Mechanisms of Cancer Chemoprevention by Hop Bitter Acids (Beer Aroma) through Induction of Apoptosis Mediated by Fas and Caspase Cascades

The bitter acids of hops (Humulus lupulus L.) mainly consist of α-acids, β-acids, and their oxidation products that contribute the unique aroma of the beer beverage. Hop bitter acids displayed a strong growth inhibitory effect against human leukemia HL-60 cells, with an estimated IC 50 value of 8.67 μg/mL, but were less effective against human histolytic lymphoma U937 cells. Induction of apoptosis was confirmed in HL-60 cells by DNA fragmentation and the appearance of a sub-G1 DNA peak, which were preceded by dissipation of mitochondrial membrane potential, cytochrome c release, and subsequent induction of pro-caspase-9 and -3 processing. Cleavages of PARP and DFF-45 were accompanied with activation of caspase-9 and -3 triggered by hop bitter acids in HL-60 cells. The change in the expression of Bcl-2, Bcl-XL, and Bax in response to hop bitter acids was studied, and the Bcl-2 protein level slightly decreased; however, the Bcl-XL protein level was obviously decreased, whereas the Bax protein level was dramatically increased, indicating that the control of Bcl-2 family proteins by hop bitter acids might participate in the disruption of mitochondrial integrity. In addition, the results showed that hop bitter acids promoted the up-regulation of Fas and FasL prior to the processing and activation of pro-caspase-8 and cleavage of Bid, suggesting the involvement of a Fas-mediated pathway in hop bitter acids-induced cells. Taken together, these findings suggest that a certain intimate link might exist between receptor- and mitochondria-mediated death signalings that committed to cell death induced by hop bitter acids. The induction of apoptosis by hop bitter acids may offer a pivotal mechanism for their chemopreventive action.