Investigation of liver fibrosis in clinical practice

The liver is composed of different hepatic fibrogenic cells: hepatic stellate cells, portal fibroblasts, fibroblasts of the Glisson capsule surrounding the liver and vascular smooth muscle cells and the second layer cells present around centrolobular veins. During liver disease, one or several populations of these cells are activated, transformed into myofibroblasts and secrete the extra-cellular matrix. There are markers to identify hepatic stellate cells either quiescent (CRBP-1) or activated (alpha-smooth muscle actin). Liver biopsy, the current "gold-standard" to estimate liver fibrosis cannot be used anymore as a "gold standard". Furthermore, it is a costly procedure with adverse effects feared by patients and clinicians. Alternative to liver biopsy using non-invasive-tests or technics include FibroTest-ActiTest, transient-elastography, hepatic vein transit time using contrast ultrasonography, magnetic resonance imaging. As a routine test, the FibroTest-ActiTest is a validated one for patients with chronic hepatitis C. The advantage of the non-invasive tests or technics is that they provide a rapid and quantitative estimation of fibrosis. With these new methods, it is possible to follow the progression of the disease and its regression either spontaneously or under treatment. In conclusion, clinicians have in their hands several painless tools to explore liver fibrosis that can be easily repeated.

A role for thrombin in liver fibrosis

BACKGROUND/AIM

Several lines of evidence incriminate the serine proteinase thrombin in liver fibrogenesis either through its procoagulant function or its signaling via cell-surface receptors. The aim of this study was therefore to evaluate the effect of thrombin inhibition on experimental liver fibrosis.

METHODS

Fibrosis was induced in rats by administration of CCl4 for either three or seven weeks. Oral administration of the thrombin antagonist SSR182289 started one week after the start of CCl4 intoxication. Fibrosis and the area occupied by alpha smooth muscle actin (ASMA) positive cells were quantified with histomorphometry. Expression of fibrosis related genes was measured by real time RT-PCR.

RESULTS

After three weeks of CCl4, treatment with SSR182289 did not significantly decrease the area of fibrosis but significantly decreased the area of ASMA positive cells by 22% (p = 0.03) and the expression of TIMP-1 mRNA by 52% (p = 0.02). There was no effect on gene expression of collagen I, MMP-2, or TIMP-2. After seven weeks of CCl4, treatment with SSR182289 resulted in a significant decrease in fibrosis (-30%, p = 0.04) and ASMA positive areas (-35%, p = 0.05). SSR182289 alone had no effect on the measured parameters. Additionally, it did not alleviate the acute toxicity of CCl4 as shown by measuring levels of serum aminotransferases and the area of necrosis.

CONCLUSIONS

These data provide evidence that thrombin antagonism can reduce liver fibrogenesis. The early effect of SSR182289 on ASMA and TIMP-1 expression suggests that it is beneficial in reducing fibrogenic cell activation.

Validation of a morphometric method for evaluating fibroblast numbers in normal and pathologic tissues

The aim of this work was to validate an image analysis method, based on cell nuclei form factor determination, for counting fibroblasts within human dermis. We first used reconstructed dermal equivalents in which fibroblasts can also be counted directly after lysis of the collagen matrix. We found a good correlation between the results of direct counting and those of image analysis from day 10 to day 28 of culture. When applied to young normal donors' skin biopsies fixed in Bouin's solution and embedded in paraffin, the image analysis method yielded mid-dermis fibroblast counts of between 2100 and 4100 per mm3 of fresh tissue. A nuclear form factor (FF) comprised between 0.35 and 0.84 was found to be a biologic marker of fibroblasts. This was confirmed after fibroblast discrimination from other cell types, which had rounder nuclei (FF >/= 0.85) and were identified either by their location (e.g. endothelial cells) or by labeling with specific antibodies (e.g. lymphocytes and monocytes/macrophages). Similar results were obtained with seven healthy donors' skin biopsies that had been frozen in nitrogen liquid and cryostat-sectioned, showing that this counting method is independent of the histologic procedure. Finally, analysis of samples of hypertrophic scars from two patients revealed that fibroblast density in some parts of the dermis was more than twice the value found in other parts presenting a fibroblast density almost normal, showing that this cell counting method can also be used to assess fibroblast heterogeneity within a given tissue.

[Mechanisms of hepatic fibrogenesis]

Hepatic fibrosis is a scaring process leading to cirrhosis, a major complication of numerous chronic liver diseases. Hepatic stellate cells play a central role in the fibrotic process. After parenchymal or biliary injury, cytokines and growth factors allow the recruitment, proliferation, and activation, of stellate cells toward myofibroblasts, which secrete the extracellular matrix. Fibrosis, resulting from the failure of the balance between synthesis and degradation of extracellular matrix, is an evolutive and potentially reversible process. Histological examination is the main investigation to quantify fibrosis. Serological tests are warranted to allow a non invasive follow up of patients. Development of antifibrotic therapies should soon permit to slow down the evolution toward cirrhosis, limiting the needs for hepatic transplantation.

Trans-resveratrol, a grapevine-derived polyphenol, blocks hepatocyte growth factor-induced invasion of hepatocellular carcinoma cells

We have shown that liver myofibroblasts stimulate in vitro invasion of hepatocellular carcinoma cell lines through a hepatocyte growth factor/urokinase-dependent mechanism. Resveratrol, a grapevine-derived polyphenol, has been shown to inhibit cellular events associated with tumor initiation, promotion and progression. The aim of this study was to evaluate the effects of trans-resveratrol on invasion of the human hepatoma cell line HepG2. Cell invasion was assessed using a Boyden chamber assay. Activation of the HGF signal transduction pathways was evaluated by Western blot with phospho-specific antibodies. Urokinase expression was measured by RT-PCR and zymography. Trans-resveratrol decreased hepatocyte growth factor-induced cell scattering and invasion. It also decreased cell proliferation without evidence for cytotoxicity or apoptosis. Trans-resveratrol did not decrease the level of the hepatocyte growth factor receptor c-met and did not impede the hepatocyte growth factor-induced increase in c-met precursor synthesis. Moreover, trans-resveratrol did not decrease hepatocyte growth factor-induced c-met autophosphorylation, or Akt-1 or extracellular-regulated kinases-1 and -2 activation. Finally, it did not decrease urokinase expression and did not block the catalytic activity of urokinase. In conclusion, our results demonstrate that trans-resveratrol decreases hepatocyte growth factor-induced HepG2 cell invasion by an as yet unidentified post-receptor mechanism.

