The role of OAT2 (SLC22A7) in the cyclic nucleotide biokinetics of human erythrocytes

The present study was conducted to characterise the transporter(s) responsible for the uptake of cyclic nucleotides to human erythrocytes. Western blotting showed that hRBC expressed OAT2 (SLC22A7), but detection of OAT1 (SLC22A6), or OAT3 (SLC22A8) was not possible. Intact hRBC were employed to clarify the simultaneous cyclic nucleotide egression and uptake. Both these opposing processes were studied. The K_m‐values for high affinity efflux was 3.5 ± 0.1 and 39.4 ± 5.7 μM for cGMP and cAMP, respectively. The respective values for low affinity efflux were 212 ± 11 and 339 ± 42 μM. The uptake was characterised with apparently low affinity and similar K_m‐values for cGMP (2.2 mM) and cAMP (0.89 mM). Using an iterative approach in order to balance uptake with efflux, the predicted real K_m‐values for uptake were 100–200 μM for cGMP and 50–150 μM for cAMP. The established OAT2‐substrate indomethacin showed a competitive interaction with cyclic nucleotide uptake. Creatinine, also an OAT2 substrate, showed saturable uptake with a K_m of 854 ± 98 μM. Unexpectedly, co‐incubation with cyclic nucleotides showed an uncompetitive inhibition. The observed K_m‐values were 399 ± 44 and 259 ± 30 μM for creatinine, in the presence of cGMP and cAMP, respectively. Finally, the OAT1‐substrate para‐aminohippurate (PAH) showed some uptake (K_m‐value of 2.0 ± 0.4 mM) but did not interact with cyclic nucleotide or indomethacin transport.

Modulation of ALDH5A1 and SLC22A7 by microRNA hsa-miR-29a-3p in human liver cells

Observed variations in drug responses among patients may result from differences in heritable genetic traits or from alterations in the epigenetic regulation of drug metabolizing enzymes and transporters (DMETs). MicroRNAs (miRNAs), a group of small non-coding RNAs, provide an epigenetic mechanism for fine-tuning the expression of targeted DMET genes by regulating the efficiency of protein translation and by decreasing mRNA stability via enhanced degradation. In the current study we systematically screened 374 important genes encoding DMETs for potential response elements to hsa-miR-29a-3p, a highly abundant miRNA in human liver. RNA electrophoresis mobility shift assays displayed direct interactions between hsa-miR-29a-3p and its cognate targets within the mRNA transcripts for the ABCC6, SLC22A7 and ALDH5A1 genes. The expression of luciferase reporter genes containing the 3'-UTRs of SLC22A7 or ALDH5A1 and the expression of endogenous SLC22A7 and ALDH5A1 were each suppressed by transfection with hsa-miR-29a-3p mimics. Importantly, chemically-induced up-regulation of hsa-miR-29a-3p correlated inversely with the expression of SLC22A7 and ALDH5A1. However, our studies failed to detect suppressive effects of hsa-miR-29a-3p on ABCC6 expression, which might be explained by the notion that the interaction of hsa-miR-29a-3p and ABCC6 mRNA was unable to recruit ribonucleoproteins to form a RNA-induced silencing complex.