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We evaluate a potential role of activating transcription factor 4 (Atf4) in invertebrate and mammalian metabolism.
With two parallel approaches—a fat body–specific green fluorescent protein enhancer trap screen in D. melanogaster and expression profiling of developing murine fat tissues—we identified Atf4 as expressed in invertebrate and vertebrate metabolic tissues. We assessed the functional relevance of the evolutionarily conserved expression by analyzing Atf4 mutant flies and Atf4 mutant mice for possible metabolic phenotypes.
Flies with insertions at the Atf4 locus have reduced fat content, increased starvation sensitivity, and lower levels of circulating carbohydrate. Atf4 null mice are also lean, and they resist age-related and diet-induced obesity. Atf4 null mice have increased energy expenditure potentially accounting for the lean phenotype. Atf4 null mice are hypoglycemic, even before substantial changes in fat content, indicating that Atf4 regulates mammalian carbohydrate metabolism. In addition, the Atf4 mutation blunts diet-induced diabetes as well as hyperlipidemia and hepatosteatosis. Several aspects of the Atf4 mutant phenotype resemble mice with mutations in components of the target of rapamycin (TOR) pathway. Consistent with the phenotypic similarities, Atf4 null mice have reduced expression of genes that regulate intracellular amino acid concentrations and lower intracellular concentration of amino acids, a key TOR input. Further, Atf4 mutants have reduced S6K activity in liver and adipose tissues.
Atf4 regulates age-related and diet-induced obesity as well as glucose homeostasis in mammals and has conserved metabolic functions in flies.
The ability to sense nutrient availability and regulate energy homeostasis is an ancient and fundamental process that, when disturbed, leads to significant metabolic derangements (1). Modern life has provided unparalleled access to food, contributing to unprecedented proportions of obesity, insulin resistance, and diabetes (1). Epidemiological evidence implicates not only increased fat consumption but also excess protein intake as causative in the intertwined epidemics of obesity and diabetes (2). Further, human trials show that amino acid infusions induce insulin resistance (3). Two evolutionarily conserved pathways, target of rapamycin (TOR) and the integrated stress response (ISR), play central roles in responding to amino acid availability (4,5).
The mechanisms whereby surplus food consumption engenders insulin resistance are an area of intense scrutiny, and several lines of evidence implicate TOR signaling as an important contributor (6). TOR signals, which couple amino acid supplies to translational efficiency, are in part conveyed through ribosomal S6 protein kinase (S6K) and eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1) (4). Recent data indicate that this evolutionarily conserved cascade links nutrient excess to insulin resistance in flies and mammals (7–10). For example, flies with mutations in 4E-BP or TOR control fat metabolism, and the TOR mutants have reduced lipid and glucose levels (7,8). Similarly, strains of 4E-BP1 mutant mice and S6K mutant mice are lean and have increased energy expenditure (9,10).
ISR is another highly conserved pathway that is important in translational control, nutrient sensing, and glucose homeostasis (11–13). The ISR is one branch of a signaling system termed the unfolded protein response (UPR) (12). Described for its role in handling stress induced by high demand of protein translation, for example during anabolic states, recent studies illuminate the notion that UPR has additional roles in metabolic physiology (11–13). For example, excessive UPR signaling leads to obesity, metabolic dysfunction, and fatty liver (14). Atf4, a bZip transcription factor, is an important component of the ISR (5). In the liver other bZip factors, C/EBPs, connect the ISR to several important metabolic functions, such as lipid, glucose, and glycogen biosynthesis, by increasing the expression of genes such as peroxisome proliferator–activated receptor γ (PPARγ) (14). The stress pathway is thought to have adaptive responses but if prolonged or excess can lead to maladaptive metabolic changes. For instance, a variety of animal studies indicate that excess stress, for example induced by knockouts of components of the UPR that rectify stress, can lead to obesity and fatty liver (15). Although the role of Atf4 in metabolic control is yet to be defined, it is known that Atf4 plays a central function in handling stress induced by amino acid imbalances (5). In addition, Atf4 regulates memory formation and is required for the appropriate development of cell lineages including blood and bone (16–18). The bone defects can be corrected by feeding the mutant animals a high protein diet, indicating that the roles of Atf4 in bone formation are nutritionally linked (19).
