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Autoimmune, Cholestatic and Biliary Disease
Article first published online: 11 FEB 2009
Copyright © 2009 American Association for the Study of Liver Diseases
Volume 49, Issue 6, pages 1982–1991, June 2009
How to Cite
Yang, H., Ramani, K., Xia, M., Ko, K. S., Li, T. W.H., Oh, P., Li, J. and Lu, S. C. (2009), Dysregulation of glutathione synthesis during cholestasis in mice: Molecular mechanisms and therapeutic implications. Hepatology, 49: 1982–1991. doi: 10.1002/hep.22908
Mouse hepatocytes were isolated by the Cell Culture Core and pathological sections and stainings were done by the Imaging Core of the USC Research Center for Liver Diseases (P30DK48522).
Potential conflict of interest: Nothing to report.
- Issue published online: 28 MAY 2009
- Article first published online: 11 FEB 2009
- Accepted manuscript online: 11 FEB 2009 12:00AM EST
- Manuscript Accepted: 5 FEB 2009
- Manuscript Received: 12 DEC 2008
- NIH. Grant Numbers: DK45334, AT1576
Glutathione (GSH) provides important antioxidant defense and regulates multiple critical processes including fibrogenesis. There are conflicting literature studies regarding changes in GSH during cholestasis. Here we examined changes in the GSH synthetic enzymes during bile duct ligation (BDL) in mice and how treatment with ursodeoxycholic acid (UDCA) and/or S-adenosylmethionine (SAMe) affects the expression of these enzymes and liver injury. The hepatic expression of glutamate-cysteine ligase (GCL) subunits and GSH synthase (GS) increased transiently after BDL but fell to 50% of baseline by 2 weeks. Nuclear factor-erythroid 2-related factor 2 (Nrf2) trans-activates gene expression by way of the antioxidant response element (ARE), which controls the expression of all three genes. Despite increased Nrf2 nuclear levels, Nrf2 nuclear binding to ARE fell 2 weeks after BDL. Nuclear levels of c-Maf and MafG, which can negatively regulate ARE, were persistently induced during BDL and the dominant proteins bound to ARE on day 14. UDCA and SAMe induced the expression of GCL subunits and raised GSH levels. They increased nuclear Nrf2 levels, prevented c-Maf and MafG induction, and prevented the fall in Nrf2 nuclear binding to ARE. Combined treatment had additive effects, reduced liver cell death, and prevented fibrosis. Conclusion: GSH synthesis falls during later stages of BDL due to lower expression of GSH synthetic enzymes. UDCA and SAMe treatment prevented this fall and combined therapy was more effective on preserving GSH levels and preventing liver injury. (HEPATOLOGY 2009.)
Glutathione (GSH) is the main nonprotein thiol in mammalian cells that has many critical cellular functions, including defending against oxidative stress and modulating cell growth and death.1 GSH synthesis occurs in the cytosol of all mammalian cells by way of two enzymatic steps: the formation of γ-glutamylcysteine from glutamate and cysteine catalyzed by glutamate-cysteine ligase (GCL); and the formation of GSH from γ-glutamylcysteine and glycine catalyzed by GSH synthase (GS).1 GCL, the rate-limiting enzyme, is made up of a catalytic (GCLC) and a modifier (GCLM) subunit.1 Oxidative stress induces a defensive coordinated up-regulation of the GCL subunits and GS.1 Transcription factors that positively regulate the expression of GCL subunits and GS in human and mouse include activator protein-1 (AP-1) and nuclear factor-erythroid 2-related factor 2 (Nrf2).1 Dysregulation of GSH synthesis has been reported to occur during aging, in diabetes mellitus, drug resistant tumors, and endotoxemia.1, 2 Given the importance of GSH in maintaining normal cell function, dysregulation in its synthesis generally worsens the disease process.
