Correction to: Mitochondrial H+-ATP synthase in human skeletal muscle: contribution to dyslipidaemia and insulin resistance.

Owing to an oversight, the authors omitted to note that Dr Taub is a co-founder of and equity holder in Cardero Therapeutics.

Mitochondrial H+-ATP synthase in human skeletal muscle: contribution to dyslipidaemia and insulin resistance.

AIMS/HYPOTHESIS: Mitochondria are important regulators of the metabolic phenotype in type 2 diabetes. A key factor in mitochondrial physiology is the H+-ATP synthase. The expression and activity of its physiological inhibitor, ATPase inhibitory factor 1 (IF1), controls tissue homeostasis, metabolic reprogramming and signalling. We aimed to characterise the putative role of IF1 in mediating skeletal muscle metabolism in obesity and diabetes. METHODS: We examined the 'mitochondrial signature' of obesity and type 2 diabetes in a cohort of 100 metabolically characterised human skeletal muscle biopsy samples. The expression and activity of H+-ATP synthase, IF1 and key mitochondrial proteins were characterised, including their association with BMI, fasting plasma insulin, fasting plasma glucose and HOMA-IR. IF1 was also overexpressed in primary cultures of human myotubes derived from the same biopsies to unveil the possible role played by the pathological inhibition of the H+-ATP synthase in skeletal muscle. RESULTS: The results indicate that type 2 diabetes and obesity act via different mechanisms to impair H+-ATP synthase activity in human skeletal muscle (76% reduction in its catalytic subunit vs 280% increase in IF1 expression, respectively) and unveil a new pathway by which IF1 influences lipid metabolism. Mechanistically, IF1 altered cellular levels of α-ketoglutarate and L-carnitine metabolism in the myotubes of obese (84% of control) and diabetic (76% of control) individuals, leading to limited ß-oxidation of fatty acids (60% of control) and their cytosolic accumulation (164% of control). These events led to enhanced release of TNF-α (10 ± 2 pg/ml, 27 ± 5 pg/ml and 35 ± 4 pg/ml in control, obese and type 2 diabetic participants, respectively), which probably contributes to an insulin resistant phenotype. CONCLUSIONS/INTERPRETATION: Overall, our data highlight IF1 as a novel regulator of lipid metabolism and metabolic disorders, and a possible target for therapeutic intervention.

The emerging role of PtdIns5P: another signalling phosphoinositide takes its place.

Of the seven phosphoinositides, PtdIns5P remains the most enigmatic. However, recent research has begun to elucidate its physiological functions. It is now clear that PtdIns5P is found in several distinct subcellular locations, and the identification of a number of PtdIns5P-binding proteins points to its involvement in a variety of key processes, including signal transduction, membrane trafficking and regulation of gene expression. Although the mechanisms that control its turnover are not yet fully understood, the existence of multiple pathways for PtdIns5P regulation is consistent with this emerging versatility.

Involvement of phosphatidylinositol 5-phosphate in insulin-stimulated glucose uptake in the L6 myotube model of skeletal muscle.

The phosphoinositide phospholipid PtdIns5P has previously been implicated in insulin-stimulated translocation of the glucose transporter GLUT4 into the plasma membrane of adipocytes, but its potential role in glucose transport in muscle has not been explored. The involvement of PtdIns5P in insulin-stimulated glucose uptake was therefore investigated in myotubes of the skeletal muscle cell line L6. Stimulation with insulin produced a transient increase in PtdIns5P, which was abolished by the over-expression of the highly active PtdIns5P 4-kinase PIP4Kα. PIP4Kα over-expression also abolished both the enhanced glucose uptake and the robust peak of PtdIns(3,4,5)P (3) production stimulated by insulin in myotubes. Delivery of exogenous PtdIns5P into unstimulated myotubes increased Akt phosphorylation, promoted GLUT4 relocalisation from internal membrane to plasma membrane fractions and its association with plasma membrane lawns and also stimulated glucose uptake in a tyrosine kinase and phosphoinositide 3-kinase (PI 3-kinase)-dependent fashion. Our results are consistent with a role for insulin-stimulated PtdIns5P production in regulating glucose transport by promoting PI 3-kinase signalling.

Myokine Regulation of Insulin Secretion: Impact of Inflammation and Type 2 Diabetes.

Skeletal muscle (SkM) secretes protein factors (myokines) that can exert multiple actions. To study the control of myokine regulation of ß-cell function, SkM biopsies were taken from non-diabetic (ND) and Type 2 diabetic (T2D) subjects and satellite cells cultured to myotubes (MT). MT were also treated with lipopolysaccharide (infectious inflammation - II) or a combination of glucose (10 mM), insulin (120 pM), and palmitate (0.4 mM) (metabolic inflammation - MI) to model the inflammatory and metabolic conditions seen in vivo with T2D. Conditioned media (CM) was collected from MT after 24 h and used to treat INS-1 cells for 24 h. Cell viability, total insulin content, glucose-stimulated insulin secretion (GSIS) and maximal (IBMX-stimulated) IS (ISmax) were monitored. Under baseline conditions, CM from ND and T2D MT had no effects on INS-1 cell viability, insulin content, GSIS, or ISmax. After exposure to II, CM from ND-MT augmented GSIS in INS-1 cells by 100 ± 25% over control (p < 0.05); T2D-CM had no effect. After exposure to MI, T2D-CM suppressed GSIS by 35 ± 5% (p < 0.05); ND-CM was without effect. Under either of these conditions cell viability, total insulin content and ISmax were unaffected. Effects of CM on GSIS were lost after CM was boiled. Both augmentation of GSIS by ND-CM from II-treated MT, and suppression by T2D-CM from MI-treated MT, were inhibited by wortmannin, Ro 31-8220, and SB203580. In summary: (1) ND-MT are able to augment GSIS when stressed, (2) T2D-MT responding to a diabetic-like environment secrete myokines that suppress GSIS, (3) Unknown protein factors exert effects specifically on GSIS, possibly through PI-3K, PKC, and/or p38 MAPK. In T2D, both insulin resistance and a suppression of adaptive increased insulin secretion are intrinsic properties of SkM that can contribute to the full T2D phenotype.

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.