Browsing by Author "Ojuka, Edward O"

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    Regulation of skeletal muscle glucose transporter 4 expression in fructose-fed exercised rats
    (2013) Goyaram, Veeraj; Ojuka, Edward O
    Several studies have found that the expression of skeletal muscle glucose transporter 4 (GLUT4) is decreased by high fructose consumption but increased by exercise. However, the amounts of fructose used in these studies were extremely high and the effects of moderate feeding protocols are not known. While it is known that exercise enhances GLUT4 expression via increased histone H3 acetylation and binding of the myocyte enhancer factor- 2A (MEF2A) transcription factor to its binding domain on the Glut4 gene promoter, the impact of fructose consumption on this interaction has not been studied. Moreover, there is no direct evidence that an increase in MEF2 binding is due to increased accessibility of the MEF2 binding domain to transcription factors. This study tested the hypothesis that both exercise and high fructose consumption affect GLUT4 expression by altering the accessibility of the MEF2 binding domain on the Glut4 gene promoter via remodelling of chromatin in that region. Male Wistar rats (n=30) were randomly assigned to three dietary groups: a) standard Chow, b) Chow + 10% fructose drink and c) Chow + 10% maltodextrin drink. All rats had access to drinking water and chow ad libitum for a period of 13 days. In the last 6 days of the experiment 5 animals in each group performed 3 x 17 min daily bouts of intermittent swimming, with a load equivalent to 5% bodyweight attached to their tails. The remaining 5 rats from each group were untrained. Animals were fasted overnight on the last day of the experiment, anaesthetized and sacrificed on the morning of day 14. Triceps muscle were harvested and used: (a) for measurement of total GLUT4 content by western blot, (b) to obtain nuclei for assessment of accessibility of a 350bp region encompassing the MEF2 element on the Glut4 gene using nuclease digestion assay, and (c) to measure the acetylation of histones H3 and bound MEF2A in the region above using chromatin immunoprecipitation (ChIP) assay. Blood was also collected and assayed for fasting serum glucose, insulin and free fatty acids.
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    Role of calcium and AMP kinase in the regulation of mitochondrial biogenesis and GLUT4 levels in muscle
    (2004) Ojuka, Edward O
    Contractile activity induces mitochondrial biogenesis and increases glucose transport capacity in muscle. There has been much research on the mechanisms responsible for these adaptations. The present paper reviews the evidence, which indicates that the decrease in the levels of high-energy phosphates, leading to activation of AMP kinase (AMPK), and the increase in cytosolic Ca(2+), which activates Ca(2+)/calmodulin-dependent protein kinase (CAMK), are signals that initiate these adaptative responses. Although the events downstream of AMPK and CAMK have not been well characterized, these events lead to activation of various transcription factors, including: nuclear respiratory factors (NRF) 1 and 2, which cause increased expression of proteins of the respiratory chain; PPAR-alpha, which up regulates the levels of enzymes of beta oxidation; mitochondrial transcription factor A, which activates expression of the mitochondrial genome; myocyte-enhancing factor 2A, the transcription factor that regulates GLUT4 expression. The well-orchestrated expression of the multitude of proteins involved in these adaptations is mediated by the rapid activation of PPAR gamma co-activator (PGC) 1, a protein that binds to various transcription factors to maximize transcriptional activity. Activating AMPK using 5-aminoimidizole-4-carboxamide-1-beta-D-riboside (AICAR) and increasing cytoplasmic Ca(2+) using caffeine, W7 or ionomycin in L6 myotubes increases the concentration of mitochondrial enzymes and GLUT4 and enhances the binding of NRF-1 and NRF-2 to DNA. AICAR and Ca-releasing agents also increase the levels of PGC-1, mitochondrial transcription factor A and myocyte-enhancing factors 2A and 2D. These results are similar to the responses seen in muscle during the adaptation to endurance exercise and show that L6 myotubes are a suitable model for studying the mechanisms by which exercise causes the adaptive responses in muscle mitochondria and glucose transport.