Enhancement of Glutathione and γ-Glutamylcysteine Synthetase, the Rate Limiting Enzyme of Glutathione Synthesis, by Chemoprotective Plant-Derived Food and Beverage Components in the Human Hepatoma Cell Line HepG2

Abstract

Glutathione (GSH) is an important antioxidant and cofactor of detoxifying metabolism. Therefore, elevation of GSH as achieved by inducing γ-glutamylcysteine synthetase (GCS), the limiting enzyme of GSH synthesis, may contribute to chemoprevention against cancer. In previous animal studies, increases in GCS were mainly found in liver and other organs that are not easily accessible in humans. Thus, employment and evaluation of alternative systems such as human-derived cell lines are encouraged. In the present experiment, we used the hepatoma cell line HepG2 to investigate the response of GCS and GSH to five plant-derived chemoprotectants contained in regularly consumed foodstuffs and beverages (kahweol/cafestol [K/C] [15.5-62.0 μM], α-angelicalactone [100-400 μM], benzyl isothiocyanate [1.7-5.0 μM], diallyl sulfide [175-700 μM], and quercetin [10-50 μM]). All treatments led to dose-dependent increases in both GCS activity and GSH concentration. Time course studies with K/C indicated that the enhancement of GCS preceded that of GSH, suggesting a causal relationship. K/C did not enhance γ-glutamyl transpeptidase, a further enzyme that assists GSH-related chemoprotection. Although GCS induction has been suggested to require an initial short-lived GSH depletion, we did not find any decrease in GSH after 3 h of incubation with K/C. In summary, HepG2 cells were shown to be a useful model to investigate the capacity of potential chemoprotectants to enhance GCS and GSH. To our knowledge, the present study is also the first to show increases in GCS by K/C and a-angelicalactone in vitro and by diallyl sulfide and quercetin in any system.