Insulin down-regulates the expression of ubiquitin E3 ligases partially by inhibiting the activity and expression of AMP-activated protein kinase in L6 myotubes

Insulin and Metformin Control Cell Proliferation by Regulating TDG-Mediated DNA Demethylation in Liver and Breast Cancer Cells

Type 2 diabetes mellitus (T2DM) is a frequent comorbidity of cancer. Hyperinsulinemia secondary to T2DM promotes cancer progression, whereas antidiabetic agents, such as metformin, have anticancer effects. However, the detailed mechanism for insulin and metformin-regulated cancer cell proliferation remains unclear. This study identified a mechanism by which insulin upregulated the expression of c-Myc, sterol regulatory element-binding protein 1 (SREBP1), and acetyl-coenzyme A (CoA) carboxylase 1 (ACC1), which are important regulators of lipogenesis and cell proliferation. Thymine DNA glycosylase (TDG), a DNA demethylase, was transactivated by c-Myc upon insulin treatment, thereby decreasing 5-carboxylcytosine (5caC) abundance in the SREBP1 promoter. On the other hand, metformin-activated AMP-activated protein kinase (AMPK) increased DNA methyltransferase 3A (DNMT3A) activity to increase 5-methylcytosine (5mC) abundance in the TDG promoter. This resulted in decreased TDG expression and enhanced 5caC abundance in the SREBP1 promoter. These findings demonstrate that c-Myc activates, whereas AMPK inhibits, TDG-mediated DNA demethylation of the SREBP1 promoter in insulin-promoted and metformin-suppressed cancer progression, respectively. This study indicates that TDG is an epigenetic-based therapeutic target for cancers associated with T2DM.

The protective role of metformin in autophagic status in peripheral blood mononuclear cells of type 2 diabetic patients

Autophagy plays an important role in the pathophysiology of type 2 diabetes (T2D). Metformin is the most common antidiabetic drug. The main objective of this study was to explore the molecular mechanism of metformin in starvation-induced autophagy in peripheral blood mononuclear cells (PBMCs) of type 2 diabetic patients. PBMCs were isolated from 10 diabetic patients and 7 non-diabetic healthy volunteers. The autophagic puncta and markers were measured with the help of monodansylcadaverine staining and western blot. Additionally, transmission electron microscopy was also performed. No significant changes were observed in the initial autophagy marker protein levels in PBMCs of T2D after metformin treatment though diabetic PBMCs showed a high level of phospho-mammalian target of rapamycin, p62 and reduced expression of phospho-AMP-activated protein kinase and lysosomal membrane-associated protein 2, indicating a defect in autophagy. Also, induction of autophagy by tunicamycin resulted in apoptosis in diabetic PBMCs as observed by caspase-3 cleavage and reduced expression of Bcl2. Inhibition of autophagy by bafilomycin rendered consistent expression of p62 indicating a defect in the final process of autophagy. Further, electron microscopic studies also confirmed massive vacuole overload and a sign of apoptotic cell death in PBMCs of diabetic patients, whereas metformin treatment reduced the number of autophagic vacuoles perhaps by lysosomal fusion. Thus, our results indicate that defective autophagy in T2D is associated with the fusion process of lysosomes which could be overcome by metformin.

A novel stable isotope tracer method to simultaneously quantify skeletal muscle protein synthesis and breakdown

Background/aims

Methodological challenges have been associated with the dynamic measurement of muscle protein breakdown (MPB), as have the measurement of both muscle protein synthesis (MPS) and MPB within the same experiment. Our aim was to use the transmethylation properties of methionine as proof-of-concept to measure rates of MPB via its methylation of histidine within skeletal muscle myofibrillar proteins, whilst simultaneously utilising methionine incorporation into bound protein to measure MPS.

Results

During the synthesis measurement period, incorporation of methyl[D₃]-¹³C-methionine into cellular protein in C2C12 myotubes was observed (representative of MPS), alongside an increase in the appearance of methyl[D₃]-methylhistidine into the media following methylation of histidine (representative of MPB). For further validation of this approach, fractional synthetic rates (FSR) of muscle protein were increased following treatment of the cells with the anabolic factors insulin-like growth factor-1 (IGF-1) and insulin, while dexamethasone expectedly reduced MPS. Conversely, rates of MPB were reduced with IGF-1 and insulin treatments, whereas dexamethasone accelerated MPB.

Conclusions

This is a novel stable isotope tracer approach that permits the dual assessment of muscle cellular protein synthesis and breakdown rates, through the provision of a single methionine amino acid tracer that could be utilised in a wide range of biological settings.

