The role of p70S6K in hepatic stellate cell collagen gene expression and cell proliferation

Inhibition of phosphatidylinositol 3-kinase signaling in hepatic stellate cells blocks the progression of hepatic fibrosis

The hepatic stellate cell (HSC) is the primary cell type in the liver responsible for excess collagen deposition during fibrosis. Following a fibrogenic stimulus the cell changes from a quiescent vitamin A-storing cell to an activated cell type associated with increased extracellular matrix synthesis and increased cell proliferation. The phosphatidylinositol 3-kinase (PI3K) signaling pathway has been shown to regulate several aspects of HSC activation in vitro, including collagen synthesis and cell proliferation. Using a targeted approach to inhibit PI3K signaling specifically in HSCs, we investigated the role of PI3K in HSCs using a rodent model of hepatic fibrosis. An adenovirus expressing a dominant negative form of PI3K under control of the smooth muscle alpha-actin (alphaSMA) promoter was generated (Ad-SMAdnPI3K). Transducing HSCs with Ad-SMAdnPI3K resulted in decreased proliferation, migration, collagen expression, and several additional profibrogenic genes, while also promoting cell death. Inhibition of PI3K signaling was also associated with reduced activation of Akt, p70 S6 kinase, and extracellular regulated kinase signaling as well as reduced cyclin D1 expression. Administering Ad-SMAdnPI3K to mice following bile duct ligation resulted in reduced HSC activation and decreased extracellular matrix deposition, including collagen expression. A reduction in profibrogenic mediators, including transforming growth factor beta, tissue inhibitor of metalloproteinase 1, and connective tissue growth factor was also noted. However, liver damage, assessed by alanine aminotransferase levels, was not reduced.

CONCLUSION

Inhibition of PI3K signaling in HSCs during active fibrogenesis inhibits extracellular matrix deposition, including synthesis of type I collagen, and reduces expression of profibrogenic factors. These data suggest that targeting PI3K signaling in HSCs may represent an effective therapeutic target for hepatic fibrosis.

The inhibitory effect of Gynostemma pentaphyllum on MCP-1 and type I procollagen expression in rat hepatic stellate cells

AIM OF THE STUDY

Gynostemma pentaphyllum is a popular folk medicine that has been used for treatment of hepatitis in Asia. Our previous study demonstrates that Gynostemma pentaphyllum n-butanol extract inhibits the onset and improves the recovery of CCl(4)-induced liver fibrogenesis in rats and inhibits PDGF-induced rat hepatic stellate cells (HSCs) proliferation. In this study, the effect of Gynostemma pentaphyllum extract on cytokines and type I procollagen expression was analyzed.

MATERIALS AND METHODS

Rat HSCs were treated with PDGF, Gynostemma pentaphyllum n-butanol extract, RP-18-Gyp fraction, rapamycin or vehicle. Rat cytokine antibody array chip or ELISA kit was used for cytokines detection. Intracellular protein expression was detected by Western blotting, mRNA expression was analyzed by RT-PCR.

RESULTS

RP-18-Gyp fraction is the more purified gypenosides fraction from Gynostemma pentaphyllum n-butanol extract. In cell proliferation, the inhibitory effect of 200 microg/ml RP-18-Gyp fraction is similar to 500 microg/ml Gynostemma pentaphyllum n-butanol extract. Furthermore, both of them have the ability of decreasing monocyte chemoattractant protein-1 (MCP-1) mRNA expression and protein release and inhibiting type I procollagen protein expression.

CONCLUSIONS

Both of Gynostemma pentaphyllum n-butanol extract and its more purified RP-18-Gyp fraction have the biological activities in the inhibition of cell proliferation, MCP-1 release and type I procollagen expression in rat HSCs. These data could provide the evidence to support for the traditional use of Gynostemma pentaphyllum in hepatitis.

Rapamycin inhibits hepatic fibrosis in rats by attenuating multiple profibrogenic pathways

