Transcriptional regulation of adipogenesis.

The past twenty years have seen great strides in our understanding of the molecular regulation of differentiation for a whole host of specific cell types. The development of muscle, bone, pituitary, and several hematopoeitic lineages have all been probed using a variety of biochemical and genetic means, and such studies have yielded both general and tissue-specific insights. Adipose tissue has also been the subject of intense scrutiny, for two important reasons. First, the establishment of immortal preadipocyte cell lines (see below) provided an experimentally accessible system in vitro, many features of which faithfully recapitulate this process in vivo. These features include morphological changes, cessation of cell growth, expression of many lipogenic enzymes, extensive lipid accumulation, and the establishment of sensitivity to most or all of the key hormones that impact on this cell type, including insulin. The second reason why adipose tissue has received so much attention is that this tissue provides a critical link in maintaining systemic energy balance. The ongoing explosion in the incidence of obesity and its ugly stepsister, type 2 diabetes, has focused attention on all aspects of adipocyte biology, including adipogenesis (Must et al. 1999). A wealth of observations on the development of adipose tissue in vivo in a variety of species have been recorded throughout the past century. Early on it was noted that fat develops in many different sites scattered throughout the body, generally occurring in areas composed of loose connective tissue, such as the subcutaneous layers between the muscle and dermis. However, fat deposits also form around the heart, kidneys, and other internal organs. The diffuse nature of adipose differentiation in vivo has made it difficult to dissect out regions of presumptive precursor cells from embryos and study these ex vivo. Similarly, a lack of molecular markers identifying the preadipocytic state has hampered investigations in living animals. These difficulties were partially circumvented in the 1970’s when Green and his colleagues established immortal fibroblast lines that readily differentiated into adipocytes when appropriate hormonal inducers were added (Green and Kehinde 1974, 1975, 1976). These lines, designated 3T3-L1 and 3T3-F442A, were isolated from nonclonal Swiss 3T3 cells and are already committed (or determined) to the adipocytic lineage. When treated with an empirically-derived prodifferentiative regimen that includes cAMP, insulin, and glucocorticoids, they undergo differentiation to mature fat cells over a 4–6 day period. Subsequently, committed preadipocyte lines have been derived independently by others (Negrel et al. 1978; Chapman et al. 1984); despite minor differences in their optimal differentiation conditions they behave very similarly to 3T3-L1 and 3T3-F442A cells. More recently, studies have been performed using pluripotent stem cells that can be induced to yield adipose tissue in addition to several other lineages. Mesenchymal stem cell lines can be differentiated to muscle, cartilage, and fat depending upon the chosen conditions (Taylor and Jones 1979), and marrow-derived stromal cells can be induced to form both bone cells and fat cells (Pittenger et al. 1999). Although stem cell technology is developing rapidly, their recent introduction and the complexity inherent in these systems has prevented them from being extensively used in the adipogenesis field. Almost all work on adipogenesis has utilized either the aforementioned predetermined clonal cell lines or cultured preadipocytes isolated from the stromal-vascular fraction of dissociated fat pads. Despite the many insights gleaned from these models, several caveats must be kept in mind when interpreting results. First, these cell lines are differentiated to white adipose tissue (WAT) almost exclusively. Mammals have a second type of fat cell called the brown adipocyte, which serves primarily to dissipate energy instead of storing it (Lowell and Flier 1997). This energy wasting is accomplished by a brown adipose tissue (BAT)-specific protein called uncoupling protein-1 (UCP-1), which generates heat by dissipating the proton gradient that is established across the inner mitochondrial membrane during the passage of electrons along the respiratory chain (Garlid et al. 1998). In physiological terms, brown fat functions in adaptation to cold and as protection against obesity. Cultured cell models of BAT do exist, but most of these do not mimic their endogenous ortholog quite as 3Corresponding authors. E-MAIL edrosen@massmed.org; FAX (617) 632-5363. E-Mail bruce spiegelman@dfci.harvard. edu; FAX (617) 632-4655.