Cell death response of U87 glioma cells on hypericin photoactivation is mediated by dynamics of hypericin subcellular distribution and its aggregation in cellular organelles

Hypericin (Hyp) is a hydrophobic natural photosensitizer that is considered to be a promising molecule for photodynamic treatment of tumor cells and photo-diagnosis of early epithelial cancers. Its hydrophobicity is the main driving force that governs its redistribution process. Low-density lipoproteins (LDL), a natural in vivo carrier of cholesterol present in the vascular system, have been used for targeted transport of Hyp to U87 glioma cells. For low Hyp-LDL ratios (<= 10 : 1), the cellular uptake of Hyp is characterized by endocytosis of the [Hyp-LDL] complex, while Hyp alone can enter cells by passive diffusion. Photo-induced cell death and the mitochondrial membrane potential, observed for glioma cells after various times of incubation with the [Hyp-LDL] complex or Hyp alone, were monitored by flow-cytometry analysis using Annexin-V-FITC propidium iodide and DiOC(6)(3) staining. Differences of the results are discussed in view of the respective dynamic subcellular distributions of the drugs that were obtained by co-localization experiments using confocal fluorescence microscopy. In order to give clear evidence of specific intracellular localization and to identify possible Hyp aggregation in cellular organelles, fluorescence resonance energy transfer (FRET) between selected fluorescent organelle probes and Hyp was also assessed. It is shown, that the observed photo-induced cell deaths can be correlated with the sub-cellular distribution of the active fluorescent monomer form of Hyp in lysosomes (as determined from steady-state fluorescence experiments), but that possible aggregation of Hyp in some organelles, as determined from FRET experiments, should be taken into account for interpretation of the real dynamics of the subcellular redistribution. Results of the present study underline the fact that photoinduced cell death processes are strongly influences by dynamics of Hyp subcellular redistribution processes involving monomer-aggregate equilibrium. Such an observation should be taken in consideration for further optimization of Hyp in vivo PDT applications.