Trans-resveratrol, a grapevine-derived polyphenol, blocks hepatocyte growth factor-induced invasion of hepatocellular carcinoma cells

We have shown that liver myofibroblasts stimulate in vitro invasion of hepatocellular carcinoma cell lines through a hepatocyte growth factor/urokinase-dependent mechanism. Resveratrol, a grapevine-derived polyphenol, has been shown to inhibit cellular events associated with tumor initiation, promotion and progression. The aim of this study was to evaluate the effects of trans-resveratrol on invasion of the human hepatoma cell line HepG2. Cell invasion was assessed using a Boyden chamber assay. Activation of the HGF signal transduction pathways was evaluated by Western blot with phospho-specific antibodies. Urokinase expression was measured by RT-PCR and zymography. Trans-resveratrol decreased hepatocyte growth factor-induced cell scattering and invasion. It also decreased cell proliferation without evidence for cytotoxicity or apoptosis. Trans-resveratrol did not decrease the level of the hepatocyte growth factor receptor c-met and did not impede the hepatocyte growth factor-induced increase in c-met precursor synthesis. Moreover, trans-resveratrol did not decrease hepatocyte growth factor-induced c-met autophosphorylation, or Akt-1 or extracellular-regulated kinases-1 and -2 activation. Finally, it did not decrease urokinase expression and did not block the catalytic activity of urokinase. In conclusion, our results demonstrate that trans-resveratrol decreases hepatocyte growth factor-induced HepG2 cell invasion by an as yet unidentified post-receptor mechanism.

Early activation of hepatic stellate cells and perisinusoidal extracellular matrix changes during ex vivo pig liver perfusion

In previous works, we observed during liver transplantation procedure, the early activation of hepatic stellate cells (HSC) which acquire alpha-smooth muscle (SM) actin expression. In this study, we evaluated changes in HSC and in perisinusoidal extracellular matrix during ex vivo pig liver perfusion. Under general anesthesia, pig livers were flushed and removed, and then perfused ex vivo for 6 h with homologous blood. Liver biopsies were taken before and after washout, at 5 min perfusion, and then hourly. Tissues were processed for immunohistochemistry, immunofluorescence, confocal microscopy, in situ hybridization and electron microscopy. Before and after liver washout, alpha-SM actin was present in vessel walls but in very few lobular HSC. After 1 h perfusion, a strong reactivity for alpha-SM actin was present in HSC, particularly along dilated sinusoids. At the ultrastructural level, numerous microfilament bundles appeared in HSC cytoplasmic processes. During perfusion, type I and type IV collagens, type III procollagen, and fibronectin acquired a looser organisation in relation with the enlargement of perisinusoidal spaces; laminin appeared in perisinusoidal spaces around portal areas and fibrillin deposits increased. In situ hybridization studies showed an increase of the type I procollagen mRNA expression mainly in portal tracts and septa. Ex vivo liver perfusion induces: 1) an early activation of HSC which acquire the expression of alpha-SM actin, and 2) significant changes in the perisinusoidal extracellular matrix. These results are compatible with the view that HSC function as liver specific pericytes participating in the regulation of sinusoidal blood pressure.

Myofibroblast: a prognostic marker and target cell in progressive renal disease

Myofibroblasts play an important role in many tissue injuries, and particularly in renal disease. The myofibroblast differentiation is an early event in the development of fibrosis. Myofibroblast-like cells express smooth muscle (SM) cytoskeletal markers (alpha-SM actin in particular) and participate actively in the production of extracellular matrix. Identification of early risk factors, particularly histological criteria, could be useful to identify patients at risk of progressive renal failure and needing a treatment. The evaluation of myofibroblast differentiation in renal tissue may reflect the intensity of tissue injury, predict long term outcome of chronic renal disease and help physicians to select therapeutic choices. More than a disease activity indicator. myofibroblasts appear to be a pivotal target for future therapies in progressive renal disease.

Expression and cellular localization of fibrillin-1 in normal and pathological human liver

BACKGROUND

The expression and the distribution of fibrillin-1 and elastin were studied in normal and pathological human liver samples.

METHODS

As controls, histologically normal/subnormal liver samples (n = 24) were used. Pathological samples corresponded to seven cirrhosis and eight hepatocellular carcinomas (HCC) developed on cirrhotic (four) or noncirrhotic (four) liver.

RESULTS

In normal liver, fibrillin-1 and elastin co-localized in vessel walls and portal tract connective tissue. Fibrillin-1 alone was detected along sinusoids and in portal spaces at the interface with the limiting hepatocytic plates and close to the basement membrane of bile ducts. By transmission electron microscopy, typical bundles of microfibrils were detected both in Disse space and in portal zones. Cirrhotic nodules were usually rich in fibrillin-1 along sinusoids; fibrillin-1 and elastin were co-localized in fibrotic septa surrounding nodules. In HCC, fibrillin-1 was present between tumoral hepatocytes; stromal reaction around the tumors contained both fibrillin-1 and elastin.

CONCLUSIONS

Fibrillin-1 was associated with elastin in portal mesenchyme and vessel walls of normal liver, in fibrotic septa around cirrhotic nodules and stromal reaction around HCC, but was expressed alone in the perisinusoidal space. The functional roles for fibrillin-1 in non-elastic tissues, such as the liver, remain to be elucidated.

Expression of hepatocyte growth factor in human hepatocellular carcinoma

BACKGROUND/AIMS

We have shown that hepatocyte growth factor, secreted by human liver myofibroblasts, promoted in vitro invasion of human hepatocellular carcinoma cell lines. The aim of this work was to measure hepatocyte growth factor expression in 29 human hepatocellular carcinomas and the corresponding peri-tumoral livers.

METHODS

We used reverse transcription-polymerase chain reaction, in situ hybridization, ELISA and Western blot.

RESULTS

Sixty-two of tested hepatocellular carcinomas were positive by reverse transcription-polymerase chain reaction. With in situ hybridization, a signal was found in every sample. In many cases, the signal was localized in cells labeled with an anti-smooth muscle alpka-actin antibody, while hepatocytes were mostly non-labeled. ELISA, performed in 15 pairs of hepatocellular carcinomas and surrounding livers, detected hepatocyte growth factor in every sample with wide variations. Hepatocellular carcinomas that had developed in non-cirrhotic livers contained essentially the same amount of hepatocyte growth factor as the matching non-tumoral liver. In cirrhotic livers, the hepatocyte growth factor content of the tumors was significantly lower than that of the surrounding cirrhotic livers.

CONCLUSIONS

These data indicate that hepatocyte growth factor is expressed at significant levels in every hepatocellular carcinoma tested and that its expression takes place in the stromal myofibroblasts.

