HepatologyVolume 51, Issue 3 p. 1027-1036 Liver Biology/PathobiologyFree Access Hepatocytes do not undergo epithelial-mesenchymal transition in liver fibrosis in mice† Kojiro Taura, Kojiro Taura Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA These authors equally contributed to this work.Search for more papers by this authorKouichi Miura, Kouichi Miura Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA These authors equally contributed to this work.Search for more papers by this authorKeiko Iwaisako, Keiko Iwaisako Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorChristoph H. Österreicher, Christoph H. Österreicher Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorYuzo Kodama, Yuzo Kodama Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorMelitta Penz-Österreicher, Melitta Penz-Österreicher Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorDavid A. Brenner, Corresponding Author David A. Brenner [email protected] Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA fax: (858) 822-00841318A Biomedical Sciences Building, 9500 Gilman Drive, La Jolla, CA, 92093-0602===Search for more papers by this author Kojiro Taura, Kojiro Taura Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA These authors equally contributed to this work.Search for more papers by this authorKouichi Miura, Kouichi Miura Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA These authors equally contributed to this work.Search for more papers by this authorKeiko Iwaisako, Keiko Iwaisako Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorChristoph H. Österreicher, Christoph H. Österreicher Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorYuzo Kodama, Yuzo Kodama Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorMelitta Penz-Österreicher, Melitta Penz-Österreicher Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CASearch for more papers by this authorDavid A. Brenner, Corresponding Author David A. Brenner [email protected] Department of Medicine, University of California, San Diego, School of Medicine, La Jolla, CA fax: (858) 822-00841318A Biomedical Sciences Building, 9500 Gilman Drive, La Jolla, CA, 92093-0602===Search for more papers by this author First published: 13 October 2009 https://doi.org/10.1002/hep.23368Citations: 244 † Potential conflict of interest: Nothing to report. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract The origin of fibrogenic cells in liver fibrosis remains controversial. We assessed the emerging concept that hepatocytes contribute to production of extracellular matrix (ECM) in liver fibrosis through epithelial-mesenchymal transition (EMT). We bred triple transgenic mice expressing ROSA26 stop β-galactosidase (β-gal), albumin Cre, and collagen α1(I) green fluorescent protein (GFP), in which hepatocyte-derived cells are permanently labeled by β-gal and type I collagen-expressing cells are labeled by GFP. We induced liver fibrosis by repetitive carbon tetrachloride (CCl4) injections. Liver sections and isolated cells were evaluated for GFP and β-gal as well as expression of α-smooth muscle actin (α-SMA) and fibroblast-specific protein 1 (FSP-1). Upon stimulation with transforming growth factor β-1, cultured hepatocytes isolated from untreated liver expressed both GFP and β-gal with a fibroblast-like morphological change but lacked expression of other mesenchymal markers. Cells from CCl4-treated livers never showed double-positivity for GFP and β-gal. All β-gal-positive cells exhibited abundant cytoplasm, a typical morphology of hepatocytes, and expressed none of the mesenchymal markers including α-SMA, FSP-1, desmin, and vimentin. In liver sections of CCl4-treated mice, GFP-positive areas were coincident with fibrotic septa and never overlapped X-gal-positive areas. Conclusion: Type I collagen-producing cells do not originate from hepatocytes. Hepatocytes in vivo neither acquire mesenchymal marker expression nor exhibit a morphological change clearly distinguishable from normal hepatocytes. Our results strongly challenge the concept that hepatocytes in vivo acquire a mesenchymal