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Peroxisome proliferator-activated receptor ␥ co-activator 1␣ (PGC-1␣) promotes mitochondrial biogenesis and slow fiber formation in skeletal muscle. We hypothesized that activation of the p38 mitogen-activated protein kinase (MAPK) pathway in response to increased muscle activity stimulated Pgc-1␣ gene transcription as part of the mechanisms for skeletal muscle adaptation. Here we report that a single bout of voluntary running induced a transient increase of Pgc-1␣ mRNA expression in mouse plantaris muscle, concurrent with an activation of the p38 MAPK pathway. Activation of the p38 MAPK pathway in cultured C2C12 myocytes stimulated Pgc-1␣ promoter activity, which could be blocked by the specific inhibitors of p38, SB203580 and SB202190, or a dominant negative p38. Furthermore, the p38-mediated increase in Pgc-1␣ promoter activity was enhanced by increased expression of the downstream transcription factor ATF2 and completely blocked by ATF2⌬N, a dominant negative ATF2. Skeletal muscle-specific expression of a constitutively active activator of p38, MKK6E, in transgenic mice resulted in enhanced Pgc-1␣ and cytochrome oxidase IV protein expression in fast-twitch skeletal muscles. These findings suggest that contractile activity-induced activation of the p38 MAPK pathway promotes Pgc-1␣ gene expression and skeletal muscle adaptation.

Skeletal muscle GLUT-4 transcription in response to treatment with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), a known activator of AMP-activated protein kinase (AMPK), was studied in rats and mice. The increase in GLUT-4 mRNA levels in response to a single subcutaneous injection of AICAR, peaked at 13 h in white and red quadriceps muscles but not in the soleus muscle. The mRNA level of chloramphenicol acyltransferase reporter gene which is driven by 1,154 or 895 bp of the human GLUT-4 proximal promoter was increased in AICAR-treated transgenic mice, demonstrating the transcriptional upregulation of the GLUT-4 gene by AICAR. However, this induction of transcription was not apparent with 730 bp of the promoter. In addition, nuclear extracts from AICAR-treated mice bound to the consensus sequence of myocyte enhancer factor-2 (from -473 to -464) to a greater extent than from saline-injected mice. Thus AMP-activated protein kinase activation by AICAR increases GLUT-4 transcription by a mechanism that requires response elements within 895 bp of human GLUT-4 proximal promoter and that may be cooperatively mediated by myocyte enhancer factor-2.

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The Raf/MEK/MAP kinase cascade plays a critical role in transducing growth signals from activated cell surface receptors. Using ⌬MEK1:ER, a conditionally active form of MEK1, we demonstrate the ability of this dual specificity protein kinase to abrogate the cytokine dependency of the murine lymphoid hematopoietic cell line FL5.12. Cytokine-independent cells were obtained from FL5.12 cells at a frequency of 1 × 10 −7 , indicating that a low frequency of cells expressing ⌬MEK1:ER were factor-independent. In general, cells that were converted to a cytokine-independent phenotype displayed a higher level of MAP kinase activity in response to ⌬MEK1:ER activation than those that remained cytokine-dependent. ⌬MEK1:ER-responsive cells could be maintained long-term in the presence of ␤-