The molecular basis of skeletal muscle atrophy--parallels with osteoporotic signaling.

does not appear to accompany muscle unloading since neither AP-1 nor NFAT activity is activated 5. JNK activity and TNF levels are unchanged in unloaded muscles further supporting a lack of an inflammatory response. Lastly, there is no evidence of complement activation in unloaded muscles 8. Results from immunoblotting of nuclear extracts and gel supershift assays suggested that the Rel proteins p50 and c-Rel could be involved in the activation of NF-kB, as well as an IkB-like family member which acts as a co-transactivator called Bcl-3. Knockout mice for each one of these three genes showed that p50 and Bcl-3 were required for the unloading-induced activation of NF-kB and muscle atrophy 7 , but c-Rel was not 6. Using mouse Affymetrix microarrays, we have been able to identify potential p50 and Bcl-3 target genes by comparing the response to unloading in muscles from wild type vs. p50 or bcl3 knockout mice (unpublished data). As expected more genes were upregulated by muscle unloading in wild type vs. knockout mice since the muscles from knockout mice atrophy only very minimally. Thus in knockout mice, the lack of change in a gene normally upreg-ulated by unloading suggests that it is a p50 or Bcl-3 target gene. The major upstream regulator of Rel proteins, IkBalpha, was then studied for its involvement in atrophy. Because knockout mice are embryonic lethal, we overexpressed a form of IkBalpha, which acts to inhibit classical NF-kB sig-naling; this mutant protein is called the IkB super repressor. It is missing amino acids 1-36 so it cannot be phosphorylated by upstream NF-kB kinases thereby blocking its degradation by the proteasome and blocking its release of bound Rel dimers which would otherwise localize to the nucleus to activate NF-kB target genes. Gene transfer of the IkB super repressor, fused to EGFP, into rat soleus muscles resulted in a 40% inhibition of muscle atrophy and a complete inhibition of NF-kB reporter gene due to 7 days of unloading 6. The unloading-induced increase in ubiquitinated protein was Hylonome Multiple triggers can lead to concomitant loss of bone and muscle including reduced biomechanical loading, aging, and systemic inflammatory conditions (e.g., cancer, rheumatoid arthritis, etc.). Given the in vivo physiological interdependence and the developmental relationships between muscle and bone it is not surprising to find at least one pathway that regulates both bone and muscle loss. It appears that nuclear factor of kappaB (NF-kB) signaling is …