Hepatic stellate cell transdifferentiation, epithelial-mesenchymal cell transition, and the ductular reaction each contribute to the development of hepatic fibrosis in cholestatic liver diseases. Inhibitors of mammalian target of rapamycin have antifibrotic properties. We evaluated the hypothesis that the antifibrotic action of rapamycin is due to attenuated myofibroblast proliferation in addition to an inhibitory effect on epithelial-mesenchymal transition and the ductular reaction. Hepatic fibrosis was induced by bile duct ligation, and rodents received 1.5 mg/kg/day rapamycin by subcutaneous infusion for 21 days. The expression of various markers of hepatic fibrosis, stellate cell transactivation, epithelial-mesenchymal transition, and the ductular reaction was compared between treated and untreated animals. Hepatic fibrosis, hepatic procollagen type 1 messenger RNA, and alpha-smooth muscle actin expression were significantly reduced in treated animals. Hepatic stellate cell procollagen expression and proliferation were also reduced by rapamycin. The following markers of epithelial-mesenchymal transition--vimentin protein expression, S100 calcium binding protein A4 and transforming growth factor beta 1 messenger RNA, and the mothers against decapentaplegic homolog signaling pathway--were all reduced after rapamycin treatment. The intensity of the ductular reaction was reduced by rapamycin as assessed by histopathological scoring and by reduced cytokeratin 19 expression. Rapamycin caused a reduction in hepatic progenitor cell proliferation. Together, these data show that multiple profibrogenic pathways are activated in an animal model of cholestasis and that rapamycin attenuates epithelial-mesenchymal transition and the ductular reaction as well as hepatic stellate cell activation.

Therapeutic targeting of the PDGF and TGF-beta-signaling pathways in hepatic stellate cells by PTK787/ZK22258

Stimulation of hepatic stellate cells (HSCs) by platelet-derived growth factor (PDGF) and transforming growth factor-beta1 (TGF-beta1) is an essential pathway of proliferation and fibrogenesis, respectively, in liver fibrosis. We provide evidence that PTK787/ZK222584 (PTK/ZK), a potent tyrosine kinase inhibitor that blocks vascular endothelial growth factor receptor (VEGFR), significantly inhibits PDGF receptor expression, as well as PDGF-simulated HSC proliferation, migration and phosphorylation of ERK1/2, Akt and p70S6 kinase. Interestingly, PTK/ZK also antagonizes the TGF-beta1-induced expression of VEGF and VEGFR1. Furthermore, PTK/ZK downregulates TGF-beta receptor expression, which is associated with reduced Akt, ERK and p38MAPK phosphorylation. Furthermore, PDGF-induced TGF-beta1 expression is inhibited by PTK/ZK. These findings provide evidence that PTK/ZK targets multiple essential pathways of stellate cell activation that provoke proliferation and fibrogenesis. Our study underscores the potential use of PTK/ZK as an antifibrotic drug in chronic liver disease.

The tumor suppressor protein PTEN inhibits rat hepatic stellate cell activation

BACKGROUND

Following a fibrogenic stimulus, the hepatic stellate cell (HSC) transforms from a quiescent to an activated cell type associated with increased proliferation, collagen and smooth muscle alpha-actin (alphaSMA) expression. Phosphatase and Tensin Homolog Deleted on Chromosome Ten (PTEN), a tumor suppressor phosphatase, has been shown to play a role in several nonmalignant diseases. Here, we investigated the role of PTEN during HSC activation.

METHODS

Rat HSCs 2 days after isolation were transduced with adenoviruses expressing either the wild-type (WT) or a dominant negative form of PTEN, and culture-associated activation of HSCs, including morphological changes, expression of alphaSMA and alpha1(I) collagen, and cell proliferation, were evaluated. Apoptosis of HSCs was detected by measuring activity of caspase 3/7. Phosphorylation status of Akt, p70(S6K), and Erk was detected by Western blotting.

RESULTS

Overexpression of WT-PTEN inhibited phenotypic changes were associated with HSC activation, including morphological changes, expression of alphaSMA and alpha1(I) collagen, and HSC proliferation, including cyclin D1 expression. WT-PTEN expression also induced apoptosis in HSCs with increased caspase 3/7 activity. Expression of WT-PTEN also caused decreased activation of Akt, p70(S6K), and Erk signaling pathways.

CONCLUSIONS

Taken together, these findings show that PTEN represents an important negative regulator for transactivation of HSCs. This may have important implications for the design of therapeutic strategies to prevent the progression of liver fibrosis.

Validation of anti-aging drugs by treating age-related diseases

Humans die from age-related diseases, which are deadly manifestations of the aging process. In order to extend life span, an anti-aging drug must delay age-related diseases. All together age-related diseases are the best biomarker of aging. Once a drug is used for treatment of any one chronic disease, its effect against other diseases (atherosclerosis, cancer, prostate enlargement, osteoporosis, insulin resistance, Alzheimer's and Parkinson's diseases, age-related macular degeneration) may be evaluated in the same group of patients. If the group is large, then the anti-aging effect could be validated in a couple of years. Startlingly, retrospective analysis of clinical and preclinical data reveals four potential anti-aging modalities.

