Expression of the hepatic specific V1 messenger ribonucleic acid of the human growth hormone receptor gene is regulated by hepatic nuclear factor (HNF)-4alpha2 and HNF-4alpha8

Insight into the practical models for prediciting the essential role of the cytochrome P450-mediated biotransformation in emodin-associated hepatotoxicity

Emodin is widely present in Chinese herbs with broad application prospects, however, the conflicting reports of its hepatotoxicity have created a concern. It was therefore aimed to develop practical models to elucidate the outcome of CYP450 biotransformation on emodin. HepG2 and rat liver microsomes (RLM) coculture system was first utilized for prediction. It was found that emodin (35 μM)-mediated cytotoxicity was alleviated only when the cofactor of CYP450 NADPH (1 mM) was present. Similarly, both the pan-CYP450 inhibitor 1-aminobenzotriazole (ABT) (2 mM) and the heat-inactivated liver microsomes completely abolished the protective effect of RLM (0.75 mg/mL). Consistently, ABT significantly increased the toxicity of emodin in primary rat liver cells. Along similar lines, only the monohydroxylation metabolite M3 that accounted for neglectable amount of the whole metabolites showed similar toxicity to emodin, both M1 and M2 exhibited far less toxcity than emodin in THLE-2 cells. In vivo study further supported that ABT (50 mg/kg, s.c.) aggravated the hepatotoxicity of emodin (80 mg/kg, i.p.) on mice, as emodin treatment only mediated slight increase of liver index and histological score likely due to the metabolic detoxication of emodin, whereas ABT co-administration resulted in severe liver injury as reflected by the dramatic increase of the liver index value, serum ALT and AST levels, and histopathological score. Moreover, it was explored that ROS generation together with the electrophilicity of emodin contributed to its hepatotoxicity. These findings not only provided a clear evidence of the metabolic detoxification of emodin, but also shed a light on the hepatotoxic mechanisms of emodin, which would lay a solid foundation for the rational application of emodin in the future.

Genomic structure and characterization of growth hormone receptors from golden pompano Trachinotus ovatus and their expression regulation by feed types

In this study, sequence analysis showed that ToGHR1 and ToGHR2 encoded polypeptides of 577 and 588 amino acids, respectively. Bioinformatics analysis showed that both ToGHR1 and ToGHR2 contain FN3 domains and transmembrane domains, which have glycosylation and phosphorylation sites. The exons of ToGHR1 and exons 4-10 of ToGHR2 are homologous to exons 2 and 4-9 in Homo sapiens genes, respectively. Only 3 SSR sites in ToGHR1 have SSR polymorphisms, and ToGHR2 has no SSR polymorphisms. ToGHR1 and ToGHR2 have high homology with GHR1 and GHR2 of many fish by BLAST. qRT-PCR was used to examine the expression profile of ToGHR mRNA in 12 normal liver and intestine tissue samples from 3 feed-type groups. The results showed that ToGHR is expressed in all 12 tissues, especially liver and muscle tissues, which showed higher ToGHR expression than that in other tissues (p < 0.05). Experiments on feed-type groups may indicate that high levels of LC-PUFA in squid bait can promote ToGHR1 expression and simultaneously inhibit ToGHR2 expression in the liver tissue. In addition, the high levels of LC-PUFA in food could inhibit intestinal ToGHR1 expression, and the intermediate levels may promote intestinal ToGHR1 expression. However, the unsaturated fatty acid content in the food does not affect the expression of intestinal ToGHR2.

Prediction of Drug-Induced Liver Injury in HepG2 Cells Cultured with Human Liver Microsomes

