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Bax and Bak can localize to the endoplasmic reticulum to initiate apoptosis.

Zong WX, Li C, Hatzivassiliou G, Lindsten T, Yu QC, Yuan J, Thompson CB - J. Cell Biol. (2003)

Bottom Line: In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells.In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak.These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Cancer Biology, Abramson Cancer Center, 421 Curie Blvd., BRB II/III, 445, Philadelphia, PA 19104-6160, USA. craig@mail.med.upenn.edu

ABSTRACT
Bax and Bak play a redundant but essential role in apoptosis initiated by the mitochondrial release of apoptogenic factors. In addition to their presence at the mitochondrial outer membrane, Bax and Bak can also localize to the ER. Agents that initiate ER stress responses can induce conformational changes and oligomerization of Bax on the ER as well as on mitochondria. In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells. In bax-/-bak-/- cells, introduction of Bak mutants selectively targeted to either mitochondria or the ER can induce apoptosis. However, ER-targeted, but not mitochondria-targeted, Bak leads to progressive depletion of ER Ca2+ and induces caspase 12 cleavage. In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak. These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

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The ER-targeted Bak can induce apoptosis in the absence of endogenous Bax and Bak. Wild-type and bax−/−bak−/− MEFs were infected with retroviruses expressing GFP or GFP together with wild-type murine Bak, Bak-ActA, Bak-cb5, or Bak-ΔC. 48 h after infection, 1 μg/ml DAPI was added to stain the apoptotic cells. (A) bax−/−bak−/− cells were photographed using a FITC or DAPI filter. (B) The percentage of dead cells was determined by the ratio of DAPI-positive cells to GFP-positive cells. In addition to the retroviral infection of the Bak mutants, bax−/−bak−/− cells were also coinfected with retrovirus expressing Bcl-xL, or infected in the presence of 20 μg/ml Z-VAD-fmk. Data shown are the average of three independent experiments. (C) 24 h after infection, bax−/−bak−/− cells were trypsinized. A portion of each sample was subjected to FACS® to determine the expression efficiency indicated by GFP-positive cells. The rest of the cells were fixed in 0.25% paraformaldehyde and stained with an anti-Bak antibody that recognizes the active form of murine Bak. The number in each dot plot represents the percentage of gated events (R1).
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fig6: The ER-targeted Bak can induce apoptosis in the absence of endogenous Bax and Bak. Wild-type and bax−/−bak−/− MEFs were infected with retroviruses expressing GFP or GFP together with wild-type murine Bak, Bak-ActA, Bak-cb5, or Bak-ΔC. 48 h after infection, 1 μg/ml DAPI was added to stain the apoptotic cells. (A) bax−/−bak−/− cells were photographed using a FITC or DAPI filter. (B) The percentage of dead cells was determined by the ratio of DAPI-positive cells to GFP-positive cells. In addition to the retroviral infection of the Bak mutants, bax−/−bak−/− cells were also coinfected with retrovirus expressing Bcl-xL, or infected in the presence of 20 μg/ml Z-VAD-fmk. Data shown are the average of three independent experiments. (C) 24 h after infection, bax−/−bak−/− cells were trypsinized. A portion of each sample was subjected to FACS® to determine the expression efficiency indicated by GFP-positive cells. The rest of the cells were fixed in 0.25% paraformaldehyde and stained with an anti-Bak antibody that recognizes the active form of murine Bak. The number in each dot plot represents the percentage of gated events (R1).

Mentions: Expression of wild-type Bak, Bak-ActA, and Bak-cb5 induced apoptosis in wild-type MEFs (Fig. 6)Figure 6.


Bax and Bak can localize to the endoplasmic reticulum to initiate apoptosis.

Zong WX, Li C, Hatzivassiliou G, Lindsten T, Yu QC, Yuan J, Thompson CB - J. Cell Biol. (2003)

The ER-targeted Bak can induce apoptosis in the absence of endogenous Bax and Bak. Wild-type and bax−/−bak−/− MEFs were infected with retroviruses expressing GFP or GFP together with wild-type murine Bak, Bak-ActA, Bak-cb5, or Bak-ΔC. 48 h after infection, 1 μg/ml DAPI was added to stain the apoptotic cells. (A) bax−/−bak−/− cells were photographed using a FITC or DAPI filter. (B) The percentage of dead cells was determined by the ratio of DAPI-positive cells to GFP-positive cells. In addition to the retroviral infection of the Bak mutants, bax−/−bak−/− cells were also coinfected with retrovirus expressing Bcl-xL, or infected in the presence of 20 μg/ml Z-VAD-fmk. Data shown are the average of three independent experiments. (C) 24 h after infection, bax−/−bak−/− cells were trypsinized. A portion of each sample was subjected to FACS® to determine the expression efficiency indicated by GFP-positive cells. The rest of the cells were fixed in 0.25% paraformaldehyde and stained with an anti-Bak antibody that recognizes the active form of murine Bak. The number in each dot plot represents the percentage of gated events (R1).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172724&req=5

fig6: The ER-targeted Bak can induce apoptosis in the absence of endogenous Bax and Bak. Wild-type and bax−/−bak−/− MEFs were infected with retroviruses expressing GFP or GFP together with wild-type murine Bak, Bak-ActA, Bak-cb5, or Bak-ΔC. 48 h after infection, 1 μg/ml DAPI was added to stain the apoptotic cells. (A) bax−/−bak−/− cells were photographed using a FITC or DAPI filter. (B) The percentage of dead cells was determined by the ratio of DAPI-positive cells to GFP-positive cells. In addition to the retroviral infection of the Bak mutants, bax−/−bak−/− cells were also coinfected with retrovirus expressing Bcl-xL, or infected in the presence of 20 μg/ml Z-VAD-fmk. Data shown are the average of three independent experiments. (C) 24 h after infection, bax−/−bak−/− cells were trypsinized. A portion of each sample was subjected to FACS® to determine the expression efficiency indicated by GFP-positive cells. The rest of the cells were fixed in 0.25% paraformaldehyde and stained with an anti-Bak antibody that recognizes the active form of murine Bak. The number in each dot plot represents the percentage of gated events (R1).
Mentions: Expression of wild-type Bak, Bak-ActA, and Bak-cb5 induced apoptosis in wild-type MEFs (Fig. 6)Figure 6.

Bottom Line: In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells.In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak.These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Cancer Biology, Abramson Cancer Center, 421 Curie Blvd., BRB II/III, 445, Philadelphia, PA 19104-6160, USA. craig@mail.med.upenn.edu

ABSTRACT
Bax and Bak play a redundant but essential role in apoptosis initiated by the mitochondrial release of apoptogenic factors. In addition to their presence at the mitochondrial outer membrane, Bax and Bak can also localize to the ER. Agents that initiate ER stress responses can induce conformational changes and oligomerization of Bax on the ER as well as on mitochondria. In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells. In bax-/-bak-/- cells, introduction of Bak mutants selectively targeted to either mitochondria or the ER can induce apoptosis. However, ER-targeted, but not mitochondria-targeted, Bak leads to progressive depletion of ER Ca2+ and induces caspase 12 cleavage. In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak. These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

Show MeSH
Related in: MedlinePlus