A variety of recent data indicate that the regulatory cascades controlling metabolism are conserved among evolutionarily diverse organisms such as worms, flies, and mammals (20). To attempt to isolate genes involved in metabolism, we characterized and compared the molecular signature of fly and murine developing metabolic tissues with the premise that conserved genes expressed in both organisms were plausible candidates. In D. melanogaster, we undertook an enhancer trap screen to isolate genes expressed in the larval fat body, a central fly metabolic organ (21), and identified Atf4. We also found that Atf4 was expressed in the embryonic day 14.5 (E14.5) mammalian fat anlagen using a combinatorial transgenic-transcriptional profiling approach. The evolutionarily conserved expression may have functional relevance because both Atf4 mutant flies and Atf4 null mice are lean and have reduced levels of circulating carbohydrate. The Atf4 mutant mice resist diet-induced and age-dependent obesity and diabetes. Further, the Atf4 mutant mice have increased energy expenditure. Liver and adipose tissues, key metabolic organs, removed from the Atf4 mutant mice have reduced expression of genes controlling intracellular amino acid concentrations. Further, tissues of Atf4 mutants contained lower amino acid levels, thereby supporting the idea that changes in amino acid metabolism are present in vivo. These tissues also have decreased activity of the TOR signaling pathway, which responds to amino acid inputs. These data support the notion that Atf4 is a conserved regulator of metabolism and carbohydrate homeostasis, thus providing a mechanistic link between nutrients, insulin resistance, and diabetes.
The X-linked enhancer trap P-element, PGawB, was mobilized to generate new insertions as described previously (22). Individual F1 larvae were screened for fat body green fluorescent protein (GFP) expression under a fluorescence-dissecting microscope, and fat body enhancer trap lines were established by crossing to balancer stocks. Adult fat body tissues were explanted under dissected microscopy, placed in PBS, and photographed with differential interference contrast or fluorescence microscopy as described (20). Insertion sites were identified by inverse PCR and/or plasmid rescue. Starvation and triglyceride assays were performed as described (20). For trehalose quantification, 2 μl of pooled hemolymph from L3 larvae was collected, diluted, heat inactivated, and treated with porcine kidney trehalase (Sigma). Glucose concentration was measured using Infinity Glucose Reagent (ThermoElectron). For Nile Red staining, whole flies or dissected fat bodies were fixed in formalin, permeabilized in 0.2% Triton X-100 solution, stained with Nile Red, and documented under a fluorescence-dissecting scope as described (20).
The aP2-GFP mice were generated by placing GFP into the 5.4-kb aP2 enhancer/promoter (23). Multiple transgenic lines were generated and screened for GFP expression in embryonic and adult fat depots with fluorescent microscopy. For this study, we selected the transgenic strain with the highest, most specific, and most consistent expression of GFP in embryonic and adult fat tissues. RNA was extracted from GFP-negative and -positive cells either after careful dissection or after dissociation and fluorescence-activated cell sorter (FACS). The Affymetrix microarrays were done by the University of Texas Southwestern Microarray Core Facility (http://microarray.swmed.edu). Atf4 heterozygous mice were backcrossed with pure inbred C57BL/6 mice, housed, and analyzed as described (18,20). Mice were fed either normal (4% fat, Teklad) or high-fat chow (60% fat, Research Diets). Fat content was measured using a minispec mq10 NMR Analyzer (Bruker). For glucose tolerance tests and insulin tolerance tests, 1.25 mg glucose or 0.9 mU Humulin R (Lilly)/1g body wt was injected intraperitoneally, and blood glucose levels were measured at the indicated intervals as described (20). Veterinary care was provided by the Division of Comparative Medicine. All animals were maintained under the guidelines of the University of Texas Southwestern Medical Center Animal Care and Use Committee according to current National Institutes of Health guidelines.
Tissues from Atf4 null and control mice were fixed with formalin solution, dehydrated with tissue processor (Microm), and embedded in paraffin. Hematoxylin-eosin was used to cut and stain 5- to 8-micron sections. For in situ hybridizations, embryos were collected from C57BL/6 female mice at E14.5, fixed in formalin, dehydrated, embedded in paraffin, and 5-micron sections hybridized with α-32P-[UTP]–labeled antisense aP2 or Atf4 probe.
