Weijuan Shao

Curcumin Prevents High Fat Diet Induced Insulin Resistance and Obesity via Attenuating Lipogenesis in Liver and Inflammatory Pathway in Adipocytes

Background Mechanisms underlying the attenuation of body weight gain and insulin resistance in response to high fat diet (HFD) by the curry compound curcumin need to be further explored. Although the attenuation of the inflammatory pathway is an accepted mechanism, a recent study suggested that curcumin stimulates Wnt signaling pathway and hence suppresses adipogenic differentiation. This is in contrast with the known repressive effect of curcumin on Wnt signaling in other cell lineages. Methodology and Principal Findings We conducted the examination on low fat diet, or HFD fed C57BL/6J mice with or without curcumin intervention for 28 weeks. Curcumin significantly attenuated the effect of HFD on glucose disposal, body weight/fat gain, as well as the development of insulin resistance. No stimulatory effect on Wnt activation was observed in the mature fat tissue. In addition, curcumin did not stimulate Wnt signaling in vitro in primary rat adipocytes. Furthermore, curcumin inhibited lipogenic gene expression in the liver and blocked the effects of HFD on macrophage infiltration and the inflammatory pathway in the adipose tissue. Conclusions and Significance We conclude that the beneficial effect of curcumin during HFD consumption is mediated by attenuating lipogenic gene expression in the liver and the inflammatory response in the adipose tissue, in the absence of stimulation of Wnt signaling in mature adipocytes.

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.

GLP-1(28–36) improves β-cell mass and glucose disposal in streptozotocin-induced diabetic mice and activates cAMP/PKA/β-catenin signaling in β-cells in vitro

Recent studies have demonstrated that the COOH-terminal fragment of the incretin hormone glucagon-like peptide-1 (GLP-1), a nonapeptide GLP-1(28-36)amide, attenuates diabetes and hepatic steatosis in diet-induced obese mice. However, the effect of this nonapeptide in pancreatic β-cells remains largely unknown. Here, we show that in a streptozotocin-induced mouse diabetes model, GLP-1(28-36)amide improved glucose disposal and increased pancreatic β-cell mass and β-cell proliferation. An in vitro investigation revealed that GLP-1(28-36)amide stimulates β-catenin (β-cat) Ser(675) phosphorylation in both the clonal INS-1 cell line and rat primary pancreatic islet cells. In INS-1 cells, the stimulation was accompanied by increased nuclear β-cat content. GLP-1(28-36)amide was also shown to increase cellular cAMP levels, PKA enzymatic activity, and cAMP response element-binding protein (CREB) and cyclic AMP-dependent transcription factor-1 (ATF-1) phosphorylation. Furthermore, GLP-1(28-36)amide treatment enhanced islet insulin secretion and increased the growth of INS-1 cells, which was associated with increased cyclin D1 expression. Finally, PKA inhibition attenuated the effect of GLP-1(28-36)amide on β-cat Ser(675) phosphorylation and cyclin D1 expression in the INS-1 cell line. We have thus revealed the beneficial effect of GLP-1(28-36)amide in pancreatic β-cells in vitro and in vivo. Our observations suggest that GLP-1(28-36)amide may exert its effect through the PKA/β-catenin signaling pathway.

Liver-specific Expression of Dominant Negative Transcription Factor 7-like 2 Causes Progressive Impairment in Glucose Homeostasis

Investigations on the metabolic role of the Wnt signaling pathway and hepatic transcription factor 7-like 2 (TCF7L2) have generated opposing views. While some studies demonstrated a repressive effect of TCF7L2 on hepatic gluconeogenesis, a recent study using liver-specific Tcf7l2−/− mice suggested the opposite. As a consequence of redundant and bidirectional actions of transcription factor (TCF) molecules and other complexities of the Wnt pathway, knockout of a single Wnt pathway component may not effectively reveal a complete metabolic picture of this pathway. To address this, we generated the liver-specific dominant-negative (DN) TCF7L2 (TCF7L2DN) transgenic mouse model LTCFDN. These mice exhibited progressive impairment in response to pyruvate challenge. Importantly, LTCFDN hepatocytes displayed elevated gluconeogenic gene expression, gluconeogenesis, and loss of Wnt-3a–mediated repression of gluconeogenesis. In C57BL/6 hepatocytes, adenovirus-mediated expression of TCF7L2DN, but not wild-type TCF7L2, increased gluconeogenesis and gluconeogenic gene expression. Our further mechanistic exploration suggests that TCF7L2DN-mediated inhibition of Wnt signaling causes preferential interaction of β-catenin (β-cat) with FoxO1 and increased binding of β-cat/FoxO1 to the Pck1 FoxO binding site, resulting in the stimulation of Pck1 expression and increased gluconeogenesis. Together, our results using TCF7L2DN as a unique tool revealed that the Wnt signaling pathway and its effector β-cat/TCF serve a beneficial role in suppressing hepatic gluconeogenesis.

