Marrow adipose tissue (MAT) accumulates in diverse clinical conditions but remains poorly understood. Here we show region-specific variation in MAT adipocyte development, regulation, size, lipid composition, gene expression, and genetic determinants. Early MAT formation in mice is conserved, while later development is strain dependent. Proximal, but not distal, MAT is lost with 21-day cold exposure. Rat MAT adipocytes from distal sites have an increased proportion of monounsaturated fatty acids and expression of Scd1/Scd2, Cebpa and Cebpb. Humans also have increased distal marrow fat unsaturation. We define proximal ‘regulated’ MAT (rMAT) as single adipocytes interspersed with active hematopoiesis, whereas distal ‘constitutive’ MAT (cMAT) has low hematopoiesis, contains larger adipocytes, develops earlier, and remains preserved upon systemic challenges. Loss of rMAT occurs in mice with congenital generalized lipodystrophy type 4, whereas both rMAT and cMAT are preserved in mice with congenital generalized lipodystrophy type 3. Consideration of these MAT subpopulations may be important for future studies linking MAT to bone biology, hematopoiesis and whole-body metabolism.

SUMMARY The adipocyte-derived hormone adiponectin promotes metabolic and cardiovascular health. Circulating adiponectin increases in lean states such as caloric restriction (CR), but the reasons for this paradox remain unclear. Unlike white adipose tissue (WAT), bone marrow adipose tissue (MAT) increases during CR, and both MAT and serum adiponectin increase in many other clinical conditions. Thus, we investigated if MAT contributes to circulating adiponectin. We find that adiponectin secretion is greater from MAT than from WAT. Notably, specific inhibition of MAT formation in mice results in decreased circulating adiponectin during CR, despite unaltered adiponectin expression in WAT. Inhibiting MAT formation also alters skeletal muscle adaptation to CR, suggesting that MAT exerts systemic effects. Finally, we reveal that both MAT and serum adiponectin increase during cancer therapy in humans. These observations identify MAT as an endocrine organ that contributes significantly to increased serum adiponectin during CR, and perhaps in other adverse states.

Dyslipidemia and insulin resistance are commonly associated with catabolic or lipodystrophic conditions (such as cancer and sepsis) and with pathological states of nutritional overload (such as obesity-related type 2 diabetes). Two common features of these metabolic disorders are adipose tissue dysfunction and elevated levels of tumour necrosis factor-alpha (TNF-a). Herein, we review the multiple actions of this pro-inflammatory adipokine on adipose tissue biology. These include inhibition of carbohydrate metabolism, lipogenesis, adipogenesis and thermogenesis and stimulation of lipolysis. TNF-a can also impact the endocrine functions of adipose tissue. Taken together, TNF-a contributes to metabolic dysregulation by impairing both adipose tissue function and its ability to store excess fuel. The molecular mechanisms that underlie these actions are discussed.

Wnt10b is an established regulator of mesenchymal stem cell (MSC) fate that inhibits adipogenesis and stimulates osteoblastogenesis, thereby impacting bone mass in vivo. However, downstream mechanisms through which Wnt10b exerts these effects are poorly understood. Moreover, whether other endogenous Wnt ligands also modulate MSC fate remains to be fully addressed. In this study, we identify Wnt6 and Wnt10a as additional Wnt family members that, like Wnt10b, are downregulated during development of white adipocytes in vivo and in vitro, suggesting that Wnt6 and/or Wnt10a may also inhibit adipogenesis. To assess the relative activities of Wnt6, Wnt10a and Wnt10b to regulate mesenchymal cell fate, we used gain- and loss-of function approaches in bipotential ST2 cells and in 3T3-L1 preadipocytes. Enforced expression of Wnt10a stabilizes β-catenin, suppresses adipogenesis and stimulates osteoblastogenesis to a similar extent as Wnt10b, whereas stable expression of Wnt6 has a weaker effect on these processes than Wnt10a or Wnt10b. In contrast, knockdown of endogenous Wnt6 is associated with greater preadipocyte differentiation and impaired osteoblastogenesis than knockdown of Wnt10a or Wnt10b, suggesting that, amongst these Wnt ligands, Wnt6 is the most potent endogenous regulator of MSC fate. Finally, we show that knockdown of β-catenin completely prevents the inhibition of adipogenesis and stimulation of osteoblast differentiation by Wnt6, Wnt10a or Wnt10b. Potential mechanisms whereby Wnts regulate fate of MSCs downstream of β-catenin are also investigated. In conclusion, this study identifies Wnt10a and Wnt6 as additional regulators of MSC fate and demonstrates that mechanisms downstream of β-catenin are required for Wnt6, Wnt10a and Wnt10b to influence differentiation of mesenchymal precursors.

