Advances in epigenetic regulation of Chinese hamster ovary cells / 生物工程学报

Chinese hamster ovary (CHO) cells play an irreplaceable role in biopharmaceuticals because the cells can be adapted to grow in suspension cultures and are capable of producing high quality biologics exhibiting human-like post-translational modifications. However, gene expression regulation such as transgene silencing and epigenetic modifications may reduce the recombinant protein production due to the decrease of expression stability of CHO cells. This paper summarized the role of epigenetic modifications in CHO cells, including DNA methylation, histone modification and miRNA, as well as their effects on gene expression regulation.

Bovine viral diarrhea virus Erns protein expressed in Chinese hamster ovary cells and its immunogenicity analysis / 生物工程学报

The aim of this study was to produce Erns protein of bovine viral diarrhea virus (BVDV) by using suspensively cultured CHO cells expression system and to analyze the immunogenicity of the purified Erns protein. In this study, the recombinant eukaryotic expression plasmid pcDNA3.1-BVDV-Erns was constructed based on the gene sequence of BVDV-1 NADL strain. The Erns protein was secreted and expressed in cells supernatant after transfecting the recombinant expression plasmid pcDNA3.1-BVDV-Erns into CHO cells. The expression and purification of the Erns protein was analyzed by SDS-PAGE, the reactivity was determined with anti-His monoclonal antibodies and BVDV positive serum with Western blotting. Immunogenicity analysis of the Erns protein was determined after immunizing New Zealand white rabbits, and the serum antibodies were tested by indirect ELISA (iELISA) and indirect immunofluorescence (IFA). The serum neutralizing titer of the immunized rabbits was determined by virus neutralization test. The concentration of the purified Erns protein was up to 0.886 mg/mL by BCA protein quantification kit. The results showed that the Erns protein could be detected with anti-His monoclonal antibodies and anti-BVDV sera. Serum antibodies could be detected by iELISA on the 7th day post-prime immunization, and the antibody level was maintained at a high titer until the 28th day post-immunization. The antibody titer was 1:128 000. Furthermore, the expression of the Erns protein in BVDV-infected MDBK cells could be detected with immunized rabbits sera by IFA. Moreover, antigen-specific neutralizing antibodies of 2.71 log10 was induced in rabbits. In this study, purified BVDV Erns protein was successfully produced using CHO suspension culture system, and the recombinant protein was proved to have a good immunogenicity, which may facilitate the development of BVD diagnosis method and novel subunit vaccine.

Recombinant porcine interferon-gamma expressed in CHO cells and its antiviral activity / 生物工程学报

The aim of this study was to produce recombinant porcine interferon gamma (rPoIFN-γ) by Chinese hamster ovarian (CHO) cells expression system and to analyze its antiviral activity. Firstly, we constructed the recombinant eukaryotic expression plasmid pcDNA3.1-PoIFN-γ and transfected into suspension cultured CHO cells for secretory expression of rPoIFN-γ. The rPoIFN-γ was purified by affinity chromatography and identified with SDS-PAGE and Western blotting. Subsequently, the cytotoxicity of rPoIFN-γ was analyzed by CCK-8 test, and the antiviral activity of rPoIFN-γ was evaluated using standard procedures in VSV/PK-15 (virus/cell) test system. Finally the anti-Seneca virus A (SVA) of rPoIFN-γ activity and the induction of interferon-stimulated genes (ISGs) and cytokines were also analyzed. The results showed that rPoIFN-γ could successfully expressed in the supernatant of CHO cells. CCK-8 assays indicated that rPoIFN-γ did not show cytotoxicity on IBRS-2 cells. The biological activity of rPoIFN-γ was 5.59×107 U/mg in VSV/PK-15 system. Moreover, rPoIFN-γ could induced the expression of ISGs and cytokines, and significantly inhibited the replication of SVA. In conclusion, the high activity of rPoIFN-γ was successfully prepared by CHO cells expression system, which showed strong antiviral activity on SVA. This study may facilitate the investigation of rPoIFN-γ function and the development of novel genetically engineered antiviral drugs.

