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Potential conflict of interest: Nothing to report.
This work was supported by funds from the Alfonso Martin Escudero Fundation (to J.E.P.), the Deutsche Forschungsgemeinschaft (DFG) (to Y.A.L.), and the National Institute Health (grant nos.: DK56621 and P20AA017067; to S.L.F.). The authors thank Dr. Vijay Shah for his generous donation of TSEC cells.
We have developed a novel model for depleting mouse hepatic stellate cells (HSCs) that has allowed us to clarify their contributions to hepatic injury and fibrosis. Transgenic (Tg) mice expressing the herpes simplex virus thymidine kinase gene (HSV-Tk) driven by the mouse GFAP promoter were used to render proliferating HSCs susceptible to killing in response to ganciclovir (GCV). Effects of GCV were explored in primary HSCs and in vivo. Panlobular damage was provoked to maximize HSC depletion by combining CCl4 (centrilobular injury) with allyl alcohol (AA) (periportal injury), as well as in a bile duct ligation (BDL) model. Cell depletion in situ was quantified using dual immunofluorescence (IF) for desmin and GFAP. In primary HSCs isolated from both untreated wild-type (WT) and Tg mice, GCV induced cell death in ∼50% of HSCs from Tg, but not WT, mice. In TG mice treated with CCl4+AA+GCV, there was a significant decrease in GFAP and desmin-positive cells, compared to WT mice (∼65% reduction; P < 0.01), which was accompanied by a decrease in the expression of HSC-activation markers (alpha smooth muscle actin, beta platelet-derived growth factor receptor, and collagen I). Similar results were observed after BDL. Associated with HSC depletion in both fibrosis models, there was marked attenuation of fibrosis and liver injury, as indicated by Sirius Red/Fast Green, hematoxylin and eosin quantification, and serum alanine/aspartate aminotransferase. Hepatic expression of interleukin-10 and interferon-gamma was increased after HSC depletion. No toxicity of GCV in either WT or Tg mice accounted for the differences in injury. Conclusion: Activated HSCs significantly amplify the response to liver injury, further expanding this cell type's repertoire in orchestrating hepatic injury and repair. (HEPATOLOGY 2013)
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Hepatic stellate cells (HSCs) are well-characterized nonparenchymal cells of the liver with established roles in fibrosis, repair, and immunity.1 During liver injury, quiescent HSCs undergo activation, secreting a repertoire of molecules involved in cell proliferation, chemotaxis, inflammation, and fibrosis, among others. Although their role in fibrogenesis is well established, the contributions of HSCs to acute hepatocellular damage and tissue homeostasis are not well understood.
Models to manipulate HSC function or number offer an appealing strategy to clarify this issue. However, only two models have been established to deplete HSCs in vivo thus far, by using gliotoxin2 or gliotoxin-coupled antibodies (Abs) against synaptophysin.3, 4 Gliotoxin reportedly induces selective apoptosis of HSCs by subverting nuclear factor kappa B– mediated survival2 and can reduce fibrosis and enhance resolution in experimental models, especially when targeted using HSC-specific Abs.3-5 However, gliotoxin also has broad actions in vivo and in culture, targeting not only HSCs, but also immune and endothelial cells (ECs) and hepatocytes.5, 6
An alternative strategy is to ectopically express the herpes simplex virus/thymidine kinase (HSV-Tk) gene in target cells, which renders them susceptible to killing by the antiviral agent, ganciclovir (GCV), but only when the cells are proliferating. This possibility was first reported as an anticancer approach7 and further refined8 in murine sarcoma and lymphoma cells, provoking both apoptotic and nonapoptotic cell death.9, 10 The approach has also been reported in liver injury models and in cultured HSCs,11 but has not been used to deplete HSCs in vivo.12
We have exploited this strategy by using mice expressing the HSV-Tk gene driven by the GFAP promoter, which is a marker of HSCs in rodent liver.1 The approach has uncovered a novel, unexpected role for HSCs in amplifying acute liver injury (ALI).
Further information is provided in the Suppprting Materials and Methods.
