Human THP-1 monocytic leukemic cells induced to undergo monocytic differentiation by bryostatin 1 are refractory to proteasome inhibitor-induced apoptosis

• Published in: Cancer research (Chicago, Ill.) Ill.), 2000-08, Vol.60 (16), p.4377-4385
• Main Authors: CHEN, C, HONG LIN, KARANES, C, PETTIT, G. R, CHEN, B. D
• Format: Article
• Language: English
• Subjects: Ageing, cell death; Antineoplastic Agents - pharmacology; Apoptosis - drug effects; Apoptosis - physiology; Biological and medical sciences; Bryostatins; Caspase 3; Caspase 9; Caspases - metabolism; Caspases - physiology; Cell Differentiation - drug effects; Cell physiology; Cysteine Endopeptidases - pharmacology; Cysteine Proteinase Inhibitors - pharmacology; Cytochrome c Group - secretion; Enzyme Activation - drug effects; Fundamental and applied biological sciences. Psychology; Humans; Lactones - pharmacology; Leukemia, Monocytic, Acute - metabolism; Leukemia, Monocytic, Acute - pathology; Leukemia, Promyelocytic, Acute - blood; Leukemia, Promyelocytic, Acute - metabolism; Leukemia, Promyelocytic, Acute - pathology; Leupeptins - pharmacology; Macrolides; Mitochondria - drug effects; Mitochondria - secretion; Molecular and cellular biology; Monocytes - cytology; Monocytes - drug effects; Monocytes - metabolism; Multienzyme Complexes - antagonists & inhibitors; Multienzyme Complexes - pharmacology; Proteasome Endopeptidase Complex; Proto-Oncogene Proteins c-bcl-2 - metabolism; Ubiquitins - antagonists & inhibitors; Ubiquitins - physiology
• ISSN: 0008-5472; 1538-7445

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.