Knockout of caspase-7 gene improves the expression of recombinant protein in CHO cell line through the cell cycle arrest in G2/M phase

BACKGROUND: Chinese hamster ovary cell line has been used routinely as a bioproduction factory of numerous biopharmaceuticals. So far, various engineering strategies have been recruited to improve the production efficiency of this cell line such as apoptosis engineering. Previously, it is reported that the caspase-7 deficiency in CHO cells reduces the cell proliferation rate. But the effect of this reduction on the CHO cell productivity remained unclear. Hence, in the study at hand the effect of caspase-7 deficiency was assessed on the cell growth, viability and protein expression. In addition, the enzymatic activity of caspase-3 was investigated in the absence of caspase-7. RESULTS: Findings showed that in the absence of caspase-7, both cell growth and cell viability were decreased. Cell cycle analysis illustrated that the CHO knockout (CHO-KO) cells experienced a cell cycle arrest in G2/M phase. This cell cycle arrest resulted in a 1.7-fold increase in the expression of luciferase in CHO-KO cells compared to parenteral cells. Furthermore, in the apoptotic situation the enzymatic activity of caspase-3 in CHO-KO cells was approximately 3 times more than CHO-K1 cells. CONCLUSIONS: These findings represented that; however, caspase-7 deficiency reduces the cell proliferation rate but the resulted cell cycle arrest leads to the enhancement of recombinant protein expression. Moreover, increasing in the caspase-3 enzymatic activity compensates the absence of caspase-7 in the caspase cascade of apoptosis.

Detection and quantitation of host cell proteins in monoclonal antibody drug products using automated sample preparation and data-independent acquisition LC-MS/MS / 药物分析学报

Ensuring the removal of host cell proteins (HCPs) during downstream processing of recombinant pro-teins such as monoclonal antibodies (mAbs) remains a challenge.Since residual HCPs might affect product stability or safety,constant monitoring is required to demonstrate their removal to be below the regulatory accepted level of 100 ng/mg.The current standard analytical approach for this procedure is based on ELISA;however,this approach only measures the overall HCP content.Therefore,the use of orthogonal methods,such as liquid chromatography-mass spectrometry (LC-MS),has been established,as it facilitates the quantitation of total HCPs as well as the identification and quantitation of the indi-vidual HCPs present.In the present study,a workflow for HCP detection and quantitation using an automated magnetic bead-based sample preparation,in combination with a data-independent acquisi-tion (DIA) LC-MS analysis,was established.Employing the same instrumental setup commonly used for peptide mapping analysis of mAbs allows for its quick and easy implementation into pre-existing workflows,avoiding the need for dedicated instrumentation or personnel.Thereby,quantitation of HCPs over a broad dynamic range was enabled to allow monitoring of problematic HCPs or to track changes upon altered bioprocessing conditions.

Site-specific integration and stable expression of exogenous protein at a novel site on CHO cell chromosome / 中国药科大学学报

@#Finding stable expression sites on the chromosomes of Chinese hamster ovary (CHO) cells is an effective method to solve the problem of unstable expression of CHO cells in long-term culture. Our group used lentiviral transfection to integrate the tracer gene (Zsgreen1) into the chromosome of CHO cells and found multiple potential stable expression sites. This study verified the ability of one of the sites located in the 148052-148157 bp region on chromosome NW_003614241.1 to stably express exogenous proteins.The expression of Zsgreen1 gene was first observed, and CRISPR/Cas9 technology was then used to integrate the enhanced green fluorescent protein (EGFP) gene into this site. Three strains of EGFP gene integrated cells were obtained. After 60 generations of suspension culture, the fluorescence intensity of the cells had no significant changes, which proved that this site can stably express the EGFP gene. The same method was used to construct recombinant CHO cell lines expressing the human serum albumin (HSA) gene, and was verified by Western blot that this site could express and secrete HSA. It shows that the above-mentioned sites can be integrated and can stably express exogenous proteins.

