Melanoma is the most aggressive form of skin cancer.

Although less common, it is the most serious due to its high potential for metastasis.

Despite the health benefits of sunlight, prolonged exposure to ultraviolet rays is the main risk factor for developing the disease.

Excessive radiation induces oxidative stress and inflammation in skin cells through photo-oxidation reactions.

Both ultraviolet and visible light can activate photosensitisers naturally present in tissues.

This process converts light energy into chemicals, generating reactive oxygen species that damage biomolecules such as the lipids in cell membranes.

However, these reactions can also be used for therapeutic purposes, such as photodynamic therapy to target tumour cells or pathogens.

At the Centre for Research on Redox Processes in Biomedicine (Redoxoma) at the University of São Paulo (USP), for example, researchers have tested some oxidising compounds that, in combination with photodynamic therapy, could lead to new strategies for fighting melanoma.

Redoxoma is a FAPESP Research, Innovation and Dissemination Centre (RIDC).

In the study, coordinated by Professor Sayuri Miyamoto of the Institute of Chemistry (IQ-USP), the group discovered that endoperoxides derived from the oxidation of ergosterol and 7-dehydrocholesterol (7-DHC), both lipids of the sterol class, induce the death of melanoma cells.

The findings were published in the journal Photochemistry and Photobiology.

“The big challenge with melanoma is its very rapid progression. Although it isn’t the first treatment option, photodynamic therapy is gaining a lot of attention because it is less invasive than conventional treatment methods such as surgery. Our focus was to optimise photodynamic therapy, and to do that we needed to understand what happens in the cell membranes,” says Megumi Nishitani Yukuyama, first author of the article.

This finding is part of a larger study aimed at understanding the mechanisms behind light-induced oxidative damage to cell membranes.

The researchers analysed and compared how the photo-oxidation of ergosterol types I and II, 7-DHC and cholesterol affects these structures.

They also identified and characterised the main products formed during these oxidation processes.

“This study is important for understanding the mechanism of oxidation of these sterols in membranes when exposed to different oxidants. It also helps us to determine what products are formed and what effects these oxidised products have on membrane integrity,” comments Miyamoto.

Sterols and photo-oxidation

A new finding of the research was that the permeability of cell membranes changes depending on the type of oxidative damage.

All cells are surrounded by a cell membrane consisting of a lipid bilayer associated with proteins.

The structural basis of the bilayer is formed by phospholipids, which are susceptible to oxidation.

These oxidations can compromise the integrity of the membrane, increasing permeability and potentially leading to cell death.

Photo-oxidation reactions are classified according to two main mechanisms.

Type I reaction produces reactive radical species such as the superoxide radical anion and hydroperoxyl radicals.

Type II produces singlet molecular oxygen, a highly reactive form of oxygen.

The study showed that the sterols ergosterol and 7-DHC provide greater membrane protection than cholesterol in radical oxidation (type I).

However, in singlet oxygen-mediated oxidation (type II), cholesterol proved to be more effective.

This suggests that cholesterol acts as an antioxidant in type II oxidations.

“It organises the membrane in such a way that singlet oxygen cannot access the unsaturated lipids that would otherwise be oxidised. This test showed that cholesterol is very important for protecting cell membranes from light-induced damage,” explains the IQ-USP professor.

However, during membrane protection, these sterols are oxidised to form various products, including endoperoxides.

The study showed that endoperoxides derived from 7-DHC and ergosterol are the most stable in these processes.

“In the paper we published last year in Nature, we showed that 7-DHC worked as an antioxidant, protecting cells from death by ferroptosis in radical oxidation reactions. But in performing this function, it is oxidised and generates various products,” says Miyamoto.

7-DHC is a precursor of cholesterol, and both are common sterols in mammals.

Ergosterol, on the other hand, is a sterol found in yeast that is similar in structure to 7-DHC.

When oxidised, it also forms endoperoxides.

According to Yukuyama, “the literature on ergosterol was somewhat controversial. That’s why we did a comparative study to clarify the mechanisms of the protective or harmful effects of these sterols.”

The group also evaluated the viability of A375 melanoma cells treated with 7-DHC, ergosterol and their endoperoxides generated by photodynamic therapy, a process that induces type I and type II oxidation.

Most interestingly, the ergosterol and 7-DHC endoperoxides generated by singlet oxygen proved to be more effective at eliminating melanoma cells than their precursor molecules.

“Next, we plan to investigate how different concentrations of endoperoxides and various doses of radiation influence their effects. The study has opened the door to several questions,” says Yukuyama.

Source: Fundação de Amparo à Pesquisa do Estado de São Paulo