BAF60a deficiency uncouples chromatin accessibility and cold sensitivity from white fat browning

Brown and beige fat share a remarkably similar transcriptional program that supports fuel oxidation and thermogenesis. The chromatin-remodeling machinery that governs genome accessibility and renders adipocytes poised for thermogenic activation remains elusive. Here we show that BAF60a, a subunit of the SWI/SNF chromatin-remodeling complexes, serves an indispensable role in cold-induced thermogenesis in brown fat. BAF60a maintains chromatin accessibility at PPARγ and EBF2 binding sites for key thermogenic genes. Surprisingly, fat-specific BAF60a inactivation triggers more pronounced cold-induced browning of inguinal white adipose tissue that is linked to induction of MC2R, a receptor for the pituitary hormone ACTH. Elevated MC2R expression sensitizes adipocytes and BAF60a-deficient adipose tissue to thermogenic activation in response to ACTH stimulation. These observations reveal an unexpected dichotomous role of BAF60a-mediated chromatin remodeling in transcriptional control of brown and beige gene programs and illustrate a pituitary-adipose signaling axis in the control of thermogenesis.

Agonist Antibody Converts Stem Cells into Migrating Brown Adipocyte-Like Cells in Heart

We present data showing that Iodotyrosine Deiodinase (IYD) is a dual-function enzyme acting as a catalyst in metabolism and a receptor for cooperative stem cell differentiation. IYD is present both in thyroid cells where it is critical for scavenging iodine from halogenated by-products of thyroid hormone production and on hematopoietic stem cells. To close the cooperative loop, the mono- and di-Iodotyrosine (MIT and DIT) substrates of IYD in the thyroid are also agonists for IYD now acting as a receptor on bone marrow stem cells. While studying intracellular combinatorial antibody libraries, we discovered an agonist antibody, H3 Ab, of which the target is the enzyme IYD. When agonized by H3 Ab, IYD expressed on stem cells induces differentiation of the cells into brown adipocyte-like cells, which selectively migrate to mouse heart tissue. H3 Ab also binds to IYD expressed on human myocardium. Thus, one has a single enzyme acting in different ways on different cells for the cooperative purpose of enhancing thermogenesis or of regenerating damaged heart tissue.

Sucrose Nonfermenting-Related Kinase Regulates Both Adipose Inflammation and Energy Homeostasis in Mice and Humans

Sucrose nonfermenting-related kinase (SNRK) is a member of the AMPK-related kinase family, and its physiological role in adipose energy homeostasis and inflammation remains unknown. We previously reported that SNRK is ubiquitously and abundantly expressed in both white adipose tissue (WAT) and brown adipose tissue (BAT), but SNRK expression diminishes in adipose tissue in obesity. In this study we report novel experimental findings from both animal models and human genetics. SNRK is essential for survival; SNRK globally deficient pups die within 24 h after birth. Heterozygous mice are characterized by inflamed WAT and less BAT. Adipocyte-specific ablation of SNRK causes inflammation in WAT, ectopic lipid deposition in liver and muscle, and impaired adaptive thermogenesis in BAT. These metabolic disorders subsequently lead to decreased energy expenditure, higher body weight, and insulin resistance. We further confirm the significant association of common variants of the SNRK gene with obesity risk in humans. Through applying a phosphoproteomic approach, we identified eukaryotic elongation factor 1δ and histone deacetylase 1/2 as potential SNRK substrates. Taking these data together, we conclude that SNRK represses WAT inflammation and is essential to maintain BAT thermogenesis, making it a novel therapeutic target for treating obesity and associated metabolic disorders.

An essential role for Tbx15 in the differentiation of brown and "brite" but not white adipocytes

The transcription factor Tbx15 is expressed predominantly in brown adipose tissue and in those white adipose depots that are capable of giving rise to brown-in-white ("brite"/"beige") adipocytes. Therefore, we have investigated a possible role here of Tbx15 in brown and brite adipocyte differentiation in vitro. Adipocyte precursors were isolated from interscapular and axilliary brown adipose tissues, inguinal white ("brite") adipose tissue, and epididymal white adipose tissue in 129/Sv mouse pups and differentiated in culture. Differentiation was enhanced by chronic treatment with the PPARγ agonist rosiglitazone plus the sympathetic neurotransmitter norepinephrine. Using short interfering RNAs (siRNA) directed toward Tbx15 in these primary adipocyte cultures, we decreased Tbx15 expression >90%. This resulted in reduced expression levels of adipogenesis markers (PPARγ, aP2). Importantly, Tbx15 knockdown reduced the expression of brown phenotypic marker genes (PRDM16, PGC-1α, Cox8b/Cox4, UCP1) in brown adipocytes and even more markedly in inguinal white adipocytes. In contrast, Tbx15 knockdown had no effect on white adipocytes originating from a depot that is not brite competent in vivo (epididymal). Therefore, Tbx15 may be essential for the development of the adipogenic and thermogenic programs in adipocytes/adipomyocytes capable of developing brown adipocyte features.

