related genotoxic species and altering the infiltration of neutrophils into tumors.

Neutrophils, nitric oxide synthase, and mutations in the mutatect murine tumor model

Mutatect MN-11 is a tumor line that can be grown subcutaneously in syngeneic C57BL/6 mice. The frequency of spontaneously arising mutants at the hypoxanthine phosphoribosyltransferase (Hprt) locus was observed to be elevated as a result of in vivo growth. The objective of the present study was to identify factors in the tumor microenvironment that might explain this increase in mutant frequency (MF). When tumors were examined histologically, neutrophils were found to be the predominant infiltrating cell type. Quantitative estimates of the number of neutrophils and MF of tumors in different animals revealed a statistically significant correlation (r = 0.63, P < 0.0001). Immunohistochemical analysis for inducible nitric oxide synthase (iNOS) demonstrated its presence, mainly in neutrophils. Biochemical analysis of tumor homogenates for nitric oxide synthase (NOS) activity indicated a statistically significant correlation with MF (r = 0.77, P < 0.0001). Nitrotyrosine was detected throughout the tumor immunohistochemically; both cytoplasmic and nuclear staining was seen. To increase the number of infiltrating neutrophils, tumors were injected with chemoattractant interleukin-8 and prostaglandin E2. This produced a statistically significant increase in neutrophil content (P = 0.005) and MF (P = 0.0002). As in control MN-11 tumors, neutrophil content and MF were strongly correlated (r = 0.63, P = 0. 003). Because neutrophils are a potential source of genotoxic reactive oxygen and/or nitrogen species, our results support the notion that these tumor-infiltrating cells may be mutagenic and contribute to the burden of genetic abnormalities associated with tumor progression.

Mutatect: a mouse tumour model for detecting radiation-induced mutations in vivo

A new mouse model (Mutatect) that permits detection of mutations at the hprt (hypoxanthine phosphoribosyltransferase) locus is described. It is highly sensitive to detection of mutants induced by clastogenic agents such as ionizing radiation. MN-11 cells are grown as a subcutaneous tumour in C57BL/6 mice for a period of 2 weeks, during which time they can be exposed to mutagenic treatments. Cells taken from the animal are cultured ex vivo and 6-thioguanine (6-TG)-resistant mutant clones can be readily identified and scored. This model system may have special utility for detecting multi-locus deletion events (chromosomal mutations) induced by high LET forms of radiation that might be encountered in space.

A myeloperoxidase-specific assay based upon bromide-dependent chemiluminescence of luminol

Measurement of myeloperoxidase (MPO; EC 1.11.1.7) activity is often used as a marker of neutrophil infiltration into tissues. However, most enzymatic assays for MPO are susceptible to interference from other peroxidases (including eosinophil peroxidase, EPX) and hemoproteins (such as hemoglobin and myoglobin) present in the tissues. In this report, we describe a bromide-dependent chemiluminescence (Br-CL) assay that uses luminol as a chemiluminescence probe. The assay can distinguish between MPO and nonspecific peroxidase reactions. The MPO-specific reaction is believed to proceed in two steps: (i) the enzymatic generation of hypobromous acid (HOBr) from KBr and H(2)O(2) at pH 5 and (ii) the spontaneous reaction of HOBr and H(2)O(2) with luminol to give a Br-CL signal. The assay is sufficiently sensitive to allow detection of MPO in <100 human neutrophils. Other peroxidases and hemoproteins do not interfere with the Br-CL signal. Although EPX can also oxidize bromide to generate HOBr, activities of MPO and EPX can be distinguished at different pHs. As a demonstration of the utility of the Br-CL assay, MPO activity was measured in murine tumors known to be infiltrated with neutrophils. A statistically significant correlation was seen between MPO activity and histological neutrophil counts in the tumors (r = 0.69, P < 0.01, n = 14). The assay should have wide application for measuring the neutrophil content of tissues.

Mutagenicity and cytotoxicity of reactive oxygen and nitrogen species in the MN-11 murine tumor cell line

There is increasing evidence that endogenously generated reactive oxygen (ROS) and reactive nitrogen (RNS) species at sites of inflammation and in tumors may be genotoxic. We have developed a murine tumor model (MN-11) in which mutations at the hypoxanthine phosphoribosyltransferase (HPRT) locus, arising both in vitro and in vivo, can be detected. In the present report, we describe an in vitro study of the ability of ROS and RNS to induce mutations in our model system. 137Cs radiation and radiomimetic drugs caused a dose-dependent increase in mutant frequency. At D0, radiation induced about 170 mutants per 10(5) viable cells, compared to 50 and 95 for streptonigrin and bleomycin, respectively. H2O2 induced a lower frequency of mutants, 20-30 per 10(5), for enzymatically generated or bolus, respectively. For the following treatments, mutant frequency at 50% survival is shown. Incubation with human granulocytes induced a low frequency of mutants (about 15 per 10(5)). RNS was tested using a series of NO-donating drugs. Spermine/NO. induced cytotoxicity but no mutants while S-nitroso-N-acetylpenicillamine induced a low level, 10 per 10(5). Both release nitrogen monoxide spontaneously, with a t1/2 < 3 h. Glyceryl trinitrate and sodium nitroprusside are two drugs that were slowly metabolized by MN-11 cells (> 12 h). Glyceryl trinitrate induced about 20 per 10(5) while nitroprusside induced 50 per 10(5). Our results indicate that RNS can readily induce mutations detectable in MN-11 cells. At equicytotoxic doses, the induced mutant frequency varied considerably for different drugs, suggesting that different states of nitrogen monoxide (such as NO+ or NO.) may be generated and these may vary in their mutagenic/cytotoxic potential.

