Role of amino acids in modulating glucose-induced desensitization of the glucose transport system

Amino acids were found to play an integral role in modulating glucose-induced desensitization of the glucose transport system (GTS). When adipocytes were treated for 6 h in a defined buffer containing 25 ng/ml insulin, 20 mM glucose, plus the 15 amino acids found in Dulbecco's modified Eagle's medium, we observed marked desensitization of the GTS, manifested by a 60% decrease in maximal insulin responsiveness (MIR) and a 2.5-fold reduction in insulin sensitivity. In contrast, little or no desensitization was seen under similar conditions in the absence of amino acids. The ability of amino acids to co-regulate the GTS appears to be directly attributable to amino acids per se since desensitization was still observed in cycloheximide-treated cells. Moreover, the action of amino acids is specific to glucose-induced desensitization since amino acids were not required for dexamethasone-induced desensitization of the GTS. Of the 15 amino acids contained in Dulbecco's modified Eagle's medium, one group of 8 amino acids was fully effective in mediating loss of both MIR and insulin sensitivity, whereas the remaining 7 amino acids were ineffective. Interestingly, this second group selectively retained the ability to modulate loss of insulin sensitivity. Upon screening the individual amino acids, we found that L-glutamine (but not D-glutamine) was as effective as total amino acids in modulating loss of MIR, whereas glycine and threonine were only partially effective. Since isoleucine and serine enhanced both MIR and insulin sensitivity of the protein synthesis system without influencing the GTS, it appears that amino acids can influence several insulin effector systems with notable differences in rapidity of action, direction of regulation, and specificity of amino acids. From these studies we conclude: 1) desensitization of the GTS requires three components--glucose, insulin, and selective amino acids; 2) insulin resistance of the GTS can be induced through several mechanisms, but only glucose-induced desensitization requires amino acids; 3) glucose-induced desensitization is mediated primarily by metabolic events independent of de novo protein synthesis; and 4) glutamine is the primary amino acid modulating glucose-induced loss of MIR. Overall, these studies reveal that amino acids play an important role in modulating insulin action at the cellular level and provide new insights into the metabolic mechanisms mediating insulin resistance of the glucose transport system.