Fructose induces mitochondrial dysfunction and triggers apoptosis in skeletal muscle cells by provoking oxidative stress.

Mitochondrial dysfunction in skeletal muscle has been implicated in the development of insulin resistance, a major characteristic of type 2 diabetes. There is evidence that oxidative stress results from the increased production of reactive oxygen species and reactive nitrogen species leads to mitochondrial dysfunction, tissue damage, insulin resistance, and other complications observed in type 2 diabetes. It has been suggested that intake of high fructose contributes to insulin resistance and other metabolic disturbances. However, there is limited information about the direct effect of fructose on the mitochondrial function of skeletal muscle, the major metabolic determinant of whole body insulin activity. Here, we assessed the effect of fructose exposure on mitochondria-mediated mechanisms in skeletal muscle cells. Exposure of L6 myotubes to high fructose stimulated the production of mitochondrial reactive oxygen species and nitric oxide (NO), and the expression of inducible NO synthase. Fructose-induced oxidative stress was associated with increased translocation of nuclear factor erythroid 2-related factor-2 to the nucleus, decreases in mitochondrial DNA content and mitochondrial dysfunctions, as evidenced by decreased activities of citrate synthase and mitochondrial dehydrogenases, loss of mitochondrial membrane potential, decreased activity of the mitochondrial respiratory complexes, and impaired mitochondrial energy metabolism. Furthermore, positive Annexin-propidium iodide staining and altered expression of Bcl-2 family members and caspases in L6 myotubes indicated that the cells progressively became apoptotic upon fructose exposure. Taken together, these findings suggest that exposure of skeletal muscle cells to fructose induced oxidative stress that decreased mitochondrial DNA content and triggered mitochondrial dysfunction, which caused apoptosis.

Design and synthesis of lupeol analogues and their glucose uptake stimulatory effect in L6 skeletal muscle cells.

Structure modifications of lupeol at the isopropylene moiety have been described via allylic oxidation using selenium dioxide. The antidiabetic efficacy of lupeol analogues were evaluated in vitro as glucose uptake stimulatory effect in L6 skeletal muscle cells. From all tested compounds, 2, 3, 4b and 6b showed significant stimulation of glucose uptake with respective percent stimulation of 173.1 (p <0.001), 114.1 (p <0.001), 98.3 (p <0.001) and 107.3 (p <0.001) at 10µM concentration. Stimulation of glucose uptake by these compounds is associated with enhanced translocation of glucose transporter 4 (GLUT4) and activation of IRS-1/PI3-K/AKT-dependent signaling pathway in L6 cells. Structure-activity relationship analysis of these analogues demonstrated that the integrity of α,ß-unsaturated carbonyl and acetyl moieties were important in the retention of glucose uptake stimulatory effect. It is therefore proposed that naturally occurring lupeol and their analogues might reduce blood glucose, at least in part, through stimulating glucose utilization by skeletal muscles.

Nod1-mediated lipolysis promotes diacylglycerol accumulation and successive inflammation via PKCδ-IRAK axis in adipocytes.

Chronic inflammation contributes to obesity mediated metabolic disturbances, including insulin resistance. Obesity is associated with altered microbial load in metabolic tissues that can contribute to metabolic inflammation. Different bacterial components such as, LPS, peptidoglycans have been shown to underpin metabolic disturbances through interaction with host innate immune receptors. Activation of Nucleotide-binding oligomerization domain-containing protein 1 (Nod1) with specific peptidoglycan moieties promotes insulin resistance, inflammation and lipolysis in adipocytes. However, it was not clear how Nod1-mediated lipolysis and inflammation is linked. Here, we tested if Nod1-mediated lipolysis caused accumulation of lipid intermediates and promoted cell autonomous inflammation in adipocytes. We showed that Nod1-mediated lipolysis caused accumulation of diacylglycerol (DAG) and activation of PKCδ in 3T3-L1 adipocytes, which was prevented with a Nod1 inhibitor. Nod1-activated PKCδ caused downstream stimulation of IRAK1/4 and was associated with increased expression of proinflammatory cytokines such as, IL-1ß, IL-18, IL-6, TNFα and MCP-1. Pharmacological inhibition or siRNA mediated knockdown of IRAK1/4 attenuated Nod1-mediated activation of NF-κB, JNK, and the expression of proinflammatory cytokines. These results reveal that Nod1-mediated lipolysis promoted accumulation of DAG, which engaged PKCδ and IRAK1/4 to augment inflammation in 3T3-L1 adipocytes.

