Endoplasmic Reticulum (ER)-stress, the accumulation of misfolded proteins in the ER, is assumed to be the cause of several diseases. Recently, enough evidence accumulated to state that it also plays an important role in idiopathic pulmonary fibrosis (IPF). The presence of a maladaptive ER-stress in the alveolar epithelial type II cells (AECII) of patient´s lungs has been demonstrated and the ability of this mechanism to cause or enhance a fibrotic phenotype has been shown. The hypothesis that AECII apoptosis induced by ER-stress or alternative mechanisms is a main trigger of this disastrous disease is therefore widely accepted. Nevertheless it was a requirement to characterize the diverse pathways of ER stress response (the unfolded protein response, UPR) in the alveolar epithelium in detail. My studies focus on, but are not limited to, the effects of the UPR- transcription factors Atf4, Atf6 and Xbp1 in vitro in the AECII-derived murine cell line MLE12 and in vivo with conditional over-expression in transgenic mouse lungs of de novo generated lines. A broad analysis of the transgene-effect in vitro was assessed, based on microarray analysis of transfected cells, including miRNA analysis and 400K-Exonarrays, which provide first evidence that regulation of miRNAs and alternative splicing events are components of the UPR. Induction of cell death by the UPR-transcription factors was of special interest, because it is a proposed mechanism in the pathogenesis of IPF. However, no increased cell death or strong induction of the pro apoptotic mediator Chop (gene for the C/EBP Homologous Protein; CHOP) was observed, the protective effects seemed to dominate in the system. Atf4, Atf6 and Xbp1 are not sufficient to mimic or generate a maladaptive ER stress situation in these cells. In experiments with drug (Thapsigargin, Brefeldin A) induced ER-stress the induction of Chop was one of the earliest effects and occurred simultaneously with the induction of Atf4 and Xbp1(s). Microarrays, addressing the very early effects of Thapsigargin (TG) treatment, could reveal an even earlier transcriptional regulation. However, it could not be determined which factors make the difference between a good and a maladaptive ER-stress. Other potential impact of ER Stress on IPF resulting from the expression data was discussed.Transgenic mice for inducible over-expression of Atf4, Atf6 and Xbp1 were generated. The investigated lines did not develop any phenotype after transgene induction. This would fit to the in vitro finding, that the transgenes do not cause AECII death. However, the extend of the transgene induction in vivo has not yet been completely assessed.
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