Effect of forkhead transcription factor O1 on free fatty acid induced insulin resistance and steatosis by sterol-regulatory element binding protein-1c pathway

Objective To study the effects and potential mechanisms of Forkhead transcription factor O1 (FoxO1) on free fatty acid (FFA) induced insulin resistance (IR) and steatosis. Methods HepG-2 cells were induced to a status of insulin resistance by being exposed to 5.0×10-4 mmol/L palmitic acid (PA) for 24 hours. HepG-2 cells were devided into 4 groups, the HepG-2 cell group cultured with normal medium, the palmitic acid group, the blank plasmid group, and the siFoxO1 group. PA was added to normal medium to a final concentration of 5.0×10-4 mol/L. The expression levels of FoxO1 in different groups were detected by RT-PCR. The glucose consumption was detected by using glucose oxidase method. MTT method was used to detect the proliferation of HepG-2 cells. Steatosis of HepG-2 cells was observed by Oil Red O staining. The mRNA and protein expression of SREBP-1c were determined by RT-PCR and Western blot. Results Compared with control, the glucose consumption of cells cultured with FFA was significantly reduced(1.17±0.56 vs 4.31±0.21, t=10.587, P<0.01). Cellular lipid accumulation, the expressions of FoxO1 mRNA(0.78±0.10 vs 0.51±0.12, t=3.629, P<0.05), SREBP-1c mRNA (0.71±0.17 vs 0.25±0.08, t=6.290, P<0.05), and the protein of SREBP-1c (0.69±0.10 vs 0.41±0.07, t=4.797, P<0.01)was increased .After transfected by siFoxO1 plasmids using Lipofectamine 2000, the mRNA expression of FoxO1 (0.38±0.06 vs 0.78±0.10, t=7.164, P<0.01), SREB9-1c (0.45±0.13 vs 0.71±0.17, t=2.479, P<0.05) and the SREBP-1c protein expression (0.41±0.06 vs 0.69±0.10, t=4.797, P<0.01) were significantly decreased after tranfected by siFoxO1 plasminds, whereas the glucose consumption obviously the glucose consumption obviously increased(2.26±0.41 vs 1.17±0.56, t=3.144, P<0.05). The cellular lipod accumulation was decrease after transfected by siFoxO1. There was no remarkable difference between the palmitic acid group, and the blank plasmid group. Conclusions Inhibiting the expression of FoxO1 could improve FFA-induced IR and steatosis by down-regulating SREBP-1c expression. Key words: Forkhead transcription factor O1; Free fatty acid; Sterol regulatory element-binding proteins 1c