Activation of two caspase cascades, caspase 8/3/6 and caspase 9/3/6, during photodynamic therapy using a novel photosensitizer, ATX-S10(Na), in normal human keratinocytes.

Background/purpose: Photodynamic therapy (PDT) is a potent treatment for skin tumors. Although the therapeutic effect of PDT is supposed to be due to cellular cytotoxicity, the precise mechanism is still unknown. ATX-S10(Na) [13,17-bis(1-carboxypropionyl)carbamoylethyl-8-ethenyl-2-hydroxy-3-hydroxyiminoethylidene-2,7,12,18-tetramethylporphyrin sodium salt], a novel hydrophilic chlorin photosensitizer, shows good accumulation in tumors and is suitable for use in PDT. In this study, we investigated the mechanism of PDT-induced cell death using ATX-S10(Na).

Methods: . Following ATX-S10(Na) treatment for 12 h, normal human keratinocytes (NHK) were irradiated using a diode laser. PDT-induced cell death and the activity of various caspases were measured. Activation of Fas antigen was also determined by immunoprecipitation analysis. The expression of Bax, cytochrome c, and apoptosis-inducing factor (AIF) was determined by Western blotting.

Results: ATX-S10(Na)-PDT had induced apoptosis of NHK by 2 h and the maximal effect was observed at 6 h following irradiation. The effect was suppressed by pretreatment of NHK with inhibitors of caspases 3, 6, 8 and 9. A caspase activity assay revealed the sequential activation of caspases 8, 3 and 6, and caspases 9, 3 and 6, respectively. Immunoprecipitation analysis indicated multimerization of Fas antigen without Fas ligand binding in ATX-S10(Na)-PDT-treated NHK. Western blotting revealed cytosolic release of cytochrome c and AIF accompanied by decreased Bax expression in the cytosol.

Conclusions: ATX-S10(Na)-PDT induces apoptosis of NHK, and this was mediated by sequential activation of two caspase cascades, caspases 8, 3 and 6, and caspases 9, 3 and 6. This was accompanied by multimerization of Fas antigen and cytosolic release of cytochrome c and AIF.