Submitted on November 12, 2008
Revised on August 14, 2009
Accepted on August 25, 2009
RESTORING AS160 PHOSPHORYLATION RESCUES SKELETAL MUSCLE INSULIN RESISTANCE AND FATTY ACID OXIDATION WHILE NOT REDUCING INTRAMUSCULAR LIPIDS
Hakam Alkhateeb1, Adrian Chabowski2, Jan F.C. Glatz3, Brendon Gurd4, Joost J.F.P. Luiken3, and Arend Bonen1*
1 University of Guelph
2 Medical University Bialystok
3 Maastricht University
4 Queens University
* To whom correspondence should be addressed. E-mail: abonen{at}uoguelph.ca.
We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance, via increased palmitate oxidation, reductions in intramuscular lipids and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2mM, 0-18h) induced insulin resistance in soleus muscle. From 12-18h palmitate was removed, or AICAR or leptin were provided while maintaining 2mM palmitate. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulin-stimulated glucose transport, plasmalemmal GLUT4, and Akt and AS160 phosphorylation were examined at 0, 6, 12, 18h. Palmitate treatment (12h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol +11%, total ceramide +18%, C16:0 ceramide +60%) and AMPK phosphorylation (+118%), while reducing fatty acid oxidation (-60%), and insulin-stimulated glucose transport (-70%), GLUT4 translocation (-50%%) and AS160 phosphorylation (-40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR- and leptin-treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation, and insulin-stimulated glucose transport, GLUT4 translocation and AS160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS160 phosphorylation were highly correlated (r=0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, ceramide), the rapid AICAR-, and leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulin-stimulated AS160 phosphorylation and GLUT4 translocation.
