SUMMARY Obesity and nutrient homeostasis are linked by mechanisms that are not fully elucidated. Here we describe a secreted protein, adropin, encoded by a gene, Energy Homeostasis Associated (Enho), expressed in liver and brain. Liver Enho expression is regulated by nutrition: lean C57BL/6J mice fed high-fat diet (HFD) exhibited a rapid increase, while fasting reduced expression compared to controls. However, liver Enho expression declines with diet-induced obesity (DIO) associated with 3 months of HFD or with genetically induced obesity, suggesting an association with metabolic disorders in the obese state. In DIO mice, transgenic overexpression or systemic adropin treatment attenuated hepatosteatosis and insulin resistance independently of effects on adiposity or food intake. Adropin regulated expression of hepatic lipogenic genes and adipose tissue peroxisome proliferator-activated receptor gamma, a major regulator of lipogenesis. Adropin may therefore be a factor governing glucose and lipid homeostasis, which protects against hepatosteatosis and hyperinsulinemia associated with obesity.

The nuclear receptors REV-ERBα (encoded by NR1D1) and REV-ERBβ (NR1D2) have remained orphans owing to the lack of identified physiological ligands. Here we show that heme is a physiological ligand of both receptors. Heme associates with the ligand-binding domains of the REV-ERB receptors with a 1:1 stoichiometry and enhances the thermal stability of the proteins. Results from experiments of heme depletion in mammalian cells indicate that heme binding to REV-ERB causes the recruitment of the co-repressor NCoR, leading to repression of target genes including BMAL1 (official symbol ARNTL), an essential component of the circadian oscillator. Heme extends the known types of ligands used by the human nuclear receptor family beyond the endocrine hormones and dietary lipids described so far. Our results further indicate that heme regulation of REV-ERBs may link the control of metabolism and the mammalian clock.REV-ERBα was originally identified as an orphan member of the nuclear hormone receptor (NHR) family on the basis of its canonical domain structure and sequence conservation 1,2 . REV-ERBβ was subsequently identified by its homology to other NHRs and its pattern of expression, which overlaps greatly with that of REV-ERBα. Both receptors have particularly high expression in the liver, adipose tissue, skeletal muscle and brain 3-8 , where they are transcribed in a circadian manner 9-11 . The REV-ERBs are unique in the NHR superfamily in that they lack the carboxy-terminal tail (helix 12) of the ligand-binding domain (LBD), which is required for coactivator recognition 12

The nuclear hormone receptor, REV-ERB, plays an essential role in adipogenesis. Rev-erbalpha expression is induced in 3T3-L1 cells during adipogenesis, and overexpression of this receptor leads to expression of adipogenic genes. We recently demonstrated that the porphyrin heme functions as a ligand for REV-ERB, and binding of heme is required for the receptor's activity. We therefore hypothesized that REV-ERB ligands may play a role in regulation of adipogenesis. We detected an increase intracellular heme levels during 3T3-L1 adipogenesis that correlated with induction of aminolevulinic acid synthase 1 (Alas1) expression, the rate-limiting enzyme in heme biosynthesis. If the increase in Alas1 expression was blocked, adipogenesis was severely attenuated, indicating that induction of expression of Alas1 and the increase in heme synthesis is critical for differentiation. Inhibition of heme synthesis during adipogenesis leads to decreased recruitment of nuclear receptor corepressor to the promoter of a REV-ERB target gene, suggesting alteration of REV-ERB activity. Treatment of 3T3-L1 cells with a synthetic REV-ERB ligand, SR6452, resulted in induction of adipocyte differentiation to a similar extent as treatment with the peroxisomal proliferator-activated receptor-gamma agonist, rosiglitazone. Combination of SR6452 and rosiglitazone had an additive effect on stimulation of adipocyte differentiation. These results suggest that heme, functioning as a REV-ERB ligand, is an important signaling molecule for induction of adipogenesis. Moreover, synthetic small molecule ligands for REV-ERB are effective modulators of adipogenesis and may be useful for treatment of metabolic diseases.

Objective: Understanding the regulation of adipocyte differentiation by cellular and extracellular factors is crucial for better management of chronic conditions such as obesity, insulin resistance and lipodystrophy. Experimental infection of rats with a human adenovirus type 36 (Ad-36) improves insulin sensitivity and promotes adipogenesis, reminiscent of the effect of thiozolinediones. Therefore, we investigated the role of Ad-36 as a novel regulator of the adipogenic process. Design and Results: Even in the absence of adipogenic inducers, infection of 3T3-L1 preadipocytes and human adipose-derived stem cells (hASC) by Ad-36, but not Ad-2 that is another human adenovirus, modulated regulatory points that spanned the entire adipogenic cascade ranging from the upregulation of cAMP, phosphatidylinositol 3-kinase and p38 signaling pathways, downregulation of Wnt10b expression, and increased expression of CCAAT/enhancer binding protein-b and peroxisome proliferator-activated receptor g2 and consequential lipid accumulation. Next, we identified that E4 open reading frame (orf)-1 gene of the virus is necessary and sufficient for Ad-36-induced adipogenesis. Selective knockdown of E4 orf-1 by RNAi abrogated Ad-36-induced adipogenic signaling cascade in 3T3-L1 cells and hASC. Compared to the null vector, selective expression of Ad-36 E4 orf-1 in 3T3-L1 induced adipogenesis, which was abrogated when the PDZ-binding domain of the protein was deleted. Conclusion: Thus, Ad-36 E4 orf-1 is a novel inducer of rodent and human adipocyte differentiation process.

