MECHANISM OF COLLAGEN RESORPTION IN REVERSIBLE HEPATIC FIBROSIS

A review of liver fibrosis and cirrhosis regression

Cirrhosis has traditionally been considered an irreversible process of end-stage liver disease. With new treatments for chronic liver disease, there is regression of fibrosis and cirrhosis, improvement in clinical parameters (i.e. liver function and hemodynamic markers, hepatic venous pressure gradient), and survival rates, demonstrating that fibrosis and fibrolysis are a dynamic process moving in two directions. Microscopically, hepatocytes push into thinning fibrous septa with eventual perforation leaving behind delicate periportal spikes in the portal tracts and loss of portal veins. Obliterated portal veins during progressive fibrosis and cirrhosis due to parenchymal extinction, vascular remodeling and thrombosis often leave behind a bile duct and hepatic artery within the portal tract. Traditional staging classification systems focused on a linear, progressive process; however, the Beijing classification system incorporates both the bidirectional nature for the progression and regression of fibrosis. However, even with regression, vascular lesions/remodeling, parenchymal extinction and a cumulative mutational burden place patients at an increased risk for developing hepatocellular carcinoma and should continue to undergo active clinical surveillance. It is more appropriate to consider cirrhosis as another stage in the evolution of chronic liver disease as a bidirectional process rather than an end-stage, irreversible state.

Regression of Hepatic Fibrosis and Evolution of Cirrhosis: A Concise Review

Fibrosis is not a unidirectional, linear process, but a dynamic one resulting from an interplay of fibrogenesis and fibrolysis depending on the extent and severity of a biologic insult, or lack thereof. Regression of fibrosis has been documented best in patients treated with phlebotomies for hemochromatosis, and after successful suppression and eradication of chronic hepatitis B and C infections. This evidence mandates a reconsideration of the term "cirrhosis," which implies an inevitable progression towards liver failure. Furthermore, it also necessitates a staging system that acknowledges the bidirectional nature of evolution of fibrosis, and has the ability to predict if the disease process is progressing or regressing. The Beijing classification attempts to fill this gap in contemporary practice. It is based on microscopic features termed "the hepatic repair complex," defined originally by Wanless and colleagues. The elements of the hepatic repair complex represent the 3 processes of fragmentation and regression of scar, vascular remodeling (resolution), and parenchymal regeneration. However, regression of fibrosis does not imply resolution of cirrhosis, which is more than just a stage of fibrosis. So far, there is little to no evidence to suggest that large regions of parenchymal extinction can be repopulated by regenerating hepatocytes. Similarly, the vascular lesions of cirrhosis persist, and there is no evidence of complete return to normal microcirculation in cirrhotic livers. In addition, the risk of hepatocellular carcinoma is higher compared with the general population and these patients need continued screening and surveillance.

Research progress on the anti-hepatic fibrosis action and mechanism of natural products

Hepatic fibrosis is the most common pathological feature of most chronic liver diseases, and its continuous deterioration gradually develops into liver cirrhosis and eventually leads to liver cancer. At present, there are many kinds of drugs used to treat liver fibrosis. However, Western drugs tend to only target single genes/proteins and induce many adverse reactions. Most of the mechanisms and active ingredients of traditional Chinese medicine (TCM) are not clear, and there is a lack of unified diagnosis and treatment standards. Natural products, which are characterized by structural diversity, low toxicity, and origination from a wide range of sources, have unique advantages and great potential in anti-liver fibrosis. This article summarizes the work done over the previous decade, on the active ingredients in natural products that are reported to have anti-hepatic fibrosis effects. The effective anti-hepatic fibrosis ingredients identified can be generally divided into flavonoids, saponins, polysaccharides and alkaloids. Mechanisms of anti-liver fibrosis include inhibition of liver inflammation, anti-lipid peroxidation injury, inhibition of the activation and proliferation of hepatic stellate cells (HSCs), modulation of the synthesis and secretion of pro-fibrosis factors, and regulation of the synthesis and degradation of the extracellular matrix (ECM). This review provides suggestions for the development of anti-hepatic fibrosis drugs.

