Wnt signaling regulates hepatic metabolism

The mechanisms of genome-wide target gene regulation by TCF7L2 in liver cells

In the liver Wnt-signaling contributes to the metabolic fate of hepatocytes, but the precise role of the TCF7L2 in this process is unknown. We employed a temporal RNA-Seq approach to examine gene expression 3–96 h following Tcf7l2 silencing in rat hepatoma cells, and combined this with ChIP-Seq to investigate mechanisms of target gene regulation by TCF7L2. Silencing Tcf7l2 led to a time-dependent appearance of 406 differentially expressed genes (DEGs), including key regulators of cellular growth and differentiation, and amino acid, lipid and glucose metabolism. Direct regulation of 149 DEGs was suggested by strong proximal TCF7L2 binding (peak proximity score > 10) and early mRNA expression changes (≤18 h). Indirect gene regulation by TCF7L2 likely occurred via alternate transcription factors, including Hnf4a, Foxo1, Cited2, Myc and Lef1, which were differentially expressed following Tcf7l2 knock-down. Tcf7l2-silencing enhanced the expression and chromatin occupancy of HNF4α, and co-siRNA experiments revealed that HNF4α was required for the regulation of a subset of metabolic genes by TCF7L2, particularly those involved in lipid and amino-acid metabolism. Our findings suggest TCF7L2 is an important regulator of the hepatic phenotype, and highlight novel mechanisms of gene regulation by TCF7L2 that involve interplay between multiple hepatic transcriptional pathways.

β-catenin links hepatic metabolic zonation with lipid metabolism and diet-induced obesity in mice

β-catenin regulates the establishment of hepatic metabolic zonation. To elucidate the functional significance of liver metabolic zonation in the chronically overfed state in vivo, we fed a high-fat diet (HFD) to hepatocyte-specific β-catenin transgenic (TG) and knockout (KO) mice. Chow-fed TG and KO mice had normal liver histologic findings and body weight. However, HFD-fed TG mice developed prominent perivenous steatosis with periportal sparing. In contrast, HFD-fed KO mice had increased lobular inflammation and hepatocyte apoptosis. HFD-fed TG mice rapidly developed diet-induced obesity and systemic insulin resistance, but KO mice were resistant to diet-induced obesity. However, β-catenin did not directly affect hepatic insulin signaling, suggesting that the metabolic effects of β-catenin occurred via a parallel pathway. Hepatic expression of key glycolytic and lipogenic genes was higher in HFD-fed TG and lower in KO mice compared with wild-type mice. KO mice also exhibited defective hepatic fatty acid oxidation and fasting ketogenesis. Hepatic levels of hypoxia inducible factor-1α, an oxygen-sensitive transcriptional regulator of glycolysis and a known β-catenin binding partner, were higher in HFD-fed TG and lower in KO mice. KO mice had attenuated perivenous hypoxia, suggesting disruption of the normal sinusoidal oxygen gradient, a major determinant of liver carbohydrate and liver metabolism. Canonical Wnt signaling in hepatocytes is essential for the development of diet-induced fatty liver and obesity.

Serum secreted frizzled-related protein 5 levels differentially decrease in patients with hepatitis B virus-associated chronic infection and hepatocellular carcinoma

The aim of this study was to investigate the characteristics of serum secreted frizzled-related protein 5 (SFRP5), an inhibitor of Wnt signaling, in hepatitis B virus (HBV)-associated infections and hepatocellular carcinoma (HCC) patients. Serum SFRP5 levels were detected in 147 patients with HBV-associated chronic infection or HCC. Compared with the non-HBV-infected and non-HCC group, the HBV-associated chronic infection and HCC groups exhibited decreased serum SFRP5 levels. A significant inverse correlation between serum SFRP5 levels and HBV DNA levels was identified in the HBV-associated chronic infection and HCC groups. Furthermore, SFRP5 levels differentially decreased in patients with HBV-associated diseases, in a manner which was dependent on liver disease status. Compared with patients exhibiting HBV-associated chronic infection, patients with HCC were found to exhibit lower serum SFRP5 levels. The results of the present study indicated that patients with HBV-associated liver infection and HCC exhibited significantly deceased serum SFRP5 levels, which were found to negatively correlate with HBV DNA levels. Serum SFRP5 levels may present a biomarker for the severity of HBV-associated liver infection, and the risk of HCC initiation and progression.

Serum secreted frizzled-related protein 5 levels differentially decrease in patients with hepatitis B virus-associated chronic infection and hepatocellular carcinoma

The aim of this study was to investigate the characteristics of serum secreted frizzled-related protein 5 (SFRP5), an inhibitor of Wnt signaling, in hepatitis B virus (HBV)-associated infections and hepatocellular carcinoma (HCC) patients. Serum SFRP5 levels were detected in 147 patients with HBV-associated chronic infection or HCC. Compared with the non-HBV-infected and non-HCC group, the HBV-associated chronic infection and HCC groups exhibited decreased serum SFRP5 levels. A significant inverse correlation between serum SFRP5 levels and HBV DNA levels was identified in the HBV-associated chronic infection and HCC groups. Furthermore, SFRP5 levels differentially decreased in patients with HBV-associated diseases, in a manner which was dependent on liver disease status. Compared with patients exhibiting HBV-associated chronic infection, patients with HCC were found to exhibit lower serum SFRP5 levels. The results of the present study indicated that patients with HBV-associated liver infection and HCC exhibited significantly deceased serum SFRP5 levels, which were found to negatively correlate with HBV DNA levels. Serum SFRP5 levels may present a biomarker for the severity of HBV-associated liver infection, and the risk of HCC initiation and progression.

T-cell factor 4 and β-catenin chromatin occupancies pattern zonal liver metabolism in mice

β-catenin signaling can be both a physiological and oncogenic pathway in the liver. It controls compartmentalized gene expression, allowing the liver to ensure its essential metabolic function. It is activated by mutations in 20%-40% of hepatocellular carcinomas (HCCs) with specific metabolic features. We decipher the molecular determinants of β-catenin-dependent zonal transcription using mice with β-catenin-activated or -inactivated hepatocytes, characterizing in vivo their chromatin occupancy by T-cell factor (Tcf)-4 and β-catenin, transcriptome, and metabolome. We find that Tcf-4 DNA bindings depend on β-catenin. Tcf-4/β-catenin binds Wnt-responsive elements preferentially around β-catenin-induced genes. In contrast, genes repressed by β-catenin bind Tcf-4 on hepatocyte nuclear factor 4 (Hnf-4)-responsive elements. β-Catenin, Tcf-4, and Hnf-4α interact, dictating β-catenin transcription, which is antagonistic to that elicited by Hnf-4α. Finally, we find the drug/bile metabolism pathway to be the one most heavily targeted by β-catenin, partly through xenobiotic nuclear receptors.