Isoalantolactone derivative promotes glucose utilization in skeletal muscle cells and increases energy expenditure in db/db mice via activating AMPK-dependent signaling.

Augmenting glucose utilization and energy expenditure in skeletal muscle via AMP-activated protein kinase (AMPK) is an imperative mechanism for the management of type 2 diabetes. Chemical derivatives (2a-2h, 3, 4a-4d, 5) of the isoalantolactone (K007), a bioactive molecule from roots of Inula racemosa were synthesized to optimize the bioactivity profile to stimulate glucose utilization in skeletal muscle cells. Interestingly, 4a augmented glucose uptake, driven by enhanced translocation of glucose transporter 4 (GLUT4) to cell periphery in L6 rat skeletal muscle cells. The effect of 4a was independent to phosphatidylinositide-3-kinase (PI-3-K)/Akt pathway, but mediated through Liver kinase B1 (LKB1)/AMPK-dependent signaling, leading to activation of downstream targets acetyl coenzyme A carboxylase (ACC) and sterol regulatory element binding protein 1c (SREBP-1c). In db/db mice, 4a administration decreased blood glucose level and improved body mass index, lipid parameters and glucose tolerance associated with elevation of GLUT4 expression in skeletal muscle. Moreover, 4a increased energy expenditure via activating substrate utilization and upregulated the expression of thermogenic transcription factors and mitochondrial proteins in skeletal muscle, suggesting the regulation of energy balance. These findings suggest the potential implication of isoalantolactone derivatives for the management of diabetes.

Glucose uptake stimulatory potential and antidiabetic activity of the Arnebin-1 from Arnabia nobelis.

The enhanced disposal of glucose by the peripheral tissue is an important mechanism to regulate hyperglycemia. Here, we investigated the effect of Arnebin-1 from Arnebia nobilis, on glucose disposal in skeletal muscle cells and explored its in vivo antihyperglycemic potential. In L6 myotubes, Arnebin-1 stimulated glucose uptake, mediated through the enhanced translocation of the glucose transporter-4 (GLUT4) to plasma membrane, without changing the amount of GLUT4 or GLUT1. These effects of Arnebin-1 were synergistic with that of insulin. The effect of Arnebin-1 on glucose uptake was abolished in presence of wortmannin, and Arnebin-1 significantly stimulated the phosphorylation of Akt and downstream marker GSK-3ß. Moreover, treatment with Arnebin-1 lowered postprandial blood glucose levels in streptozotocin-induced diabetic rats, and improved glucose tolerance and suppressed the rises in the fasting blood glucose, serum insulin, triglycerides, and total cholesterol in db/db mice, associated with enhanced expression of the major marker of the PI-3-Kinase-mediated signaling cascade in skeletal muscle. These findings suggest that Arnebin-1 exert antihyperglycemic activity through stimulating glucose disposal in peripheral tissues via PI-3-Kinase-dependent pathway.

Glycolipids: isolated from Oplismenus burmannii induce glucose uptake in L6-GLUT4myc myotube cells.

