Upregulation of CASP genes in human tumor cells undergoingetoposide-induced apoptosis

N Droin1, L Dubrez1, B Eymin1, C Renvoize 2, J Bre ard2, MT Dimanche-Boitrel1 and E Solary1

1CJF INSERM 94-08, Biology and Therapy of Cancer Group, UFR of Medicine, 7, boulevard Jeanne d'Arc, 21000 Dijon and2INSERM U461, U.F.R. of Pharmacy, Chatenay-Malabry, France

Caspases are aspartate-speci®c cysteine proteases thatplay a pivotal role in drug-induced cell death. Wedesigned RT ±PCR assays to analyse the expression ofCASP-3, CASP-4, CASP-6 and the long and shortisoforms of CASP-2 genes in human cells. These genesheterogeneously coexpress in leukemic cell lines and bonemarrow samples from patients with de novo acutemyelogenous leukemia at diagnosis. Treatment of U937and HL60 leukemic cells and HT29 colon carcinomacells with the topoisomerase II inhibitor etoposideupregulates CASP-2 and CASP-3 genes in these cellsbefore inducing their apoptosis. This e�ect of etoposideis not observed in K562 cells and bcl-2-transfected U937cells which are less sensitive to drug-induced apoptosis.Nuclear run-on experiments demonstrate that etoposideincreases CASP gene transcription in U937 cells, ane�ect that is prevented by Bcl-2 overexpression.Upregulation of CASP genes is associated with anenhanced synthesis of related procaspases that precedesthe appearance of apoptosis markers including caspase-3activation, poly(ADP-ribose) polymerase cleavage andinternucleosomal DNA fragmentation. These resultssuggest that the ability of tumor cells to upregulateCASP-2 and CASP-3 genes in response to cytotoxicdrugs could be predictive of their sensitivity to drug-induced apoptosis.

Keywords: apoptosis; caspases; etoposide; humantumor cells

Introduction

Genetic analyses performed in Caenorrhabditis elegansidenti®ed the genes that are necessary for apoptosisduring embryonic development of this nematode: ced-9and a long isoform of ced-4 encode negative regulatorsthat suppress inappropriate cellular suicide while ced-3and a short isoform of ced-4 encode e�ectorcomponents of the cell death pathway (Hentgartnerand Horvitz, 1994a; Shaham and Horvitz, 1996). Themammalian homolog of ced-9 is bcl-2 which cansubstitute functionally for ced-9 (Hentgartner andHorvitz, 1994b). The CED-3 protein demonstratessigni®cant homologies with the interleukin-1b convert-ing enzyme (ICE) family of cysteine proteases, nowreferred as caspases (Yuan et al., 1993; Alnemri et al.,1996). These aspartate-speci®c proteases exist in thecytosol as proenzymes that are proteolytically activated

at internal aspartate residues to form a heterodimericcatalytic domain (Takahashi and Earnshaw, 1996). Celldeath induced by mammalian CED-3-like caspases canbe inhibited by overexpression of Bcl-2 (Chinnaiyan etal., 1996; Meisenholder et al., 1996; Armstrong et al.,1996; Monney et al., 1996) and the Bcl-2-relatedprotein Bcl-XL (Erhardt and Cooper, 1996). While nohomolog of ced-4 has been so far identi®ed inmammals, a CED-4-like protein could mediateprotection by Bcl-2 against cell death induced byCED-3-related proteases (Chinnaiyan et al., 1997; Wuet al., 1997).At least ten caspases have been identi®ed in humans,

most of them inducing apoptosis when overexpressed(Miura et al., 1993; Kumar et al., 1994). At least invitro, some of them are capable of activating others ina manner analogous to the protease zymogens of thecoagulation or complement cascades (Fernandes-Alnemri et al., 1996; Greidinger et al., 1996; Orth etal., 1996). Despite the rapidly growing knowledge ofthe structure, expression and substrate-recognitionproperties of these proteases, it remains uncertainwhether each caspase has a speci®c substrate formediating apoptosis in vivo or if some of them areredundant, cleaving the same substrate. It is note-worthy that mice knocked-out for CASP-1 (ICE) genedevelop normally although their thymocytes demon-strate a speci®c resistance to Fas-mediated apoptosis(Kuida et al., 1995) while CASP-3-de®cient mice donot show any alteration of their thymocyte suscept-ibility to various apoptotic stimuli but show profoundalterations of their brain development and die at 1 ± 3weeks of age (Kuida et al., 1996). These results suggestthat caspase-1 is speci®cally involved in Fas-mediatedcell death while caspase-3 plays a major role in thenormal development of some cells of the brain.The epipodophyllotoxin etoposide (VP-16) is a

