Distribution and cellular origin of collagen VI during development and in cirrhosis

Platelet-derived growth factor receptor β-targeted positron emission tomography imaging for the noninvasive monitoring of liver fibrosis

PURPOSE

Noninvasive quantifying activated hepatic stellate cells (aHSCs) by molecular imaging is helpful for assessing disease progression and therapeutic responses of liver fibrosis. Our purpose is to develop platelet-derived growth factor receptor β (PDGFRβ)-targeted radioactive tracer for assessing liver fibrosis by positron emission tomography (PET) imaging of aHSCs.

METHODS

Comparative transcriptomics, immunofluorescence staining and flow cytometry were used to evaluate PDGFRβ as biomarker for human aHSCs and determine the correlation of PDGFRβ with the severity of liver fibrosis. The high affinity affibody for PDGFRβ (ZPDGFRβ) was labeled with gallium-68 (68Ga) for PET imaging of mice with carbon tetrachloride (CCl4)-induced liver fibrosis. Binding of the [68Ga]Ga-labeled ZPDGFRβ ([68Ga]Ga-DOTA-ZPDGFRβ) for aHSCs in human liver tissues was measured by autoradiography.

RESULTS

PDGFRβ overexpressed in aHSCs was highly correlated with the severity of liver fibrosis in patients and CCl4-treated mice. The 68Ga-labeled ZPDGFRβ affibody ([68Ga]Ga-DOTA-ZPDGFRβ) showed PDGFRβ-dependent binding to aHSCs. According to the PET imaging, hepatic uptake of [68Ga]Ga-DOTA-ZPDGFRβ increased with the accumulation of aHSCs and collagens in the fibrotic livers of mice. In contrast, hepatic uptake of [68Ga]Ga-DOTA-ZPDGFRβ decreased with spontaneous recovery or treatment of liver fibrosis, indicating that the progression and therapeutic responses of liver fibrosis in mice could be visualized by PDGFRβ-targeted PET imaging. [68Ga]Ga-DOTA-ZPDGFRβ also bound human aHSCs and visualized fibrosis in patient-derived liver tissues.

CONCLUSIONS

PDGFRβ is a reliable biomarker for both human and mouse aHSCs. PDGFRβ-targeted PET imaging could be used for noninvasive monitoring of liver fibrosis in mice and has great potential for clinical translation.

Tissue mechanics coevolves with fibrillar matrisomes in healthy and fibrotic tissues

Fibrillar proteins are principal components of extracellular matrix (ECM) that confer mechanical properties to tissues. Fibrosis can result from wound repair in nearly every tissue in adults, and it associates with increased ECM density and crosslinking as well as increased tissue stiffness. Such fibrotic tissues are a major biomedical challenge, and an emerging view posits that the altered mechanical environment supports both synthetic and contractile myofibroblasts in a state of persistent activation. Here, we review the matrisome in several fibrotic diseases, as well as normal tissues, with a focus on physicochemical properties. Stiffness generally increases with the abundance of fibrillar collagens, the major constituent of ECM, with similar mathematical trends for fibrosis as well as adult tissues from soft brain to stiff bone and heart development. Changes in expression of other core matrisome and matrisome-associated proteins or proteoglycans contribute to tissue stiffening in fibrosis by organizing collagen, crosslinking ECM, and facilitating adhesion of myofibroblasts. Understanding how ECM composition and mechanics coevolve during fibrosis can lead to better models and help with antifibrotic therapies.

Alpha-single chains of collagen type VI inhibit the fibrogenic effects of triple helical collagen VI in hepatic stellate cells

The interaction of extracellular matrix (ECM) components with hepatic stellate cells (HSCs) is thought to perpetuate fibrosis by stimulating signaling pathways that drive HSC activation, survival and proliferation. Consequently, disrupting the interaction between ECM and HSCs is considered a therapeutical avenue although respective targets and underlying mechanisms remain to be established. Here we have interrogated the interaction between type VI collagen (CVI) and HSCs based on the observation that CVI is 10-fold upregulated during fibrosis, closely associates with HSCs in vivo and promotes cell proliferation and cell survival in cancer cell lines. We exposed primary rat HSCs and a rat hepatic stellate cell line (CFSC) to soluble CVI and determined the rate of proliferation, apoptosis and fibrogenesis in the absence of any additional growth factors. We find that CVI in nanomolar concentrations prevents serum starvation-induced apoptosis. This potent anti-apoptotic effect is accompanied by induction of proliferation and acquisition of a pronounced pro-fibrogenic phenotype characterized by increased α-smooth muscle actin, TGF-β, collagen type I and TIMP-1 expression and diminished proteolytic MMP-13 expression. The CVI-HSC interaction can be disrupted with the monomeric α2(VI) and α3(VI) chains and abrogates the activating CVI effects. Further, functional relevant α3(VI)—derived 30 amino acid peptides lead to near-complete inhibition of the CVI effect. In conclusion, CVI serves as a potent mitogen and activating factor for HSCs. The antagonistic effects of the CVI monomeric chains and peptides point to linear peptide sequences that prevent activation of CVI receptors which may allow a targeted antifibrotic therapy.

