The influence of HIF-prolyl hydroxylase-2 during colitis and inflammation-associated colorectal carcinogenesis in mice
Hematology, Oncology
Final Report Abstract
An inappropriate low amount of O2 can be damaging for our cells and organs. Oxygen-sensing is therefore indispensable as it enables the cells to instantaneously adapt to an inappropriately low pO2 (=hypoxia). This machinery relies on the so-called oxygen sensors HIF-prolyl hydroxylases (PHDs 1-3), enzymes that need oxygen to initiate the inactivation of hypoxia inducible factor (HIF). The latter is a transcription factor, which is able to steer the expression of several hundreds of genes related to overcoming hypoxia, including erythropoietin (Epo), a hormone responsible for the production of red blood cells. PHD2 is considered to be the key oxygen sensor, and its function has been associated with several different physiological settings and in a variety of pathological disorders (reviewed by our group). Inflammatory bowel disease (IBD) is considered an inflammation-driven disorder leading to the destruction of both the structure and function of the gastrointestinal tract. Although not that many studies are available on the role of blood vessels in colon inflammation, endothelial dysfunction is considered one of the etiological factors of IBD. With this project, we unraveled the role of different hypoxia pathway proteins in a mouse model of IBD (dextran Sodium Sulfate (DSS)-induced Ulcerative colitis). In our work, we revealed that mice chronically overexpressing Epo (Epo Tg6) were much more susceptible to DSS compared to their WT littermates. Based on extensive in vivo and ex vivo approaches, we found that the dramatic inflammation and loss of the intestinal barrier in Epo Tg6 colons is clearly preceded by the enhanced proliferation and consequent leakiness of neighboring blood vessels; a phenotype that was underscored by an increase of angiogenic proteins in the colon tissue before any signs of the disease were evident. Interestingly, colon endothelial cells from untreated Epo Tg6 mice already displayed an increase of endothelial dysfunction markers, next to a dramatic loss of extracellular matrix (ECM) components and focal adhesion molecules, consequently resulting in a pro-inflammatory signature. Therefore, our data collectively demonstrate that continuous exposure to high levels of Epo trains the endothelium to obtain a pre-conditioned/inflammatory-like state, which boosts pathological angiogenesis very early in a model of IBD. Furthermore, we were able to direct this susceptibility to the binding of Epo to its Epo-receptor (EpoR) on endothelial cells, using mice lacking EpoR in their endothelium. Taken together, our data strongly suggest that excessive endothelial specific Epo/EpoR signaling plays a detrimental role in the sensitivity during IBD. To finalize this project, a profound study exploring the impact of a point-mutation in the promotor of the Epo-gene in a group of IBD patients is currently ongoing. We and others have already identified hypoxia pathway proteins as central players during different phases of tumor development. Unlike the majority of human cancers, PHD2 and 3 seem to be tumor suppressors in colorectal cancers, based on expression data from different databases. Within this project, we are the first to identify a dual-function for PHD2 during the initiation and propagation in a genetic mouse model of CRC. Initially, loss of PHD2 in colon epithelium inhibited adenoma formation, whereas the size of these tumors was bigger. Current research is ongoing to identify the molecular background of this phenomenon.