Fate-mapping evidence that hepatic stellate cells are epithelial progenitors in adult mouse livers

Liver injury activates quiescent hepatic stellate cells (Q-HSC) to proliferative myofibroblasts. Accumulation of myofibroblastic hepatic stellate cells (MF-HSC) sometimes causes cirrhosis and liver failure. However, MF-HSC also promote liver regeneration by producing growth factors for oval cells, bipotent progenitors of hepatocytes and cholangiocytes. Genes that are expressed by primary hepatic stellate cell (HSC) isolates overlap those expressed by oval cells, and hepatocytic and ductular cells emerge when HSC are cultured under certain conditions. We evaluated the hypothesis that HSC are a type of oval cell and, thus, capable of generating hepatocytes to regenerate injured livers. Because Q-HSC express glial fibrillary acidic protein (GFAP), we crossed mice in which GFAP promoter elements regulated Cre-recombinase with ROSA-loxP-stop-loxP-green fluorescent protein (GFP) mice to generate GFAP-Cre/GFP double-transgenic mice. These mice were fed methionine choline-deficient, ethionine-supplemented diets to activate and expand HSC and oval cell populations. GFP(+) progeny of GFAP-expressing precursors were characterized by immunohistochemistry. Basal expression of mesenchymal markers was negligible in GFAP(+)Q-HSC. When activated by liver injury or culture, HSC downregulated expression of GFAP but remained GFP(+); they became highly proliferative and began to coexpress markers of mesenchyme and oval cells. These transitional cells disappeared as GFP-expressing hepatocytes emerged, began to express albumin, and eventually repopulated large areas of the hepatic parenchyma. Ductular cells also expressed GFAP and GFP, but their proliferative activity did not increase in this model. These findings suggest that HSC are a type of oval cell that transitions through a mesenchymal phase before differentiating into hepatocytes during liver regeneration. Disclosure of potential conflicts of interest is found at the end of this article.

Enhanced matrix degradation after withdrawal of TGF-beta1 triggers hepatocytes from apoptosis to proliferation and regeneration

TGF-beta1 is a profibrogenic cytokine participating in deposition of extracellular matrix in fibrotic disorders. In liver, its anti-proliferative/apoptotic effect on hepatocytes promotes fibrosis. The tetracycline-controlled double-transgenic TA(LAP-2)/p(tet)TGF-beta1 mouse provides a model for reversible liver fibrosis. In livers of TGF-beta1-expressing mice, hepatocytes showed synchronous apoptosis detected by DNA laddering and active caspase-3 staining that disappeared when expression of transgenic TGF-beta1 was switched off. In these 'off' mice, perisinusoidal liver fibrosis resolved within 21 days accompanied by elevated proliferation of hepatocytes. Here, we have specified the intermediary stages (2-3 days off and 6 days off) in terms of (i) proliferation (by immunohistochemical staining of proliferating cell nuclear antigen and expression of cyclin D1 mRNA) and (ii) extracellular matrix remodelling processes (by measuring mRNA expression of matrix metalloproteinases-2 and -13 (mmp-2 and mmp-13) and tissue inhibitor of matrix metalloproteinases 1 (timp-1) and quantitative morphometric analysis. In summary, we show a rapidly declining timp-1 mRNA level together with lastingly high mmp-2 and mmp-13 mRNA levels after 2-3 days, suggesting that high matrix-degrading potential represents a prerequisite for the markedly enhanced proliferation of hepatocytes in the early stages after switching off transgenic TGF-beta1.

Regression of human cirrhosis. Morphologic features and the genesis of incomplete septal cirrhosis

CONTEXT

Cirrhosis is widely regarded as being irreversible. Recent studies have demonstrated that fibrosis may decrease with time in humans and experimental animals if the disease activity becomes quiescent. The histologic appearance of regressing cirrhosis in the human has not been described in detail.

