Hypoxia-Inducible Factor (HIF) 1α Accumulation and HIF Target Gene Expression Are Impaired after Induction of Endotoxin Tolerance

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-1 alpha accumulation and expression of HIF target genes in human monocytic cells and primary mouse peritoneal macrophages. Tolerant cells had reduced levels of HIF-1 alpha under hypoxia, which was due to lowered levels of HIF-1 alpha mRNA. HIF-1 alpha expression is under control of NF-kappa B and increased DNA binding of the p52 subunit of NF-kappa B was found in tolerant cells. Knock down of p52 abolished the effects of endotoxin tolerance on HIF-1 alpha expression, which suggest a negative regulatory role of p52 on HIF-1 alpha 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-1 alpha protein accumulation and subsequent HIF target gene expression. We conclude that endotoxin tolerance impairs HIF-1 alpha induction which reduces the ability of monocytic cells to survive and function under hypoxic conditions. The Journal of Immunology, 2009, 182: 6470-6476.