In addition to generating movement, skeletal muscle may have a function as a secretory organ. The aim of the present study was to identify novel proteins with signaling capabilities secreted from skeletal muscle cells. IL-7 was detected in media conditioned by primary cultures of human myotubes differentiated from satellite cells, and concentrations increased with incubation time. By immunoblotting and real-time RT-PCR IL-7 expression was confirmed at both protein and mRNA levels. Furthermore, with immunofluorescence and specific antisera, multinucleated myotubes were found to coexpress IL-7 and myosin heavy chain. During differentiation of human myotubes from satellite cells, IL-7 expression increased at mRNA and protein levels. In contrast, mRNA expression of the IL-7 receptor was 80% lower in myotubes compared with satellite cells. Incubations with recombinant IL-7 under differentiation conditions caused approximately 35% reduction in mRNA for the terminal myogenic markers myosin heavy chain 2 (MYH2) and myogenin (MYOG), suggesting that IL-7 may act on satellite cells to inhibit development of the muscle fiber phenotype. Alternative routes of cell development were investigated, and IL-7 increased migration of satellite cells by 40% after 48 h in a Transwell system, whereas cell proliferation remained unchanged. In vivo, real-time RT-PCR analysis of musculus vastus lateralis (n = 10) and musculus trapezius (n = 7) biopsies taken from male individuals undergoing a strength training program demonstrated that after 11 wk mean IL-7 mRNA increased by threefold (P = 0.01) and fourfold (P = 0.04), respectively. In conclusion, we have demonstrated that IL-7 is a novel myokine regulated both in vitro and in vivo, and it may play a role in the regulation of muscle cell development.

Liver X receptors (LXRs) are important regulators of cholesterol and lipid metabolism and are also involved in glucose metabolism. However, the functional role of LXRs in human skeletal muscle is at present unknown. This study demonstrates that chronic ligand activation of LXRs by a synthetic LXR agonist increases the uptake, distribution into complex cellular lipids, and oxidation of palmitate as well as the uptake and oxidation of glucose in cultured human skeletal muscle cells. Furthermore, the effect of the LXR agonist was additive to acute effects of insulin on palmitate uptake and metabolism. Consistently, activation of LXRs induced the expression of relevant genes: fatty acid translocase (CD36/FAT), glucose transporters (GLUT1 and -4), sterol regulatory element-binding protein-1c, peroxisome proliferator-activated receptor-␥, carnitine palmitoyltransferase-1, and uncoupling protein 2 and 3. Interestingly, in response to activation of LXRs, myotubes from patients with type 2 diabetes showed an elevated uptake and incorporation of palmitate into complex lipids but an absence of palmitate oxidation to CO 2 . These results provide evidence for a functional role of LXRs in both lipid and glucose metabolism and energy uncoupling in human myotubes. Furthermore, these data suggest that increased intramyocellular lipid content in type 2 diabetic patients may involve an altered response to activation of components in the LXR pathway.

We describe multiwell assays for detecting the accumulation as well as the subsequent oxidation of 14