with ill health. Such stigmatization distracts from a fascinating aspect of WAT biology: its enormous plasticity as an organ. Indeed, WAT is capable of massive expansion and contraction in response to chronic alterations in energy balance, accounting for as little as 5% body mass in extremely lean athletes ( 1 ) or as much as 60% body mass in morbidly obese individuals ( 2 ). Moreover in rodents, rabbits, and humans, WAT can regenerate following lipectomy ( 3-6 ). This striking degree of plasticity is unique among organs in adults. On a cellular level, WAT expansion is driven by both hypertrophy and hyperplasia of adipocytes ( 7-13 ). Even in nonexpanding WAT, adipocytes renew frequently to compensate for adipocyte death, with approximately 10% of adipocytes renewed annually ( 14,15 ). These data, which indicate that committed adipocyte progenitors (preadipocytes) exist within WAT, are the product of decades of research motivated largely by our desire to understand adipose tissue in the context of obesity and related diseases. Preadipocytes exist in adipose tissueThough the notion of obesity as a disease has existed since at least the time of Hippocrates ( 16 ), investigation into the origins of the adipocyte and the individual contributions of adipocyte hypertrophy and hyperplasia to adipose bulk began in earnest only after the appearance of the stem cell concept, as related to blood cells, in the early 1900s ( 17 ). In the 1940s, histologic observations of the appearance of adipocytes in chambers implanted in the ears of live rabbits suggested that de novo fat formation was Abstract White adipose tissue (WAT) is perhaps the most plastic organ in the body, capable of regeneration following surgical removal and massive expansion or contraction in response to altered energy balance. Research conducted for over 70 years has investigated adipose tissue plasticity on a cellular level, spurred on by the increasing burden that obesity and associated diseases are placing on public health globally. This work has identifi ed committed preadipocytes in the stromal vascular fraction of adipose tissue and led to our current understanding that adipogenesis is important not only for WAT expansion, but also for maintenance of adipocyte numbers under normal metabolic states. At the turn of the millenium, studies investigating preadipocyte differentiation collided with developments in stem cell research, leading to the discovery of multipotent stem cells within WAT. Such adipose tissue-derived stem cells (ASCs) are capable of differentiating into numerous cell types of both mesodermal and nonmesodermal origin, leading to their extensive investigation from a therapeutic and tissue engineering perspective. However, the insights gained through studying ASCs have also contributed to more-recent progress in attempts to better characterize committed preadipocytes in adipose tissue. Thus, ASC research has gone back to its roots, thereby expanding our knowledge of preadipocyte commitment and adipose tissue biology. -Cawthorn, W. P., E. ...

