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.

Hydrogen sulfide antagonizes formaldehyde-induced ferroptosis via preventing ferritinophagy by upregulation of GDF11 in HT22 cells

Formaldehyde (FA) has neurotoxic characteristics and causes neurodegenerative disease. Our previous study demonstrated the neuroprotective effects of hydrogen sulfide (H₂S) on FA-induced neurotoxicity in HT22 cells. Emerging evidence have supported that ferroptosis is involved in FA-induced neurotoxicity. To understand the mechanism of the protection of H₂S against FA-induced neurotoxicity, this study explored the regulatory effect of H₂S on FA-induced ferroptosis and the underlying mechanisms. The researcher found that H₂S (100, 200, and 400μM, 30min) reverses the ferroptosis induced by FA (100μM, 24h) in HT22 cells (a cell line of mouse hippocampal neurons), including decreases in free iron, reactive oxygen species (ROS), 4-hydroxy-2-trans-nominal (4-HNE), and malondialdehyde (MDA) contents, as well as an increase in glutathione (GSH) content. H₂S (100, 200, and 400μM, 30min) also inhibited ferritinaphagy in FA-exposed HT22 cells, as evidenced by the downregulation of the ferritinophagy receptor nuclear receptor coactivator 4 (NCOA4) and microtubule-associated protein 1 light chain-3B (LC3B) as well as the upregulation of the main iron storage protein ferritin heavy chain 1 (FTH1) and p62. We also found that H₂S (100, 200, and 400μM, 30min) up-regulated the expression of growth differentiation factor-11 (GDF11) in FA-exposed HT22 cells. Furthermore, knockdown of GDF11 in HT22 cells cancelled the beneficial effects of H₂S in FA-induced ferroptosis and ferritinaphagy. These data indicated that the protective mechanism underlying H₂S-prevented neurotoxicity of FA is involved in alleviating FA-induced ferroptosis via inhibiting ferritinaphagy by upregulation of GDF11.

Photobiomodulation for Alzheimer's disease: photoelectric coupling effect on attenuating Aβ neurotoxicity

Alzheimer's disease (AD) and dementia are the most worrying health problems faced by people globally today. Although the pathological features of AD consisting of amyloid-beta (Aβ) plaques in the extracellular space (ECS) and intracellular tau tangles are well established, the developed medicines targeting these two proteins have not obtained the expected clinical effects. Photobiomodulation (PBM) describes the therapeutic use of red light (RL) or near-infrared light (NIR) to serve as a noninvasive neuroprotective strategy for brain diseases. The present review discusses the mechanisms of the photoelectric coupling effect (light energy-induced special electronic transition-related alterations in protein structure) of PBM on reducing Aβ toxicity. On the one hand, RL or NIR can directly disassemble Aβ in vitro and in vivo. On the other hand, formaldehyde (FA)-inhibited catalase (CAT) and H2O2-inactived formaldehyde dehydrogenase (FDH) are formed a vicious circle in AD; however, light energy not only activates FDH to degrade excessive FA (which crosslinks Aβ monomer to form Aβ oligomers and senile plaques) but also sensitizes CAT to reduce hydrogen peroxide levels (H2O2, which can facilitate Aβ aggregation and enhance FA generation). In addition, it also activates mitochondrial cytochrome-c to produce ATP in the neurons. Clinical trials of phototherapeutics or oral coenzyme Q10 have shown positive effects in AD patients. Hence, a promising strategy combined PBM with nanopacked Q10 has been proposed to apply for treating AD.

