Signal transduction in hepatic stellate cells

Fibrogenesis I. New insights into hepatic stellate cell activation: the simple becomes complex

Hepatic stellate cell activation is a complex process. Paradoxes and controversies include the origin(s) of hepatic stellate cells, the regulation of membrane receptor signaling and transcription, and the fate of the cells once liver injury resolves. Major themes have emerged, including the dominance of autocrine signaling and the identification of counterregulatory stimuli that oppose key features of activated cells. Advances in analytical methods including proteomics and gene array, coupled with powerful bioinformatics, promise to revolutionize how we view cellular responses. Our understanding of stellate cell activation is likely to benefit from these advances, unearthing modes of regulating cellular behavior that are not even conceivable on the basis of current paradigms.

An oral endothelin-A receptor antagonist blocks collagen synthesis and deposition in advanced rat liver fibrosis

BACKGROUND & AIMS

Endothelin 1 induces contraction, proliferation, and collagen synthesis of hepatic stellate cells in vitro, which may be mediated via the endothelin A receptor. It is unknown if specific blockade of the endothelin A receptor inhibits hepatic fibrosis in vivo.

METHODS

Groups of 10-20 rats with bile duct occlusion were treated with the nonpeptide endothelin-A receptor antagonist LU 135252 at 80 mg. kg(-1). day(-1) from week 1-6 or from week 4-6, or with LU at 10 mg. kg(-1). day(-1) from week 1-6. Animals with bile duct occlusion alone and sham-operated rats without or with LU at 80 mg. kg(-1). day(-1) over 6 weeks served as controls. After 6 weeks, parameters of fibrogenesis were determined.

RESULTS

LU treatment led to improved histology, paralleled by a dose-dependence up to 60% reduction of liver collagen, even when administered at an advanced fibrosis stage. This was accompanied by a decreased messenger RNA of hepatic procollagen alpha1(I) and tissue inhibitor of metalloproteinase 1, 2 major effectors of fibrosis, and of serum procollagen type III, a surrogate marker of liver fibrogenesis.

CONCLUSIONS

Selective endothelin-A receptor blockade can dramatically reduce collagen accumulation in rat secondary biliary fibrosis, a model refractory to most potential antifibrotic agents. Endothelin-A receptor antagonists are promising antifibrotic agents in chronic liver disease.

Decorin and actin expression and distribution in patients with chronic hepatitis C following interferon-alfa-2b treatment

BACKGROUND/AIMS

Chronic hepatitis C can lead to cirrhosis and hepatocellular carcinoma. Interferon-alfa therapy may prevent the progression of the disease. The expressions of decorin and alfa-smooth muscle cell actin of the extracellular matrix play a central role in liver fibrosis. We set out to assess the expressions of these proteins in chronic hepatitis C patients, and to evaluate how they can be modified by interferon-alfa therapy.

METHODS

Twenty chronic hepatitis C patients received interferon-alfa-2b therapy for 6 months (group I) or 12 months (group II). Liver biopsy samples were taken before and after the therapy. The alfa-smooth muscle actin-positive cells were determined with a monoclonal antibody, and decorin expression was detected with a polyclonal antibody. The cells were evaluated with a semiquantitative scoring method. For statistical analysis, non-parametric methods were used.

RESULTS

Before the therapy, alfa-smooth muscle actin-labeled cells and marked decorin expression were present throughout all the acinar zones. Interferon-alfa-2b therapy resulted in significant decreases in both the number of alfa-smooth muscle actin-positive cells and the decorin expression. The alfa-smooth muscle actin-positive cells and decorin expression correlated with the histological activity index (R=0.72, p<0.03, R=0.68, p<0.05).

CONCLUSIONS

This study demonstrates that a large number of alfa-smooth muscle actin-positive cells and a marked decorin expression are frequent findings in chronic hepatitis C. Treatment with interferon-alfa-2b for 12 months reduced the number of labeled cells and the decorin expression. The results suggest that interferon-alfa-2b is capable of interfering with fibrogenesis in an early and presumably still reversible phase of chronic hepatitis C.