Paradoxical pro-invasive effect of the serine proteinase inhibitor tissue factor pathway inhibitor-2 on human hepatocellular carcinoma cells

We have previously shown that human liver myofibroblasts promote in vitro invasion of human hepatocellular carcinoma (HCC) cells through a hepatocyte growth factor (HGF)/urokinase/plasmin-dependent mechanism. In this study, we demonstrate that myofibroblasts synthesize the serine proteinase inhibitor tissue factor pathway inhibitor-2 (TFPI-2). Despite the fact that recombinant TFPI-2 readily inhibits plasmin, we show that it potentiates HGF-induced invasion of HCC cells and is capable of inducing invasion on its own. Furthermore, HCC cells stably transfected with a TFPI-2 expression vector became spontaneously invasive. HCC cells express tissue factor and specifically factor VII. Addition of an antibody to factor VII abolished the pro-invasive effect of TFPI-2. We suggest that TFPI-2 induces invasion following binding to a tissue factor-factor VIIa complex preformed on HCC cells. Our data thus demonstrate an original mechanism of cell invasion that may be specific for liver tumor cells.

Deactivation of cultured human liver myofibroblasts by trans-resveratrol, a grapevine-derived polyphenol

Liver myofibroblasts are major actors in the development of liver fibrosis and cancer progression. There is a large interest in drugs that might deactivate these cells. Many studies have shown that the grapevine-derived polyphenol, trans-resveratrol, and other stilbenes have therapeutic potential in some diseases. In this work, we have studied the effect of grapevine polyphenols on cultured human liver myofibroblasts. We have shown that trans-resveratrol profoundly affects myofibroblast phenotype. Trans-resveratrol induced morphological modifications. It markedly reduced proliferation of myofibroblasts in a dose-dependent manner. Trans-resveratrol also decreased the expression of alpha smooth muscle actin (alpha-SMA) without affecting vimentin or beta-cytoplasmic actin expression. It decreased myofibroblast migration in a monolayer wounding assay. We also showed that trans-resveratrol inhibited the messenger RNA (mRNA) expression of type I collagen. Finally, it decreased the secretion of matrix metalloproteinase 2 (MMP-2). We conclude that trans-resveratrol can deactivate human liver myofibroblasts. In the second part of this study, we have shown that neither trans-piceid (a glycosylated analog) nor trans-piceatannol (a hydroxylated analog) reproduces trans-resveratrol effects on liver myofibroblasts. We finally show that, although trans-resveratrol decreases the proliferation of skin fibroblast and vascular smooth muscle cells, it does not affect their expression of alpha-SMA, which indicates some cell specificity.

Role of myofibroblasts during normal tissue repair and excessive scarring: interest of their assessment in nephropathies

Following injury, tissue repair process takes place involving inflammation, granulation tissue formation and scar constitution. Granulation tissue develops from the connective tissue surrounding the damaged area and contains vessels, inflammatory cells, fibroblasts and myofibroblasts. Myofibroblasts play an important role in many tissue injuries and fibrocontractive diseases. The process of normal wound repair after tissue injury follows a closely regulated sequence including the activation and the proliferation of fibroblastic cells. In pathological situations, the normal resolution stages are abrogated and the proliferation of myofibroblasts continues, inducing excessive accumulation of extracellular matrix. The differentiation of fibroblastic cells into myofibroblasts is an early event in the development of tissue fibrosis. Myofibroblastic cells express smooth muscle cytoskeletal markers (alpha-smooth muscle actin in particular) and participate actively in the production of extracellular matrix. The evaluation of myofibroblast differentiation in renal biopsies would be useful for histopathologists to appreciate the intensity of tissue injury and particularly to predict the long term outcome of some nephropathies. Immunohistochemical studies for alpha-smooth muscle actin should be made systematically in renal tissue biopsies. Myofibroblastic differentiation appears to play a significant role in the progression of renal failure and seems to be a useful marker of progressive disease.

Interstitial alpha-smooth muscle actin: a prognostic marker in membranous nephropathy

AIM

Membranous nephropathy in adults causes 20% of nephrotic syndromes. Spontaneous outcome of this glomerulopathy is difficult to evaluate from clinical and histological data. Some patients can achieve complete remission, while others develop progressive renal failure. In this study we assessed alpha-smooth muscle actin (alpha-SMA) expression in renal interstitial myofibroblasts as a marker to predict the outcome of membranous nephropathy.

PATIENTS AND METHODS

Renal function tests in tandem with alpha-SMA immunolabelling were performed on 25 patients with a mean follow-up of 7.2+/-5.6 years. The intensity of interstitial alpha-SMA (ialpha-SMA) immunostaining was compared to changes in glomerular filtration rate (GFR) evaluated by inulin clearance, between the time of diagnosis (GFR1) and the end of follow-up (GFR2).

RESULTS

A significant correlation (r = 0.62, p<0.001) was found, between the intensity of interstitial myofibroblasts, immunolabeling and GFR at the end of follow-up. Moreover, the annual GFR variation and the annual percentage of GFR variation were correlated to interstitial myofibroblast labeling (respectively r = 0.62, p<0.001; r = 0.67, p<0.001). In addition, the importance of proteinuria, initial GFR impairment and fibrosis were confirmed as prognostic criteria.

CONCLUSION

This study strongly shows that ialpha-SMA expression is a useful and early prognostic marker in the evolution of membranous nephropathy.

Activation of cultured rat hepatic stellate cells by tumoral hepatocytes

Hepatocellular carcinoma (HCC) is the main type of primary liver cancer, and it develops from hepatocytes. The stroma of HCC is infiltrated by myofibroblasts. In other settings, such as liver fibrosis, myofibroblasts are derived mainly from the activation of hepatic stellate cells (HSC). In this study, we investigated whether tumoral hepatocytes were able to activate HSC. HSC were isolated from normal rats and were plated in dishes coated with Matrigel, to prevent their spontaneous activation. HSC were exposed to conditioned medium (CM) from the rat HCC lines Fao and H5. Tumor cell CM elicited major morphologic changes, such as spreading and generation of cytoplasmic processes. Fao and H5 CM increased HSC proliferation to 1.60 and 1.76 times control values, respectively. The expression of alpha-smooth muscle actin was low or undetectable in control cells and was markedly increased by both tumor cell CM but not by normal rat hepatocyte CM. Desmin expression was also enhanced. Gelatinase A secretion was significantly increased 1.20-fold by Fao CM and 1.55-fold by H5 CM. Expression of beta-type platelet-derived growth factor receptor mRNA was increased 5.8-fold by H5 CM but was decreased to 13% of control levels by Fao CM. HSC activation by tumor cell CM was not prevented by urokinase or matrix metalloproteinase inhibitors, suggesting that Matrigel degradation was not central to the activation process. Finally, a blocking antibody to transforming growth factor-beta1 did not impede Fao CM-induced activation but significantly blocked the increase in matrix metalloproteinase-2 expression induced by H5 CM. Our results show that tumoral rat hepatocyte CM is able to induce the activation of rat HSC in culture. The lack of induction of beta-type platelet-derived growth factor receptor mRNA by Fao CM indicates that, in some cases, tumor-induced activation differs from classic fibrosis-type activation. Our data thus suggest that HSC recruitment and activation in HCC could be under the control of tumor cells.

