BACKGROUND: Pretreatment with 5-LOX pathway inhibitor MK-886 potentiates cytotoxic effects of photodynamic therapy mediated by natural photosensitizer, hypericin. In this study, we focused on elucidating mechanisms beyond the increased efficacy of combined treatment. METHODS: Metabolic activity/viability, caspase-3 activation/mitochondrial membrane potential dissipation, intracellular hypericin level, glutathione level and redox status (NAD(P)H/oxidized flavins ratio) analyses, as well as drug efflux assays, were performed by flow cytometry. Changes in protein expression of ATP-binding cassette transporters, GDF-15 and other selected proteins were evaluated by Western blotting. Silencing of gdf-15 was carried out to verify its role in response to treatment. RESULTS: MK-886 pretreatment led to a concentration-dependent increase in intracellular hypericin content, accompanied by changes in ATP-binding cassette transporters levels and efflux efficiency. Intracellular accumulation of cytokine GDF-15 correlated with increased cell death markers; however, the impact of gdf-15 silencing on the evaluated markers was negligible. A marked decrease in the glutathione level of a majority of cells was observed after more toxic combination treatment. CONCLUSION: The significant increase in cell death markers after combination treatment confirms the potentiating effect of MK-886 on hypericin-mediated photodynamic therapy in HT-29 and MCF-7 cells. Although BCRP downregulation was not confirmed as leading mechanism responsible for elevated levels of hypericin content, changes in expression and efflux activity of ABC transporters caused by MK-886 suggest its potential in combination treatment with drugs that are substrates of these transporters, predominantly MRP1. However, complex cellular response to MK-886 pretreatment needs to be considered and further elucidated.
BACKGROUND: Pretreatment with 5-LOX pathway inhibitor MK-886 potentiates cytotoxic effects of photodynamic therapy mediated by natural photosensitizer, hypericin. In this study, we focused on elucidating mechanisms beyond the increased efficacy of combined treatment. METHODS: Metabolic activity/viability, caspase-3 activation/mitochondrial membrane potential dissipation, intracellular hypericin level, glutathione level and redox status (NAD(P)H/oxidized flavins ratio) analyses, as well as drug efflux assays, were performed by flow cytometry. Changes in protein expression of ATP-binding cassette transporters, GDF-15 and other selected proteins were evaluated by Western blotting. Silencing of gdf-15 was carried out to verify its role in response to treatment. RESULTS:MK-886 pretreatment led to a concentration-dependent increase in intracellular hypericin content, accompanied by changes in ATP-binding cassette transporters levels and efflux efficiency. Intracellular accumulation of cytokine GDF-15 correlated with increased cell death markers; however, the impact of gdf-15 silencing on the evaluated markers was negligible. A marked decrease in the glutathione level of a majority of cells was observed after more toxic combination treatment. CONCLUSION: The significant increase in cell death markers after combination treatment confirms the potentiating effect of MK-886 on hypericin-mediated photodynamic therapy in HT-29 and MCF-7 cells. Although BCRP downregulation was not confirmed as leading mechanism responsible for elevated levels of hypericin content, changes in expression and efflux activity of ABC transporters caused by MK-886 suggest its potential in combination treatment with drugs that are substrates of these transporters, predominantly MRP1. However, complex cellular response to MK-886 pretreatment needs to be considered and further elucidated.