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2The role of endogenous porphyrin photosensitizers in the inhibitory effect of blue light on cancer cells
V. Plavskii*, O. Dudinova, L. Plavskaya, A. Sobchuk, A. Mikulich, A. Tretyakova, R. Nahorny, T. Ananich, A. Svechko, S. Yakimchuk, I. Leusenka
B.I. Stepanov Institute ofPhysics of the National Academy of Sciences of Belarus, Minsk, Belarus
The ability of optical radiation in blue spectral region (A, = 400-485 nm) of low intensity (0.5100 mW/cm2) to inhibit the growth of cancer cells is currently practically beyond doubt. It is characteristic that in studies in which, in vitro, the effect of light of the same spectral and energy parameters on cancer and non-transformed cells was compared, the presence of pronounced differences in their response to the influence of the specified physical factor was noted. However, the reasons for the increased sensitivity of cancer cells to the action of blue light remain unclear. In our opinion, a complicating factor in resolving this issue is the lack of complete understanding of the mechanisms of photophysical and photochemical processes that determine the effects of photobiomodulation initiated by exposure to blue light. The question of the primary acceptor molecules responsible for the regulatory action of this physical factor remains the least studied and most debated [1]. The studies carried out in this work showed the important role of endogenous porphyrins (free-base and their zinc complexes) in the generation of reactive oxygen species in cells (and, above all, singlet oxygen), which are capable of influencing, by changing the redox state of cells upon absorption of blue light, the metabolic processes occurring in them. It has been shown that the decisive role of porphyrins in the effects of cell sensitization occurs despite higher, at least two orders of magnitude, concentrations of flavins (riboflavin, FMN, FAD) in cells.
The manifestation of the sensitizing properties of porphyrins in cells is facilitated by: a) higher efficacy of formation of singlet oxygen sensitized by them and higher values of their molar extinction coefficients compared to flavins; b) localization of porphyrins in mitochondria, while a significant proportion of flavins are localized in the cytosol; c) binding of flavins to proteins that perform a protective function against flavin-sensitized damage to cellular structures; d) pronounced antioxidant properties of flavins, which contributes to the quenching of ROS generated by them during photoexcitation.
The leading role of porphyrins, and not flavins, in the photobiological processes that determine cell metabolism when exposed to blue light is confirmed by a higher rate of inhibition of cell metabolic activity and higher levels of ROS formation, recorded using the chemiluminescence assay, when a cell suspension is exposed to radiation with X = 405 nm compared to X = 445 nm. In this case, radiation with a wavelength of X = 405 nm corresponds to the maximum of the absorption spectrum of protoporphyrin IX and the local minimum of the absorption spectrum of flavins, and radiation with X = 445 nm corresponds to the maximum of the absorption spectrum of flavins and the region of local minimum of the absorption spectrum of porphyrins.
For the first time, it has been shown that one of the reasons for the increased sensitivity of cancer cells compared to normal cells to the blue light is the higher concentration of endogenous porphyrin sensitizers, which is confirmed by fluorescent analysis methods. The determining role of porphyrins in the difference in the reactions of cancer and normal cells to the blue light on their suspension is also evidenced by higher levels of ROS formation and a higher rate of light-induced inhibition of the metabolic activity of HeLa cancer cells compared to normal BGM cells.
In addition to differences in the concentrations of endogenous porphyrin sensitizers in cancer and normal cells, the reason for the specificity of their reactions to exposure to blue light may lie in the fact that cancer and normal cells differ in their cellular response to the effects of ROS; and cancer cells may be more prone to apoptosis as a result of an imbalance in the intracellular antioxidant system caused by excess ROS production.
[1] V.Y. Plavskii, L.G. Plavskaya, O.N. Dudinova, A.I. Tretyakova, A.V. Mikulich, A.N. Sobchuk, R.K. Nahorny, T.S. Ananich, A.D. Svechko, S.V. Yakimchuk, I.A. Leusenko, Endogenous photoacceptors sensitizing photobiological reactions in somatic cells, J. Appl.
Spectrosc, Vol. 90, pp. 334-345, (2023).
