Mortensen, Ole Hartvig, Lis Frandsen, Peter Schjerling, Erica Nishimura, and Niels Grunnet. PGC-1␣ and PGC-1 have both similar and distinct effects on myofiber switching toward an oxidative phenotype. Am J Physiol Endocrinol Metab 291: E807-E816, 2006. First published May 23, 2006 doi:10.1152/ajpendo.00591.2005.-Peroxisome proliferator-activated receptor-␥ coactivator-1␣ and -1 (PGC-1␣ and PGC-1) were overexpressed by adenovirus-mediated gene transfer in cultures of primary rat skeletal muscle cells derived from neonatal myoblasts. Effects on muscle fiber type transition and metabolism were studied from days 5 to 22 of culture. PGC-1␣ and PGC-1 overexpression caused a three-to fourfold increase in mRNA level, a doubling of enzymatic activity of citrate synthase, a slight increase in short-chain acyl-CoA dehydrogenase mRNA, a doubling of the mRNA level, and a 30 -50% increase in enzymatic activity of glyceraldehyde-3-phosphate dehydrogenase. Lactate dehydrogenase or creatine kinase activity was unchanged. PGC-1␣ enhanced glycogen buildup twofold at 5 or 25 mM glucose, whereas PGC-1 caused a decrease. Both PGC-1␣ and PGC-1 overexpression caused a faster maturation of myotubes, as seen by mRNA downregulation of the immature embryonal and perinatal myosin heavy-chain (MHC) isoforms. PGC-1␣ or PGC-1 overexpression enhanced mRNA of the slow oxidative-associated MHC isoform MHCIb and downregulated mRNA levels of the fast glycolytic-associated MHC isoforms MHCIIX and MHCIIB. Only PGC-1 overexpression caused an increase in mRNA of the intermediary fast oxidative-associated MHC isoform MHCIIA. PGC-1␣ or PGC-1 overexpression upregulated GLUT4 mRNA and downregulated myocyte enhancer factor 2C transcription factor mRNA; only PGC-1␣ overexpression caused an increase in the mRNA expression of TRB3, a negative regulator of insulin signaling. These results show that both PGC-1␣ and PGC-1 are involved in the regulation of skeletal muscle fiber transition and metabolism and that they have both overlapping and differing effects. skeletal muscle cell culture; peroxisome proliferator-activated receptor ␥ coactivator-1; myosin heavy chain; enzyme activities; glycogen THE BENEFICIAL EFFECT of exercise on various diseases is widely acknowledged (7), and exercise has proven particularly successful as a treatment for the metabolic syndrome, including type 2 diabetes (48); however, the exact molecular pathway responsible for the effect is largely unknown. Recently, genes involved in oxidative phosphorylation in skeletal muscle were found to be downregulated in patients with type 2 diabetes (37, 43), and speculations about type 2 diabetes being caused by mitochondrial dysfunction, primarily in the type I skeletal muscle fibers, have surfaced (13,31,51). Regular exercise has been shown to induce changes in both skeletal muscle metabolism and muscle fiber type over time, most notably an increase in mitochondrial content and oxidative metabolism as well as a shift toward a more slow oxidative fiber type (8,44).