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Supplemental Figure legends and Figures Fig. S1: Levels of STRA6 and RBP in breast cancers Fig. S2: Effects of decreased STRA6 expression on oncogenic properties. Fig. S3: Immunoblots demonstrating variable RBP expression levels and effects on "wound healing". Fig. S4: Effects of decreased expression of RBP on oncogene expression.

Beige adipocytes are induced by cold temperatures or β3-adrenergic receptor (Adrb3) agonists. They create heat through glucose and fatty acid (FA) oxidation, conferring metabolic benefits. The distinct and shared mechanisms by which these treatments induce beiging are unknown. Here, we perform single-nucleus assay for transposase-accessible chromatin sequencing (snATAC-seq) on adipose tissue from mice exposed to cold or an Adrb3 agonist to identify cellular and chromatin accessibility dynamics during beiging. Both stimuli induce chromatin remodeling that influence vascularization and inflammation in adipose. Beige adipocytes from cold-exposed mice have increased accessibility at genes regulating glycolytic processes, whereas Adrb3 activation increases cAMP responses. While both thermogenic stimuli increase accessibility at genes regulating thermogenesis, lipogenesis, and beige adipocyte development, the kinetics and magnitudes of the changes are distinct for the stimuli. Accessibility changes at lipogenic genes are linked to functional changes in lipid composition of adipose. Both stimuli tend to decrease the proportion of palmitic acids, a saturated FA in adipose. However, Adrb3 activation increases the proportion of monounsaturated FAs, whereas cold increases the proportion of polyunsaturated FAs. These findings reveal common and distinct mechanisms of cold and Adrb3 induced beige adipocyte biogenesis, and identify unique functional consequences of manipulating these pathways in vivo.

BACKGROUND: Skeletal muscle progenitor cells (MPCs) repair damaged muscle postinjury. Pyruvate kinase M2 (PKM2) is a glycolytic enzyme (canonical activity) that can also interact with other proteins (noncanonical activity) to modify diverse cellular processes. Recent evidence links PKM2 to MPC proliferation. OBJECTIVES: This study aimed to understand cellular roles for PKM2 in MPCs and the necessity of PKM2 in MPCs for muscle regeneration postinjury. METHODS: Cultured, proliferating MPCs (C2C12 cells) were treated with a short hairpin RNA targeting PKM2 or small molecules that selectively affect canonical and noncanonical PKM2 activity (shikonin and TEPP-46). Cell number was measured, and RNA-sequencing and metabolic assays were used in follow-up experiments. Immunoprecipitation coupled to proteomics was used to identify binding partners of PKM2. Lastly, an MPC-specific PKM2 knockout mouse was generated and challenged with a muscle injury to determine the impact of PKM2 on regeneration. RESULTS: When the noncanonical activity of PKM2 was blocked or impaired, there was an increase in reactive oxygen species concentrations (1.6–2.0-fold, P

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.

Cold temperatures induce progenitor cells within white adipose tissue to form beige adipocytes that burn energy and generate heat; this is a potential anti-diabesity therapy. However, the potential to form cold-induced beige adipocytes declines with age. This creates a clinical roadblock to potential therapeutic use in older individuals, who constitute a large percentage of the obesity epidemic. Here we show that aging murine and human beige progenitor cells display a cellular aging, senescence-like phenotype that accounts for their age-dependent failure. Activating the senescence pathway, either genetically or pharmacologically, in young beige progenitors induces premature cellular senescence and blocks their potential to form cold-induced beige adipocytes. Conversely, genetically or pharmacologically reversing cellular aging by targeting the p38/MAPK-p16Ink4a pathway in aged mouse or human beige progenitor cells rejuvenates cold-induced beiging. This in turn increases glucose sensitivity. Collectively, these data indicate that anti-aging or senescence modalities could be a strategy to induce beiging, thereby improving metabolic health in aging humans.

