Role of interleukin 6 and corticosteroids in the regulation of expression of glutathione S-transferases in primary cultures of rat hepatocytes

Regulation of the rat glutathione S-transferase A2 gene by glucocorticoids: crosstalk through C/EBPs

Regulation of the rat glutathione S-transferase A2 (GSTA2) gene by glucocorticoids is biphasic in its concentration dependence to glucocorticoids, with concentrations of 10-100 nM repressing gene activity (GR-dependent), and concentrations above 1 microM increasing transactivation (PXR-dependent) in adult rat hepatocytes or transient transfection assays. Over-expression of either C/EBP alpha or beta negatively regulates basal and inducible expression of a 1.65 Kb GSTA2 luciferase reporter, and synergizes the response to glucocorticoids (GC). C/EBP responsive elements have been identified in the GSTA2 5'-flanking sequence, associated with the palindrominic Glucocorticoid Responsive Element (GRE), the Ah receptor response elements, and the antioxidant response element. In reporters lacking the palindromic GRE, negative regulation by GC is observed only when C/EBP alpha is co-expressed. Co-transfection of C/EBP alpha/beta induced gene expression of the GSTA2 XRE reporter, but negatively regulated the GSTA2 ARE-reporter. In contrast, the ARE from the rat NAD(P)H quinone oxidoreductase gene was induced by co-transfection of C/EBPs, but was still negatively regulated by GC. PXR-induction of the GSTA2 reporter was partially ablated by co-transfection of C/EBP alpha and enhanced by co-transfection of C/EBPbeta. We conclude that C/EBP alpha and beta are involved in GC-dependent repression of GSTA2 gene expression and ARE sequences that bind C/EBPs appears to be critical for these responses.

Repression of human GSTA1 by interleukin-1beta is mediated by variant hepatic nuclear factor-1C

Down-regulation of glutathione transferase A1 (GSTA1) expression has profound implications in cytoprotection against toxic by-products of lipid peroxidation produced during inflammation. We investigated the role of hepatic nuclear factor 1 (HNF-1) in repression of human GSTA1 expression by interleukin (IL)-1beta in Caco-2 cells. In luciferase reporter assays, overexpression of HNF-1alpha increased GSTA1 transcriptional activity via an HNF-1 response element (HRE) in the proximal promoter. In addition, constitutive mRNA levels of GSTA1 and HNF-1alpha rose concurrently in Caco-2 cells with increasing stage of confluence. IL-1beta reduced GSTA1 mRNA levels at all stages of confluence; however, HNF-1alpha mRNA levels were not altered. IL-1beta repressed GSTA1 transcriptional activity, an effect that was abolished by mutating the HRE. Similar results were observed in HT-29 and HepG2 cells. Overexpression of HNF-1alpha did not counteract IL-1beta-mediated repression of GSTA1 transcription either in reporter assays or at the mRNA level. Involvement of the transdominant repressor C isoform of variant HNF-1 (vHNF-1C) in GSTA1 repression was demonstrated, because vHNF-1C overexpression significantly reduced GSTA1 transcriptional activity. Finally, IL-1beta caused concentration-related up-regulation of vHNF-1C mRNA levels and increased binding of vHNF-1C protein to the HRE, whereas HNF-1alpha-HRE complex formation was reduced. These findings indicate that IL-1beta represses GSTA1 transcription via a mechanism involving overexpression of vHNF-1C.

Identification of a short form of ubiquitin-specific protease 3 that is a repressor of rat glutathione S-transferase gene expression