Palmitate-induced IL6 expression ameliorated by chicoric acid through AMPK and SIRT1-mediated pathway in the PBMCs of newly diagnosed type 2 diabetes patients and healthy subjects

Inhibition of inflammation is one of the possible therapeutic approaches for Insulin resistance (IR) during type 2 diabetes mellitus (T2DM). In the current study we investigated the effects of palmitate and chicoric acid (CA) on inflammation in peripheral blood mononuclear cells (PBMCs) of newly diagnosed T2DM patients and healthy subjects and explored the mechanism by which palmitate and CA influence inflammation. 20 newly diagnosed T2DM patients and 20 healthy subjects were recruited in our study. Blood sample were collected and PBMCs were isolated. Interleukin 6 (IL6), silent information regulator type 1 (SIRT1), AMP-activated protein kinase (AMPK) and phospho-AMPK (pAMPK) were evaluated both in vivo and in vitro. PBMCs were treated with palmitate and CA to investigate their effects on inflammation. IL6 and SIRT1 genes expression were evaluated by real-time PCR. The levels of IL6 in culture medium were measured by ELISA. Proteins levels of AMPK and pAMPK in PBMCs were detected by western blotting. IL6 expression was higher and SIRT1 expression and pAMPK levels were lower in PBMCs of diabetic patients and obese subjects compared to healthy subjects and non-obese subjects, respectively. CA significantly prevented against increased IL6 levels as well as its gene expression in PBMCs induced by palmitate. Also, CA returned reduction in SIRT1 expression and pAMPK levels mediated via palmitate to near control level. These findings reveal that CA reduces inflammation in PBMCs probably through upregulation of SIRT1 and pAMPK. Therefore, CA would be suggested as a novel agent for the treatment of T2DM.

Update on the synergistic effect of HSL and insulin in the treatment of metabolic disorders

Hormone-sensitive lipase (HSL) is one of the three lipases in adipose tissue present during periods of energy demand. HSL is tightly controlled by insulin regulation via the central and peripheral systems. The suppressive effects of insulin on HSL are also associated with complex crosstalk with other pathways in the metabolic network. Because impaired insulin action is the driving force behind the pathogenesis of diabetes and other metabolic complications, elucidation of the intricate relationships between HSL and insulin may provide an in-depth understanding of these pandemic diseases and potentially identify strategies to inhibit disease development. Insulin not only differentially regulates HSL isoform transcription but also post-transcriptionally affects HSL phosphorylation by stimulating PKA and endothelin (ET-1), and controls its expression indirectly via regulating the activity of growth hormone (GH). In addition, a rapid elevation of HSL levels was detected after insulin injection in patients, which suggests that the inhibitory effects of insulin on HSL can be overridden by insulin-induced hypoglycemia. Conversely, individuals with hereditary HSL deficiency, and animals with experimental HSL deletion, showed major disruptions in mRNA/protein expression in insulin signaling pathways, ultimately leading to insulin resistance, diabetes, and fatty liver. Notably, HSL inactivation could cause insulin-independent fatty liver, while insulin resistance induced by HSL deficiency may further aggravate disease progression. The common beliefs that HSL is the overall rate-limiting enzyme in lipolysis and that insulin is an inhibitor of HSL have been challenged by recent discoveries; therefore, a renewed examination of their relationships is required. In this review, by analyzing current data related to the role of, and mutual regulation between, HSL and insulin and discussing unanswered questions and disparities in different lines of studies, the authors intend to shed light on our understanding of lipid metabolism and provide a rational basis for future research in drug development.

High glucose alters tendon homeostasis through downregulation of the AMPK/Egr1 pathway

Diabetes mellitus (DM) is associated with higher risk of tendinopathy, which reduces tolerance to exercise and functional activities and affects lifestyle and glycemic control. Expression of tendon-related genes and matrix metabolism in tenocytes are essential for maintaining physiological functions of tendon. However, the molecular mechanisms involved in diabetic tendinopathy remain unclear. We hypothesized that high glucose (HG) alters the characteristics of tenocyte. Using in vitro 2-week culture of tenocytes, we found that expression of tendon-related genes, including Egr1, Mkx, TGF-β1, Col1a2, and Bgn, was significantly decreased in HG culture and that higher glucose consumption occurred. Down-regulation of Egr1 by siRNA decreased Scx, Mkx, TGF-β1, Col1a1, Col1a2, and Bgn expression. Blocking AMPK activation with Compound C reduced the expression of Egr1, Scx, TGF-β1, Col1a1, Col1a2, and Bgn in the low glucose condition. In addition, histological examination of tendons from diabetic mice displayed larger interfibrillar space and uneven glycoprotein deposition. Thus, we concluded that high glucose alters tendon homeostasis through downregulation of the AMPK/Egr1 pathway and the expression of downstream tendon-related genes in tenocytes. The findings render a molecular basis of the mechanism of diabetic tendinopathy and may help develop preventive and therapeutic strategies for the pathology.

Resveratrol regulates neuronal glucose uptake and insulin sensitivity via P21-activated kinase 2 (PAK2)

We have recently reported P21-activated kinase 2 (PAK2), a serine/threonine kinase as a negative regulator of neuronal glucose uptake and insulin sensitivity. Resveratrol (RSV), a natural polyphenol with anti-oxidative, anti-inflammatory and anti-diabetic properties, regulates PAK2 activity in HepG2 and ESC-B5 cell apoptosis. However, regulation of PAK2 by RSV in neuronal insulin signaling pathway, if any, is still unknown. In the present study, RSV treatment significantly increased PAK2 activity under insulin-sensitive and insulin-resistant condition, along with a marked decrease in glucose uptake in differentiated N2A cells. Pretreatment with AMPK inhibitor, followed by RSV treatment resulted in reduction in PAK2 activity whereas glucose uptake showed an increase. However, pretreatment with Akt inhibitor and then RSV exposure significantly increased PAK2 activity, with a corresponding decrease in glucose uptake. RSV treatment increased AMPK activity and decreased Akt activity. In conclusion, RSV negatively regulates neuronal glucose uptake and insulin sensitivity via PAK2.