Transporter gene expression data

Focused use of microarrays to generate large gene expression data sets relevant to transporters has been rare. To date these microarrays have been limited in the number of transporters present on them [8]. They have been used in an attempt to correlate pharmacokinetic properties with gene expression, for example, for valacyclovir [79], as well as to understand the transporter expression profile in different tissues or cell lines upon dietary component or xenobiotic treatment [1]. The lack of transporters on many commercially available microarrays has prompted some groups to produce arrays with a heavier emphasis on transporters. These arrays have, for example, then been used to demonstrate the upregulation of ABC transporters and dowmegulation of GST-Pi in cell lines resistant to colchicines or 

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Figure 14.5 Schematic diagram of the magnetic isolation method for rat retinal vascular endothelial cells (RVEC) (A) and the transcript level of organic anion-transporting polypeptides (Oatps) in RVEC (B). A Endothelial cells (RVEC) EC, Nonendothe-lial cells (Non-RVEC) Non-EC. B Not detected N.D. Data taken from Journal of Neurochemistry, 91, Tomi and Hosoya, Application of magnetically isolated rat retinal vascular endothelial cells for the determination of transporter gene expression levels at the inner blood-retinal barrier. 1244-1248, 2004, with permission from Blackwell Publishing.

When applying any of these models it is crucial to understand the main transport mechanisms as well as the metabolic route and characterization of the activity of the transporter/enzyme involved. It is well recognized that the activities of carrier-mediated processes in Caco-2 cells are considerably lower than in vivo [20, 42, 48] therefore, it is crucial to extrapolate in vitro cell culture data to the in vivo situation with great care [18, 20, 42, 48], This is especially important when carrier-mediated processes are involved, as evidenced by a recent report which showed significant differences in gene expression levels for transporters, channels and metabolizing enzymes in Caco-2 cells than in human duodenum [48], If an animal model is used, then potential species differences must also be considered [18, 20, 45],... 

Brown S, Chang JL, Sadee W, Babbitt PC. A semiauto-mated approach to gene discovery through expressed sequence tag data mining Discovery of new human transporter genes. AAPS PharmSci 2003 5 E1. 

From the bulk of publications and preceding sections in this and other chapters of this book, it is apparent that it may be useful to build an integrated computational approach that will (i) predict which transporters will have affinity for novel xenobiotic compounds, (ii) identify whether the role of one or more human transporters (and their polymorphisms) may be a risk factor for patients and drug developers, (iii) propose potential toxic effects, and (iv) enable the combination of predicted and experimental data (gene expression, in vitro biology, etc.) for drug transporters (Figure 14.6). 

Significant differences were revealed when comparing overall gene expression profiles and permeability data obtained from Caco-2 cells with data derived from human enterocytes (GeneChip analysis) these were consistent with observed differences in carrier-mediated transport. It has been concluded that in vivo/in vitro (Caco-2)... 

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