Search

Inflammatory bowel disease such as chronic colitis promotes colorectal cancer, which is a common cause of cancer mortality worldwide. Hypoxia is a characteristic of inflammation as well as of solid tumors and enforces a gene expression response controlled by hypoxia-inducible factors (HIFs). Once established, solid tumors are immunosuppressive to escape their abatement through immune cells. Although HIF activity is known to 1) promote cancer development and 2) drive tumor immune suppression through the secretion of adenosine, both prolyl hydroxylases and an asparaginyl hydroxylase termed factor-inhibiting HIF (FIH) negatively regulate HIF. Thus, FIH may act as a tumor suppressor in colorectal cancer development. In this study, we examined the role of colon epithelial FIH in a mouse model of colitis-induced colorectal cancer. We recapitulated colitis-associated colorectal cancer development in mice using the azoxymethane/dextran sodium sulfate model in Vil1-Cre/FIH +f/+f and wild-type siblings. Colon samples were analyzed regarding RNA and protein expression and histology. Vil1-Cre/FIH +f/+f mice showed a less severe colitis progress compared with FIH +f/+f animals and a lower number of infiltrating macrophages in the inflamed tissue. RNA sequencing analyses of colon tissue revealed a lower expression of genes associated with the immune response in Vil1-Cre/FIH +f/+f mice. However, tumor occurrence did not significantly differ between Vil1-Cre/FIH +f/+f and wild-type mice. Thus, FIH knockout in colon epithelial cells did not modulate colorectal cancer development but reduced the inflammatory response in chronic colitis., (Copyright © 2022 by The American Association of Immunologists, Inc.)

Myeloid cells recruited to sites of bacterial inflammation are exposed to low oxygen tension, hypoxia, and high concentrations of inflammatory cytokines that significantly affect myeloid cell function. Therefore, we analyzed the direct consequences of acute and severe hypoxia on monocytic adhesion to the endothelium in coculture experiments. Marked upregulation of monocytic ICAM-1, but no other monocytic adhesion molecule, was responsible for an approximately 50-fold increase in adhesion of the monocytic cells THP-1 to human and rat endothelial cells. ICAM-1 expression was rapidly induced after the onset of severe hypoxia, but it decreased after 4 h. Knockdown of ICAM-1 by siRNA in endothelial and monocytic cells abolished the adhesion, indicating that ICAM-1 expression on both cell types was indispensable for hypoxia-induced adhesion of monocytes to the endothelium. siRNA-mediated knockdown of hypoxia inducible factor (HIF)-1alpha, HIF-2alpha, and the NF-kappaB family member p65 revealed that hypoxic upregulation of ICAM-1 resulted from hypoxic NF-kappaB induction but not from activation of HIFs. Within the leukocyte-adhesion cascade, our results provide evidence for prolyl hydroxylase-dependent but HIF-independent activation of hypoxia-induced monocyte-endothelial adhesion and assign a new function to monocytic ICAM-1 under acute hypoxic conditions.

The oxygen-sensitive transcription factor hypoxia-inducible factor 1 (HIF-1) is known as the key regulator of hypoxia-induced gene expression. In addition to hypoxia, endotoxins such as bacterial LPS as well as proinflammatory cytokines have been shown to induce HIF-1, suggesting an integrative role for HIF-1 in conditions of hypoxia and inflammation. Cells can become tolerant to endotoxins by repetitive exposure to LPS. Herein, we studied the effect of endotoxin tolerance on HIF-1alpha accumulation and expression of HIF target genes in human monocytic cells and primary mouse peritoneal macrophages. Tolerant cells had reduced levels of HIF-1alpha under hypoxia, which was due to lowered levels of HIF-1alpha mRNA. HIF-1alpha expression is under control of NF-kappaB and increased DNA binding of the p52 subunit of NF-kappaB was found in tolerant cells. Knock down of p52 abolished the effects of endotoxin tolerance on HIF-1alpha expression, which suggest a negative regulatory role of p52 on HIF-1alpha transcription during endotoxin tolerance. Endotoxin tolerant cells showed diminished expression of the HIF target genes phosphoglycerate kinase 1 and adrenomedullin and reduced viability under hypoxic conditions, as well as a significantly reduced invasion. Peritoneal macrophages from endotoxin-tolerant mice made showed significantly reduced HIF-1alpha protein accumulation and subsequent HIF target gene expression. We conclude that endotoxin tolerance impairs HIF-1alpha induction which reduces the ability of monocytic cells to survive and function under hypoxic conditions.