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Repression of PKR mediates palmitate-induced apoptosis in HepG2 cells through regulation of Bcl-2.

Yang X, Chan C - Cell Res. (2009)

Bottom Line: Palmitate down-regulates the activity of PKR and thereby decreases the level of Bcl-2 protein, mediated in part by reduced activation of the NF-kappaB transcription factor.The decrease in the phosphorylation of Bcl-2 at Ser70 upon exposure to palmitate is mediated by inhibition of PKR and possibly by c-Jun N-terminal kinase (JNK), whereas the phosphorylation of Bcl-2 at Ser87 is unaffected by palmitate or PKR.In summary, PKR mediates the regulation of the protein level and the phosphorylation status of Bcl-2, providing a novel mechanism of palmitate-induced apoptosis in HepG2 cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA.

ABSTRACT
The present study shows that double-stranded RNA-dependent protein kinase (PKR) regulates the protein expression level and phosphorylation of Bcl-2 and plays an anti-apoptotic role in human hepatocellular carcinoma cells (HepG2). In various types of cells, saturated free fatty acids (FFAs), such as palmitate, have been shown to induce cellular apoptosis by several mechanisms. Palmitate down-regulates the activity of PKR and thereby decreases the level of Bcl-2 protein, mediated in part by reduced activation of the NF-kappaB transcription factor. In addition to the level of Bcl-2 protein, the phosphorylation of Bcl-2 at different amino acid residues, such as Ser70 and Ser87, is also important in regulating cellular apoptosis. The decrease in the phosphorylation of Bcl-2 at Ser70 upon exposure to palmitate is mediated by inhibition of PKR and possibly by c-Jun N-terminal kinase (JNK), whereas the phosphorylation of Bcl-2 at Ser87 is unaffected by palmitate or PKR. In summary, PKR mediates the regulation of the protein level and the phosphorylation status of Bcl-2, providing a novel mechanism of palmitate-induced apoptosis in HepG2 cells.

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Proposed signaling pathways from PKR to Bcl-2 induced by palmitateA. Reverse transfection of suspended HepG2 cells were performed with scrambled siRNA (Control) or siRNA of PKR for 24 hours and the transfected cells were cultured in regular media for another 24 hours. Cells were harvested, and western blot analysis was performed to detect the protein level of Bim. B. Summary of the signaling pathways identified in this study. First, PKR up-regulates the transcription of Bcl-2 gene through the transcription factor, likely NF-κB. Second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK. By suppressing PKR and in turn these two downstream pathways, palmitate regulates the protein level and phosphorylation of Bcl-2 at Ser70 and therefore induces apoptosis in HepG2 cells. Another Bcl-2 family protein, Bim, which also mediates palmitate-induced apoptosis (31, 32) (dash lines) and the potential effect of PKR on Bim (line with the question mark) are also included.
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Figure 11: Proposed signaling pathways from PKR to Bcl-2 induced by palmitateA. Reverse transfection of suspended HepG2 cells were performed with scrambled siRNA (Control) or siRNA of PKR for 24 hours and the transfected cells were cultured in regular media for another 24 hours. Cells were harvested, and western blot analysis was performed to detect the protein level of Bim. B. Summary of the signaling pathways identified in this study. First, PKR up-regulates the transcription of Bcl-2 gene through the transcription factor, likely NF-κB. Second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK. By suppressing PKR and in turn these two downstream pathways, palmitate regulates the protein level and phosphorylation of Bcl-2 at Ser70 and therefore induces apoptosis in HepG2 cells. Another Bcl-2 family protein, Bim, which also mediates palmitate-induced apoptosis (31, 32) (dash lines) and the potential effect of PKR on Bim (line with the question mark) are also included.

Mentions: Taken together, investigating the association between PKR and Bcl-2 revealed two different but complementary anti-apoptotic pathways that connect PKR and Bcl-2. First, PKR up-regulates the transcription of Bcl-2 gene, possibly through the transcription factor, NF-κB, and second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK (Fig. 11B). These two pathways were down-regulated in HepG2 cells upon exposure to palmitate, and may be one of the potential mechanisms by which palmitate induces apoptosis in HepG2 cells.


Repression of PKR mediates palmitate-induced apoptosis in HepG2 cells through regulation of Bcl-2.