CONCLUSIONS

β-catenin patterns the zonal liver together with Tcf-4, Hnf-4α, and xenobiotic nuclear receptors. This network represses lipid metabolism and exacerbates glutamine, drug, and bile metabolism, mirroring HCCs with β-catenin mutational activation.

Activation of cAMP signaling attenuates impaired hepatic glucose disposal in aged male p21-activated protein kinase-1 knockout mice

p21-activated protein kinase-1 (Pak1) plays a role in insulin secretion and glucagon-like peptide-1 (GLP-1) production. Pak1(-/-) mice were found to carry a defect in ip pyruvate tolerance test (IPPTT), leading us to speculate whether Pak1 represses hepatic gluconeogenesis. We show here that the defect in IPPTT became more severe in aged Pak1(-/-) mice. In primary hepatocytes, 2,2'-dihydroxy-1,1'-dinaphthyldisulfide, a potent inhibitor of group I Paks, reduced basal glucose production (GP), attenuated forskolin- or glucagon-stimulated GP, and attenuated the stimulation of forskolin on the expression of Pck1 and G6pc. In addition, the capacity of primary hepatocytes isolated from Pak1(-/-) mice in GP at the basal level is significantly lower than that of the control littermates. These in vitro observations imply that the direct effect of Paks in hepatocytes is the stimulation of gluconeogenesis and that the impairment in IPPTT in Pak1(-/-) mice is due to the lack of Pak1 elsewhere. Consecutive ip injection of forskolin for 2 weeks increased gut proglucagon expression, associated with improved IPPTT in aged Pak1(-/-) mice and wild-type controls. In addition, administration of the DPP-IV (dipeptidyl peptidase-4) inhibitor sitagliptin for 1 week reversed the defect in IPPTT in aged Pak1(-/-) mice, associated with increased plasma GLP-1 levels. Our observations indicate a potential role of Pak1 in the gut/pancreas/liver axis in controlling glucose disposal and affirmed the therapeutic application of GLP-1 and DPP-IV inhibitors in attenuating hepatic gluconeogenesis.

Wnt signaling directs a metabolic program of glycolysis and angiogenesis in colon cancer

Much of the mechanism by which Wnt signaling drives proliferation during oncogenesis is attributed to its regulation of the cell cycle. Here, we show how Wnt/β-catenin signaling directs another hallmark of tumorigenesis, namely Warburg metabolism. Using biochemical assays and fluorescence lifetime imaging microscopy (FLIM) to probe metabolism in vitro and in living tumors, we observe that interference with Wnt signaling in colon cancer cells reduces glycolytic metabolism and results in small, poorly perfused tumors. We identify pyruvate dehydrogenase kinase 1 (PDK1) as an important direct target within a larger gene program for metabolism. PDK1 inhibits pyruvate flux to mitochondrial respiration and a rescue of its expression in Wnt-inhibited cancer cells rescues glycolysis as well as vessel growth in the tumor microenvironment. Thus, we identify an important mechanism by which Wnt-driven Warburg metabolism directs the use of glucose for cancer cell proliferation and links it to vessel delivery of oxygen and nutrients.

Pigment epithelium-derived factor (PEDF) suppresses IL-1β-mediated c-Jun N-terminal kinase (JNK) activation to improve hepatocyte insulin signaling

Pigment epithelium-derived factor (PEDF) is an antiinflammatory protein that circulates at high levels in the metabolic syndrome. Metabolic studies of PEDF knockout (KO) mice were conducted to investigate the relationship between PEDF, inflammatory markers, and metabolic homeostasis. Male PEDF KO mice demonstrated a phenotype consisting of increased adiposity, glucose intolerance, and elevated serum levels of metabolites associated with the metabolic syndrome. Genome expression analysis revealed an increase in IL-1β signaling in the livers of PEDF KO mice that was accompanied by impaired IRS and Akt signaling. In human hepatocytes, PEDF blocked the effects of an IL-1β challenge by suppressing activation of the inflammatory mediator c-Jun N-terminal kinase while restoring Akt signaling. RNA interference of PEDF in human hepatocytes was permissive for c-Jun N-terminal kinase activation and decreased Akt signaling. A metabolomics profile identified elevated circulating levels of tricarboxyclic acid cycle intermediates including succinate, an inducer of IL-1β, in PEDF KO mice. Succinate-dependent IL-1β expression was blocked by PEDF in PEDF KO, but not wild-type hepatocytes. In vivo, PEDF restoration reduced hyperglycemia and improved hepatic insulin signaling in PEDF KO mice. These findings identify elevated PEDF as a homeostatic mechanism in the human metabolic syndrome.

Alternative human liver transcripts of TCF7L2 bind to the gluconeogenesis regulator HNF4α at the protein level

AIMS/HYPOTHESIS

Gene polymorphisms of TCF7L2 are associated with increased risk of type 2 diabetes and transcription factor 7-like 2 (TCF7L2) plays a role in hepatic glucose metabolism. We therefore addressed the impact of TCF7L2 isoforms on hepatocyte nuclear factor 4α (HNF4α) and the regulation of gluconeogenesis genes.

METHODS

Liver TCF7L2 transcripts were analysed by quantitative PCR in 33 non-diabetic and 31 type 2 diabetic obese individuals genotyped for TCF7L2 rs7903146. To analyse transcriptional regulation by TCF7L2, small interfering RNA transfection, luciferase reporter and co-immunoprecipitation assays were performed in human hepatoma HepG2 cells.

RESULTS

In livers of diabetic compared with normoglycaemic individuals, five C-terminal TCF7L2 transcripts showed increased expression. The type 2 diabetes risk allele of rs7903146 positively correlated with TCF7L2 expression in livers from normoglycaemic individuals only. In HepG2 cells, transcript and TCF7L2 protein levels were increased upon incubation in high glucose and insulin. Of the exon 13 transcripts, six were increased in a glucose dose-responsive manner. TCF7L2 transcriptionally regulated 29 genes related to glucose metabolism, including glucose-6-phosphatase. In cultured HepG2 cells, TCF7L2 did not regulate HNF4Α and FOXO1 transcription, but did affect HNF4α protein expression. The TCF7L2 isoforms T6 and T8 (without exon 13 and with exon 15/14, respectively) specifically interacted with HNF4α.

CONCLUSIONS/INTERPRETATION

The different levels of expression of alternative C-terminal TCF7L2 transcripts in HepG2 cells, in livers of normoglycaemic individuals carrying the rs7901346 type 2 diabetes risk allele and in livers of diabetic individuals suggest that these transcripts play a role in the pathophysiology of type 2 diabetes. We also report for the first time a protein interaction in HepG2 cells between HNF4α and the T6 and T8 isoforms of TCF7L2, which suggests a distinct role for these specific alternative transcripts.

