The plastoquinones sargaquinoic acid and sargachromenol are major components of brown alga Sargassum sagamianum and are known to be involved in neurite growth and survival. In this study, we describe their novel pro-apoptotic activities in vitro and in vivo. In vitro, treatment with sargaquinoic acid or sargachromenol promoted cell death and activation of caspase-3, caspase-8, caspase-9 and poly (ADP-ribose) polymerase (PARP) in a concentration-dependent manner. Sargaquinoic acid- or sargachromenol-induced apoptosis was enhanced by co-treatment with UVB irradiation. It showed much higher levels of cleaved caspase-3, caspase-8, caspase-9, and PARP than with sargaquinoic acid and sargachromenol alone, while it had no effect on Bcl-2 and Bax expression level. Examination by terminal deoxynucleotidyl transferase-mediated nick-end labeling (TUNEL) and immunohistochemistry showed that topical application of sargaquinoic acid (1 mg/ml) before UVB irradiation (2.5 kJ/m(2)) of hairless mice also enhanced apoptosis including activation of caspase-3. Since a combination of phototherapy using UVB with topical reagents has been clinically applied to treat hyperproliferative skin disease, these results suggest that sargaquinoic acid and sargachromenol could be effective therapeutic agents.