Role of mTOR Inhibitors in Kidney Disease.

The first compound that inhibited the mammalian target of rapamycin (mTOR), sirolimus (rapamycin) was discovered in the 1970s as a soil bacterium metabolite collected on Easter Island (Rapa Nui). Because sirolimus showed antiproliferative activity, researchers investigated its molecular target and identified the TOR1 and TOR2. The mTOR consists of mTOR complex 1 (mTORC1) and mTORC2. Rapalogues including sirolimus, everolimus, and temsirolimus exert their effect mainly on mTORC1, whereas their inhibitory effect on mTORC2 is mild. To obtain compounds with more potent antiproliferative effects, ATP-competitive inhibitors of mTOR targeting both mTORC1 and mTORC2 have been developed and tested in clinical trials as anticancer drugs. Currently, mTOR inhibitors are used as anticancer drugs against several solid tumors, and immunosuppressive agents for transplantation of various organs. This review discusses the role of mTOR inhibitors in renal disease with a particular focus on renal cancer, diabetic nephropathy, and kidney transplantation.

Urinary Dopamine as a Potential Index of the Transport Activity of Multidrug and Toxin Extrusion in the Kidney.

Dopamine is a cationic natriuretic catecholamine synthesized in proximal tubular cells (PTCs) of the kidney before secretion into the lumen, a key site of its action. However, the molecular mechanisms underlying dopamine secretion into the lumen remain unclear. Multidrug and toxin extrusion (MATE) is a H⁺/organic cation antiporter that is highly expressed in the brush border membrane of PTCs and mediates the efflux of organic cations, including metformin and cisplatin, from the epithelial cells into the urine. Therefore, we hypothesized that MATE mediates dopamine secretion, a cationic catecholamine, into the tubule lumen, thereby regulating natriuresis. Here, we show that [³H]dopamine uptake in human (h) MATE1-, hMATE-2K- and mouse (m) MATE-expressing cells exhibited saturable kinetics. Fluid retention and decreased urinary excretion of dopamine and Na⁺ were observed in Mate1-knockout mice compared to that in wild-type mice. Imatinib, a MATE inhibitor, inhibited [³H]dopamine uptake by hMATE1-, hMATE2-K- and mMATE1-expressing cells in a concentration-dependent manner. At clinically-relevant concentrations, imatinib inhibited [³H]dopamine uptake by hMATE1- and hMATE2-K-expressing cells. The urinary excretion of dopamine and Na⁺ decreased and fluid retention occurred in imatinib-treated mice. In conclusion, MATE transporters secrete renally-synthesized dopamine, and therefore, urinary dopamine has the potential to be an index of the MATE transporter activity.

Influence of ensitrelvir or nirmatrelvir/ritonavir on tacrolimus clearance in kidney transplant recipients: a single-center case series.

BACKGROUND: Among the oral antivirals used for treating patients with mild-to-moderate novel coronavirus disease 2019 (COVID-19), nirmatrelvir/ritonavir (NMV/RTV) and ensitrelvir (ESV) are inhibitors of cytochrome P450 (CYP) 3A, and therefore, can cause drug-drug interactions with concomitant medications. Tacrolimus (TAC), a substrate of CYP3A4/5, is administered for a long period to prevent rejection after kidney transplantation. TAC should be discontinued while using NMV/RTV because blood TAC levels significantly increase when these drugs are concomitantly administered. However, the influence of ESV on blood TAC levels has not yet been reported, and the management of TAC doses during the use of ESV remains unclear. CASE PRESENTATION: We experienced three kidney transplant recipients with COVID-19, whose blood trough levels of TAC increased by the concomitant use of NMV/RTV or ESV. In two patients administering NMV/RTV, blood trough levels of TAC increased more than tenfold after combination therapy, whereas in one patient administering ESV, TAC level increased approximately threefold. CONCLUSIONS: These cases suggest that TAC administration should be discontinued during NMV/RTV treatment to maintain blood TAC levels within the therapeutic range, and a reduced TAC dose is sufficient during ESV treatment.

Identification of multidrug and toxin extrusion (MATE1 and MATE2-K) variants with complete loss of transport activity.

H(+)/organic cation antiporters (multidrug and toxin extrusion: MATE1 and MATE2-K) play important roles in the renal tubular secretion of cationic drugs. We have recently identified a regulatory single nucleotide polymorphism (SNP) of the MATE1 gene (-32G>A). There is no other information about SNPs of the MATE gene. In this study, we evaluated the functional significance of genetic polymorphisms in MATE1 and MATE2-K. We sequenced all exons of MATE1 and MATE2-K genes in 89 Japanese subjects and identified coding SNPs (cSNPs) encoding MATE1 (V10L, G64D, A310V, D328A and N474S) and MATE2-K (K64N and G211V). All the variants except for MATE1 V10L showed significant decrease in transport activity. In particular, MATE1 G64D and MATE2-K G211V variants completely lost transport activities. When membrane expression level was evaluated by cell surface biotinylation, those of MATE1 (G64D and D328A) and MATE2-K (K64N and G211V) were significantly decreased compared with that of wild type. These findings suggested that the loss of transport activities of the MATE1 G64D and MATE2-K G211V variants were due to the alteration of protein expression in cell surface membranes. This is the first demonstration of functional impairment of the MATE family induced by cSNPs.