Cholestatic liver injury is a major cause of chronic liver disease, which is one of the top 15 causes of death in the United States.3 Chronic retention of toxic bile acids is a major mechanism for cell toxicity by inducing oxidative stress, apoptosis, and fibrosis leading to cirrhosis.3–5 Although there are conflicting data, many groups showed a fall in GSH level in either livers of animals subjected to experimental cholestasis or hepatocytes treated with toxic bile acids.1, 6–8 This further jeopardizes antioxidant defense and contributes to injury. Some6, 7 but not others8 showed a fall in GCL activity. One article reported a fall in both GCLC and GCLM messenger (m)RNA levels.7 Interestingly, ursodeoxycholic acid (UDCA), the only medication approved by the Food and Drug Administration for the treatment of primary biliary cirrhosis,9 a chronic cholestatic disorder, was shown to prevent the fall in GCL expression during chronic cholestasis7 and increase GCL expression in cultured rat hepatocytes.10 The molecular mechanism(s) for changes in GCL expression during cholestasis or in response to UDCA treatment remain unknown. Our current investigation examined changes in the expression of the GSH synthetic enzymes during experimental cholestasis induced by bile duct ligation (BDL). We uncovered a transient increase early on, followed by a dramatic fall in the expression of GSH synthetic enzymes at later stages of cholestasis. We also elucidated the molecular mechanism(s) for the fall in expression of these enzymes and provided new information on improving the treatment of cholestatic liver injury.
Materials and Methods
UDCA was obtained from Sigma-Aldrich (St. Louis, MO). S-adenosylmethionine (SAMe) in the form of disulfate p-toluenesulfonate dried powder was generously provided by Gnosis SRL (Cairate, Italy). α-32P-dCTP and γ-32P adenosine triphosphate (ATP) (3000 Ci/mmol) was purchased from PerkinElmer (Boston, MA). All other reagents were of analytical grade and were obtained from commercial sources.
BDL in Mice.
Three-month old male C57/B6 mice were fed chow ad libitum and housed at a constant temperature (22°C) with alternating 12 hours of light and darkness. The mouse procedure protocols, use, and the care of the animals were reviewed and approved by the Institutional Animal Care and Use Committee at the University of Southern California (USC). Following intraperitoneal injection of ketamine (80 mg/kg) and xylazine (10 mg/kg), the common bile duct was exposed through a midline abdominal incision, ligated in two places with a silk thread, and sectioned between the ligatures. Sham-operated mice had anesthesia and laparotomy only. Mice were sacrificed on days 0, 1, 3, 7, 10, 14, 21, or 28 days postsurgery and livers were harvested for the studies described below.
UDCA and SAMe Treatment in BDL Mice.
UDCA (10 to 200 mg/kg/day in 0.1 mL 2.5% sodium bicarbonate, pH 7.4) was given by gavage once per day. SAMe was given at the dose of 100 mg/kg/day (in 0.1 mL Tris buffer, pH 7.4) by gavage once per day. This dose was chosen as we showed 50 mg/kg of SAMe given intraperitoneally every 12 hours for three doses protected against the lipopolysaccharide-induced fall in GCLC and GCLM expression.2 Following BDL, mice received either vehicle, UDCA, SAMe, or UDCA plus SAMe for up to 28 days.
Necrosis, Apoptosis, and Fibrosis Determination in Liver Specimens.
Formalin-fixed liver tissues embedded in paraffin were cut and stained with hematoxylin and eosin (H&E) for the evaluation of necrosis. The percentage of necrosis was estimated by counting the number of microscopic fields with necrosis compared to the entire section in 15 different sections at 100× magnification. For apoptosis, tissue sections were stained with terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-digoxigenin nick-end labeling (TUNEL) according to the manufacturer's (In situ cell death detection kit, Roche) suggested protocol. Five random fields containing an average of 250 nuclei were counted for each TUNEL-stained tissue sample. The apoptotic index (percentage of apoptotic nuclei) of hepatocytes was calculated as (apoptotic nuclei/total nuclei) × 100%. Samples from at least three mice per treatment condition were scored. Fibrosis was determined by staining with 0.1% Sirius red (Sigma, St. Louis, MO) and quantified using a computer-assisted image analysis system (MetaMorph imaging system; Universal Imaging, Downingtown, PA) and expressed as stained area per total examined area.11
Mouse Hepatocyte Isolation and Apoptosis Determination.
Mouse hepatocytes were isolated by the Cell Culture Core of the USC Research Center for Liver Diseases. Hepatocytes were centrifuged and purified through Percoll, as previously described.12 Hepatocyte viability was detected by Trypan blue exclusion. Apoptosis was determined in hepatocytes isolated from various days following BDL by DNA fragmentation as described.13
RNA Isolation and Gene Expression Analysis.