Pathogenesis of liver fibrosis: role of oxidative stress

In the liver, the progressive accumulation of connective tissue, a complex and dynamic process termed fibrosis, represents a very frequent event following a repeated or chronic insult of sufficient intensity to trigger a "wound healing"-like reaction. The fibrotic process recognises the involvement of various cells and different factors in bringing about an excessive fibrogenesis with disruption of intercellular contacts and interactions and of extracellular matrix composition. However, Kupffer cells, together with recruited mononuclear cells, and hepatic stellate cells are by far the key-players in liver fibrosis. Their cross-talk is triggered and favoured by a series of chemical mediators, with a prominent role played by the transforming growth factor beta. Both expression and synthesis of this inflammatory and pro-fibrogenic cytokine are mainly modulated through redox-sensitive reactions. Further, involvement of reactive oxygen species and lipid peroxidation products can be clearly demonstrated in other fundamental events of hepatic fibrogenesis, like activation and effects of stellate cells, expression of metalloproteinases and of their specific inhibitors. The important outcome of such findings as regards the pathogenesis of liver fibrosis derives from the observation of a consistent and marked oxidative stress condition in many if not all chronic disease processes affecting hepatic tissue. Hence, reactive oxidant species likely contribute to both onset and progression of fibrosis as induced by alcohol, viruses, iron or copper overload, cholestasis, hepatic blood congestion.

Serum YKL-40 is increased in patients with hepatic fibrosis

BACKGROUND/AIMS

YKL-40, a mammalian member of the chitinase family, is a lectin that binds heparin and chitin. The function of YKL-40 is unknown, but it may function in tissue remodelling. The aims of this study were to assess the level of circulating YKL-40 in patients with various kinds and degree of chronic liver disease and its possible relation to liver fibrosis.

METHODS

Serum YKL-40 levels were determined by radioimmunoassay in 129 patients with suspected liver disease and related to histological findings and immunohistochemical staining of YKL-40 in a liver biopsy taken simultaneously with the blood sample.

RESULTS

The median serum YKL-40 was highest in patients with alcoholic cirrhosis (532 microg/l), in particular in patients with additional alcoholic hepatitis (740 microg/l). Patients with alcoholic cirrhosis, post-hepatitic cirrhosis (425 microg/l) and non-cirrhotic fibrosis (330 microg/l) had significantly higher serum YKL-40 than normal subjects (102 microg/l), patients with fatty liver (195 microg/l) or patients with viral hepatitis without fibrosis (174 microg/l). Serum YKL-40 was significantly (p<0.001) related to the degree of liver fibrosis with the highest levels in patients with moderate (466 microg/l) to severe (676 microg/l) fibrosis. Serum YKL-40 was also increased (p=0.018) in patients with slight fibrosis (270 microg/l) compared to patients without fibrosis. Immunohistochemical analysis demonstrated positive staining for YKL-40 antigen in areas with fibrosis, particularly areas with active fibrogenesis. YKL-40 staining was never found in hepatocytes.

CONCLUSIONS

Our study indicates that the increased serum YKL-40 in patients with liver disease of various degree and aetiology seems to reflect fibrosis and fibrogenesis.

Pathophysiology of the intrahepatic biliary epithelium

The intrahepatic bile duct epithelium modulates the fluidity and alkalinity of the primary hepatocellular bile from which it reabsorbs fluids, amino acids, glucose and bile acids, while secreting water, electrolytes and immunoglobulin A. The transport function of the intrahepatic biliary epithelium is finely regulated by a number of gastrointestinal hormones, neuropeptides and neurotransmitters that promote either secretion or absorption. The intrahepatic biliary epithelium appears to be a primary target in a broad group of chronic cholestatic disorders that represent an important cause of morbidity and mortality. The spectrum of cholangiopathies ranges from conditions in which a normal epithelium is damaged by disordered autoimmunity, infectious agents, toxic compounds or ischaemia, to genetically determined disorders arising from an abnormal bile duct biology, such as cystic fibrosis or biliary atresia. Probably as a result of the known heterogeneity in cholangiocyte function, different portions of the biliary tree appear to be preferentially affected in specific cholangiopathies. From a pathophysiological point of view, cholangiopathies are characterized by the coexistence of cholangiocyte loss (by apoptotic or lytic cell death) with cholangiocyte proliferation and various degrees of portal inflammation, fibrosis and cholestasis. These basic disease mechanisms are discussed in detail. Better understanding of cholangiocyte pathophysiology, in particular the immune regulation of cholangiocyte function, will help in designing newer genetic or pharmacological approaches to treat cholangiopathies.

Stress-activated protein kinases in the activation of rat hepatic stellate cells in culture

BACKGROUND/AIMS

The signal cascades involved in the activation of hepatic stellate cells (HSC) are largely unknown. Factors initiating activation include tumour necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, endothelin, and oxidative stress. In other cell types some of these have been reported to stimulate p38 mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK). We have therefore investigated the role of these kinases in HSC activation.

