Bcl-2 phosphorylation and proteasome-dependent degradation induced by paclitaxel treatment: consequences on sensitivity of isolated mitochondria to Bid.
Several studies have suggested that Bcl-2 phosphorylation, which occurs during mitotic arrest induced by paclitaxel, inhibits its antiapoptotic function. In the present study, we demonstrated that the level of phosphorylated Bcl-2 was threefold higher in mitochondria than in the nuclear membrane or endoplasmic reticulum. Our results show, in isolated mitochondria, that phosphorylation of Bcl-2 in mitosis does not modify either its integration into the mitochondrial membrane or the ability to release cytochrome c in response to Bid, a cytochrome c releasing agent. In HeLa cells, in which paclitaxel induces apoptosis, the nonphosphorylated form of Bcl-2 is degraded by a proteasome-dependent degradation pathway, whereas the phosphorylated forms of mitochondrial Bcl-2 appear to be resistant to proteasome-induced degradation. We found that low concentrations of recombinant Bid triggered a greater release of cytochrome c from mitochondria isolated from paclitaxel-treated HeLa cells than from mitochondria isolated from control HeLa cells. Taken together, these results show that Bcl-2 phosphorylation does not inhibit its function. On the contrary, Bcl-2 phosphorylation indirectly regulated its antiapoptotic action via protection against degradation. Indeed, in response to paclitaxel treatment, the level of Bcl-2 expression in mitochondria rather than its phosphorylation state could regulate the sensitivity of mitochondria to cytochrome c releasing agents in vitro.
LBCMCP, UMR CNRS 5088, Université Paul Sabatier, 31, Toulouse, France.
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Brichese
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Barboule
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Heliez
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Valette
D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D017209:Apoptosis / Q000502:physiology; D051023:BH3 Interacting Domain Death Agonist Protein; D002352:Carrier Proteins / Q000235:genetics / Q000494:pharmacology*; D003546:Cysteine Endopeptidases / Q000378:metabolism; D003574:Cytochrome c Group / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006367:HeLa Cells / Q000187:drug effects; D006801:Humans; D007425:Intracellular Membranes / Q000502:physiology; D008928:Mitochondria / Q000187:drug effects* / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism; D009685:Nuclear Envelope / Q000378:metabolism; D017239:Paclitaxel / Q000494:pharmacology*; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000187:drug effects / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D000972:Antineoplastic Agents, Phytogenic; D051023:BH3 Interacting Domain Death Agonist Protein; C494814:BID protein, human; D002352:Carrier Proteins; D003574:Cytochrome c Group; D009097:Multienzyme Complexes; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D017239:Paclitaxel