Project Summary Gastrointestinal syndrome (GIS) remains a significant clinical problem with no effective treatment. Radiotherapy and chemotherapy lead to bleeding and cell death resulting in release of hemoglobin and/or free heme. Accumulating data suggest that free heme stimulates the production of reactive oxygen species (ROS) and acts as a pro-inflammatory danger-associated molecular pattern (DAMP) through its binding to Toll-Like Receptor 4 (TLR4). In this study, we will focus on understanding how free heme regulates inflammation through gene regulation in myeloid or epithelial cells in the gut in the GIS models. Our data indicate there are high numbers of infiltrating heme oxygenase-1 (HO-1)-positive macrophages (Mø) in colonic biopsies from patients after radiotherapy versus non-irradiated controls or in the ischemic intestine compared to matched normal tissues. Further, we showed that myeloid cell-specific deletion of HO-1 resulted in abnormal epithelial cell proliferation and increased DNA damage (phosphorylated histone H2AX (H2AXg)) in the intestine upon irradiation. We propose that removal of free heme by the heme scavenger, hemopexin (Hx), and/or by the activity of HO-1 (or biological products) may be potential therapeutic option for patients with GIS. Our new preliminary data showed heightened levels of colonic damage and inflammation in Hx-/- mice in response to phenylhydrazine-induced hemolysis. We also demonstrated that heme affects gene expression through binding to G-quadruplex (G4) secondary structures in genomic DNA, which are key regulators of genomic stability, transcription and replication. Our state-of-the-art experimental approaches will allow us to dissect functions of free heme in myeloid and colonic epithelial cells during treatment-induced inflammation and tissue damage. Moreover, we will assess the impact of the gut microbiota in the protective effects of HO-1/Hx against GIS. Specifically, in this proposal we intend to: 1. Characterize the role of free heme in colonic epithelial and myeloid cells in vitro. 2. Determine the role of myeloid cell-expressed HO-1 and heme-regulated gut microbiota diversity in the GIS models. 3. Assess the therapeutic potential of recombinant Hx in the GIS models. 4. Characterize the staining of HO-1, Hx, DNA damage and proliferation markers and G4 in biopsies from patients with GIS and correlate immunostaining with clinical parameters. In summary, this study will pursue the definition of a novel role and mechanisms by which free heme acts as a key player in regulating gene expression upon tissues injury and gut inflammation such as observed in patients with GIS and/or colitis. This study will not only have high impact on the field of heme biology but will also contribute to developing much needed treatment strategies for GIS patients.