Total RNA was extracted using TRIzol (Invitrogen), DNaseI treated, and reverse transcribed with random hexamers. Gene expression was analyzed using 7500 Real-Time PCR System (Applied Biosystems) using SYBR Green Master Mix reagent (Applied Biosystems). The values for gene expression were normalized by β-actin expression. Primer sequences are available upon request.
Immunoblotting was performed according to standard procedures. Protein samples were extracted using phosphoSafe Extraction Reagent (Calbiochem), boiled, and separated on denaturing polyacrylamide gels (percentage varied from 8–15%) prior to transfer onto nitrocellulose. The membrane was blocked and incubated with primary antibodies. Primary antibodies were purchased from the following: β-actin (Sigma), SLC1A4 (Millipore), uncoupling protein (UCP)-1 and AARS (Abcam), and all other antibodies (Cell Signaling Technology). Subsequently, the nitrocellulose membranes were washed and incubated with secondary antibodies conjugated with horseradish peroxidase (Jackson Immunoresearch). Signals are detected with chemiluminescent kits (NEN).
Floating adipocytes were isolated from control and Atf4 mutants as described (24). Total lipids were extracted for lipid analysis. Trinonadecanoin and pentadecanoic acid were used as internal standards for analysis by gas chromatography. Palmitoleate composition of the triglyceride fraction was determined by gas chromatography with flame-ionization detection, and the identity was determined by retention time and compared with the internal standard for quantification.
Protein concentrations of plasma mixed with 5% trichloroacetic acid and homogenized high-fat diet–fed mouse tissues were determined, and then the samples were centrifuged. The supernatant was filtered through 0.2 μm polytetrafluoroethylene centrifuge filter to remove precipitated proteins and tissue debris. Free amino acid concentrations of the samples were analyzed using a Hitachi l-8800 amino acid analyzer (25).
cAMP concentration was measured using Cyclic AMP Assay Kit (R&D) according to the manufacturer's protocol. Brown adipose tissue (BAT) was homogenized using phosphoSafe Extraction Reagent (Calbiochem). cAMP concentration of each BAT sample was determined by comparing with the cAMP standard curve with duplicates of each protein sample (50 μg).
To identify genes that might regulate metabolism, we characterized the molecular signature of the D. melanogaster larval fat body, a central fly metabolic organ. We performed a two-component (minimal promoter Gal4; upstream activating sequence–GFP) enhancer trap insertional screen (22) and isolated ~600 lines with fat body GFP expression. This produced a set of 587 genes including many previously reported to be fat body specific (e.g., Adh, arg, and vkg). Remarkably, although redundant insertions were quite uncommon, two of the fat body enhancer trap lines, E8 and G74, had insertions at the CG8669 locus, encoding the fly homolog of Atf4 (Fig. 1A) (26). In larvae, E8 expression appeared relatively specific to the fat body, whereas G74 also had ectoskeletal coexpression; both lines also displayed strong expression in adult metabolic tissues (Fig. 1B).
We also found that Atf4 was expressed in E14.5 murine metabolic tissues in a screen combining fluorescent cell–marking techniques, FACS, and microarray technology. This was based upon the observations that aP2, a known marker of adult fat depots, had restricted expression in the E14.5 fat pad (Fig. 1C) (23) and that aP2-GFP transgenic mice had strong GFP expression in embryonic fat tissues (Fig. 1D). The latter facilitated isolation of pure populations of E14.5 GFP+ and GFP− cells using dissections and FACS. We performed a series of Affymetrix microarray experiments to identify genes upregulated in GFP+ cells compared with GFP− cells. We then compared those genes expressed in the E14.5 fat anlagen with those identified in the fly enhancer trap screen; Atf4 was on both lists. In situ hybridizations and RT-PCR analyses showed that Atf4 was expressed in embryonic fat depots in the developing liver, a key metabolic tissue, and also in adult fat depots (Fig. 1E and F).
To determine whether the expression of Atf4 in fly metabolic tissues might have functional relevance, we examined the phenotype of the flies with P-element insertions at the Atf4 locus and found that they had altered metabolism. For example, E8 and G74 insertion mutant flies had reduced fat content based upon Nile Red staining of fat body explants, lower triglyceride levels, and increased starvation sensitivity (Fig. 2A–C). These flies also had a trend of reduced circulating levels of trehalose, the fly glucose equivalent (Fig. 2D).
