Thermogenic genes are blunted whereas brown adipose tissue identity is preserved in human obesity

Obesity associates with a reduction in cold-induced glucose tracer uptake in brown adipose tissue in humans, suggesting loss of thermogenic capacity. We therefore hypothesized that a whitening of BAT occurs in obesity and assessed the molecular characteristics of deep neck BAT in a cohort of 24 normal weight, 24 overweight and 22 obese individuals in comparison with subcutaneous abdominal white adipose tissue (WAT). We found that the major marker of BAT thermogenesis, UCP1, was associated with central but not general obesity. We performed transcriptomic analysis of BAT in a cohort of 27 individuals classified as normal weight, over-weight or obese, and additionally four subjects with type 2 diabetes (T2DM), dispersed among the 3 BMI groups. We identified 3204 differentially expressed genes between BAT and WAT in samples from normal weight individuals, including genes involved in thermogenesis, but also revealing differences in developmental and immune system related genes. In BAT from individuals with overweight or obesity, 202 genes were downregulated and 66 of these were involved in cellular respiratory pathways, likely reflecting previously observed reduction in thermogenic function with obesity. Importantly, most BAT selective genes were not affected, and isolated adipose progenitors differentiated into thermogenic adipocytes with equal frequency regardless of BMI group. In conclusion, our data suggest a retained BAT identity, with a selective reduction of thermogenic genes, in human obesity.