Systemic Activin signaling independently regulates sugar homeostasis, cellular metabolism, and pH balance in Drosophila melanogaster

The ability to maintain cellular and physiological metabolic homeostasis is key for the survival of multicellular organisms in changing environmental conditions. However, our understanding of extracellular signaling pathways that modulate metabolic processes remains limited. In this study we show that the Activin-like ligand Dawdle (Daw) is a major regulator of systemic metabolic homeostasis and cellular metabolism in Drosophila. We find that loss of canonical Smad signaling downstream of Daw leads to defects in sugar and systemic pH homeostasis. Although Daw regulates sugar homeostasis by positively influencing insulin release, we find that the effect of Daw on pH balance is independent of its role in insulin signaling and is caused by accumulation of organic acids that are primarily tricarboxylic acid (TCA) cycle intermediates. RNA sequencing reveals that a number of TCA cycle enzymes and nuclear-encoded mitochondrial genes including genes involved in oxidative phosphorylation and β-oxidation are up-regulated in the daw mutants, indicating either a direct or indirect role of Daw in regulating these genes. These findings establish Activin signaling as a major metabolic regulator and uncover a functional link between TGF-β signaling, insulin signaling, and metabolism in Drosophila.

Activation of β-catenin by inhibitors of glycogen synthase kinase-3 ameliorates cisplatin-induced cytotoxicity and pro-inflammatory cytokine expression in HEI-OC1 cells

Cisplatin is used in the treatment of a wide variety of solid tumors, but its use is limited by its serious adverse effects, including ototoxicity. Glycogen synthase kinase-3 (GSK-3) is a ubiquitously expressed serine/threonine kinase that regulates a variety of cellular functions by phosphorylating its substrates. However, the otoprotective effect of GSK-3 inhibitors is poorly understood. Here, we investigated whether GSK-3 is involved in cisplatin-induced ototoxicity in HEI-OC1 cells and organs of Corti (OCs). GSK-3 inhibitors suppressed cisplatin-induced apoptosis determined by decreased p53 activity, and also decreased expression of PARP and p53 target genes such as p21 and PUMA. The effect of GSK-3 inhibitors was mediated by markedly increased nuclear β-catenin that in turn blocked nuclear translocation of NF-κB. siRNA-mediated β-catenin knockdown markedly increased the expression of NF-κB target genes, such as TNF-α and IL-6. Our data suggest that the GSK-3/β-catenin pathway may play a central role in cisplatin-mediated cytotoxicity in HEI-OC1 cells and hair cells of OCs in vitro.

Mice deficient in Sfrp1 exhibit increased adiposity, dysregulated glucose metabolism, and enhanced macrophage infiltration

The molecular mechanisms involved in the development of obesity and related complications remain unclear. Wnt signaling plays an important role in preadipocyte differentiation and adipogenesis. The expression of a Wnt antagonist, secreted frizzled related protein 1 (SFRP1), is increased in response to initial weight gain, then levels are reduced under conditions of extreme obesity in both humans and animals. Here we report that loss of Sfrp1 exacerbates weight gain, glucose homeostasis and inflammation in mice in response to diet induced obesity (DIO). Sfrp1^-/- mice fed a high fat diet (HFD) exhibited an increase in body mass accompanied by increases in body fat percentage, visceral white adipose tissue (WAT) mass, and adipocyte size. Moreover, Sfrp1 deficiency increases the mRNA levels of key de novo lipid synthesis genes (Fasn, Acaca, Acly, Elovl, Scd1) and the transcription factors that regulate their expression (Lxr-α, Srebp1, Chreb, and Nr1h3) in WAT. Fasting glucose levels are elevated, glucose clearance is impaired, hepatic gluconeogenesis regulators are aberrantly upregulated (G6pc and Pck1), and glucose transporters are repressed (Slc2a2 and Slc2a4) in Sfrp1^-/- mice fed a HFD. Additionally, we observed increased steatosis in the livers of Sfrp1^-/- mice. When there is an expansion of adipose tissue there is a sustained inflammatory response accompanied by adipokine dysregulation, which leads to chronic subclinical inflammation. Thus, we assessed the inflammatory state of different tissues and revealed that Sfrp1^-/- mice fed a HFD exhibited increased macrophage infiltration and expression of pro-inflammatory markers including IL-6, Nmnat, Tgf-β2, and SerpinE1. Our findings demonstrate that the expression of Sfrp1 is a critical factor required for maintaining appropriate cellular signaling in response to the onset of obesity.

GLP-1-derived nonapeptide GLP-1(28-36)amide represses hepatic gluconeogenic gene expression and improves pyruvate tolerance in high-fat diet-fed mice

Certain "degradation" products of GLP-1 were found to possess beneficial effects on metabolic homeostasis. Here, we investigated the function of the COOH-terminal fragment of GLP-1, the nonapeptide GLP-1(28-36)amide, in hepatic glucose metabolism. C57BL/6 mice fed a high-fat diet (HFD) for 13 wk were injected intraperitoneally with GLP-1(28-36)amide for 6 wk. A significant reduction in body weight gain in response to HFD feeding was observed in GLP-1(28-36)amide-treated mice. GLP-1(28-36)amide administration moderately improved glucose disposal during glucose tolerance test but more drastically attenuated glucose production during pyruvate tolerance test, which was associated with reduced hepatic expression of the gluconeogenic genes Pck1, G6pc, and Ppargc1a. Mice treated with GLP-1(28-36)amide exhibited increased phosphorylation of PKA targets, including cAMP response element-binding protein (CREB), ATF-1, and β-catenin. In primary hepatocytes, GLP-1(28-36)amide reduced glucose production and expression of Pck1, G6pc, and Ppargc1a, which was associated with increased cAMP content and PKA target phosphorylation. These effects were attenuated by PKA inhibition. We suggest that GLP-1(28-36)amide represses hepatic gluconeogenesis involving the activation of components of the cAMP/PKA signaling pathway. This study further confirmed that GLP-1(28-36)amide possesses therapeutic potential for diabetes and other metabolic disorders.

FOXOs attenuate bone formation by suppressing Wnt signaling

Wnt/β-catenin/TCF signaling stimulates bone formation and suppresses adipogenesis. The hallmarks of skeletal involution with age, on the other hand, are decreased bone formation and increased bone marrow adiposity. These changes are associated with increased oxidative stress and decreased growth factor production, which activate members of the FOXO family of transcription factors. FOXOs in turn attenuate Wnt/β-catenin signaling by diverting β-catenin from TCF- to FOXO-mediated transcription. We show herein that mice lacking Foxo1, -3, and -4 in bipotential progenitors of osteoblast and adipocytes (expressing Osterix1) exhibited increased osteoblast number and high bone mass that was maintained in old age as well as decreased adiposity in the aged bone marrow. The increased bone mass in the Foxo-deficient mice was accounted for by increased proliferation of osteoprogenitor cells and bone formation resulting from upregulation of Wnt/β-catenin signaling and cyclin D1 expression, but not changes in redox balance. Consistent with this mechanism, β-catenin deletion in Foxo null cells abrogated both the increased cyclin D1 expression and proliferation. The elucidation of a restraining effect of FOXOs on Wnt signaling in bipotential progenitors suggests that FOXO activation by accumulation of age-associated cellular stressors may be a seminal pathogenetic mechanism in the development of involutional osteoporosis.

