Published online Apr 7, 2020. doi: 10.3748/wjg.v26.i13.1450
Peer-review started: December 2, 2019
First decision: December 12, 2019
Revised: February 20, 2020
Accepted: March 9, 2020
Article in press: March 9, 2020
Published online: April 7, 2020
Endoplasmic reticulum (ER) stress-mediated hepatocyte apoptosis is associated with many liver diseases, however, the underlying mechanisms remain unknown. Calpain-2 is a Ca2+-dependent cysteine protease, which can cleave several apoptosis-related proteins and mediate the process of apoptosis. Our data indicate that calpain-2 is crucial for the aberrant ER stress-induced apoptosis of hepatocytes and may be a novel therapeutic target for liver diseases.
Calpain-2 protease is involved in multiple signaling pathways mediating apoptotic processes, including the mitochondrial pathway. However, the regulatory mechanisms by which calpain-2 regulates ER stress-mediated hepatocyte apoptosis remain unclear. Our study further assessed its potential as a therapeutic target for inhibiting hepatocyte apoptosis.
In this study, the effect of calpain-2 on ER stress-mediated hepatocyte apoptosis and the underlying regulatory mechanisms were investigated. Our data indicates that ER stress may be a novel therapeutic target, and our findings provide new evidence to demonstrate the importance of Ca2+-dependent calpain-2 in caspase-12 activation and ER stress-related apoptosis in hepatocytes.
Our study employed Western blotting, flow cytometry, confocal microscopy, and calpain-2 small interfering RNA to elucidate the mechanism of calpain-2-mediated activation of caspase-12, a key molecule in ER stress-related apoptosis. These research methods are relatively mature technically, thus ensuring the reliability of the results from this study.
The ER stress inducer dithiothreitol can significantly increase intracellular Ca2+content, calpain-2 expression and activity in hepatocytes and promote caspase-12 cleavage and apoptosis in hepatocytes. Moreover, calpain-2 silencing can mitigate dithiothreitol-enhanced calpain-2 expression, caspase-12 cleavage, and apoptosis in hepatocytes. These results indicated that enhanced calpain-2 activity promoted aberrant ER stress-mediated apoptosis of hepatocytes. In future studies, we will further investigate whether calpain-2 can mediate hepatocyte death through other mechanisms, such as regulating autophagy.
Ca2+-dependent calpain-2 activity promoted the aberrant ER stress-related apoptosis of rat hepatocytes by activating caspase-12 in the ER. ER stress may be a novel therapeutic target and our findings may provide new evidence to demonstrate the importance of Ca2+-dependent calpain-2 in caspase-12 activation and ER stress-related apoptosis in hepatocytes. ER stress may induce Ca2+ release from the ER and lead to the recruitment and activation of calpain-2 in the ER, where, calpain-2 activates caspase-12 and caspase-3, and triggers apoptosis in hepatocytes. Calpain-2 activity plays an important role in aberrant ER stress-related apoptosis in hepatocytes. Inhibition of calpain-2 expression and activity is expected to be an effective way to alleviate hepatocyte apoptosis and liver injury.
Because calpain-2 is a protease, in addition to focusing on its expression, studies should also focus on its activity. In our recent preliminary studies, we found that treatment with Z-LLY-fmk, a specific inhibitor of calpain activity, significantly mitigated DTT-induced hepatocyte apoptosis by more than 90%.