We recently identified heat shock protein 27 (HSP27) as an estrogen receptor beta (ERbeta)-associated protein and noted its role as a biomarker for atherosclerosis. The current study tests the hypothesis that HSP27 is protective against the development of atherosclerosis. HSP27 overexpressing (HSP27o/e) mice were crossed to apoE-/- mice that develop atherosclerosis when fed a high-fat diet. Aortic en face analysis demonstrated a 35% reduction (P < or =0.001) in atherosclerotic lesion area in apoE-/-HSP27o/e mice compared to apoE-/- mice, but primarily in females. Serum -HSP27levels were 10-fold higher in female apoE-/-HSP27o/e mice compared to males, and there was a remarkable inverse correlation between circulating HSP27 levels and lesion area in both male and female mice (r(2)=0.78, P < or =0.001). Mechanistic in vitro studies showed upregulated HSP27 expression and secretion in macrophages treated with estrogen or acLDL. Moreover, exogenous HSP27 added to culture media inhibited macrophage acLDL uptake and competed for the scavenger receptor A (SR-A)--an effect that was abolished with the SR-A competitive ligand fucoidan and absent in macrophages from SR-A-/- mice. Furthermore, extracellular HSP27 decreased acLDL-induced release of the proinflammatory cytokine IL-1beta and increased the release of the antiinflammatory cytokine IL-10. HSP27 is atheroprotective, perhaps because of its ability to compete for the uptake of atherogenic lipids or attenuate inflammation.