β₂-integrins control HIF1α activation in human neutrophils

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Human neutrophils migrate to inflammatory sites using β2-integrins under low oxygen and high cytokine conditions.
• The hypoxia-inducible factor (HIF) pathway plays a crucial role in cellular adaptation to low oxygen environments.

Purpose of the Study:

• To investigate the cooperative roles of prolyl hydroxylase domain-containing enzymes (PHDs) inhibition, cytokines, and β2-integrins in HIF pathway activation in neutrophils.
• To elucidate the mechanism by which β2-integrins regulate HIF-1α stabilization and its impact on neutrophil apoptosis.

Main Methods:

• Utilized roxadustat (a PHD inhibitor) and normobaric hypoxia to induce pseudohypoxia and hypoxia, respectively.
• Examined HIF-1α protein and mRNA levels in adherent and suspended neutrophils under various conditions, including inflammatory mediator stimulation and β2-integrin blockade/activation.
• Investigated downstream signaling pathways, including JAK2-STAT3 and translation initiation factors (eIF4E, 4EBP1).
• Assessed neutrophil apoptosis using fibronectin adhesion assays and pharmacological HIF-1α inhibition.

Main Results:

• HIF-1α protein accumulated in adherent neutrophils under pseudohypoxic and hypoxic conditions, with additive/synergistic effects from inflammatory mediators like GM-CSF.
• HIF-1α stabilization was dependent on β2-integrins, both in adherent neutrophils and when activated in suspended neutrophils.
• β2-integrins mediated HIF-1α mRNA translation via phosphorylation of eIF4E and 4EBP1, a process necessary but not sufficient for protein upregulation.
• HIF-1α stabilization delayed apoptosis in adherent neutrophils under low serum conditions, an effect reversed by pharmacological HIF-1α inhibition.

Conclusions:

• A novel β2-integrin-dependent mechanism for HIF-1α stabilization in human neutrophils has been identified.
• This mechanism restricts HIF-1α activation to neutrophils migrating towards inflammatory sites, linking integrin engagement to hypoxia response.
• The findings highlight the importance of the HIF pathway for neutrophil survival in inflammatory microenvironments.