The transcription rate and protein expression from both GSTA2 (glutathione S-transferase A2) and albumin genes decrease in rat liver after IL-6 (interleukin 6) plus DEX (dexamethasone) treatment of primary hepatocytes or after LPS (lipopolysaccharide)-induced acute-phase response in animals. The down-regulation is associated with the induced expression of a nuclear protein (termed IL6DEX-NP for IL-6/DEX-induced nuclear protein) that binds to a specific site on the promoter of GSTA2, leading to a decrease in transcriptional activity. IL6DEX-NP is not similar to other transcription factors, and, for identification, we functionally cloned it from a rat liver library using a yeast one-hybrid screen based on DNA-binding activity. The cloned sequence was a truncated form of USP3 (ubiquitin-specific protease 3) and the truncated USP3 protein in a yeast extract bound to DNA containing the IL6DEX-NP recognition sequence. Using 5'- and 3'-RACE (rapid amplification of cDNA ends), the complete sequence of USP3 was found in liver from LPS-treated rats. However, using Western blot analysis, only truncated forms of USP3 could be identified in nuclear extracts from LPS-treated rat livers. A GSTA2 promoter-reporter gene plasmid and USP3-expressing plasmids were transfected into rat hepatoma cells. Expression of the short form of USP3, but not the full-length protein, abolished expression from the reporter gene. Chromatin immunoprecipitation localized USP3 to the GSTA2 promoter in rat hepatocytes in vivo. We believe that the short form of USP3 is IL6DEX-NP and that it may play an important role in the negative regulation of proteins during the acute-phase response.

Regulation of drug-metabolizing enzymes and transporters in inflammation

Inflammation and infection have long been known to downregulate the activity and expression of cytochrome P450 (CYP) enzymes involved in hepatic drug clearance. This can result in elevated plasma drug levels and increased adverse effects. Recent information on regulation of human CYP enzymes is presented, as are new developments in our understanding of the mechanisms of regulation. Experiments to study the effects of modulating CYP activities on the inflammatory response have yielded possible insights into the physiological consequences, if not the purpose, of the downregulation. Regulation of hepatic flavin monooxygenases, UDP-glucuronosyltransferases, sulfotransferases, glutathione S-transferases, as well as of hepatic transporters during the inflammatory response, exhibits similarities and differences with regulation of CYPs.

Counter-regulatory effects of procalcitonin and indoxyl sulphate on net albumin secretion by cultured rat hepatocytes

BACKGROUND

Although hypoalbuminaemia is a significant predictor of mortality in haemodialysis (HD) patients, the pathophysiological mechanisms involved remain to be determined. Albumin is a negative acute-phase reactant and many proinflammatory substances are elevated in HD patients. We investigated factors that may affect liver albumin synthesis.

METHODS

Hepatocytes were isolated from rat livers and were cultured with interleukin (IL)-4, IL-6, IL-12, tumor necrosis factor (TNF)-alpha, procalcitonin (PCT), a sensitive marker of infection, and indoxyl sulphate (IS), a uraemic toxin. Albumin levels in the supernatant were measured by enzyme-linked immunosorbent assay. Albumin mRNA expression was determined by reverse transcriptase polymerase chain reaction.

RESULTS

IL-6 and TNF-alpha significantly decreased albumin levels in a dose-dependent manner (P<0.01 and P<0.05, respectively). In contrast, IL-4 and IL-12 did not modulate albumin production. PCT and IS significantly and dose-dependently increased albumin levels (both P<0.01). PCT increased albumin mRNA expression in the hepatocytes (P = 0.05) and dose-dependently abrogated IL-6-induced suppression of albumin synthesis (P<0.01). IS also blocked the IL-6-induced decrease in net albumin secretion (P<0.01).

CONCLUSION

Our findings indicate that PCT and IS protect against suppression of hepatic albumin synthesis caused by proinflammatory cytokines, suggesting their potential role in preventing hypoalbuminaemia in HD patients.

Decreased expression levels of rat liver glutathione S-transferase A2 and albumin during the acute phase response are mediated by HNF1 (hepatic nuclear factor 1) and IL6DEX-NP