Total RNA was isolated by the TRIzol reagent (Invitrogen) from liver tissues. Northern blot analysis, autoradiography, and densitometry were done as described.14–16 The mouse specific complementary (c)DNA probes for Northern blot include: GCLC, nucleotides 120-610 (NM_010295); GCLM, nucleotides 411-905 (NM_008129); GS, nucleotides 181-695 (NM_008131); c-Fos, nucleotides 243-791 (NM_010234); c-Jun, nucleotide 957-1441 (NM_010591); p50, nucleotide 541-981 (NM_008689); p65, nucleotide 541-1051 (NM_009045); Nrf1, nucleotides 301-847 (NM_010938); and Nrf2, nucleotides 251-1020 (NM_010902). Specific GCLC, GCLM, GS, c-Fos, c-Jun, p50, p65, Nrf1, Nrf2, and β-actin probes were labeled with [32P]dCTP using a random-primer kit (RediPrime DNA Labeling System; Amersham Pharmacia Biotech) as described.14–16 Results of Northern blot analysis were normalized to β-actin.
Quantitative real-time polymerase chain reaction (PCR) was used to measure collagen type I, alpha 2 (Col1A2) mRNA levels as described.2 The primers and TaqMan probes for Col1A2 and Universal PCR Master Mix were purchased from ABI (Foster City, CA). 18SrRNA was used as housekeeping genes as described.2 The delta Ct obtained was used to find the relative expression of Col1A2 according to the formula: relative expression = 2-ΔΔCt, where ΔΔCt = ΔCt of Col1A2 in experimental groups − ΔCt of Col1A2 in the control group.
Western Blot Analysis of Liver Tissues and Hepatocytes from BDL Mice.
Liver tissues and hepatocytes isolated from BDL mice were subjected to Western blot analysis as described.15, 16 Nuclear protein was isolated from liver tissues as described.16, 17 Equal amounts of total protein extracts (15 μg/well) were resolved on 12.5% sodium dodecyl sulfate (SDS)-polyacrylamide gels. Membranes were probed with antibodies to GCLC, GCLM (Novus Biologicals, Littleton, CO), GS, c-Jun, c-Fos, p50, p65, Nrf1, Nrf2, c-Myc, c-Maf, MafB, MafF, Bach1 (Santa Cruz Biotechnology, Santa Cruz, CA), MafK, and MafG (R&D Systems, Minneapolis, MN). To ensure equal loading, membranes were stripped and reprobed with anti-actin or anti-Histone 3 antibodies (Santa Cruz Biotechnology) for total versus nuclear protein levels, respectively. Blots were developed by enhanced chemiluminescence (Millipore, Billerica, MA).
Electrophoretic Mobility Shift Assay (EMSA) and Supershift Assay.
EMSAs were done as described.16 The probe was32P-end-labeled double-stranded antioxidant response element (ARE) DNA fragment (CTGGAAGACAATGACTAAGCAGAAA), corresponding to −315 to −339 basepairs relative to the translation start codon of mouse GCLM (NM_008129), with the core ARE sequence underlined. Supershift assays confirmed the identity of the binding proteins using antibodies to Nrf1, Nrf2, c-Maf, MafK, MafF, MafB, MafG, or Bach1 (Santa Cruz Biotechnology or R&D Systems) as described.16
Serum Alkaline Phosphatase (ALP), Bilirubin, and Alanine Transaminase (ALT) Levels.
Serum ALP, bilirubin (Thermo Electron, Waltham, MA), and ALT (RAICHEM, San Marcos, CA) levels were measured following the manufacturers' instruction.
Data are given as mean ± the standard error of the mean (SEM). Statistical analysis was performed using analysis of variance followed by Fisher's test for multiple comparisons. For changes in mRNA and protein levels, ratios of genes or proteins to housekeeping genes or proteins densitometric values were compared. Significance was defined as P < 0.05.
Changes in the Expression of GSH Synthetic Enzymes During BDL.
