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Trauma is a major cause of mortality in the United States. Death among those surviving the initial insult is caused by multiple organ failure (MOF) with the liver among the organs most frequently affected. We previously demonstrated in rodents that trauma complicated by hemorrhagic shock (trauma/HS) results in liver injury that can be prevented by IL-6 administration at the start of resuscitation; however, the contribution of the severity of HS to the extent of liver injury, whether or not resuscitation is required and the mechanism for the IL-6 protective effect have not been reported. In the experiments reported here, we demonstrated that the extent of liver apoptosis induced by trauma/HS depends on the duration of hypotension and requires resuscitation. We established that IL-6 administration at the start of resuscitation is capable of completely reversing liver apoptosis and is associated with increased Stat3 activation. Microarray analysis of the livers showed that the main effect of IL-6 was to normalize the trauma/HS-induced apoptosis transcriptome. Pharmacological inhibition of Stat3 activity within the liver blocked the ability of IL-6 to prevent liver apoptosis and to normalize the trauma/HS- induced liver apoptosis transcriptome. Genetic deletion of a Stat3β, a naturally occurring, dominant-negative isoform of the Stat3, attenuated trauma/HS-induced liver apoptosis, confirming a role for Stat3, especially Stat3α, in preventing trauma/HS-mediated liver apoptosis. Thus, trauma/HS-induced liver apoptosis depends on the duration of hypotension and requires resuscitation. IL-6 administration at the start of resuscitation reverses HS-induced liver apoptosis, through activation of Stat3α, which normalizes the trauma/HS-induced liver apoptosis transcriptome.
Trauma is the leading cause of death for those under 45 years old in the United States . While almost half of the deaths occur at the time of the injury, the leading cause of death among those surviving the initial insult is multiple organ failure (MOF) [2, 3]. The liver is one of the organs most frequently affected by trauma and hemorrhagic shock, and its central role in metabolism and homeostasis makes this organ a critical one for survival of the host after severe injury [4, 5].
We previously demonstrated in rats and mice that trauma complicated by hemorrhagic shock (trauma/HS) results in liver injury as evidence by hepatocyte apoptosis , liver necrosis  and elevated transaminases ; however, the contribution of the severity of hemorrhagic shock to the extent of liver injury and whether or not resuscitation is required for liver injury to occur have not been reported. We also previously demonstrated that administration of IL-6 at the start of resuscitation prevented liver apoptosis and necrosis [6, 7]. IL-6 activates two anti-apoptotic signaling pathways, one involving Akt and the other involving signal transducer and activator or transcription (STAT)3. Whether or not one or both pathways are involved in the anti-apoptotic effect of IL-6 has not been determined.
In the experiments reported here, we investigated the hypotheses: 1) that trauma/HS-induced liver apoptosis depends on the severity of hemorrhagic shock and requires resuscitation; and 2) that the protective effect of IL-6 administration is mediated by Stat3. We demonstrated that the extent of liver apoptosis induced by our model of trauma/HS depends on the duration of hypotension and requires resuscitation. We established that IL-6 administration at the start of resuscitation following the longest duration of hypotension is capable of completely reversing liver apoptosis and is associated with increased Stat3 activation. Microarray analysis of the livers showed that the main effect of IL-6 was to normalize the trauma/HS-induced apoptosis transcriptome. Pharmacological inhibition of Stat3 activity within the liver blocked the ability of IL-6 to prevent liver apoptosis and to normalize the trauma/HS-induced liver apoptosis transcriptome. Genetic deletion of a Stat3β, a naturally occurring, dominant-negative isoform of the Stat3, attenuated trauma/HS-induced liver apoptosis, confirming a role for Stat3, especially Stat3α, in preventing trauma/HS-mediated liver apoptosis. Thus, trauma/HS-induced liver apoptosis depends on the duration of hypotension and requires resuscitation. IL-6 administration at the start of resuscitation reverses HS-induced liver apoptosis, through activation of Stat3α, which normalizes the trauma/HS-induced liver apoptosis transcriptome.
These studies were approved by the Baylor College of Medicine Institutional Review Board for animal experimentation and conform to National Institutes of Health guidelines for the care and use of laboratory animals. Adult male Sprague-Dawley rats were obtained from Harlan (Indianapolis, IN). Stat3β homozygous-deficient (Stat3βΔ/Δ) mice were generated as described  and re-derived at Jackson labs. Pups from heterozygous matings were tailed and genotyped by PCR, as described, with minor modifications .
