White adipose tissue displays high plasticity. We developed a system for the inducible, permanent labeling of mature adipocytes that we called the AdipoChaser mouse. We monitored adipogenesis during development, high-fat diet (HFD) feeding and cold exposure. During cold-induced ‘browning’ of subcutaneous fat, most ‘beige’ adipocytes stem from de novo–differentiated adipocytes. During HFD feeding, epididymal fat initiates adipogenesis after 4 weeks, whereas subcutaneous fat undergoes hypertrophy for a period of up to 12 weeks. Gonadal fat develops postnatally, whereas subcutaneous fat develops between embryonic days 14 and 18. Our results highlight the extensive differences in adipogenic potential in various fat depots.

No abstract

Epithelial cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM)1. Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, a property of most malignant cells2. Detachment from ECM is associated with enhanced reactive oxygen species (ROS) due to altered glucose metabolism2. Here we identify an unconventional pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumor spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidation of both nutrients was suppressed in spheroids, whereas reductive formation of citrate from glutamine was enhanced. Reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1), because the activity was suppressed in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. This activity occurred in absence of hypoxia, a well-known inducer of reductive metabolism. Rather, IDH1 mitigated mitochondrial ROS in spheroids, and suppressing IDH1 reduced spheroid growth through a mechanism requiring mitochondrial ROS. Isotope tracing revealed that in spheroids, isocitrate/citrate produced reductively in the cytosol could enter the mitochondria and participate in oxidative metabolism, including oxidation by IDH2. This generates NADPH in the mitochondria, enabling cells to mitigate mitochondrial ROS and maximize growth. Neither IDH1 nor IDH2 was necessary for monolayer growth, but deleting either one enhanced mitochondrial ROS and reduced spheroid size, as did deletion of the mitochondrial citrate transporter protein. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism, initiated by IDH1-dependent reductive carboxylation and culminating in suppression of mitochondrial ROS.

Summary Chronic inflammation constitutes an important link between obesity and its pathophysiological sequelae. In contrast to the belief that inflammatory signals exert a fundamentally negative impact on metabolism, we show that pro-inflammatory signaling in the adipocyte is in fact required for proper adipose tissue remodeling and expansion. Three mouse models with an adipose tissue-specific reduction in pro-inflammatory potential were generated that display a reduced capacity for adipogenesis in vivo, while the differentiation potential is unaffected in vitro. Upon high fat diet exposure, the expansion of visceral adipose tissue is prominently affected. This is associated with decreased intestinal barrier function, increased hepatic steatosis and metabolic dysfunction. An impaired local pro-inflammatory response in the adipocyte leads to increased ectopic lipid accumulation, glucose intolerance and systemic inflammation. Adipose tissue inflammation is therefore an adaptive response that enables safe storage of excess nutrients and contributes to a visceral depot barrier that effectively filters gut-derived endotoxin.

Summary The expansion of white adipose tissue (WAT) in obesity involves de novo differentiation of new adipocytes; however, the cellular origin of these cells remains unclear. Here, we utilize Zfp423GFP reporter mice to characterize adipose mural (Pdgfrβ+) cells with varying levels of the preadipocyte commitment factor Zfp423. We find that adipose tissue contains distinct mural populations, with levels of Zfp423 distinguishing adipogenic from inflammatory-like mural cells. Using our “MuralChaser” lineage tracking system, we uncover adipose perivascular cells as developmental precursors of adipocytes formed in obesity, with adipogenesis and precursor abundance regulated in a depot-dependent manner. Interestingly, Pdgfrβ+ cells do not significantly contribute to the initial cold-induced recruitment of beige adipocytes in WAT; it is only after prolonged cold exposure that these cells differentiate into beige adipocytes. These results provide genetic evidence for a mural cell origin of white adipocytes in obesity, and suggest that beige adipogenesis may originate from multiple sources.

Summary The transcriptional regulators Ebf2 and Prdm16 establish and maintain the brown/beige fat cell identity. However, the mechanisms operating in white adipocytes to suppress the thermogenic gene program and maintain an energy-storing phenotype are less understood. Here, we report that the transcriptional regulator, Zfp423, is critical for maintaining white adipocyte identity through suppression of the thermogenic gene program. Zfp423 expression is enriched in white vs. brown adipocytes and suppressed upon cold exposure. Doxycycline-inducible inactivation of Zfp423 in mature adipocytes, combined with β-adrenergic stimulation, triggers a conversion of differentiated adiponectin-expressing inguinal and gonadal adipocytes into beige-like adipocytes; this reprogramming event is sufficient to prevent and reverse diet-induced obesity and insulin resistance. Mechanistically, Zfp423 acts in adipocytes to inhibit the activity of Ebf2 and suppress Prdm16 activation. These data identify Zfp423 as a molecular brake on adipocyte thermogenesis and suggest a therapeutic strategy to unlock the thermogenic potential of white adipocytes in obesity.

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The adipocyte-specific secretory molecule adiponectin has found widespread acceptance as a systemic marker that effectively integrates a number of signals associated with metabolic dysfunction at the level of adipose tissue. The widely used aP2 promoter cassette, which is frequently chosen to achieve adipocyte-specific expression of transgenes, conveys transcription in cell types other than adipocytes, such as macrophages and cardiomyocytes. To improve our ability to drive transgene expression in a more adipocyte-specific way, we aimed to define the minimal promoter segment from the adiponectin genomic locus. We generated a series of transgenic animals in which the expression of reporter genes and Cre recombinase was driven by 2, 4.9, and 5.4 kb of adiponectin promoter sequences. We found that the 5.4-kb adiponectin promoter fragment is the most effective cassette conveying adipocyte-specific expression of target genes. We therefore define a novel promoter cassette that ensures adipocyte-specific expression of passenger genes and may be used in the generation of transgenic mouse models to study gene function in vivo.