The mRNA (Fig. 1A) and protein (Fig. 1B) levels of hepatic GCLC, GCLM, and GS exhibited a transient increase from day 1 to day 7 after BDL, followed by a fall to below baseline by day 14 that persisted to day 28. To see if the change in liver expression of these genes is occurring in hepatocytes, hepatocytes were isolated after BDL and purified by centrifugation through Percoll. After Percoll purification, 75%-95% of the isolated hepatocytes were viable as estimated by Trypan blue staining. Figure 1C shows that the protein levels of GCLC, GCLM, and GS in hepatocytes exhibit a very similar pattern as whole liver with an early increase followed by a fall to 50% of baseline or lower.
Expression of Transcription Factors Important for Expression of GSH Synthetic Enzymes During BDL.
We next examined changes in the expression of transcription factors important in the regulation of GCLC, GCLM, and GS, namely, Nrf1, Nrf2, AP-1, nuclear factor kappa B (NFκB), and c-Myc.1, 14–17 Figure 2A shows that c-Fos and c-Jun mRNA levels increased rapidly but returned toward baseline by day 14. NFκB family members p50 and p65 mRNA levels also increased following BDL but peaked later and remained elevated. Nrf1 mRNA levels remained unchanged throughout the BDL time course, whereas Nrf2 mRNA levels increased later around day 7 but were back to baseline by day 10. Although Nrf1 and Nrf2 showed little change in mRNA levels, the nuclear protein levels were increased from day 1 to day 14 and fell back to baseline from day 21 to 28 (Fig. 2B). c-Myc protein levels increased by day 3 and remained elevated throughout the 28 days. Protein levels of c-Fos, c-Jun, p50, and p65 largely changed in parallel to their mRNA levels (Fig. 2B).
ARE Binding Activity During BDL.
One of the most important cis-acting elements present in the promoter of all three genes is ARE.1 Figure 3A shows that nuclear binding activity to ARE increased rapidly from day 1 to day 7. However, it fell back to baseline by day 10 and was below baseline from day 14 to day 28. At day 3, both Nrf1 and Nrf2 binding to the ARE increased (Fig. 3B) but Nrf2 binding in particular fell markedly by day 14 (Fig. 3C).
Expression and ARE Nuclear Binding Activity of Mafs and Bach1.
Nrf2 positively regulates ARE-mediated gene expression.18 However, the effect of its binding partner has been controversial. Both activation and repression of ARE-mediated gene expression have been reported with heterodimers of Nrf2 and small Maf proteins (MafG, MafK, and MafF).18 Furthermore, homodimers of these small Mafs have also been shown to repress ARE-mediated gene expression.18 In addition, c-Maf, a large Maf protein, and Bach1, a transcription factor that belongs to the cap “n” collar (CNC), b-Zip family, can also repress ARE-mediated gene expression.19, 20 We next examined the expression of these proteins and Fig. 4A shows that the nuclear protein levels of c-Maf, MafK, and MafG increased from day 1 and reached 3- to 4-fold of baseline by day 14 and remained elevated up to day 28. Nuclear protein levels of MafF and Bach1 were essentially unchanged during BDL (Fig. 4A). In terms of proteins that bind to ARE, on day 3 after BDL, whereas small Mafs, c-Maf, and Bach1 were all binding to ARE, the dominant protein bound was Nrf2 (Fig. 4B). On day 14 after BDL, this picture changed so that Nrf2 was a relatively minor protein bound to ARE, whereas the relative abundance of c-Maf and MafG bound to ARE increased (Fig. 4C).
Effects of UDCA and SAMe Treatment on Expression of GSH Enzymes and GSH Levels During BDL.
Apoptosis occurs during BDL21 and UDCA is known to protect against apoptosis induced by toxic bile acids.22 We verified that this occurs in our model, as Fig. 5A shows that apoptosis occurs by day 3 in hepatocytes isolated from BDL mice and UDCA administration exerted a dose-dependent protection against apoptosis so that no apoptosis was evident as measured by DNA fragmentation at 100 mg/kg/day dose and this was the dose used for all subsequent studies. UDCA administration increased the baseline GCLC and GCLM mRNA levels (compare sham groups in Fig. 5B) and prevented the fall during BDL (Fig. 5B). The ability of UDCA to enhance the expression of GSH synthetic enzymes may or may not have anything to do with its antiapoptotic effect noted on day 3. Recently we reported that SAMe administration prevented the fall in GCLC and GCLM mRNA levels during endotoxemia.2 We next compared administration of UDCA or SAMe alone or together during BDL. At 14 days after BDL, the protein levels of GCLC and GCLM were about 50% of baseline. Both UDCA and SAMe prevented the fall in GCLC and GCLM protein levels (in fact, they were higher than the sham control group) but together exerted an additional effect (more than 300% of control) (Fig. 5C). Hepatic GSH levels changed in parallel with changes in GCL. Hepatic GSH levels fell to 50% and 22% of baseline at 14 and 28 days after BDL, respectively, and either UDCA or SAMe treatment alone raised the GSH level above baseline, whereas combination treatment raised the level even higher (Fig. 5D).
