Regulation of hypoxia-inducible factors during inflammation.

The microenvironment of inflamed and injured tissue is characterized by low levels of oxygen and glucose and high levels of inflammatory cytokines, reactive oxygen, and nitrogen species and metabolites. The transcription factor complex hypoxia-inducible factor (HIF)-1 is regulated by hypoxia as well as by a broad variety of inflammatory mediators. In cells of the innate and adaptive immune system, HIF-1 is upregulated by bacterial and viral compounds, even under normoxic conditions. This upregulation prepares these cells to migrate to and to function in hypoxic and inflamed tissues. Once extravasated from the vasculature, the activity of cells is further enhanced by stimulation of HIF-1 by proinflammatory cytokines like interleukin (IL)-1beta (beta) and tumor necrosis factor (TNF) alpha (alpha), and locally expressed tissue factors. Crosstalk between hypoxic induction of HIF-1 and other signaling pathways activated by inflammation ensures a cell type-specific and stimulus-adequate cellular response. Prolonged activation of HIF-1 under conditions of inflammation, however, may contribute to the survival of damaged tissue and cells, thus promoting the development of tumors.