The number of Atf4 mutant offspring in the C57BL/6 (B6) background is reduced (8.9%; 15/168) compared with the expected Mendelian ratio, but mice surviving >3 months are healthy and 14% have smaller body length (control 9.4 ± 0.15 cm, n = 8; mutant 8.1 ± 0.22 cm, n = 6) (27). To evaluate a potential role of Atf4 in mammalian metabolism, we analyzed young adult cohorts (aged 3 months) of Atf4 null and control siblings fed normal chow diet. Because aging is associated with increased insulin resistance and obesity (28,29), we also studied older mice (aged 12 months). Based upon appearance, nuclear magnetic resonance fat content analyses, and examination of adipose depots, the young adult Atf4 mutants had modestly reduced fat mass compared with controls, and this effect substantially increased with age (Fig. 3A–E), indicating that the Atf4 mutants resist age-associated obesity. However, kidney and heart weights, normalized to lean body mass, approximated those of controls (Fig. 3F, supplemental Table S1 in the online appendix, available at http://diabetes.diabetesjournals.org/content/early/2009/08/13/db09-0335/suppl/DC1). At young ages, Atf4-deficient adipocytes were of similar size to or in some cases even larger than control adipocytes. However, they had a blunted hypertrophic response to age-associated obesity (Fig. 3G).
To examine the potential role that Atf4 may play in diet-induced obesity, we provided a high-fat diet to control and Atf4 mutant littermates. During the 16 weeks of high-fat diet, we followed the mice with weekly weights, normalizing them to starting weight, and found that Atf4 mutants had a significantly blunted response to high-fat diet (Fig. 4A). This resistance to diet-induced weight gain was accompanied by a marked decrease in fat accumulation as determined by nuclear magnetic resonance analyses as well as by gross observation and weights of fat depot explants (Fig. 4B–E). However, the weights of other organs were not affected (data not shown). The differences in fat content are apparent in histological sections, which show that although control mice have significant fat, Atf4 null mice have a paucity of such deposition (Fig. 4F). In addition, the adipocytes are substantially smaller than those observed in controls (Fig. 4F). Fatty liver, a leading cause of liver disease, is a manifestation of metabolic dysfunction that can be elicited by high-fat diet (30). Consistent with that, we found that high-fat diet led to significant fat deposition in the livers of wild-type mice (Fig. 4G, H). In contrast, Atf4 mutants were resistant to this pathology (Fig. 4G and H).
Next we examined control and Atf4 mutant siblings for aspects of glucose homeostasis and found that Atf4 regulated murine carbohydrate metabolism. For example, Atf4 mutants had lower random and fasting glucose levels, and glucose tolerance tests showed that normal chow–fed Atf4 null mice had improved glucose metabolism (Fig. 5A–C). Even at the young ages, the Atf4 mutant cohorts displayed glucose levels and glucose tolerance test results that were reduced compared with controls (Fig. 5B) and seemingly out of proportion to the relatively modest changes in fat content observed at these ages (Fig. 3C). This may indicate that Atf4 has direct functions in glucose metabolism that are independent of the degree of fat reduction. In support of that notion, we found that insulin levels in normal chow–fed Atf4 mutants approximated control levels (Fig. 5D). This normal insulin level, in the setting of lowered blood glucose, indicates relative insulin excess and may be consistent with the previously identified role of the ISR in β-cell apoptosis, proliferation, and insulin secretion (31–33).
The Atf4 mutation also ameliorated high-fat diet–induced hyperglycemia (Fig. 5E). However, in this setting insulin levels were significantly lower than those for controls (Fig. 5F), pointing to roles of Atf4 in metabolism that are independent of the pancreas and potentially related to the lean phenotype described above. An insulin tolerance test indicated that the Atf4 mutant mice were insulin sensitive (Fig. 5G). Diet-induced and age-related obesity can provoke hyperlipidemia (1). The Atf4 mutation appeared to ameliorate this adverse consequence as the high-fat diet Atf4 null mice and the 12-month-old normal chow–fed mutants had improved lipid profiles compared with those for controls (Fig. 5H and I). Recent evidence indicates that the adipose-generated lipid palmitoleate is a lipokine that regulates glucose homeostasis (34). This does not appear to be the mechanism whereby Atf4 impacts metabolism because control and Atf4 mutant fat depots had equivalent levels of this fatty acid (Fig. 5J).
