Corticosterone induced inhibition of OCT mediated transport

Corticosterone-induced inhibition of OCT-mediated transport is rapid and appears to involve direct interaction of the steroid with the transporter at a specific site. Volk et al. (2003) studied OCT2-mediated transport by measuring substrate-induced currents using patch clamp of Xenopus oocytes exogenously expressing rat OCT2. In these studies, corticosterone inhibited OCT2-mediated currents within 30 s of application (Volk et al., 2003). The ability of corticosterone and other steroids to inhibit OCT-mediated transport appears to involve direct binding with the transporter in a pocket deep within the substrate-binding cleft. The corticosterone binding sites in OCT1 and OCT2 were identified by the construction of chimeric transporters, interchanging segments of rOCT1 and rOCT2, and measuring both the transport activity and its sensitivity to inhibition by corticosterone. By this method, three key Retapamulin in the 10th transmembrane domain (Ala443, Leu447, and Gln448 in rOCT1 and Ile443, Tyr447, and Glu448 in rOCT2) were identified as critically involved in corticosteroid-induced inhibition of transport (Gorboulev et al., 2005). When these three amino acids from rOCT2, which are identical in rOCT2 and rOCT3, are introduced into rOCT1, they decrease the IC50 for corticosterone inhibition of rOCT1 from 150 μM to that of rOCT2, i.e., 4 μM. Subsequent studies and structural modeling provided evidence that these amino acids are found in the innermost cavity of a substrate-binding cleft in the transporter that alternates between inwardly- and outwardly-facing during transport (Fig. 1) (Volk et al., 2009). Thus, corticosterone appears to directly bind to OCT3, making the inhibition of OCT-mediated transport by corticosterone a truly nongenomic, GR-independent phenomenon. This is borne out in studies of corticosteroid-induced inhibition of uptake in bronchial tissue, which is rapid, GR-independent, and insensitive to inhibitors of transcription (Horvath et al., 2001). Together, these studies suggest that uptake2-like transport of monoamines is mediated by a family of transporters. While OCT3-mediated monoamine transport appears to be the most sensitive to inhibition by corticosteroids, the presence of species differences in steroid sensitivity, together with variations in the reported IC50s depending on the preparation (native transporter expression versus transporter exogenously expressed in cell lines), make definitive determination of relative steroid sensitivity among OCT1–3 difficult. It is clear, at least, that OCT1 is much less corticosterone-sensitive than either OCT2 or OCT3. However, roles for OCT2 in mediating the effects of stress-level corticosterone cannot be ruled out. Indeed, a recent study showed that genetic deletion of OCT2, which is expressed in the brain, including in several stress-related brain regions, resulted in increased HPA axis responses to acute stress, suggesting potential roles in corticosterone-dependent processes (Courousse et al., 2015; Courousse and Gautron, 2015). More recently, an additional uptake2-like transporter, the plasma membrane monoamine transporter (PMAT), has been identified, and its expression in the brain has been described (Engel et al., 2004). Like OCT3, PMAT is a multi-specific monoamine transporter, but PMAT-mediated transport is highly selective for DA and 5-HT. In direct comparisons, PMAT and OCT3 displayed comparable DA and histamine uptake (Vmax), while PMAT displayed significantly greater uptake of 5-HT, and OCT3 displayed greater uptake of norepinephrine and epinephrine (Duan and Wang, 2010). Transport mediated by PMAT is markedly less sensitive to inhibition by corticosteroids, with a reported IC50 of 450 μM, well outside the physiological range (Engel et al., 2004). Thus, while it is now clear that “uptake2” activity is mediated by a group of at least 4 transporters, and that any of them may contribute to effects of supraphysiological corticosteroid concentrations, OCT3 (and perhaps OCT2) are the most likely candidates for mediating rapid effects of corticosterone in the physiological range.