Docosahexaenoic acid counteracts palmitate‐induced endoplasmic reticulum stress in C2C12 myotubes: Impact on muscle atrophy

Lipid accumulation in skeletal muscle results in dysregulation of protein metabolism and muscle atrophy. We previously reported that treating C2C12 myotubes with palmitate (PA), a saturated fatty acid, increases the overall rate of proteolysis via activation of the ubiquitin‐proteasome and autophagy systems; co‐treatment with the omega‐3 polyunsaturated fatty acid docosahexaenoic acid (DHA) prevents the PA‐induced responses. Others have reported that PA induces endoplasmic reticulum (ER) stress which initiates the unfolded protein response (UPR), a collective group of responses that can lead to activation of caspase‐mediated proteolysis and autophagy. Presently, we tested the hypothesis that DHA protects against PA‐induced ER stress/UPR and its atrophy‐related responses in muscle cells. C2C12 myotubes were treated with 500 μmol/L PA and/or 100 μmol/L DHA for 24 h. Proteins and mRNA associated with ER stress/UPR, autophagy, and caspase‐3 activation were evaluated. PA robustly increased the phosphorylation of protein kinase R (PKR)‐like ER kinase (PERK) and eukaryotic initiation factor 2α (eIF2α). It also increased the mRNAs encoding activating transcription factor 4 (ATF4), spliced X‐box binding protein 1 (XBP1s), C/EBP homologous protein (CHOP), and autophagy‐related 5 (Atg5) as well as the protein levels of the PERK target nuclear factor erythroid 2‐related factor (Nrf2), CHOP, and cleaved (i.e., activated) caspase‐3. Co‐treatment with DHA prevented all of the PA‐induced responses. Our results indicate that DHA prevents PA‐induced muscle cell atrophy, in part, by preventing ER stress/UPR, a process that leads to activation of caspase‐mediated proteolysis and an increase in expression of autophagy‐related genes.