Substrate cycling between de novo lipogenesis and lipid oxidation:: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

Life is a combustion, but how the major fuel substrates that sustain human life compete and interact with each other for combustion has been at the epicenter of research into the pathogenesis of insulin resistance ever since Randle proposed a 'glucose-fatty acid cycle' in 1963. Since then, several features of a mutual interaction that is characterized by both reciprocality and dependency between glucose and lipid metabolism have been unravelled, namely: (i) the inhibitory effects of elevated concentrations of fatty acids on glucose oxidation ( via inactivation of mitochondrial pyruvate dehydrogenase or via desensitization of insulin-mediated glucose transport), (ii) the inhibitory effects of elevated concentrations of glucose on fatty acid oxidation ( via malonyl-CoA regulation of fatty acid entry into the mitochondria), and more recently (iii) the stimulatory effects of elevated concentrations of glucose on de novo lipogenesis, that is, synthesis of lipids from glucose ( via SREBP1c regulation of glycolytic and lipogenic enzymes). This paper first revisits the physiological significance of these mutual interactions between glucose and lipids in skeletal muscle pertaining to both blood glucose and intramyocellular lipid homeostasis. It then concentrates upon emerging evidence, from calorimetric studies investigating the direct effect of leptin on thermogenesis in intact skeletal muscle, of yet another feature of the mutual interaction between glucose and lipid oxidation: that of substrate cycling between de novo lipogenesis and lipid oxidation. It is proposed that this energy-dissipating substrate cycling that links glucose and lipid metabolism to thermogenesis could function as a 'fine-tuning' mechanism that regulates intramyocellular lipid homeostasis, and hence contributes to the protection of skeletal muscle against lipotoxicity.