Protective Effects of Imatinib on Ischemia/Reperfusion Injury in Rat Lung.

BACKGROUND: Ischemia/reperfusion injury (IRI) remains a significant complication after lung transplantation. Endothelial damage and inflammation contribute to its development. Imatinib has been reported to regulate vascular permeability by maintaining endothelial junctions and showing antiinflammatory effects through inhibition of the Abl kinases. We hypothesized that imatinib could have a protective effect against IRI. METHODS: Male Lewis rats were heparinized and underwent left thoracotomy, and the left hilum was clamped for 90 minutes followed by reperfusion for 120 minutes. Imatinib mesylate (50 mg/kg) and a solvent were administered intraperitoneally 20 minutes before ischemia in the imatinib group and the vehicle group, respectively (n = 7 in each group). After reperfusion, lung function, lung wet to dry weight (W/D) ratio, and histologic findings were obtained. The expression of vascular endothelial cadherin (VEC), the phosphorylation level of CrkL (pCrkL) (an exclusive target of Abl kinases), and the cytokine level were evaluated using lung tissue lysate. The imatinib concentrations of plasma and lungs after reperfusion were measured in this hilar clamp model (n = 7). RESULTS: In the imatinib group, lung function was improved with a lower W/D ratio. Perivascular edema and neutrophil infiltration were ameliorated. The imatinib group demonstrated maintained expression of VEC, inhibition of pCrkL, and a significantly higher level of interleukin (IL)-10. The imatinib concentration in both lungs showed a strong correlation with plasma concentration. CONCLUSIONS: In a rat IRI model, imatinib attenuated lung injury by an antipermeability and antiinflammatory effect. The delivery and function of imatinib in the lung was also confirmed in this model.

[The Contribution of GMP-grade Hospital Preparation to Translational Research].

Translational research is important for applying the outcomes of basic research studies to practical medical treatments. In exploratory early-phase clinical trials for an innovative therapy, researchers should generally manufacture investigational agents by themselves. To provide investigational agents with safety and high quality in clinical studies, appropriate production management and quality control are essential. In the Department of Pharmacy of Kyoto University Hospital, a manufacturing facility for sterile drugs was established, independent of existing manufacturing facilities. Manuals on production management and quality control were developed according to Good Manufacturing Practices (GMP) for Investigational New Drugs (INDs). Advanced clinical research has been carried out using investigational agents manufactured in our facility. These achievements contribute to both the safety of patients and the reliability of clinical studies. In addition, we are able to do licensing-out of our technique for the manufacture of investigational drugs. In this symposium, we will introduce our GMP grade manufacturing facility for sterile drugs and discuss the role of GMP grade hospital preparation in translational research.

Molecular Markers of Tubulointerstitial Fibrosis and Tubular Cell Damage in Patients with Chronic Kidney Disease.

In chronic kidney disease (CKD), progressive nephron loss causes glomerular sclerosis, as well as tubulointerstitial fibrosis and progressive tubular injury. In this study, we aimed to identify molecular changes that reflected the histopathological progression of renal tubulointerstitial fibrosis and tubular cell damage. A discovery set of renal biopsies were obtained from 48 patients with histopathologically confirmed CKD, and gene expression profiles were determined by microarray analysis. The results indicated that hepatitis A virus cellular receptor 1 (also known as Kidney Injury Molecule-1, KIM-1), lipocalin 2 (also known as neutrophil gelatinase-associated lipocalin, NGAL), SRY-box 9, WAP four-disulfide core domain 2, and NK6 homeobox 2 were differentially expressed in CKD. Their expression levels correlated with the extent of tubulointerstitial fibrosis and tubular cell injury, determined by histopathological examination. The expression of these 5 genes was also increased as kidney damage progressed in a rodent unilateral ureteral obstruction model of CKD. We calculated a molecular score using the microarray gene expression profiles of the biopsy specimens. The composite area under the receiver operating characteristics curve plotted using this molecular score showed a high accuracy for diagnosing tubulointerstitial fibrosis and tubular cell damage. The robust sensitivity of this score was confirmed in a validation set of 5 individuals with CKD. These findings identified novel molecular markers with the potential to contribute to the detection of tubular cell damage and tubulointerstitial fibrosis in the kidney.

Renal tubular secretion of varenicline by multidrug and toxin extrusion (MATE) transporters.

