Specific, metabolism-driven transporters in excretory epithelia and barrier tissues are important determinants of xenobiotic uptake, distribution and excretion. Along with xenobiotic metabolizing enzymes, these transporters are the first line of defense against toxicants. However, since xenobiotic transporters and enzymes do not distinguish well between toxic chemicals and therapeutic drugs, both can impede pharmacotherapy. The major focus of this lab has been on the blood-brain and blood-cerebrospinal fluid (CSF) barriers, where we are identifying and characterizing the xenobiotic transporters present and beginning to explore mechanisms that modulate expression and function with a view towards being able to manipulate barrier function in a controlled manner to improve therapy while minimizing loss of protection.? Work in the past year has focused primarily on the blood-brain barrier, which resides within the non-fenestrated brain capillary endothelium. Although originally thought to present a passive, anatomical barrier to xenobiotics, it is now clear that multispecific, xenobiotic efflux transporters are a critical feature of the barrier. Because of its luminal membrane location, high level of expression and ability to avidly transport a wide range of xenobiotics (including therapeutic drugs), one such transporter, p-glycoprotein, is the primary, selective obstacle to drug penetration at the blood-brain barrier and thus to CNS pharmacotherapy. We have been studying extracellular and intracellular signals that regulate this transporter using pharmacological tools, intact rat brain capillaries, fluorescent p-glycoprotein substrates and confocal imaging to measure transport function and Western blotting to measure transporter expression. To date, eight potential signaling pathways have been defined and partially mapped. Three pathways are triggered by elements of the brain?s innate immune response, one by reactive oxygen species, one by glutamate, one by xenobiotic (therapeutic drug and toxicant)-nuclear receptor interactions and two by elevated beta-amyloid levels. Three work over the short-term (minutes) to reduce transport function with no change in transporter expression, Five work over the long-term (hours to days). Of these, four increase both function and expression and one decreases function and expression, likely through increased transporter degradation. Signaling is complex. Several of these pathways have common signaling elements (TNF-R1, ET-B receptor, protein kinase C, NO synthase), suggesting a regulatory network. Several pathways have autocrine/paracine elements involving release of the pro-inflammatory cytokine, TNF-alpha, and the polypeptide hormone, endothelin. Finally, several steps in signaling are potential therapeutic targets that may be of importance in treating brain tumors, drug-resistant epilepsy and neurodegenerative diseases.
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