OBJECTIVES

To define histologic parameters that indicate regression of cirrhosis and to provide an interpretation of how regression occurs from a histologic point of view.

DESIGN

A patient who underwent a series of biopsies that showed apparent regression of hepatitis B cirrhosis is presented. In addition, 52 livers removed at transplantation having cirrhosis or incomplete septal cirrhosis were graded for histologic parameters that suggest progression or regression of fibrosis. Progression parameters were steatohepatitis, inflammation, bridging necrosis, and piecemeal necrosis. The regression parameters (collectively called the hepatic repair complex) were delicate perforated septa, isolated thick collagen fibers, delicate periportal fibrous spikes, portal tract remnants, hepatic vein remnants with prolapsed hepatocytes, hepatocytes within portal tracts or splitting septa, minute regenerative nodules, and aberrant parenchymal veins.

RESULTS AND CONCLUSIONS

Regression parameters were found in all livers and were prominent in the majority. Livers with micronodular cirrhosis, macronodular cirrhosis, and incomplete septal cirrhosis demonstrate a histologic continuum. A continuum of regressive changes was also seen within individual livers. These appearances allow one to understand visually how fibrous regions of hepatic parenchyma can be returned toward a normal appearance. Many examples of incomplete septal cirrhosis could be the product of regressed cirrhosis.

Gene expression of interstitial collagenase in both progressive and recovery phase of rat liver fibrosis induced by carbon tetrachloride

BACKGROUND/AIMS

Liver fibrosis is a dynamic state between matrix production and degradation. Since our report in 1974, many studies have examined collagenase and liver fibrosis, but not the identification of cells responsible for collagenase production in vivo. The aim of this study was to investigate the gene expression of interstitial collagenase in the progressive and recovery phases of experimental rat liver fibrosis by in situ hybridization.

METHODS

We examined the gene expression of interstitial collagenase (MMP-13) in the progressive and recovery phase of experimental rat liver fibrosis induced by chronic CCl4 intoxication by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. In order to identify the cells expressing MMP-13 mRNA by in situ hybridization, immunohistochemistry was performed using serial sections.

RESULTS

In normal rat liver, a faint band for MMP-13 mRNA was observed by RT-PCR, but not by in situ hybridization. The livers of rats treated with CCl4 for 4 weeks showed fatty metamorphosis but no definite fibrosis. Positive signals for MMP-13 mRNA were observed in scattered mesenchymal cells, within lobules which seem to be stellate cells from immunohistochemical staining. Once the fibrosis became prominent, the faint band for MMP-13 mRNA was detected only by RT-PCR and very few signals, if any, by in situ hybridization. On the other hand, in the recovery phase of liver fibrosis, gene expression of MMP-13 was markedly enhanced. Strong positive cells by in situ hybridization were observed mainly at the interface between the resolving fibrous septa and the parenchyma. Overlapping both images of in situ hybridization and immunohistochemical staining with the help of a computer revealed that some positive cells, but not all cells, were stellate cells stained with a-smooth muscle actin antibody.

CONCLUSIONS

MMP-13 participates in the degradation of newly-formed matrix in the recovery from rat liver fibrosis more than in the remodeling of extracellular matrix for the formation of fibrosis. Hepatic stellate cells play a crucial role in MMP-13 production in the recovery from fibrosis, though not all stellate cells were positive for MMP-13 mRNA. Further investigation into gene expression of MMP-13 in recovery will lead to new strategies for the treatment of liver cirrhosis.

