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1 Department of Kinesiology and Health Science, York University, Toronto, Ontario, Canada
2 Department of Biology, York University, Toronto, Ontario, Canada
* To whom correspondence should be addressed. E-mail: dhood{at}yorku.ca.
The early cellular signals associated with contractile activity initiate the activation and induction of transcription factors that regulate changes in skeletal muscle phenotype. The transcription factors Egr-1, Sp1, and SRF, are potentially important mediators of mitochondrial biogenesis based on the prevalence of binding sites for these transcription factors in the promoter regions of genes encoding mitochondrial proteins, including the important regulator of mitochondrial biogenesis PGC-1
. Thus, to further define a role for transcription factors at the onset of contractile activity, we examined the time dependent alterations in Egr-1, Sp1 and SRF mRNA and levels in electrically stimulated mouse C2C12 skeletal muscle cells. Early transient increases in Egr-1 mRNA levels within 30 mins (p<0.05) of contractile activity led to 3-fold increases (p<0.05) in Egr-1 protein by 60 mins. The increase in Egr-1 mRNA was not a result of increased stability since a 58% decline in Egr-1 mRNA half-life following 30 mins of stimulation was observed. Stimulation of muscle cells had no effect on Sp1 mRNA, but led to progressive increases (p<0.05) in SRF mRNA by 30 and 60 minutes. This was not matched by increases in SRF protein, but occurred coincident with increases (p<0.05) in SRF-SRE DNA binding at 30 and 60 mins as a result of SRF phosphorylation on Ser-103. To assess the importance of the recovery period, 12 hrs of continuous contractile activity was compared to 4 successive 3-hr bouts, with an intervening 21-hr recovery period following each bout. Continuous contractile activity led a 2.0-fold increase (p<0.05) in Egr-1 mRNA, no change in SRF mRNA, and a 43% decrease in Sp1 mRNA expression. The recovery period prevented the decline in Sp1 mRNA, produced a decrease in Egr-1 mRNA, and had no effect on SRF mRNA. Thus, continuous and intermittent contractile activity evoke different specific transcription factor expression patterns, which may ultimately contribute to divergent qualitative, or temporal patterns, of phenotypic adaptation in muscle.
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