topoisomerase II-reactive agent commonly used forthe treatment of various human tumors. This cytotoxicdrug produces DNA double-strand breaks by stabiliz-ing a transient intermediate of the topoisomerasereaction (Pommier, 1997). These DNA damage arethought to be a critical event for etoposide cytotoxicactivity. As many other cytotoxic drugs, etoposide killstumor cells by inducing their apoptosis (Solary et al.,1994). The ability of malignant cells to undergoapoptotic cell death following DNA damage is theultimate determinant of their sensitivity to these drugs(Pommier et al., 1994; Dubrez et al., 1995). We havepreviously identi®ed several steps in the proteolyticpathway that leads to apoptotic DNA fragmentation inhuman leukemic cells treated with etoposide (Dubrez etal., 1996). We observed that a protease sensitive to theCED-3-like caspase tetrapeptide inhibitor DEVDplayed a central role in this pathway. The relation-

Correspondence: E SolaryReceived 1 August 1997; revised 30 December 1997; accepted 30December 1997

Oncogene (1998) 16, 2885 ± 2894 1998 Stockton Press All rights reserved 0950 ± 9232/98 $12.00

http://www.stockton-press.co.uk/onc

ships between the expression of CASP genes in tumorcells and their sensitivity to etoposide-induced apopto-sis remained unknown. This question was addressed inthe present study.

Results

Heterogeneous expression of CASP genes in humanleukemic cell lines

Three groups of caspases have been identi®ed, basedon structural homologies (Alnemri et al., 1996). Wedesigned reverse transcriptase polymerase chain reac-tion (RT ±PCR) assays to study the expression ofCASP genes in tumor cells. The studied CASP geneswere selected in the three caspase groups and included:(1) the long and the short isoforms of CASP-2 geneencoding procaspase-2L (ICH-1L) and procaspase-2S(ICH-1S) respectively (Wang et al., 1994); (2) twoCASP genes encoding CED3-related procaspases,namely CASP-3 encoding procaspase-3 (CPP32/Yama/Apopain) (Fernandes Alnemri et al., 1994;Tewari et al., 1995; Nicholson et al., 1995) andCASP-6 encoding procaspase-6a (Mch-2a) (Fer-nandes-Alnemri et al., 1995a); and (3) CASP-4encoding the ICE-related procaspase-4 (ICH-2/TX/ICErel-II) (Kamens et al., 1995; Faucheu et al., 1995;Munday et al., 1995) (Table 1). Their expression was®rst analysed in ®ve human leukemic cell lines (BV173,HL60, U937, K562 and KCL22 cell lines) inexponential phase of growth without addition of anyapoptosis-inducing agent (Figure 1a and b). Theexpression of each CASP gene was related to the b2microglobulin gene expression measured in parallel inevery cell line. Ethidium bromide staining of theagarose gel after migration of the PCR productsidenti®ed a band of the expected size for each pair ofprimers (Figure 1a). The speci®city of this amplifica-tion product was systematically con®rmed both byhybridizing the generated band with a speci®c 30-meroligonucleotide and by directly sequencing the band ofthe expected size (not shown). CASP-4 gene expressionwas not detected in K562 cells. The expression of theother studied CASP genes was heterogeneous among

Table 1 Primers used for RT±PCR analysis of CASP genes in leukemic cells

Gene Primers Ampli®ed DNA fragment

CASP-2 (Ich-1) sense

antisense5'-AATGTGGAACTCCTCAACTTGCTG-3'5'-TCCATCTTGTTGGTCAACCCC-3'

813 bp

CASP-2L (Ich-1L) sense

antisense

5'-ATGGCCGCTGACAGGGGACGC-3'5'-TCATGTGGGAGGGTGTCCTGGG-3'

