N6-Formyllysine as a Biomarker of Formaldehyde Exposure: Formation and Loss of N6-Formyllysine in Nasal Epithelium in Long-Term, Low-Dose Inhalation Studies in Rats

Dual effects of endogenous formaldehyde on the organism and drugs for its removal

Endogenous formaldehyde (FA) is produced in the human body via various mechanisms to preserve healthy energy metabolism and safeguard the organism. However, endogenous FA can have several negative effects on the body through epigenetic alterations, including cancer growth promotion; neuronal, hippocampal and endothelial damages; atherosclerosis acceleration; haemopoietic stem cell destruction and haemopoietic cell production reduction. Certain medications with antioxidant effects, such as glutathione, vitamin E, resveratrol, alpha lipoic acid and polyphenols, lessen the detrimental effects of endogenous FA by reducing oxidative stress, directly scavenging endogenous FA or promoting its degradation. This study offers fresh perspectives for managing illnesses associated with endogenous FA exposure.

Approaches to Formaldehyde Measurement: From Liquid Biological Samples to Cells and Organisms

Formaldehyde (FA) is the simplest aldehyde present both in the environment and in living organisms. FA is an extremely reactive compound capable of protein crosslinking and DNA damage. For a long time, FA was considered a "biochemical waste" and a by-product of normal cellular metabolism, but in recent decades the picture has changed. As a result, the need arose for novel instruments and approaches to monitor and measure not only environmental FA in water, cosmetics, and household products, but also in food, beverages and biological samples including cells and even organisms. Despite numerous protocols being developed for in vitro and in cellulo FA assessment, many of them have remained at the "proof-of-concept" stage. We analyze the suitability of different methods developed for non-biological objects, and present an overview of the recently developed approaches, including chemically-synthesized probes and genetically encoded FA-sensors for in cellulo and in vivo FA monitoring. We also discuss the prospects of classical methods such as chromatography and spectrophotometry, and how they have been adapted in response to the demand for precise, selective and highly sensitive evaluation of FA concentration fluctuations in biological samples. The main objectives of this review is to summarize data on the main approaches for FA content measurement in liquid biological samples, pointing out the advantages and disadvantages of each method; to report the progress in development of novel molecules suitable for application in living systems; and, finally, to discuss genetically encoded FA-sensors based on existing natural biological FA-responsive elements.

LC-MS/MS Quantitation of Formaldehyde-Glutathione Conjugates as Biomarkers of Formaldehyde Exposure and Exposure-Induced Antioxidants: A New Look on an Old Topic

Humans are continuously exposed to formaldehyde via both endogenous and exogenous sources. Prolonged exposure to formaldehyde is associated with many human diseases, such as lung cancer and leukemia. The goal of this study is to develop biomarkers to measure formaldehyde exposure, which could be used to predict the risk of associated diseases. As glutathione (GSH) is well-known for its crucial role in the detoxification of a wide variety of xenobiotics, including formaldehyde, we rigorously quantitated in this study the conjugates formed when formaldehyde reacted with GSH using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) coupled with an isotope dilution method. The results showed for the first time that (S)-1-(((R)-2-amino-3-(carboxymethylamino)-3-oxopropylthio)methyl)-5-oxopyrrolidine-2-carboxylic acid (PGF) and thioproline-glycine (SPro-Gly) are major metabolites in both nonenzymatic reactions and formaldehyde-exposed human cells. In particular, over 35% of the formaldehyde from external sources was found to convert to SPro-Gly in the exposed cells. Interestingly, data showed that these exposure-induced adducts exhibited good antioxidative properties, which can protect cells from hydrogen peroxide mediated oxidative insult. It is anticipated that the findings of this study could shed light on developing PGF and SPro-Gly as dietary supplements and on the development of noninvasive methods to assess health risks associated with formaldehyde exposure.