Seven- to eight-week-old male Gfap-Tk mice (B6.Cg-Tg(Gfap-Tk)7.1Mvs/J; Jackson Laboratory, Bar Harbor, ME) were used for in vivo experiments in accord with institutional animal care and use committee protocols. Transgenic (Tg) mice express the HSV-Tk gene driven by the mouse glial fibrillary acidic protein (GFAP) promoter. HSV-Tk-negative littermates served as controls (wild type; WT).
In Vivo Liver Injury Models for HSC Depletion.
All treatment schemes are depicted in Supporting Fig. 1. CCl4 and allyl alcohol (AA) were purchased from Sigma-Aldrich (St. Louis, MO). Mice were treated with CCl4 (0.25 μL/g, intraperitoneally [IP], diluted in 50 μL of corn oil, on days 1, 4, 7, and 10) and AA (0.0125 μL/g, IP, diluted in 100 μL of 0.9% NaCl, on days 2, 5, and 8) to induce ALI and optimize HSC proliferation while evoking only modest liver damage. To induce selective killing of HSV-Tk-expressing HSCs, GCV (Cytovene; Syntex, Palo Alto, CA) was administrated from days 3 to 11 (100 μg/g, IP, diluted in 0.9% NaCl; n = 6).
For bile duct ligation (BDL), 8-12-week-old mice underwent BDL or sham surgery as previously described.13 WT and Tg animals received 100 μg/g/day of GCV (IP) diluted in 0.9% NaCl (or 0.9% NaCl alone for sham animals), beginning the day after surgery, for 11 consecutive days. At least 5 animals were treated per BDL group (n = 5; sham+GCV: n = 3; sham+saline: n = 2).
Cell Lines and Cultures.
The murine HSC line, JS1, has been previously described,14 the mouse hepatocyte cell line, AML12, was purchased from ATCC (Manassas, VA), and the immortalized EC line, TSEC, was kindly donated by Vijay Shah, M.D.15
Isolation and Culture of Primary HSCs and Hepatocytes.
Mouse HSCs were isolated by in situ perfusion of livers with collagenase and pronase as well as Percoll gradient centrifugation. Primary hepatocytes were isolated by in situ perfusion with collagenase, followed by differential centrifugation.
Histological liver analysis was performed by an expert pathologist (I.F.) using a score from 0-3 for both centrilobular and parenchymal necrosis, according to the following: 0 = none; 1 = isolated hepatocytes; 2 = groups of hepatocytes; 3 = bridging. Ballooning of hepatocytes was scored as follows: 0 = none; 1 = mild; 2 = moderate; 3 = severe. For each mouse, 10 fields at ×100 magnification were analyzed, and the average was calculated for each mouse.
Unless otherwise stated, data represent mean ± standard error of the mean. Statistical analysis was performed by SPSS software (version 17; SPSS, Inc., Chicago, IL). Significance was calculated by the Student t test or, when appropriate, by analysis of variance. Differences were considered significant if P < 0.05.
Validation of HSV-Tk Expression and Specific GCV Toxicity Toward Cultured Primary Cells.
Reverse transcriptase polymerase chain reaction (PCR) was performed on messenger RNA (mRNA) extracted from whole liver and HSCs isolated from both WT and GFAP-HSV-Tk (Tg) mice. Only Tg samples consistently expressed the HSV-Tk transcript for up to 7 days in primary culture (Supporting Fig. 2D). HSV-Tk expression was absent from both WT and Tg primary hepatocytes (data not shown).
Optimization of GCV Dose in Cultured Cells.
In initial studies, we first established a dose-dependent toxicity curve for GCV in established murine cell lines and then applied the same concentrations to primary HSCs isolated from WT and Tg mice. Both immortalized mouse stellate cells (JS1) and hepatocytes (AML12) were incubated with incremental GCV concentrations for 3 days. GCV-mediated toxicity unrelated to HSV-Tk gene expression was analyzed by assessing 3H-thymidine incorporation as well as alamarBlue assay. Cell death was determined by staining with trypan blue and determining the percentage of viable cells. Using this approach, GCV doses higher than 10 μM were toxic in cell lines (Supporting Fig. 2A-C), and subsequent experiments in primary cells therefore used 5 μM of GCV, which avoided nonspecific toxicity.