Eight-week old male Sprague-Dawley rats (200–250 gm) were used for all experiments in this study. Rats were subjected to the sham or hemorrhagic shock (HS) protocols, as described [10, 11] with modifications. Blood was withdrawn into a heparinized syringe episodically to maintain the target MAP at 35 mmHg until blood pressure compensation failed. Blood was then returned as needed to maintain the target MAP. The amount of shed blood returned (SBR) defined 5 different levels of shock severity reflected in the duration of hypotension: 0% SBR (SBR0) represented the lowest level of shock severity (duration of hypotension, 78 ± 2.5 minutes), 10% SBR (SBR10; duration of hypotension, 149 ± 41.4 minutes), 20% SBR (SBR20; duration of hypotension, 165 ± 32.7 minutes), 35% SBR (SBR35; duration of hypotension, 211 ± 7.6 minutes), and 50% SBR (SBR50; duration of hypotension, 273 ± 24.9 minutes). At the end of the hypotensive period, rats were resuscitated as described [10, 11] and humanely sacrificed 60 minutes after the start of resuscitation. Where indicated, rats received 10 μg/kg of recombinant human IL-6 in 0.1 ml PBS at the initiation of the resuscitation or PBS alone. Sham rats were anesthetized and cannulated for 250 minutes but were not subjected to hemorrhage or resuscitation. One group of rats (UHS) was subjected to the most severe hemorrhagic shock protocol (50% SBR), but not resuscitated and kept at the target MAP (35 mmHg) for an additional 60 minutes (duration of hypotension = 336 ± 10.3 minutes) before sacrifice.
Stat3βΔ/Δ mice and wild-type littermate mice were subjected to a trauma/HS protocol [8, 12], which was similar to the rat protocol except that the target MAP in the mouse was 30mm Hg and the duration of hypotension was 180 min in all mice. Sham mice were anesthetized and immobilized in a pair-wise fashion with HS mice and sacrificed at the same time as their HS companion.
Rat and mouse livers were harvested immediately after sacrifice. The right liver lobe was fixed with paraformaldehyde solution (2%) for histologic analysis and the left lobe was snap frozen in liquid nitrogen for protein and RNA extraction.
To achieve pharmacological inhibition of Stat3 activity within the lungs of rats, rats were randomized to receive by tail vein injection the G-rich, quartet-forming oligodeoxynucleotide (GQ-ODN) T40214 or nonspecific (NS)-ODN (2.5 mg ODN/kg) complexed in polyethyleneimine, as described , 24 hours prior to subjecting them to the SBR50 protocol with IL-6 treatment. The half-life of T40214 in tissues is ≥ 48 hr .
Levels of histone-associated DNA fragments (nucleosomes) were determined in liver homogenates with an ELISA method (Cell Death Detection ELISAplus; Roche Diagnostics, Manheim, Germany). Frozen livers were cut by cryotome into 5 micron sections and resuspended in cell lysis buffer using the reagents from the ELISA kit. The lysates were sonicated in ice 3 times, 10 seconds each, centrifuged and supernatants harvested. Total protein concentration of each supernatant was determined by Bradford assay (Bio-Rad Protein Assay, Bio-Rad Laboratories, Inc., Hercules, CA). Equal amounts of protein (200ug) were loaded into microtiter wells in duplicate. A positive control (lyophilized, stabilized nucleosome concentrate of known concentration, provided in the kit) and a negative control (water) were also loaded in duplicate. Serial dilutions of the positive control were loaded in duplicate and used to plot a standard curve. The nucleosome concentration for each sample was obtained by plotting each sample duplicate's OD against the standard curve. The final sample nucleosome concentration was the average of the duplicates . The rest of the assay was performed according to manufacterer's instructions.
