[Mechanisms of insulin resistance]. / Insuliiniresistenssin mekanismit.

Insulin resistance refers to an aberrant physiological response to insulin in its target tissues. Several signal transduction mechanisms sensing intracellular stress are activated in situations where energy supply exceeds the cells' energy requirements. This stress interferes with insulin-induced intracellular signal transduction and leads to an inflammatory state. The activation of inflammatory responses in peripheral tissues and central nervous system weakens the body's insulin sensitivity, glucose tolerance and predisposes to obesity. Insulin resistance is thus a crucial metabolic disorder in obesity, type 2 diabetes and cardiovascular diseases.

Palmitate and oleate exert differential effects on insulin signalling and glucose uptake in human skeletal muscle cells.

Saturated fatty acids are implicated in the development of insulin resistance, whereas unsaturated fatty acids may have a protective effect on metabolism. We tested in primary human myotubes if insulin resistance induced by saturated fatty acid palmitate can be ameliorated by concomitant exposure to unsaturated fatty acid oleate. Primary human myotubes were pretreated with palmitate, oleate or their combination for 12 h. Glucose uptake was determined by intracellular accumulation of [3H]-2-deoxy-d-glucose, insulin signalling and activation of endoplasmic reticulum (ER) stress by Western blotting, and mitochondrial reactive oxygen species (ROS) production by fluorescent dye MitoSOX. Exposure of primary human myotubes to palmitate impaired insulin-stimulated Akt-Ser473, AS160 and GSK-3ß phosphorylation, induced ER stress signalling target PERK and stress kinase JNK 54 kDa isoform. These effects were virtually abolished by concomitant exposure of palmitate-treated myotubes to oleate. However, an exposure to palmitate, oleate or their combination reduced insulin-stimulated glucose uptake. This was associated with increased mitochondrial ROS production in palmitate-treated myotubes co-incubated with oleate, and was alleviated by antioxidants MitoTempo and Tempol. Thus, metabolic and intracellular signalling events diverge in myotubes treated with palmitate and oleate. Exposure of human myotubes to excess fatty acids increases ROS production and induces insulin resistance.

Angiopoietin-like 4 mediates PPAR delta effect on lipoprotein lipase-dependent fatty acid uptake but not on beta-oxidation in myotubes.

Peroxisome proliferator-activated receptor (PPAR) delta is an important regulator of fatty acid (FA) metabolism. Angiopoietin-like 4 (Angptl4), a multifunctional protein, is one of the major targets of PPAR delta in skeletal muscle cells. Here we investigated the regulation of Angptl4 and its role in mediating PPAR delta functions using human, rat and mouse myotubes. Expression of Angptl4 was upregulated during myotubes differentiation and by oleic acid, insulin and PPAR delta agonist GW501516. Treatment with GW501516 or Angptl4 overexpression inhibited both lipoprotein lipase (LPL) activity and LPL-dependent uptake of FAs whereas uptake of BSA-bound FAs was not affected by either treatment. Activation of retinoic X receptor (RXR), PPAR delta functional partner, using bexarotene upregulated Angptl4 expression and inhibited LPL activity in a PPAR delta dependent fashion. Silencing of Angptl4 blocked the effect of GW501516 and bexarotene on LPL activity. Treatment with GW501516 but not Angptl4 overexpression significantly increased palmitate oxidation. Furthermore, Angptl4 overexpression did not affect the capacity of GW501516 to increase palmitate oxidation. Basal and insulin stimulated glucose uptake, glycogen synthesis and glucose oxidation were not significantly modulated by Angptl4 overexpression. Our findings suggest that FAs-PPARdelta/RXR-Angptl4 axis controls the LPL-dependent uptake of FAs in myotubes, whereas the effect of PPAR delta activation on beta-oxidation is independent of Angptl4.

Acute exposure to rosiglitazone does not affect glucose transport in intact human skeletal muscle.

Thiazolidinediones (TZDs) such as rosiglitazone are widely used as antidiabetic drugs. Animal studies suggest that TZDs may have direct metabolic actions in skeletal muscle. Here, we examined if acute exposure to rosiglitazone stimulates glucose transport rate and affects proximal insulin signaling in isolated skeletal muscle strips from nondiabetic men. Open muscle biopsies were obtained from musculus vastus lateralis from 15 nondiabetic men (50 +/- 3 years old, 26.9 +/- 1.1 kg/m(2)). Skeletal muscle strips were isolated and exposed to rosiglitazone (1 or 10 micromol/L), 5-aminoimidazole-4-carboxamide 1-beta-D-ribonucleoside (1 mmol/L), insulin (120 nmol/L), or a combination of insulin (120 nmol/L) and rosiglitazone (10 micromol/L) in vitro for 1 hour. Glucose transport was analyzed by accumulation of intracellular 3-O-methyl [(3)H] glucose; phosphorylation of Akt-Ser(473) and Akt-Thr(308) and phosphorylation of acetyl coenzyme A carboxylase beta were determined using phosphospecific antibodies. 5-Aminoimidazole-4-carboxamide 1-beta-d-ribonucleoside and insulin increased glucose transport rate 1.5-fold (P < .05) and 1.7-fold (P < .01) in isolated muscle strips, respectively. Exposure to rosiglitazone transiently increased phosphorylation of acetyl coenzyme A carboxylase beta, with a maximum effect at 15 minutes and return to baseline at 60 minutes. However, rosiglitazone did not affect basal or insulin-stimulated glucose transport rate, or phosphorylation of Akt-Ser(473) or Akt-Thr(308) in isolated muscle strips. In conclusion, acute exposure to rosiglitazone does not affect glucose transport in human skeletal muscle.

Globular adiponectin stimulates glucose transport in type 2 diabetic muscle.

BACKGROUND: Adiponectin acts as an insulin sensitizer in rodent models. The direct effect of adiponectin in intact type 2 diabetic muscle is unknown. We examined whether adiponectin stimulates glucose transport in isolated skeletal muscle strips from type 2 diabetic men. METHODS: We obtained open muscle biopsies from 12 men with type 2 diabetes (56 +/- 1 years, 30.5 +/- 1.1 kg/m(2)), and from 15 non-diabetic men (59 +/- 1 years, 28.0 +/- 1.0 kg/m(2)). Skeletal muscle strips were isolated and exposed to globular adiponectin (2.5 microg/mL), insulin (120 nM) and/or AICAR (1 mM) in vitro for 1 h. Glucose transport was analysed by accumulation of intracellular 3-O-methyl [(3)H] glucose, phosphorylation of Akt-Ser(473) and Akt-Thr(308) was determined using phosphospecific antibodies, and adiponectin receptor 1 and 2 content was measured using specific antibodies. RESULTS: Globular adiponectin increased glucose transport rate by 1.3-fold (P < 0.01) in type 2 diabetic, but not in non-diabetic muscle. Insulin-stimulated glucose transport rate was unaltered by exposure to globular adiponectin in either group. AICAR increased glucose transport and enhanced insulin-stimulated glucose transport in type 2 diabetic and non-diabetic muscles. Insulin-stimulated phosphorylation of Akt-Ser(473) or Akt-Thr(308) was comparable in type 2 diabetic and non-diabetic muscles, and unaltered by the addition of globular adiponectin in either group. Adiponectin receptor expression was similar in skeletal muscle from type 2 diabetic and non-diabetic men. CONCLUSIONS: Globular adiponectin directly increases glucose transport in skeletal muscle from type 2 diabetic patients. This may occur via Akt-independent signalling routes.

HobA--a novel protein involved in initiation of chromosomal replication in Helicobacter pylori.

Replication of the bacterial chromosome is initiated by the binding of the DnaA protein to a unique DNA region, called oriC. Many regulatory factors in numerous species act by controlling the ability of DnaA to bind and unwind DNA, but the Helicobacter pylori genome does not contain homologues to any of these factors. Here, we describe HobA, a novel protein essential for initiation of H. pylori chromosome replication, which is conserved among, and unique to, epsilon proteobacteria. We demonstrate that HobA interacts specifically via DnaA with the oriC-DnaA complex. We postulate that HobA is essential for correct formation and stabilization of the orisome by facilitating the spatial positioning of DnaA at oriC. Consistent with its function, overexpression of hobA had no effect on growth of H. pylori, whereas depletion of HobA led to growth arrest and failure to initiate replication. In conclusion, HobA may be the first identified of a new group of initiation factors common to epsilon proteobacteria.