CCAAT -binding factor (CBF) function in endoplasmic reticulum stress response
Perturbations that alter Endoplasmic Reticulum (ER) homeostasis can lead to the accumulation of unfolded proteins, which is called ER stress. A variety of physiological and environmental stress conditions can cause accumulation of unfolded proteins in the ER. The hepatocytes in liver contain abundant ER. A broad spectrum of insults in liver can lead to activation of the UPR pathway in hepatocytes. Prolonged activation of UPR in hepatocytes is an underlying cause of liver injury under various pathological conditions. Many ER stress response gene promoters contain multiple CBF binding sites. ATF6, an ER stress inducible transcription factor, can activate a subset of ER stress response promoter, and the binding of ATF6 to the promoter DNA is CBF-dependent. We hypothesize that CBF plays an important role in the regulation of ER stress response gene expression. The following experiments were done to test the hypothesis. To inhibit CBF activity in cultured cells, a dominant negative CBF-B (B-Mut) was expressed in HeLa cells. The results showed that inhibition of CBF activity dramatically reduced the transcription of many ER stress response genes. Chromatin immunoprecipitation (CHIP) demonstrated that loss of CBF binding resulted in inhibition of both ATF6 and TBP binding to the cellular ER stress-regulated promoters but did not change two chromatin modifications such as AcH3K9 and Tri-McH3K4. To determine the role of CBF in ER stress response pathway in hepatocytes, a conditional knockout system was used to inactivate CBF-B gene in mouse liver. We found that CBF-B knockout mice died around 4-6 weeks and had abnormal livers. The ER stress was induced in CBF-B knockout mouse liver. Expression of the major ER stress marker gene GRP78 was greatly upregulated. However, expression of one folding enzyme ERP72 and PDI were downregulated when CBF-B gene was inactivated, which might perturb ER function and alter ER homeostasis. Our results suggest that CBF is also required for normal ER function in mouse liver in vivo.
Luo, Rong, "CCAAT -binding factor (CBF) function in endoplasmic reticulum stress response" (2007). Texas Medical Center Dissertations (via ProQuest). AAI3287849.