The acute phase response is characterized by positive and negative regulation of many liver proteins including GSTs (glutathione S-transferases) and albumin. The expression of albumin and some GSTs are dependent on HNF1 (hepatic nuclear factor 1). Interleukin 6 plus dexamethasone induce a nuclear protein (IL6DEX-NP) in rat hepatocytes in vitro that binds to a promoter element adjacent to the HNF1 site of rGSTA2 and decreases its expression. We determined how HNF1 and IL6DEX-NP regulate rGSTA2 and albumin expression in rats during the acute phase response after LPS (lipopolysaccharide) treatment. Expression of rGSTA2 and albumin mRNA decreased 3 h after LPS treatment and remained low for 48 h. Transcription rates showed a similar pattern but albumin transcription was less affected. HNF1 and IL6DEX-NP binding to the rGSTA2 promoter was present in control livers but was absent at 3 and 6 h after LPS. By 12 h, HNF1 and IL6DEX-NP binding to the rGSTA2 promoter reappeared and increased to above normal at 48 h. The patterns of HNF1 and IL6DEX-NP binding to the albumin promoter were similar. Affinity of IL6DEX-NP for the albumin promoter was less than that for the rGSTA2 promoter and changes in the transcription rates were consistent with the difference. Early decreases in rGSTA2 and albumin during the acute phase response are due to decreased binding of HNF1. Later persistent decreases in transcriptional rate of rGSTA2 and to a lesser extent albumin are due to increased IL6DEX-NP binding. IL6DEX-NP appears to be an important negative regulator of gene expression in vitro and in vivo.

Down-regulation of alpha class glutathione S-transferase by interleukin-1beta in human intestinal epithelial cells (Caco-2) in culture

The influence of pro-inflammatory cytokines on alpha class glutathione S-transferase A1 and A2 (GSTA1/A2) expression was examined in human colonic epithelial cells (Caco-2) in culture. Dose-dependent reductions in GSTA1/A2 mRNA, protein, and activity levels occurred in Caco-2 cells cultured in conditioned medium (CM) from lipopolysaccharide-stimulated murine monocyte-macrophage cells (RAW 264.7). Neutralizing anti-interleukin-1beta (IL-1beta) antibodies attenuated this repression of GSTA1/A2 expression by CM. Moreover, recombinant human IL-1beta reduced GSTalpha expression at the mRNA, protein, and activity levels in a dose-related fashion. Reduction of GSTA1/A2 mRNA levels by IL-1beta was attenuated by pretreatment with IL-1 receptor antagonist. GSTA1/A2 mRNA half-lives were similar in control and IL-1beta-treated cells, indicating that IL-1beta has no effect on mRNA stability. In reporter gene studies, IL-1beta caused a dose-related reduction of luciferase activity in Caco-2 cells transfected with the full-length GSTA1 promoter-luciferase construct. Using truncated constructs, IL-1beta responsiveness was mapped to a region 286 base pairs upstream to the coding region. Deletion of a hepatic nuclear factor 1 (HNF-1) site in this region abrogated the IL-1beta-mediated repression of GSTA1 promoter activity. These results demonstrate that IL-1beta down-regulates GSTA1/A2 expression in cultured human enterocytes by a transcriptional mechanism involving an HNF-1 site.

Mechanism of negative regulation of rat glutathione S-transferase A2 by the cytokine interleukin 6

A decrease in concentration of some liver proteins, including the detoxification enzyme glutathione S-transferase A2 (rGSTA2), occurs during the acute-phase response. Interleukin 6 (IL-6) with dexamethasone (DEX) decreases transcription of rGSTA2 in rat hepatocytes. The promoter region that mediates suppression of rGSTA2 was localized to 150 bp. These 150 bp were divided and used for electrophoretic mobility-shift assays. Induction of a protein that specifically bound to an oligonucleotide from this region required new protein synthesis and IL-6 with DEX in the culture media. The protein bound to part of the hepatocyte nuclear factor 1 (HNF1) site but was different from and did not displace HNF1. A core sequence, TGATT, was required for binding. The protein also bound to an HNF1 site in the albumin promoter. We hypothesize that IL-6 along with DEX induced a novel protein that decreased transcription of rGSTA2 and possibly albumin by interfering with the transactivating function of HNF1. The protein may be an important negative regulator of transcription during the acute-phase response.