Collagen VI as a driver and disease biomarker in human fibrosis

Fibrosis of visceral organs such as the lungs, heart, kidneys and liver remains a major cause of morbidity and mortality and is also associated with many other disorders, including cancer and metabolic disease. In this review, we focus upon the microfibrillar collagen VI, which is present in the extracellular matrix (ECM) of most tissues. However, expression is elevated in numerous fibrotic conditions, such as idiopathic pulmonary disease (IPF), and chronic liver and kidney diseases. Collagen VI is composed of three subunits α1, α2 and α3, which can be replaced with alternate chains of α4, α5 or α6. The C-terminal globular domain (C5) of collagen VI α3 can be proteolytically cleaved to form a biologically active fragment termed endotrophin, which has been shown to actively drive fibrosis, inflammation and insulin resistance. Tissue biopsies have long been considered the gold standard for diagnosis and monitoring of progression of fibrotic disease. The identification of neoantigens from enzymatically processed collagen chains have revolutionised the biomarker field, allowing rapid diagnosis and evaluation of prognosis of numerous fibrotic conditions, as well as providing valuable clinical trial endpoint determinants. Collagen VI chain fragments such as endotrophin (PRO-C6), C6M and C6Mα3 are emerging as important biomarkers for fibrotic conditions.

Serum levels of endotrophin are associated with nonalcoholic steatohepatitis

BACKGROUND AND AIMS

There are no currently available biomarkers that can accurately indicate the presence of non-alcoholic steatohepatitis (NASH). We investigated the association between endotrophin, a cleavage product of collagen type 6α3, and disease severity in patients with non-alcoholic fatty liver disease (NAFLD).

METHODS

We measured serum endotrophin levels in 211 patients with NAFLD and nine healthy controls. Liver biopsy data was available for 141 (67%) of the patients. Associations between endotrophin and the presence of NASH and advanced fibrosis were investigated alone and in combination with standard clinical parameters using logistic regression.

RESULTS

A total of 211 patients were enrolled in this study, consisting of 108 (51%) men and 103 (49%) women with a mean age of 55.6 years. 58 (27%) of the patients had advanced fibrosis. Of those with biopsy data, 87 (62%) had NASH. Serum levels of endotrophin were significantly higher in patients with NAFLD than those in healthy controls (37[±12] vs. 17[±7] ng/mL, p<.001). Serum levels of endotrophin were also significantly higher in patients with NASH than in those without NASH (40[±12] vs. 32[±13] ng/mL, p<.001). A model using age, sex, body mass index and levels of alanine aminotransferase (ALT), glucose and endotrophin effectively predicted the presence of NASH in a derivation (AUROC 0.83, 95%CI = 0.74-0.92) and validation cohort (AUROC 0.71, 95%CI = 0.54-0.88). There was no significant association between serum levels of endotrophin and advanced fibrosis.

CONCLUSIONS

These data suggest that serum endotrophin could be a valuable biomarker for diagnosing NASH, but not for detecting advanced fibrosis in NAFLD.

Targeted Drug Delivery to Hepatic Stellate Cells for the Treatment of Liver Fibrosis

Liver fibrosis is caused by excessive accumulation of extracellular matrix during chronic liver injuries. Although clinical evidence suggests that liver fibrosis can be reversed, there is no standard therapy for liver fibrosis. Moreover, there is a lack of diagnostic tools to detect early-stage liver fibrosis. Activation of hepatic stellate cells (HSCs) is the key step during liver fibrogenesis, and its mechanism has been extensively studied by various cell culture and animal models. Targeted delivery of therapeutic agents to activated HSCs is therefore critical for the successful treatment of liver fibrosis. A number of protein markers have been found to be overexpressed in activated HSCs, and their ligands have been used to specifically deliver various antifibrotic agents. In this review, we summarize these HSC-specific protein markers and their ligands for targeted delivery of antifibrotic agents.

The good and the bad collagens of fibrosis - Their role in signaling and organ function

Usually the dense extracellular structure in fibrotic tissues is described as extracellular matrix (ECM) or simply as collagen. However, fibrosis is not just fibrosis, which is already exemplified by the variant morphological characteristics of fibrosis due to viral versus cholestatic, autoimmune or toxic liver injury, with reticular, chicken wire and bridging fibrosis. Importantly, the overall composition of the ECM, especially the relative amounts of the many types of collagens, which represent the most abundant ECM molecules and which centrally modulate cellular functions and physiological processes, changes dramatically during fibrosis progression. We hypothesize that there are good and bad collagens in fibrosis and that a change of location alone may change the function from good to bad. Whereas basement membrane collagen type IV anchors epithelial and other cells in a polarized manner, the interstitial fibroblast collagens type I and III do not provide directional information. In addition, feedback loops from biologically active degradation products of some collagens are examples of the importance of having the right collagen at the right place and at the right time controlling cell function, proliferation, matrix production and fate. Examples are the interstitial collagen type VI and basement membrane collagen type XVIII. Their carboxyterminal propeptides serve as an adipose tissue hormone, endotrophin, and as a regulator of angiogenesis, endostatin, respectively. We provide an overview of the 28 known collagen types and propose that the molecular composition of the ECM in fibrosis needs careful attention to assess its impact on organ function and its potential to progress or reverse. Consequently, to adequately assess fibrosis and to design optimal antifibrotic therapies, we need to dissect the molecular entity of fibrosis for the molecular composition and spatial distribution of collagens and the associated ECM.

