The transcriptional coactivator the peroxisome proliferator-activated receptor 
gamma coactivator-1alpha (PGC-1alpha) has been identified as an important 
mediator of mitochondrial biogenesis based on its ability to interact with 
transcription factors that activate nuclear genes encoding mitochondrial 
proteins. The induction of PGC-1alpha protein expression under conditions that 
provoke mitochondrial biogenesis, such as contractile activity or thyroid 
hormone (T(3)) treatment, is not fully characterized. Thus we related PGC-1alpha 
protein expression to cytochrome c oxidase (COX) activity in 1) tissues of 
varying oxidative capacities, 2) tissues from animals treated with T(3), and 3) 
skeletal muscle subject to contractile activity both in cell culture and in 
vivo. Our results demonstrate a strong positive correlation (r = 0.74; P < 0.05) 
between changes in PGC-1alpha and COX activity, used as an index of 
mitochondrial adaptations. The highest constitutive levels of PGC-1alpha were 
found in the heart, whereas the lowest were measured in fast-twitch white muscle 
and liver. T(3) increased PGC-1alpha content similarly in both fast- and 
slow-twitch muscle, as well as in the liver, but not in heart. T(3) also induced 
early (6 h) increases in AMP-activated protein kinase (AMPKalpha) activity, as 
well as later (5 day) increases in p38 MAP kinase activity in slow-twitch, but 
not in fast-twitch, muscle. Contractile activity provoked early increases in 
PGC-1alpha, coincident with increases in mitochondrial transcription factor A 
(Tfam), and nuclear respiratory factor-1 (NRF-1) protein expression, suggesting 
that PGC-1alpha is physiologically important in coordinating the expression of 
the nuclear and mitochondrial genomes. Ca(2+) ionophore treatment of muscle 
cells led to an approximately threefold increase in PGC-1alpha protein, and 
contractile activity induced rapid and marked increases in both p38 MAP kinase 
and AMPKalpha activities. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside 
(AICAR) treatment of muscle cells also led to parallel increases in AMPKalpha 
activity and PGC-1alpha protein levels. These data are consistent with 
observations that indicate that increases in PGC-1alpha protein are affected by 
Ca(2+) signaling mechanisms, AMPKalpha activity, as well as posttranslational 
phosphorylation events that increase PGC-1alpha protein stability. Our data 
support a role for PGC-1alpha in the physiological regulation of mitochondrial 
content in a variety of tissues and suggest that increases in PGC-1alpha 
expression form part of a unifying pathway that promotes both T(3)- and 
contractile activity-induced mitochondrial adaptations.