Next, primary HSCs from both WT and Tg mice were cultured for 5 days with GCV (5 and 500 μM) or saline. Specific GCV toxicity was determined by measuring 3H-thymidine incorporation and trypan blue staining (Fig. 1A,B). Only Tg HSCs exhibited significantly decreased thymidine incorporation and cell survival, indicating specific GCV-mediated killing at 5 μM, thus validating the construct for use in vivo.
To further establish the specificity of GCV, we also isolated primary hepatocytes from both WT and Tg mice, incubating them with GCV at the same concentrations (5 and 500 μM). In primary hepatocytes, 5 μM of GCV had no effect on the cells, whereas 500 μM of GCV remained toxic, highlighting the specificity of cell killing at the 5-μM concentration (Supporting Fig. 2E). Immortalized ECs (TSEC) treated with the same GCV doses behaved identically to primary hepatocytes, with no decrease in 3H-thymidine incorporation at 5 μM of GCV, but a significant effect at 500 μM (Supporting Fig. 2F).
GCV Kills HSV-Tk-Expressing HSCs by Apoptosis.
We next determined the mechanism underlying the GCV-mediated killing of Tg HSCs by measuring poly(ADP-ribose) polymerase (PARP) cleavage by western blotting as a reflection of apoptosis. Using this approach, only Tg HSCs treated with GCV displayed specific PARP cleavage (Fig. 1C). Tg HSC killing was also completely inhibited by the pan-caspase inhibitor, z-VAD-fmx, further establishing apoptosis as the underlying mechanism of GCV-mediated killing (Fig. 1D).
Optimization of HSC Depletion In Vivo After GCV Administration.
We next established the specificity of GCV effects in vivo. A dose range was performed by administering GCV in different concentrations (20-150 μg/g body weight, IP), daily for up to 10 days in WT and Tg mice. None of these mice displayed behavioral or morphological changes (data not shown) or any increase in serum alanine aminotransferase (ALT) levels (Supporting Fig. 3A). In contrast, more prolonged treatments using higher doses of GCV (≥150 μg/g) led to a significant decrease in weight in Tg, but not WT mice (Supporting Fig. 3B). This finding, together with previously published studies,16 led us to choose a final dose of 100 μg/g in subsequent experiments to deplete Tg HSCs in vivo.
Because HSCs must be proliferating to render them susceptible to GCV-mediated killing, we next optimized the method of liver injury required to maximize HSC depletion. To do so, we used CCl4 and AA to induce selective injury to the centrilobular and periportal regions, respectively. Accordingly, we performed a dose-dependent toxicity curve after four doses of AA (every 3 days), choosing 0.0125 μL/g as the final dose, based on mouse survival, extent of HSC activation (alpha smooth muscle actin [α-SMA] immunohistochemistry [IHC]), and liver damage (hematoxylin and eosin; H&E), to provoke the most widespread HSC proliferation while minimizing hepatocyte damage (Supporting Fig. 4). A dose of 0.25 μL/g of CCl4 in 50 μL of oil was used to optimize centrilobular HSC activation. The treatment scheme is depicted in Supporting Fig. 1A.
Validation of HSC Depletion in Tg Mice.
In Tg mice treated with CCl4+AA+GCV (“depletion treatment”), there was a significant decrease of desmin- and GFAP-coexpressing cells, with a reduction of ∼65% (P < 0.01), as assessed by IF colocalization analysis, compared to WT mice receiving the same treatment (Fig. 2A and Supporting Fig. 5). This decrease in protein expression was accompanied by a reduction of GFAP mRNA in mice undergoing HSC depletion (Fig. 2B).
To further analyze the efficacy of the depletion treatment, we assessed markers of HSC activation, including α-SMA IHC and β-PDGFR (platelet-derived growth factor receptor) and collagen I mRNA quantification. Figure 3A displays a >90% depletion of α-SMA-positive cells in Tg animals with HSC depletion, compared to WT mice. Consistently, β-PDGFR and collagen I mRNA expression levels were decreased in Tg mice after HSC depletion, compared to WT mice (Fig. 3B). Of interest, C-X-C chemokine receptor type 4 (CXCR4) has been recently implicated in HSC activation,17 and its mRNA expression in Tg mice undergoing HSC depletion was also reduced (Fig. 3C).
