ATP-binding cassette transporters in primary central nervous system lymphoma: Decreased expression of MDR1 P-glycoprotein and breast cancer resistance protein in tumor capillary endothelial cells

Most tumors in the central nervous system are drug resistant partly because of the presence of the blood-brain barrier (BBB) between circulating blood and tumor tissues. Primary central nervous system lymphoma (PCNSL) is one of the exceptions, as it is highly sensitive to high-dose methotrexate (MTX)-based chemotherapy. The aim of this study was to evaluate the BBB function of tumor capillary endothelial cells in PCNSL. Expression of three major drug efflux transporters that belong to the ATP-binding cassette (ABC) superfamily, P-glycoprotein encoded by the human multidrug resistance gene (MDR1 Pgp; ABCB1), breast cancer resistance protein (BCRP; ABCG2), and multidrug resistance-associated protein 1 (MRP1; ABCC1), was evaluated. Immunohistochemistry was performed in capillary endothelial cells of 30 tumor areas from 22 PCNSL cases and compared with that of 30 gliomas. The microenvironment around tumor capillaries was assessed by examining the distribution of astrocytes and by counting the number of macrophages and T-cells, the principal cytokine producers. In PCNSL, expression of MDR1 Pgp and BCRP in tumor capillary endothelial cells was decreased in 63 and 93% of tumor areas examined, respectively, and these reduction levels differed significantly from those of gliomas (P<0.05). When the PCNSLs were further segregated by way of infiltration of tumor cells into three patterns (dense, perivascular and sparse), decreased MDR1 Pgp and BCRP in tumor capillary endothelial cells were much more prominent in dense and perivascular patterns. In all tumors and non-tumor areas of the brain, MRP1 was undetected on capillary endothelial cells. Assessment of the microenvironment around the tumor capillaries suggested that dissociation from astrocytes and infiltration of macrophages and T-cells was involved in the down-regulation of MDR1 Pgp and BCRP in capillary endothelial cells. In conclusion, we report that expression of the major ABC transporters of the BBB, MDR1 Pgp and BCRP, decreases in tumor capillary endothelial cells of PCNSL. Thus, decreased expression may permit delivery of chemotherapeutic agents to the tumor tissues.