phenotype through EMT to produce the ECM in liver fibrosis. (HEPATOLOGY 2009.) Liver fibrosis is the consequence of a sustained wound-healing response to chronic liver injury, including viral, alcoholic, and autoimmune hepatitis.1 Progressive liver fibrosis leads to cirrhosis and its associated complications, including portal hypertension, hepatic encephalopathy, and hepatocellular carcinoma. Currently, several antifibrotic drugs are in development for the treatment of liver fibrosis but their efficacy has not been proven in patients.2 Further understanding of the cellular and molecular mechanism of liver fibrosis may lead to the development of more effective treatments. A key issue in liver fibrosis is the origin of fibrogenic cells or activated myofibroblasts that produce extracellular matrix (ECM) such as type I collagen. Quiescent hepatic stellate cells (HSCs) are believed to be the main source of fibrogenic cells.3 However, there is accumulating evidence suggesting that HSCs are not the only origin. Portal myofibroblasts4 or bone marrow-derived fibrocytes5 could be other cellular sources of myofibroblasts in liver fibrosis. In addition, a new concept has been proposed that hepatocytes undergo a phenotypical change called epithelial-mesenchymal transition (EMT) to acquire a fibroblastic phenotype in liver fibrosis.6 Using a cell fate tracing technique it was demonstrated that hepatocyte-derived cells exhibited a fibroblast-like morphology accompanied by expression of fibroblast specific protein 1 (FSP-1). However, it is still unknown whether those hepatocyte-derived cells contribute to production of ECM in liver fibrosis and if FSP-1 is a marker for fibroblasts in fibrotic liver. We have developed a reporter mouse in which green fluorescent protein (GFP) is expressed under the collagen α1(I) promoter, enabling tracking of collagen-producing cells in vitro and in vivo.7, 8 The aim of the present study was to directly examine whether hepatocyte-derived cells express type I collagen in vitro and in vivo. Abbreviations α-SMA, α-smooth muscle actin; CCl4, carbon tetrachloride; ECM, extracellular matrix; EMT, epithelial-mesenchymal transition; FBS, fetal bovine serum; FSP-1, fibroblast specific protein 1; GFP, green fluorescent protein; HSC, hepatic stellate cell; TGFβ-1, transforming growth factor β-1. Materials and Methods Materials. Recombinant transforming growth factor β-1 (TGFβ-1) was purchased from R&D Systems (Minneapolis, MN). An adenovirus expressing Cre recombinase (Ad Cre) was generated with the AdEasy adenoviral system (Stratagene, La Jolla, CA). Gliotoxin was purchased from Sigma-Aldrich (St. Louis, MO). Animals. ROSA26 stop β-galactosidase (β-gal) mice (stock number 003504), in which the β-gal reporter gene is expressed under the ROSA26 promoter after Cre recombinase-mediated excision of the stop codon, were purchased from the Jackson Laboratory (Bar Harbor, ME).9 Albumin-Cre (Alb Cre) mice (stock number 003574), in which Cre recombinase is expressed under the albumin promoter, were purchased from Jackson Laboratory. Coll GFP mice, in which GFP is expressed under the collagen α1(I) promoter, were as described.8 Generation of Triple Transgenic Mice. A ROSA26 stop β-gal+/+ mouse was mated with an Alb Cre+/+ mouse to generate double transgenic mice: ROSA26 stop β-gal+/−; Alb Cre+/−. These double transgenic mice were mated with Coll GFP+/− mice. The offspring were genotyped for ROSA26 stop β-gal and Alb Cre transgenes according to the protocols provided by Jackson Laboratory. Presence of Coll GFP transgene is evaluated by observation of green fluorescence of the tails. Triple transgenic mice ROSA26 stop β-gal+/−, Alb Cre+/−, and Coll GFP+/− were obtained in accordance with Mendel's law of inheritance. In these triple transgenic mice, cells that derived from albumin-expressing cells (i.e., hepatocyte-derived cells) are permanently labeled with β-gal and collagen-expressing cells are labeled with GFP. Isolation of Hepatocytes and Induction of EMT In Vitro with TGFβ-1. Hepatocytes were isolated from the triple