We have previously shown that the common feature of both pressure overload-induced
hypertrophy and
atrophy is a reactivation of the fetal gene program. Although gene expression profiles and signal transduction pathways in pressure overload
hypertrophy have been well studied, little is known about the mechanisms underlying atrophic remodeling of the unloaded heart. Here, we induced atrophic remodeling by
heterotopic transplantation of the rat heart. The activity parameters of three signal transduction pathways important in
hypertrophy, i.e.
mitogen-activated
protein (MAP)
kinase,
mammalian target of rapamycin (mTOR), and
Janus kinase/signal transducers and activators of transcription (JAK/STAT), were interrogated. Gene expression of upstream stimuli--
insulin-like growth factor 1 (IGF-1) and
fibroblast growth factor 2 (FGF-2)--and metabolic correlates, i.e.
peroxisome proliferator-activated receptor-alpha (
PPARalpha) and
PPARalpha-regulated genes, of these pathways were also measured. In addition, we measured transcript levels of genes known to regulate skeletal muscle
atrophy, all of which are negatively regulated by
IGF-1 (Mafbx/Atrogin-1, MuRF-1). Atrophic remodeling of the heart was associated with increased expression of
IGF-1 and
FGF-2. Transcript levels of the
nuclear receptor PPARalpha were decreased, as were the levels of
PPARalpha-regulated genes. Furthermore, there was phosphorylation of ERK1, STAT3, and
p70S6K with unloading. Consistent with the increase in
IGF-1, we found a decrease in Mafbx/Atrogin-1 and MuRF-1 transcript levels.
Rapamycin administration at 0.8 mg/kg/day for 7 days resulted in enhanced
atrophy and attenuated the phosphorylation of ERK1, STAT3, and
p70S6K without altering gene expression. We conclude that there is significant crosstalk between the mTOR, MAP
kinase, and JAK/STAT signaling cascades. Furthermore,
ubiquitin ligases, known to be essential for skeletal muscle
atrophy, decrease in unloading-induced cardiac
atrophy.