Free L-Lysine and Its Methyl Ester React with Glyoxal and Methylglyoxal in Phosphate Buffer (100 mM, pH 7.4) to Form Nε-Carboxymethyl-Lysine, Nε-Carboxyethyl-Lysine and Nε-Hydroxymethyl-Lysine

Glyoxal (GO) and methylglyoxal (MGO) are highly reactive species formed in carbohydrate metabolism. N^ε-Carboxymethyllysine (CML) and N^ε-carboxyethyllysine (CEL) are considered to be the advanced glycation end-products (AGEs) of L-lysine (Lys) with GO and MGO, respectively. Here, we investigated the reaction of free L-lysine (Lys) with GO and MGO in phosphate buffer (pH 7.4) at 37 °C and 80 °C in detail in the absence of any other chemicals which are widely used to reduce Schiff bases. The concentrations of Lys, GO and MGO used in the experiments were 0.5, 2.5, 5.0, 7.5 and 10 mM. The reaction time ranged between 0 and 240 min. Experiments were performed in triplicate. The concentrations of remaining Lys and of CML and CEL formed in the reaction mixtures were measured by stable-isotope dilution gas chromatography-mass spectrometry (GC-MS). Our experiments showed that CML and CEL were formed at higher concentrations at 80 °C compared to 37 °C. CML was found to be the major reaction product. In mixtures of GO and MGO, MGO inhibited the formation of CML from Lys (5 mM) in a concentration-dependent manner. The highest CML concentration was about 300 µM corresponding to a reaction yield of 6% with respect to Lys. An addition of Lys to GO, MGO and their mixtures resulted in strong reversible decreases in the Lys concentration up to 50%. It is assumed that free Lys reacts rapidly with GO and MGO to form many not yet identified reaction products. Reaction mixtures of Lys and MGO were stronger colored than those of Lys and GO, notably at 80 °C, indicating higher reactivity of MGO towards Lys that leads to polymeric colored MGO species. We have a strong indication of the formation of N^ε-(hydroxymethyl)-lysine (HML) as a novel reaction product of Lys methyl ester with MGO. A mechanism is proposed for the formation of HML from Lys and MGO. This mechanism may explain why Lys and GO do not react to form a related product. Preliminary analyses show that HML is formed at higher concentrations than CEL from Lys methyl ester and MGO. No Schiff bases or their hydroxylic precursors were identified as reaction products. In their reactions with Lys, GO and MGO are likely to act both as chemical oxidants on the terminal aldehyde group to a carboxylic group (i.e., R-CHO to R-COOH) and as chemical reductors on labile Schiff bases (R-CH=N-R to R-CH₂-NH-R) presumably via disproportionation and hydride transfer. Our study shows that free non-proteinic Lys reacts with GO and MGO to form CML, CEL and HML in very low yield. Whether proteinic Lys also reacts with MGO to form HML residues in proteins remains to be investigated. The physiological occurrence and concentration of HML in biological fluids and tissues and its relation to CML and CEL are elusive and warrant further investigations in health and disease. Chemical synthesis and structural characterization of HML are expected to advance and accelerate the scientific research in this topic.

Effect of formaldehyde exposure on bacterial communities in simulating indoor environments

Indoor formaldehyde (CH₂O) exceeding the recommended level is a severe threat to human health. Few studies have investigated its effect on indoor surface bacterial communities, affecting habitants' health. This study used 20-L glass containers to mimic the indoor environment with bacterial inputs from human oral respiration. The behavior of bacterial communities responding to CH₂O varied among the different CH₂O levels. The bacterial community structure significantly changed over time in the 0.054 mg·m^-3 CH₂O group, which varied from the 0.1 mg·m^-3 and 0.25 mg·m^-3 CH₂O groups. The Chao1 and Shannon index significantly increased in the 0.054 mg·m^-3 CH₂O group at 6 week, while they remained unchanged in the 0.25 mg·m^-3 CH₂O group. At 12 week, the Chao1 significantly increased in the 0.25 mg·m^-3 CH₂O group, while it remained unchanged in the 0.054 mg·m^-3 CH₂O group. Only a few Operational Taxonomic Units (OTUs) significantly correlated with the CH₂O concentration. CH₂O-induced OTUs mainly belong to the Proteobacteria and Firmicutes. Furthermore, bacterial communities formed at 6 or 12 weeks differed significantly among different CH₂O levels. Functional analysis of bacterial communities showed that inferred genes related to chemical degradation and diseases were the highest in the 0.25 mg·m^-3 CH₂O group at 12 weeks. The development of nematodes fed with bacteria collected at 12 weeks was applied to evaluate the bacterial community's hazards. This showed significantly impaired growth in the 0.1 mg·m^-3 and 0.25 mg·m^-3 CH₂O groups. These findings confirmed that CH₂O concentration and exposure time could affect the indoor bacterial community and formed bacterial communities with a possibly more significant hazard to human health after long-term exposure to high CH₂O levels.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

