Distinct mechanisms are believed to regulate growth of the liver during fetal development and after injury in adults, because the former relies on progenitors and the latter generally involves replication of mature hepatocytes. However, chronic liver injury in adults increases production of Hedgehog (Hh) ligands, developmental morphogens that control progenitor cell fate and orchestrate various aspects of tissue construction during embryogenesis. This raises the possibility that similar Hh-dependent mechanisms also might regulate adult liver regeneration. The current analysis of murine liver regeneration after 70% partial hepatectomy (PH), an established model of adult liver regeneration, demonstrated that PH induced production of Hh ligands and activated Hh signaling in liver cells. Treatment with a specific Hh signaling inhibitor interfered with several key components of normal liver regeneration, significantly inhibiting progenitor responses, matrix remodeling, proliferation of hepatocytes and ductular cells, and restoration of liver mass. These global inhibitory effects on liver regeneration dramatically reduced survival after PH. Conclusion: Mechanisms that mediate liver organogenesis, such as Hh pathway activation, are retained and promote reconstruction of adult livers after injury. Hepatology 2010
Healthy adult livers have enormous regenerative capacity. This permits recovery of normal tissue-specific functions and mass within weeks of 70% partial hepatectomy (PH) in humans. Liver regeneration proceeds more rapidly in rodents, which accomplish liver reconstruction within 7 to 10 days after PH.1 Thus, rodents are often used as experimental models to investigate regenerative mechanisms. Such work has consistently demonstrated that striking increases in hepatocyte DNA synthesis occur within the initial 48 hours after PH, followed by smaller (but highly significant) increases in hepatocyte mitoses and eventual recovery of liver mass, leading to consensus that liver regeneration after PH relies largely on increased replication of mature hepatocytes.2-5 Nevertheless, changes in expression of progenitor markers, such as alpha-fetoprotein (AFP) and Fn14, have long been acknowledged to occur during regeneration.6-9 Severe inhibition of liver regeneration after toxic liver injury was recently reported to occur in mice with targeted disruption of Fn14, a member of the tumor necrosis factor receptor superfamily that promotes the growth of bipotent hepatic progenitors (in other words, oval cells).10 These findings suggest that liver progenitors may have a larger role in regenerating adult livers after PH, and perhaps after other types of acute injury, than previously appreciated.
Because mature hepatocyte replication is inhibited in many types of chronic liver injury, it is generally believed that progenitor populations contribute to regeneration of chronically injured livers. However, the mechanisms that mobilize progenitor cells, and that control their fate in damaged livers, are poorly understood.11-13 Recent studies have demonstrated that Hedgehog (Hh), a fetal morphogenic signaling pathway, becomes activated in many types of chronic liver injury.14 Hh ligands generally promote the growth and viability of progenitor-type cells15-17 and have been shown to function as viability factors for human and rodent liver progenitors, including oval cells. During embryogenesis and cancer metastasis, Hh-pathway activation tends to preferentially expand stromal cell populations by retaining the primitive, migratory phenotype of existing mesenchymal cells and promoting epithelial-to-mesenchymal transitions (EMT) in certain types of immature epithelial cells.18-21 A similar process occurs when the Hh-pathway becomes activated during chronic liver injury because Hh ligands function as growth factors for myofibroblastic liver cells,15, 22 stimulate quiescent hepatic stellate cells to acquire a more myofibroblastic phenotype,23 and induce immature ductular cells to undergo EMT.13 As a result, Hh pathway activation promotes fibrogenic repair responses during chronic liver injury. Our group has also suggested that Hh pathway activation enhances the outgrowth of liver progenitors that help to replace mature hepatocytes that die, but the latter remains unproven.
The current study evaluates the hypothesis that Hh pathway activation occurs after PH and plays a role in regulating liver regeneration after a surgical insult that causes massive acute loss of mature hepatocytes. Our findings demonstrate the kinetics of Hh pathway activation after PH, identify the types of Hh-responsive cells, and characterize the effects of Hh-pathway inhibition on the regenerative process. The results support our hypothesis and identify Hh as a major regulator of liver regeneration post-PH. This, in turn, suggests that common mechanisms regulate liver growth during organogenesis and when reconstruction of adult livers is necessitated by injury.