To attempt to identify the basis of the lean phenotype, we examined eating behavior, activity, and metabolic rate comparing Atf4 null and control siblings on either normal chow or high-fat diet. Food intake was equivalent in the Atf4 mutants and controls (Fig. 6A). The leanness was not due to increased activity because X-Y movement (walking) was equivalent in the two groups and Z-axis (rearing) activity was reduced by Atf4 deficiency (Fig. 6B). Atf4 null mice did have increased energy expenditure, displaying a higher rate (~15% increase) of oxygen consumption (VO2) in both light and dark phases that was accentuated by high-fat diet (~30% increase) (Fig. 6C and D). Because adipose tissue, reduced in Atf4 mutants, and muscle have differential contributions to whole-body VO2, we repeated the metabolic chamber analyses on additional cohorts of mice and in these cases with normalized energy expenditure using lean body mass. Again, Atf4 mutant mice had elevated energy expenditure (Fig. 6E and G). We also plotted the absolute energy expenditure (a per mouse analysis, plotting lean mass vs. VO2) of control and Atf4 mutants, and the slopes are illustrated in supplemental Fig. S1 in the online appendix. Another indication of altered metabolism is the observation that Atf4 mutants had a substantially lower respiratory quotient (RQ, VCO2/VO2), a measure of the proportionate rates of substrate utilization, indicating higher fat oxidation (Fig. 6F). Core body temperature is an additional measure of energy expenditure. Consistent with the other metabolic studies, we found that Atf4 mutant mice had elevated core body temperature (Fig. 6H). Given the starvation sensitivity observed in the Atf4 mutant flies (Fig. 2C), we also measured respiratory quotient and VO2 during the fed-to-fasted transition. Initially the respiratory quotients and VO2s became similar, but with a sustained fast the mutant animal VO2 dropped precipitously and the mutants become acutely ill. This indicates a potential defect in starvation resistance, reminiscent of fly starvation intolerance (Figs. 2C, ,66F, and and66G). Next we investigated potential energy expenditure defects molecularly. Levels of peroxisome proliferator–activated γ coactivator (PGC)-1α, UCP-1, and/or cAMP often reflect metabolic rate. We determined their expression or concentration in adipose tissues. First, we examined cAMP levels in BAT, a major source of thermogenesis. We observed a trend toward increased cAMP concentrations in Atf4 mutant mice (Fig. 6I), possibly reflecting increased metabolic activity. Next, we quantified mRNA levels in BAT and white adipose tissue (WAT) using qPCR. Expression of some genes involved in mitochondrial biogenesis and energy expenditure (e.g., PGC1-α and UCP-1) was slightly increased in Atf4 mutant BAT and WAT (Fig. 6J). We then examined the levels of these two proteins using Western blots. We found that PGC1-α, a key transcriptional regulator of oxidative capacity, was modestly increased in both tissues with a greater increase in WAT. Notably, the Western blots revealed a marked increased (~10-fold) in UCP-1 protein levels (often thought of as a marker of BAT) in Atf4 mutant WATs (Fig. 6K). Consistent with that observation, we found that Atf4 mutant mice contained multilocular adipocytes, characteristics of BAT in WAT depots (Fig. 6L).
Several lines of evidence indicate that Atf4 regulates intracellular amino acid metabolism (5,25). Notably, high intake of amino acids leads to both adiposity and insulin resistance (2,3). In cellular assays, Atf4 siRNA reduces intracellular concentrations of amino acids (25). Because reduced amino acid availability could account for various aspects of the Atf4 mutant phenotype (35), we assessed expression of genes important in intracellular amino acid concentrations in control and Atf4 mutant mouse embryonic fibroblasts (MEFs) as well as adipose depots and livers because these two tissues are metabolically relevant and are affected by Atf4 deficiency. Based upon qPCR, we detected significantly reduced levels of genes regulating amino acid metabolism, and transport was in Atf4 mutant MEFs (Fig. 7A and B). We also measured protein levels of some of the salient genes. We found that alanyl-tRNA synthetase was significantly reduced, whereas solute carrier family 1 member 4 levels were equivalent. Both liver and adipose tissues from Atf4 mutant mice also showed the same trends as those detected in the MEFs (Fig. 7C and D). We extended the data by directly quantifying amino acid concentrations in plasma, liver, and adipose depots of control and Atf4 mutant siblings (Fig. 7E and supplemental Fig. S2 in the online appendix). Although plasma amino acid levels were comparable, the Atf4 mutant tissues displayed a trend toward reduced concentrations of amino acids. Several of the metabolic characteristics observed in the Atf4-deficient flies and mice mirror phenotypes observed when components of the TOR pathway are mutated in those organisms (7–10). These phenotypic similarities coupled with the Atf4-dependent regulation of amino acid availability, a key TOR input (5,36), prompted us to examine metabolic tissues of Atf4 mutants and control siblings for TOR activity. Therefore, we assessed the phosphorylation of S6K, a key TOR target, in liver and adipose tissues removed from Atf4 mutants and control siblings. We found that S6K phosphorylation was reduced in Atf4 mutant liver and adipose tissues (Fig. 7F). We observed a similar reduction in the levels of phosphorylation of S6, an S6K target (Fig. 7F). To examine the possibility that the Atf4 mutation might lead to a state of energy depletion and consequent altered TOR activity, we analyzed the activity of AMP-activated protein kinase, a central regulator and sensor of intracellular energy balance and changes in ATP levels (37). However, the levels of AMP-activated protein kinase phosphorylation in the activation loop were similar in control and Atf4 mutant tissues (Supplemental Fig. S3 in the online appendix).