OBJECTIVE-Human adenovirus type 36 (Ad-36) increases adiposity but improves insulin sensitivity in experimentally infected animals. We determined the ability of Ad-36 to increase glucose uptake by human primary skeletal muscle (HSKM) cells. RESEARCH DESIGN AND METHODS-The effect of Ad-36 on glucose uptake and cell signaling was determined in HSKM cells obtained from type 2 diabetic and healthy lean subjects. Ad-2, another human adenovirus, was used as a negative control. Gene expression and proteins of GLUT1 and GLUT4 were measured by real-time PCR and Western blotting. Role of insulin and Ras signaling pathways was determined in Ad-36 -infected HSKM cells. RESULTS-Ad-36and Ad-2 infections were confirmed by the presence of respective viral mRNA and protein expressions. In a dose-dependent manner, Ad-36 significantly increased glucose uptake in diabetic and nondiabetic HSKM cells. Ad-36 increased gene expression and protein abundance of GLUT1 and GLUT4, GLUT4 translocation to plasma membrane, and phosphatidylinositol 3-kinase (PI 3-kinase) activity in an insulin-independent manner. In fact, Ad-36 decreased insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation and IRS-1-and IRS-2-associated PI 3-kinase activities. On the other hand, Ad-36 increased Ras gene expression and protein abundance, and Ras siRNA abrogated Ad-36 -induced PI 3-kinase activation, GLUT4 protein abundance, and glucose uptake. These effects were not observed with Ad-2 infection.CONCLUSIONS-Ad-36 infection increases glucose uptake in HSKM cells via Ras-activated PI 3-kinase pathway in an insulinindependent manner. These findings may provide impetus to exploit the role of Ad-36 proteins as novel therapeutic targets for improving glucose handling.

Cholesterol is required for normal cellular and physiological function, yet dysregulation of cholesterol metabolism is associated with diseases such as atherosclerosis. Cholesterol biosynthesis is regulated by end product negative feedback inhibition where the levels of sterols and oxysterols regulate the expression of cholesterologenic enzymes. Sterol regulatory element-binding protein-2 is responsive to both sterols and oxysterols and has been shown to mediate the transcriptional response of the cholesterologenic enzymes to these lipids. Here, we show that the nuclear hormone receptor for oxysterols, the liver X receptor ␣ (LXR␣), regulates cholesterol biosynthesis by directly silencing the expression of two key cholesterologenic enzymes (lanosterol 14␣-demethylase (CYP51A1), and squalene synthase (farnesyl diphosphate farnesyl transferase 1)) via novel negative LXR DNA response elements (nLXREs) located in each of these genes. Examination of the CYP51A1 gene revealed that both the SRE and nLXRE are required for normal oxysterol-dependent repression of this gene. Thus, these data suggest that LXR␣ plays an important role in the regulation of cholesterol biosynthesis.Cholesterol is essential for maintenance of cellular membrane fluidity and is a precursor for the production of steroid hormones and bile acids. Although cholesterol is required for normal cellular and physiological function, dysregulation of cholesterol metabolism is associated with atherosclerosis and increased risk of heart disease (1). Thus, cholesterol metabolism is under strict biological regulation, and drugs inhibiting cholesterol biosynthesis such as the statins, inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) 2 (see Fig. 1a), have been successfully utilized clinically to reduce the risk of atherosclerosis. The biosynthesis of cholesterol has long been known to be regulated by end product feedback inhibition, and this regulation has been attributed to direct regulation of the expression of several cholesterol biosynthetic genes by the sterol sensing sterol regulatory element-binding protein-2 (SREBP-2) (2). The SREBPs are responsive to both alterations in cholesterol levels as well as oxidized cholesterol metabolites known as oxysterols (3).The nuclear hormone receptors LXR␣ (NR1H3) and LXR␤ (NR1H2) have been demonstrated to be physiological receptors for oxysterols (4 -6) and regulate components of the cholesterol metabolic pathway including reverse cholesterol transport and cholesterol elimination (7). No role for the LXRs in direct regulation of cholesterol biosynthesis has been described. A recent study by Wong et al. (8) suggested that 24,25-epoxycholesterol was important for "fine-tuning" acute control of cellular cholesterol biosynthesis. Because this endogenous oxysterol is known to be one of the more potent natural oxysterol ligands for the LXRs, it is possible that some of its activity may be mediated by these receptors. Here, we show that two key cholesterologenic enzyme genes are LXR target genes and that...

