Molecular mechanisms of liver fibrogenesis--a homage to the role of activated fat-storing cells

Myofibroblasts are responsible for collagen synthesis in the stroma of human hepatocellular carcinoma: an in vivo and in vitro study

BACKGROUND/AIMS

Marked changes in extracellular matrix occur in the stroma of hepatocellular carcinoma, as compared to normal or cirrhotic liver. The cell types responsible for extracellular matrix synthesis within hepatocellular carcinoma have not been clearly identified.

METHODS

In vivo collagen synthesis was studied by in situ hybridization and immunohistochemistry for types I, IV, V and VI collagen, together with immunolabeling of alpha-smooth muscle actin, a myofibroblast marker, and CD34, an endothelial cell marker. In vitro, extracellular matrix deposition by cultured myofibroblasts was studied by reticulin staining, immunocytochemistry and RNase protection.

RESULTS

All collagens studied were expressed in the stroma of the tumor, with a higher level of type VI and IV collagens than of type I and V. The majority of the cells expressing collagen transcripts in human hepatocellular carcinoma stroma were alpha-actin positive and CD 34 negative. In vitro experiments demonstrated that the hepatocellular carcinoma cell lines HepG2, HuH7 and Hep3B markedly increased extracellular matrix deposition by human liver myofibroblasts. This increase was mediated by a soluble mediator present in tumor cell conditioned medium. It was not explained by an increase in mRNA levels of extracellular matrix components, nor by a decrease in the secretion of matrix-degrading proteinases by myofibroblasts.

CONCLUSIONS

Myofibroblasts are the main source of collagens in the stroma of hepatocellular carcinoma. Our data also indicate that tumoral hepatocytes increase extracellular matrix deposition by cultured myofibroblasts, probably by post-transcriptional mechanisms. The generation of hepatocellular carcinoma stroma by myofibroblasts could thus be under control of tumoral cells.

Changes of sinusoidal basement membrane collagens in early hepatic fibrosis induced with CCl4 in cynomolgus monkeys

CCl4 was chronically administrated for 25 months to induce hepatic fibrosis in three cynomolgus monkeys so as to examine the alteration of basement membrane-related collagens during the liver injury. Although type IV collagen was immunohistochemically present along sinusoidal walls before and during the CCl4 administration, basement membrane-associated collagen (BAC), which was recognized with JK-132 monoclonal antibody, appeared around sinusoids at five to ten months of CCl4 administration. We previously developed a sandwich enzyme linked-immunosorbent assay, utilizing two monoclonal antibodies, anti-BAC antibody (JK-132) and anti-type IV collagen antibody (JK-199) [Int Hepatology Commun 1995; 4: 1-8]. The serum level of the collagen complex, which is disulfide-bridged with BAC and type IV collagen, was simultaneously monitored. The serum level of the complex at the initial stage of the examination was 19-34 ng/ml and gradually increased in relation to the intensity of immunofluorescence of BAC and type IV collagen in sinusoids and connective tissues, up to 51-57 ng/ml. The serum collagen complex levels showed a weak correlation with serum hyaluronic acid, a serum marker of hepatic fibrosis. The serum GOT, GPT, ALP and CHE levels did not reflect the alteration of sinusoids or relate to the serum collagen complex level. The increase in BAC around sinusoids and the increase of collagen complex with BAC and type IV collagen in the sera, correlate with early lesional events in hepatic fibrosis.

Vitamin E status in patients with liver cirrhosis: normal or deficient?