Multidrug and toxin extrusion (MATE) 1 and MATE2-K, H(+)/organic cation antiporters, are located at the brush-border membrane of renal proximal tubules. The present study aimed to clarify the role of MATE transporters in tubular secretion of varenicline. Varenicline at a dose of 5 mg/kg was administered to wild-type and Mate1-knockout mice via the jugular vein, and its uptake was measured by high-performance liquid chromatography. The renal secretory clearance of and systemic exposure to varenicline were significantly decreased (54.6%, p < 0.05) and increased (116%, p < 0.05) respectively, by the genetic disruption of Mate1 in mice. Uptake of varenicline and [(14)C]tetraethylammonium (TEA) was examined in HEK293 cells transiently expressing the human (h) MATE1, hMATE2-K, mouse (m) MATE1, and hOCT2 basolateral organic cation transporter. [(14)C]TEA uptake in HEK293 cells expressing MATE transporters and hOCT2 was decreased in the presence of varenicline. The calculated IC(50) values for hMATE1, hMATE2-K, mMATE1, and hOCT2 were 62.2 ± 6.5, 122.3 ± 67.6, 255.0 ± 37.9, and 1,003.9 ± 135.8 (µM; mean ± S.E. for three separate experiments), respectively. Varenicline uptake was significantly increased in HEK293 cells expressing mMATE1, hMATE1, or hMATE2-K cDNA as well as hOCT2 compared to empty vector-transfected cells. In conclusion, renal MATE transporters were found to be responsible for renal tubular secretion of varenicline.

Kidney-specific expression of human organic cation transporter 2 (OCT2/SLC22A2) is regulated by DNA methylation.

Human organic cation transporter 2 (OCT2/SLC22A2), which is specifically expressed in the kidney, plays critical roles in the renal secretion of cationic compounds. Tissue expression and membrane localization of OCT2 are closely related to the tissue distribution, pharmacological effects, and/or adverse effects of its substrate drugs. However, the molecular mechanisms underlying the kidney-specific expression of OCT2 have not been elucidated. In the present study, therefore, we examined the contribution of DNA methylation of the promoter region for the OCT2 gene to its tissue-specific expression using human tissue samples. In vivo methylation status of the proximal promoter region of OCT2 and that of OCT1, a liver-specific organic cation transporter, were investigated by bisulfite sequencing using human genomic DNA extracted from the kidney and liver. All CpG sites in the OCT2 proximal promoter were hypermethylated in the liver, while hypomethylated in the kidney. On the other hand, the promoter region of OCT1 was hypermethylated in both the kidney and liver. The level of methylation of the OCT2 promoter was especially low at the CpG site in the E-box, the binding site of the basal transcription factor upstream stimulating factor (USF) 1. In vitro methylation of the OCT2 proximal promoter dramatically reduced the transcriptional activity, and an electrophoretic mobility shift assay showed that methylation at the E-box inhibited the binding of USF1. These results indicate that kidney-specific expression of human OCT2 is regulated by methylation of the proximal promoter region, interfering with the transactivation by USF1.

Regulation of basal core promoter activity of human organic cation transporter 1 (OCT1/SLC22A1).

Human organic cation transporter 1 (OCT1/SLC22A1) plays important roles in the hepatic uptake of cationic drugs. The functional characteristics of this transporter have been well evaluated, but molecular information regarding transcriptional regulation is limited. In the present study, therefore, we examined the gene regulation of OCT1 gene focusing on basal core expression. An approximately 2.5-kb fragment of the OCT1 promoter region was isolated, and promoter activity was measured by luciferase assay in the human liver cell lines Huh7 and HepG2. Deletion analysis suggested that the region spanning -141/-69 was essential for the basal core transcriptional activity and that this region contained the sequence of a cognate E-box (CACGTG). The E-box is known to be bound by the basal transcription factors, upstream stimulating factors (USFs), and the functional involvements of USF1 and USF2 were confirmed by a transactivation effect, a mutational analysis of the E-box, and an electrophoretic mobility shift assay. The transactivation effect of USFs on the OCT1 promoter was further stimulated by hepatocyte nuclear factor 4alpha, a liver-enriched transcription factor. There were no polymorphisms in the proximal promoter region (about 400 bp) of OCT1 gene (n = 109). These findings indicated that both USF1 and USF2 bind to an E-box sequence located in the OCT1 core promoter region and are required for the basal gene expression of this transporter.

Critical roles of Sp1 in gene expression of human and rat H+/organic cation antiporter MATE1.

A H+/organic cation antiporter (multidrug and toxin extrusion 1: MATE1/SLC47A1) plays important roles in the tubular secretion of various clinically important cationic drugs such as cimetidine. We have recently found that the regulation of this transporter greatly affects the pharmacokinetic properties of cationic drugs in vivo. No information is available about the regulatory mechanisms for the MATE1 gene. In the present study, therefore, we examined the gene regulation of human (h) and rat (r) MATE1, focusing on basal expression. A deletion analysis suggested that the regions spanning -65/-25 and -146/-38 were essential for the basal transcriptional activity of the hMATE1 and rMATE1 promoter, respectively, and that both regions contained putative Sp1-binding sites. Functional involvement of Sp1 was confirmed by Sp1 overexpression, a mutational analysis of Sp1-binding sites, mithramycin A treatment, and an electrophoretic mobility shift assay. Furthermore, we found a single nucleotide polymorphism (SNP) in the promoter region of hMATE1 (G-32A), which belongs to a Sp1-binding site. The allelic frequency of this rSNP was 3.7%, and Sp1-binding and promoter activity were significantly decreased. This is the first study to clarify the transcriptional mechanisms of the MATE1 gene and to identify a SNP affecting the promoter activity of hMATE1.