Single cell FRET imaging for determination of pathway of tumor cell apoptosis induced by photofrin-PDT.

Apoptosis is an important cellular event that plays a key role in pathogeny and therapy of many diseases. Apoptosis has been associated with photodynamic therapy (PDT) and its pathway is important in the mechanistic study of PDT. We show that single cell fluorescent imaging can be used to determine the pathway of PDT-induced tumor cell apoptosis. In this study, ASTC-a-1 tumor cells transfected by plasmid DNA SCAT3 were treated by Photofrin-PDT. The intracellular distribution of Photofrin was observed using a confocal microscope. The activations of caspase-3 and caspase-8 were dynamically observed using fluorescence resonance energy transfer (FRET). Our experimental results show that the Photofrin molecules are localized in cell mitochondria, and that after PDT caspase-3 was activated rapidly while caspase-8 remained inactive. These results demonstrate that the tumor cell apoptosis induced by Photofrin-PDT was directly initiated from the mitochondrial pathway.