transgenic mice as described.10 They were plated on collagen-coated plastic plates and cultured in Waymouth's medium supplemented with 10% fetal bovine serum (FBS) and antibiotics/antimycotics solution. Twenty hours after plating the culture medium was replaced with Waymouth's medium supplemented with 0.5% FBS and 2 μg/mL insulin. The cells were cultured for 48 hours in the presence or absence of 3 ng/mL recombinant TGFβ-1. TaqMan Real-Time Reverse-Transcription Polymerase Chain Reaction (RT-PCR). Total RNA was extracted from cells by RNeasy Mini Kit with on-column DNA digestion (Qiagen, Valencia, CA). Total RNA was reverse-transcribed to complementary DNA. Quantitative real-time RT-PCR was performed using commercially available primer-probe sets and the ABI Prism 7000 Sequence Detector and software. The relative abundance of the target genes was obtained by calculating against a standard curve and normalized to 18S ribosomal RNA as an internal control. Carbon Tetrachloride (CCl4)-Induced Liver Fibrosis and Isolation of Liver Cells from Fibrotic Livers. Mice were injected intraperitoneally with 0.5 μL of CCl4 per gram mouse (Sigma-Aldrich) diluted in corn oil every 3 days to induce liver fibrosis. The CCl4-treated liver was perfused by way of the inferior vena cava sequentially with 0.04% pronase (EMD Chemicals, Gibbstown, NJ) and 0.05% collagenase (Roche, Indianapolis, IN). The liver was excised and further digested in 0.05% pronase and 0.05% collagenase solution with gentle stirring. The cell suspension was filtered through a cell strainer to obtain a “whole liver cell fraction.” An aliquot of the “whole liver cells” were centrifuged at 50g for 1 minute to obtain a “hepatocyte fraction.” The supernatant was collected and centrifuged at 800g for 5 minutes to obtain a “nonparenchymal cell fraction.” The cells were washed, plated, and cultured overnight to allow attachment on plates. All animal studies were approved by the University Committee on Use and Care of Animals at University of California, San Diego (S07088). Evaluation for GFP and β-Gal Expression at the Single Cell Level. We evaluated individual cells for both GFP expression by fluorescent microscopy and β-gal expression by X-gal staining. As GFP fluorescence is substantially interfered by 5-bromo-4-chloro-3-indolyl, the reaction product generated for X-gal staining,11 we could not photograph GFP and X-gal staining simultaneously. Thus, we photographed GFP first, then fixed the cell and reacted with X-gal. Matching fields for GFP images and X-gal images was the key issue to evaluate an individual cell for both GFP and X-gal. In order to match fields for GFP and X-gal staining, we put cross-striped scratches with a knife on the bottom of the plates before photographing for GFP. We photographed exactly matched fields for GFP and X-gal staining using the cross-stripes as guides. Individual cells were identified morphologically and scored for positivity of GFP and X-gal staining. For liver sections, the livers were fixed with 4% neutral-buffered formalin, embedded in OCT compound, and sectioned at 6 μm. The sections were thawed in phosphate-buffered saline (PBS), photographed for GFP, reacted with X-gal, and photographed for X-gal staining. Fields were matched for GFP and X-gal based on the alignment of the sections. X-gal staining was performed using the β-gal staining kit (K1465-01, Invitrogen, Carlsbad, CA) according to the manufacturer's protocol. Immunofluorescence. The cells were fixed with 4% paraformaldehyde, permeabilized with 0.05% Triton X-100, and blocked with Cytomation Protein Block Serum-Free (Dako, Glostrup, Denmark). The primary antibody against α-SMA was clone 1A4 (Dako) and that against FSP-112 was a kind gift from Dr. Eric G. Neilson (Vanderbilt University, Nashville, TN). The primary antibody for desmin (RB9014) was purchased from Lab Vision (Fremont, CA). The primary antibodies were incubated overnight at 1:200 for α-SMA and desmin and 1:300 for FSP-1, respectively. The cells were then incubated with the respective secondary antibodies conjugated