Comparison of protein precipitation methods for sample preparation prior to proteomic analysis of Chinese hamster ovary cell homogenates

BACKGROUND: Chinese hamster ovary (CHO) cells are the workhorse for obtaining recombinant proteins. Proteomic studies of these cells intend to understand cell biology and obtain more productive and robust cell lines for therapeutic protein production in the pharmaceutical industry. Because of the great importance of precipitation methods for the processing of samples in proteomics, the acetone, methanol-chloroform (M/C), and trichloroacetic acid (TCA)-acetone protocols were compared for CHO cells in terms of protein recovery, band pattern resolution, and presence on SDS-PAGE. RESULTS: Higher recovery and similar band profile with cellular homogenates were obtained using acetone precipitation with ultrasonic bath cycles (104.18 ± 2.67%) or NaOH addition (103.12 ± 5.74%), compared to the other two protocols tested. TCA-acetone precipitates were difficult to solubilize, which negatively influenced recovery percentage (77.91 ± 8.79%) and band presence. M/C with ultrasonic homogenization showed an intermediate recovery between the other two protocols (94.22 ± 4.86%) without affecting protein pattern on SDS-PAGE. These precipitation methods affected the recovery of low MW proteins (< 15 kDa). CONCLUSIONS: These results help in the processing of samples of CHO cells for their proteomic study by means of an easily accessible, fast protocol, with an almost complete recovery of cellular proteins and the capture of the original complexity of the cellular composition. Acetone protocol could be incorporated to sample-preparation workflows in a straightforward manner and can probably be applied to other mammalian cell lines as well.

2,6-and 3,5-dimethylaniline-induced mutagenesis in Chinese hamster ovary cells expressing human cytochrome P450 1A2 and sulfotransferase

Aim: The aim of this study was to test the hypothesis that human cytochrome P450 1A2 (CYP1A2) and sulfotransferase (SULT) contribute to the phase I and II bioactivation of 2,6-dimethylaniline (2,6-DMA) and 3,5-dimethylaniline (3,5-DMA) in affecting the incidence of genotoxicity.Methodology: 5P3H1 cells carrying cytochrome P450 1A2 (CYP1A2) and SULT cells were treated with various concentrations of 2,6-and 3,5-DMA for 48 hr or their N-hydroxyl and aminophenol metabolites for 1 hr in the absence or presence of 2,6-Dichloro-4-nitrophenol (DCNP). Cell lethality was assayed by trypan blue exclusion and induced mutagenesis of adenine phosphoribosyl transferase (aprt) gene was also evaluated. Results: A significant dose-dependent increase in cytotoxicity and mutant fraction was observed after treatment with 2,6- and 3,5-DMA, and their metabolites; N-hydroxy and aminophenol metabolites are more potent than the parent compounds. Addition of sulfotransferase inhibitor DCNP decreased the cytotoxic and mutagenic effects of 2,6- and 3,5-DMA, and their metabolites in a dose-dependent manner. Interpretation: This research indicate that 2,6 and 3,5-DMA are mutagenic, and their toxicity in model systems depend on metabolic activation. This activation is mediated by CYP1A2 and SULT enzymes

Increasing transgenic expression in recombinant Chinese hamster ovary cells using introns in different directions / 生物工程学报

The aim of this study is to investigate the effect of the chimeric intron in different directions on the expression of the nerve growth factor (NGF) in recombinant Chinese hamster ovary (CHO) cells. The chimeric intron that contained the splice sequence of the first intron of the human β-globin and the human immunoglobulin heavy chain variable region intron was used. NGF gene was cloned into the expression vectors containing the chimeric intron in the forward or reverse direction, followed by transfecting into CHO cells, and screened under G418 to produce the stable transfected CHO cells. Fluorescence quantitative PCR, ELISA, and Western blotting were performed to detect the recombinant NGF gene expression in CHO cells. The results showed that the chimeric introns could significantly enhance the expression of NGF in recombinant CHO cells. Moreover, the enhancing effect on NGF expression level by the intron in the forward direction showed stronger than that of the reverse direction both at mRNA and protein level. In conclusion, the chimeric intron could increase NGF expression in stably transfected CHO cells and the effect is associated with the direction of the intron insertion.

Advances in development and application of Chinese hamster resource / 中国比较医学杂志

Chinese hamsters are a kind of valuable laboratory animal resources and play an important role in medical,genetics and pharmaceutical research. More and more biological characteristics of Chinese hamsters have been discovered with in-depth research,and many Chinese hamster models have been established so far. This paper is a brief overview of the development and research progress of Chinese hamsters and their application in taxonomy and medical research.

