It is generally assumed that white adipocytes arise from resident adipose tissue mesenchymal progenitor cells. We challenge this paradigm by defining a hematopoietic origin for both the de novo development of a subset of white adipocytes in adults and a previously uncharacterized adipose tissue resident mesenchymal progenitor population. Lineage and cytogenetic analysis revealed that bone marrow progenitor (BMP)-derived adipocytes and adipocyte progenitors arise from hematopoietic cells via the myeloid lineage in the absence of cell fusion. Global gene expression analysis indicated that the BMP-derived fat cells are bona fide adipocytes but differ from conventional white or brown adipocytes in decreased expression of genes involved in mitochondrial biogenesis and lipid oxidation, and increased inflammatory gene expression. The BMP-derived adipocytes accumulate with age, occur in higher numbers in visceral than in subcutaneous fat, and in female versus male mice. BMP-derived adipocytes may, therefore, account in part for adipose depot heterogeneity and detrimental changes in adipose metabolism and inflammation with aging and adiposity.

The differentiation of preadipocytes to adipocytes is orchestrated by the expression of the "master adipogenic regulators," CCAAT/enhancer-binding protein (C/EBP) ␤, peroxisome proliferator-activated receptor ␥ (PPAR␥), and C/EBP ␣. In addition, activation of the cAMP-response element-binding protein (CREB) is necessary and sufficient to promote adipogenic conversion and prevent apoptosis of mature adipocytes. In this report we used small interfering RNAto deplete CREB and the closely related factor ATF1 to explore the ability of the master adipogenic regulators to promote adipogenesis in the absence of

Adipogenesis is the process by which mature, insulin-responsive adipocytes are generated from undifferentiated preadipocytes and mesenchymal progenitor cells (1). This process is crucial to the normal development of adipose tissue and its expansion in response to excess dietary energy intake. Alternatively, most lipodystrophic syndromes are characterized by a suppression of adipogenesis and an increase in adipocyte death.Cells destined to the adipose lineage arise late in development from multipotential stem cells of mesodermal origin (1-4). The commitment of the multipotent stem cells to the adipocyte lineage is a poorly understood process. However, once committed to the adipocyte lineage, nonproliferating preadipocytes become responsive to external stimuli that induce their differentiation to mature adipocytes. These stimuli include insulin-like growth factor-1 or insulin (which appears to work through the insulin-like growth factor-1 receptor), glucocorticoids, and agents that elevate intracellular cAMP levels (1).Exposure of these cells to adipogenic inducers initiates a temporally orchestrated cascade of gene expression events that characterize adipogenic differentiation. These agents initially induce a period of mitotic expansion during which expression of CCAAT/enhancer binding proteins (C/EBPs) 2 ␤ and ␦ is increased, whereas expression of factors like Pref-1, necdin, and Wnt10b are diminished. Following mitotic expansion, differentiation begins, during which peroxisome-proliferator-activated receptor ␥ (PPAR␥) and C/EBP ␣ are up-regulated. These transcription factors regulate the expression of many of the factors that characterize the mature adipocyte phenotype like GLUT4 (5), adiponectin, aP2 (6), and perilipin (7).We previously reported that the activity of the transcription factor CREB was stimulated by cAMP mimetics and insulin in both preadipocytes (8), suggesting that CREB might play a role in adipogenic conversion. Subsequent experiments demonstrated that ectopic expression of constitutively active forms of CREB could induce adipogenesis of 3T3-L1 cells (9, 10) and prevent their apoptotic death in response to insulin and/or serum deprivation and tumor necrosis factor ␣ (11). Alternately, ectopic expression of dominant negative forms of CREB blocked adipogenic conversion and stimulated apoptosis of mature adipocytes. Recent studies using these techniques as well as CREB-specific antisense and siRNA confirm these results and indicate that CREB may promote adipogenesis by

Tenascin-C (TN-C) is a mesenchyme-derived extracellular matrix (ECM) glycoprotein required for fetal lung branching morphogenesis. Given that the low oxygen (O 2 ) environment of the fetus is also essential for normal lung branching morphogenesis, we determined whether fetal O 2 tension supports this process by promoting TN-C expression. Initial studies showed that 15-day fetal rat lung explants cultured for 2 days at 3% O 2 not only branched well, but they also expressed higher levels of TN-C when compared to lungs maintained at 21% O 2 , which branched poorly. Antisense oligonucleotide studies demonstrated that TN-C produced in response to 3% O 2 was essential for lung branching morphogenesis. As well, exogenous TN-C protein was shown to promote branching of lung epithelial rudiments cultured at 21% O 2 . Because ECMdegrading proteinases are capable of catabolizing TN-C protein, we reasoned that 3% O 2 might promote TN-C deposition by limiting the activity of these enzymes within the fetal lung. Consistent with this idea, gelatin zymography showed that the activity of a 72-kDa gelatinase, identified as matrix metalloproteinase-2 (MMP-2), was lower at 3% O 2 vs. 21% O 2 . Furthermore, pharmacologic inhibition of MMP-2 activity in fetal lung explants cultured at 21% O 2 resulted in increased TN-C deposition within the mesenchyme, as well as enhanced branching morphogenesis. Collectively, these studies indicate that fetal O 2 tension promotes TN-C-dependent lung epithelial branching morphogenesis by limiting the proteolytic turnover of this ECM component within the adjacent mesenchyme. Developmental Dynamics 234:1-10, 2005.

Tight regulation of VEGF-A production and signaling is important for the maintenance of lung development and homeostasis. VEGF null mice have provided little insight into the role of VEGF during the later stages of lung morphogenesis. Therefore, we examined the in vitro effects of autocrine and paracrine VEGF-A production and the inhibition of VEGF-A signaling on a Flk-1-negative subset of fetal pulmonary mesenchymal cells (pMC). We hypothesized that VEGF-A receptor signaling regulates turnover of fetal lung mesenchyme in a cell cycle-dependent manner. VEGF receptor blockade with SU-5416 caused cell spreading and decreased proliferation and bcl-2 localization. Nuclear expression of the cell cycle inhibitory protein, p21, was increased with SU-5416 treatment, and p27 was absent. Autocrine VEGF production by pMC resulted in proliferation and p21/p27-dependent contact inhibition. In contrast, exogenous VEGF-A increased cell progression through the cell cycle. Selective activation of Flt by placental growth factor demonstrated the importance of this receptor/kinase in the VEGF-A responsiveness of pMC. The expression and localization of the survival factor bcl-2 was dependent on VEGF. These results provide evidence that VEGF-A plays a critical role in the regulation of fetal pulmonary mesenchymal proliferation, survival, and the subsequent development of normal lung architecture through bcl-2 and p21/p27-dependent cell cycle control.