with Alexa Fluor 594 (red) (Invitrogen). In case we needed to combine GFP fluorescence images and immunostaining, we photographed for GFP before staining, as GFP fluorescence is significantly attenuated after multiple washing steps of immunofluorescence. In order to match fields for GFP and immunofluorescence staining, we put scratches on the bottom of the plates before taking pictures for GFP. Immunocytochemistry/Immunohistochemistry Combined with X-gal Staining. As β-gal activity is attenuated after multiple washing and incubation steps of immunostaining, we performed X-gal staining first and immunostaining afterward. The cells were fixed with 4% paraformaldehyde and X-gal staining was performed. The cells were then permeabilized, blocked, and incubated with a primary antibody. The primary antibodies for α-SMA, FSP-1, and desmin were described in the previous section. The primary antibody for vimentin (JM-3634-100) was purchased from MBL International (Woburn, MA) and used at 1:200. Biotin-conjugated secondary antibodies and streptavidin-biotin complex/ horseradish peroxidase were bound. Diaminobenzidine was reacted to develop a brown color. The blue precipitation of X-gal staining (5-bromo-4-chloro-3-indolyl) was stable during immunostaining procedures. Fluorescence-Activated Cell Sorting (FACS). The CCl4-treated liver was digested and whole liver cells were sorted by flow cytometry using a MoFlo (Beckman Coulter, Miami, FL) equipped with a Coherent Enterprise II Argon laser (488 nm) and a processor module permitting doublet discrimination. GFP fluorescence was collected through a 530/40 nm bandpass filter. Cells were sorted at a rate of 30,000 events per second using a sort purify 1 drop window. GFP-positive cells were sorted into Waymouth's culture medium supplemented with 10% FBS and plated. X-gal staining was performed in the same manner as described above. Sirius Red Staining. Liver tissues were fixed in 10% buffered formalin, embedded in paraffin, and sectioned at 5 μm thickness. Sections were stained with Sirius red solution (0.1% Direct Red 80 in saturated picric acid) to visualize collagen deposition. Results Primary Cultured Hepatocytes Exhibit Fibroblast-Like Morphological Change and Activate Collagen α1(I) Promoter on Stimulation with TGFβ-1. Hepatocytes were isolated and cultured from triple transgenic mice ROSA26 stop β-gal, Alb Cre, and Coll GFP. Hepatocytes were GFP-negative before TGFβ-1 treatment or after 48 hours culture without TGFβ-1 (Fig. 1A, left and middle). Treatment of hepatocytes with 3 ng/mL TGFβ-1 for 48 hours induced a fibroblast-like morphological change (Fig. 1A, upper right), consistent with previous findings.6, 13 TGFβ-1 treatment not only induced a fibroblast-like morphological change, but also activated the collagen α1(I) promoter, as demonstrated by GFP expression in TGFβ-1-treated hepatocytes (Fig. 1A, middle right). The messenger RNA (mRNA) level of collagen α1(I) in TGFβ-1-treated hepatocytes was increased to a level comparable to culture-activated HSCs (Supporting Fig. S1A). The increase was not blocked by gliotoxin, which eliminates potentially contaminating HSCs, as demonstrated by inhibition of the level of desmin, a marker for HSCs (Supporting Fig. S1B). β-Gal expression of GFP-positive cells indicated that these cells were not contaminating nonparenchymal cells, but instead originally derived from hepatocytes (Fig. 1A, bottom, right). This result was confirmed by another experiment in which X-gal staining was followed by immunocytochemistry for GFP. Consistently, hepatocytes doubly positive for GFP and β-gal appeared upon stimulation with TGFβ-1 (Fig. 1B, right). A few GFP-positive cells were seen even in the absence of TGFβ-1 (Fig. 1B, middle). However, they were never positive for β-gal, indicating they were contaminating nonparenchymal cells. Figure 1Open in figure viewerPowerPoint Primary-cultured hepatocytes exhibit fibroblast-like morphological change and activate collagen. α1(I) promoter upon stimulation with TGFβ-1. Hepatocytes were isolated from triple transgenic