PI-3 K/AKT and ERK signaling pathways mediate leptin-induced inhibition of PPARgamma gene expression in primary rat hepatic stellate cells

Compelling evidence indicates the pro-fibrogenic action of leptin in liver. Peroxisome proliferator-activated receptor-gamma (PPARgamma) can reverse hepatic stellate cell (HSC) activation and maintain HSC quiescence. HSC activation, a key step in the development of liver fibrosis, is coupled with the up-expression of leptin and the dramatic down-expression of PPARgamma. The present study is aimed to assess the effect of leptin on PPARgamma gene expression in primary cultured rat HSCs and investigate the related mechanisms by using Western blotting analysis, real-time PCR, transient transfection approach, and cell growth analysis. The results suggest that leptin negatively regulates PPARgamma gene expression at mRNA level, protein level and PPARgamma gene promoter activity level in HSCs. The inhibitory effect of leptin on PPARgamma gene expression contributes to cell growth of activated HSCs in vitro. Phosphatidylinositol 3-kinase/AKT (PI-3 K/AKT) and extracellular signal-regulated kinase (ERK) signaling pathways mediate the leptin-induced inhibition of PPARgamma gene expression. In summary, these findings suggest that leptin down-regulates PPARgamma gene expression through activation of PI-3 K/AKT or ERK signaling pathway in primary cultured rat HSCs. Our results might provide novel insights into the mechanisms for the pro-fibrogenic action of leptin in liver.

The survival pathways phosphatidylinositol-3 kinase (PI3-K)/phosphoinositide-dependent protein kinase 1 (PDK1)/Akt modulate liver regeneration through hepatocyte size rather than proliferation

Liver regeneration comprises a series of complicated processes. The current study was designed to investigate the roles of phosphoinositide-dependent protein kinase 1 (PDK1)-associated pathways in liver regeneration after partial hepatectomy (PH) using liver-specific Pdk1-knockout (L-Pdk1KO) and Pdk1/STAT3 double KO (L-DKO) mice. There was no liver regeneration, and 70% PH was lethal in L-Pdk1KO mice. Liver regeneration was severely impaired equally in L-Pdk1KO and L-DKO mice, even after nonlethal 30% PH. There was no cell growth (measured as increase of cell size) after hepatectomy in L-Pdk1KO mice, although the post-PH mitotic response was the same as in controls. As expected, hepatectomy did not induce hepatic Akt-phosphorylation (Thr308) in L-Pdk1KO mice, and post-PH phosphorylation of Akt, mammalian target of rapamycin (mTOR), p70 ribosomal S6 kinase (p70(S6K)), and S6 were also reduced. To examine the specific role of PDK1-associated signals, a "pif-pocket" mutant of PDK1, which allows PDK1 only to phosphorylate Akt, was used. Liver regeneration was recovered in L-Pdk1KO mice with a "pif-pocket" mutant of PDK1. This re-activated Akt in L-Pdk1KO mice liver and induced post-PH cell growth, without affecting cell proliferation. Further deletion of STAT3 (L-DKO mice) did not further deteriorate liver regeneration, although this certainly reduced post-PH mitotic response. These findings indicate that PDK1/Akt contribute to liver regeneration by regulating cell size. Regarding phosphatidylinositol-3 kinase (PI3-K), immediate upstream signal of PDK1, activation of PI3-K induced cell proliferation via STAT3 activation in the liver of L-Pdk1KO mice but did not improve impaired liver regeneration. This confirmed the pivotal role of PDK1 in liver regeneration and cell growth.

CONCLUSION

PDK1/Akt-mediated responsive cell growth is essential for normal liver regeneration after PH, especially when cell proliferation is impaired.

Aging, stem cells, and mammalian target of rapamycin: a prospect of pharmacologic rejuvenation of aging stem cells

What is the relationship between stem cell aging and organismal aging? Does stem cell aging cause organismal aging or vice versa? Will stem cell aging aggravate age-related diseases? And what is stem cell aging? As suggested herein, hyperstimulation of signal transduction pathways can render cells compensatorily irresponsive. And the hallmark of stem cell aging is poor responsiveness to activating stimuli. On the basis of the hypothesis that insensitivity to stimuli is in part due to hyperactivation of the target of rapamycin (TOR), this article suggests a means of pharmacologic rejuvenation of stem cells and wound-healing cells.

TNFalpha is required for cholestasis-induced liver fibrosis in the mouse

TNFalpha, a mediator of hepatotoxicity in several animal models, is elevated in acute and chronic liver diseases. Therefore, we investigated whether hepatic injury and fibrosis due to bile duct ligation (BDL) would be reduced in TNFalpha knockout mice (TNFalpha-/-). Survival after BDL was 60% in wild-type mice (TNFalpha+/+) and 90% in TNFalpha-/- mice. Body weight loss and liver to body weight ratios were reduced in TNFalpha-/- mice compared to TNFalpha+/+ mice. Following BDL, serum alanine transaminases (ALT) levels were elevated in TNFalpha+/+ mice (268.6+/-28.2U/L) compared to TNFalpha-/- mice (105.9U/L+/-24.4). TNFalpha-/- mice revealed lower hepatic collagen expression and less liver fibrosis in the histology. Further, alpha-smooth muscle actin, an indicator for activated myofibroblasts, and TGF-beta mRNA, a profibrogenic cytokine, were markedly reduced in TNFalpha-/- mice compared to TNFalpha+/+ mice. Thus, our data indicate that TNFalpha induces hepatotoxicity and promotes fibrogenesis in the BDL model.