OBJECTIVE-Experimental infection of rats with human adenovirus type 36 (Ad-36) promotes adipogenesis and improves insulin sensitivity in a manner reminiscent of the pharmacologic effect of thiozolinediones. To exploit the potential of the viral proteins as a therapeutic target for treating insulin resistance, this study investigated the ability of Ad-36 to induce metabolically favorable changes in human adipose tissue. RESEARCH DESIGN AND METHODS-We determinedwhether Ad-36 increases glucose uptake in human adipose tissue explants. Cell-signaling pathways targeted by Ad-36 to increase glucose uptake were determined in the explants and human adipose-derived stem cells. Ad-2, a nonadipogenic human adenovirus, was used as a negative control. As a proof of concept, nondiabetic and diabetic subjects were screened for the presence of Ad-36 antibodies to ascertain if natural Ad-36 infection predicted improved glycemic control.RESULTS-Ad-36 increased glucose uptake by adipose tissue explants obtained from nondiabetic and diabetic subjects. Without insulin stimulation, Ad-36 upregulated expressions of several proadipogenic genes, adiponectin, and fatty acid synthase and reduced the expression of inflammatory cytokine macrophage chemoattractant protein-1 in a phosphotidylinositol 3-kinase (PI3K)-dependent manner. In turn, the activation of PI3K by Ad-36 was independent of insulin receptor signaling but dependent on Ras signaling recruited by Ad-36. Ad-2 was nonadipogenic and did not increase glucose uptake. Natural Ad-36 infection in nondiabetic and diabetic subjects was associated with significantly lower fasting glucose levels and A1C, respectively. CONCLUSIONS-Ad-36 proteins may provide novel therapeutic targets that remodel human adipose tissue to a more metabolically favorable profile.

The ARF tumor suppressor participates in a p53-dependent apoptotic pathway that is stimulated in response to some oncogenic stimuli. The E2F1 transcription factor is a critical downstream target of the Rb tumor suppressor and, when active, can promote proliferation as well as apoptosis. The finding that E2F1 transcriptionally regulates the ARF gene has led to the suggestion that ARF contributes to E2F1-induced apoptosis. Counter to this hypothesis, this study demonstrates not only that ARF is unnecessary for E2F1 to induce apoptosis but also that inactivation of ARF actually enhances the ability of E2F1 to promote apoptosis. Inactivation of ARF also cooperates with E2F1 activity to promote entry into the S phase of the cell cycle. This relationship between ARF and E2F1 is demonstrated in transgenic epidermis in vivo and in mouse embryo fibroblast cultures in vitro. In contrast, the ability of Myc to induce apoptosis is diminished in the absence of ARF. E2F1 induces the accumulation of p53 in the absence of ARF, and this is associated with the phosphorylation of p53 on several residues. These findings demonstrate that ARF is a negative regulator of E2F1 activity and is not required for E2F1-induced apoptosis. ARF (p19ARF in mice, p14 ARF in humans) is one of two tumor suppressors encoded at the INK4a locus, a chromosomal region frequently deleted in human tumors (21,23,36,46). Mice specifically lacking ARF, but retaining functional p16 INK4a , are predisposed to developing tumors, the most common being sarcomas and lymphomas (22,23). ARF functions as a major modifier of various p53-dependent signaling pathways through its ability to inhibit mdm2 (8,25,37,66). mdm2 antagonizes p53 by blocking its transcriptional activity, promoting its translocation to the cytoplasm, and targeting it for degradation by the proteasome (45,53). ARF may inhibit mdm2 by sequestering mdm2 in the nucleolus and promoting mdm2 degradation (44,56,62,65). The mdm2 gene is a transcriptional target of p53 and thus participates in a negativefeedback loop that normally maintains p53 at low levels (3, 63). ARF activates p53 by disrupting the p53-mdm2 feedback loop (45,53).ARF mediates the activation of p53 in response to oncogenic signals such those emanating from Ras, E1A, and Myc (8,37,66). Mouse embryonic fibroblasts (MEFs) null for ARF do not undergo replicative senescence and can be transformed by Ras alone in the absence of an immortalizing oncogene such as E1A or Myc (23, 52). In primary keratinocytes, ARF is required for the p53-dependent cell cycle arrest program induced by Ras (29). Moreover, the activation of p53 and the promotion of apoptosis by Myc and E1A are impaired in the absence of ARF (8,66). Transgenic mice expressing Myc under the control of the immunoglobulin enhancer develop B-cell lymphoma at an accelerated rate when ARF is hemizygous or nullizygous (10,51). This suggests that the ability to activate p53 in response to oncogenic signals underlies the tumor suppressor function of ARF.The mechanism by which at least some ...