小檗鹼誘導的二胺螢素的氮──乙醯化於人類乳癌細胞株(MCF-7)中，所牽涉到的氮──乙醯轉移酵素活性作用及基因表現

Effects of N-acetyltransferase activity and gene expression involved in berberine-induced Nacetylation of 2-aminofluorene in human breast cell line (MCF-7)

氮－－乙醯轉移酵素已被證實是催化異環式芳香胺形成致癌物的主要酵素。二胺螢素是一種人工合成的生物鹼，在生物體内經過氮－－乙醯轉移酵素的乙醯化反應，可被轉換成著名的致癌物－－二乙醯胺螢素。小檗鹼是一種生物鹼，存在於許多臨床上重要的藥草，包括北美黃連、野黃連、奥勒岡葡萄和小檗等。很多論文已證實小檗鹼可抑制氮－－乙醯轉移酵素的活性，但尚未有關於小檗鹼對人類乳癌細胞中氮－－乙醯轉移酵素活性的發表。因此，我們使用小檗鹼來探討對人類乳癌細胞中氮－－乙醯轉移酵素的活性抑制作用。藉由二乙醯胺螢素及殘餘二胺螢素的量，我們以高效液相層析(HPLC)來測試究氮－－乙醯轉移酵素的活性。氮－－乙醯轉移酵素蛋白的量及基因表現有否受到小檗鹼的影響則分別利用西方墨點法(Western blot)及聚合酵素連鎖反應(PCR) 來檢測。我們的研究結果顯示：(1)小檗鹼以時間及劑量的雙重依賴性抑制氮－－乙醯轉移酵素活性及蛋白的量。(2)小檗鹼是氮－－乙醯轉移酵素的不競爭(uncompetitive)抑制劑，可同時降低平衡常數(Km)及最大反應速率(Vmax)。(3) 小檗鹼可抑制人類乳癌細胞株氮－－乙醯轉移酵素mRNA的基因表現。

N-Acetyltransferase°]NAT°^ has been shown to be a major enzyme that catalyzes the heterocyclic aromatic amines to form the active carcinogens. 2-Aminofluorene(2- AF) is a synthetic arylamine which can be converted by N-acetyltransferase to a wellknown animal carcinogen, called 2-acetylaminofluorene(2-AAF). Berberine is an alkaloid present in a number of clinically important medicinal plants, including Hydrastis canadensis (goldenseal), Coptis chinensis (coptis or goldenthread), Berberis aquifolium (Oregon grape), and Berberis vulgaris (barberry). Many papers have shown that berberine can inhibit the activity of N-acetyltransferase but there are no available information addressing berberine effects on the NAT activity of human breast cancer cell. Thus, we used berberine to test its inhibition of NAT activity in human breast cancer cell line (MCF-7). The NAT activity was measured by a high performance liquid chromatography assaying for the amounts of 2-AAF and the remaining 2-AF. The protein of NAT and NAT mRNA gene expression were evaluated by Western blotting and RT-PCR methods separately. Our results demonstrated that (1) Berberine can inhibit the activity and protein level of NAT in a dose-dependent and time-dependent manner. (2) Berberine is an uncompetitive inhibitor of NAT that berberine can decrease the value of Km and Vmax. (3) Berberine can decrease NAT mRNA gene expression.