Quality control of chromosome preparation in the chromosome aberration test in vitro / 中国比较医学杂志

Objective To study the influencing factors and quality control of chromosome preparation during chromosome aberration test in vitro,and to summarize and analyze the method and key points of successful preparation of chromosome specimens in vitro. Methods Chinese hamster lung cells(CHL)were used for cell culture and chromosome preparation. Mitomycin and cyclophosphamide were used as positive mutagens. After routine hypotonic treatment,fixation, and squash preparation, finally, to read the film under the microscope. Results The CHL chromosome aberration test showed that both the chromosome aberration rates of mitomycin- and cyclophosphamide-treated cells were significantly increased(>20%),while the aberration rates in the negative control group were less than 5%, either with and without metabolic activation. The success rate was high and the prepared chromosomes were well dispersed with a moderate length. Conclusions Many factors can affect the specimen preparation in chromosome aberration test. Every step is very important,and it should be strictly following the operating procedures. It is of importance to grasp the principle of each step and to operate carefully,patiently and scientifically in order to prepare good specimens.

A pilot study of the modulation of sirtuins on arylamine -acetyltransferase 1 and 2 enzymatic activity

Arylamine -acetyltransferase (NAT; E.C. 2.3.1.5) enzymes are responsible for the biotransformation of several arylamine and hydrazine drugs by acetylation. In this process, the acetyl group transferred to the acceptor substrate produces NAT deacetylation and, in consequence, it is susceptible of degradation. Sirtuins are protein deacetylases, dependent on nicotine adenine dinucleotide, which perform post-translational modifications on cytosolic proteins. To explore possible sirtuin participation in the enzymatic activity of arylamine NATs, the expression levels of NAT1, NAT2, SIRT1 and SIRT6 in peripheral blood mononuclear cells (PBMC) from healthy subjects were examined by flow cytometry and Western blot. The activity of the sirtuins on NAT enzymatic activity was analyzed by HPLC, in the presence or absence of an agonist (resveratrol) and inhibitor (nicotinamide) of sirtuins. We detected a higher percentage of positive cells for NAT2 in comparison with NAT1, and higher numbers of SIRT1+ cells compared to SIRT6 in lymphocytes. NAT2 activity in the presence of NAM inhibitors was higher than in the presence of its substrate, but not in the presence of resveratrol. In contrast, the activity of NAT1 was not affected by sirtuins. These results showed that NAT2 activity might be modified by sirtuins.

Stable co-expression of human kappa opioid receptor and PKAcat-EGFP in CHO cells and functional identification / 中国药理学通报

Aim To establish a cell model which stably co-ex-press human kappa opioid receptor (hKOR) and enhanced green fluorescent protein ( EGFP) labeled catalytic domain of cAMP-dependent protein kinase A(PKAcat) fusion protein (PKAcat-EGFP) in Chinese hamster ovary(CHO) cells, laying the foun-dation for the high-throughput screening of hKOR drugs and drug molecular mechanisms in vitro. Methods Hygromycin B resist-ant hKOR recombinant plasmid [ pcDNA3.1/Hygro ( + ) -hKOR] was transfected into CHO cells stably expressing PKA-cat-EGFP by a lipofectin based method. Transfected cells were selected in culture medium containing hygromycin B. The posi-tive clones were selected by PKA redistribution assay. Z’ factor was used for evaluation and validation the reliability of the cell model. PKA redistribution assay and LANCE cAMP 384 Kit were used to test the function of the receptors in selected clone. Results CHO-PKAcat-EGFP/hKOR-13 cell model exhibited stable response in PKA redistribution assay and LANCE cAMP 384 Kit. Treated with 100 nmol·L-1U-50488 for 30 min, the average value of Z’ factor was 0.596, proving the reliability of the cell model. The hKOR expression in cell model remained stable after a few generations. Conclusion The CHO-PKAcat-EGFP/hKOR-13 cell model with stable co-expression of hKOR and PKAcat-EGFP has been successfully established.

Impact of sodium butyrate and mild hypothermia on metabolic and physiological behaviour of CHO TF 70R cells

