Treatment of cultured myotubes with the calcium ionophore A23187 increases proteasome activity via a CaMK II-caspase-calpain-dependent mechanism.
BACKGROUND
Previous studies suggest that increased ubiquitin-proteasome-dependent protein breakdown in various muscle-wasting conditions may at least in part be mediated by increased cellular calcium levels. The role of calcium in the regulation of proteasome activity, however, is not well understood.


METHODS
We treated cultured L6 myotubes with the calcium ionophore A23187 or thapsigargin, substances that increase intracellular calcium levels through different mechanisms, and measured proteasome activity by determining the degradation of the fluorogenic substrate LLVY-AMC.


RESULTS
Treatment of the myotubes with A23187 or thapsigargin resulted in a dose- and time-dependent increase in proteasome activity. When the myotubes were treated with metabolic inhibitors, results suggested that the A23187- and thapsigargin-induced activation of proteasome activity was at least in part regulated by calmodulin, calcium calmodulin-dependent kinase II, caspases, and calpains.


CONCLUSIONS
The present observations support a role of calcium in the regulation of proteasome-dependent protein breakdown in skeletal muscle and may explain why muscle wasting was reduced by calcium antagonists in previous studies.
Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
Michael J
MJ
Menconi
Wei
W
Wei
Hongmei
H
Yang
Curtis J
CJ
Wray
Per-Olof
PO
Hasselgren
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology*; D002154:Calpain / Q000502:physiology*; D020169:Caspases / Q000502:physiology*; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000502:physiology*; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D009097:Multienzyme Complexes / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D051381:Rats
D009097:Multienzyme Complexes; D000001:Calcimycin; D007770:L-Lactate Dehydrogenase; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002154:Calpain; D020169:Caspases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D002118:Calcium