Metformin: Targeting the Metabolo-Epigenetic Link in Cancer Biology

Metabolism can directly drive or indirectly enable an aberrant chromatin state of cancer cells. The physiological and molecular principles of the metabolic link to epigenetics provide a basis for pharmacological modulation with the anti-diabetic biguanide metformin. Here, we briefly review how metabolite-derived chromatin modifications and the metabolo-epigenetic machinery itself are both amenable to modification by metformin in a local and a systemic manner. First, we consider the capacity of metformin to target global metabolic pathways or specific metabolic enzymes producing chromatin-modifying metabolites. Second, we examine its ability to directly or indirectly fine-tune the activation status of chromatin-modifying enzymes. Third, we envision how the interaction between metformin, diet and gut microbiota might systemically regulate the metabolic inputs to chromatin. Experimental and clinical validation of metformin's capacity to change the functional outcomes of the metabolo-epigenetic link could offer a proof-of-concept to therapeutically test the metabolic adjustability of the epigenomic landscape of cancer.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Evaluation of inhaled low-dose formaldehyde-induced DNA adducts and DNA-protein cross-links by liquid chromatography-tandem mass spectrometry

As a widespread industrial chemical, formaldehyde carcinogenicity has been highly controversial. Meanwhile, formaldehyde is an essential metabolite in all living cells. Previously, we have demonstrated exogenous formaldehyde causes DNA adducts in a nonlinear manner between 0.7 and 15.2 ppm using [¹³CD₂]-formaldehyde for exposure coupled with the use of sensitive mass spectrometry. However, the responses from exposure to low doses of formaldehyde are still unknown. In this study, rats were exposed to 1, 30, and 300 ppb [¹³CD₂]-formaldehyde for 28 days (6 h/day) by nose-only inhalation, followed by measuring DNA mono-adduct (N²-HOMe-dG) and DNA-protein crosslinks (dG-Me-Cys) as formaldehyde specific biomarkers. Both exogenous and endogenous DNA mono-adducts and dG-Me-Cys were examined with ultrasensitive nano-liquid chromatography-tandem mass spectrometry. Our data clearly show that endogenous adducts are present in all tissues analyzed, but exogenous adducts were not detectable in any tissue samples, including the most susceptible nasal epithelium. Moreover, formaldehyde exposure at 1, 30 and 300 ppb did not alter the levels of endogenous formaldehyde-induced DNA adducts or DNA-protein crosslinks. The novel findings from this study provide new data for risk assessment of exposure to low doses of formaldehyde.

Improving risk assessment approaches for chemicals with both endogenous and exogenous exposures

To conduct risk assessments of exogenous chemicals for which there are also endogenous exposures, knowledge of the chemistry and biology of both types of exposures needs to be integrated into problem formulation and carried through to risk characterization. This issue is framed in a risk assessment context, highlighting the importance of quantifying increments of dose from all sources of the same or similar chemicals interacting with biological targets; understanding the influence of endogenous chemical concentrations on disease risk; and assessing total dose to targets in evaluating risk from incremental environmental exposures. Examples of recent assessments illustrate the importance of addressing this issue. Evaluations of data on blood or organ concentrations of ammonia, methanol, formaldehyde, acetaldehyde, and three gaseous signaling molecules (hydrogen sulfide, carbon monoxide, and nitric oxide) provide examples where current data are already informing perspectives on relative exposures at the portal of entry and systemically. To facilitate quality risk assessments of exogenous chemicals with endogenous exposures, a series of specific questions are presented that need to be addressed in systematic review to enhance problem formulation, improve the development of holistic conceptual models, and to facilitate the identification of priority data needs for improving risk assessments.