Blood, Vol. 94 No. 5 (September 1), 1999:
pp. 1827-1828
CORRESPONDENCE
A Combination of Retinoic Acid and Proteasome Inhibitors for the
Treatment of Leukemias Is Potentially Dangerous
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LETTER |
To the Editor:
Retinoic acid (RA) is used in the treatment of leukemias; however, the
phenomenon of RA resistance is often the cause of relapse in some
patients. In a recent report, Fanelli et al1 showed, using
an acute promyelocytic leukemia-derived cell line NB4, that the
treatment of RA-resistant cells with proteasome inhibitors restores RA sensitivity.
Proteasome inhibitors are already being tested in phase I of clinical
trials as anticancer agents2 and are proposed to be useful
in the treatment of chemoresistant and radioresistant leukemias.3 One might speculate that combination therapy of RA and proteasome inhibitors can be advantageous and may potentially overcome RA-resistant cases. A selective inhibitor of the proteasome [PSI;
N-benzyloxycarbonyl-Ile-Glu-(O-t-butyl)-Ala-leucinal]
induces apoptosis of murine lymphocytic leukemia L1210
cells,4 while RA treatment inhibits their
proliferation,5 abolishes clonogenicity, and causes G1
arrest.6 L1210 is sensitive to RA, regardless of the fact
that it does not express PML/RAR
or RA binding
proteins.5,7
We pretreated L1210 cells for 72 hours with either a control solvent
(dimethyl sulfoxide [DMSO]) or 5 µmol/L and 50 µmol/L RA. Cells
were then further treated for 24 hours with a combination of those
agents and 500 nmol/L PSI or DMSO alone as control. Treated cells were
fixed in 70% ethanol, stained with acridine orange, and analyzed using
a FACSCalibur flow cytometer (Becton Dickinson, San Diego,
CA).8
We have found that treatment with 5 µmol/L RA, which by itself is not
cytotoxic, has a cytotoxic effect when combined with 500 nmol/L PSI
(Table 1). However, a combination of 50 µmol/L RA, which is by itself cytotoxic, with 500 nmol/L PSI
surprisingly diminishes the cytotoxic effects on L1210 cells, allowing
a survival of 21.4% of the cells compared with 12.2% for PSI alone
(
2 P < .0001). Moreover, while RA or PSI
alone caused a G0/G1 block, this effect was partially abolished by a
combination of both drugs (Table 2). PSI or
5 µmol/L RA treatment did not increase granulosity of the cells, as
judged by the flow cytometric SSC-H parameter, while 50 µmol/L RA
increased mean granulosity by 62%. The combination of 50 µmol/L RA
and PSI increased mean granulosity by 50%.
View this table:
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Table 1.
Percentages of Apoptotic, Necrotic, and Living L1210
Cells, After 72 Hours' Pretreatment With Either a Control Solvent or
RA at the Indicated Concentrations, Followed by a 24-Hour PSI
or Control Treatment
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View this table:
[in this window]
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Table 2.
Percentages of L1210 Cells in Different Phases of the
Cell Cycle, After 72 Hours' Pretreatment With Either a Control
Solvent or RA at the Indicated Concentrations, Followed by a
24-Hour PSI or Control Treatment
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Although 5 µmol/L RA does not induce differentiation
it causes only
a moderate G1 phase block or retardation50 µmol/L RA treatment most
probably induces differentiation of a fraction of L1210 cells, as
judged by increased granulosity, and these cells became resistant to
proteasome inhibition. Although proteasome inhibitors cause apoptosis
of proliferating cells,4 they prevent apoptosis of some
differentiated cells, such as neurons9 or thymocytes.10 Our results suggest that any attempt to treat leukemia with a combination of RA and proteasome inhibitors should be
made with great caution. It potentially allows the survival of cells,
which after the withdrawal of the drugs can cause a fast relapse of the
leukemic disease, regardless of the presence of the PML/RMR fusion protein.
| |
ACKNOWLEDGMENT |
Supported by KBN Grant No. 4 P05A 084 16.
Cezary Wójcik
Department of Histology and Embryology
Institute
of Biostructure
Medical University of Warsaw
Warsaw,
Poland
Izabella M
ynarczuk
Department of Histology and
Embryology
Department of Immunology
Institute of Biostructure
Medical University of Warsaw
Warsaw, Poland
Grazyna Hoser
Jerzy Kawiak
Department of Clinical Cytology
Postgraduate Center of Medical Instruction
Warsaw, Poland
Tomasz Stokosa
Jakub Goab
Department of Immunology
Institute of Biostructure
Medical University of Warsaw
Warsaw,
Poland
Sherwin Wilk
Department of Pharmacology
Mount
Sinai Medical Center
New York, NY
| |
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