We confirmed these findings in a BDL model. BDL (or sham) was performed in WT and Tg mice (n = 5) (Supporting Fig. 1B). Mice received 100 μg/g/day of GCV (i.p.) (or 0.9% NaCl) for 11 days. In Tg mice that had been treated by BDL+GCV, desmin-positive cells were significantly decreased, compared to WT mice, indicating that activated GFAP-expressing fibrogenic cells proliferate after BDL as well, and that these cells can therefore be successfully depleted in BDL by GCV in Gfap-HSV-Tk+HSV mice (Supporting Fig. 6).
To determine whether apoptosis accounted for HSC depletion in vivo as in culture, we performed terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining in mice that had been treated with CCl4+AA+GCV for 4 days, instead of 10 days, because we expected the number of HSCs undergoing apoptosis to be higher at this time point. Indeed, at 4 and 7 days, HSC depletion was evident (reduction of ∼ 40% and 50%, respectively; P < 0.05), albeit to a lower extent (Supporting Fig. 7). As anticipated, a significant increase in nonparenchymal TUNEL-positive cells was evident in Tg mice, compared to WT animals, after the depletion treatment (Supporting Fig. 8A). To further establish that these nonparenchymal TUNEL-positive cells were HSCs, we analyzed serial sections with staining for TUNEL and desmin (Supporting Fig. 8B), which demonstrated apparent expression by the same cells, although double IF and confocal microscopy would be required for strict confirmation of coexpression.
Of interest, HSC depletion with CCl4+AA+GCV was not accompanied by any detectable nonliver effects of GCV on other GFAP-expressing populations. Specifically, there were no differences in animal behavior, survival at 30 days, leukocyte counts, serum creatinine, cytochrome P450 2E1 activity (which metabolizes CCl4 and AA) (Supporting Fig. 9) or macro- or microscopic gastrointestinal appearance subsequent to HSC depletion (Supporting Fig. 10). Specifically, there was no edema, necrosis, or inflammation in the bowel of either WT and Tg mice.
Functional Consequences of HSC Depletion.
Having established a specific, reproducible method of HSC depletion, we next explored the functional effect of HSC loss on fibrosis, liver injury, and hepatic inflammation in the two models.
Attenuated hepatic fibrosis
Because activated HSCs are the main collagen-producing cell during liver injury, we assessed fibrosis by collagen morphometry after their depletion. After 11 days of CCl4+AA+GCV treatment, collagen deposition was significantly decreased in GFAP-HSV-Tk mice, compared to WT animals, as determined by Sirius Red/Fast Green staining and morphometry (Fig. 4A). This finding could also be observed in mice treated with BDL+GCV (see Fig. 4B).
Reduced hepatic injury
There was a significant diminution in the extent of liver necrosis after HSC depletion in Tg by both fibrosis models, based on histology and serum chemistry. Blinded pathologic scoring revealed significantly decreased scores for necrosis for both CCl4+AA+GCV- (Fig. 5A) and BDL+GCV-treated Tg animals (Figure 5B), thus underscoring the applicability of this deletion approach to two mechanistic distinct fibrosis models. Aspartate aminotransferase (AST)/ALT levels were significantly reduced in CCl4+AA+GCV-treated animals, indicating less liver injury (Fig. 6A). The same trend could be observed for BDL+GCV-treated animals (Fig. 6B), whereas no significant differences were observed in serum total billirubin or total protein (data not shown). Consistent with the histological results, there was a significantly reduced expression of high-mobility group protein B1 (a marker of hepatic injury18) in Tg animals with HSC depletion (Supporting Fig. 12). The extent of hepatic injury (as assessed by histology scores for centrilobular necrosis, ballooning, and serum AST/ALT levels) coincided temporally with the extent of HSC depletion in mice treated for 0, 4, and 7 days with CCl4+AA+GCV, reinforcing the role of HSCs in provoking injury (Supporting Fig. 11).
