Cytotoxic and Photocytotoxic Effects of Cercosporin on Human Tumor Cell Lines

Abstract

Cercosporin is a naturally occurring perylenequinone. Although other perylenequinones have been extensively studied as photosensitizers in photodynamic therapy of cancer (PDT), cercosporin has been studied in this light only within the remits of phytopathology. Herein, we investigated the photocytotoxicity of cercosporin against two glioblastoma multiforme (T98G and U87) and one breast adenocarcinoma (MCF7) human cell lines. Cercosporin was found to be a potent singlet oxygen producer upon 532 nm excitation, while its cell loading was similar for MCF7 and U87, but approximately threefold higher for T98G cells. The subcellular localization of cercosporin was in all cases in both mitochondria and the endoplasmic reticulum. Light irradiation of cercosporin-incubated cells around 450 nm showed that T98G cells were more susceptible to cercosporin PDT, mainly due to their higher cercosporin uptake. Metabolic studies before and 1 h following cercosporin PDT showed that cercosporin PDT instigated a bioenergetic collapse in both the respiratory and glycolytic activities of all cell lines. In the dark, cercosporin exhibited a synergistic cytotoxicity with copper only in the most respiratory cell lines (MCF7 and T98G). Cercosporin is a potent photosensitizer, but with a short activation wavelength, mostly suitable for superficial PDT treatments, especially when it is necessary to avoid perforations.

Photodynamic Efficacy of Cercosporin in 3D Tumor Cell Cultures

“…In a more recent publication (14), we further saw that HYP‐PDT caused a profound decrease of both the respiration and the glycolytic rate of MCF7 human breast adenocarcinoma cells, 1 h following irradiation. This is in good agreement with our 2D cerco findings (5), where cerco‐PDT collapsed the respiration and profoundly diminished the glycolytic action in U87, T98G human glioblastoma multiforme and MCF7 human breast carcinoma cells.…”

“…In contrast the U87 spheroids, although their respiration breaks down following cerco‐PDT, apparently their quasi‐functional glycolytic ability, could be one key factor that helped the spheroids retain their viability and their capacity to grow up to the size of the nontreated controls by day 8, despite their initial decrease in size up to day 3. This is also supported from the results of our previous work on cerco PDT (5) where U87 present with the lowest respiration of the 3 cell lines investigated (T98G, MCF7 and U87) yet their glycolytic activity is comparable to that of MCF7 indicating that they rely more on glycolysis for ATP production (Warburg effect) (15) and hence survival since ATP deficiency leads to bioenergetic collapse and cell death (16,17).…”

“…One more factor that contributes to the above observations is the much looser adhesion of U87 spheroids ( vide infra ), which may apparently allow both glycolysis substrates and oxygen easier access to the spheroid inner layers. As it has been shown in our previous work (5), cerco is subcellularly located in the endoplasmic reticuli/cell mitochondria. As a result, the PDT activation of cerco may confer considerable damage to the cellular respiration by damaging the electron transport chain (ETC) constituent complexes.…”

“…Despite the fact that both our 2D cell study (5) and our present work in 3D spheroid cultures are in agreement that cerco is a quite potent photosensitizer, there are several discrepancies in the effect of cerco‐PDT on cell monolayers and 3D cultures. In 2D cell cultures, we found that U87 cells were the least efficient in uploading cercosporin as compared to MCF7 breast adenocarcinoma and T98G GBM cell lines following incubation with 4 µ m cerco for 4 h. In the case of the 3D spheroids, however, incubation under the same conditions led to U87 cells showing the most efficient loading in the single cell level, double than both the T98G and T47D cells.…”

“…The metabolic profile of the spheroids is also very different to that in 2D cell cultures (5). In cell monolayers, U87 cells exhibited much lower respiratory activity than T98G cells.…”