Inhibition of Notch uncouples Akt activation from hepatic lipid accumulation by decreasing mTorc1 stability

Increased hepatic lipid content is an early correlate of insulin resistance, and can be caused by nutrient-induced mTor activation. The latter increases basal Akt activity, leading to a self-perpetuating lipogenic cycle. We have previously shown that the developmental Notch pathway has metabolic functions in adult liver. Acute or chronic inhibition of Notch dampens hepatic glucose production and increases Akt tone, and might therefore be predicted to increase hepatic lipid content. Surprisingly, we show that constitutive liver-specific ablation of Notch signaling, or its acute inhibition with a decoy Notch1 receptor, prevents hepatosteatosis by blocking mTorc1. Conversely, Notch gain-of-function causes fatty liver through constitutive activation of mTorc1, an effect reversible by rapamycin treatment. We demonstrate that Notch signaling increases mTorc1 complex stability, augmenting mTorc1 function and Srebp1c-mediated lipogenesis. The data identify Notch as a therapeutically actionable branch point of metabolic signaling, where hepatic Akt activation can be uncoupled from steatosis.

Basic biology of skeletal aging: role of stress response pathways

Although a decline in bone formation and loss of bone mass are common features of human aging, the molecular mechanisms mediating these effects have remained unclear. Evidence from pharmacological and genetic studies in mice has provided support for a deleterious effect of oxidative stress in bone and has strengthened the idea that an increase in reactive oxygen species (ROS) with advancing age represents a pathophysiological mechanism underlying age-related bone loss. Mesenchymal stem cells and osteocytes are long-lived cells and, therefore, are more susceptible than other types of bone cells to the molecular changes caused by aging, including increased levels of ROS and decreased autophagy. However, short-lived cells like osteoblast progenitors and mature osteoblasts and osteoclasts are also affected by the altered aged environment characterized by lower levels of sex steroids, increased endogenous glucocorticoids, and higher oxidized lipids. This article reviews current knowledge on the effects of the aging process on bone, with particular emphasis on the role of ROS and autophagy in cells of the osteoblast lineage in mice.

The Paradox of Akt-mTOR Interactions

The serine threonine protein kinase, Akt, is at the central hub of signaling pathways that regulates cell growth, differentiation, and survival. The reciprocal relation that exists between the two activating phosphorylation sites of Akt, T308 and S473, and the two mTOR complexes, C1 and C2, forms the central controlling hub that regulates these cellular functions. In our previous review “PI3Kinase (PI3K)-AKT-mTOR and Wnt signaling pathways in cell cycle” we discussed the reciprocal relation between mTORC1 and C2 complexes in regulating cell metabolism and cell cycle progression in cancer cells. We present in this article, a hypothesis that activation of Akt-T308 phosphorylation in the presence of high ATP:AMP ratio promotes the stability of its phosphorylations and activates mTORC1 and the energy consuming biosynthetic processes. Depletion of energy leads to inactivation of mTORC1, activation of AMPK, FoxO, and promotes constitution of mTORC2 that leads to phosphorylation of Akt S473. Akt can also be activated independent of PI3K; this appears to have an advantage under situations like dietary restrictions, where insulin/insulin growth factor signaling could be a casualty.

WNT-LRP5 signaling induces Warburg effect through mTORC2 activation during osteoblast differentiation

WNT signaling controls many biological processes including cell differentiation in metazoans. However, how WNT reprograms cell identity is not well understood. We have investigated the potential role of cellular metabolism in WNT-induced osteoblast differentiation. WNT3A induces aerobic glycolysis known as Warburg effect by increasing the level of key glycolytic enzymes. The metabolic regulation requires LRP5 but not β-catenin and is mediated by mTORC2-AKT signaling downstream of RAC1. Suppressing WNT3A-induced metabolic enzymes impairs osteoblast differentiation in vitro. Deletion of Lrp5 in the mouse, which decreases postnatal bone mass, reduces mTORC2 activity and glycolytic enzymes in bone cells and lowers serum lactate levels. Conversely, mice expressing a mutant Lrp5 that causes high bone mass exhibit increased glycolysis in bone. Thus, WNT-LRP5 signaling promotes bone formation in part through direct reprogramming of glucose metabolism. Moreover, regulation of cellular metabolism may represent a general mechanism contributing to the wide-ranging functions of WNT proteins.

[Cross-talk between insulin signaling and cell proliferation pathways]

Epidemiological studies provide evidence for a close relationship between diabetes and cancer. Insulin is in fact a growth factor, and its binding to its membrane receptor activates intracellular signaling pathways involved in the regulation of both metabolism and cell proliferation. The balance between mitogenic and metabolic actions of insulin can be modulated by various mechanisms, including the way the ligand binds to its receptor or to the closely related insulin-like growth factor-1 (IGF-1) receptor. Cross-talks with other signaling pathways implicated in cell proliferation have also been described, like the Wnt/β catenin pathway, and involve the activation of common downstream effectors such as insulin receptor substrate-1 (IRS-1). Finally, the identification of new proteins activated by insulin and involved in intracellular signaling would allow a better understanding of the complex connections linking metabolic and proliferative regulatory pathways. As an example, the molecular adaptor Grb14, which is a specific inhibitor of insulin receptor catalytic activity, also controls insulin-induced metabolic and mitogenic signaling pathways through post-receptor mechanisms that remain to be fully elucidated.

Identification of a pathway by which glucose regulates β-catenin signalling via the cAMP/protein kinase A pathway in β-cell models

Pancreatic β-cells are highly responsive to changes in glucose, but the mechanisms involved are only partially understood. There is increasing evidence that the β-catenin signalling pathway plays an important role in regulating β-cell function, but the mechanisms regulating β-catenin signalling in these cells is not well understood. In the present study we show that β-catenin levels and downstream signalling are regulated by changes in glucose levels in INS-1E and β-TC6-F7 β-cell models. We found a glucose-dependent increase in levels of β-catenin in the cytoplasm and nucleus of INS-1E cells. Expression of cyclin D1 also increased with glucose and required the presence of β-catenin. This was associated with an increase in phosphorylation of β-catenin on Ser552, which is known to stabilize the molecule and increase its transcriptional activity. In a search for possible signalling intermediates we found forskolin and cell-permeable cAMP analogues recapitulated the glucose effects, suggesting a role for cAMP and PKA (cAMP-dependent protein kinase/protein kinase A) downstream of glucose. Furthermore, glucose caused sustained increases in cAMP. Two different inhibitors of adenylate cyclase and PKA signalling blocked the effects of glucose, whereas siRNA (small interfering RNA) knockdown of PKA blocked the effects of glucose on β-catenin signalling. Finally, reducing β-catenin levels with either siRNA or pyrvinium impaired glucose- and KCl-stimulated insulin secretion. Taken together the results of the present study define a pathway by which changes in glucose levels can regulate β-catenin using a mechanism which involves cAMP production and the activation of PKA. This identifies a pathway that may be important in glucose-dependent regulation of gene expression and insulin secretion in β-cells.

