Lipolysis-derived linoleic acid drives beige fat progenitor cell proliferation

De novo beige adipocyte biogenesis involves the proliferation of progenitor cells in white adipose tissue (WAT); however, what regulates this process remains unclear. Here, we report that in mouse models but also in human tissues, WAT lipolysis-derived linoleic acid triggers beige progenitor cell proliferation following cold acclimation, β3-adrenoceptor activation, and burn injury. A subset of adipocyte progenitors, as marked by cell surface markers PDGFRα or Sca1 and CD81, harbored cristae-rich mitochondria and actively imported linoleic acid via a fatty acid transporter CD36. Linoleic acid not only was oxidized as fuel in the mitochondria but also was utilized for the synthesis of arachidonic acid-derived signaling entities such as prostaglandin D2. Oral supplementation of linoleic acid was sufficient to stimulate beige progenitor cell proliferation, even under thermoneutral conditions, in a CD36-dependent manner. Together, this study provides mechanistic insights into how diverse pathophysiological stimuli, such as cold and burn injury, promote de novo beige fat biogenesis.

Adipose stem cells in obesity: challenges and opportunities

Adipose tissue, the storage of excessive energy in the body, secretes various proteins called adipokines, which connect the body’s nutritional status to the regulation of energy balance. Obesity triggers alterations of quantity and quality of various types of cells that reside in adipose tissue, including adipose stem cells (ASCs; referred to as adipose-derived stem/stromal cells in vitro). These alterations in the functionalities and properties of ASCs impair adipose tissue remodeling and adipose tissue function, which induces low-grade systemic inflammation, progressive insulin resistance, and other metabolic disorders. In contrast, the ability of ASCs to recruit new adipocytes when faced with caloric excess leads to healthy adipose tissue expansion, associated with lower amounts of inflammation, fibrosis, and insulin resistance. This review focuses on recent advances in our understanding of the identity of ASCs and their roles in adipose tissue development, homeostasis, expansion, and thermogenesis, and how these roles go awry in obesity. A better understanding of the biology of ASCs and their adipogenesis may lead to novel therapeutic targets for obesity and metabolic disease.

Rnf20 deficiency in adipocyte impairs adipose tissue development and thermogenesis

RNF20, an E3 ligase critical for monoubiquitination of histone H2B at lysine 120 (H2Bub), has been implicated in the regulation of various cellar processes; however, its physiological roles in adipocytes remain poorly characterized. Here, we report that the adipocyte-specific knockout of Rnf20 (ASKO) in mice led to progressive fat loss, organomegaly and hyperinsulinemia. Despite signs of hyperinsulinemia, normal insulin sensitivity and improved glucose tolerance were observed in the young and aged CD-fed ASKO mice. In addition, high-fat diet-fed ASKO mice developed severe liver steatosis. Moreover, we observed that the ASKO mice were extremely sensitive to a cold environment due to decreased expression levels of brown adipose tissue (BAT) selective genes, including uncoupling protein 1 (Ucp1), and impaired mitochondrial functions. Significantly decreased levels of peroxisome proliferator-activated receptor gamma (Pparγ) were observed in the gonadal white adipose tissues (gWAT) from the ASKO mice, suggesting that Rnf20 regulates adipogenesis, at least in part, through Pparγ. Rosiglitazone-treated ASKO mice exhibited increased fat mass compared to that of the non-treated ASKO mice. Collectively, our results illustrate the critical role of RNF20 in control of white and brown adipose tissue development and physiological function.

Dynamic control of adipose tissue development and adult tissue homeostasis by platelet-derived growth factor receptor alpha

Adipocytes arise from distinct progenitor populations during developmental and adult stages but little is known about how developmental progenitors differ from adult progenitors. Here, we investigate the role of platelet-derived growth factor receptor alpha (PDGFRα) in the divergent regulation of the two different adipose progenitor cells (APCs). Using in vivo adipose lineage tracking and deletion mouse models, we found that developmental PDGFRα+ cells are adipogenic and differentiated into mature adipocytes, and the deletion of Pdgfra in developmental adipose lineage disrupted white adipose tissue (WAT) formation. Interestingly, adult PDGFRα+ cells do not significantly contribute to adult adipogenesis, and deleting Pdgfra in adult adipose lineage did not affect WAT homeostasis. Mechanistically, embryonic APCs require PDGFRα for fate maintenance, and without PDGFRα, they underwent fate change from adipogenic to fibrotic lineage. Collectively, our findings indicate that PDGFRα+ cells and Pdgfra gene itself are differentially required for WAT development and adult WAT homeostasis.