Thermogenic beige fat found in white adipose tissue is a potential therapeutic target to curb the global obesity and diabetes epidemic. However, these inducible thermogenic beige adipocytes have been thought to be short-lived and rapidly convert to “white-like” adipocytes after discontinuing stimuli. In this study, using effective labelling techniques and genetic mouse tools, we demonstrated that a subset of UCP1+ cells that exist within white adipose tissue are able to self-divide and contribute to new beige adipocyte recruitment in response to β3 stimuli. When these cells were depleted or their adipogenic capability was blocked, β3-induced beige adipocyte formation was impaired. We also identified a cell cycle machinery of p21 and CDKN2A as a molecular basis of beige adipocyte regulation. Collectively, our findings provide new insights into the cellular and molecular mechanisms of beige adipocyte regulation and potentially therapeutic opportunities to induce beige phenotype and treat metabolic disease.

Beige/brite adipocytes are induced within white adipose tissues (WAT) and, when activated, consume glucose and fatty acids to produce heat. Classically, two stimuli have been used to trigger a beiging response: cold temperatures and β3-adrenergic receptor (Adrb3) agonists. These two beiging triggers have been used interchangeably but whether these two stimuli may induce beiging differently at cellular and molecular levels remains unclear. Here, we found that cold-induced beige adipocyte formation requires Adrb1, not Adrb3, activation. Adrb1 activation stimulates WAT resident perivascular (Acta2+) cells to form cold-induced beige adipocytes. In contrast, Adrb3 activation stimulates mature white adipocytes to convert into beige adipocytes. Necessity tests, using mature adipocyte-specific Prdm16 deletion strategies, demonstrated that adipocytes are required and are predominant source to generate Adrb3-induced, but not cold-induced, beige adipocytes. Collectively, we identify that cold temperatures and Adrb3 agonists activate distinct cellular populations that express different β-adrenergic receptors to induce beige adipogenesis., eLife digest Excess accumulation of a type of fat called white fat is associated with obesity and metabolic problems. White fat cells store energy. White fat tissue also contains some beige fat cells, which burn fats and sugars to produce heat. Cold temperatures trigger the production and activity of beige fat cells, which allows the body to stay warm. People with obesity tend to have less beige fat and more white fat. This has led scientists to test whether treatments that increase the number of beige fat cells a person has could reduce fat mass and improve metabolism. To develop treatments that increase beige fat, scientists must first understand where it comes from and how cold and other factors stimulate its growth. Recent studies have shown that smooth muscle cells, which surround blood vessel walls, make cold-induced beige fat cells. A widely used drug that turns on the β3 adrenergic receptor, which is found in the cell membrane, also boosts the creation of beige fat cells. Yet, it was not clear exactly how cold or this drug triggers the production of beige fat. Now, Jiang et al. show that drugs that target β3 adrenergic receptors cause white fat cells in mice to change into beige fat cells. The experiments also showed that cold turns on a different receptor called the β1 adrenergic receptor on smooth muscle cells causing them to make beige fat cells. This shows that there is more than one source for beige fat cells in the body and that different strategies for increasing beige fat cell numbers do not work the same way. More studies are needed to learn whether beige fat cells produced after exposure to cold or drugs behave in the same way and have similar affects on metabolism. This could help scientists determine if one of these strategies could make a better treatment for obesity or other metabolic disorders.

Adipose progenitor cells (APCs) reside in a vascular niche, located within the perivascular compartment of adipose tissue blood vessels. Yet, the signals and mechanisms that govern adipose vascular niche formation and APC niche interaction are unknown. Here we show that the assembly and maintenance of the adipose vascular niche is controlled by PPARγ acting within APCs. PPARγ triggers a molecular hierarchy that induces vascular sprouting, APC vessel niche affinity and APC vessel occupancy. Mechanistically, PPARγ transcriptionally activates PDGFRβ and VEGF. APC expression and activation of PDGFRβ promotes the recruitment and retention of APCs to the niche. Pharmacologically, targeting PDGFRβ disrupts APC niche contact thus blocking adipose tissue expansion. Moreover, enhanced APC expression of VEGF stimulates endothelial cell proliferation and expands the adipose niche. Consequently, APC niche communication and retention are boosted by VEGF thereby impairing adipogenesis. Our data indicate that APCs direct adipose tissue niche expansion via a PPARγ-initiated PDGFRβ and VEGF transcriptional axis., Adipocyte progenitor cells (APCs) are found tethered to adipose tissue blood vessel walls and can differentiate into adipocytes. Here the authors show that PPARγ controls angiogenesis by stimulating APC–blood vessel interaction and retention via a transcriptional network that includes PDGFRβ and VEGF.

