Polyamine Transport by the Polyspecific Organic Cation Transporters OCT1, OCT2, and OCT3

Abstract

Polyamines are ubiquitous organic cations implicated in many physiological processes. Because they are positively charged at physiological pH, carrier-mediated systems are necessary for effective membrane permeation, but the identity of specific polyamine transporter proteins in eukaryotic cells remains unclear. Polyspecific organic cation transporters (OCTs) interact with many natural and xenobiotic monovalent cations and have been reported to transport dicationic compounds, including the short polyamine putrescine. In this study, we used Xenopus oocytes expressing mammalian OCT1 (SLC22A1), OCT2 (SLC22A2) or OCT3 (SLC22A3) to assess binding and transport of longer-chain polyvalent polyamines. In OCT-expressing oocytes, [3H]MPP+ uptake rates were 15– to 35–fold higher than in non-injected oocytes, whereas those for [3H]spermidine increased more modestly above the background, up to 3–fold. This reflected up to 20–fold lower affinity for spermidine than for MPP +; thus, K0.5 for MPP+ was ~50 μM in OCT1, ~170 μM in OCT2, and ~60 μM in OCT3, whereas for spermidine, K0.5 was ~1 mM in OCT1, OCT2 and OCT3. Jmax values for MPP+ and spermidine were within the same range, suggesting that both compounds are transported at a similar turnover rate. To gain further insight into OCT substrate specificity, we screened a selection of structural polyamine analogs for effect on [3H]MPP+ uptake. In general, blocking potency increased with overall hydrophobic character, which indicates that, as for monovalent cations, hydrophobicity is a major requirement for recognition in polyvalent OCT substrates and inhibitors. Our results demonstrate that the natural polyamines are low affinity, but relatively high turnover, substrates for OCTs. The identification of OCTs as polyamine transport systems may contribute to further understanding of the mechanisms involved in polyamine homeostasis, and aid in the design of polyamine-like OCT-targeted drugs.

Identification of Functional Amino Acid Residues Involved in Polyamine and Agmatine Transport by Human Organic Cation Transporter 2

“…It has been also reported that putrescine and agmatine are taken up by human OCT2 (SLC22A2) [21], and spermidine is by mouse OCT2 [22]. In this communication, we studied the properties of putrescine, agmatine and spermidine uptake by hOCT2 in detail, and found that the active center of putrescine, agmatine and spermidine uptake are located on α-helices 9 to 12.…”

Involvement of Carrier-Mediated Transport at the Blood–Cerebrospinal Fluid Barrier in Spermine Clearance from Rat Brain

“…Sala-Rabanal et al have been reported that MPP + transport via mouse OCT1-2 and rat OCT3 was not significantly inhibited by 1 mM spermine, 26) although rat OCT1, 17) human OCT2, 18) and human/rat OCT3 19) accept spermine as a substrate or inhibitor. Among these OCTs, it has been reported that OCT3 is localized on the CSF side of choroid plexus epithelial cells in rats, and is involved in creatinine transport from the CSF to the cells.…”

Organic cation transporter 3 contributes to norepinephrine uptake into perivascular adipose tissue

“…Uptake of NE by aortic PVAT was reduced by inhibition of OCT3 with corticosterone. Thus, it could serve to remove excess NE (57), but there may be other physiological roles for OCT3 that may not involve blood pressure regulation, such as polyamine transport (60) and clearance of toxins (35). Therefore, OCT3 may be more necessary in the aortic PVAT due to its other roles versus mesenteric resistance PVAT.…”