Energy homeostasis is maintained by a dynamic interaction between several organ systems that coordinate food intake, energy stores, metabolic demands, and energy expenditure (38,39). The understanding of metabolism has fundamental and clinical importance, so efforts directed toward characterization of molecules controlling energy homeostasis and metabolism have therapeutic potential. Here, we provide evidence that Atf4, a bZIP class transcription factor, regulates several aspects of mammalian metabolism, including fat storage, energy expenditure, and glycemic control. Some of these functional attributes may be conserved because Atf4 is expressed in the Drosophila fat body, a major fly metabolic organ, and flies with insertions into the dAtf4 locus are lean and have reduced circulating carbohydrate levels.
Modernity has produced a marked increase in dietary protein intake, which in turn has epidemiological and experimental links to the current explosion of obesity and diabetes. Protein synthetic overload is often sensed in the endoplasmic reticulum, an organelle in which newly translated proteins are sorted and modified (13). The surplus in endoplasmic reticulum handling of proteins triggers the UPR, a central mechanism designed to handle endoplasmic reticulum stress (40). Accumulated evidence indicates that endoplasmic reticulum stress pathways couple obesity to metabolic dysfunctions such as diabetes (41,42).
The UPR is composed of several different limbs; each handles different aspects of the stress response (11–13). One arm of the UPR increases the availability of endoplasmic reticulum chaperons, which in turn remodel misfolded proteins into the appropriate structure. Another arm of the UPR, ISR, reduces endoplasmic reticulum workload by inhibiting bulk protein translation. A few mRNAs, such as those encoding the bZIP class transcription factors Atf4, C/EBPα, and C/EBPβ (14), respond to the ISR signal paradoxically, thereby increasing their translation. In turn, these proteins affect transcriptional programs that increase the ability of the cell to handle stress (12,42). Recent studies have illuminated a key role that these proteins and the ISR play in metabolism. For example, the ISR regulates pancreatic β-cell survival, proliferation, and insulin secretion, and the normal chow–fed insulin levels observed in the Atf4 mutants resonate with previous descriptions (33,43).
That ISR contributes to negative outcomes produced by overindulgence may have evolutionary connections all the way to yeast (12). The TOR pathway also regulates invertebrate and vertebrate metabolism (7–10). Here, we showed that tissues derived from Atf4 mutant mice have reduced TOR signaling, reduced expression of genes important in the intracellular concentration of amino acids, and thereby, reduced concentration of amino acids, a major TOR input. These results underscore a potential intimate relationship between Atf4 function, ISR, TOR pathway, and metabolism that could be therapeutically exploited.
J.M.G. is a founder of Reata Pharmaceuticals. No other potential conflicts of interest relevant to this article were reported.
We thank Drs. Chul Ahn, Jawed Alam, Claude Desplan, Eiichi Hinoi, Xianxin Hua, Gerard Karsenty, David Ron, Bruce Spiegelman, and Charles Vinson for invaluable assistance with techniques, reagents, and manuscript preparation, as well as for helpful discussions. We thank Tammy L. Lisi for excellent technical assistance and members of the Graff lab for expert technical assistance, reagents, and insights.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.