Protective role of downregulated MLK3 in myocardial adaptation to chronic hypoxia.

A series of protective responses could be evoked to achieve compensatory adaptation once cardiomyocytes are subjected to chronic hypoxia. MLK3/JNK/c-jun signaling pathway was previously demonstrated to be involved in this process. In the present study, we aim to further examine the performance of MLK3 in hypoxic H9C2 cells and potential mechanism. Myocardial samples of patients with congenital heart disease (CHD) were collected. H9C2 cells were cultured in hypoxic conditions for various durations. MLK3 was silenced by transfection of shRNA to evaluate its role in cell viability. We found expression of MLK3 protein was lower in patients with cyanotic CHD. In hypoxic H9C2 cells, its expression was gradually decreased in a time-dependent manner. However, there was no significant difference about expression of MLK3 mRNA. According to the results of MTT, LDH, and TUNEL, faster cell growth curve, lower death rate, and less apoptotic cells could be observed in MLK-shRNA group compared with scramble-shRNA group. Silencing of MLK3 significantly reduced expression of cleaved caspase-3, cleaved PARP, Bad, and Bax, together with increased expression of Bcl-2 and ration of Bcl-2/Bax. Both ratio of phospho-JNK/total JNK and ratio of phospho-c-jun/total c-jun were significantly decreased once MLK3 was silenced. At various reoxygenation time, MLK3 shRNA could significantly promote cell survival and decrease cell death according to MTT and LDH. Our results suggested that chronic hypoxia could reduce MLK3 expression in a posttranscriptional regulatory manner. Downregulation of MLK3 protects H9C2 cells from hypoxia-induced apoptosis and H/R injury via blocking the activation of JNK and c-jun.