TUNEL staining to enzymatically detect the free 3'-OH termini was performed using the ApopTag Plus Peroxidase in situ Apoptosis Detection Kit from Chemicon International. Slides were rehydrated from Xylene to PBS through a series of decreasing concentrations of ethanol and digested in proteinase K (20 ug/ml) for 3 minutes at 23°C. Endogenous peroxidases were quenched for 30 minutes in 3% hydrogen peroxide in PBS. TdT enzyme was diluted in TUNEL solution buffer then used as suggested by the manufacturer. Slides were counterstained with hematoxyllin. TUNEL positive cells were assessed microscopically by counting the total nuclei and the number of TUNEL-positive nuclei in twenty random 1000x fields by an experienced histologist, blinded to the treatment each rat received. Data is presented as the number of TUNEL positive cells per high power field (hpf).
Levels of STAT3 activation within the livers of rats were assessed by immunoblotting using whole-tissue extracts of liver sections with mouse monoclonal antibody to Tyr705 phosphorylated (p)STAT3 (Cell Signaling Technology, Inc., Danvers, MA; 1:1000 dilution). Briefly, frozen livers were cut by cryotome into 5 micron sections and resuspended in cell lysis buffer (Cell Death Detection ELISAplus Kit, Roche Diagnostics, Manheim, Germany). The supernatant was sonicated in ice 3 times, 10 seconds each. Samples were then centrifuged and the supernatant evaluated by Bradford assay for total protein quantification. Protein samples (60ug total protein) were separated by SDS- PAGE and transferred to a PVDF membrane. The membrane was incubated overnight with mouse monoclonal antibody and subsequently incubated with goat antimouse antibody with horseradish peroxidase (HRP) conjugate (Zymed, San Francisco, CA) for 1 hour. ECL agent (Amersham Biosciences, UK) was used for detection. The membrane was then stripped (using RestoreTM Western Blot Stripping Buffer, PIERCE, Rockford, IL) and immuoblotting performed to detect total STAT3 protein in the whole tissue extracts of livers using mouse IgG1 monoclonal antibody to STAT3 (BD Biosciences, Rockville, MD). Densitometry was performed using ImageQuant TL v2005 software (Amersham Biosciences, Buckinghamshire, England). Results are expressed as the ratio of pSTAT3 signal (after background signal subtraction) to total STAT3 signal (after background signal subtraction) for each sample.
Total RNA was isolated from 4–5 micron cryotome sections of liver using TRIzol® Reagent (Invitrogen, Carlsbad, California) single step RNA isolation protocol followed by purification with RNeasy® Mini Kit (QIAGEN, Hilden, Germany) as instructed by the RAE 230A following Affymetrix protocols used within the Baylor College of Medicine Microarray Core Facility. Gene expression profiling was performed with the Affymetrix Rat Array.
We used Affymetrics GCOS, dChip and Array Analyzer (Insightful Corporation) software packages for quality assessment and statistical analysis and annotation. Expression estimation and group comparisons were done with Array Analyzer. Low-level analyses included background correction, quartile normalization and expression estimation using GCRMA . One-way analysis of variance (ANOVA) with contrasts  was used for group comparisons on all genes and on the list of apoptosis related genes only. P-values were adjusted for multiple comparisons using the Benjamini-Hockberg method . The adjusted p-values represent false discovery rates (FDR) and are estimates of the proportion of “significant” genes that are false or spurious “discoveries”. We used a FDR=10% as cut-off.
Data are presented as mean ± standard error of the mean (SEM). Multiple group comparisons of means were done by one-way analysis of variance (ANOVA). Post hoc analysis was done by Student-Newman-Keuls test for 2-group comparisons of means. Correlation between duration of hypotension and nucleosome levels was done for each individual study animal by Pearson correlation coefficient. Goodness of fit was evaluated by R-square. All statistical analyses were done on SigmaStat 3.5 (SYSTAT Software Inc., Chicago, IL).
To confirm our previous findings that trauma/HS induces liver apoptosis, to determine if apoptosis is an early event following trauma/HS as well as to evaluate the contribution of the severity of shock in our rat model of trauma/HS, we measured histone-associated DNA fragments (nucleosomes) in the livers of rats subjected to increasing severity of shock 1 hr after the initiation of resuscitation. Nucleosome levels increased exponentially with increasing duration of shock (Pearson correlation coefficient 0.879, p<0.001) with the level of nucleosomes in the SBR50 group (1817.3 ± 105.9 units/ml) achieving a level 13.1 times higher than sham (139 ± 67 units/ml; p<0.001, ANOVA; Figure 1). Thus, trauma/HS-induced liver apoptosis occurs within 1 hr of resuscitation and depends on the severity of shock.