The emergence of cold-induced brown adipocytes in mouse white fat depots is determined predominantly by white to brown adipocyte transdifferentiation

The origin of brown adipocytes arising in white adipose tissue (WAT) after cold acclimatization is unclear. Here, we demonstrate that several UCP1-immunoreactive brown adipocytes occurring in WAT after cold acclimatization have a mixed morphology (paucilocular adipocytes). These cells also had a mixed mitochondrioma with classic "brown" and "white" mitochondria, suggesting intermediate steps in the process of direct transformation of white into brown adipocytes (transdifferentiation). Quantitative electron microscopy disclosed that cold exposure (6 degrees C for 10 days) did not induce an increase in WAT preadipocytes. beta(3)-adrenoceptor-knockout mice had a blunted brown adipocyte occurrence upon cold acclimatization. Administration of the beta(3)-adrenoceptor agonist CL316,243 induced the occurrence of brown adipocytes, with the typical morphological features found after cold acclimatization. In contrast, administration of the beta(1)-adrenoceptor agonist xamoterol increased only the number of preadipocytes. These findings indicate that transdifferentiation depends on beta(3)-adrenoceptor activation, whereas preadipocyte recruitment is mediated by beta(1)-adrenoceptor. RT-qPCR experiments disclosed that cold exposure induced enhanced expression of the thermogenic genes and of genes expressed selectively in brown adipose tissue (iBAT) and in both interscapular BAT and WAT. beta(3)-adrenoceptor suppression blunted their expression only in WAT. Furthermore, cold acclimatization induced an increased WAT expression of the gene coding for C/EBPalpha (an antimitotic protein), whereas Ccna1 expression (related to cell proliferation) was unchanged. Overall, our data strongly suggest that the cold-induced emergence of brown adipocytes in WAT predominantly reflects beta(3)-adrenoceptor-mediated transdifferentiation.

Dynamic regulation of genes involved in mitochondrial DNA replication and transcription during mouse brown fat cell differentiation and recruitment

Background

Brown adipocytes are specialised in dissipating energy through adaptive thermogenesis, whereas white adipocytes are specialised in energy storage. These essentially opposite functions are possible for two reasons relating to mitochondria, namely expression of uncoupling protein 1 (UCP1) and a remarkably higher mitochondrial abundance in brown adipocytes.

Methodology/Principal Findings

Here we report a comprehensive characterisation of gene expression linked to mitochondrial DNA replication, transcription and function during white and brown fat cell differentiation in vitro as well as in white and brown fat, brown adipose tissue fractions and in selected adipose tissues during cold exposure. We find a massive induction of the majority of such genes during brown adipocyte differentiation and recruitment, e.g. of the mitochondrial transcription factors A (Tfam) and B2 (Tfb2m), whereas only a subset of the same genes were induced during white adipose conversion. In addition, PR domain containing 16 (PRDM16) was found to be expressed at substantially higher levels in brown compared to white pre-adipocytes and adipocytes. We demonstrate that forced expression of Tfam but not Tfb2m in brown adipocyte precursor cells promotes mitochondrial DNA replication, and that silencing of PRDM16 expression during brown fat cell differentiation blunts mitochondrial biogenesis and expression of brown fat cell markers.

Conclusions/Significance

Using both in vitro and in vivo model systems of white and brown fat cell differentiation, we report a detailed characterisation of gene expression linked to mitochondrial biogenesis and function. We find significant differences in differentiating white and brown adipocytes, which might explain the notable increase in mitochondrial content observed during brown adipose conversion. In addition, our data support a key role of PRDM16 in triggering brown adipocyte differentiation, including mitochondrial biogenesis and expression of UCP1.

Transdifferentiation properties of adipocytes in the adipose organ

Mammals have two types of adipocytes, white and brown, but their anatomy and physiology is different. White adipocytes store lipids, and brown adipocytes burn them to produce heat. Previous descriptions implied their localization in distinct sites, but we demonstrated that they are mixed in many depots, raising the concept of adipose organ. We explain the reason for their cohabitation with the hypothesis of reversible physiological transdifferentiation; they are able to convert one into each other. If needed, the brown component of the organ could increase at the expense of the white component and vice versa. This plasticity is important because the brown phenotype of the organ associates with resistance to obesity and related disorders. Another example of physiological transdifferetiation of adipocytes is offered by the mammary gland; the pregnancy hormonal stimuli seems to trigger a reversible transdifferentiation of adipocytes into milk-secreting epithelial glands. The obese adipose organ is infiltrated by macrophages inducing chronic inflamation that is widely considered as a causative factor for insulin resistance. We showed that the vast majority of macrophages infiltrating the obese organ are arranged around dead adipocytes, forming characteristic crown-like structures. We recently found that visceral fat is more infiltrated than the subcutaneous fat despite a smaller size of visceral adipocytes. This suggests a different susceptibility of visceral and subcutaneous adipocytes to death, raising the concept of smaller critical death size that could be important to explain the key role of visceral fat for the metabolic disorders associated with obesity.