SummaryAdipose tissues have striking plasticity, highlighted by childhood and adult obesity. Using adipose lineage analyses, smooth muscle actin (SMA)-mural cell-fate mapping, and conditional PPARγ deletion to block adipocyte differentiation, we find two phases of adipocyte generation that emanate from two independent adipose progenitor compartments: developmental and adult. These two compartments are sequentially required for organ formation and maintenance. Although both developmental and adult progenitors are specified during the developmental period and express PPARγ, they have distinct microanatomical, functional, morphogenetic, and molecular profiles. Furthermore, the two compartments derive from different lineages; whereas adult adipose progenitors fate-map from an SMA+ mural lineage, developmental progenitors do not. Remarkably, the adult progenitor compartment appears to be specified earlier than the developmental cells and then enters the already developmentally formed adipose depots. Thus, two distinct cell compartments control adipose organ development and organ homeostasis, which may provide a discrete therapeutic target for childhood and adult obesity.

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP). At some tissues, RBP is recognized by STRA6, a plasma membrane protein that serves a dual role: it transports retinol from extracellular RBP into cells and it transduces a signaling cascade mediated by the Janus kinase JAK2 and the transcription factors STAT3 and STAT5. We show here that expression of RBP and STRA6 is markedly upregulated in human breast and colon tumors, that holo-RBP/STRA6 signaling promotes oncogenic properties, and that STRA6 expression is critical for tumor formation by colon carcinoma cells in vivo. The holo-RBP/STRA6 pathway also efficiently induces fibroblasts to undergo oncogenic transformation, rendering them highly tumorigenic. These data establish that holo-RBP and its receptor STRA6 are potent oncogenes and suggest that the pathway is a novel target for therapy of some human cancers. Cancer Res; 74(21); 6341–51. ©2014 AACR.

Adipose tissue is formed at stereotypic times and locations in a diverse array of organisms. Once formed, the tissue is dynamic, responding to homeostatic and external cues and capable of a 15-fold expansion. The formation and maintenance of adipose tissue is essential to many biological processes and when perturbed leads to significant diseases. Despite this basic and clinical significance, understanding of the developmental biology of adipose tissue has languished. In this Review, we highlight recent efforts to unveil adipose developmental cues, adipose stem cell biology and the regulators of adipose tissue homeostasis and dynamism.

Vitamin A, retinol, circulates in blood associated with retinol-binding protein 4 (RBP4). It was reported that the level of circulating RBP4 is often elevated in obese mice and humans and that, under these circumstances, the protein induces insulin resistance. Recent studies showed that, in addition to its function as a transport protein, RBP4 serves as a cytokine which, upon binding to a cognate membrane receptor termed STRA6, activates a signaling cascade mediated by the Janus kinase JAK2 and its associated transcription factors STAT3 and STAT5. In turn, activated STATs induce the expression of target genes, some of which are closely involved in the regulation of insulin responsiveness and cancer cell biology. Taken together, available information suggests that through its unexpected activity as a signaling molecule, RBP4 may be a link through which obesity results in insulin resistance and some cancers.