1308 bp

CASP-2S (Ich-1S) sense

antisense

5'-ATGCATCCTCATCATCAGGAA-3'5'-TCATAGAGCAAGAGAGGCGGT-3'

939 bp

CASP-3 (CPP32) sense

antisense

5'-ATGGAGAACACTGAAAACTCA-3'5'--TTAGTGATAAAAATAGAGTTC-3'

834 bp

CASP-4 (Tx/Ich-2/ICErel-II) sense

antisense

5'-ATGGCAGAAGGCAACCACAGA-3'5'-TCAATTGCCAGGAAAGAGGTA-3'

1134 bp

CASP-6 (Mch-2a) sense

antisense

5'-ATGAGCTCGGCCTCGGGGCTC-3'5'-TTAATTAGATTTTGGAAAGAA-3'

882 bp

b2 microglobulin sense

antisense

5'-AAGTGGAGCATTCAGACTTGTC-3'5'-CTTACATGTCTCGATCCCACTT-3'

158 bp

CASP-3

CASP-2 L

CASP-2 S

CASP-4

CASP-6

β2

a

BV

173

HL6

0 U

937

K56

2 K

C12

2

b

c

Figure 1 Expression of CASP genes in ®ve human leukemic celllines. Relationships with sensitivity to etoposide-induced apopto-sis. (a and b) The expression of ®ve CASP gene isoforms wasstudied in the ®ve indicated human leukemic cell lines by usingRT±PCR as described in Materials and methods. (a) Ethidiumbromide staining of the PCR ampli®cation products after agarosegel electrophoresis. The obtained signals were compared to theexpression of the b2 microglobulin gene (b2) analysed in parallel.Results shown are one of at least three experiments performedwith di�erent RNA preparations (b) Histograms indicate the ratioof the CASP gene signal related to the b2 microglobulin signalexpressed in arbitrary units. The lack of expression of CASP-4gene in K562 cells was con®rmed by hybridization with a speci®c30-mer oligonucleotide (not shown). (c) Apoptotic DNAfragmentation was measured by using a ®lter elution assay inthe ®ve studied cell lines (BV173: *; HL60: ~; U937: &; K562:*; KCL-22: ~) treated with 100 mM VP16 for the indicatedtimes. Results are the mean+s.d. of three di�erent experimentsperformed in triplicate

Regulation of CASP gene expressionN Droin et al

2886

the tested cell lines (Figure 1a and b). For example, theshort isoform of CASP-2 was sixfold more expressed inK562 cells compared to HL60 cells and the expressionof CASP-6 was 28-fold lower in HL60 cells comparedto KCL22 cells (Figure 1b). These results were highlyreproducible from multiple mRNA preparations. Thesensitivity of the studied cell lines to apoptotic DNAfragmentation induced by continuous exposure to100 mM etoposide was measured by the use of a ®lterelution assay (Figure 1c). These experiments con®rmedthat K562 and KCL22 cell lines were less sensitive toetoposide-induced cell death than the three otherstudied cell lines (Dubrez et al., 1995). No clearrelationship was identi®ed between the pro®le ofexpression of the studied CASP genes and the cellline sensitivity to etoposide-induced apoptosis.

Heterogeneous expression of CASP genes in clinicalbone marrow samples from de novo acute myelogenousleukemias

We checked whether the heterogeneous expression ofCASP genes identi®ed in leukemic cell lines was alsoobserved in bone marrow samples from patients withde novo acute myeloblastic leukemia (AML). Weselected three CASP gene isoforms whose expressionwas determined by RT±PCR ampli®cation in ®fteenAML bone marrow samples, two bone marrowsamples obtained from healthy donors and the humanleukemic cell line U937 (Figure 2a and b). Results wereanalysed on ethidium bromide-stained agarose gel aftermigration of PCR ampli®cation products (Figure 2a)and con®rmed by hybridization with speci®c 30-meroligonucleotides (not shown). The b2 microglobulingene expression was studied in parallel and the ratio ofthe expression of each CASP gene to the expression ofthe b2 microglobulin gene was determined for everysample studied (Figure 2b). These experimentsindicated that the expression of CASP genes was also

heterogeneous in clinical samples from patients withAML. CASP-3 gene was more expressed in U937 cellsand in most AML bone marrow samples studied thanin the two normal bone marrow samples. Conversely,the expression of the short isoform of CASP-2 genewas lower in U937 cells and in a majority of clinicalAML samples compared to normal bone marrow.