α-SMA, alpha smooth muscle actin; AFP, alpha fetoprotein; ANOVA, analysis of variance; BrdU, bromodeoxyuridine; BUN, blood urea nitrogen; EMT, epithelial-to-mesenchymal transition; Gli, glioblastoma; Hh, Hedgehog; Hip, Hh interacting protein; Ihh, Indian Hedgehog; mRNA, messenger RNA; PH, partial hepatectomy; Ptc, patched; QRT-PCR, Quantitative real-time polymerase chain reaction; SEM, standard error of the mean; sFRP1, secreted frizzled-related protein 1; Shh, Sonic Hedgehog; Smo, smoothened.
Materials and Methods
B6:129Sv (in-house strain, Spain) and C57BL/6 mice (Jackson Laboratories, Bar Harbor, ME) were maintained in respective animal facilities at the University of the Basque Country and Duke University. Animal care and surgical procedures were conducted in compliance with local institutional guidelines and those set forth in the “Guide for the Care and Use of Laboratory Animals” as published by the National Institute of Health.
To ascertain the kinetics of Hh-signaling during liver regeneration, 70% PH was performed on 8-week-old to 10-week-old female mice (n = 102), according to the method of Higgins and Anderson.1 Mice underwent surgery between 2:00 PM and 5:00 PM. The mice were sacrificed at 6 hours (n = 6), 12 hours (n = 6), 24 hours (n = 6), 48 hours (n = 12), 72 hours (n = 12), 96 hours (n = 12), 120 hours (n = 12), 144 hours (n = 12), 168 hours (n = 12), and 216 hours (n = 12) after PH. Animals were administered bromodeoxyuridine (BrdU) intraperitoneally (50 μg/g body weight) 2 hours before sacrifice. Animals were weighed before PH and at the time of sacrifice; resected quiescent liver (used as 0-hour comparisons) and regenerating liver remnants were weighed and then formalin-fixed or snap frozen in liquid nitrogen.
To determine whether inhibiting the Hh-pathway altered liver regeneration, PH was performed in an additional 100 mice (10-13-week-old males) that were injected intraperitoneally with vehicle (olive oil) or cyclopamine (15 mg/kg/day, Toronto Research Chemicals, Toronto, Canada12, 24) 24 hours before PH and daily thereafter. Liver remnants and blood were harvested for subsequent analysis. In addition to evaluating Hh-signaling and BrdU incorporation, potential toxic actions of cyclopamine were assessed by examining liver histology and levels of serum aminotransferases, blood urea nitrogen (BUN) and glucose.
Hh signaling was also assessed in primary hepatocytes isolated from another 6 adult male mice 24 or 48 hours after sham surgery (n = 2 mice) or PH (n = 4 mice). Standard in situ liver perfusion and density gradient centrifugation techniques were used to isolate hepatocytes.18 Cells were processed immediately for immunocytochemistry or plated onto plastic dishes in 10% serum-supplemented Dulbecco's modified Eagle medium (Sigma, St. Louis, MO) overnight. Medium containing nonadhered cells was removed the following morning, plates were washed with fresh medium, and adherent cells were harvested for analysis. To assess direct effects of Hh pathway inhibition on cell viability and proliferation, hepatocytes were similarly isolated from four additional mice 24 hours after sham surgery (n = 2) or PH (n = 2) and cultured with either vehicle or cyclopamine (5 μM) in the presence of BrdU for 24 hours.
Immunohistochemistry and BrdU Labeling.
Formalin-fixed paraffin-embedded livers and hepatocyte cytospins were prepared as previously described.16 A detailed protocol and antibodies used are listed in Supporting Materials and Methods.
