Physical activity is associated with retained muscle metabolism in human myotubes challenged with palmitate

The aim of this study was to investigate whether physical activity is associated with preserved muscle metabolism in human myotubes challenged with saturated fatty acids. Human muscle satellite cells were isolated from sedentary or active individuals and differentiated into myocytes in culture. Metabolic differences were then investigated in the basal state or after chronic palmitate treatment. At basal, myocytes from sedentary individuals exhibited higher CD36 and HSP70 protein expression as well as elevated phosphorylation of c-Jun NH(2)-terminal kinase (JNK) and insulin receptor substrate 1 (IRS1) serine(307) compared to myocytes from active individuals. Despite equal lipid accumulation following palmitate treatment, myocytes from sedentary individuals exhibited delayed acetyl coenzyme A carboxylase phosphorylation compared to the active group. Myocytes from sedentary individuals had significantly higher basal glucose uptake and palmitate promoted insulin resistance in sedentary myocytes. Importantly, myocytes from active individuals were partially protected from palmitate-induced insulin resistance. Palmitate treatment enhanced IRS1 serine(307) phosphorylation in myocytes from sedentary individuals and correlated positively to JNK phosphorylation. In conclusion, muscle satellite cells retain metabolic differences associated with physical activity. Physical activity partially protects myocytes from fatty acid-induced insulin resistance and inactivity is associated with dysregulation of metabolism in satellite cells challenged with palmitate. Although the benefits of physical activity on whole body physiology have been well investigated, this paper presents novel findings that both diet and exercise impact satellite cells directly. Given the fact that satellite cells are important for muscle maintenance, a dysregulated function could have profound effects on health. Therefore the effects of lifestyle on satellite cells needs to be delineated

Lifelong Physical Activity Prevents Aging-Associated Insulin Resistance in Human Skeletal Muscle Myotubes via Increased Glucose Transporter Expression.

Both aging and physical inactivity are associated with increased development of insulin resistance whereas physical activity has been shown to promote increased insulin sensitivity. Here we investigated the effects of physical activity level on aging-associated insulin resistance in myotubes derived from human skeletal muscle satellite cells. Satellite cells were obtained from young (22 yrs) normally active or middle-aged (56.6 yrs) individuals who were either lifelong sedentary or lifelong active. Both middle-aged sedentary and middle-aged active myotubes had increased p21 and myosin heavy chain protein expression. Interestingly MHCIIa was increased only in myotubes from middle-aged active individuals. Middle-aged sedentary cells had intact insulin-stimulated Akt phosphorylation however, the same cell showed ablated insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane. On the other hand, middle-aged active cells retained both insulin-stimulated increases in glucose uptake and GLUT4 translocation to the plasma membrane. Middle-aged active cells also had significantly higher mRNA expression of GLUT1 and GLUT4 compared to middle-aged sedentary cells, and significantly higher GLUT4 protein. It is likely that physical activity induces a number of stable adaptations, including increased GLUT4 expression that are retained in cells ex vivo and protect, or delay the onset of middle-aged-associated insulin resistance. Additionally, a sedentary lifestyle has an impact on the metabolism of human myotubes during aging and may contribute to aging-associated insulin resistance through impaired GLUT4 localization

Characteristics of muscle satellite cell donors.

<p>Data are means ± SD. OGTT, oral glucose tolerance test.</p>***<p>P = <0.0001 vs. young subjects.</p><p>(¤)P = 0.07 vs. middle-aged active subjects.</p

Effect of aging and physical activity on GLUT1 and GLUT4 expression.

<p>Satellite cells were isolated from vastus lateralis biopsies from young or middle-aged volunteers: sedentary or active. Cells were grown in culture until mature myotubes were formed. Expression of (A) GLUT1 mRNA and (C) GLUT4 mRNA were measured by qPCR and using the delta CT method (AU). Expression of (B) GLUT1 and (D) GLUT4 mRNA in the middle-aged groups was normalized to expression level in young group. (E) Representative immunoblot and quantification (arbitrary units) of GLUT4 protein. Values shown are the mean ±S.E.M from cells from 5 individuals for each group. An asterisk denotes a significant difference from young (*P<0.05, **P<0.01). A # denotes a significant difference from middle-aged sedentary (P<0.05, t-test). A (#) denotes a trend towards a significant difference from middle-aged sedentary (P = 0.07, t-test). The outlier in GLUT4 is shown with a circle around the value (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066628#pone-0066628-g003" target="_blank">Figure 3C</a>).</p

Effect of aging and physical activity on insulin-stimulated GLUT4 localisation.

<p>Satellite cells were isolated from vastus lateralis biopsies from young or middle-aged volunteers: sedentary or active. Cells were grown in culture until mature myotubes were formed. Myotubes were treated with insulin (100 nM) for 30 mins before fixation and immunofluorescence staining. (A) Representative immunofluorescence images of myotubes stained with anti-GLUT4 and anti-wheat germ agglutinin (Scale bar 20 µm). The plasma membrane region is a 2× zoom of the merged image. Quantification of membrane fluorescence intensity in basal state (B) or after insulin treatment as a fold change from basal (C). (B and C) Open bars = young, gray bars = middle-aged sedentary and black bars = middle-aged active. Values are mean ± S.E.M from cells from 5 individuals for each group. An asterisk denotes a significant difference from basal (*P<0.05, **P<0.01). A (*) denotes a trend towards a significant difference from young (P = 0.058).</p

Effect of aging and physical activity on insulin signaling pathway.

<p>Satellite cells were isolated from vastus lateralis biopsies from young or middle-aged volunteers: sedentary or active. Cells were grown in culture until mature myotubes were formed. Myotubes were treated with insulin (100 nM) for 30 mins before measuring glucose uptake into myotubes expressed as pmol/min/mg protein (A) or fold change from basal (B). (C) Immunoblotting of total protein lysate for phosphorylation of Akt Serine473 or GSK3 Serine21/9 and total protein amount of Akt. Equal gel loading was ascertained by immunoblotting with an antibody against β-tubulin. Protein expression of (D) Akt phosphorylation and (E) GSK3 phosphorylation were quantified and expressed as arbitrary units. Open bars = young, gray bars = middle-aged sedentary and black bars = middle-aged active. Values shown are the mean ±S.E.M from cells from 5 individuals for each group. An asterisk denotes a significant difference basal for each group (*P<0.05, **P<0.01). A ¤ denotes a significant change from young basal (¤¤ P<0.01).</p