The study aim was to compare the ratio of vitamin E to serum cholesterol with the serum vitamin E level alone as a measure of vitamin E status in patients with different degrees of liver dysfunction. Assessment of serum vitamin E and total serum cholesterol was performed in 85 patients with liver cirrhosis at Child's stage A (n = 26), B (n = 26), and C (n = 33) and 50 patients with noncirrhotic liver disease. As surrogate markers of liver function, 7alpha-hydroxycholesterol and prealbumin concentrations and the plasma prothrombin time were determined. Mean serum vitamin E concentrations in Child A, B, and C patients were 27.4%, 36.9%, and 37.3% lower, respectively, than in healthy controls (P<.01). Twelve of 26 Child A, 14 of 26 Child B, and 14 of 33 Child C patients had vitamin E deficiency with respect to the absolute values, i.e., serum levels less than 13.76 micromol/L (5% percentile of healthy controls). In contrast, only two of 26 Child A, five of 26 Child B, and five of 33 Child C patients (P<.01 for Child A/B and P<.05 for Child C) were vitamin E-deficient according to the serum vitamin E to cholesterol ratio, i.e., less than 2.86 micromol/mmol. Serum vitamin E was correlated significantly with prealbumin, 7alpha-hydroxycholesterol, and the plasma prothrombin time, but the vitamin E to cholesterol ratio was not. Correcting serum vitamin E for total serum cholesterol in patients with liver cirrhosis leads to the phenomenon of reduced serum vitamin E levels inadvertently shifted toward normal values. In patients with liver cirrhosis, the absolute vitamin E concentration correlates better with the typical clinical and biochemical findings of the disease than the vitamin E to cholesterol ratio. Therefore, a considerable number of patients with advanced liver cirrhosis might actually be vitamin E-deficient.

Expression patterns of matrix metalloproteinases and their inhibitors in parenchymal and non-parenchymal cells of rat liver: regulation by TNF-alpha and TGF-beta1

BACKGROUND/AIMS

Although matrix metalloproteinases (MMPs) and their specific inhibitors (TIMPs) play an essential role in liver injury associated with tissue remodeling, the cellular origin of MMPs/TMPs within the liver remains to be clarified.

METHODS

Different liver cell populations were analysed with respect to their expression by reverse transcription-polymerase chain reaction, Northern blot analysis and zymography.

RESULTS

MMP and TIMP coding transcripts were detectable in all liver cell types by reverse transcription-polymerase chain reaction; however, the cellular expression levels were markedly different as assessed by Northern blot analysis. Gelatinase-B was predominantly expressed in Kupffer cells, gelatinase-A in hepatic stellate cells and rat liver myofibroblasts and stromelysins-1, -2 as well as collagenase in hepatic stellate cells. Membrane type-1 MMP (MMP-14) was found in significant amounts in all liver cells. TIMP-1 coding m-RNAs were present mainly in hepatic stellate cells and rat liver myofibroblasts, TIMP-2 additionally in Kupffer cells, while TIMP-3 expression was detectable only in hepatocytes. During in vitro activation of hepatic stellate cells, MMP expression was mostly downregulated, while TIMP expression was enhanced, thereby providing an explanation for matrix accumulation co-localised with these cells during chronic liver injury. In general, TNF-alpha stimulated both MMP and TIMP expression of hepatic stellate cells, while TGF-beta1 induced TIMP expression only.

CONCLUSIONS

Collectively these data demonstrate that all resident liver cells are involved in matrix degradation to some extent and that hepatic stellate cells play an important role in matrix breakdown in addition to matrix synthesis. The cytokine-specific regulation of MMP/TIMP expression in hepatic stellate cells suggests that the initial matrix breakdown following liver injury might be enhanced by TNF-alpha, while diminished matrix degradation during chronic tissue injury might be due to the action of TGF-beta1 through TIMP induction.

Retinol uptake and metabolism, and cellular retinol binding protein expression in an in vitro model of hepatic stellate cells

Liver is a major site of retinoid metabolism and storage, and more than 80% of the liver retinoids are stored in hepatic stellate cells. These cells represent less than 1% of the total liver protein, reaching a very high relative intracellular retinoid concentration. The plasma level of retinol is maintained close to 2 microM, and hepatic stellate cells have to be able both to uptake or to release retinol depending upon the extracellular retinol status. In view of their paucity in the liver tissue, stellate cells have been studied in primary cultures, in which they loose rapidly the stored lipids and retinol, and convert spontaneously into the activated myofibroblast phenotype, turning a long-term study of their retinol metabolism impossible. We have analyzed the retinol metabolism in the established GRX cell line, representative of stellate cells. We showed that this cell line behaves very similarly, with respect the retinol uptake and release, to primary cultures of hepatic stellate cells. Moreover, we showed that the cellular retinol binding protein (CRBP-I) expression in these cells, relevant for both uptake and esterification of retinol, responds to the extracellular retinol status, and is correlated to the retinol binding capacity of the cytosol. Its expression is not associated with the overall induction of the lipocyte phenotype by other agents. We conclude that the GRX cell line represents an in vitro model of hepatic stellate cells, and responds very efficiently to wide variations of the extracellular retinol status by autonomous controls of its uptake, storage or release.