Background: To reduce costs associated with productivity of recombinant proteins in the biopharmaceutical industry, research has been focused on regulatory principals of growth and survival during the production phases of the cell culture. The main strategies involve the regulation of cell proliferation by the modulation of cell cycle control points (G1/S or G2/M) with mild hypothermia and the addition of sodium butyrate (NaBu). In this study, batch culture strategies were evaluated using CHO TF 70R cells producing the recombinant human tissue plasminogen activator (rh-tPA), to observe their individual and combined effect on the cellular physiological state and relevant kinetic parameters. Results: NaBu addition has a negative effect on the mitochondrial membrane potential (ΔΨm), the values of which are remarkably diminished in cultures exposed to this cytotoxic compound. This effect was not reflected in a loss of cell viability. NaBu and mild hypothermic conditions increased the doubling time in the cell cultures, suggesting that these strategies triggered a general slowing of each cell cycle phase in a different way. Finally, the individual and combined effect of NaBu and mild hypothermia produced an increase in the specific rh-tPA productivity in comparison to the control at 37°C without NaBu. Nevertheless, both strategies did not have a synergistic effect on the specific productivity. Conclusions: The combination of NaBu addition and mild hypothermic condition causes an impact on physiological and metabolic state of CHO TF 70R cells, decreasing cell growth rate and improving glucose consumption efficiency. These results therefore provide a promising strategy to increase specific productivity of rh-tPA.

Expression of apoptosis-related genes caspase-3, caspase-9, Bax and Bcl-2 during oral buccal mucosa carcinogenesis in Chinese hamster / 中国实验动物学报

Objective To study the expression and significance of apoptosis-related genes caspase-3,caspase-9,Bax and Bcl-2 in oral normal mucosa,oral simple hyperplasia,oral epithelial dysplasia and oral squamous cell carcinomas (OSCC) in Chinese hamsters.Methods The expressions of mRNA and protein of caspase-3,caspase-9,Bax and Bcl-2 in the oral normal mucosa,oral simple hyperplasia,oral epithelial dysplasia and OSCC tissues in Chinese hamsters were examined by immunohistochemistry and RT-PCR.Results During the process of oral carcinogenesis,the expression of Bcl-2 protein was significantly higher in OSCC than in oral normal mucosa,oral simple hyperplasia,and oral epithelial dysplasia (P<0.05).In dysplastic epithelia,the protein expressions of caspase-3,caspase-9 and Bax were more than that of normal epithelia,and along with the increased dysplasis,the expression level was decreased.Further analysis showed that expression of Bcl-2 was negatively related with the expressions of Bax,caspase-3 and caspase-9 (P<0.05).The result of RT-PCR showed that Bcl-2 was significantly increased in OSCC compared with normal mucosa,while the expressions of caspase-3,caspase-9 and Bax were decreased (P<0.05).Conclusions In the Chinese hamster squamous carcinoma,the expressions of caspase-3,caspase-9 and Bax are reduced and the Bcl-2 expression are increased,indicating that the expressions of caspase-3,caspase-9,Bax and Bcl-2 are closely related with the occurrence and development of oral squamous cell cancinoma.This study can offer some clues for gene therapy of OSCC,or can provide a reference for evaluating the biological characteristics and prognosis of OSCC.

Study of mitochondrial genomes and molecular genetic marker of microsatellite in chinese hamster / 中国比较医学杂志

Chinese hamster is an important laboratory animal in medical and biological researches,but the molecular genetic markers research was rarely reported.In our study the base composition,gene structure,genetic evolution and other characteristics of mitochondrial genome of Chinese hamster were analyzed using the methods of bioinformatics and comparative genomics,genetic quality detection system of Chinese hamster were also established.These results would supply genome data for animal models of human diseases,and lay the foundation for scientific evaluation and reasonable utilization.

Influence of ubiquitous chromatin opening element on the antibody expression in CHO cells / 中国药科大学学报

Ubiquitous chromatin opening element ( UCOE ) , composed of the promoters of human housekeeping genes, prevents transgene from silencing and produces consistent, stable and high-level gene expression irrespec-tive of the chromosomal integration site. The research studied the influence of different UCOE element parts on antibody expression in CHO cells. UCOE 1. 5 kb from chromobox homolog 3 ( CBX3 ) , UCOE 2. 5 kb from the heterogeneous ribonucleoprotein A2/B1 ( HNRPA2 B1 ) and the whole UCOE 4. 0 kb were inserted into the anti-body light and heavy chain vectors, respectively, and transfected into CHO cells using antibiotics-Zeocin and Blasticidin for pressure selection. Four groups of monoclonal cells were harvested and antibody expression of each group was detected. The monoclonal cells with UCOE 1. 5 kb and UCOE 2. 5 kb increased 1. 5 to 2-fold in the level of antibody expression, wheareas, monoclonal cells with UCOE 4. 0 kb increased 3 to 4-fold. The enhance-ment of two housekeeping promoter genes on antibody expression could stack up.

