Date of Award
Doctor of Philosophy (PhD)
Genetic and environmental factors heavily intertwine to affect metabolic homeostasis. To tease out the exact interactions between these two realms of influence, researchers often compare how one or multiple different inbred mouse strains react to various diets. An observation consistently seen across multiple strains on the same diet can reasonably be considered a general dietary effect, whereas an observation seen only in one strain of mice is more likely to result from a genetic cause or gene-by-environment interaction. Similarly to humans, a high fat diet causes many mouse strains to develop obesity and poor metabolic health, with varying degrees of hyperglycemia, hyperinsulinemia, systemic insulin resistance, hyperlipidemia, ectopic lipid deposition, and inflammation and dysfunction of metabolic tissues.
The LG/J and SM/J inbred mouse strains show disparate responses to long-term exposure to a high fat diet. While 20 weeks of high fat diet causes both strains to develop obesity, hyperglycemia and impaired glucose tolerance, the glycemic effect in SM/J mice is stronger than in LG/J mice, and SM/J mice also develop hyperinsulinemia and impaired insulin sensitivity. Interestingly, an additional 10 weeks of high fat diet exposure results in reversion of unhealthy glycemic levels in SM/J mice, while LG/J mice maintain their elevated glucose parameters. This diabetic remission in SM/J mice is not the result of weight loss; the mice continue to gain weight, and in particular show a dramatic increase in the mass of their interscapular brown adipose tissue. Brown adipose tissue is best known for its function in non-shivering thermogenesis, the release of energy in the form of heat, however it has also emerged as a potent source of cytokines that can coordinate whole-body metabolic homeostasis. Based on the concurrent timing of the two phenomena, I hypothesized that the expansion of brown adipose tissue in the high fat-fed SM/J mice directly contributes to the glycemic normalization through secretion of pro-health cytokines and by serving as a more efficient glucose sink.
Though brown adipose is primarily associated with metabolic improvement through thermogenic action, I found no evidence that the expansion of brown adipose tissue in high fat-fed SM/J mice leads to increased thermogenesis. There is no change in the expression of the thermogenic genes Ucp1, Cidea, and Eva1, nor is there any change in mitochondrial DNA content, brown adipocyte morphology and Ucp1 staining, core body temperature, or circulating levels of thermogenesis-activating catecholamines. Instead, I found that the expression levels of Irs1 and Glut4, two key members of the insulin-stimulated glucose uptake pathway, increase significantly with the brown adipose expansion. This suggested that the brown adipose expansion can act as a insulin-responsive sink for excess blood glucose, and indeed removal of the brown adipose depot before or after expansion prevents the improvement in whole-body insulin sensitivity.
To further explore the gene-by-environment interactions that cause the uniqueness of the SM/J brown adipose expansion and its association with metabolically healthy obesity, I analyzed the transcriptomic profile of the brown and white adipose tissues of high and low fat-fed LG/J and SM/J mice at 20 and 30 weeks. I performed weighted gene co-expression network analysis to identify clusters of genes whose expression in brown adipose tissue correlate with metabolic phenotypes. I identified four clusters that are enriched for genes in cell division, immune and cytokine response, organic molecule metabolism, and peroxisome function, as well as four clusters that have no significant enrichment. While other modules were identified in all cohorts, the cell division cluster was only found when the high fat-fed SM/J mice were included in the analysis. Principal components analysis of the expression of genes in this cluster shows a distinct grouping of the high fat-fed SM/J samples away from the other cohorts.
Twenty-nine genes in the cell division cluster also show significant differential expression between 20 and 30 weeks in high fat-fed SM/J brown adipose. In particular, expression of Sfrp1 (secreted frizzled-related protein 1) positively correlates with brown adipose mass across all SM/J cohorts and with improved glucose tolerance in the high fat-fed SM/J mice. Sfrp1 has previously been identified as a pro-adipogenic cytokine in white adipose that positively correlates with insulin sensitivity and declines in obesity. The Sfrp1 locus contains variants between LG/J and SM/J mice and is located within QTL for adiposity and glucose tolerance. The human SFRP1 genomic region contains a variant (rs973441) is significantly associated with type 2 diabetes adjusted for BMI.
Overall, my dissertation robustly characterizes a novel mouse model of insulin-sensitive obesity dependent on natural brown adipose tissue expansion. I have defined the first brown adipose gene co-expression clusters and their relationship to metabolic phenotypes and identified the cytokine Sfrp1 as a novel stimulator of brown adipogenesis and insulin sensitivity. Together, these studies expand our knowledge of the potent non-thermogenic ability of brown adipose tissue to promote healthy metabolism in an obese state.
Chair and Committee
Heather A. Lawson
Barak A. Cohen, Irfan J. Lodhi, Clay F. Semenkovich, David W. Piston,
Carson, Caryn Nicole, "The Good, The Brown, and The Healthy: Understanding Non-Thermogenic Brown Adipose Function in Obese Mice" (2020). Arts & Sciences Electronic Theses and Dissertations. 2305.