Narang VS, Fraga C, Kumar N, Shen J, Throm S, Stewart CF, Waters CM. Dexamethasone increases expression and activity of multidrug resistance transporters at the rat blood-brain barrier. Am J Physiol Cell Physiol 295: C440 -C450, 2008. First published June 4, 2008 doi:10.1152/ajpcell.00491.2007.-Brain edema is an important factor leading to morbidity and mortality associated with primary brain tumors. Dexamethasone, a synthetic glucocorticoid, is routinely prescribed with antineoplastic agents to alleviate pain associated with chemotherapy and reduce intracranial pressure. We investigated whether dexamethasone treatment increased the expression and activity of multidrug resistance (MDR) transporters at the blood-brain barrier. Treatment of primary rat brain microvascular endothelial cells with submicromolar concentrations of dexamethasone induced significantly higher levels of drug efflux transporters such as breast cancer resistance protein (abcg2), P-glycoprotein (P-gp; abcb1a/abcb1b), and MDR protein 2 (Mrp2; abcc2) as indicted by protein and mRNA levels as well as by functional activity. The effect of dexamethasone on transporter function was significant within 6 h of treatment, was dose dependent, and was reversible. Dexamethasone-induced upregulation of Bcrp and P-gp expression and function was partially abrogated by the glucocorticoid receptor (GR) antagonist RU486. In contrast, RU486 had no effect on the dexamethasone-induced upregulation of Mrp2, suggesting a GR-independent regulation of Mrp2, and a GR-dependent regulation of P-gp and Bcrp. In addition to the dexamethasone-induced upregulation of MDR transporters, we measured a dose-dependent and reversible increase in the expression of the nuclear transcription factor pregnane xenobiotic receptor (PXR). Administering dexamethasone to rats caused increased expression of PXR in brain microvessels within 24 h. These results suggest that adjuvant therapy with corticosteroids such as dexamethasone in the treatment of brain tumors may increase the expression of MDR transporters at the blood-brain barrier through pathways involving GR and PXR. multidrug resistance proteins; pregnane xenobiotic receptor; breast cancer resistance protein BRAIN TUMORS ACCOUNT for approximately 2-3% of all cancers and approximately 25-30% of solid pediatric tumors (20). Treatment of brain tumors and other central nervous system diseases is limited by the ability of therapeutic drugs to cross the blood-brain barrier (BBB). Beyond the tight physical barrier provided by the endothelial cells, drug penetration is substantially reduced by the presence of multidrug resistance (MDR) efflux transporters of the ATP-binding cassette family (28). Previous in vitro studies have identified functionally active MDR efflux transporters such as P-glycoprotein (P-gp, or abcb1a/abcb1b), breast cancer resistance protein (Bcrp, or abcg2), MDR-associated protein 2 (Mrp2, or abcc2), and Mrp4 (or abcc4) in primary cultures of human, rat, and bovine cerebral endothelial cells (19,26). These transporters...

Janus kinase 3 (Jak3) is a non-receptor tyrosine kinase known to be expressed in hematopoietic cells. Studies of whole organ homogenates show that Jak3 is also expressed in the intestines of both human and mice. However, neither its expression nor its function has been defined in intestinal epithelial enterocytes. The present studies demonstrate that functional Jak3 is expressed in human intestinal enterocytes HT-29 Cl-19A and Caco-2 and plays an essential role in the intestinal epithelial wound repair process in response to interleukin 2 (IL-2). Exogenous IL-2 enhanced the wound repair of intestinal enterocytes in a dose-dependent manner. Activation by IL-2 led to rapid tyrosine phosphorylation and redistribution of Jak3. IL-2-stimulated redistribution of Jak3 was inhibited by the Jak3-specific inhibitor WHI-P131. IL-2 also induced Jak3-dependent redistribution of the actin cytoskeleton in migrating cells. In these cells Jak3 interacted with the intestinal and renal epithelial cellspecific cytoskeletal protein villin in an IL-2-dependent manner. Inhibition of Jak3 activation resulted in loss of tyrosine phosphorylation of villin and a significant decrease in wound repair of the intestinal epithelial cells. Previously, we had shown that tyrosine phosphorylation of villin is important for cytoskeletal remodeling and cell migration. The present study demonstrates a novel pathway in intestinal enterocytes in which IL-2 enhances intestinal wound repair through mechanisms involving Jak3 and its interactions with villin.

Background: We previously showed that the anti-inflammatory drug, sulfasalazine (salicylazosulfapyridine, SASP), can arrest proliferation of MCF-7 and MDA-MB-231 mammary cancer cells by inhibiting uptake of cystine via the x_c^– cystine/glutamate antiporter. Here we examined SASP with regard to reduction of cellular glutathione (GSH) levels and drug efficacy-enhancing ability. Methods: GSH levels were measured spectrophotometrically. Cellular drug retention was determined with ³H-labeled methotrexate, and drug efficacy with a colony formation assay. Results: Incubation of the mammary cancer cells with SASP (0.3–0.5 mM) led to reduction of their GSH content in a time- and concentration-dependent manner. Similar to MK-571, a multidrug resistance-associated protein inhibitor, SASP increased intracellular accumulation of methotrexate. Preincubation of cells with SASP (0.3 mM) significantly enhanced the potency of the anticancer agent doxorubicin (2.5 nM). Conclusions: SASP-induced reduction of cellular GSH levels can lead to growth arrest of mammary cancer cells and enhancement of anticancer drug efficacy.

Digoxin and midazolam are routinely used as probe drugs to measure in vivo activity of P-glycoprotein (P-gp) and cytochrome P450 3A4/5 (CYP3A), respectively. We investigated whether digoxin and midazolam could be coadministered to simultaneously determine P-gp and CYP3A activity without a significant pharmacokinetic interaction. In a randomized crossover design, digoxin (0.5 mg oral) or midazolam (2.0 mg oral) was administered individually or in combination (digoxin 1 hour after midazolam) to 14 healthy volunteers. Blood and urine samples were collected for up to 48 hours. Pharmacokinetic parameters of digoxin, midazolam and 1'-OH midazolam were evaluated to determine the presence of an interaction. The geometric mean ratios of all measured pharmacokinetic parameters of digoxin and midazolam were not significantly affected by coadministration. Coadministration of digoxin and midazolam can be used to simultaneously phenotype P-gp and CYP3A activity without a significant pharmacokinetic interaction.

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