Yang X, Chan C - Cell Res. (2009)

Proposed signaling pathways from PKR to Bcl-2 induced by palmitateA. Reverse transfection of suspended HepG2 cells were performed with scrambled siRNA (Control) or siRNA of PKR for 24 hours and the transfected cells were cultured in regular media for another 24 hours. Cells were harvested, and western blot analysis was performed to detect the protein level of Bim. B. Summary of the signaling pathways identified in this study. First, PKR up-regulates the transcription of Bcl-2 gene through the transcription factor, likely NF-κB. Second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK. By suppressing PKR and in turn these two downstream pathways, palmitate regulates the protein level and phosphorylation of Bcl-2 at Ser70 and therefore induces apoptosis in HepG2 cells. Another Bcl-2 family protein, Bim, which also mediates palmitate-induced apoptosis (31, 32) (dash lines) and the potential effect of PKR on Bim (line with the question mark) are also included.
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Related In: Results  -  Collection

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Figure 11: Proposed signaling pathways from PKR to Bcl-2 induced by palmitateA. Reverse transfection of suspended HepG2 cells were performed with scrambled siRNA (Control) or siRNA of PKR for 24 hours and the transfected cells were cultured in regular media for another 24 hours. Cells were harvested, and western blot analysis was performed to detect the protein level of Bim. B. Summary of the signaling pathways identified in this study. First, PKR up-regulates the transcription of Bcl-2 gene through the transcription factor, likely NF-κB. Second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK. By suppressing PKR and in turn these two downstream pathways, palmitate regulates the protein level and phosphorylation of Bcl-2 at Ser70 and therefore induces apoptosis in HepG2 cells. Another Bcl-2 family protein, Bim, which also mediates palmitate-induced apoptosis (31, 32) (dash lines) and the potential effect of PKR on Bim (line with the question mark) are also included.
Mentions: Taken together, investigating the association between PKR and Bcl-2 revealed two different but complementary anti-apoptotic pathways that connect PKR and Bcl-2. First, PKR up-regulates the transcription of Bcl-2 gene, possibly through the transcription factor, NF-κB, and second, PKR up-regulates the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, mediated by JNK (Fig. 11B). These two pathways were down-regulated in HepG2 cells upon exposure to palmitate, and may be one of the potential mechanisms by which palmitate induces apoptosis in HepG2 cells.

Bottom Line: Palmitate down-regulates the activity of PKR and thereby decreases the level of Bcl-2 protein, mediated in part by reduced activation of the NF-kappaB transcription factor.The decrease in the phosphorylation of Bcl-2 at Ser70 upon exposure to palmitate is mediated by inhibition of PKR and possibly by c-Jun N-terminal kinase (JNK), whereas the phosphorylation of Bcl-2 at Ser87 is unaffected by palmitate or PKR.In summary, PKR mediates the regulation of the protein level and the phosphorylation status of Bcl-2, providing a novel mechanism of palmitate-induced apoptosis in HepG2 cells.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA.

ABSTRACT
The present study shows that double-stranded RNA-dependent protein kinase (PKR) regulates the protein expression level and phosphorylation of Bcl-2 and plays an anti-apoptotic role in human hepatocellular carcinoma cells (HepG2). In various types of cells, saturated free fatty acids (FFAs), such as palmitate, have been shown to induce cellular apoptosis by several mechanisms. Palmitate down-regulates the activity of PKR and thereby decreases the level of Bcl-2 protein, mediated in part by reduced activation of the NF-kappaB transcription factor. In addition to the level of Bcl-2 protein, the phosphorylation of Bcl-2 at different amino acid residues, such as Ser70 and Ser87, is also important in regulating cellular apoptosis. The decrease in the phosphorylation of Bcl-2 at Ser70 upon exposure to palmitate is mediated by inhibition of PKR and possibly by c-Jun N-terminal kinase (JNK), whereas the phosphorylation of Bcl-2 at Ser87 is unaffected by palmitate or PKR. In summary, PKR mediates the regulation of the protein level and the phosphorylation status of Bcl-2, providing a novel mechanism of palmitate-induced apoptosis in HepG2 cells.

Show MeSH
Related in: MedlinePlus