Glutathione depression by dihydropyrazine derivative

Molecular mechanism of dihydropyrazine-induced cytotoxicity: the possibility of an independent pathway from the receptor for advanced glycation end products

Dihydropyrazines (DHPs) are one of glycation products that are non-enzymatically generated in vivo and in food. We had previously revealed that 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), a methyl-substituted DHP, elicited redox imbalance and cytotoxicity in cultured cells. However, the molecular mechanisms underlying DHP-3-induced cytotoxicity remain unclear. To address this issue, we examined the involvement of the receptor for advanced glycation end products (RAGE) in DHP-3-induced cytotoxicity. To evaluate the role of RAGE, we prepared HeLa cells that constitutively expressed RAGE and its deletion mutant, which lacks the cytoplasmic domain (RAGE_Δcyto), using an episomal vector. After transfection with the vector, cells were selected following incubation with multiple concentrations of hygromycin to remove non-transfected cells. The expression of RAGE and RAGE_Δcyto in the cells was confirmed by immunoblotting. RAGE and RAGE_Δcyto were apparently expressed in transfected cells; however, there were no significant differences in DHP-3-induced cytotoxicity between these cells and mock vector-transfected cells. These results suggested that DHP-3 elicits cytotoxicity in a RAGE-independent manner.

Identification of dihydropyrazine-glutathione adducts

Dihydropyrazines (DHPs) are glycation intermediates generated both in vivo and in food. DHPs can lead to the formation of a variety of different radical species, which can lead to DNA damage and enzyme inhibition. In addition, the presence of DHPs can lead to a decrease in cellular glutathione (GSH) levels, and induce the expression of antioxidant genes. In this study, the products resulting from the reaction of DHP with GSH have been analyzed in detail, with some of the products being separated by reversed-phase HPLC. The structures of the isolated DHP-GSH adducts were determined by FAB-MS and NMR analyses. These data suggested that the reaction of DHP with a thiol moiety could be involved in oxidative stress, because an increase in the amount of DHP-GSH adducts would result in a decrease in the cellular GSH levels.

Possible involvement of glutathione balance disruption in dihydropyrazine-induced cytotoxicity on human hepatoma HepG2 cells

Dihydropyrazines (DHPs), formed by nonenzymatic glycation, are known to exert various effects in vitro and in vivo, such as generation of radical species, DNA strand breakage, enzyme inhibition, and inhibition of bacterial growth. However, their effects on mammalian cells remain elusive. To address this issue, we investigated the effects of a range of DHP concentrations on human hepatoma HepG2 cells using 2,3-dihydro-5,6-dimethylpyrazine (DHP-1), 2,3-dihydro-2,5,6-trimethylpyrazine (DHP-2), and 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3) as model compounds. All of the tested compounds exerted cytotoxic activity against HepG2 cells in the range of 10 µM-1 mM, and significantly so at the highest concentration. DHP-3 was the most effective drug, and it also caused a significant decrease in the ratio of intracellular reduced and oxidized glutathione (GSH/GSSG). In addition, the cytotoxic effect of DHP-3, but not DHP-1 and DHP-2, was enhanced by the inhibition of GSH biosynthesis using 100 µM l-buthionine-(S,R)-sulfoximine (BSO). From these results, it is suggested that the mechanisms of cytotoxicity exerted by DHP-3 are distinct from those exerted DHP-1 and DHP-2. In addition, it is possible that the disruption of intracellular glutathione balance induced by DHP-3 is related to its effect on HepG2 cells.