| Summary: | 博士 === 國立陽明大學 === 藥理學研究所 === 88 === Epidemiological studies have shown that betel quid (BQ) chewing is the main cause of oral squamous cell carcinoma (OSCC) in Taiwan. The most popular way to chew BQ in Taiwan is the combination of areca nut, Piper betle inflorescence (sometimes substituted by betel leaf), and lime paste. The BQ chewed in Taiwan contains tender areca nut with husk. On the other hand, ripe and dried areca nut without husk is preferred one in other countries. Besides Taiwan, Papua New Guinea is the only other country where Piper betle inflorescence is used, and it contains a high concentration of safrole (15.4 mg/g) and hydroxychavicol (HC) (9.7 mg/g). Consequently, chewing BQ containing Piper betle inflorescence may result in safrole (420 M in saliva during chewing) and HC exposure. In this study, we examined the genotoxic and toxic potentials of various BQ ingredients.
HC alone is slightly toxic to human hepatoma HepG2 cells. However, the cytotxicity increased significantly when HepG2 cells were pretreated with buthionine sulfoximine (BSO), suggesting that endogenous glutathione prevented HC-induced cytotoxicity. Addition of catalase or N-acetylcysteine prevented the cytotoxocity caused by BSO and HC. HC also increased 8-hydroxy-2’-deoxyguanosine (8-OH-dG) formation and apoptosis in BSO-pretreated HepG2 cells, and these increases could be suppressed by catalase. These data suggest that BSO pretreatments enhanced HC-induced cytotoxicity and apoptosis in HepG2 cells which are mediated by oxidative DNA damage.
Safrole is a known rodent hepatocarcinogen, yet its carcinogenicity in human is largely undetermined. In this study, using a 32P-postlabeling method, we have found a high frequency of safrole-DNA adducts in BQ -associated OSCC (77%, 23/30) and non-cancerous matched tissue (NCMT) (97%, 29/30). This was in contrast to the absence (< 1/109 nucleotides) of such adducts in all of non-BQ-associated OSCC and their pairing NCMT (P < 0.001). The DNA adduct levels in NCMT were significantly higher than that in OSCC (P < 0.05). These results suggest that safrole forms stable safrole-DNA adducts in humans oral tissue following BQ chewing, which may be associated with oral carcinogenesis.
Tannin and catechin are the major constituents of areca nut, which are believed to contribute to the formation of reactive oxygen species (ROS) under alkaline conditions. In this part of the study, we demonstrated that ripe ANE (areca nut extract) contains higher total tannin and catechin than tender ANE, which may cause the generation of higher levels of 8-OH-dG in hamster cheek pouch. In addition, using the conversion of phenyalanine to o-tyrosine and m-tyrosine by ROS, we have clearly demonstrated that ROS is formed in the human oral cavity during BQ chewing.
In summary, these results suggest that stable safrole-DNA adducts derived from Piper betle inflorescence and ROS formation during BQ chewing may be contributive to oral carcinogenesis.
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