Effects of UDCA and SAMe Treatment on Expression of Mafs, Nrf2, and ARE Binding.
To see if UDCA and SAMe affect the expression of GSH synthetic enzymes by way of ARE binding, we examined the nuclear protein levels of Mafs and Nrf2 on day 28 of BDL. Figure 6A shows that BDL with or without UDCA or SAMe treatment alone or in combination had little effect on the expression of MafF, MafB, or Bach1. However, the induction of c-Maf, MafG, and MafK following BDL was suppressed by treatment with UDCA or SAMe. For all three, UDCA or SAMe treatment alone suppressed partially, but combined therapy completely prevented induction (Fig. 6A). Interestingly, whereas BDL had no effect on the nuclear level of Nrf2 on day 28, UDCA or SAMe treatment alone raised its level by about 60%-70%, and when combined, raised its level by over 200%. These changes impacted nuclear binding activity to ARE such that ARE binding did not fall during BDL during UDCA or SAMe treatment and, most important, the fall in Nrf2 binding to ARE during the later stage of BDL was completely prevented (Fig. 6B,C).
Effects of UDCA and SAMe Treatment on Liver Injury, Hepatic Cell Death, and Fibrosis.
To see how UDCA and SAMe treatment during BDL affect liver injury, serum biochemical profiles and liver immunohistochemistry were examined on day 28. UDCA or SAMe treatment alone did not significantly lower serum ALT levels but the combination significantly lowered the ALT levels (Table 1). Similarly, either treatment did not significantly lower serum ALP or bilirubin levels, but combined treatment did significantly lower both as compared to the BDL group (Table 1). UDCA and combined treatment significantly lowered necrosis, whereas all treatments lowered apoptosis and the amount of liver fibrosis as determined by Sirius red stain (Fig. 7, Table 1). Still, for each of these variables, combined treatment tended to be more effective than either treatment alone. Consistently, hepatic Col1A2 mRNA levels in the combined treatment group showed no increase when compared to the sham control group (Table 1).
|Sham||BDL||UDCA||SAMe||UDCA + SAMe|
|ALT (Units/L)||38 ± 2||239 ± 25*||194 ± 14*||217 ± 15*||179 ± 13†|
|ALP (Units/L)||95 ± 16||728 ± 22*||619 ± 69*||639 ± 59*||426 ± 35†|
|Bilirubin (μmol/L)||15 ± 2||219 ± 19*||173 ± 17*||185 ± 14*||152 ± 16†|
|Necrosis (% of total)||0 ± 0||33.3 ± 1.2*||27.0 ± 2.4†||29.9 ± 2.0*||21.9 ± 1.3†‡|
|Apoptosis (% of total)||0.9 ± 0.1||15.1 ± 1.6*||6.8 ± 1.1†||7.9 ± 0.9†||5.3 ± 1.0†|
|Sirius Red (% of total)||0.5 ± 0.04||7.2 ± 0.3*||5.1 ± 0.5†||5.3 ± 0.4†||4.3 ± 0.3†|
|Liver mRNA level:|
|Col1A2 (% of sham)||100 ± 0||551 ± 163§||264 ± 55§||127 ± 18†||94 ± 21†|
Although all mammalian cells synthesize GSH, the liver has one of the highest organ contents of GSH and plays a central role in the interorgan homeostasis of GSH.23 In addition to its important roles in antioxidant defense and detoxification, GSH also modulates cell death, growth, inflammatory response, and even hepatic fibrogenesis (reviewed in1. Regarding fibrogenesis, recent studies show that transforming growth factor-β1, a potent profibrogenic factor, suppressed the expression of GCLC and lowered GSH levels in rat hepatic stellate cells (HSCs), key effectors in hepatic fibrogenesis.24 Epigallocatechin-3-gallate and curcumin were shown to block activation of HSCs by a mechanism that required raising GSH levels.24, 25 Depletion in hepatic GSH levels is well recognized in patients with liver disease and has been largely attributed to nutritional deficiency and oxidative stress. Our current work shows that GSH synthesis becomes dysregulated and impaired during chronic cholestasis. This realization has important implications for designing therapy in chronic cholestatic disorders such as primary biliary cirrhosis, which at present is limited to UDCA.9 UDCA's efficacy in other cholestatic disorders remains unproven and whether other agents can enhance the efficacy is unclear.26 The goals for this work were twofold: to elucidate the molecular mechanisms responsible for the impairment in GSH synthesis and to see how treatment may be improved.