Etoposide e�ects on the expression of CASP genes andthe appearance of apoptotic markers in U937 cells

To determine whether the expression of caspases wasregulated at the mRNA level during etoposide-inducedapoptosis, we studied the kinetics of CASP-3, CASP-2Land CASP-2S gene expression in U937 cells treated with50 mM etoposide. The expression of CASP genes eitherremained stable (CASP-2L) or increased (CASP-2S andCASP-3) during the ®rst hours of drug exposure (Figure3a), before the appearance of apoptotic markers.Procaspase-2L protein level decreased while procas-pase-2S accumulated in parallel with the related CASPgene isoform mRNA. Apoptosis markers included theactivation of procaspase-3 whose level decreased whencaspase-3-p19 and caspase-3-p17 active fragmentsappeared simultaneously with the cleavage of the p116PARP protein into a 85 kDa fragment (Figure 3b) andthe appearance of a DNA ladder (Figure 3c). Startingwith the occurrence of these markers, CASP geneexpression decreased progressively until completedisappearance in ethidium bromide-stained agarosegel, 24 h after the beginning of drug exposure, whenthe majority of cells are apoptotic (Dubrez et al., 1995).

Di�erential modulation of CASP gene expression inHL60 and K562 cells

In HL60 cells, which are as sensitive to etoposide-induced apoptosis as U937 cells, continuous exposureto 50 mM etoposide also increased the expression of

M M U 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ade novo AMLs

CASP-3

CASP-2 L

CASP-2 S

β2

b

Figure 2 Expression of CASP genes in clinical bone marrow samples from de novo acute myelogenous leukemias. (a) Ethidiumbromide staining of agarose gels after migration of indicated PCR ampli®cation products obtained from two bone marrow samplesfrom healthy donors (M), the U937 cell line (U) and 15 bone marrow samples from patients with de novo acute myelogenousleukemia at diagnosis. Results shown are one of three experiments performed with di�erent RNA preparations (b) Expression of thestudied CASP genes, expressed as the ratio of the speci®c CASP signal to the b2 microglobulin signal (arbitrary units) in the twonormal bone marrow samples (~), U937 cells (&) and the 15 studied clinical samples (*)

Regulation of CASP gene expressionN Droin et al

2887

both CASP-2S and CASP-3 genes. In this cell line,CASP-2L mRNA level also increased during the ®rsthours of drug exposure (Figure 4). In the K562 cell

line, that was much less sensitive to etoposide-inducedapoptosis as compared to U937 and HL60 cells, theexpression of the three studied CASP gene isoformsremained stable during the ®rst 48 h exposure to100 mM etoposide (Figure 4).

CASP gene expression increase correlates withprocaspase accumulation in the HT29 human coloncarcinoma cell line

Human leukemic cells are cultured as ¯oating cells. Whenexposed to etoposide at concentrations that induceapoptosis, dead cells cannot be easily separated fromnon-apoptotic cells. To compare the expression of CASPgenes and procaspases in non-apoptotic and apoptoticcells, we used the HT29 human colon carcinoma cell line.These cells grow attached in culture ¯asks and detachwhen undergoing apoptotic cell death under etoposideexposure. In attached cells, etoposide exposure induced aprogressive accumulation of CASP-2S, CASP-2L andCASP-3 mRNAs (Figure 5a, left and middle panels).Increased expression of CASP genes in these cellscorrelated with an accumulation of the related proteinsthat reached a maximum between 24 and 48 h of drugexposure (Figure 5a, right panels). In detached cells(Figure 5d), the expression of both CASP genes andprocaspases decreased while the caspase-3-p19 andcaspase-3-p17 active fragments appeared.