mice ROSA26 stop β-gal, Alb Cre, Coll GFP, and cultured in the presence or absence of 3 ng/mL recombinant TGFβ-1 (R&D Systems). (A) Phase contrast images (top) and GFP fluorescence images (middle) were obtained. Cells were fixed with 4% paraformaldehyde and X-gal staining was performed using a β-Gal staining kit (Invitrogen) (bottom). Arrows indicate double-positive cells. (B) X-gal staining was performed, followed by immunohistochemical staining for GFP using anti-GFP antibody (Rockland, Gilbertsville, PA). An arrowhead in the middle panel indicates a contaminating GFP-positive cell in the absence of TGFβ-1 stimulation. Arrows in the right panel indicate double-positive cells. Collagen-Producing Cells in CCl4-Induced Liver Fibrosis Do Not Originate from Hepatocytes. Liver fibrosis was induced in triple transgenic mice ROSA26 stop β-gal, Alb Cre, and Coll GFP by eight injections with CCl4 (Supporting Fig. S2). GFP-positive cells (collagen-expressing cells) were seen along fibrotic septa (Fig. 2, upper). However, they were never positive for β-gal, as demonstrated by the lack of double-positive cells in merged images of GFP and X-gal staining (Fig. 2, bottom). Higher-magnification images clearly show that the GFP-positive cells are present exclusively in the X-gal-negative area (Supporting Fig. S3). The absence of hepatocyte-derived collagen-expressing cells (i.e., cells double positive for GFP and β-gal) was confirmed by cell isolation; double-positive cells were not seen in the whole liver cell fraction (Fig. 3A, left), hepatocyte fraction (Fig. 3A, middle), or nonparenchymal cell fraction (Fig. 3A, right). We photographed 224 different fields and evaluated 3,522 GFP-positive cells. None of them were positive for β-gal. There were 1,737 β-gal-positive cells in the evaluated fields and none of them were GFP-positive. Nonparenchymal cells expressed β-gal upon Ad Cre-mediated excision of the stop codon (Fig. 3B), demonstrating that lack of X-gal staining in nonparenchymal cells was due to persistence of the stop codon, not due to an insufficient level of β-gal expression. This result eliminates the possibility that some GFP-positive cells were actually derived from hepatocytes but did not express sufficient β-gal to be detected by X-gal staining. The absence of double-positive cells was confirmed at different stages of liver injury: acute phase (single injection with CCl4) and chronic phase (16 times injection with CCl4), in both liver sections and in cells isolated from the injured liver (Supporting Figs. S4, S5, S6, and S7). Furthermore, GFP-positive cells were sorted by FACS and none of them (450,000 GFP-positive cells) were positive for β-gal (Fig. 4). These results clearly demonstrate that type I collagen-expressing cells in CCl4-induced liver fibrosis do not originate from hepatocytes. Figure 2Open in figure viewerPowerPoint GFP-positive areas do not overlap X-gal-positive areas in liver sections of the triple transgenic mice injected with CCl4 (8 times). Triple transgenic mice ROSA26 stop β-gal, Alb Cre, Coll GFP were given 8 injections with 0.5 μL/g CCl4 every 3 days to induce liver fibrosis. The liver was fixed and embedded in OCT compound. The sections were photographed for GFP fluorescence (top), incubated with X-gal overnight, and photographed again for X-gal (middle). Figure 3Open in figure viewerPowerPoint Collagen-producing cells do not originate from hepatocytes in mice injected with CCl4 (8 times). Triple transgenic mice ROSA26 stop β-gal, Alb Cre, Coll GFP were given 8 injections with 0.5 μL/g CCl4 every 3 days to induce liver fibrosis. Cells were isolated from the liver as described in Materials and Methods. (A) Phase contrast images (top) and GFP fluorescence images (middle) were obtained. Cells were fixed with 4% paraformaldehyde and X-gal staining was performed using a β-Gal staining kit (Invitrogen) (bottom). (B) Nonparenchymal cells from CCl4-treated double transgenic mice ROSA26 stop β-gal, Alb Cre were infected with adenovirus encoding Cre recombinase (Ad Cre) or green fluorescent protein (Ad GFP) as