Jak-STAT signaling in liver disease and repair : a way of living

Signaling through the Jak-STAT pathway is initiated when an extracellular signaling protein binds to its corresponding transmembrane receptor. This leads to activation of the Jaks. The Jaks mediate phosphorylation at the specific receptor tyrosine residues, which then serve as docking sites for the STATs. Once recruited to the receptors, the STATs become phosphorylated by the Jaks. Activated STATs will dimerize, translocate to the nucleus and target gene promoters. STAT proteins display a wide range of functions in many biological aspects. Consequently they are involved in or mediating pathological processes. This thesis describes and discusses our research, performed to understand in more detail the role of Jak-STAT signaling in liver pathophysiology. Hepatitis C virus (HCV) is a major cause of chronic liver disease. It is believed that HCV proteins interfere with interferon alpha induced Jak-STAT signaling in order to escape the interferons induced antiviral state. We therefore analyzed interferon alpha induced Jak-STAT signaling in the presence of HCV viral proteins in in vivo and in vitro models to gain more insight in this interference, as well as to identify the viral protein(s) responsible for this interference. In chapterwe show inhibition of JakSTAT signaling in transgenic mice that express HCV proteins in their liver cells. The inhibition occurred in the nucleus and blocked binding of STATs to the promoters of interferon stimulated genes. This inhibition of interferon induced signaling resulted in an enhanced susceptibility of the HCV transgenic mice to LCMV infection and the development of severe hepatitis. The results described in chapter 3, show that the combined HCV structural proteins and the core protein alone partly inhibit interferon alpha induced Jak-STAT signaling, using a panel of tetracycline-regulated cell lines inducibly expressing individual HCV proteins or in different combinations. In cells expressing HCV nonstructural proteins, interferon alpha induced Jak-STAT signaling was not impaired. The last chapters of this thesis describe the generation of conditional active gainof-function models to study STAT signaling independent of natural ligands. Fusion proteins were constructed between STAT1, STAT3 or STAT5a and the modified ligand binding domain of the estrogen receptor (ER) and subsequently expressed in mouse embryonic fibroblasts. In addition several mutants of the fusion proteins were generated. The fusion protein between wild-type STAT3 and the ER (mSTAT3wt-ER) was shown to bind DNA and mimic STAT3 signaling upon activation by 4hydroxytamoxifen (synthetic steroid ligand) only (chapter 4). mSTAT3wt-ER and mSTAT5aR618K-rER (SH-2 domain mutant) fusion protein constructs were used to generate liver-specific conditional active gain-of-function mouse models. Chapterdescribes the multistep cloning approach to prepare the constructs for injection and the analysis of the transgenic mice designated albSTAT3wt-ER and albSTAT5R618K-rER. The transgenic mice were designed to express the fusion proteins under control of the albumin promoter allowing liver-specific expression. Transgenic founders have been identified, as well as a transgenic F1 offspring. The transgenic mice appeared to be healthy and show a normal phenotype. Further characterization has to be completed. In chapterpossibilities for future use of the in chapterdescribed transgenic mice are discussed. The mice could be used to study STAT3 and STAT5a signaling in processes as liver cell regeneration, cellular transformation, bacterial infection, gluconeogenesis and cell survival.