Human triglyceride-rich lipoproteins impair glucose metabolism and insulin signalling in L6 skeletal muscle cells independently of non-esterified fatty acid levels.
OBJECTIVE
Elevated fasting and postprandial plasma levels of triglyceride-rich lipoproteins (TGRLs), i.e. VLDL/remnants and chylomicrons/remnants, are a characteristic feature of insulin resistance and are considered a consequence of this state. The aim of this study was to investigate whether intact TGRL particles are capable of inducing insulin resistance.


METHODS
We studied the effect of highly purified TGRLs on glycogen synthesis, glycogen synthase activity, glucose uptake, insulin signalling and intramyocellular lipid (IMCL) content using fully differentiated L6 skeletal muscle cells.


RESULTS
Incubation with TGRLs diminished insulin-stimulated glycogen synthesis, glycogen synthase activity, glucose uptake and insulin-stimulated phosphorylation of Akt and glycogen synthase kinase 3. Insulin-stimulated tyrosine phosphorylation of IRS-1, and IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase (PI3K) activity were not impaired by TGRLs, suggesting that these steps were not involved in the lipoprotein-induced effects on glucose metabolism. The overall observed effects were time- and dose-dependent and paralleled IMCL accumulation. NEFA concentration in the incubation media did not increase in the presence of TGRLs indicating that the effects observed were solely due to intact lipoprotein particles. Moreover, co-incubation of TGRLs with orlistat, a potent active-site inhibitor of various lipases, did not alter TGRL-induced effects, whereas co-incubation with receptor-associated protein (RAP), which inhibits interaction of TGRL particles with members of the LDL receptor family, reversed the TGRL-induced effects on glycogen synthesis and insulin signalling.


CONCLUSIONS
Our data suggest that the accumulation of TGRLs in the blood stream of insulin-resistant patients may not only be a consequence of insulin resistance but could also be a cause for it.
Pedrini
M T
MT
Clinical Department of Internal Medicine, Medical University of Innsbruck, Anichstrasse 35, 6020 Innsbruck, Austria. michael.pedrini@uibk.ac.at
Kranebitter
M
M
Niederwanger
A
A
Kaser
S
S
Engl
J
J
Debbage
P
P
Huber
L A
LA
Patsch
J R
JR
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002460:Cell Line; D005230:Fatty Acids, Nonesterified / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism*; D006003:Glycogen / Q000096:biosynthesis; D006006:Glycogen Synthase / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins; D026502:LDL-Receptor Related Proteins / Q000494:pharmacology; D007783:Lactones / Q000494:pharmacology; D008049:Lipase / Q000037:antagonists & inhibitors; D050356:Lipid Metabolism; D008074:Lipoproteins / Q000494:pharmacology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects* / Q000378:metabolism; D000077403:Orlistat; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects*; D014280:Triglycerides / Q000494:pharmacology
D005230:Fatty Acids, Nonesterified; D005947:Glucose; D006003:Glycogen; D006006:Glycogen Synthase; D038362:Glycogen Synthase Kinase 3; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D026502:LDL-Receptor Related Proteins; D007783:Lactones; D008049:Lipase; D008074:Lipoproteins; D000077403:Orlistat; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D014280:Triglycerides; C526292:IRS1 protein, human; C526296:Irs1 protein, rat; C030087:lipoprotein triglyceride; C494918:AKT1 protein, human; C494920:Akt1 protein, rat