Stem or progenitor cells are an essential component for the development, homeostasis, expansion, and regeneration of many tissues. Within white adipose tissue (WAT) reside vascular-resident adipose progenitor cells (APCs) that can proliferate and differentiate into either white or beige/brite adipocytes, which may control adiposity. Recent studies have begun to show that APCs can be manipulated to control adiposity and counteract 'diabesity'. However, much remains unknown about the identity of APCs and how they may control adiposity in response to homeostatic and external cues. Here, we discuss recent advances in our understanding of adipose progenitors and cover a range of topics, including the stem cell/progenitor lineage, their niche, their developmental and adult roles, and their role in cold-induced beige/brite adipocyte formation.

Cold temperatures induce formation of beige adipocytes, which convert glucose and fatty acids to heat, and may increase energy expenditure, reduce adiposity and lower blood glucose. This therapeutic potential is unrealized, hindered by a dearth of genetic tools to fate map, track and manipulate beige progenitors and ‘beiging'. Here we examined 12 Cre/inducible Cre mouse strains that mark adipocyte, muscle and mural lineages, three proposed beige origins. Among these mouse strains, only those that marked perivascular mural cells tracked the cold-induced beige lineage. Two SMA-based strains, SMA-CreERT2 and SMA-rtTA, fate mapped into the majority of cold-induced beige adipocytes and SMA-marked progenitors appeared essential for beiging. Disruption of the potential of the SMA-tracked progenitors to form beige adipocytes was accompanied by an inability to maintain body temperature and by hyperglycaemia. Thus, SMA-engineered mice may be useful to track and manipulate beige progenitors, beige adipocyte formation and function., Beige adipocytes are formed in response to cold and thought to contribute to organismal energy homeostasis. Here, the authors study a range of conditional and inducible RFP-expressing Cre mouse strains and find that SMA-based lines are the most useful for mapping beige adipocyte progenitor cells.

The plasma membrane protein STRA6 transports vitamin A from its blood carrier retinol binding protein (RBP) into cells, and it also functions as a cytokine receptor which activates JAK/STAT signaling. We show here that, unlike other cytokine receptors, phosphorylation of STRA6 is not simply induced upon binding of its extracellular ligand. Instead, activation of the receptor is triggered by STRA6-mediated translocation of retinol from serum RBP to an intracellular acceptor, the retinol-binding protein CRBP-I. The observations also demonstrate that the movement of retinol from RBP to CRBP-I, and thus activation of STRA6, is critically linked to the intracellular metabolism of the vitamin. Furthermore, the data show that STRA6 phosphorylation is required for retinol uptake to proceed. Hence, the observations demonstrate that STRA6 orchestrates a multicomponent “machinery” that couples vitamin A homeostasis and metabolism to activation of a signaling cascade and that, in turn, STRA6 signaling regulates the cellular uptake of the vitamin. STRA6 appears to be a founding member of a new class of proteins that may be termed “cytokine signaling transporters.”

Retinoic acid (RA) protects mice from diet-induced obesity. The activity is mediated in part through activation of the nuclear receptors RA receptors (RARs) and peroxisome proliferator–activated receptor β/δ and their associated binding proteins cellular RA binding protein type II (CRABP-II) and fatty acid binding protein type 5 in adipocytes and skeletal muscle, leading to enhanced lipid oxidation and energy dissipation. It was also reported that RA inhibits differentiation of cultured preadipocytes. However, whether the hormone suppresses adipogenesis in vivo and how the activity is propagated remained unknown. In this study, we show that RA inhibits adipocyte differentiation by activating the CRABP-II/RARγ path in preadipose cells, thereby upregulating the expression of the adipogenesis inhibitors Pref-1, Sox9, and Kruppel-like factor 2 (KLF2). In turn, KLF2 induces the expression of CRABP-II and RARγ, further potentiating inhibition of adipocyte differentiation by RA. The data also indicate that RA suppresses adipogenesis in vivo and that the activity significantly contributes to the ability of the hormone to counteract diet-induced obesity.