Drug-induced liver injury (DILI) via metabolic activation by drug-metabolizing enzymes, especially cytochrome P450 (CYP), is a major cause of drug failure and drug withdrawal. In this study, an in vitro model using HepG2 cells in combination with human liver microsomes was developed for the prediction of DILI. The cytotoxicity of cyclophosphamide, a model drug for bioactivation, was augmented in HepG2 cells cultured with microsomes in a manner dependent on exposure time, microsomal protein concentration, and NADPH. Experiments using pan- or isoform-selective CYP inhibitors showed that CYP2B6 and CYP3A4 are responsible for the bioactivation of cyclophosphamide. In a metabolite identification study employing LC-ESI-QTrap and LC-ESI-QTOF, cyclophosphamide metabolites including phosphoramide mustard, a toxic metabolite, were detected in HepG2 cells cultured with microsomes, but not without microsomes. The cytotoxic effects of acetaminophen and diclofenac were also potentiated by microsomes. The potentiation of acetaminophen cytotoxicity was dependent on CYP-dependent metabolism, and the augmentation of diclofenac cytotoxicity was not mediated by either CYP- or UDP-glucuronosyltransferase-dependent metabolism. The cytotoxic effects of leflunomide, nefazodone, and bakuchiol were attenuated by microsomes. The detoxication of leflunomide by microsomes was attributed to mainly CYP3A4-dependent metabolism. The protective effect of microsomes against nefazodone cytotoxicity was dependent on both CYP-mediated metabolism and nonspecific protein binding. Nonspecific protein binding but not CYP-dependent metabolism played a critical role in the attenuation of bakuchiol cytotoxicity. The present study suggests that HepG2 cells cultured with human liver microsomes can be a reliable model in which to predict DILI via bioactivation by drug metabolizing enzymes.

HepG2 cells as an in vitro model for evaluation of cytochrome P450 induction by xenobiotics

Although various in vitro assays have been developed to evaluate the cytochrome P450 (CYP)-inducing potential of drug candidates, there is a continuing need for the development of a reliable model in drug discovery. The objective of the present study was to compare CYP induction by chemicals in HepG2 cells with Huh7, NKNT-3, and reverted NKNT-3 cells. HepG2 cells showed more similarity to human liver than the other cell lines in comparisons of the expression of cellular proteins. In evaluation of basal CYP activity, Huh7 cells exhibited the highest CYP1A2 and CYP3A4 activity, and HepG2 cells showed the highest CYP2B6 activity. The inducibility of CYP1A2, CYP2B6, and CYP3A4 by prototypical inducers was determined using enzyme assay, immunoblot analysis, and real-time PCR. Among the cells tested, HepG2 cells were highly responsive to CYP inducers, such as 3-methylcholanthrene for CYP1A2 and phenobarbital for CYP2B6 and CYP3A4. Moreover, HepG2 cells were responsive to various CYP1A2, CYP2B6, and CYP3A4 inducers as determined using fluorogenic and LC-MS/MS substrates. Thus, HepG2 cells may be comparable to human hepatocytes for the evaluation of CYP induction or slightly less sensitive. These results suggest HepG2 cells as a cell-based model in screening for CYP inducers in drug discovery.

Regulation of human growth hormone receptor expression by microRNAs

Human GH binds to its receptor (GHR) on target cells and activates multiple intracellular pathways, leading to changes in gene expression, differentiation, and metabolism. GHR deficiency is associated with growth and metabolic disorders whereas increased GHR expression has been reported in certain cancers, suggesting that the GHR gene requires tight controls. Several regulatory mechanisms have been found within its 5'-untranslated region (UTR) promoter and coding regions. However, the 3'-UTR has not been previously examined. MicroRNAs (miRNAs) are small (19-22 nucleotides) noncoding RNAs that downregulate gene expression mainly through targeting the 3'-UTR of mRNAs and enhancing their degradation or inhibiting translation. In the present study, we investigated whether miRNAs regulate GHR expression. To define putative miRNA binding sites in the GHR 3'-UTR, we used multiple in silico prediction tools, analyzed conservation across species and the presence of parallel sites in GH/IGF axis-related genes, and searched for reports linking miRNAs to GHR-related physiological or pathophysiological activities. To test prioritized sites, we cotransfected a wild-type GHR 3'-UTR luciferase reporter vector as well as miRNA binding site mutants into HEK293 cells with miRNA mimics. Furthermore, we tested whether the miRNAs altered endogenous GHR mRNA and protein levels in HEK293 cells and in 2 cancer cell lines (MCF7 and LNCaP). Our experiments have identified miRNA (miR)-129-5p, miR-142-3p, miR-202, and miR-16 as potent inhibitors of human GHR expression in normal (HEK293) and cancer (MCF7 and LNCaP) cells. This study paves the way for the development of miRNA inhibitors as therapeutic agents in GH/GHR-related pathophysiologies, including cancer.

