Functional assessment of white and brown adipocyte development and energy metabolism in cell culture. Dissociation of terminal differentiation and thermogenesis in brown adipocytes.

We investigated the effect of insulin, triiodothyronine (T3) and dexamethasone (a synthetic glucocorticoid) on differentiation, lipid metabolism and thermogenesis of preadipocytes isolated from white fat (WAT) and brown fat (BAT) from the Siberian dwarf hamster (Phodopus sungorus). Cell cultures from WAT and BAT were chronically treated with the above hormones alone or in any combination. After differentiation (day 8 or 9 of culture) we measured the following parameters: adipogenic index (number x size of adipocytes), protein content, lipolysis, cell respiration, and expression of the uncoupling protein UCP, which is unique to mitochondria of brown adipocytes. Insulin was the most important adipogenic factor for brown and white adipocytes and necessary for terminal differentiation, whereas dexamethasone alone completely inhibited differentiation. T3 had no effect on adipogenesis in WAT cultures, but further increased insulin stimulated adipogenesis in BAT cultures. Basal lipolysis was higher in WAT than in BAT cultures except when dexamethasone was present, which stimulated lipolysis in both culture types to the same extent. T3 had a pronounced dose dependent lipolytic effect on WAT cultures but very little effect on BAT cultures. Respiration rates were generally higher in differentiated adipocytes than in fibroblast like cells. T3 had no effect on thermogenesis in WAT cultures but increased thermogenesis in BAT cultures, and this was further elevated by insulin. UCP expression in BAT cultures could be detected by western blot in insulin treated, T3 treated and insulin+T3 treated cultures with highest expression in the latter. These results imply a possible dissociation of terminal differentiation and thermogenic function of brown adipocytes. In WAT cultures there was also a low level of UCP detectable in the insulin+T3 treated cultures. Immuno-fluorescence microscopy analysis revealed the presence of UCP in 10-15% of adipocytes from WAT cultures (in BAT cultures: 90%), indicating the presence of some brown preadipocytes in typical WAT deposits.