Many biological activities of all-trans-retinoic acid (RA) are mediated by the ligand-activated transcription factors termed retinoic acid receptors (RARs), but this hormone can also activate the nuclear receptor peroxisome proliferation-activated receptor beta/delta (PPARbeta/delta). We show here that adipocyte differentiation is accompanied by a shift in RA signaling which, in mature adipocytes, allows RA to activate both RARs and PPARbeta/delta, thereby enhancing lipolysis and depleting lipid stores. In vivo studies using a dietary-induced mouse model of obesity indicated that onset of obesity is accompanied by downregulation of adipose PPARbeta/delta expression and activity. RA treatment of obese mice induced expression of PPARbeta/delta and RAR target genes involved in regulation of lipid homeostasis, leading to weight loss and improved insulin responsiveness. RA treatment also restored adipose PPARbeta/delta expression. The data indicate that suppression of obesity and insulin resistance by RA is largely mediated by PPARbeta/delta and is further enhanced by activation of RARs. By targeting two nuclear receptors, RA may be a uniquely efficacious agent in the therapy and prevention of the metabolic syndrome.

Retinoic acid (RA) displays potent anticarcinogenic activities that are mediated by the nuclear retinoic acid receptors (RARs). However, use of RA in oncology is limited by RA resistance acquired during carcinogenesis. Moreover, in some cancers, RA facilitates rather than inhibits growth. A clue to this paradoxical behavior was recently suggested by the findings that RA also activates PPARβ/δ, a receptor involved in mitogenic and anti-apoptotic activities. The observations that partitioning of RA between its two receptors is regulated by two intracellular lipid-binding proteins—CRABP-II, which targets RA to RAR, and FABP5, which delivers it to PPARβ/δ—further suggest that RA resistance may stem from the deregulation of the binding proteins, resulting in activation of PPARβ/δ rather than RAR. Here, we show that, in the RA-resistant mouse model of breast cancer MMTV-neu , RA indeed activates the nonclassical RA receptor PPARβ/δ. This behavior was traced to an aberrantly high intratumor FABP5/CRABP-II ratio. Decreasing this ratio in mammary tissue diverted RA from PPARβ/δ to RAR and suppressed tumor growth. The data demonstrate the existence of a mechanism that underlies RA resistance in tumors, indicate that CRABP-II functions as a tumor suppressor, and suggest that the inhibition of FABP5 may comprise a therapeutic strategy for overcoming RA resistance in some tumors.

Transcriptional activation of the nuclear receptor RAR by retinoic acid (RA) often leads to inhibition of cell growth. However, in some tissues, RA promotes cell survival and hyperplasia, activities that are unlikely to be mediated by RAR. Here, we show that, in addition to functioning through RAR, RA activates the "orphan" nuclear receptor PPARbeta/delta, which, in turn, induces the expression of prosurvival genes. Partitioning of RA between the two receptors is regulated by the intracellular lipid binding proteins CRABP-II and FABP5. These proteins specifically deliver RA from the cytosol to nuclear RAR and PPARbeta/delta, respectively, thereby selectively enhancing the transcriptional activity of their cognate receptors. Consequently, RA functions through RAR and is a proapoptotic agent in cells with high CRABP-II/FABP5 ratio, but it signals through PPARbeta/delta and promotes survival in cells that highly express FABP5. Opposing effects of RA on cell growth thus emanate from alternate activation of two different nuclear receptors.

Cellular retinoic acid-binding protein 2 (CRABP2) potently suppresses the growth of various carcinomas, but the mechanism(s) that underlies this activity remains incompletely understood. CRABP2 displays two distinct functions. The classical function of this protein is to directly deliver retinoic acid (RA) to RA receptor (RAR), a nuclear receptor activated by this hormone, in turn inducing the expression of multiple antiproliferative genes. The other function of the protein is exerted in the absence of RA and mediated by the RNA-binding and stabilizing protein HuR. CRABP2 directly binds to HuR, markedly strengthens its interactions with target mRNAs, and thus increases their stability and up-regulates their expression. Here we show that the anticarcinogenic activities of CRABP2 are mediated by both of its functions. Transcriptome analyses revealed that, in the absence of RA, a large cohort of transcripts is regulated in common by CRABP2 and HuR, and many of these are involved in regulation of oncogenic properties. Furthermore, both in cultured cells and in vivo, CRABP2 or a CRABP2 mutant defective in its ability to cooperate with RAR but competent in interactions with HuR suppressed carcinoma growth and did so in the absence of RA. Hence, transcript stabilization by the CRABP2-HuR complex significantly contributes to the ability of CRABP2 to inhibit tumorigenesis. Surprisingly, the observations also revealed that HuR regulates the expression of multiple genes involved in nuclear pore formation and is required for nuclear import of CRABP2 and for transcriptional activation by RAR. The data thus point at a novel function for this important protein.