Effect of pentoxifylline on early proliferation and phenotypic modulation of fibrogenic cells in two rat models of liver fibrosis and on cultured hepatic stellate cells

BACKGROUND/AIMS

During liver fibrosis, different fibroblastic cells, i.e. hepatic stellate cells (HSCs) or portal fibroblasts, are involved in the development of lesions, and acquire myofibroblastic differentiation. We investigated, in the rat, whether pentoxifylline can influence the early phase of fibrogenesis in two animal models of fibrosis induced by either carbon tetrachloride (CCl4) plus acetone (given twice) or bile duct ligation.

METHODS

The fibroproliferative response and myofibroblastic phenotypic modulation were evaluated by PCNA and alpha-smooth muscle (alpha-SM) actin immunohistochemistry, respectively, in livers taken 24 h after the last CCl4 treatment or 72 h after bile duct ligation. Desmin expression was also measured, and inflammation was evaluated by ED-1 staining. Furthermore, proliferation and alpha-SM actin expression were studied in cultured HSCs after pentoxifylline treatment.

RESULTS

In the CCl4-acetone groups, pretreatment with pentoxifylline decreased the proliferative response and expression of alpha-SM actin in the HSCs. Similarly, pentoxifylline reduced the proliferation and myofibroblastic differentiation of portal fibroblasts after bile duct ligation. Pentoxifylline reduced ED-1 expression, particularly in the CCl4 model, where there was significant inflammation. In cultured pentoxifylline-treated HSCs, both proliferation and alpha-SM actin expression were decreased.

CONCLUSIONS

In both animal models of fibrosis, during the early stages of tissue injury, pentoxifylline was able to reduce fibroproliferation and myofibroblastic differentiation and to reduce hepatocellular damage and the inflammatory response, particularly in the toxin-induced model. In culture, alpha-SM actin expression decreased in both growing and quiescent HSCs treated with pentoxifylline, indicating that the drug may also exert a direct effect on hepatic fibrogenic cells.

Long-term administration of interferon-alpha in non-responder patients with chronic hepatitis C: follow-up of liver fibrosis over 5 years

In chronic hepatitis C, previous data have shown that short-term treatment with interferon-alpha (IFN-alpha) can reduce collagen deposition in the liver independently of the viral response. The aim of this work was to determine, in non-responder patients, the long-term effect of IFN-alpha on liver fibrosis according to the total administered dose and the fibrotic stage. Fibrosis was investigated on liver biopsies from 24 non-responder patients with chronic hepatitis C retreated with successive courses of IFN-alpha. The degree of liver fibrosis was assessed on three successive biopsies, performed before IFN-alpha treatment and 1 and 5 years later, in 13 and 11 patients, respectively, treated for less (mean: 7.5 months, 313 MU) and more (mean: 21.8 months, 791 MU) than 1 year. For each biopsy, fibrosis was assessed using a histological semiquantitative fibrosis scoring system and by morphometry after picrosirius red staining. Regardless of the dose and duration of IFN-alpha therapy, a slight decrease of fibrosis was observed in patients 5 years after starting treatment. In cirrhotic patients, a short treatment induced an improvement followed by a relapse of fibrosis in 57%, and only 43% of patients showed constant collagen regression over the 5 years of follow-up. On the contrary, after prolonged therapy, a progressive and significant decrease occurred throughout the follow-up period in all patients (P = 0.045). Long-term treatment with IFN-alpha is therefore associated with regression of liver fibrosis, particularly in cirrhotic patients. These promising results need to be confirmed in a larger series of patients.

Direct evidence that hepatocyte growth factor-induced invasion of hepatocellular carcinoma cells is mediated by urokinase

BACKGROUND/AIMS

We have shown that hepatocyte growth factor secreted by human hepatic myofibroblasts increased the in vitro invasion of the hepatocarcinoma cell line HepG2 through Matrigel. Our aim in this study was to evaluate the role of urokinase in this process.

METHODS

Expression of urokinase in HepG2 cells was measured by Northern blot and zymography, and plasminogen activation was shown by a chromogenic substrate assay. Cell invasion was assayed on Matrigel-coated filters. Urokinase and urokinase receptor transcripts in hepatocarcinoma were detected by reverse transcription-polymerase chain reaction. Activated hepatocyte growth factor was detected by Western blot with a hepatocyte growth factor-beta chain-specific antibody.

RESULTS

HepG2 cells expressed urokinase mRNA and secreted active urokinase. Urokinase expression was enhanced by hepatocyte growth factor at the protein and mRNA level. Notably, cell-surface-associated urokinase was increased 22-fold by hepatocyte growth factor. Hepatocyte growth factor also increased urokinase receptor mRNA expression. B428, a urokinase inhibitor, decreased by up to 70% HepG2 invasion induced by myofibroblasts and by 90% that induced by recombinant hepatocyte growth factor. This was not due to a decrease in the generation of activated hepatocyte growth factor by myofibroblasts. Finally, all 17 hepatocarcinoma samples tested expressed urokinase and urokinase receptor transcripts.

CONCLUSION

Hepatocyte growth factor-dependent, myofibroblasts-induced invasion of HepG2 cells is secondary to the induction of urokinase expression on tumor cells.

Subcutaneous tissue fibroblasts transfected with muscle and nonmuscle actins: A good in vitro model to study fibroblastic cell plasticity

Cultured fibroblasts develop several biochemical and morphological properties of smooth muscle cells, particularly the expression of alpha-smooth muscle actin, the actin isoform typical of vascular smooth muscle cells. They resemble modified fibroblasts or myofibroblasts observed in granulation tissue during wound repair and in fibrotic situations. We have analysed by immunolabeling the fate of exogenous epitope-tagged actin isoforms by transfection of the corresponding cDNAs into fibroblasts cultured from rat subcutaneous tissue. Tagged muscle actins were efficiently integrated into stress fibers and did not produce obvious changes in cell shape of transfected cells. Transfected nonmuscle actins in contrast changed the morphology and were not or poorly incorporated into stress fibers. These cultured subcutaneous fibroblasts behave similarly to smooth muscle cells when transfected with the same actin encoding cDNAs, indicating another common characteristic of these two cell types in sorting and targeting actin isoforms. Subcutaneous fibroblasts transfected with muscle and nonmuscle actin isoforms provide a good in vitro model to analyze the intracellular sorting of isoactins and to improve our knowledge of myofibroblast characterization and differentiation during tissue repair as well as to understand the relationships between modifications of actin cytoskeleton, adhesion and extracellular matrix proteins.

