Inge Mannaerts

A role for autophagy during hepatic stellate cell activation

BACKGROUND & AIMS Autophagy is a metabolic process that degrades and recycles intracellular organelles and proteins with many connections to human disease and physiology. We studied the role of autophagy during hepatic stellate cell (HSC) activation, a key event in liver fibrogenesis. METHODS Analysis of the autophagic flux during in vitro activation of primary mouse HSCs was performed using a DsRed-GFP-LC3B encoding plasmid. The effect of autophagy inhibition by bafilomycin A1 on the in vitro activation process of human and mouse HSCs was examined by measuring proliferation, presence of activation markers by RT-qPCR, immunofluorescence, and Western blotting. Analysis of lipid droplet and microtubule-associated protein light chain 3 beta (LC3B) colocalization in the presence of PDGF-BB was investigated by immunocytochemistry. RESULTS A significant increased autophagic flux was observed during culture induced mouse HSC activation. Treatment of mouse HSCs and human HSCs with autophagy inhibitor bafilomycin A1 results in a significant decreased proliferation and expression of activation markers. In addition, lipid droplets and LC3B colocalization was increased after PDGF-BB treatment in quiescent HSCs. CONCLUSIONS During HSC activation, autophagic flux is increased. The demonstration of partly inhibition of in vitro HSC activation after treatment with an autophagy inhibitor unveils a potential new therapeutic strategy for liver fibrosis.

Chronic administration of valproic acid inhibits activation of mouse hepatic stellate cells in vitro and in vivo

Hepatic stellate cell (HSC) activation is a pivotal step in the pathogenesis of liver fibrosis. The clarification of this transdifferentiation process is therefore important for the development of effective therapies for fibrosis. We analyzed the effect of a histone deacetylase inhibitor, valproic acid (VPA), on mouse HSC transdifferentiation in vitro and in vivo. The exposure of freshly isolated mouse HSCs to 2.5 mM VPA led to increased histone H4 acetylation and inhibited cell proliferation. Expression of stellate cell activation markers analyzed by quantitative polymerase chain reaction and western blotting revealed that treatment with VPA inhibited the induction of activation markers such as Acta2, Lox, Spp1, and Myh11. Treatment of mice with VPA decreased collagen deposition and in vivo activation of stellate cells in the livers of CCl4‐treated mice. Class I histone deacetylase silencing through RNA interference in mouse HSCs only partially mimicked treatment with VPA. Conclusion: Chronic administration of VPA results in a marked decrease in stellate cell activation both in vitro and in vivo. We hypothesize that the VPA effect results partially from class I histone deacetylase inhibition, but that also non‐histone deacetylase class I VPA targets are involved in the stellate cell activation process. (HEPATOLOGY 2010.)

The Hippo pathway effector YAP controls mouse hepatic stellate cell activation

BACKGROUND & AIMS Hepatic stellate cell activation is a wound-healing response to liver injury. However, continued activation of stellate cells during chronic liver damage causes excessive matrix deposition and the formation of pathological scar tissue leading to fibrosis and ultimately cirrhosis. The importance of sustained stellate cell activation for this pathological process is well recognized, and several signalling pathways that can promote stellate cell activation have been identified, such as the TGFβ-, PDGF-, and LPS-dependent pathways. However, the mechanisms that trigger and drive the early steps in activation are not well understood. METHODS AND RESULTS We identified the Hippo pathway and its effector YAP as a key pathway that controls stellate cell activation. YAP is a transcriptional co-activator and we found that it drives the earliest changes in gene expression during stellate cell activation. Activation of stellate cells in vivo by CCl4 administration to mice or activation in vitro caused rapid activation of YAP as revealed by its nuclear translocation and by the induction of YAP target genes. YAP was also activated in stellate cells of human fibrotic livers as evidenced by its nuclear localization. Importantly, knockdown of YAP expression or pharmacological inhibition of YAP prevented hepatic stellate cell activation in vitro and pharmacological inhibition of YAP impeded fibrogenesis in mice. CONCLUSIONS YAP activation is a critical driver of hepatic stellate cell activation and inhibition of YAP presents a novel approach for the treatment of liver fibrosis.

HDAC inhibitors in experimental liver and kidney fibrosis

Histone deacetylase (HDAC) inhibitors have been extensively studied in experimental models of cancer, where their inhibition of deacetylation has been proven to regulate cell survival, proliferation, differentiation and apoptosis. This in turn has led to the use of a variety of HDAC inhibitors in clinical trials. In recent years the applicability of HDAC inhibitors in other areas of disease has been explored, including the treatment of fibrotic disorders. Impaired wound healing involves the continuous deposition and cross-linking of extracellular matrix governed by myofibroblasts leading to diseases such as liver and kidney fibrosis; both diseases have high unmet medical needs which are a burden on health budgets worldwide. We provide an overview of the potential use of HDAC inhibitors against liver and kidney fibrosis using the current understanding of these inhibitors in experimental animal models and in vitro models of fibrosis.