Targeting activated hepatic stellate cells (aHSCs) for liver fibrosis imaging

Following injurious stimuli, quiescent hepatic stellate cells (qHSCs) transdifferentiate into activated HSCs (aHSCs). aHSCs play pivotal roles in the onset and progression of liver fibrosis. Therefore, molecular imaging of aHSCs in liver fibrosis will facilitate early diagnosis, prognosis prediction, and instruction and evaluation of aHSC-targeted treatment. To date, several receptors, such as integrin αvβ3, mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR), collagen type VI receptor (CVIR), platelet-derived growth factor receptor-β (PDGFR-β), vimentin, and desmin, have been identified as biomarkers of aHSCs. Corresponding ligands to these receptors have also been developed. This review will discuss strategies for developing aHSC-targeted imaging in liver fibrosis.

Quantitative secretome analysis reveals that COL6A1 is a metastasis-associated protein using stacking gel-aided purification combined with iTRAQ labeling

In cancer metastasis, secreted proteins play an important role in promoting cancer cell migration and invasion and thus also in the increase of cancer metastasis in the extracellular microenvironment. In this study, we developed a strategy that combined a simple gel-aided protein purification with iTRAQ labeling to quantify and discover the metastasis-associated proteins in the lung cancer cell secretome. Secreted proteins associated with lung cancer metastasis were produced using CL1-0 and CL1-5 cells with different metastatic abilities. Quantitative secretomics analysis identified a total of 353 proteins, 7 of which were considered to be metastasis-associated proteins. These included TIMP1, COL6A1, uPA, and AAT, all of which were higher in CL1-5, and AL1A1, PRDX1, and NID1, which were higher in CL1-0. Six of these metastasis-associated proteins were validated with Western blot analysis. In addition, pathway analysis was performed in building the interaction network between the identified metastasis-associated proteins. Further functional analysis of COL6A1 on the metastatic abilities of CL1 cells was also carried out. An RNA interference-based knock-down of COL6A1 suppressed the metastatic ability of CL1-5 cells; in contrast, a plasmid-transfected overexpression of COL6A1 increased the metastatic ability of CL1-0 cells. This study describes a simple and high throughput sample purification method that can be used for the quantitative secretomics analysis of metastasis-associated proteins.

Targeted delivery of drugs for liver fibrosis

Liver fibrosis and its end stage disease cirrhosis are a major cause of mortality and morbidity around the world. There is no effective pharmaceutical intervention for liver fibrosis at present. Many drugs that show potent antifibrotic activities in vitro often show only minor effects in vivo because of insufficient concentrations of drugs accumulating around the target cell and their adverse effects as a result of affecting other non-target cells. Hepatic stellate cells (HSC) play a critical role in the fibrogenesis of liver, so they are the target cells of antifibrotic therapy. Several kinds of targeted delivery system that could target the receptors expressed on HSC have been designed, and have shown an attractive targeted potential in vivo. After being carried by these delivery systems, many agents showed a powerful antifibrotic effect in animal models of liver fibrosis. These targeted delivery systems provide a new pathway for the therapy of liver fibrosis. The characteristics of theses targeted carriers are reviewed in this paper.

Experimental poisoning by Senecio brasiliensis in calves: quantitative and semi-quantitative study on changes in the hepatic extracellular matrix and sinusoidal cells

Extracellular matrix plays an important role in chronic hepatic lesions and has been studied in experimental intoxication models. However in cattle, studies on chronic disease have focused on the hepatocellular damage and extracellular matrix (ECM) changes are usually overlooked. There are no specific studies on the hepatic ECM in either normal or chronically damaged bovine liver. Thus an experimental model of hepatic toxicity model using Senecio brasiliensis poisoned calves was designed. Senecio brasiliensis contains pyrrolizidine alkaloids which cause either acute or chronic progressive dose dependent liver damage. Five calves were orally fed with 0.38g of dry leaves of S. brasiliensis/kg/day for 24 days. Liver needle biopsy specimens were obtained every 15 days for 60 days. Clinical signs of digestive complications appeared at 3rd week. One calf died on 45th day and four were evaluated up to 60th day. Biopsy samples were processed for routine light microscopy, immuno-histochemistry and transmission electron microscopy. From 30th day on progressive liver damage characterized by hepatocellular ballooning, necrosis, apoptosis and megalocytosis, centrilobular, pericellular and portal fibrosis were seen by light microscopy. Quantitative and semi-quantitative measurements of hepatic ECM components were performed before and after the onset of lesions. Morphometric analysis of total collagen and elastic fiber system was conducted. Total collagen and I and III collagen types progressively increased in throughout the liver of affected calves. Changes in location, amount and disposition of the elastic fiber system were also observed. Then numbers of Kupffer cells were significantly increased at 30th day and total numbers of sinusoidal cells were significantly increased at 45th and 60th days. Liver damage was progressive and irreversible even after the exposure to the plant was discontinued. Severe fibrotic lesions occurred mainly in portal tracts, followed by veno-occlusive and pericellular fibrosis. Collagen types I and III s were present in every normal and damaged liver, with predominance of type I. In affected calves the increase of total collagen and elastic fibers system paralleled the number of total sinusoidal cells.