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP). At some tissues, holo-RBP is recognized by a plasma membrane receptor termed STRA6, which serves a dual role: it mediates transport of retinol from RBP into cells, and it functions as a cytokine receptor that, on binding holo-RBP, activates JAK2/STAT5 signaling. As STAT target genes include SOCS3, an inhibitor of insulin receptor, holo-RBP suppresses insulin responses in STRA6-expressing cells. We have shown previously that the two functions of STRA6 are interdependent. These observations suggest factors that regulate STRA6-mediated retinol transport may also control STRA6-mediated cell signaling. One such factor is retinol metabolism, which enables cellular uptake of retinol by maintaining an inward-directed concentration gradient. We show here that lecithin:retinol acyl transferase (LRAT), which catalyzes esterification of retinol to its storage species retinyl esters, is necessary for activation of the STRA6/JAK2/STAT5 cascade by holo-RBP. In accordance, LRAT-null mice are protected from holo-RBP-induced suppression of insulin responses. Hence, STRA6 signaling, which requires STRA6-mediated retinol transport, is supported by LRAT-catalyzed retinol metabolism. The observations demonstrate that STRA6 regulates key cellular processes by coupling circulating holo-RBP levels and intracellular retinol metabolism to cell signaling.—Marwarha, G., Berry, D.C., Croniger, C.M., Noy, N. The retinol esterifying enzyme LRAT supports cell signaling by retinol-binding protein and its receptor STRA6.

Vitamin A is secreted from cellular stores and circulates in blood bound to retinol-binding protein (RBP). In turn, holo-RBP associates in plasma with transthyretin (TTR) to form a ternary RBP-retinol-TTR complex. It is believed that binding to TTR prevents the loss of RBP by filtration in the kidney. At target cells, holo-RBP is recognized by STRA6, a plasma membrane protein that serves a dual role: it mediates uptake of retinol from extracellular RBP into cells, and it functions as a cytokine receptor that, upon binding holo-RBP, triggers a JAK/STAT signaling cascade. We previously showed that STRA6-mediated signaling underlies the ability of RBP to induce insulin resistance. However, the role that TTR, the binding partner of holo-RBP in blood, plays in STRA6-mediated activities remained unknown. Here we show that TTR blocks the ability of holo-RBP to associate with STRA6 and thereby effectively suppresses both STRA6-mediated retinol uptake and STRA6-initiated cell signaling. Consequently, TTR protects mice from RBP-induced insulin resistance, reflected by reduced phosphorylation of insulin receptor and glucose tolerance tests. The data indicate that STRA6 functions only under circumstances where the plasma RBP level exceeds that of TTR and demonstrate that, in addition to preventing the loss of RBP, TTR plays a central role in regulating holo-RBP/STRA6 signaling.

It currently is believed that vitamin A, retinol, functions through active metabolites: the visual chromophore 11- cis -retinal, and retinoic acids, which regulate gene transcription. Retinol circulates in blood bound to retinol-binding protein (RBP) and is transported into cells by a membrane protein termed “stimulated by retinoic acid 6” (STRA6). We show here that STRA6 not only is a vitamin A transporter but also is a cell-surface signaling receptor activated by the RBP–retinol complex. Association of RBP-retinol with STRA6 triggers tyrosine phosphorylation, resulting in recruitment and activation of JAK2 and the transcription factor STAT5. The RBP–retinol/STRA6/JAK2/STAT5 signaling cascade induces the expression of STAT target genes, including suppressor of cytokine signaling 3 ( SOCS3 ), which inhibits insulin signaling, and peroxisome proliferator-activated receptor gamma ( PPARγ ), which enhances lipid accumulation. These observations establish that the parental vitamin A molecule is a transcriptional regulator in its own right, reveal that the scope of biological functions of the vitamin is broader than previously suspected, and provide a rationale for understanding how RBP and retinol regulate energy homeostasis and insulin responses.

