Hypericin affects cancer side populations via competitive inhibition of BCRP.

OBJECTIVE: Cancer stem-like cells (CSLCs) are considered a root of tumorigenicity and resistance. However, their identification remains challenging. The use of the side population (SP) assay as a credible marker of CSLCs remains controversial. The SP assay relies on the elevated activity of ABC transporters that, in turn, can be modulated by hypericin (HYP), a photosensitizer and bioactive compound of St. John's Wort (Hypericum perforatum), a popular over-the-counter antidepressant. Here we aimed to comprehensively characterize the SP phenotype of cancer cells and to determine the impact of HYP on these cells. METHODS: Flow cytometry and sorting-based assays were employed, including CD24-, CD44-, CD133-, and ALDH-positivity, clonogenicity, 3D-forming ability, ABC transporter expression and activity, and intracellular accumulation of HYP/Hoechst 33342. The tumorigenic ability of SP, nonSP, and HYP-treated cells was verified by xenotransplantation into immunodeficient mice. RESULTS: The SP phenotype was associated with elevated expression of several investigated transporters and more intensive growth in non-adherent conditions but not with higher clonogenicity, tumorigenicity or ALDH-positivity. Despite stimulated BCRP level and MRP1 activity, HYP reversibly decreased the SP proportion, presumably via competitive inhibition of BCRP. HYP-selected SP cells acquired additional traits of resistance and extensively eliminated HYP. CONCLUSIONS: Our results suggest that SP is not an unequivocal CSLC-marker. However, SP could play an important role in modulating HYP-treatment and serve as a negative predictive tool for HYP-based therapies. Moreover, the use of supplements containing HYP by cancer patients should be carefully considered, due to its proposed effect on drug efflux and complex impact on tumor cells, which have not yet been sufficiently characterized.

Potentiation of hypericin-mediated photodynamic therapy cytotoxicity by MK-886: focus on ABC transporters, GDF-15 and redox status.

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.

Novel trisubstituted acridines as human telomeric quadruplex binding ligands.

A novel series of trisubstituted acridines were synthesized with the aim of mimicking the effects of BRACO19. These compounds were synthesized by modifying the molecular structure of BRACO19 at positions 3 and 6 with heteroacyclic moieties. All of the derivatives presented in the study exhibited stabilizing effects on the human telomeric DNA quadruplex. UV-vis spectroscopy, circular dichroism, linear dichroism and viscosimetry were used in order to study the nature of the DNA binding in more detail. The results show that all of the novel derivatives were able to fold the single-stranded DNA sequences into antiparallel G-quadruplex structures, with derivative 15 exhibiting the highest stabilizing capability. Cell cycle analysis revealed that a primary trend of the "braco"-like derivatives was to arrest the cells in the S- and G2M-phases of the cell cycle within the first 72h, with derivative 13 and BRACO19 proving particularly effective in suppressing cell proliferation. All studies derivatives were less toxic to human fibroblast cell line in comparison with HT 29 cancer cell line.

Conjunction of glutathione level, NAD(P)H/FAD redox status and hypericin content as a potential factor affecting colon cancer cell resistance to photodynamic therapy with hypericin.

BACKGROUND: Photodynamic therapy (PDT) is a highly efficient approach for tumour therapy, though it also has its drawbacks, too. There are multiple mechanisms involved in cell death regulation that can be successfully targeted for improvement of PDT in particular cases. We assumed, however, that the potential to manage radical stress might be the primary factor responsible for resistance to hypericin-mediated PDT (HY-PDT). METHODS: We compared the sensitivity of six colon-derived cancer cell lines to HY-PDT at IC50 equitoxic doses acquired by formazan-based (MTT) assay. Intracellular hypericin content, cell survival/metabolic activity, caspase-3 activation/mitochondrial membrane potential dissipation, apoptosis, glutathione level, redox status (NAD(P)H/oxidized flavins ratio) and Western blot analyses of proteins relevant in apoptosis regulation were measured to demonstrate differences between tested cell lines. RESULTS: Analyses revealed a whole spectrum of responses from insignificant to high cytotoxicity, despite the MTT-based "equitoxicity". Further critical evaluation of multiple parameters linked to cell physiology and proteomics proved that intracellular hypericin content, glutathione level or redox status demonstrate partial but not direct correlation with resistance to HY-PDT, when considered separately. However, their logical conjunction did copy the trend of cellular resistance. CONCLUSIONS: We may conclude that intracellular level of hypericin and glutathione together with redox state of the target cell represent a potential combination of parameters responsible for the primary cytotoxicity of HY-PDT. We also present evidence that cytotoxic assays, such as the MTT, should be accompanied with other tests of cell survival/cytotoxicity in order to avoid incorrect conclusions.