Hou Zhanjun, Matherly Larry H
Department of Oncology, Wayne State University School of Medicine, Detroit, Michigan, USA; Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan, USA.
Department of Oncology, Wayne State University School of Medicine, Detroit, Michigan, USA; Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan, USA; Department of Pharmacology, Wayne State University School of Medicine, Detroit, Michigan, USA.
Curr Top Membr. 2014;73:175-204. doi: 10.1016/B978-0-12-800223-0.00004-9.
This chapter focuses on the biology of the major facilitative membrane folate transporters, the reduced folate carrier (RFC), and the proton-coupled folate transporter (PCFT). Folates are essential vitamins, and folate deficiency contributes to a variety of heath disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates intestinal absorption of dietary folates. Clinically relevant antifolates such as methotrexate (MTX) are transported by RFC, and the loss of RFC transport is an important mechanism of MTX resistance. PCFT is abundantly expressed in human tumors and is active under pH conditions associated with the tumor microenvironment. Pemetrexed (PMX) is an excellent substrate for PCFT as well as for RFC. Novel tumor-targeted antifolates related to PMX with selective membrane transport by PCFT over RFC are being developed. The molecular picture of RFC and PCFT continues to evolve relating to membrane topology, N-glycosylation, energetics, and identification of structurally and functionally important domains and amino acids. The molecular bases for MTX resistance associated with loss of RFC function, and for the rare autosomal recessive condition, hereditary folate malabsorption (HFM), attributable to mutant PCFT, have been established. From structural homologies to the bacterial transporters GlpT and LacY, homology models were developed for RFC and PCFT, enabling new mechanistic insights and experimentally testable hypotheses. RFC and PCFT exist as homo-oligomers, and evidence suggests that homo-oligomerization of RFC and PCFT monomeric proteins may be important for intracellular trafficking and/or transport function. Better understanding of the structure and function of RFC and PCFT should facilitate the rational development of new therapeutic strategies for cancer as well as for HFM.
本章重点介绍主要的易化性膜叶酸转运体、还原型叶酸载体（RFC）和质子偶联叶酸转运体（PCFT）的生物学特性。叶酸是必需维生素，叶酸缺乏会导致多种健康问题。RFC广泛表达，是哺乳动物细胞和组织中的主要叶酸转运体。PCFT介导膳食叶酸的肠道吸收。临床相关的抗叶酸药物如甲氨蝶呤（MTX）由RFC转运，RFC转运功能丧失是MTX耐药的重要机制。PCFT在人类肿瘤中大量表达，在与肿瘤微环境相关的pH条件下具有活性。培美曲塞（PMX）既是PCFT也是RFC的优良底物。正在开发与PMX相关的新型肿瘤靶向抗叶酸药物，其通过PCFT而非RFC进行选择性膜转运。RFC和PCFT的分子结构在膜拓扑结构、N-糖基化、能量学以及结构和功能重要结构域与氨基酸的鉴定方面不断演变。与RFC功能丧失相关的MTX耐药以及由突变PCFT导致的罕见常染色体隐性疾病遗传性叶酸吸收不良（HFM）的分子基础已得到确定。基于与细菌转运体GlpT和LacY的结构同源性，构建了RFC和PCFT的同源模型，从而获得了新的机制性见解和可通过实验验证的假设。RFC和PCFT以同型寡聚体形式存在，有证据表明RFC和PCFT单体蛋白的同型寡聚化可能对细胞内运输和/或转运功能很重要。更好地理解RFC和PCFT的结构与功能应有助于合理开发针对癌症以及HFM的新治疗策略。