Rheumatoid factor induction in murine models of liver injury

Alcoholic liver disease and hepatitis C are associated with the production of autoantibodies such as rheumatoid factors (RF), which bind to IgG and can aid in host defence, but are also associated with pathological conditions such as rheumatoid arthritis. Because little is known about the role of RF in liver disease, we characterized the RF production that either occurred spontaneously in response to alcohol consumption or was induced by injection of an Escherichia coli glycolipoprotein in C57Bl/6 mice. Whereas severe liver damage was induced by carbon tetrachloride (CCl(4)), minimal damage was caused by chronic alcohol consumption. Liver damage was monitored by measurements of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Circulating RF was induced in response to chronic alcohol consumption; the latter probably involved Toll-like receptor ligation. In contrast, CCl(4)-induced damage was not associated with RF induction. However, concurrent treatment with an E. coli glycolipoprotein macromolecule that induced RF, protected against CCL(4)-induced liver damage as measured by a highly significant decrease (P = 0.008) at 4 weeks in AST and ALT. RF induced by E. coli glycolipoprotein correlated with 'protection' from liver damage, indicating that the RF autoimmune response does not necessarily exacerbate liver disease.

Stellate cell apoptosis by a soluble mediator from immortalized human hepatocytes

Activated hepatic stellate cells (HSCs) are the major source of extracellular matrix in fibrosis and cirrhosis. In this study, we have investigated the role of hepatitis C virus (HCV) core protein induced immortalized human hepatocytes (IHH) on HSC growth. Preferential growth of IHH and apoptosis of activated human hepatic stellate cells (LX2) were observed upon coculture of these two cell types in a dual chamber or in the presence of conditioned medium (CM) from IHH. CM did not display a growth inhibitory role on other hepatic (Huh-7, HepG2, Hep3B and THLE) and non-hepatic (HeLa, MCF-7, and BHK) epithelial cells, indicating that the soluble mediator from IHH does not have a generalized effect on cell lines examined in our study. Further studies suggested that CM from IHH increased the expression of TRAIL receptors on LX2 cell surface, and induced apoptosis by a caspase dependent mechanism. Peptide mass fingerprinting of the purified soluble mediator from CM suggested that gelsolin fragments may play a role in apoptosis of LX2 cells. Taken together, our results suggested that a soluble mediator secreted from immortalized human hepatocytes plays an important role in hepatic stellate cell growth regulation.

Regulation of hepatocyte cell cycle progression and differentiation by type I collagen structure

Cell behavior is strongly influenced by the extracellular matrix (ECM) to which cells adhere. Both chemical determinants within ECM molecules and mechanical properties of the ECM network regulate cellular response, including proliferation, differentiation, and apoptosis. Type I collagen is the most abundant ECM protein in the body with a complex structure that can be altered in vivo by proteolysis, cross-linking, and other processes. Because of collagen's complex and dynamic nature, it is important to define the changes in cell response to different collagen structures and its underlying mechanisms. This chapter reviews current knowledge of potential mechanisms by which type I collagen affects cell behavior, and it presents data that elucidate specific intracellular signaling pathways by which changes in type I collagen structure differentially regulate hepatocyte cell cycle progression and differentiation. A network of polymerized fibrillar type I collagen (collagen gel) induces a highly differentiated but growth-arrested phenotype in primary hepatocytes, whereas a film of monomeric collagen adsorbed to a rigid dish promotes cell cycle progression and dedifferentiation. Studies presented here demonstrate that protein kinase A (PKA) activity is significantly elevated in hepatocytes on type I collagen gel relative to collagen film, and inhibition of this elevated PKA activity can promote hepatocyte cell cycle progression on collagen gel. Additional studies are presented that examine changes in hepatocyte cell cycle progression and differentiation in response to increased rigidity of polymerized collagen gel by fiber cross-linking. Potential mechanisms underlying these cellular responses and their implications are discussed.

Heparin accelerates liver regeneration following portal branch ligation in normal and cirrhotic rats with increased plasma hepatocyte growth factor levels

BACKGROUND/AIMS

Heparin is widely used as a general anticoagulant, and has been recently reported to elevate plasma hepatocyte growth factor (HGF) levels by releasing HGF sequestrated in the extracellular matrix. Therefore, we investigated the effects of heparin administration on liver regeneration following portal branch ligation (PBL) in normal and cirrhotic rats.

METHODS

Dimethylnitrosamine-induced cirrhotic rats and control rats underwent portal ligation of the left lateral and median branches, followed by intraperitoneal heparin injections, every 12 h. To examine the feasibility of an extensive hepatectomy in the cirrhotic livers, cirrhotic rats with or without heparin treatment underwent resection of occluded lobes at 72 h after the PBL.

RESULTS

Heparin injections significantly augmented liver regeneration after PBL in both normal and cirrhotic rats, following an increase in hepatocellular DNA synthesis at 24 h after the PBL. The plasma HGF concentrations were elevated by heparin treatment in both groups. In addition, heparin administration dramatically improved the survival rate after an extensive hepatectomy in the cirrhotic rats.

