Title: 0347: Leucine, a potent inhibitor of cardiac glucose uptake
Abstract: It was demonstrated that branched-chain amino acids like leucine induce insulin resistance in muscle and adipose tissues. The mechanism proposed to explain leucine action involves mTOR/p70S6K signaling. This pathway can be activated by leucine and is implicated in the stimulation of an insulin negative feedback loop. Knowing that insulin-resistance participates in diabetic cardiomyopathy, we were interested in studying leucine effect in cardiomyocytes. Primary cultured adult rat cardiomyocytes were pretreated with different concentrations of leucine (from 1 to 10 mM) during different periods of time (up to 20h) before being exposed to insulin (3x10−9 M, 30 min). In absence of leucine, insulin induced a 6-fold increase in glucose uptake (0.31+/−0.04 vs. 0.05+/−0.01 μmoles/mg.h). This correlated with the increase in phosphorylation state of PKB and AS160, both known to regulate glucose transport downstream of insulin. Pre-incubation with leucine for 1 h stimulated mTOR/p70S6K pathway resulting in the inhibiting phosphorylation of IRS-1 located in the proximal insulin signaling pathway. This is accompanied by a significant decrease in PKB and AS160 phosphorylation but, surprisingly, insulin-stimulated glucose uptake was preserved (0.31+/–0.05 μmoles/mg.h). On the other hand, a longer incubation (14h) with leucine induced a drastic decrease in glucose transport (0.056+/–0.01 μmoles/mg.h). The mTOR/p70S6K inhibitor rapamycin did not prevent this inhibition. Moreover, the non-metabolized leucine analog BCH was able to stimulate mTOR/p70S6K pathway but had no effect on the insulin-mediated stimulation of glucose uptake. By contrast, intermediates of leucine catabolism, alpha-ketoisocaproate, acetoacetate and betahydroxybutyrate, inhibited glucose uptake similarly to leucine. Leucine catabolism reduces insulin-dependent glucose transport independently of insulin signaling.