HSP25 Vaccination Attenuates Atherogenesis via Upregulation of LDLR Expression, Lowering of PCSK9 Levels and Curbing of Inflammation

Abstract

Objective: Elevated HSP27 (heat shock protein 27) levels predict relative freedom from cardiovascular events. Over-expression or twice daily subcutaneous injections of human HSP27 in ApoE −/− mice reduces blood and plaque cholesterol levels, as well as inflammation and atherosclerotic plaque burden. Antibodies to HSP27 are present in human blood, and the purpose of the current studies is to explore their role. Approach and Results: Blood levels of both HSP27 and anti-HSP27 IgG antibodies are elevated in healthy controls compared with patients with cardiovascular disease. ApoE −/− mice fed a high-fat diet and vaccinated with recombinant HSP25 (rHSP25, murine ortholog) show increased levels of anti-HSP25 IgG antibodies and reductions in plasma cholesterol and atherogenesis. Moreover, rHSP25 vaccination markedly lowered serum amyloid A levels as well as hepatic macrophage abundance and inflammatory cytokine expression. The effects of the HPS25 vaccination on cholesterol metabolism are divergent: increased hepatic LDLR (low-density lipoprotein receptor) mRNA and protein expression and reduced plasma PCSK9 (proprotein convertase subtilisin/kexin type 9) levels—despite no effect on PCSK9 expression. In vitro, the HSP27 immune complex upregulates hepatocyte LDLR mRNA and protein expression independent of intracellular cholesterol levels and increases LDLR promoter activity. The increase in LDLR expression by the HSP27 immune complex is dependent upon activation of the NF-κB (nuclear factor κ light chain enhancer of activated B cells) pathway. Hepatocyte PCSK9 protein levels are reduced after HSP27 immune complex treatment in vitro despite only minor transient effects on gene expression. Conclusions: HSP27 immunotherapy represents a novel means of lowering cholesterol and PCSK9 levels, primarily due to augmentation of LDLR expression and is associated with marked reductions in inflammation.