To determine the specific contribution of resuscitation to liver apoptosis, we assessed nucleosome levels as well as number of TUNEL-positive cells in the livers of rats subjected to HS without resuscitation (UHS group) and compared these results with those obtained in the sham and the resuscitated SBR50 groups. The level of nucleosomes in the UHS group (193 ± 36 units/mg total protein) was statistically indistinguishable from that of the sham group (139 ± 67 units/mg total protein; Figure 2A). Similar results were obtained when liver apoptosis was assessed by TUNEL staining. The number of TUNEL- positive nuclei/hpf in the UHS group (1.4 ± 0.5; Figure 2B, C) similar to that of the sham group (0.9 ± 0.4). In addition, histological evaluation of the cells containing TUNEL-positive nuclei revealed that more than 80% were hepatocytes, a key parenchyma cell. Thus, apoptosis within the liver following trauma/HS requires resuscitation. The fact that no liver apoptosis occurs without resuscitation suggests that complete prevention of liver apoptosis may occur with an appropriate intervention introduced at the start of resuscitation.
In our mouse model of HS, we have previously demonstrated that IL-6 administration at the beginning of resuscitation prevented the development of HS-induced liver apoptosis detected 24 hrs after HS . To confirm these findings and to gain an improved molecular and cellular understanding of the anti-apoptotic effects of IL-6, we measured apoptotic cell death in rats subjected to trauma/HS with the most severe HS protocol (50% SBR) and randomly assigned to receive either PBS (SBR50) or IL-6 (10 μg/kg, SBR50/IL-6) at the beginning of resuscitation. Nucleosome levels in the SBR50/IL-6 group (264 ± 36 units/ml) were decreased 3.3 times compared to those of the SBR50 group (874 ± 127 units/ml, p<0.001) and were similar to sham levels (139 ± 67 units/ml; Figure 2A). TUNEL staining confirmed these results. The number of TUNEL-positive nuclei/hpf in the SBR50/IL-6 group (1.9 ± 0.5) was decreased 14.2 times compared to the SBR50 group (27 ± 3.6, p<0.001), to levels statistically similar to those of the sham group (0.9 ± 0.4; Figures 2B and 2C). Thus, IL-6 administration at the beginning of resuscitation prevents trauma/HS-induced liver apoptosis occurring 1 hr after trauma/HS in rats as well as 24 hrs after trauma/HS in mice .
IL-6 binding to IL-Rα and gp130 results in gp130 dimerization and phosphorylation of gp130-associated protein-tyrosine kinases Jak1, Jak2, and Tyk2, which is followed by activation of two major signaling pathways within cells—Stat3 and SHP-2/Grb-2/ERK . The SHP-2/Grb2/ERK pathway bifurcates resulting in activation of p38MAPK and PI-3K. Stat3 and P-I3K/Akt activation, but not p38MAPK, link to anti-apoptotic effects within cells. Stat3 mediates its anti-apoptotic effect in cancer cells through its ability to up-regulate anti-apoptotic genes such as Bcl-xL, Bcl-2 and Mcl-1 . Akt is a highly promiscuous kinase with a large number of binding partners and targets  that posttranslationally modify transcription factor systems such as Forkhead [22, 23], IκB/NF-κB and cyclic AMP response element binding protein (CREB) , which together result in increased transcription of survival genes and decreased transcription of apoptotic genes . To assess if the anti-apoptotic effects of IL-6 in the liver is mediated by Stat3 activation, we first determined if Stat3 is activated in the livers of rats resuscitated with IL-6. Extracts of cryotome sections of the liver harvested 1 hour after IL-6 treatment were examined by immunoblotting with mouse monoclonal antibody to Tyr705 phosphorylated (p)Stat3 (Figure 3A). Densitometric analysis of the signal intensity of the pStat3 bands normalized for total Stat3 indicated that Stat3 activity is increased 1.7 fold in the livers of IL- 6-treated rats compared to placebo-treated rats (p=0.002, ANOVA; Figure 3B).