a negative control at 500 multiplicity of infection (MOI). The cells were stained with X-gal after 7 days. Figure 4Open in figure viewerPowerPoint FACS-sorted GFP-positive cells from the triple transgenic mice injected with CCl4 do not express β-gal. Triple transgenic mice ROSA26 stop β-gal, Alb Cre, Coll GFP were given 16 injections with 0.5 μL/g CCl4. The liver was digested and GFP-positive cells were sorted. Phase contrast images (top) and GFP fluorescence images (middle) were obtained. Cells were fixed with 4% paraformaldehyde and X-gal staining was performed using a β-Gal staining kit (Invitrogen) (bottom). Hepatocytes Do Not Express Mesenchymal Markers in CCl4-Induced Liver Fibrosis. To address if hepatocytes express mesenchymal markers during liver injury we performed immunostaining following X-gal staining. No cells were double-positive for α-SMA and β-gal in CCl4-treated liver of double transgenic mice (Fig. 5A). Similarly, no cells were double-positive for FSP-1, desmin, or vimentin and β-gal in CCl4-treated liver of double transgenic mice (Fig. 5B; Supporting Fig. S8). β-Gal-positive cells in nonparenchymal cell fraction were never positive for α-SMA, FSP-1, desmin, or vimentin and must represent rare contaminating hepatocytes. Immunohistochemical staining and X-gal staining of liver sections supported the absence of hepatocyte-derived α-SMA or FSP-1 positive cells in CCl4-treated liver (Supporting Fig. S9). Figure 5Open in figure viewerPowerPoint Hepatocytes do not express mesenchymal markers in CCl4-induced liver fibrosis in mice. Double transgenic mice ROSA26 stop β-gal, Alb Cre were given 8 injections with 0.5 μL/g CCl4 every 3 days to induce liver fibrosis. Cells were isolated from the liver as described in the Materials and Methods. The cells were fixed with 4% paraformaldehyde and stained with X-gal using a β-Gal staining kit (Invitrogen). Thereafter, the cells were permeabilized and stained for α-SMA (A) or FSP-1 (B). Upper: whole liver cells; lower: NPC-enriched fraction. Hepatocytes Do Not Express Mesenchymal Markers Even on Stimulation with TGFβ-1 In Vitro. Hepatocytes cultured in the presence of TGFβ-1 exhibited fibroblast-like morphological changes (Fig. 6A, upper) and expressed GFP driven by the collagen α1(I) promoter (Fig. 6A, second). Although some α-SMA positive cells were detected in the primary culture of hepatocytes (Fig. 6A, third), they appeared even in the absence of TGFβ-1 (data not shown) and were never positive for β-gal (Fig. 6A, bottom), demonstrating that they were contaminating nonparenchymal cells and did not derive from hepatocytes. Similarly, hepatocytes treated with TGFβ-1 did not express FSP-1 (Fig. 6B, third) or desmin (Supporting Fig. S10, third). A few FSP-1 or desmin-positive cells were present in the primary cultures of hepatocytes, but were never positive for β-gal (Fig. 6B, bottom, and Supporting Fig. S10, bottom). The mRNA level of FSP-1 in hepatocyte culture was neither increased in a time-dependent manner nor enhanced by addition of TGFβ-1 (Supporting Fig. S11). This indicates that FSP-1 is not expressed by hepatocytes cultured on plastic dishes and FSP-1 is not induced by stimulation with TGFβ1. Figure 6Open in figure viewerPowerPoint Hepatocytes do not express mesenchymal markers even upon stimulation with TGFβ-1 in vitro. Hepatocytes were isolated from triple transgenic mice ROSA26 stop β-gal, Alb Cre, Coll GFP, and stimulated with 3 ng/mL recombinant TGFβ-1 (R&D Systems) for 2 days. Phase contrast images (top panels) and GFP fluorescence images (second panels) were obtained. Cells were fixed with 4% paraformaldehyde, permeabilized, and stained for α-SMA (A) or FSP-1 (B) (third panels). Separately, the TGFβ-1-treated hepatocytes were fixed, stained with X-gal, permeabilized, and stained for α-SMA or FSP-1 (bottom panels). Arrows and arrowheads, respectively, indicate α-SMA- and FSP-1-positive but β-gal-negative cells appearing in the primary culture of hepatocytes in the presence of 3 ng/mL TGFβ-1, representing contaminating nonparenchymal cells. Discussion