Endotoxin provokes cardiac dysfunction, and induction of tolerance to endotoxin has therapeutic potential. Heat shock protein 70 (HSP70) can induce endotoxin tolerance in macrophages. We recently found that heat shock cognate protein 70 (HSC70) induces pro-inflammatory cytokines via activation of TLR4 in macrophages and the myocardium. We hypothesize that HSC70 preconditioning induces cardiac tolerance to endotoxin. Pretreatment of peritoneal macrophages with HSC70 for 24h reduced TNF-alpha levels following endotoxin stimulation. Preconditioning of mice with HSC70 24h prior to endotoxin attenuated endotoxemic cardiac dysfunction. HSC70 preconditioning reduced TNF-alpha levels in plasma and heart tissue by 33.3% and 35.4%, respectively, and decreased ICAM-1 levels in heart tissue by 63.5% following endotoxin challenge. The effect of HSC70 on TNF-alpha was less robust than endotoxin preconditioning (79.7% and 75.0% reduction in TNF-alpha levels in plasma and heart tissue, respectively); however, HSC70 and endotoxin preconditioning had comparable effects on ICAM-1 levels in heart tissue. While HSC70 preconditioning had no effect on myocardial TLR4 protein levels, it suppressed NF-kappaB activation induced by endotoxin. We conclude that HSC70 preconditioning (1) attenuates the TNF-alpha response to endotoxin in macrophages in vitro, (2) induces cardiac functional tolerance to endotoxin and (3) reduces NF-kappaB activity, and TNF-alpha and ICAM-1 levels in heart tissue. Thus, the mechanism of HSC70-induced cardiac tolerance to endotoxin appears to involve down-regulation of myocardial TLR4 signaling and inflammatory responses.
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