Quantitative real-time reverse transcription polymerase chain reaction (QRT-PCR) was performed using established protocols15; details are provided in Supporting Materials and Methods and Supporting Table 1.
Proteins were isolated from whole liver tissue or primary hepatocytes. After quantification, equal amounts of protein were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and western blot analysis was performed. A detailed list of antibodies is given in Supporting Materials and Methods.
Statistical analyses were performed using GraphPad Prism v5.02 for Windows (GraphPad Software, San Diego, CA). Results of gene expression are grouped into pre-replicative (0-36 hours post-PH), replicative (36–100 hours post-PH), and post-replicative (100–216 hours post-PH) sets, and compared with 0-hour (quiescent) samples. Statistical significance was determined using one-way analysis of variance (ANOVA) and Bonferroni's multiple comparisons test. Where mice were treated with either vehicle or cyclopamine, results were compared with respective vehicle-treated or cyclopamine-treated, 0-hour (quiescent) samples. Statistical significance was determined using unpaired, two-tailed Student t test. Significance was accepted at the 5% level, *P < 0.05, **P < 0.01, or ***P < 0.001.
Hedgehog Pathway Activation Follows PH.
Hepatic expression of messenger RNAs (mRNAs) that encode Hh-pathway inhibitors (for example, Hh interacting protein [Hip]), Hh ligands (Indian Hh [Ihh], and sonic Hh [Shh]), the receptors that repress (patched [Ptc]) or promote (smoothened [Smo]) Hh signaling, and Hh-inducible transcription factors (glioblastoma [Gli]1 and Gli2) were evaluated at several distinct time points after PH. For each gene, results were normalized to expression in the liver at the time of PH (0 hours) and graphed as a function of time to demonstrate how gene expression varied during the prereplicative period, period of maximal liver cell replication, and post-replicative period (Fig. 1A).
PH abruptly reduced hepatic expression of Hip, and Hip mRNA levels generally remained below pre-PH values during the prereplicative, replicative, and postreplicative periods after PH. Reduced Hip expression was accompanied by increased expression of Hh ligands. Messenger RNA levels of Ihh began to increase during the prereplicative period, remained at their highest values during the replicative period, then gradually declined. Expression of Shh did not increase until the middle to the end of the replicative period but remained high throughout the postreplicative period after PH.
The relative abundance of Ptc and Smo mRNAs changed after PH, such that expression of Smo (the signaling competent Hh co-receptor) was greater than that of Ptc (the inhibitory Hh receptor) throughout the replicative and postreplicative periods. Together with the reciprocal changes in mRNA expression of Hh ligand antagonists and Hh ligands, the predominance of Smo relative to Ptc suggested that Hh signaling would increase after PH. Changes in expression of Gli1 and Gli2 support this concept. Levels of Gli1 began to increase in the prereplicative period and remained at high levels until the end of the postreplicative period. Increases in Gli1 expression were followed by increases in mRNA levels of Gli2, a Gli-regulated gene.20 Gli2 expression began to increase during the replicative period, peaked somewhat later, and then remained high throughout the postreplicative period. Increased Gli1 and Gli2 mRNA levels were accompanied by increased levels of Gli1 and Gli2 proteins at 48 hours post-PH (the time point of maximal mRNA expression of these genes during the replicative period) (Fig. 1B), and followed by increased mRNA expression of secreted frizzled-related protein 1 (sFRP1), a Gli-regulated, Hh-target gene (Fig. 1C).25 Hence, PH led to dramatic increases in Hh signaling, particularly during the intervals when liver cell replication and remodelling responses are known to occur in the regenerating liver tissues.
Hh Pathway Activation Is Accompanied by Increased Liver Epithelial Progenitors and Fibrogenic Repair After PH.
