B8_Transcription factors

Institute for Cell Biology (University of Tuebingen)
Prof. Dr. Alfred Nordheim (PL), Stefan Ohrnberger (PhD student)

DKFZ (Heidelberg)
Prof. Dr. Günther Schütz (PL), Tabea Arnsperger (TA)

Center for Data Analysis and Modelling (University of Freiburg)
Prof. Dr. Jens Timmer (PL), Dr. Kilian Bartholomé (PhD)

Project summary:

We identified by molecular genetic means the transcriptions factors GR and SRF as two new essential regulatory proteins of hepatocyte function. Combined, GR and SRF represent transcriptional control modules that direct mRNA expression profiles in response to at least four signaling pathways, namely signaling by steroids, Stat5-, MAPK, and RhoA/actin. As already initiated with some of the genetically deficient hepatocytes, genome-wide mRNA expression profiling begins to uncover previously unrecognized cross-talk between GR and Stat5 signaling. Altered mRNA expression profiles in GR-, Stat-5, and SRF-deficient adult hepatocytes will be investigated in normal liver physiology and during liver regeneration. Obtained gene profiles and signal cascade activation states will be modeled over time and differing physiological states (e. g. liver regeneration) to obtain new systems biological insight into hepatocyte function. Glucocorticoid receptor (GR): Our major focus is to analyze how steroid hormones and their receptors control gene expression in both liver development and adult hepatocyte physiology. GR regulates transcription through DNA binding and crosstalk with other transcription factors, e.g. Stat5. We found that GR activity in hepatocytes is essential for normal postnatal growth of mice (Tronche et al. 2004). Immunoprecipitation (IP) and chromatin-IP of GR suggested that the effect of the receptor is due to its interaction with Stat5, which is a critical mediator of growth hormone (GH) signaling. This reveals an important role of Stat5-GR interactions in vivo, specifically in controlling postnatal body growth. Interestingly, hepatocytespecific Stat5 null mice display a similar growth deficiency as GR null mice. To investigate the importance of GR/Stat5 interactions in hepatocyte-specific transcription, we compared the mRNA expression profiles in wildtype versus GR- or Stat5-deficient hepatocytes. Interestingly, we uncovered that a large proportion (about 1/3) of the GR and Stat5 target genes are shared (unpublished), suggesting that coactivation of Stat5 by GR is a common mode of both glucocorticoid action and GH-dependent growth regulation. Serum response factor (SRF): SRF-dependent target genes are subject to regulation by two types of intracellular signaling cascade, namely signaling by MAPK and by Rho-dependent actin dynamics. Our previous work suggests SRF to have an essential role in hepatocytes during embryonal liver development. This observation fits our prior understanding of SRF controlling migration and survival of embryonic cells. It remains to be investigated at what stage in embryonic liver development and for what molecular (or cellular) reason hepatocyte-specific SRF-deficiency leads to embryonic lethality. Also, the role of SRF in adult hepatocyte physiology needs to be investigated, specifically with regard to the process of liver regeneration. This study will reveal the relevance of SRF-directed MAPK signaling and RhoA/actin signaling, both during liver development and adult liver growth and regeneration.