Improved knowledge of all aspects of adipose biology will be required to counter the burgeoning epidemic of obesity. Interest in adipogenesis has increased markedly over the past few years with emphasis on the intersection between extracellular signals and the transcriptional cascade that regulates adipocyte differentiation. Many different events contribute to the commitment of a mesenchymal stem cell to the adipocyte lineage including the coordination of a complex network of transcription factors, cofactors and signalling intermediates from numerous pathways.

There has been an upsurge of interest in the adipocyte coincident with the onset of the obesity epidemic and the realization that adipose tissue plays a major role in the regulation of metabolic function. The past few years in particular have seen significant changes in the way we classify adipocytes, and how we view adipose development and differentiation. We have new perspective on the roles played by adipocytes in a variety of homeostatic processes, and the mechanisms used by adipocytes to communicate with other tissues. Finally, there has been significant progress in understanding how these relationships are altered during metabolic disease, and how they might be manipulated to restore metabolic health.

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Insulin resistance is a cardinal feature of type 2 diabetes and is characteristic of a wide range of other clinical and experimental settings. Little is known about why insulin resistance occurs in so many contexts. Do the various insults that trigger insulin resistance act through a common mechanism? Or, as has been suggested, do they use distinct cellular pathways? Here we report a genomic analysis of two cellular models of insulin resistance, one induced by treatment with the cytokine tumour-necrosis factor-alpha and the other with the glucocorticoid dexamethasone. Gene expression analysis suggests that reactive oxygen species (ROS) levels are increased in both models, and we confirmed this through measures of cellular redox state. ROS have previously been proposed to be involved in insulin resistance, although evidence for a causal role has been scant. We tested this hypothesis in cell culture using six treatments designed to alter ROS levels, including two small molecules and four transgenes; all ameliorated insulin resistance to varying degrees. One of these treatments was tested in obese, insulin-resistant mice and was shown to improve insulin sensitivity and glucose homeostasis. Together, our findings suggest that increased ROS levels are an important trigger for insulin resistance in numerous settings.

Adipocytes have been studied with increasing intensity as a result of the emergence of obesity as a serious public health problem and the realization that adipose tissue serves as an integrator of various physiological pathways. In particular, their role in calorie storage makes adipocytes well suited to the regulation of energy balance. Adipose tissue also serves as a crucial integrator of glucose homeostasis. Knowledge of adipocyte biology is therefore crucial for understanding the pathophysiological basis of obesity and metabolic diseases such as type 2 diabetes. Furthermore, the rational manipulation of adipose physiology is a promising avenue for therapy of these conditions.

The process of adipogenesis is known to involve the interplay of several transcription factors. Activation of one of these factors, the nuclear hormone receptor PPAR gamma, is known to promote fat cell differentiation in vitro. Whether PPAR gamma is required for this process in vivo has remained an open question because a viable loss-of-function model for PPAR gamma has been lacking. We demonstrate here that mice chimeric for wild-type and PPAR gamma null cells show little or no contribution of null cells to adipose tissue, whereas most other organs examined do not require PPAR gamma for proper development. In vitro, the differentiation of ES cells into fat is shown to be dependent on PPAR gamma gene dosage. These data provide direct evidence that PPAR gamma is essential for the formation of fat.

DNA methylation is a defining feature of mammalian cellular identity and essential for normal development1,2. Most cell types, except germ cells and pre-implantation embryos3–5, display relatively stable DNA methylation patterns with 70–80% of all CpGs being methylated6. Despite recent advances we still have a too limited understanding of when, where and how many CpGs participate in genomic regulation. Here we report the in depth analysis of 42 whole genome bisulfite sequencing (WGBS) data sets across 30 diverse human cell and tissue types. We observe dynamic regulation for only 21.8% of autosomal CpGs within a normal developmental context, a majority of which are distal to transcription start sites. These dynamic CpGs co-localize with gene regulatory elements, particularly enhancers and transcription factor binding sites (TFBS), which allow identification of key lineage specific regulators. In addition, differentially methylated regions (DMRs) often harbor SNPs associated with cell type related diseases as determined by GWAS. The results also highlight the general inefficiency of WGBS as 70–80% of the sequencing reads across these data sets provided little or no relevant information regarding CpG methylation. To further demonstrate the utility of our DMR set, we use it to classify unknown samples and identify representative signature regions that recapitulate major DNA methylation dynamics. In summary, although in theory every CpG can change its methylation state, our results suggest that only a fraction does so as part of coordinated regulatory programs. Therefore our selected DMRs can serve as a starting point to help guide novel, more effective reduced representation approaches to capture the most informative fraction of CpGs as well as further pinpoint putative regulatory elements.

PPAR␥ and C/EBP␣ are critical transcription factors in adipogenesis, but the precise role of these proteins has been difficult to ascertain because they positively regulate each other's expression. Questions remain about whether these factors operate independently in separate, parallel pathways of differentiation, or whether a single pathway exists. PPAR␥ can promote adipogenesis in C/EBP␣-deficient cells, but the converse has not been tested. We have created an immortalized line of fibroblasts lacking PPAR␥, which we use to show that C/EBP␣ has no ability to promote adipogenesis in the absence of PPAR␥. These results indicate that C/EBP␣ and PPAR␥ participate in a single pathway of fat cell development with PPAR␥ being the proximal effector of adipogenesis. Received September 27, 2001; revised version accepted November 9, 2001. Adipogenesis is the process by which undifferentiated precursor cells differentiate into fat cells. This has become one of the most intensively studied developmental processes for at least two reasons: the increasing prevalence of obesity in our society has focused attention on many aspects of fat cell biology, and the availability of good cell culture models of adipocyte differentiation has permitted detailed studies not possible in other systems. Experiments using these in vitro models of adipogenesis, which include the 3T3-L1 and 3T3-F442A lines, have illustrated the transcriptional cascade that promotes fat cell differentiation (Rosen et al. 2000). Representatives of several transcription factor families have been implicated in this process, including the CCAAT/enhancer binding proteins C/EBP␣, C/EBP␤, and C/EBP␦; the nuclear hormone receptor peroxisome proliferator-activated receptor ␥ (PPAR␥); and the basic helix-loop-helix protein ADD1/SREBP1c. Studies in adipogenic cell lines have shown that hormonal induction of differentiation is rapidly followed by expression of C/EBP␤ and C/EBP␦ (Cao et al. 1991;Yeh et al. 1995). Within the next day or so, levels of these proteins peak and then begin to drift downward, coincident with a rise in C/EBP␣ and PPAR␥. These latter factors induce gene expression changes characteristic of mature adipocytes and remain elevated for the life of the cell. In the present model of the transcriptional cascade leading to adipogenesis, C/EBP␤ and C/EBP␦ induce low levels of PPAR␥ and C/EBP␣, which are then able to induce each other's expression in a positive feedback loop that promotes and maintains the differentiated state. This model is consistent with gain-offunction data showing that the addition of either PPAR␥ or C/EBP␣ can promote adipogenesis in fibroblast cell lines (Lin and Lane 1994;Tontonoz et al. 1994).Loss-of-function studies have shown convincingly that PPAR␥ is required for adipogenesis in vivo and in vitro, and cells lacking PPAR␥ express greatly reduced levels of C/EBP␣ (Barak et al. 1999;Kubota et al. 1999;Rosen et al. 1999). Similarly, fibroblasts lacking C/EBP␣ have reduced adipogenic potential, and express reduced levels of PPAR␥ (Wu...