Loss of glutathione S-transferase (GST) mu phenotype in colorectal adenocarcinomas from patients with a GSTM1 positive genotype

Glutathione S-transferase (GST) mu phenotype was assessed in colon tissue from patients with ulcerative colitis and colorectal neoplasms that were positive for GSTM1 genotype. GST mu protein (enzyme linked immunosorbent assay) was absent in 2/9 unaffected colon tissue (22.3%), 4/13 tissues with chronic ulcerative colitis (CUC) (30.7%), 4/11 adenomas (36.4%) and 7/14 adenocarcinomas (50.0%; P<or=0.05). GST-3H-trans-stilbene oxide activity was detected in all tissues except 1/13 (7.6%) CUC tissues and 7/14 (50.0%, P<or=0.05) colorectal adenocarcinomas. Immunoreactive GST mu was observed in colonic epithelial cells but not in adjacent neoplastic cells by immunohistochemistry. Two-dimensional electrophoresis revealed several mu class isoforms in cytosol from unaffected colon that were absent in matched tumor cytosol. These results indicate that GSTM1 genotype may not necessarily reflect GST mu phenotype in colorectal tumors.

MER53, a non-autonomous DNA transposon associated with a variety of functionally related defense genes in the human genome

We report a new medium reiteration frequency repeat MER53 present in human and mammalian genomes. A 189 bp MER53 consensus sequence has been reconstructed based on the computer analysis of GenBank sequences. TA target site duplication and terminal inverted repeats indicate that the MER53 repeat is a non-autonomous DNA transposon related to the mariner family. Two MER53 repeats were found integrated within different mobile elements. We have found that most of the genes harboring the MER53 repeat are involved in the host defense system. The reasons for this non-random distribution of the repeat are discussed.

Regulation of glutathione S-transferase enzymes in primary cultures of rat hepatocytes maintained under various matrix configurations

Primary rat hepatocytes were cultured under various matrix and media conditions and examined after 1 week for the expression and regulation of cytosolic glutathione S-transferase (GST) enzymes. Striking effects on cell morphology were observed in relation to the different matrix conditions, whereas media effects were less prominent. Hepatocytes cultured in serum-free Dulbecco's modified Eagle's medium (DMEM) or modified Chee's medium (MCM) maintained similar levels of total GST protein regardless of the matrix configuration or corresponding cell integrity. However, HPLC analysis showed a differential expression pattern of individual GST subunits in both a time- and medium-dependent fashion. A variable, but pronounced, matrix and medium effect was observed on the induction of total GST expression by various prototypical inducers. Dexamethasone (10 microM) induced subunits A2, M1 and M2 in a medium- and matrix-dependent fashion, whereas phenobarbital (100 microM) induced significantly only subunit A2. beta-Naphthoflavone (50 microM) suppressed all GST subunit expression except subunit P1, which was induced in a matrix- and medium-dependent fashion. These studies show that total basal level expression of GSTs in vitro is reflective of a concomitant increase in mu and pi class subunits and a decrease in alpha class subunits. Moreover, the matrix and medium conditions influence both the basal and inducible expression of GST subunits in cultured rat hepatocytes.

UT-A urea transporter protein expressed in liver: upregulation by uremia

In perfused rat liver, there is phloretin-inhibitable urea efflux, but whether it is mediated by the kidney UT-A urea transporter family is unknown. To determine whether cultured HepG2 cells transport urea, thiourea influx was measured. HepG2 cells had a thiourea influx rate of 1739 +/- 156 nmol/g protein per min; influx was inhibited 46% by phloretin and 32% by thionicotinamide. Western analysis of HepG2 cell lysate using an antibody to UT-A1, UT-A2, and UT-A4 revealed two protein bands: 49 and 36 kD. The same bands were detected in cultured rat hepatocytes, freshly isolated rat hepatocytes, and in liver from rat, mouse, and chimpanzee. Both bands were present when analyzed by native gel electrophoresis, and deglycosylation of rat liver lysate had no effect on either band. Differential centrifugation of rat liver lysate showed that the 49-kD protein is in the membrane fraction and the 36-kD protein is in the cytoplasm. To determine whether the abundance of these UT-A proteins varies in vivo, rats were made uremic by 5/6 nephrectomy. The 49-kD protein was significantly increased 5.5-fold in livers from uremic rats compared to pair-fed control rats. It is concluded that phloretin-inhibitable urea flux in liver may occur via a 49-kD protein that is specifically detected by a UT-A antibody. Uremia increases the abundance of this 49-kD UT-A protein in rat liver in vivo.