Cytokines and renin-angiotensin system signaling in hepatic fibrosis

Hepatic fibrosis is the result of a complex interplay between resident hepatic cells, infiltrating inflammatory cells, and a number of locally acting peptides called cytokines. Key mediators include transforming growth factor b1, vasoactive substances, adipokines, inflammatory cytokines and chemokines. Angiotensin II, the main effector of the renin-angiotensin system, is a true cytokine that plays a major role in liver fibrosis. Angiotensin II is locally synthesized in the injured liver and induces profibrogenic actions in hepatic stellate cells. Drugs blocking the renin-angiotensin system are promising antifibrotic agents. There are multiple signal transduction pathways involved in cytokine signaling. Drugs interfering intracellular pathways involved in increased collagen production are potential therapies for liver fibrosis.

Genesis of hepatic fibrosis and its biochemical markers

Liver fibrosis is characterized by an abnormal hepatic accumulation of extracellular matrix (ECM) that results from both increased deposition and reduced degradation of collagen fibres. Fibrotic liver injury results in activation of the hepatic stellate cell (HSC). Surrogate markers are gradually being substituted for biomarkers that reflect the complex balance between synthesis and degradation of the extracellular matrix. Once the hepatic stellate cell is activated, the preceding matrix changes and recurrent injurious stimuli will perpetuate the activated state. The ECM directs cellular differentiation, migration, proliferation and fibrogenic activation or deactivation. The metabolism of the extracellular matrix is closely regulated by matrix metalloproteinases (MMP) and their specific tissue inhibitors (TIMP). Although liver biopsy combined with connective tissue stains has been a mainstay of diagnosis, there is a need for less invasive methods. These diagnostic markers should be considered in combination with liver function tests, ultrasonography and clinical manifestations.

Role of TLR9 in hepatic stellate cells and experimental liver fibrosis

Accumulating evidence indicates that bacteria and bacterial products promote hepatic fibrogenesis. The activation of hepatic stellate cells (HSC) plays a central role in hepatic fibrosis. Here, we demonstrate that HSC express toll-like receptor 9 (TLR9), a pattern recognition receptor that is activated by CpG motifs present specifically in bacterial DNA. Upon CpG stimulation human as well as murine HSC isolated from wild-type (TLR9+/+) mice express increased levels of the profibrogenic chemokine monocyte chemotactic protein 1 (MCP-1). In contrast, HSC isolated from TLR9 deficient (TLR9-/-) mice lacked CpG motif induced MCP-1 expression indicating the functionality of TLR9 in HSC. Bile duct ligation revealed significantly lower hepatic MCP-1 and collagen expression and less hepatic fibrosis in TLR9-/- compared to TLR9+/+ mice. In addition, the expression of hepatic alpha-smooth-muscle actin, a known marker for HSC activation, was reduced in TLR9-/- mice indicating that bacterial DNA induces the activation of HSC and therefore promotes hepatic fibrosis.

Impact of rapamycin on liver regeneration

The remarkable capacity of the liver to regenerate after injury and the prospects of organ self-renewal have attracted much interest in the understanding and modulation of the underlying molecular events. We investigated the effect of mammalian target of rapamycin (mTOR) inhibitor rapamycin (RAPA) on liver by correlating intravital microscopy, immunohistochemistry, and reverse transcriptase polymerase chain reaction in a rat model of 2/3 hepatectomy. RAPA significantly retarded proliferation of hepatocytes, endothelial cells, and hepatic stellate cells (HSCs) mostly between days 2 and 4 after hepatectomy and downregulated major cytokines and growth factors (tumor necrosis factor alpha, hepatocyte growth factor, platelet-derived growth factor, platelet-derived growth factor receptor, insulin-like growth factor-1, transforming growth factor beta 1) important for liver regeneration. These effects were almost absent at later time points. RAPA also had a transient, but broad effect on angiogenesis, and impaired sinusoidal density as well as mRNA levels of vascular endothelial growth factor, vascular endothelial growth factor receptor 1, vascular endothelial growth factor receptor 2, and angiopoietin-1. Activation of HSC was also transiently suppressed as observed by smooth muscle protein 1 alpha protein expression and intercellular adhesion molecule-1 mRNA levels. The rate of apoptosis in liver was significantly increased by RAPA between day 3 and day 7. The effect of RAPA on liver repair, angiogenesis, and HSC activation is confined to the phase of active cell proliferation. This transient effect might allow further exploration of mTOR inhibitors in clinical situations that involve liver regeneration, and seems to have implications beyond immunosuppression.