The Wnt signaling pathway effector TCF7L2 controls gut and brain proglucagon gene expression and glucose homeostasis

The type 2 diabetes risk gene TCF7L2 is the effector of the Wnt signaling pathway. We found previously that in gut endocrine L-cell lines, TCF7L2 controls transcription of the proglucagon gene (gcg), which encodes the incretin hormone glucagon-like peptide-1 (GLP-1). Whereas peripheral GLP-1 stimulates insulin secretion, brain GLP-1 controls energy homeostasis through yet-to-be defined mechanisms. We aim to determine the metabolic effect of a functional knockdown of TCF7L2 by generating transgenic mice that express dominant-negative TCF7L2 (TCF7L2DN) specifically in gcg-expressing cells. The gcg-TCF7L2DN transgenic mice showed reduced gcg expression in their gut and brain, but not in pancreas. Defects in glucose homeostasis were observed in these mice, associated with attenuated plasma insulin levels in response to glucose challenge. The defect in glucose disposal was exacerbated with high-fat diet. Brain Wnt activity and feeding-mediated hypothalamic AMP-activated protein kinase (AMPK) repression in these mice were impaired. Peripheral injection of the cAMP-promoting agent forskolin increased brain β-cat Ser675 phosphorylation and brain gcg expression and restored feeding-mediated hypothalamic AMPK repression. We conclude that TCF7L2 and Wnt signaling control gut and brain gcg expression and glucose homeostasis and speculate that positive cross-talk between Wnt and GLP-1/cAMP signaling is an underlying mechanism for brain GLP-1 in exerting its metabolic functions.

LRP6 enhances glucose metabolism by promoting TCF7L2-dependent insulin receptor expression and IGF receptor stabilization in humans

Common genetic variations in Wnt signaling genes have been associated with metabolic syndrome and diabetes by mechanisms that are poorly understood. A rare nonconservative mutation in Wnt coreceptor LRP6 (LRP6(R611C)) has been shown to underlie autosomal dominant early onset coronary artery disease, type 2 diabetes, and metabolic syndrome. We examined the interplay between Wnt and insulin signaling pathways in skeletal muscle and skin fibroblasts of healthy nondiabetic LRP6(R611C) mutation carriers. LRP6 mutation carriers exhibited hyperinsulinemia and reduced insulin sensitivity compared to noncarrier relatives in response to oral glucose ingestion, which correlated with a significant decline in tissue expression of the insulin receptor and insulin signaling activity. Further investigations showed that the LRP6(R611C) mutation diminishes TCF7L2-dependent transcription of the IR while it increases the stability of IGFR and enhances mTORC1 activity. These findings identify the Wnt/LRP6/TCF7L2 axis as a regulator of glucose metabolism and a potential therapeutic target for insulin resistance.

Association between donor and recipient TCF7L2 gene polymorphisms and the risk of new-onset diabetes mellitus after liver transplantation in a Han Chinese population

BACKGROUNDS & AIMS

New-onset diabetes mellitus (NODM) is a frequent and serious complication arising after liver transplantation (LT). Transcription factor 7-like 2 (TCF7L2) polymorphisms have been reported to strongly associate with type 2 diabetes. In addition, the donor liver plays a vital role in regulating blood glucose levels. In this study, we aim at evaluating the association between donor and recipient TCF7L2 gene polymorphisms with NODM after LT.

METHODS

A total of 125 patients undergoing primary LT, without a history of diabetes were included. Four single nucleotide polymorphisms (rs290487, rs7903146, rs11196205, and rs12255372), closely associated with type 2 diabetes in the Eastern Asia population, were genotyped and analyzed.

RESULTS

Both donor and recipient rs290487 polymorphisms (CC vs. TT genotype) were found to be significantly associated with NODM. In multivariate analysis, donor rs290487 genetic variation (OR = 2.172 per each C allele, p = 0.015), blood tacrolimus levels at 1 month post-LT >10 ng/ml (OR = 3.264, p = 0.017), and recipient age >55 years (OR = 2.638, p = 0.043) were identified as independent risk factors of NODM. Furthermore, donor rs290487 CC genotype could predict a high probability (>40%) of the onset of NODM. Predictive model containing donor rs290487 polymorphism showed a significantly higher prognostic ability on NODM than the model with only clinical parameters (p = 0.031).

CONCLUSIONS

Donor TCF7L2 rs290487 polymorphism is associated with an increased risk of NODM after LT and has a potential clinical value for the prediction of NODM.

WISP2 regulates preadipocyte commitment and PPARγ activation by BMP4

Inability to recruit new adipose cells following weight gain leads to inappropriate enlargement of existing cells (hypertrophic obesity) associated with inflammation and a dysfunctional adipose tissue. We found increased expression of WNT1 inducible signaling pathway protein 2 (WISP2) and other markers of WNT activation in human abdominal s.c. adipose tissue characterized by hypertrophic obesity combined with increased visceral fat accumulation and insulin resistance. WISP2 activation in the s.c. adipose tissue, but not in visceral fat, identified the metabolic syndrome in equally obese individuals. WISP2 is a novel adipokine, highly expressed and secreted by adipose precursor cells. Knocking down WISP2 induced spontaneous differentiation of 3T3-L1 and human preadipocytes and allowed NIH 3T3 fibroblasts to become committed to the adipose lineage by bone morphogenetic protein 4 (BMP4). WISP2 forms a cytosolic complex with the peroxisome proliferator-activated receptor γ (PPARγ) transcriptional activator zinc finger protein 423 (Zfp423), and this complex is dissociated by BMP4 in a SMAD-dependent manner, thereby allowing Zfp423 to enter the nucleus, activate PPARγ, and commit the cells to the adipose lineage. The importance of intracellular Wisp2 protein for BMP4-induced adipogenic commitment and PPARγ activation was verified by expressing a mutant Wisp2 protein lacking the endoplasmic reticulum signal and secretion sequence. Secreted Wnt/Wisp2 also inhibits differentiation and PPARγ activation, albeit not through Zfp423 nuclear translocation. Thus adipogenic commitment and differentiation is regulated by the cross-talk between BMP4 and canonical WNT signaling and where WISP2 plays a key role. Furthermore, they link WISP2 with hypertrophic obesity and the metabolic syndrome.

FOXOs: signalling integrators for homeostasis maintenance

Forkhead box O (FOXO) transcription factors are involved in the regulation of the cell cycle, apoptosis and metabolism. In model organisms, FOXO activity also affects stem cell maintenance and lifespan as well as age-related diseases, such as cancer and diabetes. Multiple upstream pathways regulate FOXO activity through post-translational modifications and nuclear-cytoplasmic shuttling of both FOXO and its regulators. The diversity of this upstream regulation and the downstream effects of FOXOs suggest that they function as homeostasis regulators to maintain tissue homeostasis over time and coordinate a response to environmental changes, including growth factor deprivation, metabolic stress (starvation) and oxidative stress.