METHODS

HSC were isolated from male Wistar rats. Quiescent experiments were performed on day 2 HSC and transformed experiments on day 15 passage 1 HSC. Kinase activities were determined by immunoprecipitation and phosphorylation of specific substrate proteins and alpha-smooth muscle actin (SMA) expression by immunoblotting.

RESULTS

The constitutive activity of p38 MAP kinase was higher in transformed versus quiescent cells. In quiescent cells TNFalpha stimulated p38 MAP kinase and JNK activities 12- and 4-fold respectively and this was halved by 2-mercaptoethanol, an indirect antioxidant. Endothelin-1 activated both kinases in quiescent cells via the endothelin-B receptor, while TGFbeta had no effect. Both 2-mercaptoethanol and a p38 inhibitor (SB202190) inhibited alpha-SMA expression by day 5 cells.

CONCLUSIONS

The activation of p38 MAP kinase and JNK by TNFalpha and endothelin, together with the inhibition of this activation by 2-mercaptoethanol, provides indirect evidence supporting their role in HSC transformation. Direct evidence for a role for p38 MAP kinase is provided by the observations that its constitutive activity is higher in transformed versus quiescent cells and that its inhibitor reduces HSC activation in culture as assessed by alpha-SMA expression.

Role of the Ets-1 transcription factor during activation of rat hepatic stellate cells in culture

During liver tissue repair, hepatic stellate cells (HSCs), a pericyte-like nonparenchymal liver cell population, transform from a quiescent status (resting HSCs) into myofibroblast like cells (activated HSCs); the latter is the principal matrix-synthesizing cell of the liver. Although several factors have been shown to be involved in this important process, the molecular mechanisms regulating HSC activation are still under investigation. To identify key regulatory proteins involved in the HSC activation process, we used different mRNA display technologies, with cDNAs prepared from HSCs at different stages of in vitro activation. With the latter technique, the transcription factor Ets-1 was detected through its down-regulation during activation. As confirmed by Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, mRNAs coding for Ets-1 were present in the highest amounts in freshly isolated HSCs and in HSCs 2 days after plating (classified as resting HSCs/early activated HSCs) and were diminished in HSCs 7 days after plating (activated cells). Ets-1 protein was present in HSC-lysates, as assessed by Western blot, and bound to an oligonucleotide containing the Ets-1 consensus cis-acting motif, as demonstrated by electrophoretic mobility shift assay. Ets-1 binding activity peaked in nuclear extracts prepared from resting/early activated cells and was diminished in extracts derived from fully activated cells. In contrast, binding activity of the transcription factors TFIID, AP-1, and SP-1 was highest in activated HSCs and only barely detectable in resting/early activated HSCs. By Northern blot and RT-PCR analysis, Ets-1-specific transcripts were present in parenchymal and other nonparenchymal liver cells too, illustrating that hepatic Ets-1 expression is not specific or restricted to HSCs. However, the unique pattern of Ets-1 binding activity present in resting versus activated HSCs and its known implications for cellular differentiation and tissue remodeling suggest that Ets-1 could be of crucial importance for HSC activation and hepatic tissue repair.

Stellate cell activation in alcoholic fibrosis--an overview

There has been remarkable progress in our understanding of how chronic alcohol ingestion may lead to hepatic injury and scarring, or fibrosis. Hepatic fibrosis represents the liver's wound healing response and is characterized by accumulation of interstitial matrix, or scar. Fibrosis in the liver results from the activation of stellate cells, or resident mesenchymal cells. Stellate cell activation is a dramatic phenotype transition whose net effect is the replacement of normal liver matrix by scar. Features of stellate cell activation include increased cell accumulation from proliferation and directed migration, increased matrix production, enhanced contractility, accelerated degradation of the normal liver matrix, release of profibrogenic cytokines, and loss of cellular vitamin A. Alcohol may enhance fibrogenesis through stimulation of stellate cells by hypoxia, generation of lipid peroxides from damaged hepatocytes, production of acetaldehyde that may have direct fibrogenic activity, and through activation of Kupffer cells or resident macrophages. Unanswered questions remain to be studied, but the clarification of underlying mechanisms of fibrosis portends continued progress in our ability to treat alcoholic liver fibrosis.