Content of dolichol and retinol in isolated rat non-parenchymal liver cells

The liver sinusoids, that are considered as a functional unit, harbour four types of sinusoidal cells (Ito, Kupffer, endothelial and pit cells). Dolichol content has been determined in many tissues and subcellular compartments, alteration has been reported in many types of liver injury, but until now no data are available on its content in every type of sinusoidal non-parenchymal liver cells. Dolichol and retinol metabolism might intersect in their traffic in biological membranes. Intercellular as well as intracellular exchange of retinoids is an essential element of important processes occurring in liver cells. It has been suggested that the role of dolichol, besides being a carrier of oligosaccharides in the biosynthesis of N-linked glycoproteins, may be to modify membrane fluidity and permeability, and facilitate fusion of membranes. Dolichol in the membrane is intercalated between the two-halves of the phospholipid bilayer, but its exact disposition is not known and the movement and distribution of retinoid in membranes may vary with the geometry of the membranes. Therefore the aim of this study is to obtain a global understanding of the sinusoidal system regarding dolichol and retinol content in each type of isolated rat liver sinusoidal cell, in normal conditions and after vitamin A administration. The information that can be drawn from the present results is that with normal vitamin A status of the animal, the dolichol content is almost uniform in all liver cells. After vitamin A supplementation, a great increase of dolichol, together with the known increase of retinol, can be measured only in a subpopulation of the Ito cells, the Ito-1 subfraction. Therefore in the cells that are present in the hepatic sinusoid, different pools of dolichol may have separate functions. Because retinol traffic among cells, membranes and plasma still remains to be fully understood, roles of dolichol in the exchange of vitamin A among sinusoidal liver cells are discussed.

Identification, culture, and characterization of pancreatic stellate cells in rats and humans

BACKGROUND & AIMS

Until now, the basic matrix-producing cell type responsible for pancreas fibrosis has not been identified. In this report, retinoid-containing pancreatic stellate cells (PSCs) in rat and human pancreas are described, and morphological and biochemical similarities to hepatic stellate cells are shown.

METHODS

Electron and immunofluorescence microscopy (collagen types I and III, fibronectin, laminin, alpha-actin, and desmin) was performed using pancreatic tissue and cultured PSCs. Extracellular matrix synthesis was shown using quantitative immunoassay and Northern blot analysis.

RESULTS

PSCs are located in interlobular areas and in interacinar regions. Early primary cultured PSCs contain retinol and fatty acid retinyl-esters. Addition of retinol to passaged cells resulted in retinol uptake and esterification. During primary culture, the cells changed from a quiescent fat-storing phenotype to a highly synthetic myofibroblast-like cell expressing iso-alpha-smooth muscle actin (>90%) and desmin (20%-40%) and showing strong positive staining with antibodies to collagen types I and III, fibronectin, and laminin. As determined on protein and messenger RNA level, serum growth factors stimulated the synthesis of collagen type I and fibronectin.

CONCLUSIONS

The identification of PSCs, particularly in fibrotic areas, and the similarities of these cells to hepatic stellate cells suggest that PSCs participate in the development of pancreas fibrosis.

Transformation-dependent susceptibility of rat hepatic stellate cells to apoptosis induced by soluble Fas ligand

