Tumor-Specific Expression of Organic Anion-Transporting Polypeptides: Transporters as Novel Targets for Cancer Therapy

Members of the organic anion transporter family (OATP) mediate the transmembrane uptake of clinical important drugs and hormones thereby affecting drug disposition and tissue penetration. Particularly OATP subfamily 1 is known to mediate the cellular uptake of anticancer drugs (e.g., methotrexate, derivatives of taxol and camptothecin, flavopiridol, and imatinib). Tissue-specific expression was shown for OATP1B1/OATP1B3 in liver, OATP4C1 in kidney, and OATP6A1 in testis, while other OATPs, for example, OATP4A1, are expressed in multiple cells and organs. Many different tumor entities show an altered expression of OATPs. OATP1B1/OATP1B3 are downregulated in liver tumors, but highly expressed in cancers in the gastrointestinal tract, breast, prostate, and lung. Similarly, testis-specific OATP6A1 is expressed in cancers in the lung, brain, and bladder. Due to their presence in various cancer tissues and their limited expression in normal tissues, OATP1B1, OATP1B3, and OATP6A1 could be a target for tumor immunotherapy. Otherwise, high levels of ubiquitous expressed OATP4A1 are found in colorectal cancers and their metastases. Therefore, this OATP might serve as biomarkers for these tumors. Expression of OATP is regulated by nuclear receptors, inflammatory cytokines, tissue factors, and also posttranslational modifications of the proteins. Through these processes, the distribution of the transporter in the tissue will be altered, and a shift from the plasma membrane to cytoplasmic compartments is possible. It will modify OATP uptake properties and, subsequently, change intracellular concentrations of drugs, hormones, and various other OATP substrates. Therefore, screening tumors for OATP expression before therapy should lead to an OATP-targeted therapy with higher efficacy and decreased side effects.