Meilian Liu

Transcriptional and post-translational regulation of adiponectin

Adiponectin is an adipose-tissue-derived hormone with anti-diabetic, anti-atherogenic and anti-inflammatory functions. Adiponectin circulates in the bloodstream in trimeric, hexameric and high-molecular-mass species, and different forms of adiponectin have been found to play distinct roles in the regulation of energy homoeostasis. The serum levels of adiponectin are negatively correlated with obesity and insulin resistance, yet the underlying mechanisms remain elusive. In the present review, we summarize recent progress made on the mechanisms regulating adiponectin gene transcription, multimerization and secretion. We also discuss the potential relevance of these studies to the development of new clinical therapy for insulin resistance, Type 2 diabetes and other obesity-related metabolic disorders.

Adiponectin sensitizes insulin signaling by reducing p70 S6 kinase-mediated serine phosphorylation of IRS-1

Adiponectin functions as an insulin sensitizer, and yet the underlying molecular mechanism(s) remains largely unknown. We found that treating C2C12 myotubes with adiponectin or rapamycin enhanced the ability of insulin to stimulate IRS-1 tyrosine phosphorylation and Akt phosphorylation, concurrently with reduced p70 S6 kinase phosphorylation at Thr389 as well as IRS-1 phosphorylation at Ser302 and Ser636/639. Overexpression of dominant-negative AMP kinase (AMPK), but not dominant-negative p38 MAPK, reduced the insulin-sensitizing effect of adiponectin. Rapamycin, but not adiponectin, enhanced insulin-stimulated Akt phosphorylation in HeLa cells, which lack LKB1, and exogenous expression of LKB1 in HeLa cells rescued the insulin-sensitizing effect of adiponectin. Finally, overexpression of wild-type Rheb (Ras homology-enriched in brain) or the TSC2 mutant lacking the AMPK phosphorylation site (TSC2S1345A) inhibited the insulin-sensitizing effect of adiponectin in C2C12 cells. These results indicate that activation of the LKB1/AMPK/TSC1/2 pathway alleviates the p70 S6 kinase-mediated negative regulation of insulin signaling, providing a mechanism by which adiponectin increases insulin sensitivity in cells.

A disulfide-bond A oxidoreductase-like protein (DsbA-L) regulates adiponectin multimerization

Impairments in adiponectin multimerization lead to defects in adiponectin secretion and function and are associated with diabetes, yet the underlying mechanisms remain largely unknown. We have identified an adiponectin-interacting protein, previously named GST-kappa, by yeast 2-hybrid screening. The adiponectin-interacting protein contains 2 thioredoxin domains and has very little sequence similarity to other GST isoforms. However, this protein shares high sequence and secondary structure homology to bacterial disulfide-bond A oxidoreductase (DsbA) and is thus renamed DsbA-like protein (DsbA-L). DsbA-L is highly expressed in adipose tissue, and its expression level is negatively correlated with obesity in mice and humans. DsbA-L expression in 3T3-L1 adipocytes is stimulated by the insulin sensitizer rosiglitazone and inhibited by the inflammatory cytokine TNFα. Overexpression of DsbA-L promoted adiponectin multimerization while suppressing DsbA-L expression by RNAi markedly and selectively reduced adiponectin levels and secretion in 3T3-L1 adipocytes. Our results identify DsbA-L as a key regulator for adiponectin biosynthesis and uncover a potential new target for developing therapeutic drugs for the treatment of insulin resistance and its associated metabolic disorders.

Resveratrol inhibits mTOR signaling by promoting the interaction between mTOR and DEPTOR