To address potential sources of decreased liver damage, we analyzed a marker of lipid peroxidation, 4-hydroxy-2-nonenal (4-HNE), by immunoblotting in whole liver lysates from CCl4+AA+GCV animals, which revealed a significant decrease in 4-HNE-modified proteins in Tg mice undergoing HSC depletion (Fig. 6C).
We also determined whether HSC depletion could be maintained for up to 1 month by continuing a depletion treatment with reduced doses of CCl4 and GCV to evaluate the effect on survival rates, nonliver effects, fibrosis, and damage (treatment summarized in Supporting Fig. 1C). Consistent with previous results, there remained a statistically significant decrease in fibrosis (Fig. 7A) and liver damage, as assessed by histological necrosis scores (Fig. 7B) and ALT levels (Fig. 7C) in Tg mice. Interestingly, an increase in ballooning degeneration was evident in Tg mice, whereas there was a decrease in centrilobular necrosis. There were no significant differences in overall survival or weight loss in Tg or WT mice (data not shown).
Altered intrahepatic leukocyte population and intrahepatic cytokine expression
An initial screen using a cytokine bead array assay revealed induction of two key anti-inflammatory cytokines, interleukin (IL)-10 and interferon-gamma (IFN-γ) in CCl4+AA+GCV HSC-depleted Tg mice (Fig. 8A), as well as of IL-10, but not IFN-γ, in BDL+GCV-treated Tg mice (Fig. 8B). No changes in IL-6 or tumor necrosis factor alpha concentrations were observed (data not shown). To characterize possible sources of IL-10 and IFN-γ, we analyzed intrahepatic leukocyte populations and performed polychromatic flow cytometry analysis. Dendritic cells (DCs), natural killer (NK) cells, and CD4+ and CD8+ T cells, major potential sources of IFN-γ, were significantly increased in Tg HSC-depleted mice. Among immune cells that produce IL-10, both T-regulatory cells (Tregs) and Ly6C+/F4/80+/CD11b+ cells were significantly recruited to the liver during HSC depletion (Supporting Fig. 13).
Ongoing efforts have attempted to target HSCs with cell-specific reagents as a potential diagnostic or therapeutic tool. Concomitantly, cell-specific depletion has been exploited in other cell types to establish their contribution to organ homeostasis (e.g., macrophages), with a few studies examining HSC depletion.2-5 To date, these investigations have reinforced the HSC's known role in fibrogenesis, but have not expanded their repertoire of potential contributions to liver injury and inflammation. Gliotoxin, even when targeted to HSCs by coupling to Ab to synaptophysin, could have broad actions in vivo on immune cells that have not yet been characterized thoroughly, for example, by analyzing for macrophage markers other than F4/80+ (e.g., CD68) or by fluorescence-activated cell sorting analysis of intrahepatic leukocytes.3, 4
Here, we report on a new murine model of HSC depletion that uncovers a previously unknown role in amplifying liver injury using mice expressing the HSV-Tk gene driven by the mouse GFAP promoter. This system restricts cell depletion to proliferating HSCs, thereby uncovering the effect of only activated HSCs to liver injury and repair, because quiescent, nonproliferating HSCs are not affected.
Initial analyses confirmed reduced HSC proliferation (∼50%) and increased apoptosis in isolated, cultured HSCs from Tg mice when treated with GCV, consistent with previous studies utilizing the HSV-Tk “suicide gene” strategy,12 and mimicking the natural fate of HSC during resolution acute liver damage.19 Of note, approximately 70% of HSCs express GFAP,20 so that GCV-mediated killing affects the majority of, but not all, HSCs. Importantly, neither hepatocytes from either WT or Tg mice nor immortalized sinusoidal ECs were depleted by the same treatment, reinforcing the cellular specificity of this model. Because GFAP-HSV-Tk is expressed in specific cells outside the liver (e.g., enteric glial cells), we excluded the possibility that the liver effects resulted from the loss of GFAP-expressing cells in other tissues or altered metabolism. Lethal effects of GCV16 were reported after 14 days of continuous treatment with GCV (100 μg/g/day) by a subcutaneous pump in these mice. Therefore, we limited the GCV treatment to 11 days administered once-daily IP. Based on its pharmacokinetics, toxic serum levels of GCV are expected to be of a much shorter duration, therefore minimizing adverse effects. Indeed, we did not observe increased lethality or mortality, or altered small bowel pathology, with our treatment scheme.
