Trafficking and other regulatory mechanisms for organic anion transporting polypeptides and organic anion transporters that modulate cellular drug and xenobiotic influx and that are dysregulated in disease

Abstract

Organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs), encoded by a number of solute carrier (SLC)22A and SLC organic anion (SLCO) genes, mediate the absorption and distribution of drugs and other xenobiotics. The regulation of OATs and OATPs is complex, comprising both transcriptional and post-translational mechanisms. Plasma membrane expression is required for cellular substrate influx by OATs/OATPs. Thus, interest in post-translational regulatory processes, including membrane targeting, endocytosis, recycling and degradation of transporter proteins, is increasing because these are critical for plasma membrane expression. After being synthesized, transporters undergo N-glycosylation in the endoplasmic reticulum and Golgi apparatus and are delivered to the plasma membrane by vesicular transport. Their expression at the cell surface is maintained by de novo synthesis and recycling, which occurs after clathrin- and/or caveolin-dependent endocytosis of existing protein. Several studies have shown that phosphorylation by signalling kinases is important for the internalization and recycling processes, although the transporter protein does not appear to be directly phosphorylated. After internalization, transporters that are targeted for degradation undergo ubiquitination, most likely on intracellular loop residues. Epigenetic mechanisms, including methylation of gene regulatory regions and transcription from alternate promoters, are also significant in the regulation of certain SLC22A/SLCO genes. The membrane expression of OATs/OATPs is dysregulated in disease, which affects drug efficacy and detoxification. Several transporters are expressed in the cytoplasmic subcompartment in disease states, which suggests that membrane targeting/internalization/recycling may be impaired. This article focuses on recent developments in OAT and OATP regulation, their dysregulation in disease and the significance for drug therapy.

Figure 1

Predicted secondary structures of human (A) OAT1 and (B) OATP1A2. Both transporters contain 12 TMDs, intracellular N‐ and C‐termini and several intracellular and extracellular loops. Asparagine residues that are subject to N‐glycosylation (G) are shown in the extracellular loops between TMDs 1 and 2 of OAT1 and between TMDs 3/4 and 9/10 of OATP1A2 (Tanaka et al., 2004a; Yao et al., 2012). Lysine residues in OAT1 that are subject to ubiquitination in the intracellular loop between TMDs 6 and 7 are shown (U; Li et al., 2013). Cysteine residues, with the potential for involvement in oligomerization, in OATP1A2 are shown as the green ellipses in the extracellular loop between TMDs 9 and 10. The yellow box shows the PDZ domain in the C‐terminal region of OATP1A2.

Figure 2

(A) Tissue‐ and cell‐specific transcriptional regulation of full length OATP1B3 in kidney, and the truncated cancer‐specific OATP1B3 isoform (cs‐OATP1B3) in liver. Green and red boxes indicate the activation and repression of transcription via epigenetic mechanisms respectively. cs‐OATP1B3 transcription is activated in some tumour cell types (red boxes) and repressed in others (green boxes). (B) CYP7A1 converts cholesterol to bile acids that are ligands for the FXR. FXR‐RXR dimers activate a number of genes, including SHP1. Over‐production of bile acids in cholestasis suppresses SHP and the liver‐specific transcription factor HNF4α; down‐regulation of HNF4α also suppresses HNF1α. Loss of both HNF isoforms decreases the expression of OATP1B1 and OATP1B3 that mediate bile acid influx and CYP7A1 to suppress further bile acid production (Jung et al., 2001; Jung and Kullak‐Ublick, 2003; Kamiyama et al., 2007).

Figure 3

Overview of potential pathways of OAT/OATP transporter trafficking. N‐glycosylation in the ER‐Golgi apparatus is followed by vesicular delivery to the plasma membrane. Subsequent internalization from the plasma membrane occurs via clathrin‐coated pits and caveolae that deliver transporters to the early endosome. From the early endosome evidence suggests that certain transporters could be delivered to the late endosome for lysosomal degradation, or to the recycling endosome and the Golgi apparatus for targeted delivery back to the plasma membrane. Individual OAT/OATP transporters have been detected in each of the subcellular locations indicated. However, each pathway has not been uniformly established for all transporters. Greater mechanistic detail along each of the proposed pathways is also required. Key: vertical blue bars indicate unglycosylated transporter monomers in the Golgi apparatus; G indicates N‐glycosylated transporter monomers.