DHA induces dose- and time-dependent activation of caspase-3 in living cells which was attenuated by Z-VAD.fmk (10 μM). a The emission spectral analysis of the SCAT3 in living cells stably expressing SCAT3 after 0, 5, 10, 20 μg/ml DHA treatment for 48 h. The emission peak at 524 nm gradually decreased after 5 and 10 μg/ml DHA treatment, and completely disappeared after 20 μg/ml DHA treatment, while the peak at 481 nm increased to the maximum, implying that DHA activated caspase-3 which cleaved SCAT3. b 20 μg/ml DHA was adopted to determine the time-dependent activation of caspase-3 for indicated time (0, 12, 24, 48 h). The Fluorescence spectrum of SCAT3 didn't have an obvious change at 12 h, but the emission peak at 524 nm decreased at 24 h and completely disappeared after 48 h DHA treatment. d The inhibition of DHA-induced caspase-3 activation was triggered by Z-VAD.fmk. The reoccurrence of FRET effect was indicative of the inhibition of DHA-mediated caspase-3 activation under the co-treatment of DHA and Z-VAD.fmk as well as 1 μM STS treatment (c: as a positive control) for 6 h.