Zhang Y
Department of Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Carcinogenesis. 2000 Jun;21(6):1175-82.
Isothiocyanates (ITCs) are abundant in the human diet. Many potently inhibit tumorigenesis induced by a wide variety of chemical carcinogens in rodents. Recently, we observed that several ITCs accumulated to very high concentrations in cultured cells and that their accumulated levels were closely related to their potencies in inducing phase II enzymes [NAD(P)H:quinone reductase and glutathione transferases] that detoxify carcinogens. To elucidate the molecular mechanism responsible for this accumulation, the intracellular chemical identities of two ITCs, sulforaphane [SF, 1-isothiocyanato-(4R,S)-(methylsulfinyl)butane] and benzyl-ITC, were investigated in murine hepatoma cells. Both ITCs accumulated very rapidly to high intracellular concentrations, but, remarkably, most of the intracellular forms of the ITCs were dithiocarbamates resulting from conjugation with reduced glutathione (GSH). For example, the intracellular concentration reached 6.4 mM when cells were exposed to 100 microM SF for 30 min at 37 degrees C and 95% of the accumulated product was the GSH conjugate. Cellular accumulation of each ITC was accompanied by a profound reduction in cellular GSH levels. These findings, together with our previous observation that accumulation of ITCs depended on cellular GSH levels, strongly suggest that intracellular conjugation of ITCs with GSH is mainly responsible for ITC accumulation. Surprisingly, rapid accumulation to high concentrations also occurred when cells were exposed to the GSH-ITC conjugates. However, these conjugates were apparently not absorbed intact, but were hydrolyzed extracellularly to free ITCs that were taken up by the cells. This conclusion is supported by the finding that suppression of dissociation of the conjugates by excess GSH or other thiols blocks accumulation of the conjugates.
异硫氰酸盐（ITCs）在人类饮食中含量丰富。许多异硫氰酸盐能有效抑制啮齿动物中多种化学致癌物诱导的肿瘤发生。最近，我们观察到几种异硫氰酸盐在培养细胞中积累到非常高的浓度，并且它们的积累水平与它们诱导参与致癌物解毒的Ⅱ相酶[烟酰胺腺嘌呤二核苷酸（磷酸）：醌还原酶和谷胱甘肽转移酶]的能力密切相关。为了阐明这种积累的分子机制，我们在小鼠肝癌细胞中研究了两种异硫氰酸盐——萝卜硫素[SF，1-异硫氰酸基-（4R，S）-（甲基亚磺酰基）丁烷]和苄基异硫氰酸盐的细胞内化学特性。两种异硫氰酸盐都能非常迅速地积累到较高的细胞内浓度，但值得注意的是，异硫氰酸盐的大多数细胞内形式都是与还原型谷胱甘肽（GSH）结合形成的二硫代氨基甲酸盐。例如，当细胞在37℃下暴露于100μM萝卜硫素30分钟时，细胞内浓度达到6.4 mM，积累产物的95%是谷胱甘肽结合物。每种异硫氰酸盐的细胞积累都伴随着细胞内谷胱甘肽水平的显著降低。这些发现，连同我们之前观察到的异硫氰酸盐积累依赖于细胞内谷胱甘肽水平，强烈表明异硫氰酸盐与谷胱甘肽的细胞内结合是异硫氰酸盐积累的主要原因。令人惊讶的是，当细胞暴露于谷胱甘肽-异硫氰酸盐结合物时，也会迅速积累到高浓度。然而，这些结合物显然不是完整吸收的，而是在细胞外水解为游离的异硫氰酸盐，然后被细胞摄取。这一结论得到以下发现的支持：过量的谷胱甘肽或其他硫醇抑制结合物的解离会阻止结合物的积累。
