Calpain‐2 protects against heat stress‐induced cardiomyocyte apoptosis and heart dysfunction by blocking p38 mitogen‐activated protein kinase activation

Abstract Cardiovascular dysfunction is a common complication among heatstroke patients, but its underlying mechanism is unclear. This study was designed to investigate the role of calpain‐2 and its downstream signal pathway in heat stress‐induced cardiomyocyte apoptosis and heart dysfunction. In cultured primary mouse neonatal cardiomyocytes (MNCs), heat stress (43°C for 2 hr) induced a heat‐shock response, as indicated by upregulated heat‐shock protein 27 (HSP27) expression and cellular apoptosis, as indicated by increased caspase‐3 activity, DNA fragmentation and decreased cell viability. Meanwhile, heat stress decreased calpain activity, which was accompanied by downregulated calpain‐2 expression and increased phosphorylation of p38, extraceIIuIar signaI‐reguIated protein kinase (ERK1/2) and c‐Jun N‐terminaI kinase (JNK). Calpain‐2 overexpression abrogated heat stress‐induced apoptosis and phosphorylation of p38 and JNK, but not of ERK1/2. Blocking only p38 prevented heat stress‐induced apoptosis in MNCs. In cardiac‐specific calpain‐2 overexpressing transgenic mice, p38 phosphorylation and cardiomyocyte apoptosis were decreased in the heart tissue of heatstroke mice, as revealed by western blot and terminal deoxynucleotidyl transferase dUTP nick end labelling assays, respectively. M‐mode echocardiography also demonstrated that calpain‐2 overexpression significantly improved heatstroke‐induced decreases in ventricular end‐diastolic volume and cardiac output. In conclusion, our study suggests that heat stress reduces calpain‐2 expression, which then activates p38, leading to cardiomyocyte apoptosis and heart dysfunction.