Biological role of the proapoptotic transcription factor C/EBP homologous protein (CHOP) in Idiopathic Pulmonary Fibrosis (IPF)

Introduction: We have recently identified severe and pro-apoptotic Endoplasmic Reticulum (ER) stress as major pathomechanism for alveolar epithelial cell (AEC) injury in Idiopathic Pulmonary Fibrosis (IPF). In line with this, the pro-apoptotic transcription factor CHOP, which functions as a crucial mediator of ER stress-induced apoptosis, was strongly induced in AECII from patients with IPF. We therefore aimed to fully disclose the transcriptional regulation and biological role of epithelial CHOP expression in AECII-like cells in vitro, and in pulmonary fibrosis in vivo. Methods: We performed promotor analysis of the human CHOP gene in silico and transiently overexpressed FLAG- and c-myc-tagged CHOP constructs in alveolar epithelial (and AECII-like) cell lines A549 (human) and MLE12/15 (murine), followed by the assessment of CHOP expression and post-translational modification and regulation of apoptosis. Results: The in silico-analysis of the 2.7-kb 5'-flanking region of the human CHOP gene revealed approximately 15 previously unknown different putative transcription factor binding sites, in addition to the already known ER stress-response elements (ERSE) and Amino-Acid-response elements (AARE). CHOP could be successfully overexpressed in MLE12, MLE15 and A549 cells, with a maximum of protein expression 48–72 hours after transfection. Parallel to the expression of CHOP, caspase-3 activation and hence induction of apoptosis could be encountered in these cell lines. Moreover, overexpression studies suggest the presence of peptide bound post-translational modifications such as mono-ubiquitination or sumoylation in CHOP proteins. Conclusion: Based on the data generated herein, we expect to be able to develop novel, AECII specific, antiapoptotic treatment strategies for treatment of IPF.