Proliferation of adipocyte precursors and their differentiation into mature adipocytes contributes to the development of obesity in mammals. IGF-I is a potent mitogen and important stimulus for adipocyte differentiation. The biological actions of IGFs are closely regulated by a family of IGF-binding proteins (IGFBPs), which exert predominantly inhibitory effects. IGFBP-2 is the principal binding protein secreted by differentiating white preadipocytes, suggesting a potential role in the development of obesity. We have generated transgenic mice overexpressing human IGFBP-2 under the control of its native promoter, and we show that overexpression of IGFBP-2 is associated with reduced susceptibility to obesity and improved insulin sensitivity. Whereas wild-type littermates developed glucose intolerance and increased blood pressure with aging, mice overexpressing IGFBP-2 were protected. Furthermore, when fed a high-fat/high-energy diet, IGFBP-2-overexpressing mice were resistant to the development of obesity and insulin resistance. This lean phenotype was associated with decreased leptin levels, increased glucose sensitivity, and lower blood pressure compared with wildtype animals consuming similar amounts of high-fat diet. Our in vitro data suggest a direct effect of IGFBP-2 preventing adipogenesis as indicated by the ability of recombinant IGFBP-2 to impair 3T3-L1 differentiation. These findings suggest an important, novel role for IGFBP-2 in obesity prevention. Diabetes 56:285-294, 2007

Peroxisome proliferator-activated receptor-γ (PPARγ) is a master transcriptional regulator of adipogenesis. Hence, the identification of PPARγ coactivators should help reveal mechanisms controlling gene expression in adipose tissue development and physiology. We show that the non-coding RNA, Steroid receptor RNA Activator (SRA), associates with PPARγ and coactivates PPARγ-dependent reporter gene expression. Overexpression of SRA in ST2 mesenchymal precursor cells promotes their differentiation into adipocytes. Conversely, knockdown of endogenous SRA inhibits 3T3-L1 preadipocyte differentiation. Microarray analysis reveals hundreds of SRA-responsive genes in adipocytes, including genes involved in the cell cycle, and insulin and TNFα signaling pathways. Some functions of SRA may involve mechanisms other than coactivation of PPARγ. SRA in adipocytes increases both glucose uptake and phosphorylation of Akt and FOXO1 in response to insulin. SRA promotes S-phase entry during mitotic clonal expansion, decreases expression of the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1, and increases phosphorylation of Cdk1/Cdc2. SRA also inhibits the expression of adipocyte-related inflammatory genes and TNFα-induced phosphorylation of c-Jun NH2-terminal kinase. In conclusion, SRA enhances adipogenesis and adipocyte function through multiple pathways.

Tumour necrosis factor-a (TNF-a), a proinflammatory cytokine, is a potent negative regulator of adipocyte differentiation. However, the mechanism of TNF-a-mediated antiadipogenesis remains incompletely understood. In this study, we first confirm that TNF-a inhibits adipogenesis of 3T3-L1 preadipocytes by preventing the early induction of the adipogenic transcription factors peroxisome proliferator-activated receptor-c (PPARc) and CCAAT/enhancer binding protein-a (C/EBPa). This suppression coincides with enhanced expression of several reported mediators of antiadipogenesis that are also targets of the Wnt/b-catenin/T-cell factor 4 (TCF4) pathway. Indeed, we found that TNF-a enhanced TCF4-dependent transcriptional activity during early antiadipogenesis, and promoted the stabilisation of b-catenin throughout antiadipogenesis. We analysed the effect of TNF-a on adipogenesis in 3T3-L1 cells in which b-catenin/TCF signalling was impaired, either via stable knockdown of b-catenin, or by overexpression of dominant-negative TCF4 (dnTCF4). The knockdown of b-catenin enhanced the adipogenic potential of 3T3-L1 preadipocytes and attenuated TNF-a-induced antiadipogenesis. However, b-catenin knockdown also promoted TNF-a-induced apoptosis in these cells. In contrast, overexpression of dnTCF4 prevented TNF-a-induced antiadipogenesis but showed no apparent effect on cell survival. Finally, we show that TNF-a-induced antiadipogenesis and stabilisation of b-catenin requires a functional death domain of TNF-a receptor 1 (TNFR1). Taken together these data suggest that TNFR1-mediated death domain signals can inhibit adipogenesis via a b-catenin/TCF4-dependent pathway.