An update on the role of free radicals and antioxidant defense in human disease

Mounting clinical and experimental evidence indicates that free radicals play important roles in many physiological and pathological conditions. The wider application of free radical measurement has increased awareness of functional implications of radical-induced impairment of the oxidative/antioxidative balance. In the following review, the role of oxygen free radicals in some human and experimental pathological conditions is described, with particular emphasis on the mechanisms by which they produce oxidative damage to lipids, proteins, and nucleic bases. The role of free radicals and the activation of the antioxidant systems in arteriosclerosis and ageing, diabetes, ischemia/reperfusion injury, ethanol intoxication, and liver steatosis is discussed. Therapeutic approaches to the use of antioxidants have been described and prospects for clinical use have been considered.

Intraportal administration of glyceryl trinitrate or nitroprusside exerts more systemic than intrahepatic effects in anaesthetised cirrhotic rats

BACKGROUND/AIMS

Increased intrahepatic vascular tone can be pharmacologically manipulated in isolated cirrhotic livers. Intrahepatic endothelial dysfunction may lead to a decreased production of the potent endogenous vasodilator nitric oxide in cirrhotic livers. The aims of the study were to determine whether portal pressure can be lowered in vivo by injecting nitric oxide donors glyceryl trinitrate or nitroprusside directly in the portal vein and whether this is related to a decrease in intrahepatic resistance.

METHODS

In anaesthetised CCl4 cirrhotic rats, intraportal doses of glyceryl trinitrate 0.5, 1 or 5 microg/kg/ min or nitroprusside 1, 5 or 10 microg/kg/min did not decrease portal pressure but only arterial pressure. Systemic and splanchnic haemodynamics were measured before and during 15 min intraportal infusion of glyceryl trinitrate 10 microg/kg/min or nitroprusside 20 microg/kg/min.

RESULTS

Glyceryl trinitrate decreased portal pressure from 14.0+/-1.1 to 11.8+/-1.4 mm Hg, splanchnic perfusion pressure from 102+/-10 to 74+/-5 mm Hg and portal sinusoidal flow from 2.11+/-0.38 to 1.70+/-0.35 ml/min/g liver (all p<0.05). Nitroprusside did not decrease portal pressure significantly but led to a reduction of the splanchnic perfusion pressure (104+/-9 to 66+/-7 mm Hg) and the portal sinusoidal flow (2.39+/-0.50 to 1.77+/-0.31 ml/min/g liver; all p<0.05). Portal sinusoidal resistance was not altered by either drug.

CONCLUSIONS

Intraportal infusion of nitric oxide donors decreased arterial pressure more than portal pressure. Portal sinusoidal resistance remained unaffected, but the liver parenchyma became less perfused with high doses. The systemic effects of nitric oxide donating drugs prevailed.

Strategy of liver-directed gene therapy: present status and future prospects

The liver is particularly amenable to gene therapy as it is the site of many metabolic diseases and malignancies. Thus, liver-directed gene therapy is being actively pursued and developed as a method of treatment for various liver diseases. Strategies of liver-directed gene therapy include drug delivery to the liver, compensation of the defective gene(s), anti-tumor activity, anti-viral therapy, and immunomodulation. The strategy chosen for liver-directed gene therapy depends on the genetic basis of the disease. Many aspects are key factors to the success of the chosen strategy: intervention of genes, efficient gene delivery system, stable transgene expression, transgene regulation, target cell transfection, and timing of transgene expression. Several tactics can be used to overcome problems in the above, and these include the use of a gene switch to exogenously regulate transgene expression, targeting at the transcriptional level, circumvention of the immune response (as in the use of adenovirus vector to achieve long-term correction of genetic diseases), and genetically engineered antibodies in gene transfer. At the present rate of research activity and development, gene therapies may soon be more efficient than current standard treatments for some liver diseases.

Liver fibrogenesis and the role of hepatic stellate cells: new insights and prospects for therapy

Hepatic fibrosis is a wound-healing response to chronic liver injury, which if persistent leads to cirrhosis and liver failure. Exciting progress has been made in understanding the mechanisms of hepatic fibrosis. Major advances include: (i) characterization of the components of extracellular matrix (ECM) in normal and fibrotic liver; (ii) identification of hepatic stellate cells as the primary source of ECM in liver fibrosis; (iii) elucidation of key cytokines, their cellular sources, modes of regulation, and signalling pathways involved in liver fibrogenesis; (iv) characterization of key matrix proteases and their inhibitors; (v) identification of apoptotic mediators in stellate cells and exploration of their roles during the resolution of liver injury. These advances have helped delineate a more comprehensive picture of liver fibrosis in which the central event is the activation of stellate cells, a transformation from quiescent vitamin A-rich cells to proliferative, fibrogenic and contractile myofibroblasts. The progress in understanding fibrogenic mechanisms brings the development of effective therapies closer to reality. In the future, targeting of stellate cells and fibrogenic mediators will be a mainstay of antifibrotic therapy. Points of therapeutic intervention may include: (i) removing the injurious stimuli; (ii) suppressing hepatic inflammation; (iii) down-regulating stellate cell activation; and (iv) promoting matrix degradation. The future prospects for effective antifibrotic treatment are more promising than ever for the millions of patients with chronic liver disease worldwide.