Cytokine-driven activation of hepatic stellate cells (HSC) in tissue injury and inflammation is a key pathogenetic event in liver fibrogenesis leading to an expanded pool of matrix producing myofibroblasts (MFB) which represent the transformed counterpart of HSC. We hypothesize that expansion of the pool of MFB might also be accomplished by modulation of apoptosis, which plays an opposite and complementary role to mitosis in the cellular homeostasis. We characterized the susceptibility of HSC in primary culture and of MFB in secondary culture to apoptosis induced by the soluble Fas ligand (sFasL) and related the effects to the expression levels of Fas (APO-1/CD95) and some major proapoptotic and contra-apoptotic protooncogenes. MFB showed a dose-dependent apoptotic reaction upon exposure to sFasL as evidenced by a strong increase of nucleosomal DNA fragments, loss of cellular DNA, positive TUNEL reaction, and annexin staining. The effect was found only if protein synthesis (cycloheximide) or RNA synthesis (actinomycin D) were arrested. HSC maintained for various times in primary culture were completely resistant to sFasL in combination with cycloheximide, but in late primary cultures (day 7 onward) an increasing susceptibility to sFasL-mediated apoptosis was developed. By semiquantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis and alkaline phosphatase-anti-alkaline phosphatase staining Fas receptor was identified both in HSC and MFB at comparable expression levels. The expression of the contra-apoptotic protooncogenes bcl-2 and bcl-xl was found to be much stronger in early HSC than in late HSC and MFB as shown by ribonuclease protection assay. The expression of bcl-2 was additionally confirmed by semiquantitative RT-PCR and immunoblotting. Proapoptotic bax was found in comparable quantities at the RNA level in HSC and MFB but at the protein level MFB showed increased bax expression. It is concluded that transformation of HSC to MFB is paralleled by an increasing sensitivity to sFasL-mediated apoptosis, which might be related to a strong decrease of bcl-2 and bcl-xl expression, leading to a preponderance of proapoptotic gene expression in MFB. Modulation of apoptotic susceptibility of transforming HSC could be an important complementary pathway in the pathogenesis of fibrosis.

An intravital fluorescence microscopic study of hepatic microvascular and cellular derangements in developing cirrhosis in rats

Quantitative data defining the relationship between the hepatic microcirculation and the development of liver pathological changes could provide a basis for a better understanding of fibrogenic processes, such as cirrhosis. Therefore, we established the technique of intravital fluorescence microscopy and computer-assisted microcirculation analysis systems in developing cirrhosis in rats with the aim of quantitatively assessing the association of hepatic microvascular morphology with its disordered acinar architecture, and nonparenchymal cell transformation with collagen deposition, parenchymal cell loss, and liver dysfunction. In animals chronically exposed to carbon tetrachloride (CCl4), the most significant microvascular changes progressively observed in vivo were the concomitant appearance of 1) sinusoid-free space around dilated postsinusoidal venules with 2) substituting occurrence of yellow-green autofluorescent collagen deposition, 3) reduction in sinusoidal density, but 4) increase of vascular lumen caused by the formation of shunting vessels bypassing the sinusoids. Present on-line analysis further indicated the local coincidence of changed spatial distribution of Ito cells (accumulation of vitamin A ultraviolet autofluorescence in zone 3) with fibrotic autofluorescent septa, causing significant collapse of parenchymal tissue (hepatocellular bis-benzamide fluorescence) and diminution of hepatocellular excretory function (bile flow). Regression analysis revealed strong correlations between loss of parenchymal tissue and both collagen deposition and sinusoidal rarefication, as well as between sinusoidal rarefication and collagen deposition. Thus, sequential in vivo analysis presented herein provides the new information on the concomitant onset of cellular, fibrotic, and microvascular changes in developing fibrosis/cirrhosis, excluding that distinct cellular or fibrotic alterations are a prerequisite for the manifestation of microcirculatory and vascular derangements or vice versa.

Alcohol and the pancreas

Alcoholic pancreatitis is a major, often lethal complication of alcohol abuse. Until recently it was generally accepted that alcoholic pancreatitis was a chronic disease from the outset. However, there is now emerging evidence in favour of the necrosis-fibrosis hypothesis that alcoholic pancreatitis begins as an acute process and that repeated acute attacks lead to chronic pancreatitis, resulting in exocrine and endocrine failure. Over the past 10-15 years, the focus of research into the pathogenesis of alcoholic pancreatitis has shifted from possible sphincteric and ductular abnormalities to the acinar cell itself which has increasingly been implicated as the initial site of injury. Recent studies have shown that the acinar cell can metabolize alcohol at rates comparable to those observed in hepatocytes. In addition, it has been demonstrated that alcohol and its metabolites exert direct effects on the pancreatic acinar cell which may promote premature digestive enzyme activation and oxidant stress. The challenge remains to identify predisposing and triggering factors in this disease.

