Yong-Tae Jeong

Inhibitory cross-talk between the AMPK and ERK pathways mediates endoplasmic reticulum stress-induced insulin resistance in skeletal muscle

Endoplasmic reticulum (ER) stress has been implicated in the pathogeneses of insulin resistance and type 2 diabetes, and extracellular signal‐regulated kinase (ERK) antagonist is an insulin sensitizer that can restore muscle insulin responsiveness in both tunicamycin‐treated muscle cells and type 2 diabetic mice. The present study was undertaken to determine whether the chemical or genetic inhibition ER stress pathway targeting by ERK results in metabolic benefits in muscle cells.

Low molecular weight fucoidan improves endoplasmic reticulum stress-reduced insulin sensitivity through AMP-activated protein kinase activation in L6 myotubes and restores lipid homeostasis in a mouse model of type 2 diabetes.

Low molecular weight fucoidan (LMWF) is widely used to treat metabolic disorders, but its physiologic effects have not been well determined. In the present study, we investigated the metabolic effects of LMWF in obese diabetic mice (leptin receptor–deficient db/db mice) and the underlying molecular mechanisms involved in endoplasmic reticulum (ER) stress-responsive L6 myotubes. The effect of LMWF-mediated AMP-activated protein kinase (AMPK) activation on insulin resistance via regulation of the ER stress-dependent pathway was examined in vitro and in vivo. In db/db mice, LMWF markedly reduced serum glucose, triglyceride, cholesterol, and low-density lipoprotein levels, and gradually reduced body weights by reducing lipid parameters. Furthermore, it effectively ameliorated glucose homeostasis by elevating glucose tolerance. In addition, the phosphorylation levels of AMPK and Akt were markedly reduced by ER stressor, and subsequently, glucose uptake and fatty acid oxidation were also reduced. However, these adverse effects of ER stress were significantly ameliorated by LMWF. Finally, in L6 myotubes, LMWF markedly reduced the ER stress-induced upregulation of the mammalian target of rapamycin–p70S61 kinase network and subsequently improved the action of insulin via AMPK stimulation. Our findings suggest that AMPK activation by LMWF could prevent metabolic diseases by controlling the ER stress-dependent pathway and that this beneficial effect of LMWF provides a potential therapeutic strategy for ameliorating ER stress-mediated metabolic dysfunctions.

AMP-activated protein kinase negatively regulates FcεRI-mediated mast cell signaling and anaphylaxis in mice.

BACKGROUND Aggregation of FcεRI activates a cascade of signaling events leading to mast cell activation, followed by inhibitory signals that turn off the activating signals. However, the overall view of negative signals in mast cells is still incomplete. Although AMP-activated protein kinase (AMPK), which is generally known as a regulator of energy metabolism, is also associated with anti-inflammation, little is known about the role of AMPK in mast cells. OBJECTIVES We investigated the role of AMPK and its regulatory mechanism in mast cells. METHOD The roles of AMPK in FcεRI-dependent activation of bone marrow-derived mast cells (BMMCs) were evaluated by using chemical agents, small interfering RNAs (siRNAs), or adenovirus that modulated the activity or expression of AMPK signaling components. In addition, AMPKα2(-/-) mice were used to verify the role of AMPK in anaphylactic models. RESULTS FcεRI signaling and associated effector functions in BMMCs were suppressed by the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) and were conversely augmented by siRNA knockdown of AMPKα2 or liver kinase B1 (LKB1), an upstream kinase of AMPK. Furthermore, AMPKα2 deficiency led to increased FcεRI-mediated BMMC activation and anaphylaxis that were insensitive to AICAR, whereas enforced expression of AMPKα2 in AMPKα2(-/-) BMMCs reversed the hypersensitive FcεRI signaling to normal levels. Pharmacologic inhibition or siRNA knockdown of Fyn mimicked AMPK activation, suggesting that Fyn counterregulates the LKB1-AMPK axis. Mechanistically, Fyn controlled AMPK activity by regulating LKB1 localization. CONCLUSIONS The Fyn-regulated LKB1-AMPK axis acts as a novel inhibitory module for mast cell activation, which points to AMPK activators as therapeutic drugs for allergic diseases.