CONCLUSIONS

Heparin treatment significantly accelerated liver regeneration following the PBL, with an increase in the plasma HGF levels in both normal and cirrhotic rats. Heparin administration may make an extensive hepatectomy clinically feasible even for cirrhotic livers.

[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.

Successful targeting to rat hepatic stellate cells using albumin modified with cyclic peptides that recognize the collagen type VI receptor

The key pathogenic event in liver fibrosis is the activation of hepatic stellate cells (HSC). Consequently, new antifibrotic therapies are directed toward an inhibition of HSC activities. The aim of the present study was to develop a drug carrier to HSC, which would allow cell-specific delivery of antifibrotic drugs thus enhancing their effectiveness in vivo. We modified human serum albumin (HSA) with 10 cyclic peptide moieties recognizing collagen type VI receptors (C*GRGDSPC*, in which C* denotes the cyclizing cysteine residues) yielding pCVI-HSA. In vivo experiments showed preferential distribution of pCVI-HSA to both fibrotic and normal rat livers (respectively, 62 +/- 6 and 75 +/- 16% of the dose at 10 min after intravenous injection). Immunohistochemical analysis demonstrated that pCVI-HSA predominantly bound to HSC in fibrotic livers (73 +/- 14%). In contrast, endothelial cells contributed mostly to the total liver accumulation in normal rats. In vitro studies showed that pCVI-HSA specifically bound to rat HSC, in particular to the activated cells, and showed internalization of pCVI-HSA by these cells. In conclusion, pCVI-HSA may be applied as a carrier to deliver antifibrotic agents to HSC, which may strongly enhance the effectiveness and tissue selectivity of these drugs. This approach has the additional benefit that such carriers may block receptors that play a putative role in the pathogenesis of liver fibrosis.

Soluble collagen VI drives serum-starved fibroblasts through S phase and prevents apoptosis via down-regulation of Bax

We previously showed that soluble, pepsin-solubilized collagen VI increases de novo DNA synthesis in serum-starved HT1080 and 3T3 fibroblasts up to 100-fold compared with soluble collagen I, reaching 80% of the stimulation caused by 10% fetal calf serum. Here we show that collagen VI also inhibits apoptotic cell death in serum-starved cells as evidenced by morphological criteria, DNA laddering, complementary apoptosis assays (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, enzyme-linked immunosorbent assay, and fluorescence-activated cell sorting), and quantification of apoptosis-regulating proteins. In the presence of starving medium alone or collagen I, the proapoptotic Bax was up-regulated 2-2.5-fold, compared with soluble collagen VI and fetal calf serum, whereas levels of the antiapoptotic Bcl-2 protein remained unaffected. In accordance with its potent stimulation of DNA synthesis, soluble collagen VI carries serum-starved HT1080 and Balb 3T3 fibroblasts through G(2) as shown by fluorescence-activated cell sorting analysis, whereas cells exposed to medium and collagen I where arrested at G(1)-S. This was accompanied by a 2-3-fold increase in cyclin A, B, and D1 protein expression. Collagen VI-induced inhibition of apoptotic cell death may be operative during embryogenesis, wound healing, and fibrosis when elevated tissue and blood levels of collagen VI are observed, thus initiating a feedback loop of mesenchymal cell activation and proliferation.

Two assays for measuring fibrosis: reverse transcriptase-polymerase chain reaction of collagen alpha(1) (III) mRNA is an early predictor of subsequent collagen deposition while a novel serum N-terminal procollagen (III) propeptide assay reflects manifest fibrosis in carbon tetrachloride-treated rats

Using a novel quantitative reverse transcriptase-polymerase chain reaction assay, we have determined the amount of specific mRNA for procollagen alpha(1) (III) (PIIIP) in the carbon tetrachloride (CCl(4)) model of liver fibrosis in rats. After a single week of CCl(4) application, the amount of PIIIP mRNA was increased approximately 10 times over the untreated control group and continued to increase to approximately 30 times after 7 weeks of intoxication. In this model substantial fibrosis was demonstrated by computer-aided morphometry after 5 to 7 weeks of treatment. Using recombinant murine N-terminal procollagen alpha(1) (III) propeptide (PIIINP), a novel sensitive immunoassay for the measurement of circulating PIIINP in rodent sera was established. An increase in PIIINP serum levels was observed after 5 to 7 weeks of CCl(4) intoxication. Our results suggest PIIIP gene expression is an early marker of tissue fibrosis. Early PIIIP gene expression is correlated with the extent of the subsequent fibrosis. PIIIP mRNA levels increase much earlier than conventional histological examination or PIIINP levels. PIIINP measurements with our new serum assay, on the other hand, are a good noninvasive marker of manifest fibrosis but are a poor marker of fibrogenesis.