The molecular mechanism of gypenosides-induced G1 growth arrest of rat hepatic stellate cells

AIM OF THE STUDY

Gypenosides, the saponins extract derived from Gynostemma pentaphyllum Makino, have been used for treating hepatitis and cancer in Asia. Our previous study demonstrates that gypenosides inhibit the onset and improve the recovery of liver fibrosis induced by CCl4 in rats. In this study, we used the isolated rat hepatic stellate cells (HSCs) as a model to study the cellular mechanism of gypenosides-inhibited liver fibrosis.

MATERIALS AND METHODS

Rat HSCs was treated with PDGF, gypenosides or vehicle. Cell viability was assessed by trypan blue staining. Apoptosis and cell cycle were evaluated by flow cytometry. The activation or inhibition of signal molecules was detected by Western blotting.

RESULTS

Our results showed that 500 microg/ml gypenosides decreased PDGF-induced rat HSCs numbers (8750+/-2629 versus 103,000+/-6683, p<0.001, 95% confidence interval) and arrested cells at the G1 phase without the presence of sub-G1 fraction. Analysis of PDGF-induced proliferative molecules including phosphorylation of Akt and p70 S6K, gypenosides inhibited the activation of this signal pathway. Furthermore, gypenosides down-regulated the protein expression of cell cycle G1-specific cyclin D1 and D3.

CONCLUSIONS

Gypenosides inhibited PDGF-induced HSCs proliferation by inhibiting the signal pathway of PDGF-Akt-p70 S6K and down-regulation of cyclin D1 and D3 expression.

Akt blockade downregulates collagen and upregulates MMP1 in human dermal fibroblasts

Acutely transforming retrovirus AKT8 in rodent T-cell lymphoma (Akt) is a serine/threonine kinase that plays important roles in survival, cell-cycle progression, and cell proliferation, and has recently been implicated in collagen regulation. The aim of this study was to determine the role of Akt in collagen deposition by normal dermal fibroblasts, and to determine the sensitivity of cultured systemic sclerosis (SSc) fibroblasts to Akt inhibition. We show that blockade of Akt using pharmacological inhibitors, small interfering RNA (siRNA), and a dominant-negative Akt mutant led to inhibition of the basal type I collagen production. Furthermore, inhibition of Akt upregulated basal matrix metalloproteinase 1 (MMP1) production and reversed the inhibitory effect of transforming growth factor-beta (TGF-beta) on MMP1 gene expression. In addition, SSc fibroblasts were more sensitive to Akt inhibition, with respect to collagen and MMP1 production. These findings suggest that in human dermal fibroblasts, Akt has dual profibrotic effects, increasing collagen synthesis and decreasing its degradation via downregulation of MMP1. Akt could directly contribute to elevated collagen in SSc fibroblasts and it may represent an attractive target for therapy of SSc fibrosis.

High molecular weight adiponectin inhibits proliferation of hepatic stellate cells via activation of adenosine monophosphate-activated protein kinase

Adiponectin is an adipocyte-derived, antidiabetic, antiatherogenic adipocytokine that is present in serum as 3 isoforms. Decreased plasma adiponectin levels are closely associated with the severity of nonalcoholic fatty liver diseases. This study was designed to elucidate a role of adiponectin and its mediator adenosine monophosphate-activated protein kinase (AMPK) on proliferation of activated hepatic stellate cells (HSCs), the key cells promoting fibrosis. Immortalized human HSC line hTERT and primary rat HSCs were stimulated with platelet-derived growth factor (PDGF) with or without pretreatment with AMPK activator 5-aminoimidazole-4-carboxamide-1-4-ribofuranoside (AICAR), metformin, or high molecular weight (HMW) adiponectin. HMW adiponectin dose-dependently suppressed PDGF-induced HSC proliferation. Adenoviral transduction with dominant-negative AMPK (DN-AMPK) abolished the suppressive effect of adiponectin in HSCs. AICAR, metformin, or transduction of constitutively active AMPK attenuated PDGF-induced [(3)H]thymidine incorporation, which was abolished by either a chemical AMPK inhibitor or transduction of DN-AMPK, consistent with an antiproliferative effect of AMPK. The suppressive effect of AMPK on HSC proliferation is mediated through multiple mechanisms, including (1) an inhibition of the AKT pathway, (2) inhibition of NADPH oxidase-dependent reactive oxygen species (ROS) production via induction of antioxidant enzymes, and (3) an increase in the expression of the cyclin-dependent kinase inhibitors p27(kip1) and p21(cip1).

