Dominici Silvia, Pieri Lisa, Comporti Mario, Pompella Alfonso
Department of Experimental Pathology, University of Pisa Medical School, Via Roma 55, 56126 Pisa, Italy.
Cancer Cell Int. 2003 May 14;3(1):7. doi: 10.1186/1475-2867-3-7.
The molecular mechanisms by which iron is physiologically transported trough the cellular membranes are still only partially understood. Several studies indicate that a reduction step of ferric iron to ferrous is necessary, both in the case of transferrin-mediated and transferrin-independent iron uptake. Recent studies from our laboratory described gamma-glutamyltransferase activity (GGT) as a factor capable to effect iron reduction in the cell microenvironment. GGT is located on the outer aspect of plasma membrane of most cell types, and is often expressed at high levels in malignant tumors and their metastases. The present study was aimed at verifying the possibility that GGT-mediated iron reduction may participate in the process of cellular iron uptake. RESULTS: Four distinct human tumor cell lines, exhibiting different levels of GGT activity, were studied. The uptake of transferrin-bound iron was investigated by using 55Fe-loaded transferrin, as well as by monitoring fluorimetrically the intracellular iron levels in calcein-preloaded cells. Transferrin-independent iron uptake was investigated using 55Fe complexed by nitrilotriacetic acid (55Fe-NTA complex).The stimulation of GGT activity, by administration to cells of the substrates glutathione and glycyl-glycine, was generally reflected in a facilitation of transferrin-bound iron uptake. The extent of such facilitation was correlated with the intrinsic levels of the enzyme present in each cell line. Accordingly, inhibition of GGT activity by means of two independent inhibitors, acivicin and serine/boric acid complex, resulted in a decreased uptake of transferrin-bound iron. With Fe-NTA complex, the inhibitory effect - but not the stimulatory one - was also observed. CONCLUSION: It is concluded that membrane GGT can represent a facilitating factor in iron uptake by GGT-expressing cancer cells, thus providing them with a selective growth advantage over clones that do not possess the enzyme.
铁在生理状态下通过细胞膜转运的分子机制目前仍仅被部分理解。多项研究表明，无论是在转铁蛋白介导的铁摄取还是非转铁蛋白依赖的铁摄取过程中，三价铁还原为二价铁这一步骤都是必要的。我们实验室最近的研究表明，γ-谷氨酰转移酶活性（GGT）是能够在细胞微环境中影响铁还原的一个因素。GGT位于大多数细胞类型的质膜外侧，并且在恶性肿瘤及其转移灶中常常高水平表达。本研究旨在验证GGT介导的铁还原可能参与细胞铁摄取过程的可能性。结果：研究了四种具有不同GGT活性水平的人肿瘤细胞系。通过使用负载55Fe的转铁蛋白研究转铁蛋白结合铁的摄取，同时通过荧光法监测钙黄绿素预负载细胞内的铁水平。使用次氮基三乙酸络合的55Fe（55Fe-NTA络合物）研究非转铁蛋白依赖的铁摄取。通过向细胞给予底物谷胱甘肽和甘氨酰甘氨酸来刺激GGT活性，这通常表现为促进转铁蛋白结合铁的摄取。这种促进程度与每个细胞系中该酶的固有水平相关。相应地，通过两种独立的抑制剂阿西维辛和丝氨酸/硼酸络合物抑制GGT活性，导致转铁蛋白结合铁的摄取减少。对于Fe-NTA络合物，也观察到了抑制作用，但未观察到刺激作用。结论：得出的结论是，膜GGT可以成为表达GGT的癌细胞摄取铁的促进因素，从而使其相对于不具有该酶的克隆具有选择性生长优势。