Extracellular matrix deposition, lysyl oxidase expression, and myofibroblastic differentiation during the initial stages of cholestatic fibrosis in the rat

Early studies showed that during hepatic fibrosis induced by bile duct ligation, fibroblasts within the portal tracts proliferate and express alpha-smooth muscle (SM) actin, suggesting that they may be involved in the deposition of extracellular matrix components in cholestatic fibrosis. Thus, we investigated the deposition of extracellular matrix components (laminin, fibronectin EIIIA, collagen I and IV, procollagen III, elastin, tenascin) as well as the expression of lysyl oxidase and of alpha-SM actin in the portal zone at 24, 48, and 72 hours and 7 days after ligation of the common bile duct. Rat liver tissues were processed for immunofluorescence, in situ hybridization, immunohistochemistry, and for electron and immunoelectron microscopy. At all times examined after bile duct ligation, laminin was observed essentially in the basal membrane of vessels and portal ductules. In sham-operated animals, the fibronectin EIIIA was present exclusively in vessels; at 24 hours postinjury, fibronectin EIIIA expression appeared in both the portal zone and along sinusoids. Two days after ligation, increased expressions of collagen I and IV, procollagen III, and elastin were observed within the portal zone, compared with sham-operated animals. The deposition of these components increased thereafter. Tenascin expression increased soon after bile duct ligation in stroma surrounding proliferating ductules, reaching a maximum at 48 hours; thereafter, expression was restricted to the periphery of proliferating ductules. By in situ hybridization, procollagen I and tissue inhibitor of metalloproteinase-1 mRNA expression was greatly increased in periductular areas at 24 hours postligation and remained elevated throughout the experiment. At 24 hours, a strong reactivity for lysyl oxidase appeared in the portal zone, and, as in controls, alpha-SM actin expression was restricted to vascular SM cells. In the stroma adjacent to proliferating ductules, alpha-SM actin appeared at 48 hours, and the number of alpha-SM actin-positive cells increased until the 7th day. Lysyl oxidase staining increased until 72 hours after bile duct ligation, when it was located in areas surrounding the myofibroblastic cells. At 7 days, lysyl oxidase expression was restricted around myofibroblastic cells present at the periphery of the reactive tissue and appeared to extend into the surrounding parenchyma. These results show that after bile duct ligation, extracellular matrix deposition, and lysyl oxidase expression occur very early in portal connective tissue surrounding proliferating ductules, and precede myofibroblastic differentiation, ie, alpha-SM actin expression. In addition, the data are compatible with the suggestion that in the bile duct ligation model, myofibroblastic differentiation represents an adaptive response to modification of the extracellular matrix environment.

Apoptosis during wound healing, fibrocontractive diseases and vascular wall injury

Following injury, tissue repair involves inflammation, granulation tissue formation and scar constitution. Granulation tissue develops from the connective tissue surrounding the damaged or missing area and contains mainly small vessels, inflammatory cells, fibroblasts and myofibroblasts. As the wound closes and evolves into a scar, there is a striking decrease in cellularity, including disappearance of typical myofibroblasts. The question arises as to what process is responsible for granulation tissue cell disappearance. Our results (in cutaneous wounds) and results of other laboratories (particularly in lungs and kidney) suggest that apoptosis is the mechanism responsible for the evolution of granulation tissue into a scar. During excessive scarring (hypertrophic scar or fibrosis), it is conceivable that the process of apoptosis cannot take place. After experimental endothelial injury in an artery, accumulation of smooth muscle cells participates in the formation of intimal thickening. Apoptotic features have been observed in cells of intimal thickening and also within human atherosclerotic plaques. In the case of atherosclerosis, apoptosis could be detrimental: since smooth muscle cells participate in plaque stability, apoptosis could lead to weakening and rupture of the plaque. These results underline the fact that both increased cell survival or excessive cell death can be associated with pathological disorders. Specific therapies devised to enhance or decrease the susceptibility of individual cell types to apoptosis development could modify the evolution of a variety of human diseases.

Relationship between alpha-smooth muscle actin expression and fibrotic changes in human kidney

The alpha-smooth muscle (alpha-SM) actin isoform is expressed normally by vascular SM cells and by stromal fibroblastic cells in pathological conditions leading to fibrosis. In order to investigate the relation between kidney fibrosis and alpha-SM actin expression, we studied 51 renal biopsies from 45 patients: 30 with various forms of glomerulonephritis; 1 with acute tubular necrosis; 1 with acute interstitial nephritis, and 13 renal transplant recipients. The presence of alpha-SM actin was examined by using anti-alpha SM-1, a mouse monoclonal antibody (IgG2 alpha) specific for alpha-SM actin. alpha-SM actin scores were estimated semiquantitatively, as were glomerulosclerosis and interstitial fibrosis. In acute tubular necrosis and in well-functioning grafts, alpha-SM actin expression was limited to vascular SM cells. In glomerular diseases, alpha-SM actin expression was upregulated in mesangial area in 25 of 36 biopsies, and even more frequently in the periglomerular and peritubular interstitium (34 of 36 cases, chi 2 = 7.6, P < 0.01). Whereas glomerular alpha-SM actin expression seemed to decrease as glomerulosclerosis progressed, there was a positive correlation between interstitial alpha-SM actin scores and the degree of interstitial fibrosis. Similarly, interstitial alpha-SM actin expression was found in acutely or chronically rejected kidneys, but not in well-functioning grafts. We conclude that upregulation of alpha-SM actin in the glomerulus indicates mesangial cell activation and is not always correlated with the degree of glomerulosclerosis. In contrast, interstitial upregulation of alpha-SM actin which indicates myofibroblast activation is correlated with the degree of interstitial fibrosis.

