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- <i>SLC16A1</i> (1)
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Organic cation transporter 1 (OCT1, SLC22A1) is localized in the sinusoidal membrane of human hepatocytes and mediates hepatic uptake of weakly basic or cationic drugs and endogenous compounds. Common amino acid substitutions in OCT1 were associated with altered pharmacokinetics and efficacy of drugs like sumatriptan and fenoterol. Recently, the common splice variant rs35854239 has also been suggested to affect OCT1 function. rs35854239 represents an 8 bp duplication of the donor splice site at the exon 7-intron 7 junction. Here we quantified the extent to which this duplication affects OCT1 splicing and, as a consequence, the expression and the function of OCT1. We used pyrosequencing and deep RNA-sequencing to quantify the effect of rs35854239 on splicing after minigene expression of this variant in HepG2 and Huh7 cells and directly in human liver samples. Further, we analyzed the effects of rs35854239 on OCT1 mRNA expression in total, localization and activity of the resulting OCT1 protein, and on the pharmacokinetics of sumatriptan and fenoterol. The 8 bp duplication caused alternative splicing in 38% (deep RNA-sequencing) to 52% (pyrosequencing) of the minigene transcripts when analyzed in HepG2 and Huh7 cells. The alternatively spliced transcript encodes for a truncated protein that after transient transfection in HEK293 cells was not localized in the plasma membrane and was not able to transport the OCT1 model substrate ASP+. In human liver, however, the alternatively spliced OCT1 transcript was detectable only at very low levels (0.3% in heterozygous and 0.6% in homozygous carriers of the 8 bp duplication, deep RNA-sequencing). The 8 bp duplication was associated with a significant reduction of OCT1 expression in the human liver, but explained only 9% of the general variability in OCT1 expression and was not associated with significant changes in the pharmacokinetics of sumatriptan and fenoterol. Therefore, the rs35854239 variant only partially changes splicing, causing moderate changes in OCT1 expression and may be of only limited therapeutic relevance.
Membrane monocarboxylate transporter 1 (SLC16A1/MCT1) plays an important role in
hepatocyte homeostasis, as well as drug handling. However, there is no available information
about the impact of liver pathology on the transporter levels and function. The study was aimed to
quantify SLC16A1 mRNA (qRT-PCR) and MCT1 protein abundance (liquid chromatography–tandem
mass spectrometry (LC¬¬–MS/MS)) in the livers of patients diagnosed, according to the standard
clinical criteria, with hepatitis C, primary biliary cirrhosis, primary sclerosing hepatitis, alcoholic liver
disease (ALD), and autoimmune hepatitis. The stage of liver dysfunction was classified according to
Child–Pugh score. Downregulation of SLC16A1/MCT1 levels was observed in all liver pathology
states, significantly for ALD. The progression of liver dysfunction, from Child–Pugh class A to C,
involved the gradual decline in SLC16A1 mRNA and MCT1 protein abundance, reaching a clinically
significant decrease in class C livers. Reduced levels of MCT1 were associated with significant
intracellular lactate accumulation. The MCT1 transcript and protein did not demonstrate significant
correlations regardless of the liver pathology analyzed, as well as the disease stage, suggesting
posttranscriptional regulation, and several microRNAs were found as potential regulators of MCT1
abundance. MCT1 membrane immunolocalization without cytoplasmic retention was observed in all
studied liver pathologies. Overall, the study demonstrates that SLC16A1/MCT1 is involved in liver
pathology, especially in ALD
Transmembrane drug transport in hepatocytes is one of the major determinants of drug pharmacokinetics. In the present study, ABC transporters (P-gp, MRP1, MRP2, MRP3, MRP4, BCRP, and BSEP) and SLC transporters (MCT1, NTCP, OAT2, OATP1B1, OATP1B3, OATP2B1, OCT1, and OCT3) were quantified for protein abundance (LC-MS/MS) and mRNA levels (qRT-PCR) in hepatitis C virus (HCV)-infected liver samples from the Child–Pugh class A (n = 30), B (n = 21), and C (n = 7) patients. Protein levels of BSEP, MRP3, MCT1, OAT2, OATP1B3, and OCT3 were not significantly affected by HCV infection. P-gp, MRP1, BCRP, and OATP1B3 protein abundances were upregulated, whereas those of MRP2, MRP4, NTCP, OATP2B1, and OCT1 were downregulated in all HCV samples. The observed changes started to be seen in the Child–Pugh class A livers, i.e., upregulation of P-gp and MRP1 and downregulation of MRP2, MRP4, BCRP, and OATP1B3. In the case of NTCP, OATP2B1, and OCT1, a decrease in the protein levels was observed in the class B livers. In the class C livers, no other changes were noted than those in the class A and B patients. The results of the study demonstrate that drug transporter protein abundances are affected by the functional state of the liver in hepatitis C patients.