Activation of rat hepatic stellate cells leads to loss of glutathione S-transferases and their enzymatic activity against products of oxidative stress

Oxidative stress, mediated partly by lipid peroxidation products, may lead to increased collagen synthesis by hepatic stellate cells (HSC). Stellate cells are protected from oxidative stress by enzymes of detoxication such as the glutathione S-transferases (GSTs), which form glutathione conjugates with lipid peroxidation products (e.g., 4-hydroxy-2-nonenal [HNE]). To better understand the role of GSTs in stellate cell biology, we examined the expression and enzymatic activity of GSTs in normal and activated (both culture- and in vivo-activated) stellate cells. Normal stellate cells contained numerous isoforms of GST including those that detoxify HNE. High levels of enzymatic activity toward 1-chloro-2,4-dinitrobenzene (CDNB) and HNE were present in normal stellate cells and were similar to levels present in whole liver. Following activation by growth in culture, the expression of several GSTs (rGSTA1/A2, A3, and M1) was lost. Also, enzymatic activities toward CDNB and HNE fell approximately 90%. However, expression of rGSTP1 was maintained. A similar loss of rGSTA1/A2, A3, and M1 with persistent expression of rGSTP1 was present after activation in vivo. Furthermore, we identified 2 subpopulations of activated stellate cells with different GST phenotypes from injured livers. In summary, activated stellate cells lose most forms of GST and associated enzymatic activities that are present in normal stellate cells. The findings raise the possibility that activated stellate cells have less ability to detoxify lipid peroxidation products and may be susceptible to oxidative stress. Additionally, we propose that the phenotypic change in GSTs is a sensitive marker of stellate cell activation.

Phenobarbital induction of CYP2B1/2 in primary hepatocytes: endocrine regulation and evidence for a single pathway for multiple inducers

Phenobarbital (PB) and many structurally unrelated chemicals induce the protein and mRNA of P450 cytochromes CYP2B1, CYP2B2, CYP3A1, and specific phase II enzymes to a greater extent in Fischer 344 (F344) than in Wistar Furth (WF) female rats. This sex- and strain-dependent polymorphism can be partly attributed to suppressive effects of thyroid hormone (TH) on WF but not F344 females. We show here that this strain difference was largely retained in primary hepatocyte cultures and could be resolved into two components; (1) Expression of PB-inducible genes-WF hepatocytes had inherently lower basal and PB-induced levels of CYP2B1/2B2 protein and mRNA and UDPGT mRNA; and (2) TH sensitivity-in WF hepatocytes, PB induction, but not basal expression, of CYP2B1/2B2 was three- to fivefold more susceptible to inhibition by TH when the hormone was added to the medium. This second component explains the selective effect of in vivo treatment with methimazole, which lowers circulating TH and partially improves PB induction in WF female rats. Following transfection of a reporter construct containing a PB-responsive unit (PBRU), the plasmid was activated by PB to similar extents in hepatocytes from both rat strains. TH treatment did not inhibit PB-mediated induction of the plasmid in either cell type. Thus, neither of the components determining the strain polymorphism are linked to trans-activating factors contributing to this PBRU activity. The PB-like inducers, 2,2',4,4',5, 5'-hexachlorobiphenyl (HCB) and 1,1-dichloro-2, 2-bis(p-chlorophenyl)ethane (o,p-DDD), proportionally induced the CYP2B1/2B2 and UDPGT genes and activated the plasmid (HCB = PB > DDD). CYP2B1/2B2 expression following induction by PB and HCB was subject to identical patterns of inhibition by okadaic acid, cAMP, and GH. Together, these data suggest that PB-like inducers utilize the same polymorphic pathway to affect the same PBRU-activating factors.

