An Evi1-C/EBPß complex controls peroxisome proliferator-activated receptor γ2 gene expression to initiate white fat cell differentiation.

Fibroblastic preadipocyte cells are recruited to differentiate into new adipocytes during the formation and hyperplastic growth of white adipose tissue. Peroxisome proliferator-activated receptor γ (PPARγ), the master regulator of adipogenesis, is expressed at low levels in preadipocytes, and its levels increase dramatically and rapidly during the differentiation process. However, the mechanisms controlling the dynamic and selective expression of PPARγ in the adipocyte lineage remain largely unknown. We show here that the zinc finger protein Evi1 increases in preadipocytes at the onset of differentiation prior to increases in PPARγ levels. Evi1 expression converts nonadipogenic cells into adipocytes via an increase in the predifferentiation levels of PPARγ2, the adipose-selective isoform of PPARγ. Conversely, loss of Evi1 in preadipocytes blocks the induction of PPARγ2 and suppresses adipocyte differentiation. Evi1 binds with C/EBPß to regulatory sites in the Pparγ locus at early stages of adipocyte differentiation, coincident with the induction of Pparγ2 expression. These results indicate that Evi1 is a key regulator of adipogenic competency.

Prdm16 determines the thermogenic program of subcutaneous white adipose tissue in mice.

The white adipose organ is composed of both subcutaneous and several intra-abdominal depots. Excess abdominal adiposity is a major risk factor for metabolic disease in rodents and humans, while expansion of subcutaneous fat does not carry the same risks. Brown adipose produces heat as a defense against hypothermia and obesity, and the appearance of brown-like adipocytes within white adipose tissue depots is associated with improved metabolic phenotypes. Thus, understanding the differences in cell biology and function of these different adipose cell types and depots may be critical to the development of new therapies for metabolic disease. Here, we found that Prdm16, a brown adipose determination factor, is selectively expressed in subcutaneous white adipocytes relative to other white fat depots in mice. Transgenic expression of Prdm16 in fat tissue robustly induced the development of brown-like adipocytes in subcutaneous, but not epididymal, adipose depots. Prdm16 transgenic mice displayed increased energy expenditure, limited weight gain, and improved glucose tolerance in response to a high-fat diet. shRNA-mediated depletion of Prdm16 in isolated subcutaneous adipocytes caused a sharp decrease in the expression of thermogenic genes and a reduction in uncoupled cellular respiration. Finally, Prdm16 haploinsufficiency reduced the brown fat phenotype in white adipose tissue stimulated by ß-adrenergic agonists. These results demonstrate that Prdm16 is a cell-autonomous determinant of a brown fat-like gene program and thermogenesis in subcutaneous adipose tissues.

Transcriptional control of preadipocyte determination by Zfp423.

The worldwide epidemic of obesity has increased the urgency to develop a deeper understanding of physiological systems related to energy balance and energy storage, including the mechanisms controlling the development of fat cells (adipocytes). The differentiation of committed preadipocytes to adipocytes is controlled by PPARgamma and several other transcription factors, but the molecular basis for preadipocyte determination is not understood. Using a new method for the quantitative analysis of transcriptional components, we identified the zinc-finger protein Zfp423 as a factor enriched in preadipose versus non-preadipose fibroblasts. Ectopic expression of Zfp423 in non-adipogenic NIH 3T3 fibroblasts robustly activates expression of Pparg in undifferentiated cells and permits cells to undergo adipocyte differentiation under permissive conditions. Short hairpin RNA (shRNA)-mediated reduction of Zfp423 expression in 3T3-L1 cells blunts preadipocyte Pparg expression and diminishes the ability of these cells to differentiate. Furthermore, both brown and white adipocyte differentiation is markedly impaired in Zfp423-deficient mouse embryos. Zfp423 regulates Pparg expression, in part, through amplification of the BMP signalling pathway, an effect dependent on the SMAD-binding capacity of Zfp423. This study identifies Zfp423 as a transcriptional regulator of preadipocyte determination.

PRDM16 controls a brown fat/skeletal muscle switch.

Brown fat can increase energy expenditure and protect against obesity through a specialized program of uncoupled respiration. Here we show by in vivo fate mapping that brown, but not white, fat cells arise from precursors that express Myf5, a gene previously thought to be expressed only in the myogenic lineage. We also demonstrate that the transcriptional regulator PRDM16 (PRD1-BF1-RIZ1 homologous domain containing 16) controls a bidirectional cell fate switch between skeletal myoblasts and brown fat cells. Loss of PRDM16 from brown fat precursors causes a loss of brown fat characteristics and promotes muscle differentiation. Conversely, ectopic expression of PRDM16 in myoblasts induces their differentiation into brown fat cells. PRDM16 stimulates brown adipogenesis by binding to PPAR-gamma (peroxisome-proliferator-activated receptor-gamma) and activating its transcriptional function. Finally, Prdm16-deficient brown fat displays an abnormal morphology, reduced thermogenic gene expression and elevated expression of muscle-specific genes. Taken together, these data indicate that PRDM16 specifies the brown fat lineage from a progenitor that expresses myoblast markers and is not involved in white adipogenesis.