During chronic liver injury, Hh pathway activation promotes accumulation of liver epithelial progenitor cells and myofibroblasts and stimulates fibrogenic repair. Hepatic expression of progenitor markers, such as AFP and Fn14, increase after PH.8, 9 We confirmed these observations (Fig. 2). Fn14 increased 40-fold during the early prereplicative period and remained at least fivefold above basal values throughout the entire postreplicative period, although expression of the Fn14 ligand, tumor necrosis factor-like weak inducer of apoptosis (TWEAK), remained relatively constant after PH. Early increases in Fn14 were followed by increases in AFP expression, which peaked sharply (at 160-fold above basal values) late in the replicative period (Fig. 2A). Hepatic progenitor populations are known to be heterogeneous. Therefore, to verify that increased mRNA levels of Fn14 and AFP were actually accompanied by hepatic accumulation of progenitors, two other progenitor markers, AE1/AE3 and muscle pyruvate kinase,26, 27 were evaluated with immunohistochemistry. Results demonstrated that cells expressing these markers increased steadily in regenerating livers from 12 to 48 hours post-PH (Fig. 2B). These findings show that Hh pathway activation is associated with accumulation of liver progenitors before and during the replicative period after PH, as is known to occur when the pathway becomes activated during chronic liver injury.14
Accumulation of myofibroblastic cells and increased production of collagen matrix has also been demonstrated after PH.28-30 Our studies confirmed these findings (Fig. 3). Alpha-smooth muscle actin (α-SMA) mRNA levels increased steadily after PH, peaking at more than 12-fold above basal levels early during the postreplicative period and remaining in this range until the end of the study (216 hours post-PH). Collagen expression also increased significantly, with collagen1α1 mRNA peaking approximately 15-fold above basal values 96 hours post-PH (Fig. 3A). Immunohistochemistry and morphometry confirmed hepatic accumulation of α-SMA–immunoreactive cells and Sirius red fibrils in regenerating livers (Fig. 3B,C). Therefore, Hh pathway activation post-PH is accompanied by progressive matrix accumulation, as is known to occur during fibrogenic repair of chronic liver injuries.31
Hepatocytes and Bile Ductular Cells Express Hh-Target Genes After PH.
In healthy adult livers, mature hepatocytes generally do not express Hh-target genes, such as Gli2, although Gli2 can be demonstrated in occasional ductular cells in bile ducts and canals of Hering.14 Thus, it was important to determine whether these cell types became Hh-responsive when Hh pathway activity increased after PH. Immunohistochemistry demonstrated Gli2 staining in both hepatocytic and ductular cells in regenerating livers (Fig. 4A). Numbers of Gli2(+) hepatocytes began to increase in the prereplicative period, peaked at 48 hours post-PH, and remained at high levels throughout the postreplicative period. The number of Gli2(+) ductular cells increased significantly post-PH, but peak accumulation occurred a bit later (in other words, 72 hours post-PH).
To verify the unanticipated discovery that hepatocytes express Hh-target genes after PH, six additional mice were subjected to sham surgery (n = 2 mice) or PH (n = 4 mice), and primary hepatocytes were isolated 24 hours and 48 hours later. Cellular expression of Hh-target genes was then assessed. Western blot analysis demonstrated that primary hepatocytes from 24 hours and 48 hours post-PH livers expressed much higher levels of Gli1 and Gli2 proteins than sham-operated mice (Fig. 4B). Immunocytochemistry showed that 100% of the analyzed cells expressed albumin, validating the purity of the preparation (Supporting Fig. 1A). Some (<10%) of these albumin-expressing cells also expressed Gli1 or Gli2 (Supporting Fig. 1B). Interestingly, a subset of the hepatocytic cells from the regenerating livers (but none of the cells from sham-operated mice) stained positively for AFP, and these AFP(+) cells generally co-expressed the Hh-target gene, Gli2 (Supporting Fig. 1C). Immunoblot analysis confirmed that hepatocyte populations from regenerating livers were enriched with cells that expressed keratin (K)7, a marker of immature hepatocytes (Fig. 4B). Hepatocytic cells from regenerating livers also expressed Ihh and Shh ligands (Fig. 4B), and immunostaining of 48-hour cytospins from regenerating (but not sham-operated) livers co-localized expression of Shh and albumin (Supporting Fig. 1D). Thus, the aggregate data provide conclusive evidence that hepatocytic cells expressing progenitor markers and Hh ligands or target genes increase as the liver regenerates after PH.