CONCLUSION

Adiponectin inhibits HSC proliferation via activation of AMPK. AMPK activation by AICAR or metformin inhibits HSC proliferation via suppression of ROS production and subsequent inhibition of AKT pathway. Thus, adiponectin and AMPK inhibit HSC proliferation and hepatic fibrosis via multiple molecular mechanisms.

Molecular mechanisms of hepatic fibrogenesis

Liver fibrosis, a wound-healing response to a variety of chronic stimuli, is characterized by excessive deposition of extracellular matrix (ECM) proteins, of which type I collagen predominates. This alters the structure of the liver leading to organ dysfunction. The activated hepatic stellate cell (HSC) is primarily responsible for excess collagen deposition during liver fibrosis. Two important aspects are involved in mediating the fibrogenic response: first the HSC becomes directly fibrogenic by synthesizing ECM proteins; second, the activated HSC proliferates, effectively amplifying the fibrogenic response. Although the precise mechanisms responsible for HSC activation remain elusive, substantial insight is being gained into the molecular mechanisms responsible for ECM production and cell proliferation in the HSC. The activated HSC becomes responsive to both proliferative (platelet-derived growth factor) and fibrogenic (transforming growth factor-beta[TGF-beta]) cytokines. It is becoming clear that these cytokines activate both mitogen-activated protein kinase (MAPK) signaling, involving p38, and focal adhesion kinase-phosphatidylinositol 3-kinase-Akt-p70 S6 kinase (FAK-PI3K-Akt-p70(S6K)) signaling cascades. Together, these regulate the proliferative response, activating cell cycle progression as well as collagen gene expression. In addition, signaling by both TGF-beta, mediated by Smad proteins, and p38 MAPK influence collagen gene expression. Smad and p38 MAPK signaling have been found to independently and additively regulate alpha1(I) collagen gene expression by transcriptional activation while p38 MAPK, but not Smad signaling, increases alpha1(I) collagen mRNA stability, leading to increased synthesis and deposition of type I collagen. It is anticipated that by understanding the molecular mechanisms responsible for HSC proliferation and excess ECM production new therapeutic targets will be identified for the treatment of liver fibrosis.

Sirolimus-induced pneumonitis following liver transplantation

Sirolimus-induced pneumonitis has emerged as a potentially serious complication in renal transplantation but only single case reports of this condition have been described after liver transplantation (LT), where experience with sirolimus is relatively limited. We report our experience, the largest to date, of sirolimus-induced pneumonitis following LT. Between 1999 and 2006, 186 liver transplant patients received sirolimus-based immunosuppression, after initial therapy with calcineurin inhibitors (CNIs). All cases of sirolimus-induced pneumonitis were recorded and a retrospective review of the case notes of such patients was undertaken for the purpose of this analysis. Of 186 liver transplant patients receiving sirolimus, 4 (2.2%) developed pneumonitis that was attributed to the drug; the time from starting sirolimus to presentation was varied (1.5-30 months). The most common presenting symptoms were dyspnea, cough and fatigue. The median sirolimus level at the time of diagnosis was 9.7 ng/mL (range, 7-19.5 ng/mL). All patients in the series underwent thoracic computed tomography, which showed similar changes in all patients, and lung biopsy, which revealed features consistent with a drug-induced pneumonitis. In all 4 patients, sirolimus-induced pneumonitis resolved following cessation of therapy but took weeks to months for complete recovery. In conclusion, sirolimus-induced pneumonitis occurred in at least 2% of liver transplant recipients and should be suspected in patients who develop respiratory symptoms while on sirolimus. Although it may be life threatening, early recognition and cessation of sirolimus can lead to complete resolution of pneumonitis.

The forkhead transcription factor FoxO1 regulates proliferation and transdifferentiation of hepatic stellate cells

BACKGROUND & AIMS

The Forkhead box gene, group O (FoxO) family of Forkhead transcription factors is phopsphorylated and inactivated by the phosphatidylinositol 3-kinase (PI3K)/AKT pathway and regulates a variety of cellular functions. Hepatic stellate cells (HSCs) play a crucial role in liver fibrosis. A fibrotic stimulus causes HSCs to transdifferentiate from a quiescent phenotype to a collagen-producing myofibroblast-like phenotype and to proliferate.

METHODS

Mutation/deletion mutants of FoxO1 were introduced into primary rat, mouse, and immortalized human HSCs and assessed for activation, proliferation, and signal transduction. The role of FoxO1 in experimental liver fibrosis was assessed in FoxO1(+/-) and FoxO1(+/+) mice.

