Role of Caspases-9 and -4 in Glycochenodeoxycholic Acid-induced Apoptosis in HepG2 Cells

Abstract

It is believed that hepatocellular damage in human chronic cholestatic liver diseases is caused by the accumulation of hydrophobic bile acids, such as glycochenodeoxycholic acid(GCDCA). Previous reports have shown that GCDCA-induced apoptosis is promoted by both mitochondria-mediated and endoplasmic reticulum(ER)stress-associated pathways in HepG2 cells and that these two pathways are linked by the action of caspase-8 on BAP31. However, the interdependence of multiple pathways remains poorly understood and the role of caspases is unclear. Thus, in the present study, we investigated the interactions among the executioner caspases in GCDCA-induced apoptotic HepG2 cells. Cells were treated for 1-24 h with GCDCA(300μM), in the presence or absence of inhibitors of caspase-9, -4 or -3(each at 30μM). Pretreatment of cells with the caspase-9 inhibitor significantly suppressed GCDCA-induced apoptosis ; however, pretreatment of cells with the caspase-4 inhibitor had no effect. Furthermore, pretreatment of cells with the caspase-9 inhibitor significantly reduced GCDCA-induced increases in caspase-3 and -4 activity, as well as the mRNA expression of BiP, a molecular chaperone located in the ER. In contrast, pretreatment of cells with the caspase-4 inhibitor had no effect. These results suggest that, in GCDCA-treated HepG2 cells, caspase-4 acts downstream of both the proapoptotic Bcl-2 protein Bax and caspase-9. Because the major mechanism underlying GCDCA-induced apoptosis involves a mitochondria-mediated pathway, it is unlikely that caspase-4 has a major role in the initiation and promotion of GCDCA-induced apoptosis.