Covering by a flap induces apoptosis of granulation tissue myofibroblasts and vascular cells

It has recently been shown that during the healing of an open wound, apoptosis mediates the decrease in cellularity during the transition between granulation tissue and scar. Because reduced contraction and a decrease in the number of fibroblastic cells have been described in wounds covered with a successful skin graft, we hypothesized that apoptosis could be responsible for these phenomena. Using in situ labeling of fragmented DNA, immunohistochemistry for alpha-smooth muscle actin, and electron microscopy, we have studied in rats the evolution of 10-day-old wound tissue covered with a total skin flap (containing epidermis, dermis, and the cutaneous muscle). In 10-day-old wound tissue, few apoptotic vascular cells and rare apoptotic myofibroblasts were present; the number of apoptotic cells increased slightly 72 hours later. In wounds covered with total skin flaps, the number of apoptotic vascular and myofibroblastic cells increased drastically 6 hours after flap application with a maximum at 24 and 48 hours, respectively. A decrease of apoptotic cell number was noted at 72 hours; at this time, the size of the granulation tissue was greatly reduced and showed extracellular matrix remodeling. Total flaps were more efficient in the induction of granulation tissue cell apoptosis compared with dermo-epidermal flaps. Moreover, the control application of full-thickness skin autografts, which were not viable 7 days later, did not induce apoptosis 24 hours after implantation. Our results indicate that covering granulation tissue with a skin flap results in a massive apoptotic process, possibly by means of a (some) locally released substance(s).

Proliferation and phenotypic modulation of portal fibroblasts in the early stages of cholestatic fibrosis in the rat

The animal model of hepatic fibrosis induced by bile duct ligation represents an experimental model of human chronic biliary fibrosis. Much attention has been given to the hepatic stellate cell (HSC), or perisinusoidal cell, as the source of the extracellular matrix proteins. However, in the bile duct ligation model, mesenchymal cells other than HSC may be involved in the early stages of fibrosis development. The current study examined, in Sprague-Dawley rats, proliferation in different liver cell subpopulations as well as expression of alpha-smooth muscle (SM) actin and desmin in portal fibroblasts and HSC at 6 hours and 1, 2, 3, and 7 days after bile duct ligation. Kinetics of liver cell proliferation and of phenotypic modulation of portal fibroblasts and HSC (expression of alpha-SM actin and desmin) was evaluated by immunocytochemistry, immunofluorescence, and immunoelectron microscopy using immunogold technique. In sham-operated animals, the evaluation of proliferation in various liver cell subpopulations revealed nonsignificant changes compared with nonoperated rats. alpha-SM actin was detected in vessel walls but was absent in cells of portal tract and parenchyma. Desmin was expressed in vessel walls and in some fibroblastic cells of portal stroma (8.2 cells/unit area) as well as in HSC in acinar Zones 1 and 3 (15.6 cells/unit area and 7.1 cells/unit area, respectively). In bile duct-ligated rats, 24 and 48 hours after ligation, marked proliferations of bile duct epithelial cells (labeling indices 36.8% and 29.5%, respectively) and of periductular fibroblasts (labeling indices 16.7% and 31.0%, respectively) were observed; thereafter, proliferation decreased for both populations (labeling indices at 7 days 12.0% and 11.6%, respectively). HSC proliferation increased gradually until the third day (labeling index 18.6%) and then leveled off. Immunocytochemistry and immunoelectron microscopy revealed a significant number of cells expressing alpha-SM actin 72 hours after bile duct ligation in the stroma adjacent to proliferating ductules. The number of alpha-SM actin-positive cells increased until the seventh day (251.6 cells/unit area). At all times examined, the distribution of alpha-SM actin was restricted to the connective tissue stroma adjacent to proliferating ductules; alpha-SM actin was not expressed in HSC of the lobule. An expansion of desmin expression was noted in fibroblastic cells in stroma surrounding proliferating ductules until 72 hours after bile duct ligation (74.7 cells/unit area) followed by a plateau. At this time, desmin expression increased also in HSC; as in controls, the number of positive cells was greater in Zone 1 (31.8 cells/unit area) than in Zone 3 (18.5 cells/unit area). Double immunofluorescence staining detected by confocal microscopy showed that the majority of portal fibroblastic cells expressing alpha-SM actin was desmin negative 48 hours after bile duct ligation. From 72 hours, portal fibroblastic cells coexpressing alpha-SM actin and desmin appeared, and their proportion increased until 7 days. The present findings indicate that in the early phase of bile duct ligation, there is a marked and transient proliferation of bile duct epithelial cells associated with proliferation of portal periductular fibroblasts, which rapidly express alpha-SM actin. This fibroblastic population may play a dominant role in the early portal fibrosis after bile duct ligation.

Bcl-2+ tonsillar plasma cells are rescued from apoptosis by bone marrow fibroblasts

Plasma cells represent the final stage of B lymphocyte differentiation. Most plasma cells in secondary lymphoid tissues live for a few days, whereas those in the lamina propria of mucosa and in bone marrow live for several weeks. To investigate the regulation of human plasma cell survival, plasma cells were isolated from tonsils according to high CD38 and low CD20 expression. Tonsillar plasma cells express CD9, CD19, CD24, CD37, CD40, CD74, and HLA-DR, but not CD10, HLA-DQ, CD28, CD56, and Fas/CD95. Although plasma cells express intracytoplasmic Bcl-2, they undergo swift apoptosis in vitro and do not respond to CD40 triggering. Bone marrow fibroblasts and rheumatoid synoviocytes, however, prevented plasma cells from undergoing apoptosis in a contact-dependent fashion. These data indicate that fibroblasts may form a microenvironment favorable for plasma cell survival under normal and pathological conditions.

Factors influencing myofibroblast differentiation during wound healing and fibrosis

Granulation tissue fibroblasts (myofibroblasts) develop several ultrastructural and biochemical features of smooth muscle (SM) cells, including the presence of microfilaments bundles and the expression of alpha-SM actin, the actin isoform typical of contractile vascular SM cells. Myofibroblasts have been suggested to play a role in wound contraction and in retractile phenomena observed during fibrotic diseases. When granulation tissue evolves into a scar, myofibroblasts containing alpha-SM actin disappear, probably as a result of apoptosis. In contrast myofibroblasts expressing alpha-Sm actin persist in excessive scarring and in fibrotic conditions. The mechanisms leading to the development of myofibroblastic features remain to be investigated. Studies on the factors regulating the phenotype of myofibroblasts will be necessary for understanding their behavior in vivo, and possibly modifying this behavior during the different clinical settings.