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

In preadipocytes, retinoic acid (RA) regulates gene expression by activating the nuclear RA receptor (RAR) and its cognate intracellular lipid-binding protein CRABP-II. It was previously reported that RA inhibits adipocyte differentiation but only when administered early during the differentiation program. The data presented here indicate that the diminished ability of RA to activate RAR following induction of differentiation stems from down-regulation of CRABP-II. The observations show that expression of CRABP-II in preadipocytes is repressed by all three components of the classical hormonal mixture that induces adipocyte differentiation, i.e. isobutylmethylxanthine, insulin, and dexamethasone. Isobutylmethylxanthine-dependent activation of protein kinase A triggered the phosphorylation of the transcription factor cAMP-response element-binding protein, which induced the expression of the cAMP-response element-binding protein family repressor cAMP-response element modulator. In turn, cAMP-response element modulator was found to associate with a cognate response element in the CRABP-II promoter and to repress CRABP-II expression. The data further show that CRABP-II is a direct target gene for the glucocorticoid receptor and that it is subjected to dexamethasone-induced glucocorticoid receptor-mediated repression during adipogenesis. Finally, the observations demonstrate that permanent repression of CRABP-II in mature adipocytes is exerted by the master regulator of adipocyte differentiation CCAAT/enhancer-binding protein alpha and is directly mediated through CCAAT/enhancer-binding protein alpha-response elements in the CRABP-II promoter. Taken together, the observations emphasize the important role of CRABP-II in regulating the transcriptional activity of RA through RAR, and they demonstrate that repression of this gene is critical for allowing adipogenesis to proceed.

The broad ligand-binding characteristic of PPARβ/δ has long hampered identification of physiologically-meaningful ligands for the receptor. The observations that the activity of PPARβ/δ is supported by fatty acid binding protein 5 (FABP5), which directly delivers ligands from the cytosol to the receptor, suggest that bona fide PPARβ/δ ligands both activate the receptor, and trigger the nuclear translocation of FABP5. Using these criteria, it was recently demonstrated that all-trans-retinoic acid (RA), the activator of the classical retinoic acid receptor RAR, also serves as a ligand for PPARβ/δ. Partitioning of RA between its two receptors was found to be regulated by FABP5, which delivers it to PPARβ/δ, and cellular RA binding protein II (CRABP-II), which targets it to RAR. Consequently, RA activates PPARβ/δ in cells that display a high FABP5/CRABP-II expression ratio. It remains to be clarified whether compounds other than RA may also serve as endogenous activators for this highly promiscuous protein.

The broad ligand-binding characteristic of PPARbeta/delta has long hampered identification of physiologically-meaningful ligands for the receptor. The observations that the activity of PPARbeta/delta is supported by fatty acid binding protein 5 (FABP5), which directly delivers ligands from the cytosol to the receptor, suggest that bona fide PPARbeta/delta ligands both activate the receptor, and trigger the nuclear translocation of FABP5. Using these criteria, it was recently demonstrated that all-trans-retinoic acid (RA), the activator of the classical retinoic acid receptor RAR, also serves as a ligand for PPARbeta/delta. Partitioning of RA between its two receptors was found to be regulated by FABP5, which delivers it to PPARbeta/delta, and cellular RA binding protein II (CRABP-II), which targets it to RAR. Consequently, RA activates PPARbeta/delta in cells that display a high FABP5/CRABP-II expression ratio. It remains to be clarified whether compounds other than RA may also serve as endogenous activators for this highly promiscuous protein.