Despite intense study, the origin of cells that produce the ECM in liver fibrosis is still a matter of debate. In the normal liver HSCs are characterized by vitamin A-containing lipid droplets and reside in a quiescent state in the space of Disse. Upon liver injury, HSCs acquire a myofibroblastic phenotype in response to inflammatory and profibrogenic stimuli, express activation markers including α-SMA, and synthesize ECM including type I collagens. Currently, HSCs are considered to be the major source of myofibroblasts in the liver.3, 14 However, studies have demonstrated that HSCs are not the only cell type that contributes to the accumulation of ECM in liver fibrosis. Beaussier et al.4 suggested that portal mesenchymal cells contribute to liver fibrosis in ischemic or cholestatic liver injury. We recently identified fibrocytes as another potential source of ECM-producing population in the liver. These collagen-expressing cells derive from the bone marrow and migrate to the liver in response to injury.5 An additional concept suggests that myofibroblasts can also originate from epithelial cells through phenotypical changes called EMT. Pioneering studies on EMT in the field of organ fibrosis was accomplished in the kidney,15 lens,16 and lung.17, 18 It was recently proposed that EMT also contributes to liver fibrosis.6, 13 Zeisberg et al.6 demonstrated that cells that formerly expressed albumin (that is, hepatocytes) acquire expression of FSP-1 in response to CCl4 in vivo or TGFβ-1 in vitro. Kaimori et al.13 reported that hepatocytes express collagen α1(I) in response to TGFβ-1 in vitro, and that Smad signaling mediates EMT, which was also reported by Dooley et al.19 In addition, biliary epithelial cells were suggested to undergo EMT.20-22 Our current study did not investigate if cholangiocytes undergo EMT and contribute the pool of ECM-producing cells in the injured liver. To the best of our knowledge, none of the above-mentioned studies provide evidence that hepatocyte-derived cells express ECM genes and contribute to ECM production in liver fibrosis in vivo. For example, Zeisberg et al.6 relied solely on FSP-1 expression as a marker for fibrogenic cells. However, FSP-1 positive cells in the liver have not been characterized so far, and it is unknown if these cells synthesize ECM in vivo. Kaimori et al.13 showed that primary cultures of hepatocytes stimulated with TGFβ-1 express collagen α1(I) together with morphological changes consistent with EMT in vitro. However, as in all experiments with primary cultured cells, contamination with some other populations cannot be ruled out. Although this study also observed collagen α1(I) expression in a hepatocyte cell line as well, the increase in the expression level precedes and does not parallel the morphological changes of EMT. The transient increase in collagen α1(I) expression observed in the cell line might reflect a temporal activation of collagen α1(I) promoter irrelevant to EMT. Results from previous studies suggesting that cholangiocyte undergo EMT are inconclusive because they entirely rely on double immunofluorescence staining for cholangiocyte markers and surrogate markers for mesenchymal cells including FSP-1.20-22 As pointed out above, it remains unknown if FSP-1-positive cells express collagen and therefore also contribute to the ECM-producing cells in vivo. Cell fate tracking for cholangiocytes has not been performed so far and genetic evidence that cholangiocytes lose their epithelial characteristics and acquire a mesenchymal phenotype and start to synthesize ECM is therefore still missing. Our current study did not address the role of cholangiocytes in EMT. To analyze if hepatocyte-derived cells indeed contribute to ECM production in liver fibrosis, we utilized a reporter mouse in which GFP is expressed under the murine collagen α1(I) promoter/enhancer in combination with a cell fate tracing technique used in the previous study by Zeisberg et al.6 Our in vitro results using primary cultured hepatocytes initially appeared to support the concept of EMT. Hepatocyt