Motor Function Deficits Following Chronic Prenatal Ethanol Exposure are Linked to Impairments in Insulin/IGF, Notch and Wnt Signaling in the Cerebellum

BACKGROUND

Fetal alcohol spectrum disorder (FASD) is associated with deficits in cerebellar function that can persist through adolescence. Previous studies demonstrated striking inhibition of insulin and insulin-like growth factor (IGF) signaling in ethanol-exposed cerebella.

OBJECTIVES

We sought to determine if FASD-induced impairments in motor function were associated with deficits in insulin/IGF signaling in juvenile cerebella. Given the growing evidence that insulin/IGF pathways cross-talk with Notch and Wnt to promote brain development and maturation; we also examined the integrity of canonical Wnt and Notch signaling networks in the brain following chronic prenatal ethanol exposure.

METHODS

Pregnant Long Evans rats were fed isocaloric liquid diets containing 0% or 24% ethanol by caloric content from gestation day 6 through delivery. Pups were subjected to rotarod testing on postnatal days (P) 15-16 and sacrificed on P30. Cerebella were used for molecular and biochemical analysis of insulin/IGF-1, canonical Wnt, and Notch signaling mechanisms.

RESULTS

Prenatal ethanol exposures impaired rotarod performance, inhibited signaling through insulin and IGF-1 receptors, IRS-1, and Akt, increased activation of GSK-3β, and broadly suppressed genes mediating the canonical Wnt and Notch networks.

CONCLUSIONS

Abnormalities in cerebellar function following chronic prenatal ethanol exposure are associated with inhibition of insulin/IGF, canonical Wnt, and Notch networks that cross-talk via GSK-3β. Effective therapeutic measures for FASD may require multi-pronged support of interrelated signaling networks that regulate brain development.

The Wnt signaling pathway effector TCF7L2 is upregulated by insulin and represses hepatic gluconeogenesis

Certain single nucleotide polymorphisms (SNPs) in transcription factor 7-like 2 (TCF7L2) are strongly associated with the risk of type 2 diabetes. TCF7L2 and β-catenin (β-cat) form the bipartite transcription factor cat/TCF in stimulating Wnt target gene expression. cat/TCF may also mediate the effect of other signaling cascades, including that of cAMP and insulin in cell-type specific manners. As carriers of TCF7L2 type 2 diabetes risk SNPs demonstrated increased hepatic glucose production, we aimed to determine whether TCF7L2 expression is regulated by nutrient availability and whether TCF7L2 and Wnt regulate hepatic gluconeogenesis. We examined hepatic Wnt activity in the TOPGAL transgenic mouse, assessed hepatic TCF7L2 expression in mice upon feeding, determined the effect of insulin on TCF7L2 expression and β-cat Ser⁶⁷⁵ phosphorylation, and investigated the effect of Wnt activation and TCF7L2 knockdown on gluconeogenic gene expression and glucose production in hepatocytes. Wnt activity was observed in pericentral hepatocytes in the TOPGAL mouse, whereas TCF7L2 expression was detected in human and mouse hepatocytes. Insulin and feeding stimulated hepatic TCF7L2 expression in vitro and in vivo, respectively. In addition, insulin activated β-cat Ser⁶⁷⁵ phosphorylation. Wnt activation by intraperitoneal lithium injection repressed hepatic gluconeogenic gene expression in vivo, whereas lithium or Wnt-3a reduced gluconeogenic gene expression and glucose production in hepatic cells in vitro. Small interfering RNA-mediated TCF7L2 knockdown increased glucose production and gluconeogenic gene expression in cultured hepatocytes. These observations suggest that Wnt signaling and TCF7L2 are negative regulators of hepatic gluconeogenesis, and TCF7L2 is among the downstream effectors of insulin in hepatocytes.

Genome-wide analysis of FoxO1 binding in hepatic chromatin: potential involvement of FoxO1 in linking retinoid signaling to hepatic gluconeogenesis

The forkhead transcription factor FoxO1 is a critical regulator of hepatic glucose and lipid metabolism, and dysregulation of FoxO1 function has been implicated in diabetes and insulin resistance. We globally identified FoxO1 occupancy in mouse hepatic chromatin on a genome-wide level by chromatin immunoprecipitation coupled with high-throughput DNA sequencing (ChIP-seq). To establish the specific functional significance of FoxO1 against other FoxO proteins, ChIP-seq was performed with chromatin from liver-specific FoxO1 knockout and wild-type mice. Here we identified 401 genome-wide FoxO1-binding locations. Motif search reveals a sequence element, 5′ GTAAACA 3′, consistent with a previously known FoxO1-binding site. Gene set enrichment analysis shows that the data from FoxO1 ChIP-seq are highly correlated with the global expression profiling of genes regulated by FoxO1, demonstrating the functional relevance of our FoxO1 ChIP-seq study. Interestingly, gene ontology analysis reveals the functional significance of FoxO1 in retinoid metabolic processes. We show here that FoxO1 directly binds to the genomic sites for the genes in retinoid metabolism. Notably, deletion of FoxO1 caused a significantly reduced induction of Pck1 and Pdk4 in response to retinoids. As Pck1 and Pdk4 are downstream targets of retinoid signaling, these results suggest that FoxO1 plays a potential role in linking retinoid metabolism to hepatic gluconeogenesis.

β-Catenin regulates hepatic mitochondrial function and energy balance in mice

BACKGROUND & AIMS

Wnt signaling regulates hepatic function and nutrient homeostasis. However, little is known about the roles of β-catenin in cellular respiration or mitochondria of hepatocytes.

METHODS

We investigated β-catenin's role in the metabolic function of hepatocytes under homeostatic conditions and in response to metabolic stress using mice with hepatocyte-specific deletion of β-catenin and their wild-type littermates, given either saline (sham) or ethanol (as a model of binge drinking and acute ethanol intoxication).

RESULTS

Under homeostatic conditions, β-catenin-deficient hepatocytes demonstrated mitochondrial dysfunctions that included impairments to the tricarboxylic acid cycle and oxidative phosphorylation (OXPHOS) and decreased production of adenosine triphosphate (ATP). There was no evidence for redox imbalance or oxidative cellular injury in the absence of metabolic stress. In mice with β-catenin-deficient hepatocytes, ethanol intoxication led to significant redox imbalance in the hepatocytes and further deterioration in mitochondrial function that included reduced OXPHOS, fatty acid oxidation (FAO), and ATP production. Ethanol feeding significantly increased liver steatosis and oxidative damage, compared with wild-type mice, and disrupted the ratio of nicotinamide adenine dinucleotide. β-catenin-deficient hepatocytes also had showed disrupted signaling of Sirt1/peroxisome proliferator-activated receptor-α signaling.