E�ects of bcl-2 gene overexpression upon CASP geneexpression in etoposide-treated U937 cells

To further analyse the relationship between cellsensitivity to drug-induced cell death and CASP gene

a

b

c

0 1.5 3 4.5 6 24U937

CASP-2 L

CASP-2 S

CASP-3

β2

3

2

1

0

3

2

1

0

3

2

1

0

0 1.5 3 4.5 6 24

Procaspase-2L

Procaspase-3

Procaspase-2S

Caspase-3-p19Caspase-3-p17

PARP-p116PARP-p85

0 1.5 3 4.5 6 24

CA

SP

gen

e I β

2 (a

rbit

ary

un

its)

Figure 3 Expression of CASP genes, activation of procaspases,PARP cleavage and apoptotic DNA fragmentation in etoposide-treated U937 cells: U937 cells were treated for indicated timeswith 50 mM etoposide. (a) RT±PCR analysis of the expression ofindicated CASP genes. Ethidium bromide-stained PCR amplifica-tion products after agarose gel electrophoresis are shown. b2microglobulin gene expression was used as an internal standard tomeasure the CASP to b2 microglobulin gene expression ratio fromthese gels. Results shown are one of three experiments performedwith di�erent RNA preparations (b) Western blot analysis ofprocaspase-2L and procaspase 2S, procaspase-3 and its cleavageproducts (caspase-3-p19 and caspase-3-p17) and poly(ADP-ribose)polymerase (PARP-p116) and its cleavage product(PARP-p85) in U937 cells. (c) Agarose gel electrophoresis ofDNA obtained from etoposide-treated U937 cells

0 1 2 3 6 24

CASP-3

CASP-2 S

CASP-2 L

β2

3

2

1

0

3

2

1

0

3

2

1

0

CA

SP

gen

e I β

2 (a

rbit

ary

un

its)

3

2

1

0

3

2

1

0

3

2

1

0

HL-600 6 24 48 72

K562

Figure 4 Expression of CASP genes in HL60 and K562 cell linestreated with etoposide: HL60 and K562 cells were treated forindicated times with 50 and 100 mM etoposide, respectively.Ethidium bromide-stained PCR ampli®cation products afteragarose gel electrophoresis are shown. b2 microglobulin geneexpression was used as an internal standard to measure the CASPto b2 microglobulin gene expression ratio from these gels. Resultsshown are onw of three experiments performed with di�erentRNA preparations

Regulation of CASP gene expressionN Droin et al

2888

upregulation, we tested the e�ect of BCL-2 proteinoverexpression on etoposide-mediated CASP genesmodulation. BCL-2 protein has been demonstrated toinhibit apoptosis downstream of speci®c DNA damageinduced by topoisomerase inhibitors (Kamesaki et al.,1993) and upstream of the activation of CED-3-likeprocaspases (Chinnaiyan et al., 1996; Meisenholder etal., 1996). To address the question whether over-expression of BCL-2 could modify CASP genemodulation in etoposide-treated cells, we transfectedbcl-2 gene in the U937 cell line. One of the clones (U4)in which BCL-2 protein was clearly overexpressed(Renvoize et al., 1997) was selected for furtherexperiments. Overexpression of BCL-2 protein signifi-cantly delayed procaspase-3 activation into caspase-3-p19 and caspase-3-p17 fragments, PARP cleavage intoits p85 fragment and apoptotic DNA fragmentationoccurrence (Figure 6b and c). In etoposide-treated U4cells, BCL-2 overexpression prevented both CASP-2Sand CASP-3 mRNA level increase that occurs duringthe ®rst hours of treatment in the parental U937 cells(Figure 6a).

Mechanisms of CASP gene modulation in etoposide-treated U937 cells

Nuclear run-on transcription experiments, performedin etoposide-treated U937 cells, demonstrated that thetranscription of the short isoform of the CASP-2 genewas about sixfold increased when studied 4.5 h aftercell exposure to 50 mM etoposide. Transcription of twoother studied CASP genes, namely CASP-3 andCASP-6, also increased, although more slightly, inresponse to etoposide exposure (Figure 7a). This

treatment did not modify a-actin and Bcl-XL genetranscription (Figure 7a). The same experiments wereperformed in the U4 clone that overexpresses BCL-2protein. These experiments demonstrated that BCL-2overexpression prevented CASP-2s and CASP-3upregulation in these cells (Figure 7b).The permanent peptides Z-VAD-CH2F and Z-

DEVD-CH2F, which are homologous to sequencesspeci®cally targeted by ICE-like caspases and CED-3-like caspases, respectively, both prevented etoposide-induced apoptotic DNA fragmentation and PARPcleavage in etoposide-treated U937 cells (Dubrez etal., 1996). Both Z-VAD and Z-DEVD peptides alsoe�ciently suppressed CASP gene expression decreaseassociated with the cell death process in etoposide-treated U937 cells (Figure 8).

