Adipogenesis involves differentiation of pre-adipocytes into mature adipocytes by clonal cell expansion and it is drivendetermined by two major adipogenic transcription factors, ccaat/enhancer binding protein (c/ebp) and peroxisome proliferator-activated receptor (ppar)
Answers
Obese individuals are more likely than their lean counterparts to develop cardiovascular disease and type 2 diabetes. The increase in adiposity in these individuals results from an upsurge in both adipocyte number and size of individual fat cells. Additionally, the disproportionate increase in the visceral adipose depots in some individuals is linked to development of certain metabolic disorders. Consequently, understanding the mechanisms regulating adipose formation should provide valuable information in the fight to combat the growing incidence of obesity in the modern world.
During the last several years, investigators have embarked on a detailed and systematic endeavor to define the transcriptional events regulating preadipocyte differentiation (adipogenesis) and adipocyte function. The differentiation of preadipocytes into adipocytes is regulated by an elaborate network of transcription factors that coordinate expression of hundreds of proteins responsible for establishing the mature fat-cell phenotype. At the center of this network are the two principal adipogenic factors, PPARγ and C/EBPα, which oversee the entire terminal differentiation process. PPARγ in particular is considered the master regulator of adipogenesis; without it, precursor cells are incapable of expressing any known aspect of the adipocyte phenotype (Rosen et al., 2002). On the other hand, cells deficient in C/EBPα are capable of adipocyte differentiation; however, these C/EBPα-deficient cells are insulin resistant (El-Jack et al., 1999; Wu et al., 1999). Much of our knowledge of this complex network and the importance of PPARγ and C/EBPα comes from studies performed in established preadipocyte cell lines as well as mesenchyme-derived precursor cells. More recently, data from a variety of knockout mice have confirmed these in vitro studies showing that many components of this network are required regulators of adipocyte development and function.
The 3T3-L1 and 3T3-F422A preadipocyte cell lines originally established by Green and associates have greatly facilitated our knowledge of the molecular mechanisms controlling adipogenesis (Green and Kehinde, 1975, 1976). Although committed to the adipocyte lineage, proliferating 3T3-L1 preadipocytes exert characteristics similar to those of other 3T3 fibroblasts. Confluent 3T3-L1 preadipocytes differentiate upon exposure to the adipogenic inducers fetal bovine serum (FBS), dexamethasone, isobutylmethylxanthine, and insulin. This cocktail activates an adipogenic program, which occurs in two well-defined phases. The stimulated cells immediately reenter the cell cycle and progress through at least two cell-cycle divisions, a phase often referred to as clonal expansion. During this time, the cells express specific adipogenic transcription factors as well as cell-cycle regulators that together facilitate expression of PPARγ and C/EBPα. Following this event, the committed cells undergo terminal differentiation manifested by production of lipid droplets as well as expression of multiple metabolic programs characteristic of mature fat cells. The validity of this 3T3-L1 system as an appropriate model of adipocyte formation in the animal has been supported by many studies performed in both mouse and human tissue.