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 (T-3) and dexa methasone (a synthetic glucocorticoid) on differentiation, lipid metab olism and thermogenesis of preadipocytes isolated from white fat (WAT) and brown fat (BAT) from the Siberian dwarf hamster (Phodopus sungoru s), Cell cultures from WAT and BAT were chronically treated with the a bove hormones alone or in any combination. After differentiation (day 8 or 9 of culture) we measured the following parameters: adipogenic in dex (number x size of adipocytes), protein content, lipolysis, cell re spiration, and expression of the uncoupling protein UCP, which is uniq ue to mitochondria of brown adipocytes. Insulin was the most important adipogenic factor for brown and white adipocytes and necessary for te rminal differentiation, whereas dexamethasone alone completely inhibit ed differentiation. T-3 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 d examethasone was present, which stimulated lipolysis in both culture t ypes to the same extent, T-3 had a pronounced dose dependent lipolytic effect on WAT cultures but very little effect on BAT cultures, Respir ation rates were generally higher in differentiated adipocytes than in fibroblast like cells, T-3 had no effect on thermogenesis in WAT cult ures 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, T-3 treated and insulin + T-3 tre ated cultures with highest expression in the latter. These results imp ly a possible dissociation of terminal differentiation and thermogenic function of brown adipocytes. In WAT cultures there was also a low le vel of UCP detectable in the insulin + T-3 treated cultures. Immuno-fl uorescence 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.