Human growth hormone receptor gene expression is regulated by Gfi-1/1b and GAGA cis-elements

Human growth hormone receptor (hGHR) gene regulation is complex: mRNAs are transcribed from multiple variant (V) 5'UTR exons, several ubiquitously while others only in the postnatal hepatocyte. The liver-specific V1 exon promoter contains Gfi-1/1b repressor sites adjacent to a GAGA box, a GH response element (GHRE) in several mammalian genes. GAGA boxes are also present in the ubiquitously expressing V3 exon promoter. Heterologous sites in bovine, ovine and murine GHR genes suggest conserved roles. GAGA factor stimulated V1 and V3 promoters while Gfi-1/1b repressed basal and GAF-stimulated V1 transcription. HGH treatment of HepG2 cells resulted in a new complex forming with V3 GAGA elements, suggesting a functional GHRE. Data suggest liver-specific V1 transcription is regulated by inhibitory Gfi-1/1b and stimulatory GAGA cis-elements and Gfi-1/1b may control the lack of V1 expression in fetal liver, hepatic tumours and non-hepatic tissues. In addition, hGH may regulate hGHR expression through V3 GAGA boxes.

In vivo targeting of the growth hormone receptor (GHR) Box1 sequence demonstrates that the GHR does not signal exclusively through JAK2

GH is generally believed to signal exclusively through Janus tyrosine kinases (JAK), particularly JAK2, leading to activation of signal transducers and activators of transcription (STAT), ERK and phosphatidylinositol 3-kinase pathways, resulting in transcriptional regulation of target genes. Here we report the creation of targeted knock-in mice wherein the Box1 motif required for JAK2 activation by the GH receptor (GHR) has been disabled by four Pro/Ala mutations. These mice are unable to activate hepatic JAK2, STAT3, STAT5, or Akt in response to GH injection but can activate Src and ERK1/2. Their phenotype is identical to that of the GHR(-/-) mouse, emphasizing the key role of JAK2 in postnatal growth and the minimization of obesity in older males. In particular, they show dysregulation of the IGF-I/IGF-binding protein axis at transcript and protein levels and decreased bone length. Because no gross phenotypic differences were evident between GHR(-/-) and Box1 mutants, we undertook transcript profiling in liver from 4-month-old males. We compared their transcript profiles with our 391-GHR truncated mice, which activate JAK2, ERK1/2, and STAT3 in response to GH but not STAT5a/b. This has allowed us for the first time to identify in vivo Src/ERK-regulated transcripts, JAK2-regulated transcripts, and those regulated by the distal part of the GHR, particularly by STAT5.

Structure and activity of the human growth hormone receptor (hGHR) gene V2 promoter

Human GH (hGH) has important effects on growth as well as carbohydrate, fat, and protein metabolism. These actions require the presence of normal levels of a functional hGH receptor (hGHR) on the surface of target cells. hGHR gene expression is characterized by the use of several 5'-noncoding exons and alternative splicing, resulting in the generation of multiple mRNA isoforms. The hGHR V2 transcript is predominant in most tissues, including human fat. However, factors regulating its ubiquitous expression have remained unidentified. The present study was aimed at characterizing the mechanisms regulating hGHR V2 transcription. Two major V2 transcriptional start sites were identified by primer extension assays. The V2 proximal promoter is TATA-less, with several characteristics of a housekeeping gene promoter. Transient transfection analyses of 2.6 kb of the 5'-flanking region of V2 confirmed its promoter activity in multiple primate cell lines. Similar promoter activity patterns were observed in human SGBS preadipocytes and mature adipocytes but with much higher V2 promoter activity in mature adipocytes, suggesting that changes in the availability of specific factors during adipocyte differentiation play a role in V2 promoter regulation. Serial deletion and mutation analyses revealed that transcription of hGHR V2 in different cell types, including adipocytes, is determined by a core promoter and distinct inhibitory and activation domains in the 5'-promoter region as well as within the V2 exon. Our data suggest that V2 transcription is the result of a complex interplay involving multiple factors, to ensure appropriate expression of hGHR in different hGH target cells.