Hypoxia Is Present in Murine Atherosclerotic Plaques and Has Multiple Adverse Effects on Macrophage Lipid Metabolism

Abstract

Rationale: Human atherosclerotic plaques contain large numbers of cells deprived of O 2 . In murine atherosclerosis, because the plaques are small, it is controversial whether hypoxia can occur. Objective: To examine if murine plaques contain hypoxic cells, and whether hypoxia regulates changes in cellular lipid metabolism and gene expression in macrophages. Methods and Results: Aortic plaques from apolipoprotein-E–deficient mice were immunopositive for hypoxia-inducible transcription factor (HIF-1α) and some of its downstream targets. Murine J774 macrophages rendered hypoxic demonstrated significant increases in cellular sterol and triglycerides. The increase in sterol content in hypoxic macrophages correlated with elevated 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase activity and mRNA levels. In addition, when macrophages were incubated with cholesterol complexes, hypoxic cells accumulated 120% more cholesterol, predominately in the free form. Cholesterol-efflux assays showed that hypoxia significantly decreased efflux mediated by ATP-binding cassette subfamily A member 1 (ABCA1), whose sub cellular localization was altered in both J774 and primary macrophages. Furthermore, in vivo expression patterns of selected genes from cells in hypoxic regions of murine plaques were similar to those from J774 and primary macrophages incubated in hypoxia. The hypoxia-induced accumulation of sterol and decreased cholesterol efflux was substantially reversed in vitro by reducing the expression of the hypoxia-inducible transcription factor, HIF-1α. Conclusion: Hypoxic regions are present in murine plaques. Hypoxic macrophages have increased sterol content due to the induction of sterol synthesis and the suppression of cholesterol efflux, effects that are in part mediated by HIF-1α.