Cellular phototoxicity induced by UVA irradiation and its potential application to therapy has been reported. In the present study, the induction of apoptosis induced by β-carotene and dimethyl tetrasulfide (Me₂S₄) assisted by UVA irradiation in HL-60 cells was assessed. β-carotene assisted by UVA significantly decreased the cell viability and induced DNA fragmentation in HL-60 cells. Me₂S₄ combined with β-carotene and assisted by UVA significantly inhibited the cell viability, and enhanced the caspase-3 activity which was completely inhibited by N-acety-L-cysteine. β-carotene was significantly degraded by UVA, but this was not accelerated by Me₂S₄ in a cell culture system. The photodegradation products of β-carotene prepared by UVA irradiation regardless of the addition of Me₂S₄ showed lower cytotoxicity than β-carotene itself in HL-60 cells. These results suggest that the ROS- and caspase-3-dependent apoptosis induced by β-carotene and Me₂S₄ assisted by UVA was due to a synergistic action rather than to the sole effect of the photodegradation products of β-carotene in HL-60 cells.

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