Tisdale, P.
(2011)
The regulation of metabolic gene expression in humans.
PhD thesis, University of Nottingham.
Abstract
The regulation of metabolic gene expression is fundamental to maintaining energy balance. Changes in substrate availability can alter metabolic gene expression, in order to modify the utilisation of nutrients appropriately. Metabolic gene expression can also be dysregulated in disease states. The work in this thesis examines several situations in which energy balance or metabolic substrate supply was altered, and investigates how metabolic gene expression adapted and contributed to the phenotypes observed following these interventions. All experiments in this work looked at metabolic regulation from a human perspective; with either biopsy material or cells cultured from biopsy. These experiments included;
1. The influence of postprandial fat-oxidising capacity during a calorie restricted diet of either high- or low-fat content in obese subjects.
2. Transcriptional profiling of adipose tissue in obese subjects with high- or low-postprandial fat oxidising capacity
3. High-glucose treatment of primary human myotubes (as a model of hyperglycaemia).
4. Increased PDC activation and hence carbohydrate oxidation in vivo, through administration of dichloroacetate.
Postprandial fat-oxidising capacity did not affect weight-loss during a calorie restricted diet, and there was no affect of diet composition. However, changes in metabolic gene expression were observed between groups over the course of the 10-week intervention. The groups which showed the greatest changes in gene expression were the low fat-oxidisers on a high fat diet and the high fat-oxidisers on a low fat diet, possibly due to a mismatch between diets and fat oxidising capacity, which required greater adaptation. Covariate analysis revealed interactions between gene expression and other phenotypic parameters. SREBP-1c showed a relationship with FFA concentrations and insulin-resistance, whilst HSL and apM1 were associated with FFA concentrations and Insulin resistance respectively, which underlines the importance of looking for underlying structures in data.
Transcriptional profiling of adipose tissue in obese subjects with high- or low-postprandial fat oxidising capacity, revealed significant differences in the expression of metabolic genes, and highlighted the importance of several transcripts; including RXRA, SREBP-1c and GLUT4 in determining the phenotype of adipose tissue. The major differences observed in gene expression between high and low fat oxidizers indicated that genes involved glucose metabolism and lipogenesis rather than beta-oxidation were the major processes that differed between the two groups. Genomic data indicated that the expression of these genes was not influenced to a major degree by polymorphisms within the population.
High-glucose treatment of primary myotubes, demonstrated the significance of ChREBP and some of its targets, in inducing the expression of lipogenic enzymes, which may be linked to the accumulation of intramyocellular triglyceride. However, these data also indicated the potential for the cell to initiate protective mechanisms, of substrate handling and lipid clearance, in response to carbohydrate oversupply. Conversely, increasing PDC activity and hence carbohydrate oxidation without altering substrate availability via infusion of dichloroacetate, did not alter the expression of metabolic genes in skeletal muscle. This reflects a capacity to deal with acute changes in the activity in metabolic genes without altering their expression.
In conclusion, the studies from this thesis show that important differences in metabolic gene expression can be observed during situations where energy balance and substrate availability are altered. However, flexibility within the metabolic networks means that acute changes can be countered without the need for induction or suppression of metabolic genes, and that during chronic alterations in nutrient supply, rapid adaptations and protective mechanisms are activated.
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