Fibrogenesis and fibrolysis in collagenous colitis. Patterns of procollagen types I and IV, matrix-metalloproteinase-1 and -13, and TIMP-1 gene expression

Collagenous colitis is characterized by the deposition of a superficial subepithelial collagenous layer, the pathogenesis of which is unknown. Because the excess matrix deposition is potentially reversible, a labile imbalance between fibrogenesis and fibrolysis may be suspected. Expression of procollagen alpha1(I) and alpha1(IV), matrix-metalloproteinase (MMP)-1 and -13, and tissue inhibitor of metalloproteinase (TIMP)-1 genes was semiquantitated by in situ hybridization on serial biopsies of 12 patients with collagenous colitis and compared to controls. Collagen types I, III, IV, and VI, tenascin, undulin/collagen XIV, and alpha-actin were localized by immunohistology. The superficial collagen layer stained strongly for collagen types I, III, and VI, and particularly for tenascin, but not for undulin. Elevated procollagen alpha1(I), procollagen alpha1(IV), and TIMP-1 transcript levels were found in alpha-actin-positive cells with linear distribution underneath the superficial collagenous layer, whereas MMP-1 RNA expression was variable and restricted to cell clusters. MMP-13 expression was undetectable. The patterns of procollagen alpha1(I)- and alpha1(IV)-specific labeling, combined with an intense tenascin- but absent undulin-specific staining, indicate deposition of an immature interstitial matrix that may be susceptible to degradation. The restricted MMP-1 RNA expression, counteracted by increased TIMP-1 expression, suggests locally impaired fibrolysis as a relevant factor in the pathogenesis of collagenous colitis.

MMP2 activation by collagen I and concanavalin A in cultured human hepatic stellate cells

Fibrosis occurs in most chronic liver injuries and results from changes in the balance between synthesis and degradation of extracellular matrix components. In fibrotic livers, there is a markedly increased activity of matrix metalloproteinase 2 (MMP2), a major enzyme involved in extracellular matrix remodeling. We have previously shown that hepatic stellate cells secrete latent MMP2 and that MMP2 activation occurs in coculture of hepatic stellate cells and hepatocytes concomitantly with matrix deposition. In the present work we investigated the effects of various extracellular matrix components and concanavalin A, an inducer of immune-mediated liver injuries, on MMP2 activation in cultured human hepatic stellate cells. Collagen I induced a dose-dependent MMP2 activation, which was not blocked by both actinomycin and cycloheximide. Collagen VI, laminin, and a reconstituted basement membrane (matrigel) were ineffective in inducing activation. Specific antibodies against the subunits of alpha2beta1 integrins, the major collagen I receptor, induced partial inhibition of MMP2 activation. Treatment of cells with concanavalin A resulted in a marked activation of MMP2 that correlated with the proteolytic processing of MT1-MMP, the MMP2 activator, from a Mr=60 kd toward a Mr=43 kd polypeptide. Actinomycin and cycloheximide inhibited the MMP2 activation induced by concanavalin A. Recombinant tissue inhibitor of metalloproteinase-2 and the MMP inhibitor BB-3103, but not PMSF, blocked MMP2 activation induced by collagen I or concanavalin A, and MT1-MMP processing to its Mr-43 kd form. These results suggest that the accumulation of collagen I may specifically contribute to the remodeling of extracellular matrix in fibrotic livers by inducing MMP2 activation.

Matrix protein mRNA levels in canine meniscus cells in vitro

The resident cells of the meniscus synthesize a fibrocartilaginous extracellular matrix in vivo composed predominantly of type I collagen fibers. To increase our understanding of matrix biosynthesis by meniscus cells in vitro, we examined matrix protein mRNA levels in cultured meniscus cells isolated from skeletally mature dogs. The mRNA levels of five matrix protein genes (COL1A1, COL2A1, aggrecan, COL6A1, and fibronectin) were measured in meniscus cells by Northern blotting and compared with those of patellar tendon fibroblasts and femoral articular cartilage chondrocytes. In freshly isolated cells (Day 0 cells), COL1A1, COL2A1, and aggrecan mRNA levels were low or undetectable in both meniscus cells and tendon fibroblasts. In intact meniscus tissue, COL1A1 mRNA levels were also low or undetectable. COL2A1 and aggrecan mRNA transcripts were readily observed, however, in Day 0 articular chondrocytes. The levels of expression of COL6A1 and fibronectin mRNA transcripts in Day 0 meniscus cells were intermediate between higher articular chondrocyte levels and lower tendon fibroblast levels. After 1 week in monolayer culture (Day 7 cells), meniscus cells expressed readily detectable levels of COL1A1 mRNA transcripts, similar to that observed for cultured tendon fibroblasts. COL1A1 mRNA transcripts were either not detected or detected at very low levels in monolayer cultures of articular chondrocytes. COL2A1 and aggrecan mRNA transcripts were readily detected in cultured articular chondrocytes but not in meniscus cells or in tendon fibroblasts. All three types of cells continued to express COL6A1 and fibronectin mRNA transcripts after 1 week in culture. These results demonstrate that the patterns of expression of COL1A1 and COL2A1 mRNA transcripts by meniscus cells are similar to those of tendon fibroblasts and dissimilar to those of articular chondrocytes both in freshly isolated cells and in monolayer cultured cells. This mRNA expression pattern supports the idea that monolayer culture of meniscus cells results in the expression of a predominantly fibroblastic phenotype.

Morphology of sites of adhesion between hepatic stellate cells (vitamin A-storing cells) and a three-dimensional extracellular matrix

BACKGROUND

Hepatic stellate cells lie in the perisinusoidal space in a three-dimensionally distributed extracellular matrix (ECM). This three-dimensional structure of the ECM regulates the proliferation, morphology, and functions of the stellate cell. To investigate how the three-dimensional structure of ECM regulates behavior of the cells, we cultured stellate cells two- or three-dimensionally and examined the morphology of the cells in both cases as well as the localization of cell-surface adhesion molecules specific for the ECM.

