Obesity results from the accumulation of excess white adipose tissue, and its increasing rates worldwide pose a significant risk to overall health due to its multiple comorbidities, like type 2 diabetes. A novel treatment strategy is to increase energy expenditure through activation of brown and beige adipocytes, which can use uncoupling protein 1 (UCP1) for thermogenesis. In addition to having different developmental lineages, brown and beige adipocytes differ functionally regarding the additional mechanisms by which they generate heat. Most human fat depots are not easily accessible, so little is known about their developmental and functional characteristics. In this study, we collected adipose tissue from 11 patients (age 16-84, 4 female/7 male) who had undergone autopsy. We sampled from 7 anatomically distinct fat depots: subcutaneous (sc), omental (om), retroperitoneal (rp), pericardial (pc), periadrenal (pa), paraspinal (ps), and supraclavicular (sv). We measured mRNA levels of lineage and functional genes associated with brown, beige, and white adipocytes. Energy storing white adipocytes were defined by high leptin expression, while energy expending brown and beige cells were defined by high UCP1; these were further distinguished as having a brown lineage using Zic1 and beige via Tbx1. Principal component analysis showed three different clusters of genes consistent with the brown, beige, and white lineages. Functionally, based on expression of leptin and UCP1 for white and brown fat, respectively, the sc and om were white (P<0.05), while rp, pc, pa, ps, and sv were mostly brown (P<0.05). Regarding lineage of the UCP1+ depots, ps had both beige and brown cells, while rp, pa, ps, and sv were predominantly brown.
In summary, human adipose tissue is not merely white or brown but instead is composed of many depots with distinct developmental lineages and functional capacities. These differences will be important when trying to use brown and beige adipocyte thermogenesis to treat obesity and diabetes.
J.W. Johnson: None. C. Cero: None. A. O'Mara: None. J.D. Linderman: None. A.S. Baskin: None. A. Cypess: None.