Hsueh C T, Dolnick B J
Department of Experimental Therapeutics, Grace Cancer Drug Center, Roswell Park Cancer Institute, Buffalo, NY 14263.
Biochem Pharmacol. 1993 Jun 22;45(12):2537-45. doi: 10.1016/0006-2952(93)90235-o.
Folate-binding protein (FBP), a high-affinity folate receptor, is responsible for cellular accumulation of folate and folate analogs such as methotrexate in human KB (nasopharyngeal carcinoma) cells. Both FBP and FBP mRNA increase 3- to 5-fold when KB cells are grown in folate-deficient (less than 10 nM folate) medium (KB-FD), compared with growth in standard folate-replete medium containing at least 2 microM folate (KB-FR). The possible mechanisms of enhanced FBP gene expression in KB-FD were examined in this study. Southern blot analysis revealed no significant change in the FBP gene organization or copy number in the KB-FD DNA. While hypomethylation of the FBP gene was observed in KB-FD DNA, relative to KB-FR DNA, exposure of KB-FR to the DNA methylation inhibitors did not result in elevated FBP mRNA levels. The transcriptional rate of the FBP gene was the same in KB-FR and KB-FD. RNA half-life studies indicated that the half-life of FBP mRNA in KB-FD was increased approximately 2.5-fold, compared with KB-FR. Thus, the increase in the steady-state levels of FBP mRNA in KB-FD can be attributed partly to increased FBP mRNA stability.
叶酸结合蛋白（FBP）是一种高亲和力叶酸受体，负责叶酸及叶酸类似物（如甲氨蝶呤）在人KB（鼻咽癌）细胞中的细胞内蓄积。与在含有至少2 μM叶酸的标准叶酸充足培养基（KB-FR）中生长相比，当KB细胞在叶酸缺乏（叶酸浓度低于10 nM）培养基（KB-FD）中生长时，FBP及其mRNA均增加3至5倍。本研究探讨了KB-FD中FBP基因表达增强的可能机制。Southern印迹分析显示，KB-FD DNA中FBP基因的组织或拷贝数无显著变化。虽然相对于KB-FR DNA，在KB-FD DNA中观察到FBP基因的低甲基化，但将KB-FR暴露于DNA甲基化抑制剂并不会导致FBP mRNA水平升高。FBP基因在KB-FR和KB-FD中的转录率相同。RNA半衰期研究表明，与KB-FR相比，KB-FD中FBP mRNA的半衰期增加了约2.5倍。因此，KB-FD中FBP mRNA稳态水平的增加部分可归因于FBP mRNA稳定性的增加。