RESULTS

Platelet-derived growth factor (PDGF) or insulin phosphorylates FoxO1 and induces FoxO1 translocation from the nuclei to the cytosol via the PI3K/AKT pathway in HSCs. Constitutively active FoxO1 inhibits proliferation via cell cycle arrest at the G1 phase, whereas dominant-negative FoxO1 enhances proliferation of HSCs even in the presence of the PI3K inhibitor LY294002. In addition, the phosphorylation of FoxO1 is increased during transdifferentiation of HSCs. The transdifferentiation is also inhibited by constitutively active FoxO1 and is accelerated by dominant-negative FoxO1. FoxO1 directly induces the expression of p27(kip1) and manganese superoxide dismutase (MnSOD). After bile duct ligation for 3 weeks, FoxO1(+/-) mice are more susceptible to liver fibrosis, consistent with our in vitro results.

CONCLUSIONS

FoxO1 plays a crucial role in the transdifferentiation and proliferation of HSCs in liver fibrosis. Hyperinsulinemia inactivates FoxO1 in HSCs, resulting in HSC activation and may result in the fibrosis in nonalcoholic fatty liver disease.

Signal transduction and activation of hepatic stellate cells

Liver fibrosis, which leads to cirrhosis, occurs as a result of various injurious processes and it is the common pathologic basis of all the chronic hepatic diseases. At present, a good many researches demonstrate that the activation of hepatic stellate cells play a critical role in fibrogenesis. Prolonged liver injury results in hepatocyte damages and secretion of many fibrogenic cytokines such as transforming growth factor-beta 1, angiotensin, and leptin, which triggers the activation of hepatic stellate cells through different intracellular signal transduction pathways. In this article, we reviewed the research advancement in the signal transduction pathway of nuclear receptor and membrane receptor during the activation of hepatic stellate cells.

Loss of TGF-beta dependent growth control during HSC transdifferentiation

Liver injury induces activation of hepatic stellate cells (HSCs) comprising expression of receptors, proliferation, and extracellular matrix synthesis triggered by a network of cytokines provided by damaged hepatocytes, activated Kupffer cells and HSCs. While 6 days after bile duct ligation in rats TGF-beta inhibited DNA synthesis in HSCs, it was enhanced after 14 days, indicating a switch from suppression to DNA synthesis stimulation during fibrogenesis. To delineate mechanisms modulating TGF-beta function, we analyzed crosstalk with signaling pathways initiated by cytokines in damaged liver. Lipopolysaccharide and tumor necrosis factor-alpha enhanced proliferation inhibition of TGF-beta, whereas interleukin-6, oncostatin M, interleukin-1alpha, and interleukin-1beta did not. Hepatocyte growth factor (HGF) counteracted TGF-beta dependent inhibition of DNA synthesis in quiescent HSCs. Since expression of c-met is induced during activation of HSCs and HGF is overrepresented in damaged liver, crosstalk of HGF and TGF-beta contributes to loss of TGF-beta dependent inhibition of DNA synthesis in HSCs.

Low-dose oral rapamycin treatment reduces fibrogenesis, improves liver function, and prolongs survival in rats with established liver cirrhosis

BACKGROUND/AIMS

Mammalian target of rapamycin (mTOR) signalling is central in the activation of hepatic stellate cells (HSCs), the key source of extracellular matrix (ECM) in fibrotic liver. We tested the therapeutic potential of the mTOR inhibitor rapamycin in advanced cirrhosis.

METHODS

Cirrhosis was induced by bile duct-ligation (BDL) or thioacetamide injections (TAA). Rats received oral rapamycin (0.5 mg/kg/day) for either 14 or 28 days. Untreated BDL and TAA-rats served as controls. Liver function was quantified by aminopyrine breath test. ECM and ECM-producing cells were quantified by morphometry. MMP-2 activity was measured by zymography. mRNA expression of procollagen-alpha1, transforming growth factor-beta1 (TGF-beta1) and beta2 was quantified by RT-PCR.

RESULTS

Fourteen days of rapamycin improved liver function. Accumulation of ECM was decreased together with numbers of activated HSCs and MMP-2 activity in both animal models. TGF-beta1 mRNA was downregulated in TAA, TGF-beta2 mRNA was downregulated in BDL. 28 days of rapamycin treatment entailed a survival advantage of long-term treated BDL-rats.

CONCLUSIONS

Low-dose rapamycin treatment is effectively antifibrotic and attenuates disease progression in advanced fibrosis. Our results warrant the clinical evaluation of rapamycin as an antifibrotic drug.