Proliferative Activity Parallels Enrichment of Liver Epithelial Cell Populations with Hh-Responsive Cells.
Hh ligands are known to promote the proliferation of various progenitors. Therefore, it was important to determine whether the proliferative activity of hepatocytic or ductular cells increased as these populations became enriched with Hh-responsive cells. Mice received a single injection of BrdU 2 hours before sacrifice to label cells that were engaged in DNA synthesis. The numbers of hepatocytes and ductular cells with BrdU nuclear staining increased significantly after PH (Fig. 4C, D). As with Gli2-staining (Fig. 4A), BrdU nuclear staining peaked first in hepatocytes, and then in ductular cells. PH also increased nuclear accumulation of Ki67, another S-phase marker, in both cell populations (Supporting Fig. 2A, B). Thus, increased proliferative activity in hepatocyte and ductular cell populations closely paralleled their enrichment with Hh-responsive cells.
Hh Signaling Promotes Progenitor Cell Accumulation and Fibrogenic Liver Repair After PH.
To determine how Hh-pathway activation impacts regenerative responses, post-PH, mice were treated with cyclopamine, a specific Smo antagonist that abrogates Hh signal transduction32 or vehicle (olive oil) before PH and at regular intervals (every 24 hours) after PH. As expected, cyclopamine did not prevent induction of Hh ligands (data not shown). However, it attenuated induction of Gli1 and Gli2 mRNAs (Fig. 5A) and proteins (Fig. 5B), and inhibited mRNA/protein expression of sFRP1 (Fig. 5A) and Ptc (Fig. 5B), two other Gli-regulated genes. Inhibiting Hh signaling also reduced mRNA or protein expression of various progenitor markers, such as AFP, Fn14, and keratin (K)19 after PH (Fig. 5C), and prevented cells that expressed AE1/AE3 or muscle pyruvate kinase (Mpk) (other progenitor markers) from accumulating in the liver (Fig. 5D). In addition, it attenuated fibrogenic repair, as evidenced by decreased expression of α-SMA and collagen mRNAs, α-SMA protein, and picrosirius red staining (Fig. 5E).
Hh Signaling Promotes Liver Cell Replication and Is Required for Survival and Liver Regeneration After PH.
Cyclopamine inhibition of Hh-regulated responses was associated with significantly reduced survival after PH. Increased mortality was evident as early as 48 hours after PH and worsened over the subsequent 24 hours, such that only 1 of the 50 cyclopamine-treated mice survived to 3 days post-PH, compared with almost 90% of the 50 vehicle-treated controls (P = 0.0001). Liver histology and serum aminotransferase values were no different in surviving cyclopamine-treated mice than controls at 24 and 48 hours after PH, suggesting that cyclopamine was not directly hepatotoxic. This interpretation was validated by evidence that adding cyclopamine to cultures of regenerating hepatocytes from 24-hour and 48-hour post-PH mice abrogated Hh signaling, as evidenced by reduced expression of Gli1 protein and sFRP1 mRNA (both P < 0.01 versus vehicle) but did not reduce cell viability. Because pancreatic beta cells and some renal cells are known to be Hh-responsive, levels of blood urea nitrogen and serum glucose were assessed. No differences were noted between cyclopamine-treated and vehicle-treated mice, suggesting that the increased cyclopamine-related mortality was not attributable to pancreatic or renal toxicity. Further study is needed to assure that cyclopamine did not exert other nonspecific toxic actions that might have reduced post-PH survival.