OBJECTIVE— Increased availability of fatty acids is important for accumulation of intracellular lipids and development of insulin resistance in human myotubes. It is unknown whether different types of fatty acids like eicosapentaenoic acid (EPA) or tetradecylthioacetic acid (TTA) influence these processes. RESEARCH DESIGN AND METHODS— We examined fatty acid and glucose metabolism and gene expression in cultured human skeletal muscle cells from control and type 2 diabetic individuals after 4 days of preincubation with EPA or TTA. RESULTS— Type 2 diabetes myotubes exhibited reduced formation of CO 2 from palmitic acid (PA), whereas release of β-oxidation products was unchanged at baseline but significantly increased with respect to control myotubes after preincubation with TTA and EPA. Preincubation with TTA enhanced both complete (CO 2 ) and β-oxidation of palmitic acid, whereas EPA increased only β-oxidation significantly. EPA markedly enhanced triacylglycerol (TAG) accumulation in myotubes, more pronounced in type 2 diabetes cells. TAG accumulation and fatty acid oxidation were inversely correlated only after EPA preincubation, and total level of acyl-CoA was reduced. Glucose oxidation (CO 2 formation) was enhanced and lactate production decreased after chronic exposure to EPA and TTA, whereas glucose uptake and storage were unchanged. EPA and especially TTA increased the expression of genes involved in fatty acid uptake, activation, accumulation, and oxidation. CONCLUSIONS— Our results suggest that 1 ) mitochondrial dysfunction in diabetic myotubes is caused by disturbances downstream of fatty acid β-oxidation; 2 ) EPA promoted accumulation of TAG, enhanced β-oxidation, and increased glucose oxidation; and 3 ) TTA improved complete palmitic acid oxidation in diabetic myotubes, opposed increased lipid accumulation, and increased glucose oxidation.

We evaluated the effects of partly substituting lard with marine n-3 fatty acids (FA) on body composition and weight, adipose tissue distribution and gene expression in five adipose depots of male Wistar rats fed a high-fat diet. Rats were fed diets including lard (19·5 % lard) or n-3 FA (9·1 % lard and 10·4 % Triomare) for 7 weeks. Feed consumption and weight gain were similar, whereas plasma lipid concentrations were lower in the n-3 FA group. Magnetic resonance imaging revealed smaller visceral (mesenteric, perirenal and epididymal) adipose depots in the n-3 FA-fed animals (35, 44 and 32 % reductions, respectively). n-3 FA feeding increased mRNA expression of cytokines as well as chemokines in several adipose depots. Expression of Adipoq and Pparg was enhanced in the mesenteric adipose depots of the n-3 FA-fed rats, and fasting plasma insulin levels were lowered. Expression of the lipogenic enzymes Acaca and Fasn was increased in the visceral adipose depots, whereas Dgat1 was reduced in the perirenal and epididymal depots. Cpt2 mRNA expression was almost doubled in the mesenteric depot and liver. Carcass analyses showed similar body fat (%) in the two feeding groups, indicating that n-3 FA feeding led to redistribution of fat away from the visceral compartment.

Our data support the hypothesis that TTA feeding may increase hepatic fatty acid beta-oxidation, and thereby reduce the size of adipose tissues. The functional importance of ectopic hepatic UCP3 is unknown, but might be associated with enhanced energy expenditure and thus the reduced feed efficiency.

This review focuses on the effect of exogenous factors known to be of importance for the development of insulin resistance in differentiated human myotubes. Recent data from our laboratory on the effects of fatty acid pre-treatment and chronic glucose oversupply on fatty acid and glucose metabolism, without and with acute insulin are presented, and discussed in the context of other recent publications in the field. Pre-treatment of myotubes with palmitate, chronic hyperglycaemia, and acute high concentrations of insulin changed fatty acid metabolism in favour of accumulation of intracellular lipids. Acute insulin exposure increased (14)C-oleate uptake and levels of free fatty acids (FFA) and triacylglycerol (TAG). Palmitate pre-treatment further increased oleate uptake, both under basal conditions and in the presence of insulin, with a marked increase in the phospholipid (PL) fraction, with a concomitant reduction in oleate oxidation. Chronic hyperglycaemia also promoted increased lipogenesis and elevated levels of cellular lipids. Changes in fatty acid metabolism in human muscle, in particular fatty acid oxidation, are probably crucial for the molecular mechanism behind skeletal muscle insulin resistance and impaired glucose metabolism. Differentiated human skeletal muscle cells may be an ideal system to further explore the mechanisms regulating lipid metabolism.

We demonstrate for the first time that TTA attenuates dyslipidaemia in patients with type 2 diabetes mellitus. These effects may occur through mechanisms involving PPAR-alpha and PPAR-delta activation, resulting in increased mitochondrial fatty acid oxidation.