To further evaluate the role of Stat3 downstream of IL-6 in mediating its anti-apoptotic effects in the liver, we examined whether or not these effects of IL-6 could be reversed by pretreatment of rats with a G-rich oligodeoxynucleotide, G-quartet (GQ)-ODN, T40214, a novel Stat3 inhibitor, that forms a rigid G-quartet structure within cells and inhibits the growth of tumors in which Stat3 is constitutively activated [13, 14, 26, 27]. Rats were treated in a blinded fashion with GQ-ODN (SBR50/IL-6/G group) or nonspecific (NS) ODN (SBR50/IL-6/N group) 24 hours prior to being subjected to HS and resuscitation with IL-6. Pre-treatment with GQ-ODN reduced Stat3 activity within the livers of HS/I/G rats 1.9-fold compared to HS/I/N rats (Figures 3A, B). Importantly, the inhibition of Stat3 activation within the livers of the SBR50/IL-6/G rats was accompanied by a return of nucleosomes (1556 ± 241 units/ml) to levels similar to those of the placebo treated (SBR50) group (1874 ± 127 units/ml, p>0.05) and 11.2 fold higher that those of the IL-6 treated (SBR50/IL-6) group (264 ± 36 units/ml, p<0.001; Figure 2A). Similarly, the number of TUNEL-positive nuclei/hpf in livers of rats from the SBR50/IL-6/G group (12.3 ± 1.1) was 6 fold higher than that of the SBR50/IL-6 group (1.9 ± 0.5, p<0.0001); Figures 2B and C). Nucleosome levels and number of TUNEL- positive nuclei/hpf in livers of rats pre-treated with NS-ODN were indistinguishable from those of the SBR50/IL-6 group (Figures 2A and B). Thus, pharmacological inhibition of Stat3 using GQ-ODN in rats subjected to severe HS resuscitated with IL-6 markedly attenuated IL-6- mediated Stat3 activation and prevention of liver apoptosis.
Two isoforms of Stat3 are expressed in all cells—α (p92) and β (p83)—both derived from a single gene by alternative mRNA splicing with Stat3α predominating . Stat3α functions as an oncogene  in part through inhibiting apoptosis, while Statβ antagonizes the oncogenic function of Stat3α . While mice deficient in both isoforms of Stat3 are embryonic lethal at day 6.5 to 7  and mice deficient in Stat3α die within 24 hr of birth, mice deficient in Stat3β have normal survival and fertility . To further support the hypothesis that Stat3, in particular Stat3α, contributes to resistance to apoptosis within the liver in the setting of HS, we subjected Stat3β homozygous-deficient (Stat3βΔ/Δ) mice and their littermate control wild type mice to a severe HS protocol (target MAP 30 mm Hg for 5 hr) and examined their livers for nucleosome levels 1 hr after the start of resuscitation. As expected, nucleosome levels in wild type HS mice (1027.3 ± 273.3 mU/mg total protein) were increased compared to wild type sham mice (210.3 ± 29.8; p < 0.01; Figure 4B). In contrast, however, nucleosome levels in the livers of Stat3βΔ/Δ HS (463.9 ± 3.9) mice were reduced 2.2 times compared to wild type HS mice and were similar to wild type sham mice (Figure 4B). These findings indicate that Stat3, in particular Stat3α, protects the liver from apoptosis in the setting of trauma/HS.
In addition to increasing the transcription of anti-apoptotic genes (Bcl-xL, Bcl-2, and Mcl-1) [29, 32–39], Stat3 has been shown to decrease transcription of pro-apoptotic genes (Bad, Bnip3l, Casp3). To evaluate the role of Stat3 downstream of IL-6 at the transcriptome level, and to identify genes altered within the livers of animals subjected to trauma/HS especially those involved in apoptosis in a global and unbiased manner, we performed Affymetrix oligonucleotide microarray analysis with RAE 230A chips. Fifteen chips were hybridized using mRNA isolated from 4 livers each from sham, SBR50, and SBR50/IL-6 groups, and 3 livers from SBR50/IL-6/G groups. All fifteen chips were included in the normalization and expression estimation steps of the analysis and were included in the statistical analysis and differential expression comparison. The 15,866 probesets on the RAE 230A chip represent 9,818 annotated genes or expressed sequence tags, including 860 apoptosis-related genes. The list of 860 apoptosis-related genes present on the RAE 230A (Table 1) was created by combining gene lists obtained by querying annotation databases provided in GeneSpring and dChip, which were derived from the Gene Ontology (GO) Consortium.
