Triggering of apoptosis by Puma is determined by the threshold set by prosurvival Bcl-2 family proteins.
Puma (p53 upregulated modulator of apoptosis) belongs to the BH3 (Bcl-2 homology 3)-only protein family of apoptotic regulators. Its expression is induced by various apoptotic stimuli, including irradiation and cytokine withdrawal. Using an inducible system to express Puma, we investigated the nature of Puma-induced apoptosis. In BaF(3) cells, expression of Puma caused rapid caspase-mediated cleavage of ICAD (inhibitor of caspase-activated deoxyribonuclease) and Mcl-1 (myeloid cell leukemia 1), leading to complete loss of cell viability. Surprisingly, Puma protein levels peaked within 2 h of its induction and subsequently declined to basal levels. Maximal Puma abundance coincided with the onset of caspase activity. Subsequent loss of Puma was prevented by the inhibition of caspases, indicating that its degradation was caspase dependent. In cells expressing transfected Bcl-2, induced Puma reached significantly higher levels, but after a delay, caspases became active and cell death occurred. Puma co-immunoprecipitated endogenous Bcl-2 and Mcl-1 but not Bax and Bak, suggesting that Puma did not associate with either Bax or Bak in these cells to initiate cell death. In mouse embryonic fibroblasts (MEFs), the amount of Puma peaked within 4 h of its induction. In contrast, in bax/bak double-knockout MEFs, Puma was stably expressed following its induction and was unable to trigger apoptosis even at very high levels. Overexpression of Bcl-2 in wild-type MEFs, like in BaF(3) cells, resulted in higher levels of Puma being reached but did not prevent cell death from occurring. These results demonstrate that the level of the Bcl-2 prosurvival family sets the threshold at which Puma is able to indirectly activate Bax or Bak, leading in turn to activation of caspases that not only cause cell death but also rapidly induce Puma degradation.
Department of Biochemistry, La Trobe University, Victoria 3086, Australia. b.callus@latrobe.edu.au
Bernard A
BA
Callus
Donia M
DM
Moujallad
John
J
Silke
Robert
R
Gerl
Anissa M
AM
Jabbour
Paul G
PG
Ekert
David L
DL
Vaux
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism*; D061945:Caspase Inhibitors; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D051379:Mice; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D061988:Proteasome Inhibitors; D011485:Protein Binding / Q000187:drug effects; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000172:deficiency / Q000378:metabolism; D051028:bcl-2-Associated X Protein / Q000172:deficiency / Q000378:metabolism
D051017:Apoptosis Regulatory Proteins; C438378:BBC3 protein, human; D061945:Caspase Inhibitors; D004791:Enzyme Inhibitors; C579094:Mcl1 protein, mouse; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D061988:Proteasome Inhibitors; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein