Glucose phosphorylation is required for insulin-dependent mTOR signalling in the heart.
OBJECTIVE
Insulin regulates both glucose uptake and postnatal cardiac growth. The anabolic effects of insulin are mediated by the mammalian target of rapamycin (mTOR), an evolutionarily conserved kinase which is also a convergence point between nutrient sensing and cell growth. We postulated that mTOR signalling in the heart requires the metabolism of glucose.


METHODS
We interrogated the insulin-mediated mTOR signalling pathway in response to different metabolic interventions regulating substrate metabolism in the isolated working rat heart and in isolated cardiomyocytes.


RESULTS
Although insulin enhanced Akt activity, phosphorylation of mTOR and its downstream targets (p70S6K and 4EBP1) required the addition of glucose. Glucose-dependent p70S6K phosphorylation was independent of the hexosamine biosynthetic pathway, the AMP kinase pathway, and the pentose phosphate pathway. However, inhibition of glycolysis downstream of hexokinase markedly enhanced p70S6K phosphorylation. Furthermore, 2-deoxyglucose activated p70S6K suggesting that phosphorylation of glucose is required for carbohydrate-mediated mTOR signalling in the heart. Lastly, we also found enhanced p70S6K phosphorylation in the hearts of diabetic rats.


CONCLUSIONS
Phosphorylation of glucose is necessary for insulin-dependent mTOR activity in the heart, suggesting a link between intermediary metabolism and cardiac growth.
Sharma
Saumya
S
Division of Cardiology, The University of Texas Houston Medical School, Houston, Texas 77030, USA.
Guthrie
Patrick H
PH
Chan
Suzanne S
SS
Haq
Syed
S
Taegtmeyer
Heinrich
H
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D015153:Blotting, Western / Q000379:methods; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D005947:Glucose / Q000378:metabolism*; D006019:Glycolysis; D006595:Hexosamines / Q000096:biosynthesis; D007328:Insulin / Q000378:metabolism*; D008297:Male; D032383:Myocytes, Cardiac / Q000378:metabolism*; D010428:Pentosephosphates / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases
D048868:Adaptor Proteins, Signal Transducing; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D006595:Hexosamines; D007328:Insulin; D010428:Pentosephosphates; D010750:Phosphoproteins; D011494:Protein Kinases; C546842:MTOR protein, human; C546845:mTOR protein, rat; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D005947:Glucose