Eleven members of the human organic anion transporter (OATP) family (grouped 
into six families) facilitate the Na(+)- independent transmembrane transport of 
various endo- and xenobiotics (bile acids, bilirubin, steroid hormone 
conjugates, thyroid hormones, prostaglandins, clinically used drugs, and 
toxins). OATPs are 12-transmembrane glycoproteins (643-722 amino acids) and 
contain many conserved structural features, for example, eleven cysteines in the 
large extracellular loop 5. They are important for proper transport, for which 
translocation of substrates through a central, positively-charged pore in a 
rocker-switch-type mechanism has been proposed. Although OATPs are expressed in 
various cells and tissues, some members show a more restricted pattern 
(well-studied OATP1B1/OATP1B3 in liver, OATP4C1 in kidney, and OATP6A1 in 
testis). In cancer, the distribution pattern is no longer maintained, and OATPs, 
like OATP1B3, become upregulated in malignant tissues (colon, breast, prostate). 
Studies in cell lines and animal models further revealed that the expression of 
OATPs is regulated in a cell- and tissue-specific way by cytokines and 
activation of nuclear receptors (LXR, FXR, PXR, CAR, HNF4). Also epigenetic 
mechanisms and postranslational modifications influence their expression and 
function. Therefore, changes in the expression of OATPs under pathological 
conditions will influence transport processes causing an altered accumulation of 
OATP substrates in cells of excretory organs (intestine, liver, kidney) and on 
various blood/organ barriers (such as brain, testis, placenta). For drugs, this 
may result in increased toxicity and adverse drug reactions. Therefore, it is 
important to improve the knowledge on the regulation and function of individual 
OATPs, and to apply it for therapeutic considerations.