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This Article Full Text (PDF) All Versions of this Article: 293/1/F20    most recent 00126.2007v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Sweet, D. H. Search for Related Content PubMed PubMed Citation Articles by Sweet, D. H.

EDITORIAL FOCUS

Kinase signaling pathways regulate fine tuning of transporter activity

Department of Pharmaceutical Sciences, Medical University of South Carolina, Charleston, South Carolina

OVER THE LAST DECADE, THERE has been a surge of information in the field of transporters analogous to that observed previously for the cytochrome P-450s. Indeed, with the completion of the human genome project it has been estimated that 3–5% of the human genome encodes transporter genes, potentially producing over 2,000 different proteins. In the 1990s, using the Xenopus laevis oocyte expression assay system, the first member of the organic cation/anion/zwitterion (SLC22) family of transporters was cloned from the kidney. This allowed the renal transport field to move beyond asking purely physiological questions and to begin to pursue more molecular/biochemical questions like transporter expression patterns (among tissues, species, and sexes), amino acid sequence prediction and motif searches, structure/topology prediction, substrate specificity, kinetics, driving forces, and even homology screens to isolate more family members. The introduction of cloned transporters into heterologous expression systems provided key experimental models used to generate initial transporter characterizations. However, while overexpressing cell lines, membrane vesicles, and other in vitro models are convenient systems by which to begin to understand what a transporter might be capable of in vivo, we need to return to more intact tissue systems and comparative models (e.g., kidney and liver slices; intact choroid plexus; intact rabbit, mouse, and killifish proximal tubule segments; knockout mice) to confirm whether the transport properties observed in vitro are relevant to transporter function in vivo.

We now know that transporters can influence every aspect of administration, distribution, metabolism, elimination (ADME): some transporters expressed in the gut effectively impede absorption; transporters expressed in barrier epithelia (e.g., blood-brain and blood-cerebrospinal fluid barriers) are major determinants of drug distribution; active transport of some compounds into the liver alters various P-450 expression levels affecting rates of metabolism; and transporters are behind the powerful secretory system of the renal proximal tubule, which effectively eliminates drugs and their metabolites from the body. In fact, the importance of transporters in defining ADME, drug-drug, and drug-food interactions has become so germane that in September 2006 the US Food and Drug Administration released a draft guidance for industry on drug interaction studies that foreshadows new regulations regarding transporter-based drug interactions. Specifically, the document mentions P-glycoprotein (P-gp), organic anion transporters (OATs), organic cation transporters (OCTs), and multidrug resistance-associated proteins (MRPs) as being potential transporters to evaluate during the drug development process.

As the transporter field is advancing, studies are now turning toward defining the mechanisms governing transporter membrane targeting and regulation of transporter expression (both message and protein) and function. For members of the SLC22 family, preliminary studies indicate that stimulation of the protein kinases PKA and PKC results in the modulation of transporter activity. Depending on the transporter, this can be an up- or downregulation of activity and may involve movement of transporters in and out of the plasma membrane. In an effort to clarify and further define the signaling pathways involved in the regulation of organic cation transporter function, Soodvilai et al. (1) have investigated the modulation of Oct2 (Slc22a2)-mediated transport by kinase activation. The effects of kinase activation and inhibition on organic cation transport in Chinese hamster ovary K1 cells stably expressing rabbit Oct2 were examined. Transport was reduced by treatment with a MEK inhibitor and a PKA inhibitor supporting regulation of Oct2 by a pathway utilizing these kinases. Furthermore, inhibition of PKA and MEK/MAPK was not additive, suggesting that PKA and MAPK are part of the same signaling pathway affecting Oct2 function. PKA activation increased transport, and this increase in activity was sensitive to PKA inhibition, but insensitive to MEK/MAPK inhibition, suggesting that PKA is downstream of MAPK. Kinetic experiments indicated that inhibition of Oct2 activity is accompanied by a decrease in J_max, but no change in substrate affinity (K_t), suggesting regulation of transport capacity via this signaling pathway is accomplished by altering the number of active transporters in the membrane. An important aspect of this study is the comparison of these results obtained in a heterologous cell culture model with those from intact rabbit renal proximal tubule S2 segments. This is a particularly appropriate model system, as previous work has determined that Oct2 is the dominant OCT in the basolateral membrane of the rabbit S2 segment. As observed with the Oct2-expressing cells, both PKA and MAPK activation stimulated organic cation transport in rabbit S2 segments and inhibition of each reduced transport. Together, these data indicate that function of Oct2 expressed in the basolateral membrane of renal proximal tubule cells is subject to regulation via a signaling cascade involving MAPK and PKA.

As more therapeutics come on the market that target components of signaling cascades, it is becoming more critical to understand how the body responds in terms of modulating the activity of the transport systems responsible for the absorption, distribution, and elimination of drugs and xenobiotics. Such knowledge is especially pertinent under circumstances of polypharmacy. For instance, if an individual is taking an epidermal growth factor receptor inhibitor in combination with a chemotherapeutic agent that interacts with Oct2, what effect would blocking the MEK/MAPK signaling pathway have on Oct2 activity, and hence transport rates of the chemotherapeutic agent? Such interactions may have a tremendous impact on drug efficacy and toxicity, and a more complete understanding of this interplay will lead to better predictability and management of therapeutic outcomes.

	FOOTNOTES

 

Address for reprint requests and other correspondence: D. H. Sweet, Dept. of Pharmaceutical Sciences, Medical University of South Carolina, Charleston, SC 29425 (e-mail: sweetd{at}musc.edu)

	REFERENCE

TOP REFERENCE

 

1. Soodvilai S, Chatsudthipong A, Chatsudthipong V. Role of MAPK and PKA in regulation of rbOCT2-mediated renal organic cation transport. Am J Physiol Renal Physiol. First published February 27, 2007; doi:10/1152/ajprenal.00043.2007.

This Article Full Text (PDF) All Versions of this Article: 293/1/F20    most recent 00126.2007v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Sweet, D. H. Search for Related Content PubMed PubMed Citation Articles by Sweet, D. H.