To identify genes differentially expressed among the experimental groups, the data were filtered to remove genes with nearly uniformly low expression (absent on ≥ 80% of chips). Of the 860 apoptosis-related genes represented on the chips, 731 genes met the requirement of this filtering process and were included in the analysis. One-way ANOVA (see Materials and Methods) was then performed which identified 350 apoptosis genes with differential expression among four experimental groups– sham, SBR50, SBR50/IL-6, and SBR50/IL-6/G– at a False Discovery Rate (FDR) = 10% (Table 2). Of the 350 apoptosis pathway genes whose expression was altered among the four groups, 311 were altered in the SBR50 vs. sham comparison (Figure 5A and Table 2). Among the genes whose differential expression was altered in the SBR50 vs. sham comparison, the transcripts of the majority of these genes (193 genes) were increased in SBR50 vs. sham by 4.2 ± 1.2 fold (range = 1.02 to 92.3 fold) while transcripts of 118 genes were decreased in SBR50 vs. sham by 1.5 ± 1.2 fold (range = 1.1 to 3.3 fold; Figure 5B). Importantly, 106 of the 193 genes (55%) that were increased in the SBR50 vs. sham group, were decreased significantly in the SBR50/IL- 6 vs. SBR50 group by 1.8 ± 0.9 fold (range = 1.3 to 14.3 fold) and 108 of the 118 genes (92%) that were decreased in SBR50 group, were increased significantly in the SBR50/IL-6 group by 1.4 ± 0.7 fold (range = 1.2 to 2.9 fold; Figure 5B). Thus, of the genes whose transcript levels were altered in SBR50 vs. sham group, 214 of 311 (69%) returned to sham level or were “normalized” in the SBR50/IL-6 group.
One hundred and twenty-five of the 214 genes altered in the SBR50 vs. sham comparison, and normalized in the SBR50/IL-6 vs. SBR50 comparison, were also altered in the SBR50/IL-6 vs. SBR50/IL-6/G comparison. Fifty seven of these 125 genes (46%) were altered in the opposite direction as the SBR50/IL-6 vs. SBR50 comparison, consistent with the hypothesis that IL-6 normalizes the trauma/HS-induced lung apoptosis transcriptome in part through activation of Stat3.
Apoptosis-related genes consist of those encoding proteins that prevent apoptosis (anti-apoptotic genes) and those encoding proteins that induce apoptosis (pro-apoptotic genes). To identify candidate apoptosis-related genes whose altered expression caused trauma/HS- induced AEC apoptosis, we focused on anti-apoptotic genes whose transcript levels were decreased by trauma/HS and on pro-apoptotic genes whose transcript levels were increased by trauma/HS. Among the genes differentially expressed in the SBR50 vs. sham comparison, 69 anti-apoptotic genes were decreased and 90 pro-apoptotic genes were increased (Figure 5C). Expression levels of 65 out of 69 (94%) anti-apoptotic genes decreased by trauma/HS were increased by IL-6 treatment; 76 of 90 (84%) of the pro-apoptotic genes that were increased by trauma/HS, were decreased by IL-6 treatment. Finally, the expression of 46% of anti-apoptotic genes increased by IL-6 treatment were decreased by pre-treatment with T40214; conversely 63% of pro-apoptotic genes decreased by IL-6 treatment were increased by T40214 pre-treatment (Figure 5C).
Pro-apoptotic genes whose expression was increased ≥ 6-fold by trauma/HS (≥2.5-fold upregulated) were EGL nine homolog 3 (Egln3; 17.3-fold), dual specificity phosphatase 6 (Dusp6; 15.5-fold), tumor necrosis factor receptor superfamily 12a (Tnfrsf12a; 9.8- fold), interferon regulatory factor 1 (Irf1; 9.3- fold), oxidized low density lipoprotein receptor 1 (Oldlr1; 9.0-fold), growth arrest and DNA- damage-inducible 45 alpha (Gadd45a; 6.3- fold), and BCL2 adenovirus E1B interacting protein 3 (Bnip3; 6.1-fold, Table 2). The expression of each was decreased in the IL-6- treated group by 1.7–14.3 fold (Table 2).