Biological characteristics of Chinese hamster infected with Babesia / 中国比较医学杂志

Objective To establish a Chinese hamster model of babesia infection, to find the changing pattern of organs and biochemical parameters in Chinese hamster infected with Babesia, and to promote the detection and treatment of babesiosis.Methods Healthy 5-week old Chinese hamsters were infected by intraperitoneal injection of blood containing Babesia.Blood samples were collected at 0, 2, 4, 6, 8, 10, 12, 14, 16, 23, 30, and 37 days after infection from 5 hamsters at each time point.Blood smears were prepared to detect the parasites using Giemsa staining.ELISA assay was employed to test the IL-2 concentration.The blood biochemical indexes were detected using an automatic biochemical analyzer.DNA was extracted from the whole blood and REAL-TIME RCR was performed to determine the reproduction of Babesia.Aftert the animals were sacrificed, the heart, lung, spleen, liver, and kidney were taken to analyze the changes of organ coefficients.Results The highest level of Babesia in the hamsters occurred on day 4 after the Babesia injection, and then showing a decreasing tendency.However, there was a transient increase on the 12th day after infection.The liver and spleen displayed most extensive response to the infection showing hepatomegaly and splenomegaly, but the variation of heart and kidneys coefficients was within the norm.There were prominent changes of blood cells, especially leucocytes, with two peaks at day 10 and 23 after the Babesia infection.The peak changes of blood biochemical indexes occurred at day 12 after infection.The concentration of serum IL-2 reached a peak on the 10th day after infection.Conclusions The Chinese hamsters display typical characteristics of tick-borne diseases such as hepatomegaly and splenomegaly.The immunological system is activated along with the infection and reaches a highest stage in the second week.Afterwards the Babesia can live in the hamster body for a long period of time.The results of this study provide useful information supporting further studies on the detection, treatment and prevention of Babesiosis.

Establishment of recombinant cell line stably expressing humanα2A-adrenoceptor / 中国药理学与毒理学杂志

OBJECTIVE To establish a new cell line that can stably express humanα2A-adrenoceptor (α2A- AR). METHODS Recombinant plasmid of α2A- AR with hygromycin B (Hygro) resistance (pcDNA3.1/Hygro-HA-α2A-AR)was stably transfected into Chinese hamster ovary(CHO)cells which had expressed protein kinase A catalytic subunits(PKAcat) with labeling of enhanced green fluorescent protein(EGFP)by a Lipofectamine based method. A single positive clone expressingα2A-AR was selected through cultivation in the presence of 200 mg · L-1 hygromycin B followed by PKA redistribution assay. The transcriptional expression ofα2A-AR was detected by quantitative real-time PCR(qRT-PCR). Time-resolved fluorescence resonance energy transfer immunoassay was used to identify the function of inhibiting cAMP accumulation of α2A-AR. RESULTS The CHO-PKAcat-α2A-AR cell line No.7 exhibited stable response in PKA redistribution assay. qRT-PCR analysis demonstrated that the high expression ofα2A-AR in the cell line remained stable after a few generations compared with CHO-PKAcat-EGFP cells (P<0.01). The cAMP accumulation caused by forskolin was significantly inhibited by α2A-AR agonist in CHO-PKAcat-α2A-AR cells(P<0.01). CONCLUSION CHO-PKAcat-α2A-AR cell line is constructed successfully, which provides an effective model for drug screening and studies of mechanisms.

Renal drug transporters and their significance in drug-drug interactions

The kidney is a vital organ for the elimination of therapeutic drugs and their metabolites. Renal drug transporters, which are primarily located in the renal proximal tubules, play an important role in tubular secretion and reabsorption of drug molecules in the kidney. Tubular secretion is characterized by high clearance capacities, broad substrate specificities, and distinct charge selectivity for organic cations and anions. In the past two decades, substantial progress has been made in understanding the roles of transporters in drug disposition, efficacy, toxicity and drug-drug interactions (DDIs). In the kidney, several transporters are involved in renal handling of organic cation (OC) and organic anion (OA) drugs. These transporters are increasingly recognized as the target for clinically significant DDIs. This review focuses on the functional characteristics of major human renal drug transporters and their involvement in clinically significant DDIs.