Valproic Acid Attenuates Proteinuria and Kidney Injury

Inhibitors of histone deacetylase (HDAC) have anti-inflammatory and antifibrotic effects in several organs and tissues, but their effect on the progression of renal disease is unknown. Here, we studied the effect of valproic acid in adriamycin-induced nephropathy in mice. Administration of valproic acid before kidney injury prevented the development of proteinuria and the onset of glomerulosclerosis. Even after postponing treatment until the peak of adriamycin-induced proteinuria, valproic acid rapidly decreased the quantity of proteinuria and attenuated the progression of renal disease. Valproic acid abrogated the decrease in glomerular acetylation observed during adriamycin-induced nephropathy. Furthermore, valproic acid attenuated the significant upregulation of profibrotic and proinflammatory genes, the deposition of collagen, and the infiltration of macrophages into the kidney. Valproic acid decreased glomerular apoptosis and proliferation induced by adriamycin. Ultrastructural studies further supported the protective effect of valproic acid on podocytes in this model. Taken together, these data suggest that HDACs contribute to the pathogenesis of renal disease and that HDAC inhibitors may have therapeutic potential in CKD.

FXR agonist obeticholic acid reduces hepatic inflammation and fibrosis in a rat model of toxic cirrhosis

Hepatic inflammation drives hepatic stellate cells (HSC), resulting in liver fibrosis. The Farnesoid-X receptor (FXR) antagonizes inflammation through NF-κB inhibition. We investigated preventive and therapeutic effects of FXR agonist obeticholic acid (OCA) on hepatic inflammation and fibrosis in toxic cirrhotic rats. Cirrhosis was induced by thioacetamide (TAA) intoxication. OCA was given during or after intoxication with vehicle-treated rats as controls. At sacrifice, fibrosis, hemodynamic and biochemical parameters were assessed. HSC activation, cell turn-over, hepatic NF-κB activation, pro-inflammatory and pro-fibrotic cytokines were determined. The effect of OCA was further evaluated in isolated HSC, Kupffer cells, hepatocytes and liver sinusoidal endothelial cells (LSEC). OCA decreased hepatic inflammation and fibrogenesis during TAA-administration and reversed fibrosis in established cirrhosis. Portal pressure decreased through reduced intrahepatic vascular resistance. This was paralleled by decreased expression of pro-fibrotic cytokines (transforming growth-factor β, connective tissue growth factor, platelet-derived growth factor β-receptor) as well as markers of hepatic cell turn-over, by blunting effects of pro-inflammatory cytokines (e.g. monocyte chemo-attractant protein-1). In vitro, OCA inhibited both LSEC and Kupffer cell activation; while HSC remained unaffected. This related to NF-κB inhibition via up-regulated IκBα. In conclusion, OCA inhibits hepatic inflammation in toxic cirrhotic rats resulting in decreased HSC activation and fibrosis.

Class II HDAC Inhibition Hampers Hepatic Stellate Cell Activation by Induction of MicroRNA-29

Background The conversion of a quiescent vitamin A storing hepatic stellate cell (HSC) to a matrix producing, contractile myofibroblast-like activated HSC is a key event in the onset of liver disease following injury of any aetiology. Previous studies have shown that class I histone deacetylases (HDACs) are involved in the phenotypical changes occurring during stellate cell activation in liver and pancreas. Aims In the current study we investigate the role of class II HDACs during HSC activation. Methods We characterized the expression of the class II HDACs freshly isolated mouse HSCs. We inhibited HDAC activity by selective pharmacological inhibition with MC1568, and by repressing class II HDAC gene expression using specific siRNAs. Results Inhibition of HDAC activity leads to a strong reduction of HSC activation markers α-SMA, lysyl oxidase and collagens as well as an inhibition of cell proliferation. Knock down experiments showed that HDAC4 contributes to HSC activation by regulating lysyl oxidase expression. In addition, we observed a strong up regulation of miR-29, a well-known anti-fibrotic miR, upon treatment with MC1568. Our in vivo work suggests that a successful inhibition of class II HDACs could be promising for development of future anti-fibrotic compounds. Conclusions In conclusion, the use of MC1568 has enabled us to identify a role for class II HDACs regulating miR-29 during HSC activation.