Hepatic fibrosis, glomerulosclerosis, and a lipodystrophy-like syndrome in PEPCK-TGF-beta1 transgenic mice

Transgenic mice overexpressing a constitutively active human TGF-beta1 under control of the rat phosphoenolpyruvate carboxykinase regulatory sequences developed fibrosis of the liver, kidney, and adipose tissue, and exhibited a severe reduction in body fat. Expression of the transgene in hepatocytes resulted in increased collagen deposition, altered lobular organization, increased hepatocyte turnover, and in extreme cases, hemorrhage and thrombosis. Renal expression of the transgene was localized to the proximal tubule epithelium, and was associated with tubulointerstitial fibrosis, characterized by excessive collagen deposition and increased fibronectin and plasminogen activator inhibitor-1 immunoreactivity. Pronounced glomerulosclerosis was evident, and hydronephrosis developed with low penetrance. Expression of TGF-beta1 in white and brown adipose tissue resulted in a lipodystrophy-like syndrome. All white fat depots and brown fat pads were severely reduced in size, and exhibited prominent fibroplasia. This reduction in WAT was due to impaired adipose accretion. Introduction of the transgene into the ob/ob background suppressed the obesity characteristic of this mutation; however, transgenic mutant mice developed severe hepato- and splenomegaly. These studies strengthen the link between TGF-beta1 expression and fibrotic disease, and demonstrate the potency of TGF-beta1 in modulating mesenchymal cell differentiation in vivo.

Pharmacological aspects of pentoxifylline with emphasis on its inhibitory actions on hepatic fibrogenesis

1. Pentoxifylline (PTX), a derivative of the methylxanthine theobromine, has been used for many years in the treatment of peripheral vascular diseases. Increased red blood cell flexibility, reduction of blood viscosity, and decreased potential of platelet aggregation are the basic actions of PTX, resulting in therapeutic benefits due to improved microcirculation and tissue oxygenation. 2. PTX's generally accepted mechanism of action is the inhibition of phosphodiesterases, leading to increased intracellular levels of cyclic adenosine monophosphate (cAMP). 3. A number of studies have shown PTX's effects on the cytokine network. The most relevant clinical results are the therapeutic benefits of PTX in attenuating the effects of tumor necrosis factor-alpha (TNF-alpha) in conditions such as septic shock. 4. PTX also has been found to exert antifibrogenic actions, using cultured fibroblasts or animal models of fibrosis, including liver fibrosis. 5. In hepatic stellate cell culture PTX has been shown to inhibit the basic reactions of liver fibrogenesis, being effective on cytokines and growth factors relevant in fibrogenesis of the liver, too. 6. Therefore, PTX might be an effective drug with few side effects in the treatment of liver fibrosis. Further clinical studies have to be done to establish the real therapeutic benefits of PTX in liver fibrosis and cirrhosis.

TGF-beta-mediated hepatocellular apoptosis by rat and human hepatoma cells and primary rat hepatocytes

BACKGROUND/AIMS

Primary cultures of rat hepatocytes, rat (FAO) and human (HepG2) hepatoma cells were studied by immunocytochemistry for expression of transforming growth factor (TGF)-beta, for the release of TGF-beta into the medium, and generation of hepatocellular apoptosis by the respective cell-conditioned media.

METHODS/RESULTS

Using the alkaline-phosphatase anti-alkaline-phosphatase technique, intense TGF-beta immunostaining was shown in all cell types. The cytokine is released almost entirely in the latent form into the culture medium; only the FAO-cells had a substantial fraction of bioactive TGF-beta in the native (unacidified) culture fluid. Exposure of hepatocytes with the respective cell-conditioned media in the activated, but not in the native form (except for FAO-cell media), induced severe detrimental effects as evidenced by: (i) gross morphological alterations, (ii) functional impairment (reduction of WST-1 test, detachment of cells, lactate dehydrogenase increase in the medium), and (iii) generation of apoptosis. The latter phenomenon was confirmed by an increase of internucleosomal DNA fragments, positive TUNEL reaction, and intense binding of the fluorochrome Hoechst 33342 to fragmented nuclei. All these effects, which were mimicked by addition of recombinant human TGF-beta 1, were almost entirely antagonized by pre-incubation of the conditioned media with latency associated peptide. In contrast to hepatocytes, both types of hepatoma cells were completely resistant to the multiple actions of TGF-beta and activated conditioned media.

CONCLUSIONS

It is concluded that hepatocytes might have the ability to induce autocrine, TGF-beta-mediated apoptosis, whereas hepatoma cells, because of their TGF-beta resistance, might generate TGF-beta-mediated peritumorous apoptosis of hepatocytes in a paracrine way, which could facilitate their expansion in situ. Both mechanisms, however, are critically dependent on extracellular TGF-beta activation.

