Objective: Macrophage activation contributes importantly to the pathogenesis of inflammatory diseases including atherosclerosis. Macrophages exist chronically under moderate hypoxia (2% to 5% O(2)) in inflamed tissues such as atherosclerotic plaques. However, macrophage phenotypes in such environments remain incompletely understood. This study tested the hypothesis that chronic moderate hypoxia induces macrophage activation and explored the underlying mechanisms.
Methods and results: We cultured primary human macrophages derived from peripheral blood monocytes in moderate hypoxia (2% O(2) tension) or normoxia (21% O(2)) for 10 days. Moderate hypoxia did not affect macrophage differentiation assessed via expression levels of scavenger receptor A. Chronic moderate hypoxia, but not normoxia, activated Akt and inactivated GSK-3beta, a negative effector of Akt, thus allowing nuclear translocation of beta-catenin. 2% O(2) tension increased accumulation of hypoxia-inducible factors 1 alpha (HIF-1 alpha) transiently at 3 to 5 days. Hypoxia induced mRNA expression of the beta-catenin-associated genes: MMP-7, CD44, and c-Myc. RNAi of TCF7L2, a cofactor of beta-catenin, suppressed MMP-7 expression induced by hypoxia. Inhibition of Akt phosphorylation with LY294002 abolished hypoxia-induced GSK-3beta inactivation, beta-catenin activation, and MMP-7 expression. Macrophages under hypoxia were more resistant for oxLDL-induced apoptosis. Moreover, phospho-Akt colocalized with MMP-7 and CD44 expression in macrophages of human atherosclerotic plaques.
Conclusions: Chronic moderate hypoxia induces macrophage activation via the Akt and beta-catenin pathways, providing new insight into the pathogenesis of inflammatory diseases.