Molecular mechanisms of adipocyte differentiation.

Within the past decade, our scientific notions of adipose tissue have changed dramatically, from the simplistic idea that fat serves only to store excess energy to the realization that adipose tissue is a central regulator of metabolic physiolo y. We know that too much fat leads to a variety of metabolic derangements, including insulin resistance and type 2 diabetes, while too little fat, as in the lipodystrophic conditions, is equally pathological. Adipocytes store e rgy in the form of triglyceride ; they release this energy in a regulated way as the body demands it, such as during fasting. The functions of fat go far beyond energy storage, however, and there has been increasing recognition of fat tissue as an endocrine organ as well. This began with the discovery of leptin in the mid 1990’s and continues through the present, as more secreted factors are being discovered in adipose tissue which affect a bewildering array of biologically important pathways ranging from metabolism, blood pressure control, coagulation, and immunity. The process by which f t cells develop is called adipogenesis. Adipogenesis is believed to begin with pluripot nt fibroblastic stem cells ; these cells have the ability to differentiate into muscle, cartilage, and bone as well as adipocytes if stimulated with the appropriate inducers [19, 25]. In response to as yet poorly characterized cues, these stem cells become committed to the adipocytic lineage. These cues might be hormones and/or they might be based in the extracellular matrix of a developing fat pad. egardless, this process is called “determination” and it marks the loss of multipotency. Cells that have been determined can only become adipocytes, even though th y may look identical to their precursor cells at the microscopic level. Mammals actually have two distinct types of adipocytes, wich make up brown adipose tissue (BAT) and white adipose tissue (WAT). WAT is the type of fat that most of us think of when we think of adipose tissue. It is the classic tissue of energy storage that marbles our steaks and accumulates around our mid-sections as we age. Brown fat cells have smaller collections of lipid and are very rich in mitochondria [16]. They serve primarily to generate heat through the actions of an uncoupling protein (UCP-1), which dissipates the proton gradient established across the inner mitochondrial membrane during respiration without generating ATP. Small animals like rodents have a large collection of brown fat located in a discrete pad between the scapulae. Humans, on the other hand, have small deposits of brown fat as infants in the thorax and around the kidneys This disappears as we age, until one can only find occasional brown fat cells scattered among the white adipocytes in our other depots as adults. We know very little about how BAT develops ; it may undergo adipogenesis in the same overall way as WAT, with extra factors being turned on to induce BAT-specific proteins like UCP-1. Alternatively, BAT could develop along entirely unique pathways. Finally, it is not known if BAT and WAT can « trans-differentiate » into one another. Fat cells have also served as a well-studied model of cellular differentiation. In part, this is due to the importance of this tissue in physiology and pathology, as has just been mentioned, but progress has been aided by th availability of immortalized cell lines that recapitulate the process of adipogenesis in vitro . These cell lines, most notably the 3T3-L1 and 3T3-F442A systems, have provided an invaluable reagent for investigators studying the mechanism of adipogenesis [9, 10]. With few exceptions, the phenotype of the cultured adipocytes matches that of endogenous adipocytes in vivo remarkably well. These in vitro models are induced to differentiate in the presence of a hormonal cocktail ; in the case of 3T3L1 cells this would include a glucocorticoid, insulin, and a cAMP analog or phosphodiesterase inhibitor. When treated with such a mixture, these cells undergo a stereotypical procession of events that includes morphological changes such as « rounding up » of the spindly preadipocytes, gene expression changes, accumulation of lipid, and the development of insulin sensitivity. Early on, it was noted that members of the CAAT/enhancer binding protein (C/EBP) family of transcription factors were induced during the differentiation of fat cells in vitro [2, 32]. Specifically, C/EBP β and C/EBP δ were turned on very early after the addition of the hor-