DIFFERENTIAL DISTRIBUTION OF 70-KD HEAT-SHOCK PROTEIN IN ATHEROSCLEROSIS - ITS POTENTIAL ROLE IN ARTERIAL SMC SURVIVAL

Smooth muscle cell death may contribute to necrotic plaque rupture and subsequent thromboembolus. Stress-induced synthesis of heat-shock pro teins (HSPs) normally protects cells from death, but vascular HSPs may become insufficient as cytotoxicity increases in advanced plaques. To determine whether vascular HSP content is altered near necrosis, we c ompared 70-kD HSP (HSP70) distribution between fibrotic and necrotic p laques in immunostained carotid endarterectomy specimens. Average leve ls of HSP70 immunoreactivity were compared by video densitometry betwe en fibrotic and necrotic plaques or between their underlying media. Bo th necrotic plaques and their underlying media contained significantly more HSP70 staining than did fibrotic tissues. To test whether cellul ar HSP70 correlated with resistance to toxicity in vitro, aortic smoot h muscle cells (aSMCs) were heat shocked to induce endogenous HSPs or given 2 to 50 mu g/mL purified HSP70. Cells were then serum deprived o r exposed to 12 to 96 mu mol/L cholestanetriol (C3o1) or 25-hydroxycho lesterol, and survival was determined. Cellular HSP70 content was assa yed by immunoblotting, and protein synthesis was monitored by S-35 rad iolabeling. Serum deprivation inhibited general protein synthesis but induced HSP70; C3o1 exposure inhibited both overall protein and HSP70 synthesis, including post-heat shock. Induction of endogenous HSPs or 10 mu g/mL exogenous HSP70 improved viability of serum-deprived cells (P<.05 and P<.01, respectively), while only exogenous HSP70 protected against C3o1 (P<.002). The results suggest that insufficient HSP70 acc umulates in aSMCs residing near necrosis to protect against plaque tox icity; aSMC death might then occur, allowing resident macrophages to d egrade and destabilize the matrix, leading to rupture.