CONCLUSIONS

β-catenin has an important role in the maintenance of mitochondrial homeostasis, regulating ATP production via the tricarboxylic acid cycle, OXPHOS, and fatty acid oxidation; β-catenin function in these systems is compromised under conditions of nutrient oxidative stress. Reagents that alter Wnt-β-catenin signaling might be developed as a useful new therapeutic strategy for treatment of liver disease.

The involvement of the wnt signaling pathway and TCF7L2 in diabetes mellitus: The current understanding, dispute, and perspective

The Wnt signaling pathway was initially discovered for its role in tumorigenesis and the development of Drosophila and other eukaryotic organisms. The key effector of this pathway, the bipartite transcription factor β-cat/TCF, is formed by free β-catenin (β-cat) and a TCF protein, including TCF7L2. Extensive recent investigations have highlighted the role of the Wnt signaling pathway in metabolic homeostasis and its implication in diabetes and other metabolic diseases. Genome-wide association studies have shown that several key components of the Wnt signaling pathway are implicated in metabolic homeostasis and the development of type 2 diabetes (T2D). Despite controversial observations regarding the role of Wnt signaling in the development and function of pancreatic islets, the discovery of the association between certain single nucleotide polymorphisms of TCF7L2 and T2D susceptibility has fueled great efforts to explore the role of Wnt signaling in the function of pancreatic β-cells and glucose homeostasis. Here we have introduced our basic understanding of the canonical Wnt signaling pathway, summarized our current knowledge on its implication in metabolic homeostasis and T2D, discussed the work on TCF7L2 as a T2D susceptibility gene, and presented the controversial role of Wnt signaling and TCF7L2 in pancreatic islets as well as their potential metabolic function in other organs. We then expanded our view into the crosstalk among Wnt, insulin and FOXO signaling cascades, which further illustrates the complexity of the Wnt signaling pathway in metabolic homeostasis. Finally, we have presented our perspectives.

The role of the Wnt signaling pathway in incretin hormone production and function

Glucose metabolism is tightly controlled by multiple hormones and neurotransmitters in response to nutritional, environmental, and emotional changes. In addition to insulin and glucagon produced by pancreatic islets, two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP, also known as glucose-dependent insulinotropic peptide), also play important roles in blood glucose homeostasis. The incretin hormones mainly exert their regulatory effects via their corresponding receptors, which are expressed in pancreatic islets as well as many other extra-pancreatic organs. Recent studies have shown that the genes which encode these two incretin hormones can be regulated by the effectors of the Wnt signaling pathway, including TCF7L2, a transcription factor identified recently by extensive genome wide association studies as an important type 2 diabetes risk gene. Interestingly, TCF7L2 and β-catenin (β-cat), another effector of Wnt signaling pathway, may also mediate the function of the incretin hormones as well as the expression of their receptors in pancreatic β-cells. In this review, we have introduced the incretin hormones and the Wnt signaling pathway, summarized recent findings in the field, and provided our perspectives.

Aging mechanisms in bone

Advancing age and loss of bone mass and strength are closely linked. Elevated osteoblast and osteocyte apoptosis and decreased osteoblast number characterize the age-related skeletal changes in humans and rodents. Similar to other tissues, oxidative stress increases in bone with age. This article reviews current knowledge on the effects of the aging process on bone and its cellular constituents, with particular emphasis on the role of reactive oxygen species (ROS). FoxOs, sirtuins and the p53/p66^shc signaling cascade alter osteoblast number and bone formation via ROS-dependent and -independent mechanisms. Specifically, activation of the p53/p66^shc signaling increases osteoblast/osteocyte apoptosis in the aged skeleton and decreases bone mass. FoxO activation in osteoblasts prevents oxidative stress to preserve skeletal homeostasis. However, while defending against stress FoxOs bind to β-catenin and attenuate Wnt/T-cell cell factor transcriptional activity and osteoblast generation. Thus, pathways that impact longevity and several diseases of ageing might also contribute to age-related osteoporosis.

Reduction in Tcf7l2 expression decreases diabetic susceptibility in mice

Objective: The WNT signaling pathway effector gene TCF7L2 has been associated with an increased risk of type 2 diabetes. However, it remains unclear how this gene affects diabetic pathogenesis. The goal of this study was to investigate the effects of Tcf7l2 haploinsufficiency on metabolic phenotypes in mice.

Experimental Design:Tcf7l2 knockout (Tcf7l^-/-) mice were generated. Because of the early mortality of Tcf7l2^-/- mice, we characterized the metabolic phenotypes of heterozygous Tcf7l2^+/- mice in comparison to the wild-type controls. The mice were fed a normal chow diet or a high fat diet (HFD) for 9 weeks.

Results: The Tcf7l2^+/- mice showed significant differences from the wild-type mice with regards to body weight, fasting glucose and insulin levels. Tcf7l2^+/- mice displayed improved glucose tolerance. In the liver of Tcf7l2^+/- mice fed on the HFD, reduced lipogenesis and hepatic triglyceride levels were observed when compared with those of wild-type mice. Furthermore, the Tcf7l2^+/- mice fed on the HFD exhibited decreased peripheral fat deposition. Immunohistochemistry in mouse pancreatic islets showed that endogenous expression of Tcf7l2 was upregulated in the wild-type mice, but not in the Tcf7l2^+/- mice, after feeding with the HFD. However, the haploinsufficiency of Tcf7l2 in mouse pancreatic islets resulted in little changes in glucose-stimulated insulin secretion.

Conclusion: These results suggest that decreased expression of Tcf7l2 confers reduction of diabetic susceptibility in mice via regulation on the metabolism of glucose and lipid.

The many faces and functions of β-catenin

β-Catenin (Armadillo in Drosophila) is a multitasking and evolutionary conserved molecule that in metazoans exerts a crucial role in a multitude of developmental and homeostatic processes. More specifically, β-catenin is an integral structural component of cadherin-based adherens junctions, and the key nuclear effector of canonical Wnt signalling in the nucleus. Imbalance in the structural and signalling properties of β-catenin often results in disease and deregulated growth connected to cancer and metastasis. Intense research into the life of β-catenin has revealed a complex picture. Here, we try to capture the state of the art: we try to summarize and make some sense of the processes that regulate β-catenin, as well as the plethora of β-catenin binding partners. One focus will be the interaction of β-catenin with different transcription factors and the potential implications of these interactions for direct cross-talk between β-catenin and non-Wnt signalling pathways.

