Colorectal carcinoma is one of the most frequent occurring cancers in the world with increased mortality due to liver and lung metastasis. Changes in the tumor microenvironment increase cell aggressiveness and immune evasion that promote tumor progression. Inflammation and hypoxia are key factors that modulate the tumor microenvironment. Inflammation recruits and stimulate immune cells that influence tumor development. Hypoxia induces HIF stabilization, which mediates transcriptional gene activation required for angiogenesis, proliferation and invasiveness. We used a syngeneic orthotopic murine model of colorectal cancer to evaluate the role of an impaired tumor-specific hypoxic response (MC-38-HIF-1-KD) in leukocyte recruitment, angiogenesis and tumor progression.
HIF-1a-KD tumors were smaller with reduced infiltration of granulocytes but increased recruitment of inflammatory monocytes. HIF-1a-KD tumors were less hypoxic and had a stabilized and functional vasculature. RNA sequencing data of sorted tumor cells revealed downregulation of pro-angiogenic molecules (Cyr61 and COX-2) and upregulation of vessel-stabilizing factors (Pdgfa and Endostatin) in HIF-1a-KD cells.
TRAF6 was predicted to interact with HIF-1a in silico. Binding was confirmed by immunoprecipitation of either TRAF6 or HIF-1a. Role of TRAF6 in tumor progression was evaluated in vivo. MC-38-TRAF6-KD cells showed a reduced metastatic potential and rarely grew in orthotopic model. In subcutaneous model, LPS stimulation impaired TRAF6-KD tumor development by reducing cell proliferation and increasing apoptosis. TRAF6-KD tumors had increased infiltration of myeloid cells with anti-tumorigenic phenotype that suggests an intact tumor-specific inflammatory response is required for modulation of the tumor microenvironment.