Levels of DNA strand breaks and superoxide in phorbol ester-treated human granulocytes

Phorbol ester treatment of granulocytes triggers release of superoxide (O2.-) and a concomitant burst of DNA strand breaks. The relationship between the amount of O2.- and the number of DNA breaks has not previously been explored. To quantify the relatively large amount of O2.- generated over a 40-min period by 1 x 10(6) granulocytes/mL, a discontinuous "10-min pulse" method employing cytochrome c was used; 140 nmol O2.- per 1 x 10(6) cells was detected. DNA strand breaks were quantified by fluorimetric analysis of DNA unwinding (FADU). To vary the level of O2.- released by cells, inhibitors of the respiratory burst were used. Sodium fluoride (1-10 mM) and staurosporine (2-10 nM) both inhibited O2.- production. In both cases, however, inhibition of strand breakage was considerably more pronounced than inhibition of O2.-. Zinc chloride (50-200 microM) inhibited both O2.- and DNA breaks, approximately equally. Dinophysistoxin-1 (okadaic acid) inhibited O2.- production more effectively than it inhibited DNA breaks. O2.- dismutes to H2O2, a reactive oxygen species known to cause DNA breaks. The addition of catalase to remove extracellular H2O2 had no effect on DNA breakage. Using pulse field gel electrophoresis, few double-stranded breaks were detected compared to the number detected by FADU, indicating that about 95% of breaks were single-stranded. The level of DNA breaks is not directly related to the amount of extracellular O2.- or H2O2 in PMA-stimulated granulocytes. We conclude that either an intracellular pool of these reactive oxygen species is involved in breakage or that the metabolic inhibitors are affecting a novel strand break pathway.

Hprt mutants in a transplantable murine tumour arise more frequently in vivo than in vitro

A model system was developed to allow investigation of the frequency at which clastogenic and/or mutagenic events occur in situ in a transplantable murine fibrosarcoma tumour (MC1A-C1) compared with in vitro culture. The marker selected for detecting these events was the X-linked hprt (hypoxanthine-guanine phosphoribosyltransferase) gene. We found that the hprt gene in MC1A-C1 was not suitable for this purpose, most likely because multiple active copies were present. To circumvent the problem, HPRT- [6-thioguanine (6-TG)-resistant] clones were isolated by inactivating all hprt genes with methylnitrosourea. Spontaneous revertants to hypoxanthine/aminopterin/thymidine resistance (HATR) were isolated and found to be approximately 1000 times more sensitive than the parental tumour to induction of 6-TGR mutants by cobalt-60 gamma-rays. This sensitivity is expected for a heterozygous marker, these revertants may therefore possess only one functional hprt locus but two or more active X chromosomes. A clone with a stable hprt gene was identified and a neo gene was introduced. The resulting cell line (MN-11) could be grown as a subcutaneous tumour in syngeneic C57BL/6 animals. The frequency of mutations arising in vivo in the marker hprt gene could be estimated by culturing explanted tumour cells in the presence of 6-TG, using G418 selection to distinguish tumour from host cells. The frequency of mutants in MN-11 cells grown as tumours was found to be 3.4-fold higher than in tissue culture for an equivalent period of time. These data provide the first direct evidence for the existence of mutagenic factors in a tumour environment that might contribute to tumour progression.

Fluorometric determination of DNA in agarose gels: usefulness for measurement of double-strand breaks in nonlabeled cells by pulsed-field gel electrophoresis

Quantitative measurement of DNA in agarose gels, particularly as needed for measurement of double-strand breaks induced by agents such as radiation, usually involves the use of radioactively labeled DNA. Thus its usefulness is usually limited to growing cells which incorporate radiolabeled thymidine into DNA. To circumvent this problem, we have developed a fluorometric technique for quantitative estimation of DNA in the presence of large amounts of agarose. Gel slices are solubilized with concentrated sodium perchlorate and DNA is selectively precipitated with cadmium chloride. The amount of DNA can then be estimated with 3,5-diaminobenzoic acid. Determination of DNA is linear in the range 10 ng to 1 microgram or more. We describe the application of this technique to the measurement of 60Co gamma-ray-induced double-strand breaks by pulsed-field gel electrophoresis. Our results are essentially identical to those obtained using radiolabeled DNA.