Differential up-regulation of metallothionein isoforms in well-differentiated nasopharyngeal cancer cells in vitro by photoactivated hypericin

Photoactivation of hypericin is known to generate reactive oxygen species and induce phototoxic effects. However, modulation of the cellular antioxidant defense would influence the extent and severity of the photodynamic effects. We have previously shown that hypericin-mediated photodynamic therapy (PDT) induced a significant reduction of Glutathione S-transferase activity. In this study, we investigated the phototoxic effects of hypericin-mediated PDT in nasopharyngeal cancer (NPC) in vitro and analyzed the expression of metallothionein (MT), a family of potential free radical scavengers. HK1 NPC cells were subjected to PDT treatment in vitro, and the effects on cell death were analyzed by flow cytometry (using propidium iodide and Annexin V staining) and transmission electron microscopy. The expression profile of MT-1E and MT-2A isoforms (the only functional MT isoforms expressed in HK1 NPC cells) were determined by quantitative real-time RT-PCR. The results showed that hypericin PDT induced necrotic cell death as evidenced by the absence of a subdiploid peak and decreased Annexin-V fluorescence. Ultrastructural examination verified the presence of cell necrosis. There was a significant up-regulation of MT-1E and MT-2A isoforms six hours following PDT, with an approximately 50-fold rise in the expression level of MT-1E and a 15-fold increase of MT-2A. Hence, despite the up-regulation of MT, cells still succumbed to PDT-induced necrosis. It appears that the oxidative stress induced by PDT overwhelmed the antioxidant defense mechanism such as the alteration of MT levels in tumor cells.