IBioBA (CONICET – Max Planck Partner) researcher Lucas Pontel and his team have just published a paper in Nature Communications describing a new mechanism of damage caused by a group of carcinogenic compounds known as aldehydes.
They identified that formaldehyde attacks the antioxidant defenses of cells, and described the factors that prevent this damage. This mechanism could explain the origin of some human diseases and lead to the development of new preventive strategies based on natural antioxidants.
Published today in the prestigious journal Nature Communications, the paper “Endogenous formaldehyde scavenges cellular glutathione resulting in redox disruption and cytotoxicity”, demonstrates that formaldehyde, in reaction with glutathione -the main cellular antioxidant- destabilizes its capacity to act as an antioxidant, causing oxidative stress and leading to general cellular toxicity. The work was led by the head of the “Cancer Metabolism” group at the Buenos Aires Biomedical Research Institute (IBioBA-CONICET-Max Planck), Lucas Pontel, together with his PhD students Carla Umansky, Agustín Morellato, and undergraduate student Marco Scheidegger.
Our cells process dietary nutrients generating numerous molecules necessary for growth and development. This set of reactions is known as metabolism, and can also give rise to unwanted compounds or by-products that damage the cell and ultimately initiate a pathological process. Among these unwanted metabolites we found aldehydes such as formaldehyde. This metabolite is produced in our cells as part of normal metabolism and from dietary components such as the sweetener aspartame or some fruit juices. Environmental pollution and cigarette smoke can also contribute to an increase in cellular formaldehyde. This aldehyde is highly reactive and is considered by the World Health Organization (WHO) to be a carcinogenic compound.
What was already known: prior knowledge about formaldehyde
It has been known for years that formaldehyde is a mutagen, meaning that it can cause damage -mutations- in DNA, the place where we store all the information we need to grow and develop. “There is a lot of evidence that formaldehyde can cause DNA damage and that cells have systems that repair this damage; hereditary mutations in systems that repair DNA damage caused by formaldehyde can lead to the development of tumors and diseases such as Fanconi’s Anemia”, explains Lucas Pontel.
On the other hand, to prevent formaldehyde from accumulating, our cells have a ‘machinery’ composed mainly of the enzyme alcohol dehydrogenase 5 (ADH5). Recently, Lucas was involved in identifying patients with inherited mutations in genes encoding enzymes that detoxify formaldehyde, showing that formaldehyde accumulation can lead to a carcinogenic process even in the presence of DNA repair systems (Dingler et al 2020 Mol Cell). “So we wondered how formaldehyde could be damaging cells when DNA-repairing systems are functioning normally”, Lucas recounts.
“We explored in what way formaldehyde was causing that damage, and we found that a storm of reactive oxygen species, or oxidative stress, occurs. This is because formaldehyde attacks the cell’s antioxidant defenses by damaging the main cellular antioxidant known as glutathione”, says Pontel. In other words, in addition to damaging the genome, it damages the antioxidant defenses.
The work process: technologies, experiments and collaborations
First, the researchers evaluated markers of DNA damage in cells without defects in DNA repair systems. These cells died in the presence of formaldehyde, but showed no evidence of DNA damage. Using genetic engineering (CRISPR/Cas9) they generated cell models derived from colon cancer and leukemia that lacked the main formaldehyde metabolizing enzyme (ADH5). These cells showed increased oxidative stress that correlated with formaldehyde toxicity. “Not only were we able to show that this is the case using a wide variety of probes (which gives robustness to our conclusions), but we were also able to identify the importance of a mechanism that is dedicated to its detoxification. This mechanism is driven by ADH5 and depends on the constant synthesis of glutathione in order to function properly”, adds the doctoral fellow, Agustín Morellato.
In collaboration with the scientist Matthias Rieckher together with the CECAD research group leader Prof. Björn Schumacher at the University of Cologne, Germany, the researchers identified the orthologous gene for ADH5 in the C. elegans worm, generating a laboratory model that will make it possible in the future to establish the role of formaldehyde in phenomena such as aging: “We inactivated the main enzyme that detoxifies formaldehyde and we saw that when we exposed these animals -which are widely used for research- to formaldehyde, markers were turned on that indicate that oxidative stress accumulates”, says Pontel.
To reveal the mechanism by which formaldehyde was causing oxidative stress, the researchers collaborated with the Bioanalytical Mass Spectrometry group led by María Eugenia Monge at the Bionanosciences Research Center (CIBION). In this sense, they found that in cells formaldehyde reacts with glutathione generating the intermediate S-hydroxymethylglutathione, which blocks the antioxidant capacity of the tripeptide glutathione. This intermediate is processed by ADH5 restoring the antioxidant capacity of glutathione: “As ADH5 converts it into formate -a less reactive compound- and restores glutathione”, explains Agustin.
What the research group proposes is that the mechanism discovered is indirect: formaldehyde inactivates glutathione and this makes it unable to prevent oxidative stress from being generated in cells, leading to widespread damage and cell death.
The importance of avoiding oxidative stress
When by some process (such as the transformation of a cell into a cancer cell, infection with a pathogen, etc.), cells begin to produce excessive amounts of oxidative species, these can be very harmful to the cell. “In general, this excess of oxidative species and the damage they cause is known as oxidative stress. This is because, like formaldehyde, oxidative species are very reactive and can cause great damage to cellular macromolecules (DNA, proteins and lipids), interfering with their function. That is why a balance must be maintained in the oxidizing species, allowing adequate signaling without reaching the point where the molecules begin to be damaged”, says Umansky.
From the therapeutic point of view, increasing the toxicity of formaldehyde in tumor cells could contribute to reducing their growth: “The importance of this lies in the fact that cancer cells, having an accelerated metabolism due to their uncontrolled growth, consume more nutrients and produce more waste (including formaldehyde). So, if we know the tools with which the cells defend themselves against formaldehyde, we can design therapeutic strategies that inactivate those tools and leave the cancer cells more defenseless”, Agustín thinks about the future.
In this line, what the team suggests is to increase the consumption of antioxidants -which have the property of performing the function naturally carried out by glutathione- in healthy people: “If we incorporate antioxidant compounds in our diet to restore the oxidative balance of the cell, then formaldehyde stops being so harmful. Our work suggests that simply increasing the consumption of natural antioxidants can counteract the toxicity of this compound and delay the onset of diseases such as cancer in healthy people”, concludes Pontel.