Apoptosis participates in cellularity regulation during rat aortic intimal thickening

Intimal thickening induced after endothelial denudation of rat aorta is though to be due to migration and proliferation of smooth muscle cells (SMC). When the reendothelialization is achieved, intimal thickening shows an important decrease in cellularity. Using in situ end labeling of fragmented DNA and electron microscopy, we show that this remodeling is accompanied by apoptosis of SMC. The number of apoptotic SMC becomes important 15 days after endothelial injury and reaches a maximum at 20 days; at 45 days the intimal thickening is reendothelialized and no more apoptotic SMC are detected. Apoptotic SMC show nuclear and cytoplasmic condensation as well as cytoplasmic vacuolization. Our results indicate that apoptosis is an important mechanism in the regulation of intimal thickening evolution.

Apoptosis mediates the decrease in cellularity during the transition between granulation tissue and scar

Granulation tissue formation and contraction is an important step of second intention wound healing. Granulation tissue develops from the connective tissue surrounding the damaged or missing area and its cellular components are mainly small vessel and inflammatory cells as well as fibroblasts and myofibroblasts. As the wound closes and evolves into a scar, there is an important decrease in cellularity; in particular myofibroblasts disappear. The question arises as to which process is responsible for this cellular loss. During a previous investigation on the expression of alpha-smooth muscle actin in myofibroblasts (Darby I, Skalli O, Gabbiani G, Lab Invest, 1990, 63:21-29), we have observed that in late phases of wound healing, many myofibroblasts show changes compatible with apoptosis and suggested that this type of cell death could be responsible for the disappearance of myofibroblasts. We have now tested this hypothesis by means of morphometry at the electron microscopic level and by in situ end labeling of fragmented DNA. Our results indicate that the number of myofibroblastic and vascular cells undergoing apoptosis increases as the wound closes and support the assumption that this is the mechanism of granulation tissue evolution into a scar. The regulation of apoptotic phenomena during wound healing may be important in scar establishment and development of pathological scarring.

Heterogeneity of myofibroblast phenotypic features: an example of fibroblastic cell plasticity

Granulation tissue fibroblasts (myofibroblasts) develop several ultrastructural and biochemical features of smooth muscle (SM) cells, including the presence of microfilament bundles and the expression of alpha-SM actin, the actin isoform present in SM cells and myoepithelial cells and particularly abundant in vascular SM cells. Myofibroblasts have been suggested to play a role in wound contraction and in retractile phenomena observed during fibrotic diseases. When contraction stops and the wound is fully epithelialized, myofibroblasts containing alpha-SM actin disappear, probably as a result of apoptosis, and the scar classically becomes less cellular and composed of typical fibroblasts with well-developed rough endoplasmic reticulum but with no more microfilaments. In contrast, alpha-SM actin expressing myofibroblasts persist in hypertrophic scars and in fibrotic lesions of many organs, including stroma reaction to epithelial tumours, where they are allegedly involved in retractile phenomena as well as in extracellular matrix accumulation. The mechanisms leading to the development of myofibroblastic features remain to be investigated. In vivo and in vitro investigations have shown that gamma-interferon exerts an antifibrotic activity at least in part by decreasing alpha-SM actin expression whereas heparin increases the proportion of alpha-SM actin positive cells. Recently, we have observed that the subcutaneous administration of transforming growth factor-beta 1 to rats results in the formation of a granulation tissue in which alpha-SM actin expressing myofibroblasts are particularly abundant. Other cytokines and growth factors, such as platelet-derived growth factor, basic fibroblast growth factor and tumour necrosis factor-alpha, despite their profibrotic activity, do not induce alpha-SM actin in myofibroblasts. In conclusion, fibroblastic cells are relatively undifferentiated and can assume a particular phenotype according to the physiological needs and/or the microenvironmental stimuli. Further studies on fibroblast adaptation phenomena appear to be useful for the understanding of the mechanisms of development and regression of pathological processes such as wound healing and fibrocontractive diseases.

Characterization of rat aortic smooth muscle cells resistant to the antiproliferative activity of heparin following long-term heparin treatment

Vascular smooth muscle cells (SMC) do not represent a homogeneous population (Schwartz et al., 1990, Am. J. Pathol. 136: 1417-1428). Cellular clones resistant to the antiproliferative activity of heparin were isolated from rat aortic SMC cultures (Pukac et al., 1990, Cell Regul., 1:435-443; San Antonio et al., 1993, Arterioscler. Thromb., 13:748-757) and from explant of human arterial restenotic lesions (Chan et al., 1993, Lancet, 341:341-342). We have shown in the present study that long-term treatment (growth medium supplemented with 200 micrograms/ml heparin, from the second to the tenth passage) of rat aortic SMC, without cell cloning, resulted in a significant loss of sensitivity to the growth inhibition by heparin and its derivatives. The heparin resistance was stable after growing cells for two passages in heparin-free medium, suggesting the selection of a particular phenotype. We tried to characterize these cells and to determine the causes of the resistance to the growth inhibition by heparin. Heparin-treated SMC (HT-SMC) were smaller than their control culture at the same passage, expressed less alpha-SM actin, and did not overgrow after reaching confluence. As in the heparin-resistant clones (San Antonio et al., 1993, Cell Regul., 1:435-443) expression of alpha-SM actin could be increased in HT-SMC by heparin addition before Western blotting. Heparin resistance was associated with a tenfold decrease in [3H]-heparin binding capacity (Bmax = 1.9 x 10(6) sites per cell) compared to control cultures (Bmax = 1.7 x 10(7) sites per cell), which was irreversible after growing the cells for two additional passages in heparin-free medium. We also investigated protein kinase C (PKC) in HT-SMC in terms of both enzymatic activity and protein expression (evaluated by [3H]-staurosporine and [3H]-phorbol-12,13-dibutyrate binding). We found that HT-SMC had only half the PKC activity and expression as control SMC. Therefore, long-term treatment of rat aortic SMC with heparin allowed the selection of a less differentiated subpopulation of cells, exhibiting low sensitivity to the growth inhibition by heparin, which could be related to the low capacity of binding heparin and to a lower PKC activity and/or expression.

Morphological and immunochemical differences between keloid and hypertrophic scar

There are two types of excessive scarring, keloid and hypertrophic scar. Contrary to hypertrophic scars, keloids do not regress with time, are difficult to revise surgically, and do not provoke scar contractures. These two lesions require different therapeutic approaches but are often confused because of an apparent lack of morphological differences. We have investigated the collagen organization and the possible presence of alpha-smooth muscle (SM) actin-expressing myofibroblasts in these conditions. Keloids contain large, thick collagen fibers composed of numerous fibrils closely packed together. In contrast hypertrophic scars exhibit modular structures in which fibroblastic cells, small vessels, and fine, randomly organized collagen fibers are present. We confirm that such nodular structures are always present in hypertrophic scar and rarely in keloid. Furthermore, only nodules of hypertrophic scars contain alpha-SM actin-expressing myofibroblasts. Electron microscopic examination supports the above-mentioned differences in collagen organization and in fibroblastic features and shows the presence of an amorphous extracellular material surrounding fibroblastic cells in keloid. The presence in hypertrophic scar myofibroblasts of alpha-SM actin, the actin isoform typical of vascular SM cells, may represent an important element in the pathogenesis of contraction. Interestingly, when placed in culture fibroblasts from hypertrophic scars and keloid express similar amounts of alpha-SM actin, suggesting that local microenvironmental factors influence in vivo the expression of this protein. Thus several morphological and immunohistochemical differences exist between hypertrophic scar and keloid that are useful for the biological and pathological characterization of the two lesions.