Emodin Isolated from Polygoni cuspidati Radix Inhibits TNF-α and IL-6 Release by Blockading NF-κB and MAP Kinase Pathways in Mast Cells Stimulated with PMA Plus A23187

Emodin, a naturally occurring anthraquinone derivative isolated from Polygoni cuspidati radix, has several beneficial pharmacologic effects, which include anti-cancer, anti-diabetic, and anti-inflammatory activities. In this study, the authors examined the effect of emodin on the production of proinflammatory cytokines, such as, tumor necrosis factor (TNF)-α and interleukin (IL)-6, in mouse bone marrow-derived mast cells (BMMCs) stimulated with phorbol 12-myristate 13-acetate (PMA) plus the calcium ionophore A23187. To investigate the mechanism responsible for the regulation of pro-inflammatory cytokine production by emodin, the authors assessed its effects on the activations of transcriptional factor nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPKs). Emodin attenuated the nuclear translocation of (NF)-κB p65 and its DNA-binding activity by reducing the phosphorylation and degradation of IκBα and the phosphorylation of IκB kinase B (IKK). Furthermore, emodin dose-dependently attenuated the phosphorylations of MAPKs, such as, extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAP kinase, and the stress-activated protein kinases (SAPK)/c-Jun-N-terminal kinase (JNK). Taken together, the findings of this study suggest that the anti-inflammatory effects of emodin on PMA plus A23187-stimulated BMMCs are mediated via the inhibition of NF-κB activation and of the MAPK pathway.

Tanshinone IIA improves endoplasmic reticulum stress-induced insulin resistance through AMP-activated protein kinase.

The aim of the present study was to determine the effect of Tanshinone IIA (Tan IIA) on endoplasmic reticulum (ER) stress-induced insulin resistance in L6 myotubes and db/db mice. ER stress markers, RNA-activated protein kinase-like ER resident kinase (PERK), JNK, and AMPK activity were determined in tunicamycin-treated L6 myotubes. Insulin resistance was monitored using glucose uptake assays in vitro and blood glucose levels in vivo. Tan IIA clearly suppressed the phosphorylations of PERK and JNK and potentiated insulin-mediated Akt phosphorylation as well as glucose uptake via AMPK activation under ER stress. Furthermore, these effects are completely abrogated by siRNA-mediated knockdown of AMPK or LKB1. In addition, Tan IIA reduced blood glucose levels and body weights in db/db mice without altering food intake. These findings suggest that Tan IIA enhances insulin sensitivity and improves glucose metabolic disorders by increasing AMPK activity and attenuating ER stress-induced insulin resistance.

Glyceollin improves endoplasmic reticulum stress-induced insulin resistance through CaMKK-AMPK pathway in L6 myotubes.

Glyceollin has been shown to have antidiabetic properties, although its molecular mechanism is not known. Here, we have investigated the metabolic effects of glyceollin in animal models of insulin resistance and in endoplasmic reticulum (ER) stress-responsive muscle cells. db/db mice were treated with glyceollin for 4weeks and triglycerides, total cholesterol, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) levels were measured. Glyceollin reduced serum insulin and triglycerides and increased HDL levels in db/db mice. Furthermore, glyceollin caused a significant improvement in glucose homeostasis without altering body weight and food intake in db/db mice. In muscle cells, glyceollin increased the activity of AMP-activated protein kinase (AMPK) as well as cellular glucose uptake. Fatty acid oxidation was also increased. In parallel, phosphorylation of acetyl-CoA carboxylase (ACC) at Ser-79 was increased, consistent with decreased ACC activity. An insulin-resistant state was induced by exposing cells to 5μg/ml of tunicamycin as indicated by decreased insulin-mediated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and glucose uptake. Inhibition of insulin-mediated tyrosine phosphorylation of IRS-1 and glucose uptake under ER stress was prevented by glyceollin. Strikingly, glyceollin reduced ER stress-induced, c-Jun NH2-terminal kinase activation and subsequently increased insulin signaling via stimulation of AMPK activity in L6 myotubes. Pharmacologic inhibition or knockdown of Ca(2+)/calmodulin-dependent protein kinase kinase blocked glyceollin-increased AMPK phosphorylation and insulin sensitivity under ER stress conditions. Taken together, these results indicate that glyceollin-mediated enhancement of insulin sensitivity under ER stress conditions is predominantly accomplished by activating AMPK, thereby having beneficial effects on hyperglycemia and insulin resistance.

