Impact of insulin on lactate metabolism

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Project Summary/Abstract The goal of this research is to explore the hypothesis that insulin promotes lactate burning, and to lay the groundwork for future investigation of the importance of such regulation to glucose homeostasis and diabetes. Actively transformed to each other by glycolysis and gluconeogenesis, glucose and lactate metabolism are tightly connected. Imbalances between lactate production and consumption lead to altered lactate concentrations, which can affect glycemia. Moreover, lactate accumulation (hyperlactatemia) is one of the most frequently encountered metabolic alterations in critically ill patients. Understanding the control of lactate production and utilization is therefore medically relevant, especially for more completely elucidating the pathophysiology of type 2 diabetes. Unlike for glucose, however, the mechanisms regulating lactate homeostasis are largely unknown. Here I will investigate the systemic and tissue-specific impact of insulin on lactate metabolism, test the hypothesis that insulin promotes lactate oxidation via TCA cycle, and explore the underlying biochemical mechanisms. To this end, Aim 1 will investigate the impact of insulin on whole-body and organ-specific TCA cycle activity. To measure the effect of insulin on whole body energy metabolism, insulin will be given to mice after short fasting, and oxygen consumption and carbon dioxide production will be measured by indirect calorimetry. To understand the impact of insulin on organ-specific TCA metabolism, TCA flux in individual organs will be quantified, using a novel dynamic isotope tracing approach coupled to LC-MS and flux modeling. Aim 2 will explore the mechanisms by which insulin changes tissue TCA metabolism. In particular, I will assess whether increased lactate contribution to TCA cycle depends on (i) insulin’s induction of glucose uptake, (ii) insulin’s inhibition of adipose lipolysis and associated decrease in circulating free fatty acids as alternative fuels, or (iii) insulin’s direct stimulation of lactate burning, likely via regulation of pyruvate dehydrogenase (PDH) activity. These alternatives will be rigorously examined using genetically engineered mouse models and in vivo metabolic flux analysis. Completion of these studies is poised to substantially advance understanding of mammalian metabolism, by illuminating a potentially major regulatory mechanism of likely importance to the pathogenesis of metabolic syndrome. The applicant brings unique pre-existing strengths in metabolic flux analysis at the cellular and subcellular level, but has not previously worked in vivo. Accordingly, the training program is designed to build physiological knowledge, animal handling skills, and organismal-level quantitative modeling capacity. Acquisition of this technical knowledge, combined with further growth in leadership abilities, will position the applicant to eventually become a pioneering independent scientist, studying metabolism quantitatively across spatial scales, from subcellular to organismal.