Bile Acid-induced Apoptosis in Hepatocytes Is Caspase-6-dependent

Abstract

Apoptosis induced by hydrophobic bile acids is thought to contribute to liver injury during cholestasis. Caspase-6 is an executioner caspase that also appears to have regulatory functions in hematopoetic cell lines. We aimed to elucidate the role of caspase-6 in bile acid-induced apoptosis. The major human hydrophobic bile acid, glycochenodeoxycholic acid (GCDCA, 75 mol/liter), rapidly induced caspase-6 cleavage in HepG2-Ntcp human hepatoma cells. GCDCA-induced, but not tumor necrosis factor ␣-or etoposide-induced activation of effector caspases-3 and -7 was significantly reduced by 50% in caspase-6-deficient HepG2-Ntcp cells as well as in primary rat hepatocytes pretreated with a caspase-6 inhibitor. Inhibition of caspase-9 reduced GCDCA-induced activation of caspase-6, whereas inhibition of caspase-6 reduced activation of caspase-8 placing caspase-6 between caspase-9 and caspase-8. GCDCA also induced apoptosis in Fas-deficient Hep3B-Ntcp and HuH7-Ntcp hepatoma cells. In addition, GCDCA-induced apoptosis was reduced by 50% in FADD-deficient HepG2-Ntcp cells, whereas apoptosis induced by tumor necrosis factor ␣ was reduced by 90%. Collectively, these observations suggest that GCDCA can induce hepatocyte apoptosis in the absence of death receptor signaling, presumably by a compensatory mitochondrial pathway. In conclusion, caspase-6 appears to play an important regulatory role in the promotion of bile acid-induced apoptosis as part of a feedback loop.Cholestasis is a feature of many chronic human liver diseases like primary biliary cirrhosis (1). Hepatocyte damage by toxic bile acids is assumed to represent a key event for progression of cholestatic liver diseases (2). Toxic bile acids induce hepatocellular apoptosis, thereby providing a cellular mechanism for bile acid-mediated liver injury and fibrogenesis (3-6). The glycine and taurine conjugates of chenodeoxycholic acid (GCDCA, TCDCA) 2 are the predominant dihydroxy bile acids in cholestatic patients and have been held responsible for cholestasisassociated liver injury (7). GCDCA is thought to induce hepatocyte apoptosis by Fas death receptor-dependent signaling that is independent of Fas ligand (8). In a recently proposed model, activation of NADPH oxidase isoforms by GCDCA is followed by generation of reactive oxygen species, which results in epidermal growth factor receptor-dependent tyrosine phosphorylation of Fas (5, 9). GCDCA-induced oligomerization of Fas then causes activation of the initiator caspase-8, which requires the adaptor protein FADD. In hepatocytes, caspase-8 then would cause apoptosis via the mitochondrial pathway (10).In cell-free systems, however, toxic bile acids like GCDCA can directly cause mitochondrial damage leading to cytochrome c leakage, which is followed by the activation of caspase-9 via the apoptosome (11). GCDCA can dose-dependently induce the mitochondrial permeability transition in isolated human mitochondria (12). In addition, a marked increase in reactive oxygen species production was observed within minutes in the m...

Bile acids: regulation of apoptosis by ursodeoxycholic acid

“…Curiously, caspase-6 activation is less important in apoptosis induced by other stimuli in the same cells. Finally, although mitochondrial cytochrome c release is FADD/caspase-8 dependent during hepatocyte apoptosis, pro-apoptotic bile acids, at concentrations similar to those found in cholestatic liver injury, are still capable of inducing apoptosis by the intrinsic pathway when death receptor activation is inhibited ( 63 ). Recently, apoptosis via the endoplasmic reticulum (ER) stress-mediated pathway was also found in GCDCA-induced apoptosis of liver cells, although to a lesser extent ( 64,65 ).…”

“…Novel fi ndings in the fi eld point to a critical role of caspase-6 in bile-acid-mediated apoptosis of human liver cell lines and primary rat hepatocytes ( 63 ). Caspase-6, generally considered an executioner caspase, has recently been identifi ed as a potential activator of caspase-8 in models of bile-acid-induced apoptosis.…”

Caspase-6 activity in the CA1 region of the hippocampus induces age-dependent memory impairment

“…Unlike the other effector caspases, Casp3 and Casp7, active Casp6 does not directly induce apoptotic cell death. 1,22,23 However, in many cell types, Casp6 can activate Casp3-mediated apoptosis [24][25][26] and Casp3 can activate Casp6. 27 Casp6 cleaves Lamin A and is responsible for chromatin condensation in apoptotic cells.…”

Lack of Glycoprotein 130/Signal Transducer and Activator of Transcription 3-Mediated Signaling in Hepatocytes Enhances Chronic Liver Injury and Fibrosis Progression in a Model of Sclerosing Cholangitis

“…They have recently been shown to induce apoptosis and to antagonize protective IL-6 signals in hepatocytes. 24,25 In addition, the death ligand tumor necrosis factor-related apoptosis-inducing ligand and its receptor DR5 are known to be up-regulated during cholestasis. 26 Thus, this pathway could be directly involved in triggering apoptosis in the liver of DDCtreated animals, especially if antiapoptotic pathways are inhibited.…”

“…TNF-␣-dependent gene transcription via nuclear factor-B in hepatocytes and nonparenchymal cells plays an important role in triggering the expression of cytokines and chemokines but also the activation of hepatic stellate cells. 24,25 In gp130 ⌬hepa and gp130…”