In vitro migratory potential of rat quiescent hepatic stellate cells and its augmentation by cell activation

In liver injury, hepatic stellate cells are considered to depart from the sinusoidal wall and accumulate in the necrotic lesion through migration and proliferation. In this study, we investigated the migratory capacity of quiescent stellate cells in vitro and analyzed the relationship with proliferative response. Freshly isolated stellate cells that were seeded in the upper chamber of Cell Culture Insert (Becton Dickenson, Franklin Lakes, NJ) started to migrate to the lower chamber at 1 day and increased in migration index to 19% at 2 days. Cells in the lower chamber were stretched in shape with many lipid droplets and showed quiescent properties, i.e., negative expression of alpha-smooth muscle actin (alpha-SMA) or platelet-derived growth factor receptor-beta (PDGFR-beta). Migratory capacity in quiescent cells was also shown in the Matrigel-coated insert. Matrix metalloproteinase-2 (MMP-2) messenger RNA expression was low just after isolation, but was enhanced as migration became prominent. Migrating cells further showed higher proliferative activity than resting ones. The presence of PDGF/BB and Kupffer cells accelerated stellate cell migration by the chemotactic mechanism and concurrently augmented proliferation, whereas that of dexamethasone and interferon-gamma (IFN-gamma) attenuated migration as a result of general suppression effects. Compared with quiescent ones, alpha-SMA and PDGFR-beta-positive activated stellate cells obtained by 14-day culture exhibited more rapid and prominent migration, being regulated by mediators in a similar manner as described previously. These data indicate that quiescent stellate cells undergo migration, which is linked to proliferation and enhanced by PDGF/BB and Kupffer cells, suggesting the involvement of this function in the initial phase of development of postnecrotic fibrosis.

Pathology of the liver

Among the topics of recent investigation in liver pathology were an examination of normal portal tract structures in needle liver biopsies, computer reconstructions of the intrahepatic biliary tree, identification of oval cells (presumed progeny of hepatic stem cells) in a variety of biliary and nonbiliary diseases and tumors, the features and pathogenesis of nonalcoholic steatohepatitis, and further characterization of proliferating bile ductules. A morphometric study of portal structures in normal needle liver biopsies found that approximately one third in a given specimen may not show a portal vein and that a bile duct may not be seen in 7%. Apoptosis is a critical mechanism for the death of hepatocytes in viral hepatitis and also in endothelial injury in the cold perfusion-warm reperfusion sequence in liver transplantation. The results of two studies examining the relationship of steatosis to chronic hepatitis C virus infection in native and transplanted livers suggest that fatty change is a specific virus-mediated lesion. In the field of hepatic neoplasia, liver cell dysplasia (large cell change), long thought to be a premalignant lesion, was hypothesized to represent abnormal hepatocyte polyploidization.

Transformation-dependent calcium influx by voltage-operated calcium channels in stellate cells of rat liver

BACKGROUND/AIMS

The transformation of hepatic stellate cells into myofibroblasts is a key step in the pathogenesis of fibrotic liver diseases. The intracellular signaling associated with hepatic stellate cell transformation becomes a point of interest, especially the role of cytosolic free calcium concentration ([Ca2+]i). The aim of the study was to investigate possible differences between various transformation phenotypes of hepatic stellate cells with regard to the calcium influx mediated by L-type voltage-operated calcium channels (L-type VOC).

METHODS

Hepatic stellate cells were isolated from rat liver by pronase-collagenase reperfusion and cultured under standard conditions. The transformation of hepatic stellate cells was stimulated by treatment with transforming growth factor-beta (TGF-beta) or inhibited with interferon-gamma (IFN-gamma) and characterized by immunocytochemistry for smooth muscle alpha-actin and determination of hyaluronan in the culture media with a ligand binding assay. [Ca2+]i was measured in individual cells with fluorescence microscopy using fura-2. VOCs were activated by the standard procedure of extracellular potassium elevation, to achieve depolarization, and identified by various controls.

