Acute Hypoxia Induces HIF-Independent Monocyte Adhesion to Endothelial Cells through Increased Intercellular Adhesion Molecule-1 Expression: The Role of Hypoxic Inhibition of Prolyl Hydroxylase Activity for the Induction of NF-κB

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.