Cathepsins as effector proteases in hepatocyte apoptosis

Conclusion

Cathepsins B and D appear to act as part of the effector protease cascade in hepatocyte apoptosis, both in bile salt-induced apoptosis and CPT-induced apoptosis of hepatocellular cancer cell lines. It is important to note that these proteases do not appear to participate in many models of apoptosis studied to date; in fact, cathepsin inhibitors have been used as negative controls to show that enzymes other than caspases are not involved in apoptosis. In particular, it has been shown that cathepsin B inhibitors do not prevent many models of apoptosis in lymphocytes (43). Further, our experiments have shown that not all models of hepatocyte apoptosis are mediated by cathepsins. For example, staurosporine-induced apoptosis is not inhibited by cathepsin B inhibitors in primary hepatocytes or in cell lines stably transfected with the cathepsin B antisense construct. Although the signaling pathways leading to activation of cathepsins B and D in hepatocyte apoptosis are not completely understood, we hypothesize that a caspase 8-like protein may be involved proximal to cathepsins D and B (Fig. 6). The precise mechanism by which cathepsin B is translocated from lysosomes to “apoptotic targets” is currently under investigation in our laboratory. Because of the relative promiscuity of cathepsin B as protease, it is likely that it is involved in nonspecific protein degradation in apoptotic bodies; however, cathepsin B has also been shown to degrade certain specific proteins, such as histones, which may be directly relevant to the apoptotic process. Further evaluation of the role of cathepsins B and D in apoptosis should include the determination of specific proteolytic targets that result in the biochemical and morphologic manifestations of apoptosis.