Fibrosis in chronic rejection of human liver allografts: expression patterns of transforming growth factor-TGFbeta1 and TGF-beta3

Activation and transformation of lipocytes (Ito cells, stellate cells) into alpha-actin-positive myofibroblast-like cells is an essential step in the initiation of liver fibrosis. Transforming growth factor-beta (TGF-beta) is considered an important mediator of this process. In order to determine mechanisms of fibrotic deposition in a hepatic transplant setting, we analyzed 10 chronically rejected human liver allografts for the expression of extracellular matrix (ECM) molecules, myofibroblast-like cells (alpha-actin), macrophages, and TGF-beta1 and -beta3. Using single- and double-immunohistochemical staining techniques, all specimens investigated showed increased deposition of the ECM proteins fibronectin, tenascin, undulin, and collagen VI with a characteristic densification especially in pericentral areas. Likewise, strong accumulation of alpha-actin-positive cells and TGF-beta1-expressing macrophages was observed in the same fields, supporting the concept of lipocyte activation/transformation and subsequent ECM production fostered by macrophage-derived TGF-beta1. In contrast, TGF-beta3 was found to be mainly expressed by a markedly increased number of lipocytes. Interestingly, distribution of TGF-beta3 corresponded to that of tenascin, an ECM molecule known to be involved in early matrix organization, suggesting that TGF-beta3 may likewise act mainly in early stages of fibrogenesis. Furthermore, TGF-beta3 restriction to high numbers of a single cell type (i.e., lipocytes) implied a possible role in cell proliferation through autocrine loops. In conclusion, fibrosis in chronic rejection seems to follow similar mechanisms as in non-transplanted livers but additionally suggests differential temporal and functional roles for the TGF-beta isoforms 1 and 3 in the course of a multistep process leading to lipocyte transformation and ECM production.

Dolichol and vitamin A content in rat liver Ito (perisinusoidal) cells

Dolichol has been determined in many tissues but to date no data are available on liver Ito (fat storing) cells. In this note dolichol was determined in two subpopulations of liver Ito cells isolated from rats pretreated with vitamin A: Ito-1, vitamin A enriched and Ito-2, relatively poor of vitamin A. Differences were observed in the behaviour of the two fractions after vitamin A pretreatment of rats. In fact, in Ito-1 fraction dolichol increases with the increase of vitamin A, while in Ito-2 fraction it does not change significantly with the increase of vitamin A. These results, while confirming the heterogeneity of fat storing cells, are discussed as to the possible role of dolichol and vitamin A metabolism.

Characterization of insulin-like growth factor (IGF)-I-receptor binding sites during in vitro transformation of rat hepatic stellate cells to myofibroblasts

Insulin-like growth factors are growth-promoting peptides structurally related to insulin which possess autocrine and paracrine activities. IGF-I is mainly synthesized in hepatocytes, but little is known about its paracrine action on hepatocyte-adjacent perisinusoidal hepatic stellate cells, the principal matrix producing precursor cell type in the liver. IGF-I might stimulate proliferation and phenotypical transformation of hepatic stellate cells into myofibroblasts, the cell type responsible for the excessive production of connective tissue elements during fibrogenesis. In this study we investigated the expression and function of the IGF-I receptor during transformation of isolated and cultured hepatic stellate cells. The respective stages of transformation of hepatic stellate cells were defined by determination of cellular smooth muscle iso-alpha-actin and retinyl-palmitate content, respectively. IGF-I receptor protein decreased stage-dependently down to 0.5 at the 4th day and about 0.17 at the 8th day. The number of IGF-I receptors was determined to be initially 1.3 x 10(5)/cell. Their quantity decreased to 0.8 x 10(5) sites/cell (4th day) down to 0.5 x 10(5) sites/cell at the 7th day and remained constant thereafter at 0.7 x 10(5) sites/cell. Dissociation constants (KD) for IGF-I range from 0.32-0.57 nmol/l showing constantly high receptor affinity. Northern blot analyses revealed distinct upregulation of IGF-I receptor mRNA level during culture. It is concluded that hepatocyte-generated IGF-I and/or IGF-I binding proteins are candidate mediators of hepatic stellate cell activation during the initial period of transformation to myofibroblasts. After completion of transformation the cell becomes relatively refractory to the action of IGF-I as judged from receptor density.