Once in vivo HSC depletion was achieved, its functional effect was assessed by measuring markers of HSC activation. There was a dramatic decrease in α-SMA-positive cells in Tg mice undergoing HSC depletion, together with other markers of HSC proliferation (i.e., β-PDGFR and collagen I), indicating that depletion affected those HSCs most critical to fibrogenesis and repair (i.e., activated HSCs). Of interest, CXCR4 expression was also decreased in Tg mice undergoing HSC depletion. This cytokine receptor is another feature of activated HSCs, which also contributes to profibrogenic and proliferative responses.17
The findings reinforce the rationale for therapeutic HSC depletion, albeit not necessarily by a suicide-gene strategy. Moreover, not only was fibrosis reduced, but acute damage was attenuated, suggesting that depletion of activated HSCs could have dual salutary effects on both the amount of fibrosis and extent of injury. Correlated with attenuated injury was a reduction in 4-HNE consistent with decreased oxidant stress, although the source(s) of these pro-oxidants in both WT and Tg mice are not clarified by our findings. Specifically, reduction in 4-HNE could reflect decreased release by HSCs because of their depletion, or loss of paracrine signals from HSCs to other cell types that generate 4-HNE, including hepatocytes or inflammatory cells. Moreover, 4-HNE interacts directly with c-Jun N-terminal kinase (JNK) isoforms in human HSCs to stimulate procollagen type I expression and synthesis.21 Thus, reduced collagen production could also result from a feedback loop in which less 4-HNE leads to less JNK-mediated collagen expression. Of note, previous studies using gliotoxin did not uncover an effect of HSC depletion on injury,2, 5 possibly because effects of gliotoxin are not as specific, and concurrent effects of gliotoxin on other cell types might have attenuated the phenotype.
The mechanism of attenuated injury in the setting of HSC depletion is not fully clarified, but the increase in IL-10 and IFN-γ likely contribute to reduced injury, because these two cytokines both down-regulate HSC activation and fibrosis production.22 Polychromatic flow cytometry for intrahepatic immune cell populations revealed increased numbers of well-known cellular sources of IL-10 (Tregs and monocytes)23 as well as for IFN-γ (DC, NK, and CD4+ and CD8+ T cells).24 Specifically, the increase in Tregs, along with an increase of IL-10, which, unlike IFN-γ, was evident in both HSC-depletion models, indicates a potential role for Tregs in the attenuation of liver injury, despite increased numbers of several immune cell populations in the absence of HSCs. Further studies to analyze these preliminary findings and to identify the responsible immune cell population by specific depletion studies in vivo are currently underway.
Importantly, injury was attenuated after HSC depletion not only in acute, but also in chronic injury (30 days after HSC depletion with continued CCl4 and GCV) as well as in the BDL fibrosis model, indicating that the results are generalizable and not restricted to a single model of injury. Mice with HSC depletion after chronic injury all survived, attesting to the practicality of chronic HSC depletion with this strategy. Interestingly, the reduced injury in Tg mice was associated with more hepatocyte ballooning, raising the prospect that ballooning degeneration, but not necrosis, could be beneficial, because ballooning has been previously proposed to indicate a better chance of cellular recovery after injury.25
Our studies further suggest that HSCs (and not portal myofibroblasts, which reportedly do not express GFAP26 and are therefore not ablated in this model) are the major fibrogenic cell population in BDL-induced fibrosis, consistent with an earlier study analyzing HSCs in different models by microarray.27
In conclusion, we describe a new approach to HSC depletion that has confirmed the primacy of these cells in fibrosis production, but has also revealed an unexpected role in amplifying hepatocellular liver damage and decreasing protective cytokines. The model offers the prospect of exploring other features of liver homeostasis that may depend on HSCs, including their repopulation from extrahepatic sources and their contribution to hepatic regeneration and neoplasia.
The authors thank Dr. Virginia Hernandez Gea, Dr. Feng Hong, and Stephanie Gillespie for their technical support and Dr. Inma Castilla de Cortázar for her helpful advice.