BIOT 233 |
| Saturated free fatty acid (FFA, e.g. palmitate) induced apoptosis is associated with the development of a variety of diseases. Palmitate has been shown to induce apoptosis in cardiac cells, pancreatic beta cells, breast cancer cells, hepatocytes, and many other cell types, while unsaturated FFAs (e.g. oleate) have been shown to protect cells from apoptosis induced by saturated FFAs. Palmitate induces apoptosis in liver cells by decreasing the protein level of Bcl-2, however, the intermediates that mediate this affect have not been identified. In the present study we determined that the double-stranded RNA-dependent protein kinase (PKR) exploited an anti-apoptotic role in human hepatocellular carcinoma cell (HepG2) by regulating the Bcl-2 protein. We showed for the first time that palmitate and tumor necrosis factor (TNF)-α suppressed the activity of PKR, and PKR is required to maintain the level of Bcl-2 in HepG2 cells. We propose that the transcription factor, NF-κB, is involved in mediating the effect of PKR on the protein level of Bcl-2. In addition, the phosphorylation of Bcl-2 at different amino acid residues, such as Ser70 and Ser87, is critical in determining the role of Bcl-2 in regulating apoptosis. We found that a decrease in the phosphorylation of Bcl-2 at Ser70 was mediated by PKR and JNK, while the phosphorylation of Bcl-2 at Ser87 was not affected by palmitate and TNF-α. Furthermore, we showed that eIF-2 and PP2A, the most well-known substrates of PKR, are not involved in mediating the apoptosis induced by palmitate and TNF-α. In summary, we identified the signaling pathways that regulate the level and phosphorylation status of the Bcl-2 protein and thereby apoptosis in HepG2 cells upon exposure to palmitate are mediated by PKR. |
|
Poster Session
5:30 PM-7:30 PM, Wednesday, August 22, 2007 BCEC -- Exhibit Hall - B2, Poster
Division of Biochemical Technology |