Hydrogen sulfide prevents arecoline-induced neurotoxicity via promoting leptin/leptin receptor signaling pathway

Arecoline, a major alkaloid of the areca nut, has potential toxicity to the nervous system. Our previous study reveals that the neurotoxicity of arecoline involves in inhibited endogenous hydrogen sulfide (H₂ S) generation. Therefore, the present study investigated whether exogenous H₂ S protects against arecoline-induced neurotoxicity and further explore the underlying mechanisms focusing on leptin/leptin receptor signaling pathway. The cell viability was measured by CCK-8 kit. The apoptosis were detected by Hoechst 33258 and Annexin V/PI (propidium iodide) staining. The protein expressions were determined by Western blot analysis. Our results demonstrated that NaHS, an exogenous H₂ S donor, significantly increases the cell viability, decreases apoptosis ratio, and reduces caspase-3 activity as well as Bax/Bcl-2 ratio in PC12 cells exposed to arecoline, indicating the protection of H₂ S against arecoline-induced cytotoxicity and apoptosis. Also, NaHS attenuated arecoline-induced endoplasmic reticulum (ER) stress, as evidenced by the decreases in the expressions of glucose-regulated protein 78 (GRP78), C/EBP homologous protein (CHOP), and Cleaved caspase-12. Meanwhile, NaHS promoted leptin/leptin receptor signaling pathway in arecoline-exposed PC12 cells, as illustrated by upregulations of leptin and leptin receptor expressions. Furthermore, leptin tA, an antagonist of leptin receptor, obviously abolished the inhibitory effects of NaHS on arecoline-induced cytotoxicity, apoptosis, and ER stress in arecoline-exposed PC12 cells. Taken together, these results suggested that H₂ S prevents arecoline-induced neurotoxicity via enhancing the leptin/leptin receptor signaling pathway.

Formation and disappearance of aldehydes during simulated gastrointestinal digestion of fried clams

The formation and disappearance of aldehydes during simulated gastrointestinal digestion (SD) of fried clams was investigated in order to shed light on the underlying mechanism. Results from the thiobarbituric acid reactive substance (TBARS) and fluorometric assays using a specific aldehyde probe indicated that the SD (with lipase) of fried clams initially reduced (at the gastric stage), but subsequently increased (mainly at the intestinal stage) the contents of total aldehydes. Meanwhile, eight specific aldehydes including propanal, acrolein, trans-2-pentenal, hexanal, trans,trans-2,4-octadienal, trans,trans-2,4-decadienal, 4-hydroxy-hexenal and 4-hydroxy-nonenal in the digested meal were determined by using a high-performance liquid chromatography-tandem electrospray ionization mass spectrometry (HPLC-ESI-MS/MS) method. Results indicated that the changes in the trend of the contents of the eight aforementioned aldehydes were similar to those of total aldehydes during SD (with lipase) of fried clams. However, a similar SD process without lipase time-dependently reduced the contents of total and individual aldehydes. Moreover, lipid classes and free fatty acids (FFAs) in the digested meal were determined to reveal the degree of hydrolysis of lipids during the SD process. Results indicated that the SD (with lipase) of fried clams significantly hydrolyzed triacylglycerols (TAG) and polar lipids (PL) and produced FFAs, but the SD process without lipase resulted in negligible lipid hydrolysis. Thus, our results demonstrated a positive correlation between lipid hydrolysis and aldehyde generation during the SD of fried clams. Alternatively, unsaturated FFAs instead of TAG and PL could have served as the main precursors for aldehyde generation due to their high oxidative susceptibility.

Formaldehyde induces ferroptosis in hippocampal neuronal cells by upregulation of the Warburg effect

