[Hypoxic preconditioning induces endoplasmic reticulum stress-related cardioprotection mediated by p38...

Calreticulin (CRT), an important Ca(2+)-binding molecular chaperone in the endoplasmic reticulum (ER), and caspase-12, a pivotal molecule mediating ER-initiated apoptosis, are involved in the ER stress (ERS). Using primary cultured neonatal cardiomyocytes, CRT and caspase-12 expression and activation during hypoxic preconditioning (HPC) and hypoxia/reoxygenation (H/R) were studied to explore the role of ERS in cardioprotection by HPC. And by using SB203580 and SP600125 [the specific inhibitors of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK)] separately, the role of p38 MAPK in HPC-induced ERS was also detected. Neonatal cardiomyocytes were prepared from Sprague-Dawley rats aged 24 h, and cultured in DMEM medium containing 10% fetal bovine serum, and then randomly divided into six groups as follows: H/R, HPC+H/R, SB203580+HPC+H/R, SP600125+HPC+H/R, HPC and control groups. H/R was produced by 2-hour hypoxia/14-hour reoxygenation, and HPC by 20-minute hypoxia/24-hour reoxygenation. Morphological studies, estimation of lactate dehydrogenase (LDH) leakage and flow cytometry were employed to assess cell apoptosis and necrosis. CRT and caspase-12 expression and activation, levels of phospho-p38 MAPK and phospho-JNK were detected by Western blot. All experiments were repeated at least four separate times. The results obtained are as follows: (1) HPC relieved the cell injury caused by H/R. Compared with that in H/R group, cellso survival rate in HPC+H/R group increased by 6.4%, and the apoptosis rate and LDH leakage in the cell culture medium decreased by 6.6% and 70.0%, respectively. (2) H/R induced caspase-12 activation (33.2-fold increase in comparison with control) and CRT expression (8.1-fold increase in comparison with control). HPC itself resulted in mild CRT up-regulation (2.6-fold increase in comparison with control), but the extent of up-regulation was lower than that induced by H/R. HPC before H/R was found to relieve the over-expression of CRT induced by H/R (72.4% decrease), and to inhibit the activation of caspase-12 (59.6% decrease). (3) The protection of HPC and HPC-induced up-expression of CRT and inhibition of caspase-12 activation were almost eliminated when the inhibitor of p38 MAPK, not of JNK, was present before HPC. These results suggest that HPC protects the neonatal cardiomyocytes from severe ERS-induced apoptosis during sustained H/R through pre-invoking proper ERS response. Mild up-expression of CRT and inhibition of caspase-12 activation induced by HPC, which are important protection factors, are mediated by p38 MAPK, not by JNK.