Molecular mechanism of the reversibility of hepatic fibrosis: with special reference to the role of matrix metalloproteinases

The participation of matrix metalloproteinases (MMP) and their specific inhibitors, the tissue inhibitors of matrix metalloproteinases (TIMP), in both the formation and degradative recovery processes of liver fibrosis were mainly reviewed from the molecular biological aspect. Since authors first reported increased activity of interstitial collagenase in the early stage of hepatic fibrosis in rats induced by chronic CCl4 intoxication, in baboons fed alcohol chronically and in patients with alcoholic fibrosis, other investigators have also demonstrated increased activity biologically and histochemically. However, species-specific differences in response have been found and gene-level research on the rat model has not demonstrated increased mRNA transcription of collagenase. It has also been clarified that activated stellate cells can also produce matrix components. Very recently, authors observed the participation of interstitial collagenase in the recovery from experimental hepatic fibrosis by using polymerase chain reaction northern blotting and in situ hybridization. The in situ hybridization findings not only demonstrated the cells responsible for interstitial collagenase, but also suggested a great deal about the mechanism of recovery from fibrosis. Hepatic stellate cells are activated via the expression of c-myb and nuclear factor-kappaB (NFkappaB) which is induced by oxidative stress, and inhibited by antioxidant (1-alpha-tocopherol) and butylated hydroxytoluene. The activation mechanism is now being revealed. The relationship between the activation mechanism of stellate cells and the production and secretion of MMP and TIMP in the formation and recovery process of hepatic fibrosis should be investigated from the promoter gene level. This approach might help develop a new strategy for the treatment of liver fibrosis.

Effects of the prolyl 4-hydroxylase proinhibitor HOE 077 on human and rat hepatocytes in primary culture

Alterations in extracellular matrix occur in many chronic liver diseases leading to the formation of hepatic fibrosis. We have studied the effects of the putative hepatoselective fibrosuppressive compound HOE 077, a proinhibitor of prolyl 4-hydroxylase, on normal adult human and rat hepatocytes in primary culture. In human hepatocyte cultures, the cytotoxicity of HOE 077 was assessed after a 20-h treatment at concentrations ranging from 0.125 to 2 mg/ml of medium. No significant change was found in cell morphology, neutral red uptake, red oil staining, lactate dehydrogenase release, tetrazolium salt reduction, ethoxyresorufin O-deethylase activity and protein synthesis; however, HOE 077 slightly decreased DNA synthesis at 2 mg/ml. In rat hepatocyte cultures, the cytotoxicity of the compound was assessed by testing the same parameters after a daily exposure of cultures for 2 days or 4 days, at concentrations ranging from 0.25 to 4.5 mg/ml of medium. Whatever the concentration, the compound had no obvious morphological effect. However, hepatocytes were less spread at the concentration of 4.5 mg/ml. HOE 077 at 2 mg/ml slightly decreased neutral red uptake but was without obvious effect on protein synthesis after 2 days. By contrast, on day 4, protein synthesis was markedly reduced in hepatocyte cultures exposed to HOE 077 at 4.5 mg/ml. Hydroxyproline content determination in media from 4-day-old hepatocyte cultures incubated with HOE 077 at 0.5 to 4.5 mg/ml, showed a dose-dependent decrease in the hydroxyproline/proline ratio in acetic acid soluble material. By indirect immunoperoxidase, intracellular collagen IV was found to be inhibited in hepatocyte cultures after 4 days of exposure to 4.5 mg/ml HOE 077.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of ethanol induced hepatic injury

Ethanol is hepatotoxic through redox changes produced by the NADH generated in its oxidation via the alcohol dehydrogenase pathway, which in turn affects the metabolism of lipids, carbohydrates, proteins and purines. Ethanol is also oxidized in liver microsomes by an ethanol-inducible cytochrome P-450 (P-450IIE1) which contributes to ethanol metabolism and tolerance, and activates xenobiotics to toxic radicals thereby explaining increased vulnerability of the heavy drinker to industrial solvents, anesthetic agents, commonly prescribed drugs, over-the-counter analgesics, chemical carcinogens and even nutritional factors such as vitamin A. Induction also results in energy wastage and increased production of acetaldehyde. Acetaldehyde, in turn, causes injury through the formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, and alterations in microtubules, plasma membranes and mitochondria with a striking impairment of oxygen utilization. Acetaldehyde also causes glutathione depletion and lipid peroxidation, and stimulates hepatic collagen synthesis, thereby promoting fibrosis.