Sustained activation of the mammalian target of rapamycin nutrient sensing pathway is associated with hepatic insulin resistance, but not with steatosis, in mice.
OBJECTIVE
Activation of nutrient sensing through mammalian target of rapamycin (mTOR) has been linked to the pathogenesis of insulin resistance. We examined activation of mTOR-signalling in relation to insulin resistance and hepatic steatosis in mice.


METHODS
Chronic hepatic steatosis and hepatic insulin resistance were induced by high-fat feeding of male C57BL/6Jico mice for 6 weeks. In addition, acute hepatic steatosis in the absence of insulin resistance was induced by pharmacological blockade of beta-oxidation using tetradecylglycidic acid (TDGA). mTOR signalling was examined in liver homogenates.


RESULTS
High-fat feeding caused obesity (p<0.001), hepatic steatosis (p<0.05) and hepatic insulin resistance (p<0.05). The phosphorylation of mTOR and its downstream targets p70S6 kinase and S6 ribosomal protein was two-fold higher in mice on a high-fat diet than in mice fed standard chow (all p<0.05) and associated with enhanced rates of protein synthesis. Acute induction of hepatic steatosis with TDGA had no effect on mTOR activity. The increased activity of the mTOR pathway in livers from mice on a high-fat diet could not be ascribed to diet-induced alterations in known modulators of mTOR activity such as circulating plasma leucine levels, phosphorylation of protein kinase B and AMP-activated protein kinase, and changes in mitochondrial function.


CONCLUSIONS
High-fat diet induces increase of the mTOR nutrient sensing pathway in association with hepatic insulin resistance, but not with hepatic lipid accumulation as such.
Korsheninnikova
E
E
Department of Molecular Cell Biology, Leiden University Medical Centre, Postzone S1-P, Postbus 9600, 2300 RC, Leiden, The Netherlands. Elena@lumc.nl
van der Zon
G C M
GC
Voshol
P J
PJ
Janssen
G M
GM
Havekes
L M
LM
Grefhorst
A
A
Kuipers
F
F
Reijngoud
D-J
DJ
Romijn
J A
JA
Ouwens
D M
DM
Maassen
J A
JA
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D004272:DNA, Mitochondrial / Q000235:genetics; D004041:Dietary Fats; D003576:Electron Transport Complex IV / Q000235:genetics / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism; D005234:Fatty Liver / Q000503:physiopathology*; D007328:Insulin / Q000097:blood; D007333:Insulin Resistance; D007930:Leucine / Q000097:blood; D008099:Liver / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000502:physiology*; D012269:Ribosomal Proteins / Q000235:genetics; D058570:TOR Serine-Threonine Kinases
D001786:Blood Glucose; D004272:DNA, Mitochondrial; D004041:Dietary Fats; D003576:Electron Transport Complex IV; D005227:Fatty Acids; D007328:Insulin; D007930:Leucine; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D012269:Ribosomal Proteins; D058570:TOR Serine-Threonine Kinases; C546843:mTOR protein, mouse