Effect of gamma-interferon on the clinical and biologic evolution of hypertrophic scars and Dupuytren's disease: an open pilot study

Hypertrophic scars and Dupuytren's disease are characterized by the presence of modified fibroblasts or myofibroblasts which are allegedly responsible for tissue retraction and excessive connective tissue production. gamma-Interferon, a cytokine produced by T-helper lymphocytes, has been shown to decrease fibroblast replication, alpha-smooth-muscle actin (the actin isoform characterizing myofibroblasts) expression, and collagen production. We have investigated in an open pilot study the possibility that intralesional injections of gamma-interferon exert a beneficial effect on the evolution of hypertrophic scars and Dupuytren's disease. In the 14 selected patients, gamma-interferon decreased the symptoms and the size of the lesions of both diseases; in hypertrophic scars, immunofluorescence examination showed that alpha-smooth-muscle actin expression also was decreased in myofibroblasts. Moreover, in fibroblasts cultured from 4 patients with hypertrophic scars, gamma-interferon decreased replication and alpha-smooth-muscle actin expression in vitro. Our results suggest that gamma-interferon could represent a useful adjunct to the nonsurgical therapy of hypertrophic scars and Dupuytren's disease. Larger controlled clinical studies, however, should test the validity of these preliminary observations.

Phenotypic and functional features of myofibroblasts in sheep fetal wounds

The myofibroblast is a mesenchymal cell with functional and structural characteristics in common with fibroblasts and smooth muscle cells. These cells play a critical role in wound closure and in the pathologic sequelae of healing. It has been shown in adult humans and experimental animals that the myofibroblast expresses alpha -smooth muscle actin (ASMA) temporarily during wound contraction and more persistently during fibrocontractive diseases; however, it is unclear whether this cell makes any contribution to tissue repair in utero. Experimental work in fetal animal models has demonstrated that wound repair in fetal skin occurs by reconstitution of epidermal appendages and organized restoration of the dermal collagen network. Fetal lamb wound healing studies have shown that a transition from scarless tissue repair to healing with scar formation occurs late in gestation. In this study we examined the ontogeny of myofibroblasts in fetal lamb wounds at early through late gestation, using transmission electron microscopy (TEM) and ASMA immunohistochemistry. Dramatic differences were observed in ASMA content of early as compared to late gestation fetal wound granulation tissue: ASMA was absent in wounds made at 75 days gestation but was present in progressively greater amounts in wounds made at 100 and 120 days gestation (term = 145 days). TEM studies also demonstrated progressive development and organization of microfilament bundles. Early in development microfilament bundles were sparse and disorganized, but as gestation progressed the bundles became more prevalent and formed tightly parallel arrangements. The organization of microfilament bundles was also accompanied by fibronexus formation.(ABSTRACT TRUNCATED AT 250 WORDS)

GM-CSF-induced granulation tissue formation: relationships between macrophage and myofibroblast accumulation

We have studied the formation of granulation tissue around osmotic minipumps delivering granulocyte macrophage-colony stimulating factor (GM-CSF) chronologically in the rat using electron microscopy and immunohistochemistry at the light and electron microscopic levels, with specific antibodies against alpha-smooth muscle (SM) actin and rat macrophages. At 2 and 3 days after pump implantation, GM-CSF application produced an extensive inflammatory reaction characterized by edema and the accumulation of polymorphonuclear cells and macrophages. Gradually, polymorphonuclear cells decreased in number and macrophages became arranged in large clusters. The expression of alpha-SM actin in fibroblastic cells of the granulation tissue started from the 4th day after pump implantation and progressed up to the 7th day. Double immunofluorescence staining showed macrophage clusters in relation to alpha-SM actin-rich fibroblastic cells. Electron microscopic examination confirmed that the fibroblasts containing alpha-SM actin-positive stress fibers were found initially in close proximity to clustered macrophages. The delivery of platelet-derived growth factor (PDGF) and tumor necrosis factor-alpha (TNF-alpha) by the osmotic minipump induced an accumulation of macrophages, but in a much smaller number compared with those seen after GM-CSF application; these macrophages were never assembled in clusters and, furthermore, TNF-alpha and PDGF did not stimulate alpha-SM actin expression in fibroblastic cells. Our results suggest that after GM-CSF administration, the cluster-like accumulation of macrophages plays an important role in stimulating alpha-SM actin expression in myofibroblasts. Our results may be relevant to the understanding of the processes leading to granulation tissue formation in this and other experimental models.

Transforming growth factor-beta 1 induces alpha-smooth muscle actin expression in granulation tissue myofibroblasts and in quiescent and growing cultured fibroblasts

Granulation tissue fibroblasts (myofibroblasts) develop several ultrastructural and biochemical features of smooth muscle (SM) cells, including the presence of microfilament bundles and the expression of alpha-SM actin, the actin isoform typical of vascular SM cells. Myofibroblasts have been proposed to play a role in wound contraction and in retractile phenomena observed during fibrotic diseases. We show here that the subcutaneous administration of transforming growth factor-beta 1 (TGF beta 1) to rats results in the formation of a granulation tissue in which alpha-SM actin expressing myofibroblasts are particularly abundant. Other cytokines and growth factors, such as platelet-derived growth factor and tumor necrosis factor-alpha, despite their profibrotic activity, do not induce alpha-SM actin in myofibroblasts. In situ hybridization with an alpha-SM actin probe shows a high level of alpha-SM actin mRNA expression in myofibroblasts of TGF beta 1-induced granulation tissue. Moreover, TGF beta 1 induces alpha-SM actin protein and mRNA expression in growing and quiescent cultured fibroblasts and preincubation of culture medium containing whole blood serum with neutralizing antibodies to TGF beta 1 results in a decrease of alpha-SM actin expression by fibroblasts in replicative and non-replicative conditions. These results suggest that TGF beta 1 plays an important role in myofibroblast differentiation during wound healing and fibrocontractive diseases by regulating the expression of alpha-SM actin in these cells.