Parkin inhibits BAK and BAX apoptotic function by distinct mechanisms during mitophagy.

The E3 ubiquitin ligase Parkin is a key effector of the removal of damaged mitochondria by mitophagy. Parkin determines cell fate in response to mitochondrial damage, with its loss promoting early onset Parkinson's disease and potentially also cancer progression. Controlling a cell's apoptotic response is essential to co-ordinate the removal of damaged mitochondria. We report that following mitochondrial damage-induced mitophagy, Parkin directly ubiquitinates the apoptotic effector protein BAK at a conserved lysine in its hydrophobic groove, a region that is crucial for BAK activation by BH3-only proteins and its homo-dimerisation during apoptosis. Ubiquitination inhibited BAK activity by impairing its activation and the formation of lethal BAK oligomers. Parkin also suppresses BAX-mediated apoptosis, but in the absence of BAX ubiquitination suggesting an indirect mechanism. In addition, we find that BAK-dependent mitochondrial outer membrane permeabilisation during apoptosis promotes PINK1-dependent Parkin activation. Hence, we propose that Parkin directly inhibits BAK to suppress errant apoptosis, thereby allowing the effective clearance of damaged mitochondria, but also promotes clearance of apoptotic mitochondria to limit their potential pro-inflammatory effect.

Human Bcl-2 cannot directly inhibit the Caenorhabditis elegans Apaf-1 homologue CED-4, but can interact with EGL-1.

Although the anti-apoptotic activity of Bcl-2 has been extensively studied, its mode of action is still incompletely understood. In the nematode Caenorhabditis elegans, 131 of 1090 somatic cells undergo programmed cell death during development. Transgenic expression of human Bcl-2 reduced cell death during nematode development, and partially complemented mutation of ced-9, indicating that Bcl-2 can functionally interact with the nematode cell death machinery. Identification of the nematode target(s) of Bcl-2 inhibition would help clarify the mechanism by which Bcl-2 suppresses apoptosis in mammalian cells. Exploiting yeast-based systems and biochemical assays, we analysed the ability of Bcl-2 to interact with and regulate the activity of nematode apoptosis proteins. Unlike CED-9, Bcl-2 could not directly associate with the caspase-activating adaptor protein CED-4, nor could it inhibit CED-4-dependent yeast death. By contrast, Bcl-2 could bind the C. elegans pro-apoptotic BH3-only Bcl-2 family member EGL-1. These data prompt us to hypothesise that Bcl-2 might suppress nematode cell death by preventing EGL-1 from antagonising CED-9, rather than by inhibiting CED-4.

Caspase 8 is absent or low in many ex vivo gliomas.

BACKGROUND: Better treatments are required urgently for patients with malignant glioma, which currently is incurable. Death ligands, such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), may offer promise for the treatment high-grade glioma if such ligands induce apoptotic signaling in vivo in glioma cells. Caspase 8 is required for death ligand signaling, and its levels may influence the sensitivity of glioma cells to death ligands. It also may act as a tumor suppressor protein. The authors analyzed caspase 8 expression levels in ex vivo glioma specimens and explored potential mechanisms of its regulation. METHODS: Eleven glioblastomas, 5 anaplastic astrocytomas, and 3 low-grade astrocytomas were studied. The levels of caspase 8, caspase 10, cellular FLICE inhibitory protein (c-FLIP), and signal transducer and activator of transcription (STAT)-1 were assayed using quantitative immunoblotting. Caspase 8 mRNA was measured by Northern blot analysis. The methylation status of the caspase 8 gene was determined by bisulfate modification of genomic DNA, cloning, and sequencing. Statistical analyses were performed using nonparametric (Spearman) correlations. RESULTS: Some ex vivo glioma samples lacked detectable caspase 8, with many expressing barely detectable levels. No tumors expressed significant amounts of caspase 10 or c-FLIP. A strong association was found between caspase 8 mRNA and protein levels. Neither expression of the transcription factor STAT-1 nor caspase 8 gene methylation correlated with caspase 8 levels. CONCLUSIONS: The absence of caspase 8 protein in many resected glioma samples implied that many patients with glioma may not benefit from death ligand-based treatments, unless caspase 8 (or caspase 10) protein expression can be elevated. Demethylating agents are unlikely to boost caspase 8 levels in glioma cells, but treatments that increase caspase 8 mRNA levels may up-regulate expression of the protein.

Caspase-8 levels affect necessity for mitochondrial amplification in death ligand-induced glioma cell apoptosis.

Fifty percent of high-grade glioma patients die within a year of diagnosis and less than two percent survive five years postdiagnosis. Elucidating apoptosis signaling pathways may assist in designing better adjuvant therapies. Preliminary characterizations suggested that glioma cells may either employ mitochondrial-independent or -dependent death receptor-induced apoptotic pathways, characteristic of cells termed type I and type II, respectively. In the present study, we generated panels of clonal transfectants overexpressing various levels of Bcl-2, in two parental glioma cell lines. These cells were used to explore molecular factors determining the necessity for mitochondrial amplification of death receptor signaling. Moderate Bcl-2 expression was sufficient to render one glioma cell line (D270) resistant to apoptosis induced by Fas ligand or TRAIL, consistent with these cells being type II. However, expression of even very high levels of Bcl-2 in a second line (D645) did not affect death ligand sensitivity, indicative of a type I phenotype. D270 cells expressed much less caspase-8 protein than D645 cells. Enforced overexpression of caspase-8 (or cytoplasmic Diablo/Smac) in D270 cells overcame Bcl-2 inhibition of death ligand-induced apoptosis, converting them from type II to type I. This indicates that caspase-8 levels can influence the requirement for mitochondrial involvement in death receptor apoptotic signaling in glioma cells.