St John's Wort photomedicine for Melonoma

Doctoral Thesis


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The use of photomedicine in ancient civilizations dates back 4000 years ago but it wasn't until the beginning of the 20th century that photodynamic therapy was discovered by man. The “trinity” of photodynamic therapy (PDT) comprises a photosensitizer, light and molecular oxygen. Following cellular uptake of the photosensitizer, its activation by light produces reactive oxygen species in the presence of oxygen. The resulting cytotoxic oxidative stress elicits cancer cell death by various mechanisms including apoptosis, necrosis and autophagy. Hypericin, an extract from St John's Wort, is a promising photosensitizer in the context of clinical photodynamic therapy due to its excellent photosensitizing properties and tumoritropic characteristics. However, limited reports on the efficacy of this photomedicine for the treatment of melanoma have been published. South Africa has the second highest incidence of malignant melanoma skin cancer in the world; a highly aggressive tumor due to its metastasizing potential and resistance to conventional cancer therapies. The aim of this study was to investigate the response mechanisms of melanoma cells to hypericinPDT in an in vitro tissue culture model. This investigation was three-fold. Firstly, the susceptibility of melanoma cells to the treatment was determined using cell viability assays. We found a dose of 3 µM light-activated hypericin was effective in reducing cell viability to 50 % or less than the control, for all melanoma cells employed in this study. We therefore used this killing-dose for further experiments. Next, hypericin uptake and its specific association with intracellular organelles was characterized using organelle-specific fluorescent-fusion proteins and dyes, in conjunction with the red fluorescent nature of hypericin and visualization by live confocal fluorescent microscopy. The intracellular localization of a photosensitizer directly influences its cytotoxic action and is thus crucial for effective cell death induction. Hypericin was taken up by all melanoma cells and co-localized with lysosomes and variably with melanosomes, the pigment producing organelles. No co-localization with the cell membrane, mitochondria, endoplasmic reticulum or nucleus was found. Investigating intracellular hypericin after treatment revealed a time-dependent decrease in all melanoma cells. Finally, melanoma cell death mechanisms were elucidated in response to the killing-dose of lightactivated hypericin. Ultrastructural examination of the cells with transmission electron microscopy 2 revealed extensive cytoplasmic vacuolisation, at 4 hours after treatment. In pigmented melanoma cells, the treatment furthermore induced the formation of glycogen aggregations. Fluorescent activated cell sorting analyses revealed a time-dependent increase in phosphatidylserine exposure, indicating apoptosis, in conjunction with a loss of cell membrane integrity, indicating necrosis, in all melanoma cells. An initial early necrotic population was found which decreased with time after treatment, whereas the late apoptotic/necrotic population increased. Minimal early apoptotic populations were found in all cell lines. In addition, melanoma cells showed a decrease in cellular size accompanied by an increase in granularity/pigmentation after treatment. Western blot analyses of proteins involved in specific cell death cascades furthermore verified the induction of apoptosis in melanoma cells by hypericin-PDT. The extrinsic apoptotic cascade was initiated in unpigmented A375 melanoma cells at 24 hours after treatment, mediated by activation of the suicidal proteases caspase 8 and caspase 3. Intrinsic apoptosis was found in pigmented UCT Mel-1 cells at 4 and 7 hours, mediated by activation of caspase 3 and cleavage of poly(ADP-ribose)polymerase 1 (PARP1). Induction of apoptosis by cleavage of PARP1 was furthermore evident in 501mel cells at 7 hours after treatment; however this cleavage was not mediated by caspase 3. Apoptosis inducing factor was found in its vital form in all melanoma cells, indicating that caspase-independent apoptosis or regulated necrosis by parthanatos were not induced by hypericin-PDT. In summary, this study demonstrated the effectiveness of hypericin-PDT in killing both unpigmented and pigmented melanoma cells by the induction of apoptosis. Further investigations into the exact mechanisms of the cell death response, including the observed loss of cell membrane integrity and the involvement of lysosomes and melanosomes are interesting avenues to explore in future studies. Translation of hypericin-PDT into a three-dimensional skin model with melanoma invasion is of particular interest, to further simulate the natural environment of this aggressive cancer and thereby enable the identification of enhanced treatment options.