Insulin treatment and high-fat diet feeding reduces the expression of three Tcf genes in rodent pancreas

Specific single-nucleotide polymorphisms in intronic regions of human TCF7L2 are associated with an elevated risk of developing type 2 diabetes. Whether Tcf7l2 is expressed in pancreatic islets of rodent species at a considerable level, however, remains controversial. We used RT-PCR and quantitative RT-PCR to examine Tcf7l2 expression in rodent gut, pancreas, isolated pancreatic islets, and cultured cell lines. The expression level of Tcf7l2 was relatively lower in the pancreas compared to the gut or the pancreatic β-cell line Ins-1. Immunostaining did not detect a Tcf7l2 signal in mouse pancreatic islets. Endogenous canonical Wnt activity was not appreciable in the pancreas of TOPGAL transgenic mice. Both Tcf7 and Tcf7l1, but not Lef1, were expressed in the pancreas. The expression of the three Tcf genes (Tcf7, Tcf7l1, and Tcf7l2) in the pancreas was reduced by treatment with insulin or high-fat diet feeding, in contrast to the stimulation of Tcf7l2 expression by insulin in the gut. We suggest that hyperinsulinemia represses Tcf gene expression in the pancreas. Whether and how this reduction alters the function of pancreatic β cells during hyperinsulinemia deserves further investigation.

Acute Wnt pathway activation positively regulates leptin gene expression in mature adipocytes

Genome-wide association studies (GWAS) have revealed the implication of several Wnt signaling pathway components, including its effector transcription factor 7-like 2 (TCF7L2) in diabetes and other metabolic disorders. As TCF7L2 is expressed in adipocytes, we investigated its expression and function in rodent fat tissue and mature adipocytes. We found that TCF7L2 mRNA expression in C57BL/6 mouse epididymal fat tissue was up-regulated by feeding but down-regulated by intraperitoneal insulin injection. In high-fat diet (HFD) fed mice, db/db mice and Zucker (fa/fa) rats, epididymal fat TCF7L2 mRNA levels were lower than the corresponding controls. Treating rat adipocytes with 100nM insulin repressed TCF7L2 mRNA and protein levels, associated with the repression of leptin mRNA level. The treatment with 1nM insulin, however, stimulated TCF7L2 and leptin mRNA levels. This stimulation could be attenuated by iCRT14, an inhibitor of β-catenin/TCF-responsive transcription. Wnt3a stimulated leptin mRNA level, which was also blocked by iCRT14 co-treatment. Utilizing the leptin-expressing cell line HTR8 as a tool, we defined an evolutionarily conserved CREB binding motif that mediated Wnt3a activation. Although Wnt activation is known to repress the differentiation of 3T3-L1 cells towards mature adipocytes, short-term Wnt3a treatment of differentiated 3T3-L1 cells stimulated leptin mRNA levels. Thus, wnt pathway plays a dual function in adipocytes, including the well-known repressive effect on adipogenesis and the stimulation of leptin production in mature adipocytes in response to nutritional status.

The expression of dominant negative TCF7L2 in pancreatic beta cells during the embryonic stage causes impaired glucose homeostasis.

