title: c-Myc proteolysis by the ubiquitin-proteasome pathway: stabilization of c-Myc in Burkitt's lymphoma cells.
abstract: The c-Myc oncoprotein is a transcription factor which is a critical regulator of cellular proliferation. Deregulated expression of c-Myc is associated with many human cancers, including Burkitt's lymphoma. The c-Myc protein is normally degraded very rapidly with a half-life of 20 to 30 min. Here we demonstrate that proteolysis of c-Myc in vivo is mediated by the ubiquitin-proteasome pathway. Inhibition of proteasome activity blocks c-Myc degradation, and c-Myc is a substrate for ubiquitination in vivo. Furthermore, an increase in c-Myc stability occurs in mitotic cells and is associated with inhibited c-Myc ubiquitination. Deletion analysis was used to identify regions of the c-Myc protein which are required for rapid proteolysis. We found that a centrally located PEST sequence, amino acids 226 to 270, is necessary for rapid c-Myc degradation, but not for ubiquitination. Also, N-terminal sequences, located within the first 158 amino acids of c-Myc, are necessary for both efficient c-Myc ubiquitination and subsequent degradation. We found that c-Myc is significantly stabilized (two- to sixfold) in many Burkitt's lymphoma-derived cell lines, suggesting that aberrant c-Myc proteolysis may play a role in the pathogenesis of Burkitt's lymphoma. Finally, mutation of Thr-58, a major phosphorylation site in c-Myc and a mutational hot spot in Burkitt's lymphoma, increases c-Myc stability; however, mutation of c-Myc is not essential for stabilization in Burkitt's lymphoma cells.
authors.affiliation: Department of Cell Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2175, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Gregory
authors.forename: S R
authors.initials: SR
authors.lastname: Hann
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D002051:Burkitt Lymphoma / Q000201:enzymology*; D019556:COS Cells; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D006801:Humans; D051379:Mice; D008938:Mitosis; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism*; D009154:Mutation; D009363:Neoplasm Proteins / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D017384:Sequence Deletion; D014162:Transfection; D014407:Tumor Cells, Cultured; D014452:Ubiquitins / Q000378:metabolism*
chemical_list: D015853:Cysteine Proteinase Inhibitors; D009097:Multienzyme Complexes; D009363:Neoplasm Proteins; D016271:Proto-Oncogene Proteins c-myc; D014452:Ubiquitins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex