Delivery of therapeutic agents to the brain and its neoplasms depends on the presence of membrane transport proteins in the blood-brain barrier and in the target cells. The cellular and subcellular localization of these membrane transporters determines the drug accessibility to the brain and its tumors. We therefore analyzed the expression and localization of six members of the multidrug resistance protein family of ATP-dependent efflux pumps (ABCC1-ABCC6, formerly MRP1-MRP6) and of six organic anion uptake transporters (OATP1A2, OATP1B1, OATP1B3, OATP1C1, OATP2B1, and OATP4A1) in 61 human glioma specimens of different histologic subtypes. Real-time PCRs indicated expressions of ABCC1, ABCC3, ABCC4, and ABCC5. In addition, we detected expressions of the OATP uptake transporter genes SLCO1A2, SLCO1C1, SLCO2B1, and SLCO4A1. At the protein level, however, only OATP1A2 and OATP2B1 were detectable by immunofluorescence microscopy in the luminal membrane of endothelial cells forming the blood-brain barrier and the blood-tumor barrier, but not in the glioma cells. ABCC4 and ABCC5 proteins were the major ABCC subfamily members in gliomas, localized both at the luminal side of the endothelial cells and in the glioma cells of astrocytic tumors and in the astrocytic portions of oligoastrocytomas. These results indicate that expression of ABCC4 and ABCC5 is associated with an astrocytic phenotype, in accordance with their expression in astrocytes and with the higher chemoresistance of astrocytic tumors as compared with oligodendrogliomas. Our data provide a basis for the assessment of the role of uptake transporters and efflux pumps in the accessibility of human gliomas for chemotherapeutic agents.

Hepatobiliary elimination of many organic anions is initiated by OATP1B1 (OATP2, LST-1, OATP-C), OATP1B3 (OATP8), and OATP2B1 (OATP-B), which are the predominant uptake transporters of human hepatocytes. Thereafter, the unidirectional efflux pump ABCC2 (multidrug resistance protein 2) mediates the transport of organic anions, including glutathione conjugates and glucuronosides, into bile. In this study, we generated a Madin-Darby canine kidney (MDCKII) cell line stably expressing recombinant OATP1B1, OATP1B3, and OATP2B1 in the basolateral membrane and ABCC2 in the apical membrane. Double-transfected MDCKII cells stably expressing ABCC2 together with OATP1B1, OATP1B3, or OATP2B1 served as control cells. The quadruple-transfected cells exhibited high rates of vectorial transport of organic anions, including bromosulfophthalein, cholecystokinin peptide (CCK-8), and estrone 3-sulfate. The quadruple-transfected cells enabled the identification of substrates for uptake or vectorial transport that may be missed in studies with a double-transfected cell line, as exemplified by CCK-8, which is a substrate for OATP1B3 but not for OATP1B1 or OATP2B1. The broad substrate spectrum covered by the three hepatocellular OATP transporters enables representative analyses of the uptake of many organic anions into human hepatocytes. The broad spectrum of organic anions transported vectorially by the quadruple-transfected cells also provides valuable information on the substrate selectivity of ABCC2, without the need for studies in inside-out membrane vesicles containing the ABCC2 protein. The quadruple-transfected MDCKII-ABCC2/OATP1B1/1B3/2B1 cells may thus be useful for the identification of substrates and inhibitors, including drug candidates, undergoing uptake and secretion by human hepatocytes, under conditions that may be better defined than in primary cultures of human hepatocytes.

In rat, liver transport proteins of the Oatp and Mrp family are expressed and regulated in a gender-specific manner according to sexual differences in the hepatic metabolism of steroids and drugs. These findings may partly explain the well-known sex differences in hepatic handling of organic anions.

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Dehydroepiandrosterone (DHEA), the main adrenal steroid in humans and a precursor in androgen and estrogen biosynthesis, acts as a peroxisome proliferator and as a hepatocarcinogen in rats. Neoplasms emerge from a glycogenotic/amphophilic/basophilic preneoplastic cell lineage. A higher female tumor incidence suggests a relevant influence of sex hormones. DHEA enhances hepatocarcinogenesis induced by N-nitrosomorpholine (NNM), which is characterized by the glycogenotic/basophilic cell lineage. The tumor promoting effect is related to an additional amphophilic/basophilic preneoplastic lesion sequence and to faster proliferation of the basophilic preneoplastic lesions. Nevertheless, hepatocellular carcinomas provided under DHEA treatment seem to have a less malignant phenotype compared to tumors induced by NNM only. Further, DHEA treatment reduces growth and generation of glycogen storage foci (GSF) in initial NNM-treated rats. Thus, DHEA treatment results in both, a growth stimulation of the late basophilic lesion type with an additional amphophilic lesion sequence, and in a growth inhibition of early preneoplastic lesions, addressing especially GSF. DHEA also inhibits the growth of physiologically proliferating liver tissue. This might be explained by a DHEA related cellular metabolism, which results in significant energy consumption. Additionally, a DHEA-induced alteration of cytokine levels might contribute to this growth inhibition as well.