The mechanisms underlying formaldehyde (FA)-induced neurotoxicity have not yet been fully clarified. Ferroptosis is a novel regulatory cell death and the Warburg effect is involved in regulating neural function. In this study, we investigated whether FA-induced neurotoxicity is implicated in neuronal ferroptosis and determined whether the Warburg effect mediates FA-induced neuronal ferroptosis. We found that FA (0.1, 0.5 and 1.0 mM, 6 h) induced cell death in HT22 cells (a cell line of mouse hippocampal neuron), as evidenced by a decrease in cell viability and an increase in cell mortality; enhanced oxidative stress, as evidenced by a decrease in glutathione (GSH) and increases in malondialdehyde (MDA), 4-Hydroxynonenal (4-HNE), as well as reactive oxygen species (ROS); increased the iron content; and upregulated the ferroptosis-associated genes, including Ptgs2 (prostaglandin-endoperoxide synthase 2), GLS2 (glutaminase 2), solute carrier family 1 member 5 (SLC1A5), and solute carrier family 38 member 1 (SLC38A1) in HT22 cells, indicating the inductive role of FA in the ferroptosis of HT22 cells. Meanwhile, we found that FA (0.1, 1, 10 μmol) decreased the cross-sectional of mitochondria, increased the level of lipid ROS and iron content in primary hippocampal cells. We showed that FA (0.1, 0.5 and 1.0 mM, 6 h) upregulated the Warburg effect in HT22 cells, as evidenced by up-regulations of pyruvate kinase M2 (PKM2), pyruvate dehydrogenase kinase 1(PDK-1), and lactate dehydrogenase (LDHA) proteins; down-regulation of pyruvate dehydrogenase (PDH); and an increase in lactate production. Also, we found that FA (0.1, 1, 10 μmol, 7 d) upregulated the Warburg effect in hippocampal tissue, as evidenced by up-regulations of PKM2, PDK-1, and LDHA proteins; down-regulation of PDH. Furthermore, the inhibition of the Warburg effect by dichloroacetate (DCA) protected HT22 cells against FA-induced ferroptosis and cell death. Collectively, these data indicated that FA induces ferroptosis in hippocampal neuronal cells by upregulation of the Warburg effect.

Formaldehyde toxicity reports from in vitro and in vivo studies: a review and updated data

Formaldehyde (FA) is a xenobiotic air pollutant and its universal distribution causes a widespread exposure to humans. This review aimed to bring updated information concerning FA toxicity in humans and animals based on in vitro and in vivo studies from 2013 to 2019. Researches were carried out in Pubmed, Scopus, and Science Direct databases to determine the effects of FA exposure on inflammation, oxidative stress and genotoxicity in experimental studies with animals (rats and mice) and humans. Besides, in vitro studies assessing FA cytotoxicity focusing on cell viability and apoptosis in different cell line cultures were reviewed. Studies with humans gave evidence regarding significant deleterious effects on health associated to chronic FA occupational exposure. Evaluations carried out in experimental studies showed toxic effects on different organs as lung, upper respiratory tract, bone marrow and brain as well as in cells. In summary, this study demonstrates that knowing the mechanisms underlying FA toxicity is essential to understand the deleterious effects that this xenobiotic causes on biological systems.

Protective effect of hydrogen sulfide on oxidative stress-induced neurodegenerative diseases

Hydrogen sulfide is an antioxidant molecule that has a wide range of biological effects against oxidative stress. Balanced oxidative stress is also vital for maintaining cellular function in biological system, where reactive oxygen species are the main source of oxidative stress. When the normal redox balance is disturbed, deoxyribonucleic acid, lipid, and protein molecules are oxidized under pathological conditions, like diabetes mellitus that leads to diabetic peripheral neuropathy. In diabetes mellitus-induced diabetic peripheral neuropathy, due to hyperglycemia, pancreatic beta cell (β cell) shows resistance to insulin secretion. As a consequence, glucose metabolism is disturbed in neuronal cells which are distracted from providing proper cell signaling pathway. Not only diabetic peripheral neuropathy but also other central damages occur in brain neuropathy. Neurological studies regarding type 1 diabetes mellitus patients with Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis have shown changes in the central nervous system because high blood glucose levels (HbA1c) appeared with poor cognitive function. Oxidative stress plays a role in inhibiting insulin signaling that is necessary for brain function. Hydrogen sulfide exhibits antioxidant effects against oxidative stress, where cystathionine β synthase, cystathionine γ lyase, and 3-mercaptopyruvate sulfurtransferase are the endogenous sources of hydrogen sulfide. This review is to explore the pathogenesis of diabetes mellitus-induced diabetic peripheral neuropathy and other neurological comorbid disorders under the oxidative stress condition and the anti-oxidative effects of hydrogen sulfide.