Hepatitis B virus encoded X protein suppresses apoptosis by inhibition of the caspase-independent pathway.

Hepatitis B virus (HBV) encoded X protein (HBx) has been implicated in apoptotic and related pathogenic events during hepatocellular carcinoma. However, the underlying molecular mechanism through which HBx acts is largely unclear. We used tandem affinity purification under mild conditions to gain insight into the HBx interactome in HBV-producing HepG2.2.15 cells and identified 49 proteins by mass spectrometry that are potentially associated with HBx. Two of the key proteins of the caspase-independent apoptosis pathway were newly identified, apoptosis-inducing factor (AIF) and the homologous AMID (AIF-homologue mitochondrion-associated inducer of death). We confirmed the interactions of HBx with AIF and with AMID by reciprocal coimmunoprecipitation experiments, respectively. We observed the expression of HBx-reduced AIF-mediated apoptosis and HBx colocalization with AIF and AMID, principally in the cytoplasm. Furthermore, the elevated cytoplasmic levels of HBx could inhibit mitochondrion-to-nucleus translocation of AIF. Here, we present the first detailed molecular evidence that HBx can repress apoptosis via inhibition of the caspase-independent apoptosis pathway. This inhibition of apoptosis involves the repression of the mitochondrion-to-nucleus translocation of AIF, although tests with AMID were not conclusive. These findings provide important insights into the new mechanism of the apoptosis inhibition by HBV.

Dibenzylbutane- and butyrolactone-type lignans as apoptosis inducers in human hepatoma HuH-7 cells.

Seven lignans, previously isolated from Pycnanthus angolensis or obtained by derivatization, namely the dibenzylbutane-type lignans threo-4,4'-dihydroxy-3-methoxylignan (1), 4'-hydroxy-3,3',4-trimethoxylignan (2), (-)-dihydroguaiaretic acid (3), 3,3',4,4'-tetramethoxylignan (4), 4,4'-diacetyl-3,3'-dimethoxylignan (5), heliobuphthalmin (6) and the butyrolactone lignan hinokinin (7), were evaluted for their ability as apoptosis inducers in human hepatoma HuH-7 cells. Cell viability assays, morphological evaluation of apoptosis and enzymatic analyses of caspase activity in HuH-7 cells were carried out. Using the lactate dehydrogenase lactate dehydrogenase (LDH) assay, it was demonstrated that the lignans (1-7) tested significantly reduced viability of HuH-7 cells. Morphologic evaluation of HuH-7 cells using Hoechst staining and fluorescence microscopy revealed that lignans 1-7 were strong inducers of apoptosis. In fact, HuH-7 cells developed morphological changes of apoptosis, including chromatin condensation, nuclear fragmentation and formation of apoptotic bodies. However, lignans 2 and 7 were the most promising compounds in this study, inducing 2.4- and 2.5-fold increases in apoptotic cells as compared to controls. Caspase-3-like activity assays confirmed the morphologic data.

Cytotoxic and apoptosis-inducing activities of triterpene acids from Poria cocos.

Six lanostane-type triterpene acids (1a-6a), isolated from Poria cocos , and their methyl ester (1b-6b) and hydroxy derivatives (1c-6c) were prepared. Upon evaluation of the cytotoxic activity of these compounds against leukemia (HL60), lung (A549), melanoma (CRL1579), ovary (NIH:OVCAR-3), breast (SK-BR-3), prostate (DU145), stomach (AZ521), and pancreas (PANC-1) cancer cell lines, 11 compounds (5a, 6a, 2b-5b, 1c, and 3c-6c) exhibited activity with single-digit micromolar IC(50) values against one or more cell lines. Poricotriol A (1c), a hydroxy derivative of poricoic acid A (1a), exhibited potent cytotoxicities against six cell lines with IC(50) values of 1.2-5.5 µM. Poricotriol A induced typical apoptotic cell death in HL60 and A549 cells on evaluation of the apoptosis-inducing activity by flow cytometric analysis. Western blot analysis in HL60 cells showed that poricotriol A activated caspases-3, -8, and -9, while increasing the ratio of Bax/Bcl-2. This suggested that poricotriol A induced apoptosis via both mitochondrial and death receptor pathways in HL60. On the other hand, poricotriol A did not activate caspases-3, -8, and -9, but induced translocation of apoptosis-inducing factor (AIF) from mitochondria and increased the ratio of Bax/Bcl-2 in A549. This suggested that poricotriol A induced apoptosis via the mitochondrial pathway mostly by translocation of AIF, independent from the caspase pathway in A549. Furthermore, poricotriol A was shown to possess high selective toxicity in lung cancer cells since it exhibited only weak cytotoxicity against a normal lung cell line (WI-38).

Mitochondrial m-calpain plays a role in the release of truncated apoptosis-inducing factor from the mitochondria.

Calpains, calcium-dependent cysteine proteases, are involved in a variety of cellular processes. We have reported on the characteristics of mitochondrial mu-calpain and have shown that ERp57-associated mitochondrial mu-calpain cleaves the apoptosis-inducing factor (AIF) to a truncated form (tAIF). In addition, we found an unknown mitochondrial calpain. In this study, we identified and characterized this undescribed mitochondrial calpain in rat liver mitochondrial intermembrane space. The mitochondrial mu- and unknown calpains were separated by DEAE-Sepharose column chromatography. We immunoprecipitated the unknown calpain with anti-calpain small subunit and identified it as calpain 2 (m-calpain large subunit) by nanoflow-LC-MS/MS analysis and database searching. Because the identified mitochondrial calpain was stained with anti-m-calpain large subunit antibody, we named it mitochondrial m-calpain. The Ca(2+) dependency of mitochondrial m-calpain was similar to that of cytosolic m-calpain. Immunoprecipitation analyses showed that mitochondrial m-calpain is associated with a 75-kDa glucose-regulated protein, a member of the heat shock protein 70 family. We also investigated the involvement of mitochondrial m-calpain in the release of tAIF from mitochondria. Calpain inhibitor, PD150606, an anti-voltage-dependent anion channel (VDAC), and anti-Bax antibodies prevented the release of tAIF from mitochondria. In addition, we found that mitochondrial m-calpain truncated VDAC in Ca(2+)-dependent manner. This cleavage of VDAC promotes the mitochondrial accumulation of Bax and the release of tAIF from mitochondria. The accumulated Bax in mitochondrial outer membrane was co-immunoprecipitated with VDAC. Our results demonstrated that mitochondrial m-calpain plays a role in the release of tAIF from mitochondria by cleaving VDAC, and tAIF is released through VDAC-Bax pores.

Bioinformatic and image analyses of the cellular localization of the apoptotic proteins endonuclease G, AIF, and AMID during apoptosis in human cells.

We studied the cellular localization of the apoptotic proteins endonuclease G, AIF, and AMID in silico using three prediction tools and in living cells using both single-cell colocalization image analysis and nuclear translocation analysis. We confirmed the mitochondrial localization of endonuclease G and AIF by prediction analysis and by single-cell colocalization image analysis. We found the AMID protein to be cytoplasmic, most probably incorporated into the cytoplasmic side of the membranes of various organelles. The highest concentration of AMID was observed associated with the Golgi. Colocalization of AMID with lysosomes was also indirectly confirmed by analysis of AMID-rich vesicle velocity using manual tracking analysis. Bioinformatic analysis also detected nuclear localization signals in endonuclease G and AIF, but not in AMID. A novel analysis of time-lapse fluorescence image data during staurosporine-induced apoptosis revealed nuclear translocation only for endonuclease G and AIF.