Resveratrol (RSV) is a naturally occurring polyphenol that has been found to exert antioxidant, anti-inflammatory, and neuroprotective properties. However, how RSV exerts its beneficial health effects remains largely unknown. Here, we show that RSV inhibits insulin- and leucine-stimulated mTOR signaling in C2C12 fibroblasts via a Sirt1-independent mechanism. Treating C2C12 cells with RSV dramatically inhibited insulin-stimulated Akt, S6 kinase, and 4E-BP1 phosphorylation but had little effect on tyrosine phosphorylation of the insulin receptor and activation of the p44/42 MAPK signaling pathway. RSV treatment also partially blocked mTOR and S6 kinase phosphorylation in TSC1/2-deficient mouse embryonic fibroblasts, suggesting the presence of an inhibitory site downstream of TSC1/2. Knocking out PDK1 or suppressing AMP-activated protein kinase had little effect on leucine-stimulated mTOR signaling. On the other hand, RSV significantly increased the association between mTOR and its inhibitor, DEPTOR. Furthermore, the inhibitory effect of RSV on leucine-stimulated mTOR signaling was greatly reduced in cells in which the expression levels of DEPTOR were suppressed by RNAi. Taken together, our studies reveal that RSV inhibits leucine-stimulated mTORC1 activation by promoting mTOR/DEPTOR interaction and thus uncover a novel mechanism by which RSV negatively regulates mTOR activity.

Yin-Yang regulation of adiponectin signaling by APPL isoforms in muscle cells

APPL1 is a newly identified adiponectin receptor-binding protein that positively mediates adiponectin signaling in cells. Here we report that APPL2, an isoform of APPL1 that forms a dimer with APPL1, can interacts with both AdipoR1 and AdipoR2 and acts as a negative regulator of adiponectin signaling in muscle cells. Overexpression of APPL2 inhibits the interaction between APPL1 and AdipoR1, leading to down-regulation of adiponectin signaling in C2C12 myotubes. In contrast, suppressing APPL2 expression by RNAi significantly enhances adiponectin-stimulated glucose uptake and fatty acid oxidation. In addition to targeting directly to and competing with APPL1 in binding with the adiponectin receptors, APPL2 also suppresses adiponectin and insulin signaling by sequestrating APPL1 from these two pathways. In addition to adiponectin, metformin also induces APPL1-APPL2 dissociation. Taken together, our results reveal that APPL isoforms function as an integrated Yin-Yang regulator of adiponectin signaling and mediate the cross-talk between adiponectin and insulin signaling pathways in muscle cells.

DsbA-L Alleviates Endoplasmic Reticulum Stress–Induced Adiponectin Downregulation

OBJECTIVE Obesity impairs adiponectin expression, assembly, and secretion, yet the underlying mechanisms remain elusive. The aims of this study were 1) to determine the molecular mechanisms by which obesity impairs adiponectin multimerization and stability, and 2) to determine the potential role of disulfide-bond-A oxidoreductase-like protein (DsbA-L), a recently identified adiponectin interactive protein that promotes adiponectin multimerization and stability in obesity-induced endoplasmic reticulum (ER) stress and adiponectin downregulation. RESEARCH DESIGN AND METHODS Tauroursodeoxycholic acid (TUDCA), a chemical chaperone that alleviates ER stress, was used to study the mechanism underlying obesity-induced adiponectin downregulation in db/db mice, high-fat diet-induced obese mice, and in ER-stressed 3T3-L1 adipocytes. The cellular levels of DsbA-L were altered by RNAi-mediated suppression or adenovirus-mediated overexpression. The protective role of DsbA-L in obesity- and ER stress–induced adiponectin downregulation was characterized. RESULTS Treating db/db mice and diet-induced obese mice with TUDCA increased the cellular and serum levels of adiponectin. In addition, inducing ER stress is sufficient to downregulate adiponectin levels in 3T3-L1 adipocytes, which could be protected by treating cells with the autophagy inhibitor 3-methyladenine or by overexpression of DsbA-L. CONCLUSIONS ER stress plays a key role in obesity-induced adiponectin downregulation. In addition, DsbA-L facilitates adiponectin folding and assembly and provides a protective effect against ER stress–mediated adiponectin downregulation in obesity.

Adiponectin is critical in determining susceptibility to depressive behaviors and has antidepressant-like activity

Depression is a debilitating mental illness and is often comorbid with metabolic disorders such as type 2 diabetes. Adiponectin is an adipocyte–derived hormone with antidiabetic and insulin-sensitizing properties. Here we show that adiponectin levels in plasma are reduced in a chronic social-defeat stress model of depression, which correlates with decreased social interaction time. A reduction in adiponectin levels caused by haploinsufficiency results in increased susceptibility to social aversion, “anhedonia,” and learned helplessness and causes impaired glucocorticoid-mediated negative feedback on the hypothalamic–pituitary–adrenal (HPA) axis. Intracerebroventricular (i.c.v.) injection of an adiponectin neutralizing antibody precipitates stress-induced depressive-like behavior. Conversely, i.c.v. administration of exogenous adiponectin produces antidepressant-like behavioral effects in normal-weight mice and in diet-induced obese diabetic mice. Taken together, these results suggest a critical role of adiponectin in depressive-like behaviors and point to a potential innovative therapeutic approach for depressive disorders.

