Daniel C. Berry

All-trans-Retinoic Acid Represses Obesity and Insulin Resistance by Activating both Peroxisome Proliferation-Activated Receptor β/δ and Retinoic Acid Receptor

ABSTRACT 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 β/δ (PPARβ/δ). 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 PPARβ/δ, 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 PPARβ/δ expression and activity. RA treatment of obese mice induced expression of PPARβ/δ and RAR target genes involved in regulation of lipid homeostasis, leading to weight loss and improved insulin responsiveness. RA treatment also restored adipose PPARβ/δ expression. The data indicate that suppression of obesity and insulin resistance by RA is largely mediated by PPARβ/δ 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.

Overcoming retinoic acid-resistance of mammary carcinomas by diverting retinoic acid from PPARβ/δ to RAR

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.

Signaling by vitamin A and retinol-binding protein regulates gene expression to inhibit insulin responses.

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.

The developmental origins of adipose tissue

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.

Retinoic acid upregulates preadipocyte genes to block adipogenesis and suppress diet-induced obesity.

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.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

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.

The STRA6 receptor is essential for retinol-binding protein-induced insulin resistance but not for maintaining vitamin A homeostasis in tissues other than the eye.

Background: STRA6 transports retinol into cells and activates cell signaling. Results: Ablation of Stra6 does not impair vitamin A homeostasis in tissues other than the eye but protects mice against RBP-induced insulin resistance. Conclusion: One major function of STRA6 is to control cell signaling. Significance: The data point at a new function for vitamin A and its blood carrier RBP. The plasma membrane protein STRA6 is thought to mediate uptake of retinol from its blood carrier retinol-binding protein (RBP) into cells and to function as a surface receptor that, upon binding of holo-RBP, activates a JAK/STAT cascade. It was suggested that STRA6 signaling underlies insulin resistance induced by elevated serum levels of RBP in obese animals. To investigate these activities in vivo, we generated and analyzed Stra6-null mice. We show that the contribution of STRA6 to retinol uptake by tissues in vivo is small and that, with the exception of the eye, ablation of Stra6 has only a modest effect on retinoid homeostasis and does not impair physiological functions that critically depend on retinoic acid in the embryo or in the adult. However, ablation of Stra6 effectively protects mice from RBP-induced suppression of insulin signaling. Thus one biological function of STRA6 in tissues other than the eye appears to be the coupling of circulating holo-RBP levels to cell signaling, in turn regulating key processes such as insulin response.

Independent stem cell lineages regulate adipose organogenesis and adipose homeostasis

Adipose 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.

Is PPARβ/δa Retinoid Receptor?

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.

Cross Talk between Signaling and Vitamin A Transport by the Retinol-Binding Protein Receptor STRA6

ABSTRACT 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.”