Anti-inflammatory activity of hexane extracts from bones and internal organs of Anguilla japonica suppresses cyclooxygenase-2-dependent prostaglandin D2 generation in mast cells and anaphylaxis in mice

The purpose of this study is to investigate the effects of n-hexane extracts from bones and internal organs of Japanese eel, Anguilla japonica (HEE), on cyclooxygenase-2 (COX-2)-dependent prostaglandin D₂(PGD₂) generation in stem cell factor (SCF), IL-10, plus LPS-induced mouse bone marrow-derived mast cells (BMMCs) and on passive cutaneous anaphylaxis (PCA) in mice. HEE suppressed SCF/IL-10/LPS-induced PGD₂ generation, and concomitantly reduced COX-2 protein expression dose-dependently. To understand the mechanistic basis for the inhibition of PGD₂ generation by HEE, we examined the effects of HEE on upstream signaling pathways essential for COX-2 induction. HEE was found to inhibit the translocation of nuclear factor-κB (NF-κB) p65 subunit to the nucleus and its DNA-binding ability through the inhibition of TAK1, IKK and IκB phosphorylation. Furthermore, HEE also attenuated mitogen-activated protein kinase (MAPK)-mediated regulation of DNA binding of activator protein-1 (AP-1). Moreover, oral administration of HEE inhibited anti-dinitrophenyl (DNP) IgE-induced PCA in a dose dependent manner. Taken together, the present study provides new insights into the anti-inflammatory activity of HEE, which could be a promising candidate to be used for an inflammatory therapy.

Pinusolide improves high glucose-induced insulin resistance via activation of AMP-activated protein kinase.

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) plays a crucial role in the maintenance of cellular energy homeostasis, and several natural compounds that activate AMPK possibly enhance glucose uptake by muscle cells. In this study, we found that pinusolide stimulated AMPK phosphorylation and glucose uptake and these effects were significantly reduced by siRNA LKB1 or compound C, suggesting that enhanced glucose uptake by pinusolide is predominantly accomplished via an LKB1-mediated AMPK activation pathway. An insulin resistance state was induced by exposing cells to 30mM glucose, as indicated by reduced insulin-stimulated tyrosine phosphorylation of IRS-1 and glucose uptake. Under these conditions, the phosphorylation of AMPK and ACC were decreased. Surprisingly, disrupted insulin signaling and decreased AMPK activity by high glucose concentrations were prevented by pinusolide. Moreover, this treatment increased insulin-stimulated glucose uptake via AMPK activation. Taken together, our findings suggest a link between high glucose and insulin resistance in muscle cells, and provide further evidence that pinusolide attenuates blockade of insulin signaling by enhancing IRS-1 tyrosine phosphorylation by the activating the AMPK pathway. In addition, this study indicates the targeting of AMPK represents a new therapeutic strategy for hyperglycemia-induced insulin resistance and type 2 diabetes.

Ethylacetate extracts of the muscles of Anguilla japonica suppress glucose levels in db/db mice via activation of AMP-activated protein kinase

Hypoglycemic effects of ethylacetate extracts of Anguilla japonica (EMA) muscles in db/db mice were investigated. To understand the mechanism responsible for the hypoglycemic effects of EMA, the effects of EMA on AMP-activated protein kinase (AMPK) activation in L6 myotubes and in vivo using type II diabetic db/db mice were analyzed. In L6 myotubes, the phosphorylation degrees of AMPK and acetyl-CoA carboxylase (ACC) were markedly increased and glucose uptake was significantly (p<0.001) increased by EMA, compared with untretaed L6 myotubes. However, in L6 myotubes, these effects were abolished by compound C, an AMPK inhibitor. Moreover, EMA significantly reduced non-fasting blood glucose and serum insulin levels, and strongly induced AMPK phosphorylation in skeletal muscle tissues of db/db mice. EMA regulates glucose levels in L6 myotubes and in diabetic mice by activation of AMPK. Beneficial effects for diabetes treatment are indicated.

Protective effect of butanol extracts of skin of Anguilla japonica against endoplasmic reticulum stress-induced insulin resistance via the AMPK pathway in L6 myotubes

The effect of butanol extracts of the skin of Anguilla japonica (BESA) on endoplasmic reticulum (ER) stress-induced insulin resistance in L6 myotubes was investigated. Western blotting revealed that BESA increased phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase, and stimulated glucose uptake in L6 myotubes. Stimulation of AMPK and glucose uptake by BESA were significantly (p<0.05) reduced by siRNA LKB1 or siRNA AMPK, compared with controls, suggesting that enhanced glucose uptake by BESA was predominantly accomplished via an LKB1-mediated AMPK activation pathway. In addition, BESA effectively reduced phosphorylation of ER stress markers, RNA-activated protein kinase-like ER resident kinase, JNK, and significantly (p<0.01) increased glucose uptake via AMPK activation in tunicamycin-treated L6 myotubes, compared with controls. Improvement of insulin sensitivity under ER stress conditions by BESA is predominantly accomplished via an LKB1-AMPK-dependent pathway.