2′,4′,6′-Tris(methoxymethoxy) chalcone attenuates hepatic stellate cell proliferation by a heme oxygenase-dependent pathway

Proliferation of hepatic stellate cells (HSCs) is central for the development of fibrosis during liver injury. We have shown previously that butein (3,4,2',4'-tetrahydroxychalcone) suppresses myofibroblastic differentiation of rat HSCs. Our aim in this study was to determine whether a new synthetic chalcone derivative, 2',4',6'-tris(methoxymethoxy) chalcone (TMMC) inhibits HSC proliferation induced by serum- or platelet-derived growth factor (PDGF). TMMC significantly inhibited growth factor-induced HSC proliferation. The inhibition of PDGF-induced proliferation by TMMC was associated with the phosphatidylinositol 3-kinase-Akt-p70(S6K) pathways. TMMC induced the expression of heme oxygenase 1 (HO-1) in HSCs. Using the chemical inhibitor tin protoporphyrin, we also found that the inhibitory action of TMMC on PDGF-induced proliferation is mediated by HO-1. Glutathione (GSH) depletion produced by TMMC activated extracellular signal-regulated kinase (ERK), which led to c-Fos expression and transactivation of activator protein 1 (AP-1) and HO-1 gene expression in the HSCs. These results demonstrate that TMMC preferentially activates ERK and that this activation leads to the transcriptional activation of AP-1 and consequently to HO-1 expression. HO-1 expression might be responsible for the antiproliferative effect of TMMC on HSCs.

Leucine stimulates procollagen alpha1(I) translation on hepatic stellate cells through ERK and PI3K/Akt/mTOR activation

The essential amino acid leucine has been described to specifically activate signaling pathways leading to the activation of the translational machinery and the increase of total protein synthesis. Regulation of type I collagen production by hepatic stellate cells (HSC) is a multistep process involving transcriptional and post-transcriptional mechanisms. In the present work we studied the effect of leucine on translation regulation of collagen alpha1(I) production in HSC and the signaling pathways involved. Treatment of HSC with 5 mM leucine did not alter half-life or steady state levels of procollagen alpha1(I) mRNA, but caused an increase in procollagen alpha1(I) protein that correlated with changes of components involved in translational regulation, like enhanced 4E-BP1, Mnk-1, and eIF4E phosphorylation. Leucine also induced mTOR, ERK, and Akt phosphorylation in HSC, without affecting p38 and JNK activation. Pre-treatment of HSC with PD098059, wortmannin, or rapamycin prevented the profibrogenic action of leucine due to the inhibition of different molecular mechanisms. These results suggest leucine is a profibrogenic agent for HSC, activating signaling pathways that lead to an enhancement of collagen alpha1(I) production through translational regulation.

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Coordinate activation of intracellular signaling pathways by insulin-like growth factor-1 and platelet-derived growth factor in rat hepatic stellate cells

Proliferation of activated hepatic stellate cells (HSC) is an important event in the development of hepatic fibrosis. Insulin-like growth factor-1 (IGF-1) has been shown to be mitogenic for HSC, but the intracellular signaling pathways involved have not been fully characterized. Thus, the aims of the current study were to examine the roles of the extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3-K) and p70-S6 kinase (p70-S6-K) signaling pathways in IGF-1- and platelet-derived growth factor (PDGF)-induced mitogenic signaling of HSC and to examine the potential crosstalk between these pathways. Both IGF-1 and PDGF increased ERK, PI3-K and p70-S6-K activity. When evaluating potential crosstalk between these signaling pathways, we observed that PI3-K is required for p70-S6-K activation by IGF-1 and PDGF, and is partially responsible for PDGF-induced ERK activation. PDGF and IGF-1 also increased the levels of cyclin D1 and phospho-glycogen synthase kinase-3beta. Coordinate activation of ERK, PI3-K and p70-S6-K is important for perpetuating the activated state of HSC during fibrogenesis.

Mechanisms of liver fibrosis

Liver fibrosis represents a significant health problem worldwide of which no acceptable therapy exists. The most characteristic feature of liver fibrosis is excess deposition of type I collagen. A great deal of research has been performed to understand the molecular mechanisms responsible for the development of liver fibrosis. The activated hepatic stellate cell (HSC) is the primary cell type responsible for the excess production of collagen. Following a fibrogenic stimulus, HSCs change from a quiescent to an activated, collagen-producing cell. Numerous changes in gene expression are associated with HSC activation including the induction of several intracellular signaling cascades, which help maintain the activated phenotype and control the fibrogenic and proliferative state of the cell. Detailed analyses in understanding the molecular basis of collagen gene regulation have revealed a complex process offering the opportunity for multiple potential therapeutic strategies. However, further research is still needed to gain a better understanding of HSC activation and how this cell maintains its fibrogenic nature. In this review we describe many of the molecular events that occur following HSC activation and collagen gene regulation that contribute to the fibrogenic nature of these cells and provide a review of therapeutic strategies to treat this disease.