# HIF-1α as a Central Regulator of Monocyte Responses to Hypoxia

## TL;DR

This paper reviews how HIF-1α regulates monocyte responses to low oxygen, impacting inflammation and tissue health.

## Contribution

The paper provides a comprehensive review of HIF-1α mechanisms in monocytes during hypoxia, highlighting differences from macrophages.

## Key findings

- Acute hypoxia stabilizes HIF-1α in monocytes through post-translational and calcium-dependent mechanisms.
- HIF-1α interacts with NF-κB to modulate inflammation and cell survival under hypoxia.
- Epigenetic factors influence HIF-1α activity and trained immunity in hypoxic conditions.

## Abstract

Hypoxia is a condition in which cells and tissues experience a decrease in molecular oxygen levels. Hypoxia causes cells to reorganize metabolically and transcriptionally to adapt to the new stressful situation. Blood monocytes are sentinel cells of our immune system that respond early to hypoxia. A key regulator of this adaptation is hypoxia-inducible factor-1α (HIF-1α), a protein that controls genes involved in metabolism, inflammation, and cell survival. The strategies monocytes adopt allow for the restoration of homeostasis as soon as the hypoxic condition ends. Conversely, if hypoxia persists for a long time or if monocytes fail to adapt, a pathological condition can develop.

Hypoxia is a common feature of inflamed and ischemic tissues and represents an important regulatory signal for innate immune cells. The master regulator of this response is hypoxia-inducible factor-1α (HIF-1α), a transcription factor whose stabilization and activity are tightly regulated by the presence of oxygen, inflammatory signaling, and cellular metabolism. Monocytes, key players in innate immunity, rapidly sense oxygen deprivation and display specific responses during acute hypoxia, primarily aimed at adapting and maintaining cellular homeostasis. Unlike macrophages, in which HIF-1α activity is known, the mechanisms regulating HIF-1α stabilization, subcellular localization, and transcriptional activity in circulating monocytes remain incompletely elucidated. Recent studies indicate that acute hypoxia primarily triggers post-translational stabilization of HIF-1α, calcium- and PKC-dependent signaling, metabolic reprogramming, and early inflammatory responses, while transcriptional activation of HIF-1α may require additional inflammatory or stress-related signals. Furthermore, extensive crosstalk between HIF-1α and NF-κB integrates hypoxic and inflammatory signals, modulating cytokine production, cell migration, and survival. Epigenetic regulators can also modulate these responses and contribute to hypoxia-induced trained immunity. In this review, we summarize current knowledge of the mechanisms controlling the stabilization, localization, and function of HIF-1α in human monocytes and monocyte–macrophages during acute hypoxia, highlighting the key differences between these cell types and discussing their implications for inflammation, tissue homeostasis, and disease.