Cellular Mechanisms Of Xenobiotic Transport

Environmental Health Sciences

Investigators

Linked publications & trials

Abstract

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 impeded 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. The blood-brain barrier 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 and that one transporter in particular, p-glycoprotein, is a major impediment to CNS pharmacotherapy. We have defined in freshly isolated brain capillaries three signaling pathways through which p-glycoprotein expression and transport activity are modulated. First, p-glycoprotein expression and activity are upregulated by xenobiotics acting through the ligand-activated nuclear receptor, PXR. Second, p-glycoprotein activity is rapidly (minutes) and reversibly down regulated when the innate immune system is activated by the inflammogen, lipopolysaccharide, the cytokine, TNF-alpha, and the polypeptide hormone, endothelin-1 (ET-1). Third, p-glycoprotein expression and activity is upregulated after long-term (hours) exposure to TNF-alpha or ET-1. These studies are beginning to identify signals that may be used to open the selective blood-brain barrier for pharmacotherapy or tighten the barrier for increased protection. The choroid plexus is responsible for removal of potentially toxic xenobiotics and metabolites from the CSF and delivering them to the blood for eventual excretion in bile and urine. In this tissue our work has been focused primarily on identifying and characterizing transporters that mediate the initial step in transport, concentrative uptake of anionic xenobiotics and metabolic wastes from CSF. Using transporter-selective inhibitors and an Oat3-null mouse model, we have shown that Oat3 is responsible for a large portion of the Na-dependent uptake of a number or organic anions. In contrast, Oatp3 appears to be responsible for a portion of organic anion Na-independent uptake. However, confocal imaging studies with fluorescent organic anions indicate concentrative uptake that cannot be accounted for by the transporters/mechanisms known to be present in the tissue. Finally, we are also using confocal imaging to identify for the first time efflux mechanisms at the blood side of the tissue; both electrical potential-dependent and ATP-driven processes have been found.