Protease inhibitors restore radiation-induced apoptosis to Bcl-2-expressing lymphoma cells.
The proteasome pathway is important for the turnover of many regulatory proteins. This pathway has recently become a target for antitumor agents and several research groups have demonstrated that inhibitors with specificities for the proteasome are potent apoptosis-inducing agents. Many mechanisms by which proteasome inhibitors exert their effects have been suggested, including inhibition of NF-kappa B activity and stabilization of the p53 tumor suppressor protein. We investigated the ability of inhibitors with specificities for the proteasome and for another protein degradation enzyme, calpain, to sensitize a murine B-cell lymphoma with constitutive NF-kappa B1 homodimer activity and high expression of Bcl-2 protein to radiation-induced apoptosis. Protease inhibitors tested were calpain inhibitor I, calpain inhibitor II, calpeptin, MG132, and Lactacystin. All five inhibitors induced apoptosis and sensitized cells to radiation despite the maintenance of Bcl-2 protein levels throughout the course of treatment. An electrophoretic migration shift assay for NF-kappa B1 activity provided evidence that reversal of NF-kappa B activity was not required for induction of cell death; however, p53 levels were elevated for all inhibitors tested. HL-60 cells, devoid of p53, could not be sensitized to radiation by MG132 treatment, suggesting that p53 was important for cell death induced by combined treatment with protease inhibitors and radiation. We concluded that protease inhibitors are capable of overcoming the protective effects of Bcl-2 to induce apoptosis and suggest that protease inhibitor treatment, when combined with ionizing radiation, leads to p53-mediated apoptosis.
Kurland
J F
JF
Department of Experimental Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA.
Meyn
R E
RE
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects*; D003546:Cysteine Endopeptidases / Q000502:physiology; D006801:Humans; D008223:Lymphoma / Q000473:pathology / Q000532:radiotherapy*; D051379:Mice; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors* / Q000502:physiology; D016328:NF-kappa B / Q000378:metabolism; D011480:Protease Inhibitors / Q000494:pharmacology*; D046988:Proteasome Endopeptidase Complex; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000032:analysis* / Q000502:physiology; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000032:analysis
D009097:Multienzyme Complexes; D016328:NF-kappa B; D011480:Protease Inhibitors; D019253:Proto-Oncogene Proteins c-bcl-2; D016159:Tumor Suppressor Protein p53; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex