Lack of Effect of Leptin on Glucose Transport, Lipoprotein Lipase, and Insulin Action in Adipose and Muscle Cells

The effect of leptin on glucose transport, lipogenesis, and lipoprotein lipase activity was studied in cultured rat adipocytes and 3T3-L1 adipocytes. Leptin had no effect on basal and insulin stimulated glucose transport in isolated adipocytes from the rat and the genetically obese mouse. The incorporation of glucose into lipids was also unaffected. Lipoprotein lipase (LPL) activity remained unchanged in response to leptin in these cells, as well as in minced adipose tissue. Leptin also had no effect on both basal and insulin-stimulated glucose transport in cultured rat and human skeletal muscle cells. These studies showed that leptin had no effect on glucose transport, lipoprotein lipase activity, and insulin action in fat and muscle cells in vitro

Effects of Tumor Necrosis Factor-α on Glucose Metabolism in Cultured Human Muscle Cells from Nondiabetic and Type 2 Diabetic Subjects

The effects of tumor necrosis factor-a (TNFα) on glucose uptake and glycogen synthase (GS) activity were studied in human skeletal muscle cell cultures from nondiabetic and type 2 diabetic subjects. In nondiabetic muscle cells, acute (90-Min) exposure to TNFα (5 ng/ml) stimulated glucose uptake (73 ± 14% increase) to a greater extent than insulin (37 ± 4%; P \u3c 0.02). The acute uptake response to TNFα in diabetic cells (51 ± 6% increase) was also greater than that to insulin (31 ± 3%; P \u3c 0.05). Prolonged (24-h) exposure of nondiabetic muscle cells to TNFα resulted in a further stimulation of uptake (152 ± 31%; P \u3c 0.05), whereas the increase in cells from type 2 diabetics was not significant compared with that in cells receiving acute treatment. After TNFα treatment, the level of glucose transporter-1 protein was elevated in nondiabetic (4.6-fold increase) and type 2 (1.7-fold) cells. Acute TNFα treatment had no effect on the fractional velocity of GS in either nondiabetic or type 2 cells. Prolonged exposure reduced the GS fractional velocity in both nondiabetic and diabetic cells. In summary, both acute and prolonged treatment with TNFα up-regulate glucose uptake activity in cultured human muscle cells, but reduce GS activity. Increased skeletal muscle glucose uptake in conditions of TNFα excess may serve as a compensatory mechanism in the insulin resistance of type 2 diabetes

Activation of G proteins by GIV-GEF is a pivot point for insulin resistance and sensitivity.

Insulin resistance (IR) is a metabolic disorder characterized by impaired insulin signaling and cellular glucose uptake. The current paradigm for insulin signaling centers upon the insulin receptor (InsR) and its substrate IRS1; the latter is believed to be the sole conduit for postreceptor signaling. Here we challenge that paradigm and show that GIV/Girdin, a guanidine exchange factor (GEF) for the trimeric G protein Gαi, is another major hierarchical conduit for the metabolic insulin response. By virtue of its ability to directly bind InsR, IRS1, and phosphoinositide 3-kinase, GIV serves as a key hub in the immediate postreceptor level, which coordinately enhances the metabolic insulin response and glucose uptake in myotubes via its GEF function. Site-directed mutagenesis or phosphoinhibition of GIV-GEF by the fatty acid/protein kinase C-theta pathway triggers IR. Insulin sensitizers reverse phosphoinhibition of GIV and reinstate insulin sensitivity. We also provide evidence for such reversible regulation of GIV-GEF in skeletal muscles from patients with IR. Thus GIV is an essential upstream component that couples InsR to G-protein signaling to enhance the metabolic insulin response, and impairment of such coupling triggers IR. We also provide evidence that GIV-GEF serves as therapeutic target for exogenous manipulation of physiological insulin response and reversal of IR in skeletal muscles

Role of Myotonic Dystrophy Protein Kinase (DMPK) in Glucose Homeostasis and Muscle Insulin Action

Myotonic dystrophy 1 (DM1) is caused by a CTG expansion in the 3′-unstranslated region of the DMPK gene, which encodes a serine/threonine protein kinase. One of the common clinical features of DM1 patients is insulin resistance, which has been associated with a pathogenic effect of the repeat expansions. Here we show that DMPK itself is a positive modulator of insulin action. DMPK-deficient (dmpk−/−) mice exhibit impaired insulin signaling in muscle tissues but not in adipocytes and liver, tissues in which DMPK is not expressed. Dmpk−/− mice display metabolic derangements such as abnormal glucose tolerance, reduced glucose uptake and impaired insulin-dependent GLUT4 trafficking in muscle. Using DMPK mutants, we show that DMPK is required for a correct intracellular trafficking of insulin and IGF-1 receptors, providing a mechanism to explain the molecular and metabolic phenotype of dmpk−/− mice. Taken together, these findings indicate that reduced DMPK expression may directly influence the onset of insulin-resistance in DM1 patients and point to dmpk as a new candidate gene for susceptibility to type 2-diabetes

Altered Myokine Secretion Is an Intrinsic Property of Skeletal Muscle in Type 2 Diabetes

Skeletal muscle secretes factors, termed myokines. We employed differentiated human skeletal muscle cells (hSMC) cultured from Type 2 diabetic (T2D) and non-diabetic (ND) subjects to investigate the impact of T2D on myokine secretion. Following 24 hours of culture concentrations of selected myokines were determined to range over 4 orders of magnitude. T2D hSMC released increased amounts of IL6, IL8, IL15, TNFa, Growth Related Oncogene (GRO)a, monocyte chemotactic protein (MCP)-1, and follistatin compared to ND myotubes. T2D and ND hSMC secreted similar levels of IL1ß and vascular endothelial growth factor (VEGF). Treatment with the inflammatory agents lipopolysaccharide (LPS) or palmitate augmented the secretion of many myokines including: GROa, IL6, IL8, IL15, and TNFa, but did not consistently alter the protein content and/or phosphorylation of IkBa, p44/42 MAPK, p38 MAPK, c-Jun N-terminal kinase (JNK) and NF-kB, nor lead to consistent changes in basal and insulin-stimulated glucose uptake or free fatty acid oxidation. Conversely, treatment with pioglitazone or oleate resulted in modest reductions in the secretion of several myokines. Our results demonstrate that altered secretion of a number of myokines is an intrinsic property of skeletal muscle in T2D, suggesting a putative role of myokines in the response of skeletal muscle to T2D

Adipose tissue from subjects with type 2 diabetes exhibits impaired capillary formation in response to GROα: involvement of MMPs-2 and -9

Type 2 Diabetes (T2D) is associated with impaired vascularization of adipose tissue (AT) . IL8, GROα and IL15 are pro-angiogenic myokines, secreted at elevated levels by T2D myotubes. We explored the direct impact of these myokines on AT vascularization. AT explants from subjects with T2D and without diabetes (non-diabetic, ND) were treated with rIL8, rGROα and rIL15 in concentrations equal to those in conditioned media (CM) from T2D and ND myotubes, and sprout formation evaluated. Endothelial cells (EC) were isolated from T2D and ND-AT, treated with rGROα and tube formation evaluated. Finally, we investigated the involvement of MMP-2 and -9 in vascularization. ND and T2D concentrations of IL8 or IL15   caused similar stimulation of sprout formation in ND- and T2D-AT. GROα exerted a similar effect in ND-AT. When T2D-AT explants were exposed to GROα, sprout formation in response to T2D concentrations was reduced compared to ND. Exposure of EC from T2D-AT to GROα at T2D concentrations resulted in reduced tube formation. Reduced responses to GROα in T2D-AT and EC were also seen for secretion of MMP-2 and -9. The data indicate that skeletal muscle can potentially regulate AT vascularization, with T2D-AT having impairments in sensitivity to GROα, while responding normally to IL8 and IL15