METHODS

Isolated rat stellate cells and human stellate cells were cultured in Dulbecco's modified Eagle's medium. Rat stellate cells were cultured in non-coated polystyrene culture dishes, or on or in type I collagen gels. The morphology of cell-ECM adhesion was examined under transmission and scanning electron microscopes. Localization of integrin alpha2 and integrin beta1 in human stellate cells was examined by immunoelectron microscopy. Immunostaining was performed with a mouse monoclonal anti-human integrin alpha2 or integrin beta1 antibody and goat anti-mouse IgG coupled with 10-nm immunogold.

RESULTS

Hepatic stellate cells cultured in polystyrene dishes spread well. However, the cells cultured on or in the type I collagen gel became slender. The cells extended long cellular processes onto or into the gel. The cellular processes were entangled three-dimensionally with the type I collagen fibers and directly adhered to these fibers. The cells inoculated in type I collagen gels formed a large number of adhesive structures that resembled focal adhesions. These adhesive structures were distributed not only on the lower side but also on the upper side of both the cell bodies and cellular processes. Moreover, each adhesive area formed a face but not a point. Integrin alpha2 and integrin beta1 were detected on the surfaces of cell bodies, cellular processes, and microprojections.

CONCLUSIONS

The cells cultured in type I collagen gel develop a three-dimensional adhesive structure.

Type VI collagen gene expression in experimental liver fibrosis: quantitation and spatial distribution of mRNAs, and immunodetection of the protein

Type VI collagen is a minor but essential matrix component in the liver. In this study, we utilized an acute and a chronic injury model to clarify the process of liver fibrosis in rats by administration of carbon tetrachloride. Collagen gene expression, with particular emphasis on type VI collagen, was studied by molecular hybridization techniques. The alpha 2(VI) collagen mRNA levels were markedly elevated on day 3 of acute injury and were approximately at the same high level at 7 and 14 weeks of chronic injury, as determined by Northern hybridizations and slot-blot analyses. Marked enhancement of type I collagen gene expression was similarly noted at these time points. The activation of collagen gene expression in acute injury, as determined by in situ hybridization, was particularly prominent in the vicinity of the central veins. Indirect immunofluorescence demonstrated marked accumulation of type VI collagen protein as early as day 3 of acute injury, and the reaction appeared to be initiated in the proximity of central veins. These results indicate that type VI collagen gene expression, together with other connective tissue components, including type I collagen, is activated in the early stages of the fibrotic process. Type VI collagen accumulation may contribute to the distorted architecture and functional impairment of the liver in hepatic fibrosis.

Bone marrow and serum connective tissue polypeptides in idiopathic myelofibrosis

The distribution of collagen type VI and tenascin has been determined in both normal and myelofibrotic bone marrow by immunohistological techniques. In normal sections positivity was demonstrated in the periosteum (collagen type VI and tenascin) and in the walls of small blood vessels (tenascin). In contrast, myelofibrotic bone marrow showed an increased deposition of both proteins, especially collagen type VI, although this increase was restricted to the later fibrotic stages of the disease. Serum concentrations of collagen type I (PICP), collagen type III (PIIIP) and laminin (laminin P1) related polypeptides were determined in a further 26 patients. PIIIP levels were significantly raised, in contrast to PICP and laminin P1 concentrations. All three markers, however, were significantly elevated in patients with active/transforming disease. Laminin P1 and PICP levels showed a strong correlation, indicating a relationship between basement membrane and interstitial collagen metabolism, although they do not appear to offer any advantage over PIIIP for the monitoring of disease activity.

Expression of von Willebrand factor in normal and diseased rat livers and in cultivated liver cells

Von Willebrand factor (vWf) is an adhesive glycoprotein known to play an important role in hemostasis and in tissue injury. Because the latter process resembles hepatic fibrogenesis, we studied the tissue distribution of vWf in diseased livers. In normal rat liver vWf was strongly expressed in the vascular endothelium and as small spots or fiber-like structures in the hepatic parenchyma. During acute liver injury, pronounced staining was observed within the area of necrosis. In fibrotic livers vWf deposits were distributed predominantly at the scar-parenchyma interface but also within the septum and in sinusoidal lining cells. Testing different liver cell populations in vitro demonstrated that vWf gene expression was limited to endothelial cells (ECs) and, therefore, the latter cell population might represent the vWf-positive cells detected in vivo. The distribution of vWf within fibrotic septa suggests that vWf becomes a component of the extracellular matrix (ECM) in fibrotic livers.

