A novel cleavage product formed by autoxidation of lycopene induces apoptosis in HL-60 cells

Abstract

Dietary carotenoids have been thought to have beneficial effects on human health through their antioxidant activity, provitamin A activity, and effects on cancer cell propagation. Recent studies suggest that oxidation products or metabolites are involved in biological activities of carotenoids. We previously reported that an autoxidation mixture of lycopene induced apoptosis in HL-60 human promyelocytic leukemia cells, but lycopene alone did not. In the present study, bioassay-directed fractionations of autoxidized lycopene led to isolation of a novel cleavage product of lycopene. Spectral analyses elucidated its structure as ( E, E, E)-4-methyl-8-oxo-2,4,6-nonatrienal (MON), suggesting the formation through the oxidative cleavages at the 5, 6- and 13, 14-double bonds of lycopene. MON was proved to cause a dose-dependent reduction of viability in HL-60 cells with morphological changes such as chromatin condensation and nuclear fragmentation. Treatment of HL-60 cells with MON could induce DNA fragmentation and increase apoptotic cells in a time- and dose-dependent manner. The MON treatment could enhance both caspase 8 and caspase 9 activities. Moreover, it reduced the expression of Bcl-2 and Bcl-X L proteins, whereas it had no effect on the level of Bax protein. These results clearly indicated that MON induced apoptosis in HL-60 cells, associated with the down regulation of Bcl-2 and Bcl-X L and the activation of caspase cascades. The concentration of MON attained by treatment of the autoxidized lycopene preparation was far less than the IC 50 (10 μM) value of MON alone in reducing the viability of HL-60 cells. The fractionation of the oxidized lycopene indicated the presence of other active oxidation products. Thus, unidentified products as well as MON would be responsible for the apoptosis-inducing activity of the autoxidized lycopene.