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

Abstract

In the present study we found that double-stranded RNA-dependent protein kinase (PKR) regulates the protein expression level and the phosphorylation of Bcl-2 and exploits an anti-apoptotic role in human hepatocellular carcinoma cells (HepG2). Saturated free fatty acids (FFAs), e.g. palmitate, have been shown to induce cellular apoptosis in various types of cells by different mechanisms. We found palmitate down-regulates the activity of PKR, and thereby decreases the protein level of Bcl-2, mediated, in part, by the NF-κB transcription factor. In addition to the protein level of Bcl-2, 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 PKR and possibly JNK, while the phosphorylation of Bcl-2 at Ser87 is not affected 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.

The role of fatty acid unsaturation in minimizing biophysical changes on the structure and local effects of bilayer membranes

“…1, which represents our current results, and other recent in vitro studies, saturated FFAs significantly induce toxic effects on various cells types, however, unsaturated FFAs are shown to reduce and/or prevent toxic effects [9, 10, 11, 12, 13, 14]. This effect of saturated FFAs (i.e., palmitate) is dose dependent, inducing significant cytotoxicity and cell death at much lower FFA concentrations of 0.4 mM [15]. …”

“…Previously we found that the level of cytotoxicity in HepG2 cells induced by saturated FFA (i.e., palmitate), as measured by the relative amount of LDH released, increased in a dose-dependent manner [14, 15]. Here we determined whether unsaturated FFAs (i.e., oleate and linoleate) would induce significant levels of cytotoxicity, even at the highest concentration of palmitate that was used previously [14, 15].…”

The Double-stranded RNA–dependent Protein Kinase Differentially Regulates Insulin Receptor Substrates 1 and 2 in HepG2 Cells

“…Here, we show that IKK also is activated by PKR. Silencing PKR (Yang and Chan, 2009) significantly reduced the phosphorylation of IKKα/β at Ser176/180 (Figure 3A), which indicates IKK activity. To further confirm the involvement of JNK and IKK in mediating the effect of PKR on the phosphorylation of IRS1 at Ser312 and Tyr941, we overexpressed PKR in HepG2 cells and silenced or inhibited JNK or IKK.…”

Bip overexpression, but not CHOP inhibition, attenuates fatty-acid-induced endoplasmic reticulum stress and apoptosis in HepG2 liver cells

“…It has been shown that chronic treatment with saturated fatty acids causes numerous other cellular alterations that can initiate apoptosis including generation of reactive oxygen species (Srivastava and Chan, 2007), mitochondrial perturbations (Maestre et al, 2003) and generation of ceramide . Recently double-stranded RNA-dependent protein kinase (PKR) and Bcl-2 pathway was reported to play an important role in Pa-induced apoptosis in HepG2 cells (Yang and Chan, 2009). CHOP-induced GADD34 transcription and dephosphorylates eIF2a (Lee and Glimcher, 2009), the excessive phosphorylation of these proteins may potentiate fattyacid-induced ER stress and apoptosis (Cnop et al, 2007).…”