Autophagy-mediated insulin receptor down-regulation contributes to endoplasmic reticulum stress-induced insulin resistance.

Endoplasmic reticulum (ER) stress is associated with obesity-induced insulin resistance, yet the underlying mechanisms remain to be fully elucidated. Here we show that ER stress-induced insulin receptor (IR) down-regulation may play a critical role in obesity-induced insulin resistance. The expression levels of IR are negatively associated with the ER stress marker C/EBP homologous protein (CHOP) in insulin target tissues of db/db mice and mice fed a high-fat diet. Significant IR down-regulation was also observed in fat tissue of obese human subjects and in 3T3-L1 adipocytes treated with ER stress inducers. ER stress had little effect on IR tyrosine phosphorylation per se but greatly reduced IR downstream signaling. The ER stress-induced reduction in IR cellular levels was greatly alleviated by the autophagy inhibitor 3-methyladenine but not by the proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132). Inhibition of autophagy prevented IR degradation but did not rescue IR downstream signaling, consistent with an adaptive role of autophagy in response to ER stress-induced insulin resistance. Finally, chemical chaperone treatment protects cells from ER stress-induced IR degradation in vitro and obesity-induced down-regulation of IR and insulin action in vivo. Our results uncover a new mechanism underlying obesity-induced insulin resistance and shed light on potential targets for the prevention and treatment of obesity-induced insulin resistance and type 2 diabetes.

Up-regulation of Adiponectin by Resveratrol THE ESSENTIAL ROLES OF THE Akt/FOXO1 AND AMP-ACTIVATED PROTEIN KINASE SIGNALING PATHWAYS AND DsbA-L

The natural polyphenol resveratrol (RSV) displays a wide spectrum of health beneficial activities, yet the precise mechanisms remain to be fully elucidated. Here we show that RSV promotes the multimerization and cellular levels of adiponectin in 3T3-L1 adipocytes. The stimulatory effect of RSV was not affected by knocking out Sirt1, but was diminished by suppressing the expression levels of DsbA-L, a recently identified adiponectin-interactive protein that promotes adiponectin multimerization. Suppression of the Akt signaling pathway resulted in an increase in the expression levels of DsbA-L and adiponectin. On the other hand, knocking out FOXO1 or suppressing the activity or expression levels of the AMP-activated protein kinase (AMPK) down-regulated DsbA-L and adiponectin. The stimulatory effect of RSV on adiponectin and DsbA-L expression was completely diminished in FOXO1-suppressed and AMPK-inactivated 3T3-L1 adipocytes. Taken together, our results demonstrate that RSV promotes adiponectin multimerization in 3T3-L1 adipocytes via a Sirt1-independent mechanism. In addition, we show that the stimulatory effect of RSV is regulated by both the Akt/FOXO1 and the AMPK signaling pathways. Last, we show that DsbA-L plays a critical role in the promoting effect of RSV on adiponectin multimerization and cellular levels.

Adipose tissue in control of metabolism

Adipose tissue plays a central role in regulating whole-body energy and glucose homeostasis through its subtle functions at both organ and systemic levels. On one hand, adipose tissue stores energy in the form of lipid and controls the lipid mobilization and distribution in the body. On the other hand, adipose tissue acts as an endocrine organ and produces numerous bioactive factors such as adipokines that communicate with other organs and modulate a range of metabolic pathways. Moreover, brown and beige adipose tissue burn lipid by dissipating energy in the form of heat to maintain euthermia, and have been considered as a new way to counteract obesity. Therefore, adipose tissue dysfunction plays a prominent role in the development of obesity and its related disorders such as insulin resistance, cardiovascular disease, diabetes, depression and cancer. In this review, we will summarize the recent findings of adipose tissue in the control of metabolism, focusing on its endocrine and thermogenic function.