[Time-sequential expression of lnc AK079912 during adipose tissue development and browning in mice]

OBJECTIVE

To investigate the time-sequential expression of a novel long non-coding RNA, lnc AK079912, in metabolically related tissues and during adipose tissue development and browning in mice.

METHODS

The interscapular brown adipose tissue (iBAT), subcutaneous white adipose tissue (sWAT), epididymal white adipose tissue (eWAT), liver tissues and muscular tissues were collected from 8-week-old C57BL/6J mice. The iBAT, sWAT and eWAT were also collected from the mice during development (0 day, 21 days, 8 weeks and 6 months after birth) and from 8- to 10-week- mice with cold exposure (4 ℃) and intraperitoneal injections of CL316, 243 (1 μg/g body weight) for 1 to 5 days. Trizol was used to extract the total RNA from the tissues, and RT-qPCR was performed to detect the expressions of lnc AK079912. Isolated mouse preadipocytes in primary culture were induced for adipogenic differentiation for 9 days and then treated with CL316, 243 (2 μmol/L) for different durations (no longer than 24 h); the expression of lnc AK079912 in the cells was detected using RT-qPCR at different time points of the treatment.

RESULTS

Lnc AK079912 was highly expressed in mouse adipose tissues, the highest in iBAT, followed by the muscular tissue, but was hardly detected in the liver tissue. The expression level of lnc AK079912 increased progressively in iBAT and sWAT during development of the mice, while its expression in eWAT showed an initial increase followed by a reduction at 8 weeks (P < 0.001). No significant difference was found in the expression of lnc AK079912 in the iBAT, sWAT or eWAT in mice with cold stimulation for 1 to 5 days (P > 0.05). The expression of lnc AK079912 was significantly decreased in iBAT and eWAT (P < 0.05) but increased in eWAT from mice with intraperitoneal injection of CL316, 243 for 1 to 5 days (P < 0.05). The expression level in the adipocytes in primary culture was significantly increased in response to treatment with CL316, 243 (P < 0.05).

CONCLUSIONS

Lnc AK079912 is highly expressed in mouse adipose tissue, and its expression gradually increases with the development of adipose tissue but with a depot-specific difference. Lnc AK079912 is significantly elevated in the early stage of adipose tissue browning, indicating its important role in the development and browning of adipose tissue.

Emerging Complexities in Adipocyte Origins and Identity

The global incidence of obesity and its comorbidities continues to rise along with a demand for novel therapeutic interventions. Brown adipose tissue (BAT) is attracting attention as a therapeutic target because of its presence in adult humans and high capacity to dissipate energy as heat, and thus burn excess calories, when stimulated. Another potential avenue for therapeutic intervention is to induce, within white adipose tissue (WAT), the formation of brown-like adipocytes called brite (brown-like-in-white) or beige adipocytes. However, understanding how to harness the potential of these thermogenic cells requires a deep understanding of their developmental origins and regulation. Recent cell-labeling and lineage-tracing experiments are beginning to shed light on this emerging area of adipocyte biology. We review here adipocyte development, giving particular attention to thermogenic adipocytes.

Inactivation of Plin4 downregulates Plin5 and reduces cardiac lipid accumulation in mice

Plin4 is a lipid droplet protein (LDP) found predominantly in white adipose tissue (WAT). The Plin4 gene is immediately downstream of the Plin5 gene; the two genes exhibit distinct though overlapping tissue expression patterns. Plin4 is absent in brown adipose tissue (BAT) and liver and expressed at low levels in heart and skeletal muscle, whereas Plin5 is highly expressed in these oxidative tissues but at a low level in WAT. The physiological role of Plin4 remains unclear. We have generated Plin4(-/-) mice by gene targeting. Loss of Plin4 has no effect on body weight or composition or on adipose mass or development. However, the triacylglycerol (TAG) content in heart, but not other oxidative tissues such as BAT, soleus muscle, and liver, is markedly reduced in Plin4(-/-) mice. The heart of Plin4(-/-) mice displays reduced Plin5 mRNA and protein levels (by ~38 and 87%, respectively, vs. wild-type) but unchanged mRNA levels of other perilipin family genes (Plin2 and Plin3) or genes involved in glucose and lipid metabolism. Despite reduced cardiac TAG level, both young and aged Plin4(-/-) mice maintain normal heart function as wild-type mice, as measured by echocardiography. Interestingly, Plin4 deficiency prevents the lipid accumulation in the heart that normally occurs after a prolonged (48-h) fast. It also protects the heart from cardiac steatosis induced by high-fat diet or when Plin4(-/-) mice are bred into Lep(-/-) obese background. In conclusion, inactivation of Plin4 downregulates Plin5 and reduces cardiac lipid accumulation in mice.