Irwin J Kurland MD PhD
Diabetes is an increasing concern not only for Western countries, where diet and lifestyle promote expanding waistlines and insulin resistance, but also for developing countries in which the effects of changing diet on the health of their populations are already visible. Diabetics suffer an impairment of the body's ability to switch between glucose and fat as energy sources. Even early in the evolution of diabetes (i.e., in the pre-diabetic state referred to as metabolic syndrome), individuals are unable to make this fuel switch, a physiological maladaptation termed metabolic inflexibility.
My laboratory has developed stable isotope based flux phenotyping methodology based on gas chromatography/mass spectrometry (GC/MS) to follow the “history” of glucose and other labeled substrates as they are exchanged between different organs or within the same organ, for in vivo assessment of tissue specific metabolic flexibility. These GC/MS flux profiling tests have demonstrated that some forms of insulin resistance are associated with unique, partially compensatory mechanisms of substrate re-cycling between liver, fat and muscle, as well as assessing neural influences over peripheral fuel utilization. In order to understand how dysregulations in metabolism may “feedback” to impact the proteins/enzymes that govern how fuels are selected/used, we have examined how a metabolite common to glucose, fatty acid and amino acid metabolism, acetyl CoA, acts as a “sensor” for regulating fuel usage in metabolic networks. Changes in acetyl CoA in cellular compartments help modify the acetylation of key proteins/enzymes in metabolic networks in the fasted to re-fed transition. Our hypothesis is that metabolic pathway specific insulin resistance may be a result of dysregulation of the acetylation of proteins in tissue specific metabolic networks, in either the fasted and/or re-fed states. One focus at present, in collaboration with the Accili and Haeusler laboratories, is the inter-relationship of hepatic Akt2, FoxOs and SirT1 on the fasted/re-fed metabolic and acetylome response.
We have, and are, proving the utility of these combined ‘omics methodologies, in a variety of KO mouse models of key genes regulating glucose and fatty acid metabolism. We contend that the association of a unique plasma flux profile with unique plasma/tissue metabolite profiles enables the interpretation of the metabolic network acetylome, and that the combined flux, metabolite and acetylome determination reflects the underlying pathophysiology in models of impaired glucose tolerance and metabolic inflexibility. For animal models, we have constructed a tiered framework (Kurland et al Application of combined 'omics platforms to accelerate biomedical discovery in diabesity, Ann. N.Y. Acad. Sci. (2013) ISSN 0077-8923) in which commonly used measures of metabolism (e.g., phenotyping tests such as calorimetry and body composition analysis) and a novel deuterated glucose tolerance test (termed the hepatic recycling deuterated glucose tolerance test, or HR-dGTT) that assesses peripheral versus hepatic glucose disposal, are performed first to help determine which specific 'omics experiments to do next. By beginning with these simple whole body measurements, one can decide, step by step, on hypothesis driven, multi omic characterizations aimed towards understanding mechanisms at the molecular level.