Comparison of phototoxic effects of hypericin-mediated photodynamic therapy in HT-29 and Caco-2 colon cancer cells

Hypericin (HYP) is a plant-derived photosensitizer. HYP is preferentially taken up by tumor cells. We designed this study to compare HYP-mediated photodynamic therapy (PDT) in HT-29 and Caco-2 colon cell lines. Cells were treated with 0.04, 0.08, and 0.15 μM HYP concentrations and irradiated. The effect of HYP on metabolic profiles, alterations in lactate dehydrogenase (LDH) leakage, and cell cycle progression was investigated for the first time. Changes in glucose consumption, lactate production, and LDH leakage were analyzed. HYP-induced cell death was quantified by double staining (acridine orange/propidium iodide) and alterations in cell cycle regulation were analyzed with flow cytometry (using propidium iodide). LDH leakage and the number of dead cells were elevated, and glucose consumption and lactate production decreased in a dose- and time-dependent manner. PDT resulted in an induction of apoptosis, mostly at the 0.08 μM HYP concentration. Apoptosis and/or necrosis were increased in the 0.15 μM HYP group. The accumulation of cells in the G2/M phase might account for the growth inhibition in HT-29 and Caco-2 cells with 0.08 μM HYP photoactivation. The observed G2/M arrest suggested that HYP may slow down the growth of colon cancer cells by regulating the cell cycle, leading either to growth inhibition or to initiation of apoptotic pathways.