title: Evidence for involvement of calpain in c-Myc proteolysis in vivo.
abstract: Precise control of the level of c-Myc protein is important to normal cellular homeostasis, and this is accomplished in part by degradation through the ubiquitin-proteasome pathway. The calpains are a family of calcium-dependent proteases that play important roles in proteolysis of some proteins, and their possible participation in degradation of intracellular c-Myc was therefore investigated. Activation of calpain with the cell-permeable calcium ionophore A23187 in Rat1a-myc or ts85 cells in culture induced rapid cleavage of c-Myc. This degradation was both calpain- and calcium-dependent since it was inhibited by preincubation with either the calpain-inhibitory peptide calpeptin or the calcium-chelating agent EGTA. A23187-induced c-Myc cleavage occurred in a time-dependent manner comparable to that of FAK, a known calpain substrate, and while calpeptin was able to significantly protect c-Myc from degradation, inhibitors of the proteasome or caspase proteases could not. Exposure of Rat1a-myc or ts85 cells in culture to calpeptin, or to the thiol-protease inhibitor E64d, resulted in the accumulation of c-Myc protein without an impact on ubiquitin-protein conjugates. Using an in vitro assay, calpain-mediated degradation occurred rapidly with wild-type c-Myc as the substrate, but was significantly prolonged in some c-Myc mutants with increased transforming activity derived from lymphoma patients. Those mutants with a prolonged half-life in vitro were also more resistant to A23187-induced cleavage in intact cells. These studies support a role for calpain in the control of c-Myc levels in vivo, and suggest that mutations impacting on sensitivity to calpain may contribute to c-Myc-mediated tumorigenesis.
authors.affiliation: The Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
authors.forename: George W
authors.initials: GW
authors.lastname: Small
authors.forename: Teh-Ying
authors.initials: TY
authors.lastname: Chou
authors.forename: Chi V
authors.initials: CV
authors.lastname: Dang
authors.forename: Robert Z
authors.initials: RZ
authors.lastname: Orlowski
mesh_terms: D000818:Animals; D015153:Blotting, Western; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002154:Calpain / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002460:Cell Line; D002471:Cell Transformation, Neoplastic / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004151:Dipeptides / Q000378:metabolism / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D006801:Humans; D007476:Ionophores / Q000494:pharmacology; D008223:Lymphoma / Q000378:metabolism*; D008325:Mammary Neoplasms, Experimental / Q000378:metabolism*; D051379:Mice; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D009154:Mutation; D010447:Peptide Hydrolases / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D051381:Rats
chemical_list: D002614:Chelating Agents; D004151:Dipeptides; D004791:Enzyme Inhibitors; D007476:Ionophores; D009097:Multienzyme Complexes; D016271:Proto-Oncogene Proteins c-myc; C071482:calpeptin; D000001:Calcimycin; D011505:Protein-Tyrosine Kinases; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; C496050:PTK2 protein, human; C496051:Ptk2 protein, mouse; C496052:Ptk2 protein, rat; D010447:Peptide Hydrolases; D002154:Calpain; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D002118:Calcium