Identification of glutathione S-transferase isozymes and gamma-glutamylcysteine synthetase as negative acute-phase proteins in rat liver

Because acute infection and inflammation affect drug metabolism and drug-metabolizing enzymes, the effect of the acute-phase response on the expression of glutathione S-transferase (GST) isoenzymes, glutathione synthesis, and several antioxidant enzymes was investigated. Hepatic expression of GST isozymes, positive and negative acute-phase reactants, and antioxidant enzymes were determined by Northern blotting and hybridization with gene-specific oligonucleotide probes after lipopolysaccharide treatment of rats. Lipopolysaccharide caused the expected acute-phase response as judged by the increased expression of positive and decreased expression of negative acute-phase proteins. The messenger RNA (mRNA) expression of the major hepatic rat GST isozymes A1, A2, A3, M1, and M2 was decreased 50% to 90%. Total hepatic GST activity toward 1-chloro-2,4-dinitrobenzene was also significantly decreased. mRNA expression of gamma-glutamylcysteine synthetase (GCS) large subunit and catalase was reduced by approximately 60%. GCS enzyme activity was also decreased, resulting in a 35% decrease in the hepatic content of reduced glutathione 4 days after lipopolysaccharide challenge. Mn-Superoxide dismutase expression was increased 13-fold, and thioredoxin level was elevated 3-fold after lipopolysaccharide challenge. The expression of all parameters determined returned to near control levels 7 days after treatment. Together, these data show that GSTs and GCS are negative acute-phase proteins and that decreased GCS activity results in a decrease in hepatic glutathione content. Thus, in addition to the phase I drug-metabolizing enzymes known to be decreased during the acute-phase response, some phase II enzymes involved in the elimination of xenobiotics and carcinogens are also decreased.

Dexamethasone protection of rat intestinal epithelial cells against oxidant injury is mediated by induction of heat shock protein 72

Although the therapeutic actions of glucocorticoids are largely attributed to their anti-inflammatory and immunosuppressive effects, they have been implicated in enhancing tissue and cellular protection. In this study, we demonstrate that dexamethasone significantly enhances viability of IEC-18 rat small intestinal cells against oxidant-induced stress in a dose-dependent fashion. This protective action is mediated by induction of hsp72, the major inducible heat shock protein in intestinal epithelial cells. Dexamethasone stimulates a time- and dose-dependent response in hsp72 protein expression that parallels its effects on cell viability. Furthermore, the induction of hsp72 is tissue dependent, as nonintestinal epithelioid HeLa cells show differential induction of hsp72 expression in response to the same dexamethasone treatment. Antisense hsp72 cDNA transfection of IEC-18 cells abolishes the dexamethasone-induced hsp72 response, without significantly affecting constitutive expression of its homologue, hsc73. Dexamethasone treatment also significantly induces hsp72 protein expression in rat intestinal mucosal cells in vivo. These data demonstrate that glucocorticoids protect intestinal epithelial cells against oxidant-induced stress by inducing hsp72.

Ischaemia and reperfusion injury of rat liver increases expression of glutathione S-transferase A1/A2 in zone 3 of the hepatic lobule

Effects of ischaemia-reperfusion injury (I/R) of liver on expression of rat glutathione S-transferase (rGST) isoenzymes that metabolize products of oxidative stress were examined. Rats underwent lobar liver ischaemia for 30 min followed by reperfusion. In ischaemic lobes, rGSTA1/A2 transcript levels increased significantly 12 h after I/R (2.94-fold) and protein levels increased significantly at 24 h (1.45-fold); increased transcript levels were also observed in nonischaemic lobes (1.78-fold). Superoxide dismutase prevented I/R and the increases in transcript and protein levels in ischaemic and non-ischaemic lobes. By in-situ hybridization, increases in transcript levels at 6 h were present in zones 2 and 3 of the ischaemic lobes and peaked at 12 h (2.5-fold zone 2, 4.5-fold zone 3). Significant increases in transcript levels also were observed at 24 h in zones 2 (2.0-fold) and 3 (2.9-fold) of non-ischaemic lobes. Nuclear run-off assays showed a 1.8-fold increase in rGSTA1/A2 transcription rates in ischaemic lobes at 3 h. We conclude that I/R causes increased rGSTA1/A2 expression in the zone of the hepatic lobule most susceptible to oxidative injury and that this expression may be an important defence against injury.