Discussion

The basal level of several CASP mRNAs was analysedfor the ®rst time in a series of human leukemic celllines and bone marrow samples from patients with denovo AML. It was previously shown that human CASPgenes co-expressed in a variety of tissues (Wang et al.,1994). These genes are more expressed in embryoniccompared to adult tissues and are poorly expressed inadult brain compared to any other adult tissue(Fernandes-Alnemri et al., 1994, 1995b, 1996; Kamenset al., 1995; Munday et al., 1995; Duan et al., 1996;Duan and Dixit, 1997). In the human HL60 leukemiccell line, RT ±PCR identi®ed transcripts for at leastnine di�erent caspases (Martins et al., 1997). Here, wecon®rm coexpression of CASP genes from various

A

D

A

D

A

D

Procaspase-3

Caspase-3-p19 Caspase-3-p17Procaspase-3

Caspase-3-p19 Caspase-3-p17

Procaspase-2L

Procaspase-2S

A

D

A

D

A

D

A

D

0 6 24 48 720 3 6 24 48 72

β2

CASP-3

CASP-2L

CASP-2S

Figure 5 Expression of CASP genes and procaspases in etoposide-treated HT29 cells: HT29 human colon carcinoma cells weretreated for indicated times with 50 mM etoposide. Left panels: RT ±PCR analysis of the expression of indicated CASP genes inattached (A) and detached (D) cells. Ethidium bromide-stained PCR ampli®cation products after agarose gel electrophoresis areshown. Middle panels: b2 microglobulin gene expression was used as an internal standard to measure the CASP to b2 microglobulingene expression ratio from these gels. Results shown are one of two experiments performed with di�erent RNA preparations. Rightpanels: Western blot analysis of procaspase-2L, procaspase 2S and procaspase-3 and its cleavage products (caspase-3-p19 andcaspase-3-p17) in attached (A) and detached (D) etoposide-treated HT29 cells

Regulation of CASP gene expressionN Droin et al

2889

groups of caspases in tumor cells. We show that thebasal mRNA level of the tested CASP genes is very

heterogeneous from one cell line to another and fromone patient to another. The expression of each CASPgene seems to be independent of that of other CASPgenes since no correlation was observed between thema

0 1.5 3 4.5 6 24U4

b

c

CASP-2 L

CASP-2 S

CASP-3

β2

3

2

1

0

3

2

1

0

3

2

1

0

0 1.5 3 4.5 6 24

Procaspase-2L

Procaspase-3

Procaspase-2S

Caspase-3-p19Caspase-3-p17

PARP-p116PARP-p85

0 1.5 3 4.5 6 24

CA

SP

gen

e I β

2 (a

rbit

ary

un

its)

Figure 6 Expression of CASP genes, activation of procaspases,PARP cleavage and apoptotic DNA fragmentation in etoposide-treatedU4 cells:U4 cells, that overexpressBcl-2 protein,were treatedfor indicated times with 50 mM etoposide. (a) RT±PCR analysis ofthe expression of indicated CASP genes. Ethidium bromide-stainedPCR ampli®cation products after agarose gel electrophoresis areshown. b2 microglobulin gene expression was used as an internalstandard to measure the CASP to b2 microglobulin gene expressionratio from these gels. Results shown are one of three experimentsperformed with di�erent RNA preparations. (b) Western blotanalysis of procaspase-2L and procaspase 2S, procaspase-3 and itscleavage products (caspase-3-p19 and caspase-3-p17) andpoly(ADP-ribose)polymerase (PARP-p116) and its cleavage pro-duct (PARP-p85) inU4 cells. (c)Agarose gel electrophoresis ofDNAobtained from etoposide-treated U4 cells