RESULTS

In transformed myofibroblasts the activation of VOCs by potassium elevation from 5.4 mmol/l to 50.4 mmol/l led to a 19% increase in [Ca2+]i in contrast to 0.2% in hepatic stellate cells cultured for 3 days. In 7-day old hepatic stellate cells, after stimulation of cell transformation with TGF-beta-1, an enhanced [Ca2+]i response to potassium elevation was detected, while inhibition of transformation with IFN-gamma for the same time caused a decreased calcium signal compared with untreated control cultures. Short-term treatment with the cytokines (1 day) did not influence depolarization-dependent calcium signals.

CONCLUSION

The results show the [Ca2+]i increase via L-type VOCs to be dependent on the transformation level of hepatic stellate cells into myofibroblasts which can be influenced by the long-term treatment of hepatic stellate cells with TGF-beta or IFN-gamma. In contrast, there is no evidence for direct regulation of VOC activity by TGF-beta or IFN-gamma after short-term exposure.

Organic osmolyte transport in quiescent and activated rat hepatic stellate cells (Ito cells)

Activation of hepatic stellate cells (HSCs) results in multiple alterations of cell function, but nothing is known about organic osmolytes in these cells. Organic osmolyte transport and transporter messenger RNA (mRNA) expression was studied in quiescent rat HSCs and after their transformation into alpha1-smooth muscle actin-positive myofibroblastlike cells. Quiescent stellate cells expressed in an osmosensitive manner the mRNA levels of the transporters for taurine (TAUT) and myoinositol (SMIT), whereas that for betaine was not detectable. However, these cells showed osmosensitive uptake not only of taurine and myoinositol but also of betaine. Osmosensitive betaine uptake was mediated by amino acid transport system A. After transformation into myofibroblasts, taurine and myoinositol uptake increased 5.5-fold and 4.5-fold, respectively, together with the respective transporter mRNA levels. Betaine uptake increased twofold because of osmosensitive induction of BGT1 expression. In both quiescent and activated HSCs, hypoosmotic cell swelling induced a rapid and 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid-sensitive osmolyte efflux. In quiescent HSCs, hyperosmotic exposure increased the messenger RNA (mRNA) level of cyclooxygenase-2, which was counteracted by taurine but not by betaine or myoinositol. The study identifies taurine, myoinositol, and betaine as osmolytes in HSCs. Transformation of HSCs is accompanied by enhanced osmolyte transport activity and induction of the BGT1 transporter, which may be another activation marker of HSCs.

Liver fibrosis

Knowledge on the development and progression of liver fibrosis has grown exponentially in the past decade. At present, liver fibrogenesis is referred to as a dynamic process involving complex cellular and molecular mechanisms, resulting from the chronic activation of the tissue repair mechanisms that follows reiterated liver tissue injury. The identification and characterization of the cell types and of the different mediators involved in this process has allowed a "re-visitation" of several issues related to liver cirrhosis and its immediate consequences. Among these, evaluation of the relationships occurring between fibrogenesis and portal hypertension, cholestasis and the development of hepatocellular carcinoma, represent some of the hottest areas of research in this field of hepatology. The elucidation of many of the cellular and molecular mechanisms responsible for the progression of liver fibrosis has provided a sound basis for the development of pharmacological strategies able to modulate this important pathophysiological process.

4-Hydroxynonenal as a biological signal: molecular basis and pathophysiological implications

Reactive oxygen intermediates (ROI) and other pro-oxidant agents are known to elicit, in vivo and in vitro, oxidative decomposition of omega-3 and omega-6 polyunsaturated fatty acids of membrane phospholipids (i.e, lipid peroxidation). This leads to the formation of a complex mixture of aldehydic end-products, including malonyldialdehyde (MDA), 4-hydroxy-2,3-nonenal (HNE), and other 4-hydroxy-2,3-alkenals (HAKs) of different chain length. These aldehydic molecules have been considered originally as ultimate mediators of toxic effects elicited by oxidative stress occurring in biological material. Experimental and clinical evidence coming from different laboratories now suggests that HNE and HAKs can also act as bioactive molecules in either physiological and pathological conditions. These aldehydic compounds can affect and modulate, at very low and nontoxic concentrations, several cell functions, including signal transduction, gene expression, cell proliferation, and, more generally, the response of the target cell(s). In this review article, we would like to offer an up-to-date review on this particular aspect of oxidative stress--dependent modulation of cellular functions-as well as to offer comments on the related pathophysiological implications, with special reference to human conditions of disease.