β-catenin is essential for ethanol metabolism and protection against alcohol-mediated liver steatosis in mice

The liver plays a central role in ethanol metabolism, and oxidative stress is implicated in alcohol-mediated liver injury. β-Catenin regulates hepatic metabolic zonation and adaptive response to oxidative stress. We hypothesized that β-catenin regulates the hepatic response to ethanol ingestion. Female liver-specific β-catenin knockout (KO) mice and wild-type (WT) littermates were fed the Lieber-Decarli liquid diet (5% ethanol) in a pairwise fashion. Liver histology, biochemistry, and gene-expression studies were performed. Plasma alcohol and ammonia levels were measured using standard assays. Ethanol-fed (EtOH) KO mice exhibited systemic toxicity and early mortality. KO mice exhibited severe macrovesicular steatosis and 5 to 6-fold higher serum alanine aminotransferase and aspartate aminotransferase levels. KO mice had a modest increase in hepatic oxidative stress, lower expression of mitochondrial superoxide dismutase (SOD2), and lower citrate synthase activity, the first step in the tricarboxylic acid cycle. N-Acetylcysteine did not prevent ethanol-induced mortality in KO mice. In WT livers, β-catenin was found to coprecipitate with forkhead box O3, the upstream regulator of SOD2. Hepatic alcohol dehydrogenase and aldehyde dehydrogenase activities and expression were lower in KO mice. Hepatic cytochrome P450 2E1 protein levels were up-regulated in EtOH WT mice, but were nearly undetectable in KO mice. These changes in ethanol-metabolizing enzymes were associated with 30-fold higher blood alcohol levels in KO mice.

CONCLUSION

β-Catenin is essential for hepatic ethanol metabolism and plays a protective role in alcohol-mediated liver steatosis. Our results strongly suggest that integration of these functions by β-catenin is critical for adaptation to ethanol ingestion in vivo.

Liver Patt1 deficiency protects male mice from age-associated but not high-fat diet-induced hepatic steatosis

Patt1 is a newly identified protein acetyltransferase that is highly expressed in liver. However, the role of Patt1 in liver is still unclear. We generated Patt1 liver-specific knockout (LKO) mice and mainly measured the effect of hepatic Patt1 deficiency on lipid metabolism. Hepatic Patt1 deficiency in male mice markedly decreases fat mass and dramatically alleviates age-associated accumulation of lipid droplets in liver. Moreover, hepatic Patt1 abrogation in male mice significantly reduces the liver triglyceride and free fatty acid levels, but it has no effect on liver cholesterol level, liver weight, and liver function. Consistently, primary cultured Patt1-deficient hepatocytes are resistant to palmitic acid-induced lipid accumulation, but hepatic Patt1 deficiency fails to protect male mice from high-fat diet-induced hepatic steatosis. Further studies show that hepatic Patt1 deficiency decreases fatty acid uptake, reduces lipid synthesis, and enhances fatty acid oxidation, which may contribute to the attenuated hepatic steatosis in Patt1 LKO mice. These results demonstrate that Patt1 plays an important role in hepatic lipid metabolism and have implications toward resolving age-associated hepatic steatosis.

Low density lipoprotein (LDL) receptor-related protein 6 (LRP6) regulates body fat and glucose homeostasis by modulating nutrient sensing pathways and mitochondrial energy expenditure

Body fat, insulin resistance, and type 2 diabetes are often linked together, but the molecular mechanisms that unify their association are poorly understood. Wnt signaling regulates adipogenesis, and its altered activity has been implicated in the pathogenesis of type 2 diabetes and metabolic syndrome. LRP6(+/-) mice on a high fat diet were protected against diet-induced obesity and hepatic and adipose tissue insulin resistance compared with their wild-type (WT) littermates. Brown adipose tissue insulin sensitivity and reduced adiposity of LRP6(+/-) mice were accounted for by diminished Wnt-dependent mTORC1 activity and enhanced expression of brown adipose tissue PGC1-α and UCP1. LRP6(+/-) mice also exhibited reduced endogenous hepatic glucose output, which was due to diminished FoxO1-dependent expression of the key gluconeogenic enzyme glucose-6-phosphatase (G6pase). In addition, in vivo and in vitro studies showed that loss of LRP6 allele is associated with increased leptin receptor expression, which is a likely cause of hepatic insulin sensitivity in LRP6(+/-) mice. Our study identifies LRP6 as a nutrient-sensitive regulator of body weight and glucose metabolism and as a potential target for pharmacological interventions in obesity and diabetes.

Recombinant R-spondin2 and Wnt3a up- and down-regulate novel target genes in C57MG mouse mammary epithelial cells

R-spondins (Rspos) comprise a family of four secreted proteins that have important roles in cell proliferation, cell fate determination and organogenesis. Rspos typically exert their effects by potentiating the Wnt/β-catenin signaling pathway. To systematically investigate the impact of Rspo/Wnt on gene expression, we performed a microarray analysis using C57MG mouse mammary epithelial cells treated with recombinant Rspo2 and/or Wnt3a. We observed the up- and down-regulation of several previously unidentified target genes, including ones that encode proteins involved in immune responses, effectors of other growth factor signaling pathways and transcription factors. Dozens of these changes were validated by quantitative real time RT-PCR. Time course experiments showed that Rspo2 typically had little or no effect on Wnt-dependent gene expression at 3 or 6 h, but enhanced expression at 24 h, consistent with biochemical data indicating that Rspo2 acts primarily to sustain rather than acutely increase Wnt pathway activation. Up-regulation of gene expression was inhibited by pre-treatment with Dickkopf1, a Wnt/β-catenin pathway antagonist, and by siRNA knockdown of β-catenin expression. While Dickkopf1 blocked Rspo2/Wnt3a-dependent down-regulation, a number of down-regulated genes were not affected by β-catenin knockdown, suggesting that in these instances down-regulation was mediated by a β-catenin-independent mechanism.

Oxidants, metabolism, and stem cell biology

Adult stem cells persist throughout the lifetime of the organism and may therefore require specific mechanisms to limit the effects of chronic oxidative stress. Recently, several instructive genetic mouse models have demonstrated the unique susceptibility of stem cells to perturbations in metabolic or redox homeostasis. These results have implications not only for stem cell biology but also suggest a mechanistic link between intracellular oxidants and the decline in regenerative function that occurs as a normal consequence of aging.

Unraveling the role of FoxOs in bone--insights from mouse models

The FoxO subfamily of forkhead transcription factors plays a critical role in a variety of physiological processes including metabolism, differentiation, proliferation, apoptosis and protection from stress. FoxO activity is inhibited by growth factors and the insulin signaling pathways and stimulated by nutrient depletion and a plethora of reactive oxygen species (ROS)-induced post-translational modifications. Recent studies have uncovered a fundamental role for FoxOs in skeletal homeostasis. In cells of the osteoblast lineage, FoxOs modulate redox balance, protein synthesis, and differentiation through the activation of specific gene programs and interaction with other transcription factors and co-factors such as β-catenin, ATF-4, and Runx2. FoxO activation also attenuates osteoclastogenesis through both cell autonomous and indirect mechanisms. In this review I discuss recent advances in the understanding of FoxO specific actions in osteoblast progenitors, osteoblasts, and osteoclast, as well as the implications of FoxO activation for age-related skeletal involution.