title: Human THP-1 monocytic leukemic cells induced to undergo monocytic differentiation by bryostatin 1 are refractory to proteasome inhibitor-induced apoptosis.
abstract: The ubiquitin-proteasome pathway is the principal mechanism for the degradation of short-lived proteins in eukaryotic cells. We demonstrated that treatment of THP-1 human monocytic leukemia cells with Z-LLL-CHO, a reversible proteasome inhibitor, induced cell death through an apoptotic pathway. Apoptosis in THP-1 cells induced by Z-LLL-CHO involved a cytochrome c-dependent pathway, which included the release of mitochondrial cytochrome c, activation of caspase-9 and -3, and cleavage of Bcl-2 into a shortened 22-kDa fragment. Induction of apoptosis by protease inhibitor also was detected in U937 and TF-1 leukemia cell lines and cells obtained from acute myelogenous leukemia patients but not in normal human blood monocytes. Treatment of human blood monocytes with Z-LLL-CHO did not induce apoptosis or Bcl-2 cleavage in these cells that rarely proliferate. Interestingly, when THP-1 cells were induced to undergo monocytic differentiation by bryostatin 1, a naturally occurring protein kinase C activator, they were no longer susceptible to apoptosis induced by Z-LLL-CHO. Bryostatin 1-induced differentiation of THP-1 cells was associated with growth arrest, acquisition of adherent capacity, and expression of membrane markers characteristic of blood monocytes. Likewise, differentiated THP-1 cells were refractory to Z-LLL-CHO-induced cytochrome c release, caspase activation, and Bcl-2 cleavage. Resistance to Z-LLL-CHO-induced apoptosis in differentiated THP-1 cells was not due to cell cycle arrest. These findings show that the action of proteasome inhibitors is mediated primarily through a cytochrome c-dependent pathway and induces apoptosis in leukemic cells that are not differentiated.
authors.affiliation: Division of Hematology-Oncology, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
authors.forename: C
authors.initials: C
authors.lastname: Chen
authors.forename: H
authors.initials: H
authors.lastname: Lin
authors.forename: C
authors.initials: C
authors.lastname: Karanes
authors.forename: G R
authors.initials: GR
authors.lastname: Pettit
authors.forename: B D
authors.initials: BD
authors.lastname: Chen
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology; D054713:Bryostatins; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects; D003546:Cysteine Endopeptidases / Q000494:pharmacology; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology*; D003574:Cytochrome c Group / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D007783:Lactones / Q000494:pharmacology*; D007948:Leukemia, Monocytic, Acute / Q000378:metabolism / Q000473:pathology; D015473:Leukemia, Promyelocytic, Acute / Q000097:blood / Q000378:metabolism / Q000473:pathology; D007976:Leupeptins / Q000494:pharmacology*; D018942:Macrolides; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D009000:Monocytes / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D014452:Ubiquitins / Q000037:antagonists & inhibitors* / Q000502:physiology
chemical_list: D000970:Antineoplastic Agents; D054713:Bryostatins; D015853:Cysteine Proteinase Inhibitors; D003574:Cytochrome c Group; D007783:Lactones; D007976:Leupeptins; D018942:Macrolides; D009097:Multienzyme Complexes; D019253:Proto-Oncogene Proteins c-bcl-2; D014452:Ubiquitins; C046785:bryostatin 1; C505407:CASP3 protein, human; C506433:CASP9 protein, human; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde