Mechanisms Underlying Adipose Tissue Thermogenesis

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In collaboration with Dr. Tseng's group at the Joslin Diabetes Center / Harvard Medical School in Boston, we generated clonal cell lines from human neck fat and characterized their adipogenic differentiation and metabolic function in vitro and in vivo after transplantation into immune deficient nude mice. Using clonal analysis and gene expression profiling, we identified unique sets of gene signatures in human preadipocytes that could predict the thermogenic potential of these cells once matured in culture into adipocytes. These data highlight the cellular heterogeneity in human BAT and WAT and provide novel gene targets to prime preadipocytes for thermogenic differentiation. We also used these immortalized human adipocytes to describe the adrenergic receptor expression and functional profiling that will guide the develop of pharmacological compounds that can be used to activate human BAT and treat obesity and metabolic disease. With ATCC, we are developing an additional collection of human adipose tissue cell populations and cell lines from additional depots. These will help enrich the functional and anatomical mapping of human adipose tissue. In parallel, we tested the effects of diet-induced obesity (DIO) on brown adipose tissue (BAT). We fed 6-week-old C57BL/6 mice either a normal chow diet (NCD) or a high-fat diet (HFD). After 16 additional weeks, we measured body fat, WAT, and BAT mRNA expression, glucose tolerance, and rates of glucose uptake in response to insulin and the beta3-AR agonist mirabegron. We saw that compared with NCD, HFD increased body fat and impaired glucose tolerance. Both WAT and BAT had higher mRNA levels of markers of inflammation, including TNF and F4/80. Insulin signaling in BAT and WAT was reduced, with decreased Akt phosphorylation. Diet-normalized BAT glucose uptake rates were lower in response to mirabegron. These results support a model in which DIO leads to BAT inflammation and insulin resistance, leading to a broader impairment of BAT function. More recently, working again with Dr. Tseng, we utilized mass-spectrometry-based lipidomics in mice and humans to demonstrated that cold and 3-adrenergic stimulation could promote the biosynthesis and release of 12-lipoxygenase (LOX) metabolites from BAT. Moreover, 12-LOX ablation in mouse brown adipocytes impaired glucose uptake and metabolism, resulting in blunted adaptation to the cold in vivo. The cold-induced 12-LOX product 12-Hydroxyeicosapentaenoic acid (12-HEPE) was found to be a batokine that improves glucose metabolism by promoting glucose uptake into adipocytes and skeletal muscle through activation of an insulin-like intracellular signaling pathway. Cheryl Cero, a senior postdoctoral fellow in the group has been working with human-derived brown adipocytes and showed that silencing of the 3-AR compromises genes essential for thermogenesis, fatty acid metabolism, and mitochondrial mass. Functionally, reduction lowered agonist-mediated increases in cAMP levels, lipolysis, and thermogenic capacity. Furthermore, mirabegron, a selective human beta3-AR agonist, stimulated BAT lipolysis and thermogenesis, and both processes were lost after silencing 3-AR expression. This study highlights that beta3-ARs in human brown/beige adipocytes are required to maintain multiple components of lipolytic and thermogenic cellular machinery and that beta3-AR agonists could be used to achieve metabolic benefit in humans. In the past year, we have expanded upon these discoveries to show that human brown and white adipocytes may be capable of generating bile acids and their precursors, a new role for these tissues in human physiology.