During BDL, hepatic GCLC, GCLM, and GS mRNA and protein levels were transiently increased early on, followed by a fall to about 50% of baseline levels. This pattern may explain why there are conflicting data regarding hepatic GSH levels and expression of GSH synthetic enzymes during experimental cholestasis.1, 6–8 Given the fact that the magnitude of change in mRNA levels and protein levels are so similar, we reasoned that the changes are occurring at the mRNA level, either transcriptional or posttranscriptional. GCL subunits and GS are mainly regulated transcriptionally, particularly in response to oxidative stress.1 We therefore examined the expression of transcription factors that are known to regulate their expression. In human and rodents, the most important factor is Nrf2, by way of trans-activation of the ARE.1 In addition, AP-1, NFκB, and c-Myc have also been shown to positively regulate the coordinated expression of these genes.1, 14–16, 27 In the first 3 days after BDL, the nuclear protein levels of c-Fos, c-Jun, Nrf1, Nrf2, and c-Myc increased. These are likely to contribute to the up-regulation of all three genes. NFκB subunit protein levels increased later during BDL, thus are unlikely to participate in the early increase in gene expression. The induction in GSH synthetic enzymes at the early timepoint represents an adaptive defense response to oxidative stress that is well characterized at this timepoint.5, 7, 22 Indeed, Nrf2 activation and induction of GCLC and GCLM expression occurred in vitro in hepatocytes treated with toxic bile acids.28
Later in the course of BDL (day 28), the hepatic mRNA and protein levels of GCLC, GCLM, and GS fell to 50% of baseline, despite the fact that the nuclear protein levels of the transcription factors important in maintaining their expression, namely, Nrf1, Nrf2, NFκB, AP-1, and c-Myc, were essentially unchanged (Nrf1, Nrf2, c-Fos, c-Jun) or increased (c-Myc, p50, and p65). Because all three genes have functional AREs,1 we next investigated nuclear ARE binding and found a transient increase in ARE nuclear binding activity followed by a fall to below baseline by day 14 after BDL, paralleling changes in the expression of the GSH synthetic enzymes. Despite increased nuclear Nrf2 levels (189% of baseline) on day 14 after BDL, the binding activity of Nrf2 to ARE was markedly impaired. This prompted us to examine factors known to influence Nrf2 binding activity to the ARE.
Nrf1 and Nrf2 are members of the CNC-bZIPs that can trans-activate ARE.18 Nrf2 is kept in the cytosol by Keap1 under nonstressful conditions and undergoes proteosomal degradation.29 Upon recognition of signals imparted by oxidative and electrophilic molecules, Nrf2 is released from Keap1, escapes proteosomal degradation, and translocates to the nucleus to induce genes involved in defense and survival.29 Although Nrf1 is similar to Nrf2 structurally, Nrf1's activity is not controlled by Keap1.30 Instead, Nrf1 is primarily localized to the membrane of the endoplasmic reticulum and is released and translocates to the nucleus during endoplasmic reticulum stress.30 Nrf1 and Nrf2 bind to ARE but not as homodimers or heterodimers with each other.18 Instead they require another b-ZIP protein to heterodimerize for binding to ARE. June (c-Jun, Jun-D, and Jun-B) and small Maf (MafG, MafK, and MafF) proteins that have been shown to heterodimerize with both Nrf1 and Nrf2 and activate ARE-mediated gene expression.18 However, there is controversy, as both activation and repression have been reported with heterodimers of Nrf2 and small Maf proteins.18 Interestingly, small Mafs can form homodimers, which can repress ARE-mediated gene expression.18, 31 Overexpression of MafG and MafK were shown to negatively regulate ARE-mediated gene expression.31 Although the small Mafs do not have transcriptional activation domain, they have emerged as crucial regulators of mammalian gene expression.32 These genes are under complex control, both transcriptional and posttranslational, and are responsive in particular to stressful stimuli.32 In addition to the small Maf proteins, another subgroup of Maf proteins are the large Mafs, including c-Maf and MafB.32 The large Mafs are major regulators of tissue-specific gene expression and cellular differentiation in mammals.32 Interestingly, c-Maf can bind to ARE as homodimers and heterodimers with small Maf, but not with Nrf2, to repress ARE-mediated gene expression.19 Another protein that can negatively regulate ARE is Bach1, which also belongs to the CNC, b-ZIP family of proteins.20 Overexpression of Bach1 was shown to compete with Nrf2 for heterodimerization with small Mafs and repress ARE-mediated gene expression.20 It is clear that ARE binding and trans-activation is under a complex regulatory control that involves Nrfs, Mafs, and Bach1 so that the functional outcome depends on the balance of these proteins. To understand what is responsible for the fall in Nrf2 nuclear binding to ARE then requires knowing what happens to these proteins during BDL.