Induction of hepatic Ito cell nitric oxide production after acute endotoxemia

Nitric oxide is a highly reactive mediator released in the liver by hepatocytes, Kupffer cells and endothelial cells during endotoxin-induced inflammation. In this study we determined whether Ito cells also produce nitric oxide after exposure to endotoxin. For induction of endotoxemia, rats were injected intravenously with Escherichia coli lipopolysaccharide (2.5 mg/kg). Ito cells were isolated from the animals 48 hr later by means of in situ perfusion of the liver with protease and collagenase followed by purification on an arabinogalactan gradient. Ito cells from untreated and endotoxemic rats were found to produce low levels of nitric oxide in response to interferon-gamma. In both cell types, this response depended on L-arginine and was blocked by NG-monomethyl-L-arginine, a specific nitric oxide synthase inhibitor. Cells from rats treated with endotoxin produced significantly more nitric oxide than did cells from untreated animals; this was due, at least in part, to increased expression of protein for an inducible form of nitric oxide synthase. These cells also responded to stimulation with lipopolysaccharide in vitro, as well as the combination of interferon-gamma and lipopolysaccharide, which was synergistic in stimulating nitric oxide production. Tumor necrosis factor-alpha and macrophage colony-stimulating factor were also found to stimulate nitric oxide production by Ito cells from endotoxemic rats. In addition, in these cells, tumor necrosis factor-alpha synergized with interferon-gamma in inducing nitric oxide production. The combination of interferon-gamma and lipopolysaccharide was also found to inhibit Ito cell DNA synthesis, as measured on the basis of [3H]-thymidine uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Undulin RNA and protein expression in normal and fibrotic human liver

We have analyzed the distribution, gene expression and cellular origin of undulin, a large extracellular matrix glycoprotein associated with mature collagen fibrils, in human liver by immunohistochemistry, Northern-blot analysis and in situ hybridization. In normal liver, undulin was distributed as densely packed fibers in portal tract stroma, and as fine fibers along sinusoids, and around central veins. Undulin ribonucleic acid expression was low in normal liver, and confined to mesenchymal cells of portal tract stroma, vessel walls and perisinusoidal space. In fibrotic liver, undulin deposition and gene expression were enhanced in fibrotic stroma and areas of fibrogenesis identified by the presence of active septa and inflammatory infiltrate. Undulin gene expression in fibrotic liver was exclusively localized in mesenchymal cells that could be identified by staining for vimentin, and partially for alpha-smooth muscle actin as (myo)fibroblasts, and possibly fat-storing cells. These data suggest that undulin is a constituent of the hepatic extracellular matrix of normal human liver, and that it participates in the rearrangement of connective tissue occurring in hepatic fibrosis.

Endothelial cell heterogeneity in the normal human liver acinus: in situ immunohistochemical demonstration

While a certain degree of structural and functional intra-lobular heterogeneity of sinusoidal endothelial cells has been observed in rodents, little information is available about the zonal characteristics of sinusoidal endothelial cells in the human liver acinus. We have therefore examined the intra-acinar distribution of a panel of endothelial markers in the normal human liver, including: (a) structural markers of continuous and sinusoidal endothelia (PECAM-1, CD-34 protein, VE-cadherin, 1F10 antigen), (b) functional markers specific for sinusoidal endothelial cells, as previously determined in the laboratory (CD4 protein, the lipopolysaccharide-binding protein receptor (CD 14), aminopeptidase N, ICAM-1, receptors II and III for the Fc fragment of immunoglobulins G), (c) endothelial cell-matrix adhesion proteins and leukocyte-endothelial cell adhesion molecules. We observed a heterogeneous distribution for: (a) the 1F10 antigen, whose distribution in the human liver acinus was restricted to vessels situated along the axis of acinar zone 1, (b) the lipopolysaccharide-binding protein receptor and the receptor III for the Fc fragment of IgG, not expressed or only barely expressed in acinar zone 1. The distribution of the other markers tested did not display significant intra-lobular variation. Our in situ results suggest the existence of a degree of zonal heterogeneity in the structural and functional characteristics of sinusoidal endothelial cells in the human liver acinus. This might contribute to the constitution of distinct microenvironments within the human liver parenchyma.

Increased expression of type VI collagen genes in drug-induced gingival enlargement

Fibrotic gingival enlargements induced by phenytoin or nifedipine were examined with special reference to type VI collagen expression. Immunolocalization studies showed abnormal accumulation of type VI collagen around the collagen fiber bundles in the fibrotic gingival enlargements. Examination of total RNA extracted from fibroblasts and tissues of enlarged gingivae demonstrated increased type VI collagen steady-state mRNA levels. These results suggest that excessive deposition of type VI collagen in drug-induced gingival enlargement is attributed to increased expression of the collagen genes.

New challenges in hepatic fibrosis

Treatment of hepatic fibrosis by simple and inexpensive therapies is a new challenge for the near future. Hepatic fibrosis which may lead to cirrhosis, is indeed associated with most chronic liver diseases and affects millions of people. During the last decade, major breakthroughs have been accomplished in the field of hepatic fibrosis including the discovery of key components of the extracellular matrix, the cellular origin of most matrix proteins, the molecular mechanisms involved in their synthesis and degradation, and the role of cytokines in fibrogenesis. Most of this progress came from the development of new techniques including in vitro model systems which have proven useful for investigating the molecular bases of fibrogenesis. From basic research to clinical application, two major fields are now actively explored: the search for reliable serum markers of fibrogenesis and the discovery of drugs that prevent cirrhosis. A recent approach to treat hepatic fibrosis is to use cytokines, e.g. interferons, that modulate extracellular matrix synthesis.