Objective Disruption of TCF7L2 in mouse pancreatic β-cells has generated different outcomes in several investigations. Here we aim to clarify role of β-cell TCF7L2 and Wnt signaling using a functional-knockdown approach. Methods Adenovirus-mediated dominant negative TCF7L2 (TCF7L2DN) expression was conducted in Ins-1 cells. The fusion gene in which TCF7L2DN expression is driven by PTRE3G was utilized to generate the transgenic mouse line TCF7L2DNTet. The double transgenic line was created by mating TCF7L2DNTet with Ins2-rtTA, designated as βTCFDN. β-cell specific TCF7L2DN expression was induced in βTCFDN by doxycycline feeding. Results TCF7L2DN expression in Ins-1 cells reduced GSIS, cell proliferation and expression of a battery of genes including incretin receptors and β-cell transcription factors. Inducing TCF7L2DN expression in βTCFDN during adulthood or immediately after weaning generated no or very modest metabolic defect, while its expression during embryonic development by doxycycline feeding in pregnant mothers resulted in significant glucose intolerance associated with altered β-cell gene expression and reduced β-cell mass. Conclusions Our observations support a cell autonomous role for TCF7L2 in pancreatic β-cells suggested by most, though not all, investigations. βTCFDN is a novel model for further exploring the role of TCF7L2 in β-cell genesis and metabolic homeostasis.

Diet polyphenol curcumin stimulates hepatic Fgf21 production and restores its sensitivity in high fat diet fed male mice

We found previously that short-term curcumin gavage stimulated mouse hepatic fibroblast growth factor 21 (Fgf21) expression. Here we conducted mechanistic exploration and investigated the potential pathophysiological relevance on this regulation. Fgf21 stimulation was observed at messenger RNA and protein levels in mice with daily curcumin gavage for 4 or 8 days and in primary hepatocytes with curcumin treatment. Using peroxisome proliferator-activated receptor α (PPARα) agonist and antagonist, along with luciferase reporter and chromatin immune-precipitation approaches, we determined that curcumin stimulates Fgf21 transcription in a mechanism involving PPARα activation. High-fat diet (HFD) feeding also increased mouse hepatic and serum Fgf21 levels, whereas dietary curcumin intervention attenuated these increases. We found that HFD feeding reduced hepatic expression levels of genes that encode FGFR1 and βKlotho, PGC1α, and the targets of the PPARα-PGC1α axis, whereas concomitant curcumin intervention restored or partially restored their expression levels. Importantly, hepatocytes from HFD-fed mice showed a loss of response to FGF21 treatment on Erk phosphorylation and the expression of Egr1 and cFos; this response was restored in hepatocytes from HFD-fed mice with curcumin intervention. This investigation expanded our mechanistic understanding of the metabolic beneficial effects of dietary curcumin intervention involving the regulation of Fgf21 production and the attenuation of HFD-induced Fgf21 resistance.

New insight into the mechanisms underlying the function of the incretin hormone glucagon-like peptide-1 in pancreatic β-cells: The involvement of the Wnt signaling pathway effector β-catenin

During the past two decades, the exploration of function of two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), has led to the development of two categories of novel therapeutic agents for diabetes and its complications, known as GLP-1 receptor (GLP-1R) agonists and DPP-IV inhibitors. Mechanisms underlying the function of GLP-1, however, still need to be further explored. GLP-1 not only functions as an incretin hormone in stimulating insulin secretion in response to nutritional, hormonal and neuronal stimulations, but also acts as an “insulin-like” factor in β-cell and extra-pancreatic organs. In addition to these insulinotropic and insulinomimetic effects, GLP-1 was shown to exert its protective effect in β-cell by repressing the expression of TxNIP, a mediator of glucolipotoxicity. A number of recent studies have shown that the Wnt signaling pathway effector, the bipartite transcription factor β-catenin/TCF, controls not only the production of GLP-1, but also the function of GLP-1. Furthermore, previously assumed “degradation” products of GLP-1(7–36)amide, including GLP-1(9–36)amide and GLP-1(28–36)amide, have been shown to exert beneficial effect in pancreas and extra-pancreatic tissues or cell lineages. Here we summarized our current knowledge on the metabolic, proliferative and protective effects of GLP-1(7–36)amide and its cleavage fragments, mainly focusing on pancreatic β-cells and the involvement of the Wnt signaling pathway effector β-catenin.

Dietary Curcumin Intervention Targets Mouse White Adipose Tissue Inflammation and Brown Adipose Tissue UCP1 Expression: Curcumin Intervention Targets Both WAT and BAT

This study aimed to determine whether dietary curcumin intervention targets both white adipose tissue (WAT) inflammation and brown adipose tissue (BAT)‐mediated energy expenditure.