Notably, surviving cyclopamine-treated mice failed to recover liver weight (P = 0.01). Hence, liver-to-body weight ratios in surviving cyclopamine-treated mice were lower than in vehicle-treated mice at 48 hours post-PH (P = 0.03) (Table 1). The poor survival and restitution of liver mass in the cyclopamine-treated animals suggested that Hh-pathway inhibition impaired liver cell proliferation post-PH. Ki67-immunostaining and BrdU incorporation data supported this interpretation. At 48 hours post-PH, incorporation of BrdU was reduced by 90% in hepatocytes, and by approximately 40% in ductular cells of cyclopamine-treated mice compared with vehicle-treated controls (Fig. 6A, B). Moreover, when primary hepatocyte cultures isolated from mice 24 hours post-PH were treated with cyclopamine in vitro for 24 hours, BrdU incorporation was inhibited by approximately 60% (P < 0.01 versus tomatidine-treated controls) (Fig. 6C). In contrast, cyclopamine had no effect on BrdU incorporation of hepatocyte cultures from sham-operated mice. Thus, cyclopamine specifically inhibited the proliferative activity of hepatocyte cultures that were enriched with Hh-responsive cells expressing progenitor markers (Fig. 4B and Supporting Fig. 1).
Table 1. Mean Liver and Body Weights of Cyclopamine Versus Vehicle-Treated Mice 12 to 48 Hours Post-PH
Initial BW (g)
Final BW (g)
Final LW (g)
P < 0.05; mean ± SEM.
V, vehicle; C, cyclopamine; BW, body weight; LW, liver weight; LW/BW, liver to body weight ratio in grams × 100; ns, nonsignificant.
*Estimated LW, considering that the resected LW is 70% of the original liver mass.
This study demonstrates that Hh pathway activation is critical for liver regeneration to occur after PH. The mechanisms mediating regrowth of the adult liver after a surgical insult that causes massive acute loss of mature hepatocytes have been investigated for decades.1 Several key growth regulators for this process have been identified, including hepatocyte mitogens, cytokines, pathogen-associated molecular pattern receptors, and intracellular factors involved in inflammatory and metabolic stress.33 Although a recent transcriptome analyses had documented that mRNA expression of certain Hh signaling components change after PH,34 information on the kinetics of Hh pathway activation and cellular targets of Hh signaling post-PH, as well as evidence that Hh pathway directly regulates liver regeneration after acute liver injury, are entirely novel. Additional research is now needed to clarify whether and how Hh signaling interacts with other mechanisms that are known to modulate regenerative responses to PH.
The vast majority of the earlier work in regenerating livers post-PH had focused on mature hepatocyte replication, and attention was largely restricted to the time interval that immediately spans peak replicative activity in these cells (in other words, 0–72 hours post-PH).35-37 The current study encompassed a much longer time period (from 0 hours to 216 hours post-PH) and monitored distinct components of the regenerative response. In addition to assessing hepatocyte proliferative activity, progenitor and stromal responses were analyzed concurrently, revealing a role for the Hh pathway in each of these activities. The aggregate results demonstrate that Hh signaling plays a pivotal role in integrating and coordinating various aspects of adult liver repair after acute injury. In retrospect, this discovery is not surprising because Hh pathway activation is well known to orchestrate tissue construction during fetal development,38, 39 and it provides a similar function during remodeling of chronically damaged livers.14 However, the new data in the PH model will undoubtedly spark controversy because liver regeneration post-PH is believed to be driven predominately by replication of mature hepatocytes,4, 33 whereas other types of tissue growth that rely on Hh signaling are known to involve progenitor populations.17, 39
In this regard, it is important to emphasize that earlier studies of regenerating livers after PH have demonstrated increased expression of progenitor markers,7, 9 suggesting that immature liver cells may accumulate during this process. Feng et al.9 reported that mRNA levels of Fn14 increased dramatically within 2 to 4 hours of PH and remained at high levels for the next 2 to 3 days, although they were unable to detect Fn14 mRNA by northern blot analysis of primary hepatocytes or healthy adult livers pre-PH.9 The authors suggested that up-regulation of Fn14 expression in regenerating livers contributed to increased hepatocyte proliferative activity because they detected striking induction of Fn14 in many hepatoma cell lines and in human hepatocellular carcinoma samples. Subsequently, another group discovered that ductular type progenitors express Fn14. Moreover, they demonstrated that stimulating liver progenitors with the Fn14 ligand, TWEAK, increased cell proliferation, whereas knocking out Fn14 in mice virtually eliminated proliferation of bipotent hepatic progenitors (oval cells) in an in vivo model of oval cell–dependent liver regeneration. Thus, they concluded that Fn14 controlled regenerative activity of liver progenitors.10
