The phenotypic approach (2) – advanced metabolic profiling in rodents

Dr Peter Voshol

Summer School 11-14 July 2006
The Møller Centre, Storeys Way, Cambridge, UK

For several years now there has been considerable focus on the dramatic increase in the prevalence of Type 2 Diabetes (T2DM). T2DM is a complex end-stage of an even more complex syndrome called the metabolic syndrome or insulin resistance syndrome. The metabolic syndrome has been identified as a constellation of metabolic and non-metabolic disorders. Many of these are related to defects in insulin sensitivity and eventually lead to an increased prevalence of cardiovascular disease (CVD). The insulin resistance is defined as the decreased ability of insulin to act effectively on peripheral target tissues (especially muscle, adipose tissue, and liver), and results from environmental as well as genetic susceptibility. Obesity is a shared characteristic, which almost certainly contributes to the distinctive elevated triglyceride and free fatty acid levels associated with both conditions.

The cause of insulin resistance is under debate and not fully understood, but it is probably the consequence of alteration in lipid metabolism. The most commonly accepted lipid mediator is the chronically elevated level of free fatty acids (FFAs) that impairs insulin-signaling pathways. The question remains whether FFAs are equally distributed to liver, adipose tissue or muscle. The transport of fatty acids (FAs) in plasma occurs in two ways: via (albumin-bound) FFAs and via triglycerides (TGs) that are transported in TG-rich lipoproteins. TGs in lipoproteins are hydrolyzed into FAs by the action of lipoprotein lipase (LPL), an enzyme expressed by tissues in need of the FA that is located at the capillary endothelium. The regulation of LPL is tissue-specific and dependent on the nutritional status, reflecting the FA requirements of the respective tissues at a specific time. Thus, in the fight against the metabolic syndrome, more insight in to the regulation of FA partitioning in relation to the pathogenesis of metabolic risk is required.

Beyond the assessment of gene expression (genomics), protein expression (proteomics), metabolite profiling (metabolomics) lays the physiology. In search of the physiological impact of gene modulation, assessment of nutrient fluxes is one of the key goals. This is referred to as fluxomics. During this part of the workshop, the focus will be on assessing quantitative glucose and lipid (FFAs and TGs) fluxes in vivo. Glucose fluxes (and insulin sensitivity) are assessed using the ‘gold standard’ hyperinsulinemic euglycemic clamp technique. By understanding nutrient fluxes throughout the body we will be better able to unravel the pathogenic pathways leading to many features of the metabolic syndrome, e.g. cardiometabolic risk.

The consequences of impaired PPARg function on glucose fluxes/insulin sensitivity have been examined using a homologous murine model of the P467L PPARg dominant negative mutation previously described in humans. This mouse model recapitulates several aspects of the human phenotype as it displays impaired postprandial lipid metabolism, exacerbated fatty liver in response to high fat diet and hypertension – but what happens to insulin sensitivity? Studies, which have addressed this important question in P465L mice, will be used to demonstrate some of the techniques that are available for detailed metabolic profiling in rodent models of human disease.

The opinions expressed in this paper are those of the speaker and do not necessarily reflect the views of the Society

Revised: 24-Aug-2006