Detection of caspase-3 activation in single cells by fluorescence resonance energy transfer during photodynamic therapy induced apoptosis

In many apoptosis pathways, activation of caspase-3 is considered the final stage. In this study, we studied PDT induced caspase-3 activation with fluorescence resonance energy transfer (FRET) technique. A recombinant caspase-3 substrate, SCAT3, was used as the FRET probe. FRET fluorescence images were collected after PDT or TNF-α induced apoptosis. By analyzing the dynamic changes of FRET fluorescence, the results indicate that the caspase-3 activation started immediately after the PDT treatment. In contrast, FRET disruption caused by caspase-3 activation started at 3 h after TNF-α treatment, due to different signaling pathway. The results have proofed, for the first time, that FRET is a sensitive technique that can be used to investigate PDT-induced activation of caspase-3 in real-time and in single cells. By choosing appropriate recombinant substrates as FRET probes, it is likely that FRET technique will provide a new real-time means to study the mechanism of PDT at single cell level.