In the current study, QRT-PCR and western blot analysis demonstrated that inhibiting Hh signaling with cyclopamine partially blocked induction of various progenitor markers (such as Fn14, AFP, K7, and K19) after PH. Immunohistochemistry showed that cyclopamine also prevented AE1/AE3-stained cells and Mpk(+) cells from accumulating in livers post-PH. Thus, several independent lines of evidence indicate that Hh pathway activation expands heterogeneous populations of hepatic progenitors after PH. Interestingly, blocking Hh signaling also inhibited proliferation of hepatocytes and ductular cells post-PH. In addition, post-PH recovery of liver mass and survival were negatively impacted by cyclopamine treatment. When considered in light of the aforementioned data about Fn14, these new results suggest that progenitor populations (which likely include Fn14[+] oval cells) give rise to some of the hepatocytes and ductular cells that proliferate to regenerate the liver after PH. Moreover, Hh signaling appears to be necessary for these processes to occur. This is consistent with earlier evidence that Hh ligands function as viability factors for oval cells and other hepatic progenitors.17, 39
The new data also demonstrate that the timing of progenitor accumulation after PH closely coincides with significant induction of mesenchymal markers, such as α-SMA and collagen 1αI. Others have reported that hepatic progenitor populations include multipotent cells that co-express epithelial and mesenchymal markers.26, 40, 41 The latter is characteristic of cells that undergo EMT or mesenchymal-to-epithelial transition.42 During fetal development, organogenesis involves repetitive waves of EMT/mesenchymal-to-epithelial transition.43 The Hh-pathway is known to promote EMT during development and cancer metastasis,20, 44, 45 and we recently reported that it stimulates adult liver ductular cells to undergo EMT.13 Interestingly, both hepatic epithelial progenitors and mesenchymal stem cells10, 40, 46 express Fn14, raising the possibility that some Fn14(+) epithelial progenitors that help to regenerate the liver post-PH may be derived from hepatic stromal cells. This theory is particularly intriguing because PH is accompanied by dramatic expansion of myofibroblastic cells, as well as matrix remodeling, which results in transient, but significant, accumulation of fibrous matrix.30 Hh pathway activation has been shown to promote expansion of mesenchymal-type progenitor cells in several tissues17, 38 and mediates fibrogenic repair during chronic liver injury.14 The current study provides compelling evidence that Hh signaling is involved in expanding populations of myofibroblastic cells that contribute to hepatic matrix deposition after PH. Additional research is now justified to determine whether Hh-regulated EMT/mesenchymal-to-epithelial transition responses are involved in the post-PH matrix remodeling process or participate in repopulating the liver epithelial compartment after PH.
In summary, the current study provides evidence that a developmental morphogen that regulates progenitor and stromal cell fate (in other words, Hh) is required for optimal regeneration of the adult liver post-PH. This discovery complements growing evidence that Hh signaling guides repair of chronically injured livers, and it suggests that common mechanisms mediate fetal liver development and repair of adult liver injury. Therefore, progress in delineating how Hh-responsive mechanisms regulate liver growth and development might help to unravel conserved mechanisms that control regeneration of injured livers in adults. Such knowledge has important implications for patients with various types of acute and chronic liver damage.
The authors thank Dr. Xiaoling Wang (Gastroenterology, Duke University) and Dr. Gregory Michelotti (Anesthesiology, Duke University) for technical assistance, and Dr. Jiawen Huang (Gastroenterology, Duke University) for animal care assistance. The authors thank W. C. Stone (Gastroenterology, Duke University) for his administrative support to this work.