In these studies, we demonstrated that trauma/HS-induced liver apoptosis occurs within 1 hr of resuscitation from trauma/HS and depends on the severity of shock with the degree of apoptosis increasing exponentially with increased duration of shock. There was an absolute requirement for resuscitation in order for apoptosis to occur; the finding that no apoptosis occurred in the absence of resuscitation suggested that complete prevention could be achieved by an appropriate intervention introduced at the start of resuscitation. IL-6 administration at the start of resuscitation completely prevented trauma/HS-induced liver apoptosis and was accompanied by increased levels of Stat3 activity within the liver. Pharmacological inhibition of Stat3 using the G-quartet oligodeoxynucleotide (GQ-ODN) T40214 markedly attenuated IL-6-mediated Stat3 activation and prevention of liver apoptosis. Mice deficient in Stat3β, an endogenous naturally occurring, dominant negative isoform of Stat3, were protected from trauma/HS- induced liver apoptosis confirming a role for Stat3, especially Stat3α, in protection against trauma/HS-induced liver apoptosis. Liver microarray analysis showed that 48% of known apoptosis–related genes were altered in trauma/HS. IL-6 “normalized” the expression of 69% of these genes; Stat3 was responsible for this normalization in 46% of the cases. Further examination of the microarray results indicated that the effect of IL-6 in the apoptosis transcriptome was two- fold; IL-6 increased levels of 96% of the anti-apoptotic gene transcripts whose levels were decreased by trauma/HS and also decreased transcript levels of 84% of the pro-apoptotic genes whose levels were increased by trauma/HS.
The liver is susceptible to injury following insults such as hemorrhagic shock. Since it is responsible for maintaining energy homeostasis, hepatic injury and dysfunction associated with hemorrhagic shock can affect other organs and lead to multiple organ failure and death [40–42]. We have previously demonstrated in mice that trauma/HS induces liver apoptosis detected 24 hrs after the resuscitation . In the current study, using our rat model of trauma/HS we found that liver apoptosis occur as early as 1 hour after reperfusion, and for the first time, that its severity depends on the duration of hypotension and requires resuscitation. Liver apoptosis due to ischemia/reperfusion injury has been shown to occur in humans and in experimental models of liver transplantation , as well as during hemorrhagic shock [41, 42]. In these studies, release of ROS during reperfusion was implicated in the pathogenesis of liver apoptosis during reperfusion in models of liver transplantation [42, 43], and may be contributing to liver apoptosis in our model.
Interventions aiming at preventing liver apoptosis have been shown to prevent parenchymal injury and improve animal survival in ischemic injury during liver transplant . We previously demonstrated that exogenous administration of IL-6 decreased liver apoptosis in mice following trauma/HS , however, the mechanism for the anti-apoptotic effects of IL-6 was not determined. Kovalovich et al. provided evidence that IL-6 protects hepatocytes from carbon tetrachloride-induced apoptosis  as well as from Fas-mediated apoptosis  through prevention of rapid degradation of the anti-apoptotic proteins Bcl-2, Bcl-xL and FLIP in the livers. While this may contribute to the anti-apoptotic effect of IL-6 administration in our rat trauma/HS model, our finding that the IL-6 preventive effect was blocked by a specific Stat3 inhibitor, T40214, and could be replicated in mice by genetic deletion of Stat3β, a naturally occurring, dominant-negative isoform of Stat3, indicates that a large portion of the effect of IL-6 in our trauma/HS model is being mediated by Stat3 which acts at the transcriptional and not the post-translational level.
IL-6-mediated activation of Stat3 has been shown to protect against toxin-mediated hepatocyte apoptosis through mechanisms that involve increased levels of anti-apoptotic proteins such as Bcl-xL and Bcl-2 in the liver [46, 47]. Stat3 activation mediates increased transcription of anti-apoptotic genes like Bcl- xL, Bcl-2, and Mcl-1, as well as decreased transcription of pro-apoptotic genes, including Bad, Bnip3 and Casp3 [29, 32–39]. Microarray analysis of the liver apoptosis transcriptome revealed that the expression levels of the pro- apoptotic genes Bnip3 and Casp3 were increased by 6- and 2-fold, respectively, following trauma/HS and were decreased by 5.4- and 4.1-fold respectively after IL-6 treatment. However, only the expression of Casp3 was increased by Stat3 pharmacological inhibition with T40214. None of the other apoptosis-related genes known to be modulated by Stat3 to promote apoptosis protection in other settings of liver injury were affected by IL-6 treatment or by pre-treatment with T40214, suggesting that other apoptosis-related genes are involved in the anti-apoptotic effects of IL-6-mediated Stat3 activation in our model of trauma/HS.
