Rolle der Aktivierung des <em>Peroxisome proliferator-activated receptor gamma</em> durch Pioglitazon in der hepatischen Immunregulation

Der Peroxisome proliferator-activated receptor gamma (PPARγ) gehört zur Superfamilie nukleärer Faktoren, die nach Ligandenbindung Genexpression regulieren. PPARγ steuert durch Bindung an PPARγ-responsive Elemente die Expression metabolischer Gene und reguliert durch Stabilisierung transkriptioneller Repressoren die Expression proinflammatorischer Gene in Immunzellen. In Modellen entzündlicher Erkrankungen des Darmes oder des zentralen Nervensystems, trägt die PPARγ Aktivierung zu einem verzögerten oder abgeschwächten Krankheitsverlauf bei. In dieser Arbeit wurde untersucht, ob sich die pharmakologische Aktivierung von PPARγ durch den synthetischen PPARγ Liganden Pioglitazon (Pio) protektiv auf die Entwicklung einer immunvermittelten Hepatitis auswirkt. PPARγ Aktivierung in sinusoidalen Endothelzellen der Leber (LSEC) regulierte zwar Toll-like rezeptorinduzierte Interleukin 6 Expression, hatte aber keinen Einfluss auf deren toleranzinduzierende Funktion als antigenpräsentierende Zellen. Um den Einfluss von PPARγ Aktivierung auf den Verlauf einer immunvermittelten Hepatitis zu untersuchen, wurden zwei Modelle einer experimentellen Hepatitis verwendet. Obwohl PPARγ Aktivierung die Expression von Interferon γ (INFγ) und Tumor Necrosis Factor (TNF) in T Zellen nach Stimulation mit Concanavalin A (ConA) in vitro reduzierte, hatte sie keinen Einfluss auf den Verlauf einer Hepatitis in vivo. In Pio behandelten Tieren kam es sogar zu einer verstärkten Expression von TNF, des zentralen, schädigenden Zytokins in der ConA Hepatitis. Deshalb wurde die Rolle von PPARγ in einem weiteren TNF-abhängigen Hepatitis Modell, der GalN/LPS Hepatitis, untersucht. In diesem Modell verursachte die PPARγ Aktivierung eine Verschlechterung der Hepatitis, was mit einer erhöhten intrahepatischen TNF Expression in Kupffer Zellen und einer gesteigerten Sensitivität von Hepatozyten gegenüber TNF einher ging. Diese Ergebnisse zeigen, dass PPARγ keinen Einfluss auf die tolerogene Funktion von LSEC hat und keine organ-protektive Rolle in leberspezifischen Entzündungsmodellen besitzt. Die Verschlechterung immunvermittelter Leberschäden durch PPARγ ist in ihrem molekularen Mechanismus noch nicht verstanden, bildet aber eine neue Erklärung für die Entwicklung medikamentenbedingter Leberschäden, die im Kontext lokaler Entzündung in der Leber auftreten können.Peroxisome proliferator-activated receptor gamma (PPARγ) belongs to the superfamily of nuclear receptors regulating gene expression upon ligand binding. After binding to responsive elements PPARγ induces the expression of metabolic genes and regulates the expression of proinflammatory cytokines in immune cells by stabilisation of transcriptional repressors. In models of inflammatory gut diseases and diseases of the central nervous system, PPARγ-activation contributes to the delay of disease onset and ameliorates disease severity. Here we investigated whether the pharmacological activation of PPARγ by the synthetic PPARγ ligand Pioglitazone (Pio) also provides protection from immune-mediated hepatitis. Although PPARγ activation in liver sinusoidal endothelial cells (LSEC) regulated toll-like receptor induced Interleukin 6 expression it did not influence their tolerogenic function as antigen-presenting cells. To further investigate the function of PPARγ activation during immune-mediated hepatitis, two models of experimental hepatitis were used. While PPARγ activation reduced Concanavalin A (ConA) induced Interferon γ (IFNγ) expression as well as Tumor Necrosis Factor (TNF) expression in T cells in vitro, it did not influence the course of disease during hepatitis in vivo. Pio treated animals showed even higher expression of TNF, the main harmful cytokine during ConA hepatitis. Therefore, the role of PPARγ in GalN/LPS hepatitis, another TNF-dependent hepatitis model, was investigated. In this model PPARγ activation led to an increase of hepatic injury, which was associated with increased intrahepatic TNF expression in Kupffer cells and enhanced sensitivity of hepatocytes towards TNF. These results demonstrate that PPARγ does not influence the tolerogenic function of LSEC but has no organ-protective function in models of liver associated inflammation as well. The aggravation of immune mediated liver injury by PPARγ is not understood in molecular detail yet but offers a novel explanation for the development of drug induced liver injury appearing in context of local liver inflammation

Pioglitazone-mediated peroxisome proliferator-activated receptor γ activation aggravates murine immune-mediated hepatitis

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) regulates target gene expression upon ligand binding. Apart from its effects on metabolism, PPARγ activity can inhibit the production of pro-inflammatory cytokines by several immune cells, including dendritic cells and macrophages. In chronic inflammatory disease models, PPARγ activation delays the onset and ameliorates disease severity. Here, we investigated the effect of PPARγ activation by the agonist Pioglitazone on the function of hepatic immune cells and its effect in a murine model of immune-mediated hepatitis. Cytokine production by both liver sinusoidal endothelial cells (IL-6) and in T cells ex vivo (IFNγ) was decreased in cells from Pioglitazone-treated mice. However, PPARγ activation did not decrease pro-inflammatory tumor necrosis factor alpha TNFα production by Kupffer cells after Toll-like receptor (TLR) stimulation ex vivo. Most interestingly, although PPARγ activation was shown to ameliorate chronic inflammatory diseases, it did not improve hepatic injury in a model of immune-mediated hepatitis. In contrast, Pioglitazone-induced PPARγ activation exacerbated D-galactosamine (GalN)/lipopolysaccharide (LPS) hepatitis associated with an increased production of TNFα by Kupffer cells and increased sensitivity of hepatocytes towards TNFα after in vivo Pioglitazone administration. These results unravel liver-specific effects of Pioglitazone that fail to attenuate liver inflammation but rather exacerbate liver injury in an experimental hepatitis model