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

Abstract

Lipid accumulation in skeletal muscle results in dysregulation of protein meta- bolism and muscle atrophy. We previously reported that treating C2C12 myo- tubes 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 pro- tein response (UPR), a collective group of responses that can lead to activa- tion of caspase-mediated proteolysis and autophagy. Presently, we tested the hypothesis that DHA protects against PA-induced ER stress/UPR and its atro- phy-related responses in muscle cells. C2C12 myotubes were treated with 500 lmol/L PA and/or 100 lmol/L DHA for 24 h. Proteins and mRNA asso- ciated with ER stress/UPR, autophagy, and caspase-3 activation were evalu- ated. PA robustly increased the phosphorylation of protein kinase R (PKR)- like ER kinase (PERK) and eukaryotic initiation factor 2a (eIF2a). 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.