Integrative miRNA and Gene Expression Profiling Analysis of Human Quiescent Hepatic Stellate Cells

Unveiling the regulatory pathways maintaining hepatic stellate cells (HSC) in a quiescent (q) phenotype is essential to develop new therapeutic strategies to treat fibrogenic diseases. To uncover the miRNA-mRNA regulatory interactions in qHSCs, HSCs were FACS-sorted from healthy livers and activated HSCs (aHSCs) were generated in vitro. MiRNA Taqman array analysis showed HSCs expressed a low number of miRNAs (n = 259), from which 47 were down-regulated and 212 up-regulated upon activation. Computational integration of miRNA and gene expression profiles revealed that 66% of qHSC-associated miRNAs correlated with more than 6 altered target mRNAs (17,28 ± 10,7 targets/miRNA) whereas aHSC-associated miRNAs had an average of 1,49 targeted genes. Interestingly, interaction networks generated by miRNA-targeted genes in qHSCs were associated with key HSC activation processes. Next, selected miRNAs were validated in healthy and cirrhotic human livers and miR-192 was chosen for functional analysis. Down-regulation of miR-192 in HSCs was found to be an early event during fibrosis progression in mouse models of liver injury. Moreover, mimic assays for miR-192 in HSCs revealed its role in HSC activation, proliferation and migration. Together, these results uncover the importance of miRNAs in the maintenance of the qHSC phenotype and form the basis for understanding the regulatory networks in HSCs.

Comparison of trichostatin A and valproic acid treatment regimens in a mouse model of kidney fibrosis

Histone deacetylase (HDAC) inhibitors are promising new compounds for the therapy of fibrotic diseases. In this study we compared the effect of two HDAC inhibitors, trichostatin A and valproic acid, in an experimental model of kidney fibrosis. In mice, doxorubicin (adriamycin) can cause nephropathy characterized by chronic proteinuria, glomerular damage and interstitial inflammation and fibrosis, as seen in human focal segmental glomerulosclerosis. Two treatment regimens were applied, treatment was either started prior to the doxorubicin insult or delayed until a significant degree of proteinuria and fibrosis was present. Pre-treatment of trichostatin A significantly hampered glomerulosclerosis and tubulointerstitial fibrosis, as did the pre-treatment with valproic acid. In contrast, the development of proteinuria was only completely inhibited in the pre-treated valproic acid group, and not in the pre-treated trichostatin A animals. In the postponed treatment with valproic acid, a complete resolution of established doxorubicin-induced proteinuria was achieved within three days, whereas trichostatin A could not correct proteinuria in such a treatment regimen. However, both postponed regimens have comparable efficacy in maintaining the kidney fibrosis to the level reached at the start of the treatments. Moreover, not only the process of fibrosis, but also renal inflammation was attenuated by both HDAC inhibitors. Our data confirm a role for HDACs in renal fibrogenesis and point towards a therapeutic potential for HDAC inhibitors. The effect on renal disease progression and manifestation can however be different for individual HDAC inhibitors.

Genome-wide analysis of DNA methylation and gene expression patterns in purified, uncultured human liver cells and activated hepatic stellate cells

Background & Aims Liver fibrogenesis – scarring of the liver that can lead to cirrhosis and liver cancer – is characterized by hepatocyte impairment, capillarization of liver sinusoidal endothelial cells (LSECs) and hepatic stellate cell (HSC) activation. To date, the molecular determinants of a healthy human liver cell phenotype remain largely uncharacterized. Here, we assess the transcriptome and the genome-wide promoter methylome specific for purified, non-cultured human hepatocytes, LSECs and HSCs, and investigate the nature of epigenetic changes accompanying transcriptional changes associated with activation of HSCs. Material and methods Gene expression profile and promoter methylome of purified, uncultured human liver cells and culture-activated HSCs were respectively determined using Affymetrix HG-U219 genechips and by methylated DNA immunoprecipitation coupled to promoter array hybridization. Histone modification patterns were assessed at the single-gene level by chromatin immunoprecipitation and quantitative PCR. Results We unveil a DNA-methylation-based epigenetic relationship between hepatocytes, LSECs and HSCs despite their distinct ontogeny. We show that liver cell type-specific DNA methylation targets early developmental and differentiation-associated functions. Integrative analysis of promoter methylome and transcriptome reveals partial concordance between DNA methylation and transcriptional changes associated with human HSC activation. Further, we identify concordant histone methylation and acetylation changes in the promoter and putative novel enhancer elements of genes involved in liver fibrosis. Conclusions Our study provides the first epigenetic blueprint of three distinct freshly isolated, human hepatic cell types and of epigenetic changes elicited upon HSC activation.