Bioactivity guided separation of combined n-hexane and chloroform extracts of Oplismenus burmannii resulted in the isolation and characterization of five new glycoglycerolipids, (2S)-1,2,6'-tri- O-hexadecanoyl-3-O-ß-D-galactopyranosyl glycerol (1a), (2S)-1,2,6'-tri-O-[(9Z,12Z)-octadeca-9,12- dienoyl]-3-O-ß-D-galactopyranosyl glycerol (1b), (2S)-1,6'-di-O-[(9Z,12Z)-octadeca-9,12-dienoyl]-3- O-ß-D-galactopyranosyl glycerol (2b), (2S)-1,6'-di-O-[(9Z,12Z,15Z)-octadeca-9,12,15-trienoyl]-3-O-ß-D-galactopyranosyl glycerol (2c), and (2S)-1,2-di-O-[(9Z,12Z)-octadeca-9,12-dienoyl]-3-O-(6- sulpho-α-D)-quinovopyranosyl glycerol (3b) along with five known glycoglycerolipids (1c, 2a, 3a, 3c and 4), a cerebroside (5), three monoacylglycerols (6a-c) and α-linoleic acid (7). The isolated compounds, 1-5 were in-vitro tested for their antihyperglycemic potential in terms of increase in 2-deoxyglucose uptake in L6-GLUT4myc myotube cells. The results showed that compounds, 1-5 were showing 1.52 (P<0.05), 1.50 (P<0.05), 1.28, 1.49 (P<0.05) and 1.50 (P<0.05) fold increase in the glucose uptake at concentration of 10 µg/mL and 1.71 (P<0.001), 1.74 (P<0.001), 1.50 (P<0.05), 1.76 (P<0.001) and 1.74 (P<0.001) fold increase in the glucose uptake at concentration of 25 µg/mL respectively. However, standard drug Rosiglitazone increases the glucose uptake by 1.59 fold at the concentration of 10µM. Further work on optimization of the anti-diabetic lead is under progress.

Deoxyandrographolide promotes glucose uptake through glucose transporter-4 translocation to plasma membrane in L6 myotubes and exerts antihyperglycemic effect in vivo.

Skeletal muscle is the principal site for postprandial glucose utilization and augmenting the rate of glucose utilization in this tissue may help to control hyperglycemia associated with diabetes mellitus. Here, we explored the effect of Deoxyandrographolide (DeoAn) isolated from the Andrographis paniculata Nees on glucose utilization in skeletal muscle and investigated its antihyperglycemic effect in vivo in streptozotocin-induced diabetic rats and genetically diabetic db/db mice. In L6 myotubes, DeoAn dose-dependently stimulated glucose uptake by enhancing the translocation of glucose transporter 4 (GLUT4) to cell surface, without affecting the total cellular GLUT4 and GLUT1 content. These effects of DeoAn were additive to insulin. Further analysis revealed that DeoAn activated PI-3-K- and AMPK-dependent signaling pathways, account for the augmented glucose transport in L6 myotubes. Furthermore, DeoAn lowered postprandial blood glucose levels in streptozotocin-induced diabetic rats and also suppressed the rises in the fasting blood glucose, serum insulin, triglycerides and LDL-Cholesterol levels of db/db mice. These findings suggest the therapeutic efficacy of the DeoAn for type 2 diabetes mellitus and can be potential phytochemical for its management.

NOD2 activation induces oxidative stress contributing to mitochondrial dysfunction and insulin resistance in skeletal muscle cells.

Nucleotide-binding oligomerization domain protein-2 (NOD2) activation in skeletal muscle cells has been associated with insulin resistance, but the underlying mechanisms are not yet clear. Here we demonstrate the implication of oxidative stress in the development of mitochondrial dysfunction and insulin resistance in response to NOD2 activation in skeletal muscle cells. Treatment with the selective NOD2 ligand muramyl dipeptide (MDP) increased mitochondrial reactive oxygen species (ROS) generation in L6 myotubes. MDP-induced ROS production was associated with increased levels of protein carbonyls and reduction in citrate synthase activity, cellular ATP level, and mitochondrial membrane potential, as well as altered expression of genes involved in mitochondrial function and metabolism. Antioxidant treatment attenuated MDP-induced ROS production and restored mitochondrial functions. In addition, the presence of antioxidant prevented NOD2-mediated activation of MAPK kinases and the inflammatory response. This was associated with reduced serine phosphorylation of insulin receptor substrate-1 (IRS-1) and improved insulin-stimulated tyrosine phosphorylation of IRS-1 and downstream activation of Akt phosphorylation. These data indicate that oxidative stress plays a role in NOD2 activation-induced inflammatory response and that MDP-induced oxidative stress correlates with impairment of mitochondrial functions and induction of insulin resistance in skeletal muscle cells.