The hepatic nuclear protein levels of c-Maf, MafK, and MafG are markedly increased from day 14 on to day 28 of BDL. This led to a relative switch in abundance of proteins bound to ARE so that early on (day 3) Nrf2 dominated, but later (14 days), c-Maf and MafG dominated over Nrf2. Nuclear protein levels and ARE binding activity of MafF and Bach1 remained essentially unchanged during BDL. Taken together, these results suggest that the high expression of c-Maf and MafG competed against Nrf2 for ARE binding and is at least partially responsible for the down-regulation in GCLC, GCLM, and GS.
Treatment of cholestatic liver injury has remained very limited and there is clearly room for improvement. UDCA is the mainstay of therapy, and consistent a with previous report,7 it is able to prevent the fall in GCLC and GCLM expression. SAMe is a nutritional supplement widely available in the United States but is used therapeutically elsewhere in the world to treat acute cholestatic liver disorders.33 Individually, each agent prevented the fall in GCLC and GCLM expression during BDL. However, most important, combination of UDCA and SAMe exerted an additive effect on GCLC and GCLM expression and GSH levels. This suggests that these agents work by different mechanisms. This is further reinforced when ARE nuclear binding activity was examined with these treatments. UDCA and SAMe either partially or completely prevented the increase in c-Maf and MafG protein levels during BDL and, when combined, the effect was even better. Interestingly, UDCA and SAMe treatment individually also raised Nrf2 nuclear levels and exerted an additive effect when combined. These changes also translated to ARE nuclear binding activity so that, individually, each prevented the fall in Nrf2 binding to ARE but, when combined, raised Nrf2 binding to ARE further. Taken together, these results support an important role of the changes in Mafs and Nrf2 expression and ARE binding as the cause of changes in the expression of GSH synthetic enzymes during BDL and in the therapeutic effects of UDCA and SAMe.
GSH participates in the regulation of cell death by apoptosis and necrosis as well as fibrosis. In support of this, combined treatment lowered apoptosis, necrosis, and completely prevented fibrosis. Biochemical parameters of injury were also lower with combined treatment. Although both agents exert multiple other effects besides raising GSH levels, it is more than likely that raising the GSH levels can play an important contributory role to these actions. Much more work will be required to elucidate how toxic bile acids, UDCA, and SAMe influence the expression of Mafs and Nrf2.
In conclusion, our study revealed a previously unrecognized impairment in the expression of GSH synthetic enzymes during the late stages of cholestasis. This is due to a dramatic fall in Nrf2 nuclear binding activity to ARE, which positively controls all three genes. UDCA and SAMe prevent this fall by inducing the expression of Nrf2 and suppressing the expression of Maf proteins that can repress ARE. They do so by different mechanisms so that, when combined, they exerted additional protective effects on GSH as well as on liver cell death and fibrosis.
- 3Centers for Disease Control. National Vital Statistics Reports. 2004; 53: 5.
Additional Supporting Information may be found in the online version of this article.
|HEP_22908_sm_SupTab.doc||36K||Supplement Table 1. Densitometric Values for Figure 1 (% of day 0)|
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