Development ofPhodopus Sungorus Brown Preadipocytes in Primary Cell Culture: Effect ofan Atypical Beta-adrenergic Agonist, Insulin, and Triiodothyronin on Differentiation, Mitochondrial Development, and Expression ofthe Uncoupling Protein UCP

A new cellular model for the study of brown adipocyte development and differentiation in vitro is presented . Preadipocytes isolated from brown adipose tissue (BAT) of the djungarian dwarf hamster Phodopus sungorus are able to proliferate and differentiate in vitro into true brown adipocytes able to express the BAT marker protein the uncoupling protein (UCP) . Whereas basal UCP expression is very low, its mRNA levels as well as the UCP detected by immunoblotting are highly increased by 0-adrenergic stimulation . The novel, atypical 0-adrenergic compound D7114 (ICI Pharmaceuticals, Macclesfield, Cheshire, Two types of adipose tissue with quite different functions are known to exist in mammals : firstly the white adipose tissue whose main function is energy storage, secondly the so-called brown adipose tissue (BAT)', which functions as a heat dissipating organ in small and newborn mammals . This function is due to the uncoupling protein (UCP), uniquely expressed in BAT, which is located in the inner mitochondrial membrane (for review see 24) . UCP functions as a proton translocator and can thus short circuit the coupling between the respiratory chain and ATP production, resulting in elevated respiration rates and dissipation of the energy as heat (28) . Contrary to white fat, the BAT is highly innervated and vascularized and brown adipocytes have very high respiratory capacities, i .e ., a high content of mitochondria. Brown fat is considered to play an important function in energy balance, by dissipating excess energy intake as heat in certain animal models (38) . It is also known that many types of obesity in animals are associated with a defective BAT function (17) . Despite their quite contrary physiological function, it is not yet clear if brown and white adipocytes should be considered as two distinctly different cell types or iftheyjust represent two different differentiation or maturation states of the same cell type . According to our present knowledge, every adipocyte that expresses UCP is 1 . Abbreviations used in this paper : BAT, brown adipocyte tissue; COX, cytochrome-c-oxidase ; LPL, lipoprotein lipase ; UCP, uncoupling protein. © The Rockefeller University Press, 0021-9525/91/12/1783/8 $2 .00 The Journal of Cell Biology, Volume 115, Number6, December 19911783-1790 England) was found to increase the number of adipocytes as well as UCP mRNA and UCP content of mitochondria, indicating the involvement of an atypical or /33 receptor. Insulin was found to play an important role in brown adipocyte differentiation and mitochondria) development, whereas T3 seemed to be implicated more directly in UCP expression . In a defined, serum-free medium a synergistic stimulatory action of insulin and T3 on UCP expression was found, which seems to involve a pathway different from that of 0-adrenergic UCP stimulation . considered a brown adipocyte, whereas an adipocyte not expressing UCP is not automatically a white adipocyte. In small mammals, like rodents, BAT preserves throughout the animal life its thermogenic ability, i .e ., the ability to express UCP. But in larger mammals, like bovine and ovine species, itwas shown that fat depots ofa true brown natureare present at birth, which very rapidly lose their "brown" nature and become typical white depots which will never express UCP anymore (9) . We don't know yet ifthis means that brown adipocytes become white adipocytes or if new white adipocytes are recruited which replace brown adipocytes. This leads to two crucial questions . (a) Do brown and white adipocytes originate from different precursor cells? (b) How is the gene expression of UCP, the only true marker for brown adipocytes, regulated? Numerous in vivo studies have shown that acute UCP function as well as UCP gene expression are stimulated by Noradrenaline and numerous 0-adrenergic agents (for reviews see 28, 35, 37) . Studies on the cellular level, however, had been unsuccessful for many years . Only recently the group of B. Cannon found expression of UCP in primary cultures of preadipocytes, isolated from mouse BAT (31) . Since then, several reports from the same group (16, 32) and others (20, 27) showed that UCP expression of mouse brown adipocytes differentiated in vitro can be stimulated by various 0-adrenergic agonists . In this study we introduce a new animal model for the study of brown adipocyte development and differentiation, 1783 on A ril 9, 2017 D ow nladed fom Published December 15, 1991