A complete assessment of the liver apoptosis transcriptome using oligonucleotide microarray analysis determined that trauma/HS altered the expression of 48% of apoptosis-related genes, of which 56% were anti-apoptotic and 44% were pro-apoptotic genes (Figure 5B). The overall effect of trauma/HS in the liver apoptosis transcriptome was to increase the expression of apoptosis-related genes. IL-6 administration induced the opposite effect by decreasing the expression of apoptosis-related genes whose expression was increased by trauma/HS, and by increasing the expression of apoptosis-related genes whose expression was decreased by trauma/HS (Figure 5C), suggesting a “normalizing” effect of IL-6 in the trauma/HS-induced liver apoptosis transcriptome. Stat3 inhibition, by GQ-ODN administration, reversed the IL-6 “normalizing” effect on gene expression in 46% of the apoptosis-related genes (Figure 5C). These results suggest that IL-6 administration promotes protection from trauma/HS-induced liver apoptosis by opposing the effects of trauma/HS in the liver apoptosis transcriptome in part by Stat3 activation. Anti- and pro-apoptotic subsets of genes were analyzed separately to determine the effect of trauma/HS in each subset of transcripts. Gene transcript levels in both, anti- and pro- apoptotic subsets were increased by trauma/HS. Interestingly, IL-6 normalized the expression of genes in both subsets of genes, an effect that was reversed by pre-treatment with Stat3 inhibitor (Figure 5C).
In our model of trauma/HS, pro-apoptotic genes upregulated by trauma/HS are likely to be mediators of trauma/HS-induced liver apoptosis, which is likely prevented by the IL-6 normalizing effect on their expression through Stat3 activation. Among the transcripts most up-regulated following trauma/HS and downregulated in the IL-6 treatment group were Egln3, Dusp6, Tnfrsf12, Irf1, Oldlr1, Gadd45a, and Bnip3. Egln3 is a prolyl hydroxylase which is induced in sympathetic neurons after nerve growth factor withdrawal, and induces apoptosis when overexpressed in pheochromocytoma cells . Dusp6 is a cytosolic phosphatase with pancreatic tumor suppressive properties and mediates apoptosis and cell growth arrest by specifically inactivating extracellular signalregulated kinase (ERK) . Tnfrs12 is known to sensitize melanoma cells to chemotherapy-induced apoptosis . Irf1 is activated by serum from human patients with sepsis, and mediates apoptosis in fetal myocytes . Oldlr1 is expressed in highly vascularized organs such as the lung and placenta, and in endothelial cells, smooth muscle cells, cardiomyocytes and activated macrophages. Oldlr1 is a type II glycoprotein and acts as a receptor for oxidized low-density lipoprotein (ox-LDL). Interaction with ox- LDL induces ROS, reduces NO and activates NFκB. It also increases expression of Bax and decreases expression of Bcl-2. Oldlr1 is known to induce apoptosis of vascular endothelial cells and vascular smooth muscle cells in a model of cardiac ischemia reperfusion . Gadd45g belongs to a family of proteins involved in DNA damage response and cell growth arrest. It is ubiquitously expressed in all normal adult and fetal tissues. Gadd45g activates MTK1 kinase activity in response to environmental stresses leading to apoptosis through the p38/c-Jun kinase pathway. Downregulation of Gadd45g prevents apoptosis of cancer cells .
The protective effects of IL-6 in trauma/HS-induced liver injury were mediated in large measure by Stat3 through its ability to normalize the apoptosis transcriptome. Our findings provide evidence that support the use of IL-6 as a potential therapeutic agent to protect against liver injury and dysfunction by blocking apoptosis early after reperfusion. Such an intervention has the potential to prevent multiple organ failure and improve survival in the setting of severe hemorrhagic shock.
This work was supported, in part, by grant HL07619 (DJT) and T32-HL66991 (AM) from the National Heart, Lung and Blood Institute of the National Institutes of Health and H48839 (AM) from the American Lung Association.