Fatty acid palmitate suppresses FoxO1 expression via PERK and IRE1 unfolded protein response in C2C12 myotubes

Forkhead Box O1 (FoxO1) is a transcription factor with a unique fork head domain that indirectly participates in a variety of physiological processes and plays an important role in type 2 diabetes. Palmitate as the most abundant free fatty acid, accounting for 28-32% of total free fatty acids in human plasma. There is a direct relationship between palmitate and insulin resistance-induced type 2 diabetes. In addition, palmitate can activate the unfolded protein response signaling pathway induced by endoplasmic reticulum (ER) stress. This study aimed to investigate the response of FoxO1 to palmitate and the relationship with ER stress in C2C12 myotubes. Treatment of palmitate or tunicamycin promoted ER stress-related genes expression but suppressed FoxO1 expression, while 4-phenylbutyrate presented the opposite activity in palmitate-pretreated C2C12 myotubes, indicating that ER stress might be closely associated with FoxO1 expression. Moreover, palmitate-suppressed FoxO1 expression was reversed in C2C12 cells when the PERK and IRE-1 signaling pathway was inhibited by treatment with GSK2656157 or 4μ8C. However, no differences were observed when the ATF6 signaling pathway was suppressed by knockout of the ATF6 gene. These findings suggest that palmitate suppressed FoxO1 expression via the PERK and IRE1 signaling pathways.