Palmitate activates mTOR/p70S6K through AMPK inhibition and hypophosphorylation of raptor in skeletal muscle cells: Reversal by oleate is similar to metformin.
Excessive saturated free fatty acids (SFFAs; e.g. palmitate) in blood are a pathogenic factor in diabetes, obesity, cardiovascular disease and liver failure. In contrast, monounsaturated free fatty acids (e.g. oleate) prevent the toxic effect of SFFAs in various types of cells. The mechanism is poorly understood and involvement of the mTOR complex is untested. In the present study, we demonstrate that oleate preconditioning, as well as coincubation, completely prevented palmitate-induced markers of inflammatory signaling, insulin resistance and cytotoxicity in C2C12 myotubes. We then examined the effect of palmitate and/or oleate on the mammalian target of rapamycin (mTOR) signal path and whether their link is mediated by AMP-activated protein kinase (AMPK). Palmitate decreased the phosphorylation of raptor and 4E-BP1 while increasing the phosphorylation of p70S6K. Palmitate also inhibited phosphorylation of AMPK, but did not change the phosphorylated levels of mTOR or rictor. Oleate completely prevented the palmitate-induced dysregulation of mTOR components and restored pAMPK whereas alone it produced no signaling changes. To understand this more, we show activation of AMPK by metformin also prevented palmitate-induced changes in the phosphorylations of raptor and p70S6K, confirming that the mTORC1/p70S6K signaling pathway is responsive to AMPK activity. By contrast, inhibition of AMPK phosphorylation by Compound C worsened palmitate-induced changes and correspondingly blocked the protective effect of oleate. Finally, metformin modestly attenuated palmitate-induced insulin resistance and cytotoxicity, as did oleate. Our findings indicate that palmitate activates mTORC1/p70S6K signaling by AMPK inhibition and phosphorylation of raptor. Oleate reverses these effects through a metformin-like facilitation of AMPK.
Department of Neurology, Rhode Island Hospital, Warren Alpert Medical School of Brown University, Providence, RI 02903, USA.
Bumsup
B
Kwon
Department of Neurology, Rhode Island Hospital, Warren Alpert Medical School of Brown University, Providence, RI 02903, USA. Electronic address: henry_querfurth@brown.edu.
Henry W
HW
Querfurth
D055372:AMP-Activated Protein Kinases / Q000378:metabolism*; D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D007333:Insulin Resistance / Q000502:physiology; D008687:Metformin / Q000494:pharmacology; D051379:Mice; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D019301:Oleic Acid / Q000378:metabolism / Q000494:pharmacology; D010168:Palmitates / Q000378:metabolism* / Q000633:toxicity; D010766:Phosphorylation; D000076223:Regulatory-Associated Protein of mTOR; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D010168:Palmitates; D000076223:Regulatory-Associated Protein of mTOR; C540162:Rptor protein, mouse; D019301:Oleic Acid; D008687:Metformin; C546843:mTOR protein, mouse; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases
AMPK; Insulin resistance; Oleate; Plamitate; Raptor; mTOR