ABSTRACT
MRP1 is a 190-kDa membrane glycoprotein that confers multidrug resistance (MDR) to tumor cells. MRP1 is characterized by an N-terminal transmembrane domain (TMD(0)), which is connected to a P-glycoprotein-like core region (DeltaMRP) by a cytoplasmic linker domain zero (L(0)). It has been demonstrated that GSH plays an important role in MRP1-mediated MDR. However, the mechanism by which GSH mediates MDR and the precise roles of TMD(0) and L(0) are not known. We synthesized [(125)I]11-azidophenyl agosterol A ([(125)I]azidoAG-A), a photoaffinity analog of the MDR-reversing agent, agosterol A (AG-A), to photolabel MRP1, and found that the analog photolabeled the C-proximal molecule of MRP1 (C(932-1531)) in a manner that was GSH-dependent. The photolabeling was inhibited by anticancer agents, reversing agents and leukotriene C(4). Based on photolabeling studies in the presence and absence of GSH using membrane vesicles expressing various truncated, co-expressed, and mutated MRP1s, we found that L(0) is the site on MRP1 that interacts with GSH. This study demonstrated that GSH is required for the binding of an unconjugated agent to MRP1 and suggested that GSH interacts with L(0) of MRP1. The photoanalog of AG-A will be useful for identifying the drug binding site within MRP1, and the role of GSH in transporting substrates by MRP1.
Subject(s)
DNA-Binding Proteins/metabolism , Glutathione/metabolism , Multidrug Resistance-Associated Proteins , Sterols/metabolism , Base Sequence , DNA Primers , DNA-Binding Proteins/chemistry , Humans , MutS Homolog 3 Protein , Photoaffinity Labels , Protein BindingABSTRACT
Thymidine phosphorylase (TP) has strong angiogenic activity and is overexpressed in a wide variety of malignant tumors. To elucidate the role of TP in human astrocytic tumors, we immunohistochemically investigated the expression of TP in 62 astrocytic tumors (12 astrocytomas, 12 anaplastic astrocytomas and 38 glioblastomas). Fifty-five astrocytic tumors (88.7%) were immunopositive for TP. The level of TP-expression was significantly correlated with the malignancy grade of astrocytic tumors; most of malignant gliomas highly expressed TP, while a small number of cells were positive in low grade astrocytomas (p < 0.001). Using double-immunostaining, we clarified that TP-expression was predominantly detectable in macrophages. There was no significant correlation between MIB-1 labeling index and TP-expression. However, TP-expression and the microvessel density were well correlated. These suggest that TP, mainly produced by the infiltrated macrophages, may play an important role in the progression of astrocytic tumors via neovascularization. Inhibitor of TP may represent a therapeutic modality for treating malignant gliomas.