CASP-2 S

CASP-3

actin

CASP-6

bcl-X L

U937 U4

0 4.5 0 4.5a b

Figure 7 Transcription of CASP genes in etoposide-treatedU937 and U4 cell: Nuclear run-on assay measuring thetranscription of indicated genes in untreated (left panels) andetoposide-treated (right panels, 50 mM for 4.5 h) U937 (a) and U4(b) cells

CASP-3

CASP-2

β2

a

b

VP-16 VP-16 +

DEVD

VP-16 +

zVAD

0 1.5

3 4.5

6 12

24 6 12

24 6 12

24

Figure 8 In¯uence of tetrapeptide inhibitors Z-VAD and Z-DEVD upon etoposide-induced CASP genes modulation in U937cells. (b) U937 cells were treated with 50 mM etoposide for theindicated times (hours) in the absence (&) or in the presence of Z-VAD-CH2F (*) or Z-DEVD-CH2F (~) permanent inhibitors.(a) Ethidium bromide-stained PCR ampli®cation products afteragarose gel electrophoresis. b2 microglobulin gene expression wasused as a control. (b) CASP gene to b2 microglobulin gene ratiowas measured from the gels shown on a. Results shown are one oftwo experiments performed with di�erent RNA preparations

Regulation of CASP gene expressionN Droin et al

2890

in the ®fteen tested clinical samples and the ®veleukemic cell lines.

CASP-3 gene, that encodes a pro-apoptotic protease(Fernandes-Alnemri et al., 1994; Tewari et al., 1995;Nicholson et al., 1995), was generally more expressedin fresh leukemic cells and U937 cells compared tonormal bone marrow samples. Conversely, the shortisoform of CASP-2, that encodes an anti-apoptoticprotein (Wang et al., 1994), was less expressed inleukemic cells than in normal cells. Among the fourgenes whose expression was analysed in leukemic celllines, only one, CASP-4, was not detected in one cellline, K562. The low sensitivity of K562 cells to avariety of apoptotic stimuli has been previously relatedto bcr ± abl overexpression (McGahon et al., 1994). Byanalogy with thymocytes from CASP-1-null micewhich show defective Fas-mediated apoptosis (Kuidaet al., 1995), the lack of CASP-4 gene expression couldcontribute to the low sensitivity of K562 cells toapoptotic stimuli. Actually, no clear relationship wasidenti®ed between the ability of leukemic cell lines toundergo apoptosis when treated with etoposide and thebasal expression of the studied CASP genes in thesecells. Several reasons could account for this lack ofcorrelation. First, leukemic cell sensitivity to drug-induced cell death could be regulated upstream ofprocaspase activating cleavage, for example by theBCL-2 family of proteins (Chinnayian et al., 1996;Meisenholder et al., 1996; Armstrong et al., 1996;Monney et al., 1996) and other oncogenes (McGahonet al., 1994; Chapman et al., 1994). Second, functionalredundancy in the substrate recognition of caspasescould protect the cells from the consequences of thedeletion of one or several CASP genes (Kumar et al.,1994). For example, PARP can be cleaved in vitro by avariety of caspases including caspase-1, -2, -3, -4, -6, -7and -8 although cleavage by caspase-1, -2 and -4appears to require high enzyme concentrations(Kaufmann et al., 1993; Nicholson et al., 1995;Fernandes-Alnemri et al., 1995b; Gu et al., 1995a).Third, heterodimerization between pro-apoptotic andanti-apoptotic isoforms of caspases (Gu et al., 1995b);alternative processing of the proenzymes (Alnemri etal., 1995) and/or posttranslational modi®cations of theprocessed subunits Martins et al., 1997) rather thanCASP gene mRNA level could determine whether acell live or die by apoptosis in response to a givenstimulus. For example, two-dimensional analysisidenti®ed multiple forms of procaspase-3 duringetoposide-induced apoptosis in the human leukemiccell line HL60 (Martins et al., 1997).Upregulation of the Nedd2 gene, the mouse CASP-

2S gene, has been recently described in an in vivomodel of neuronal cell death (Kinoshita et al., 1997).The present study suggests a relationship betweenenhanced expression of several CASP genes and cellsensitivity to etoposide-induced apoptosis. In U937cells, upregulation of CASP-2S and CASP-3 mRNA isrelated to an increased transcription of these genes thatprecedes the appearance of apoptotic markers. Thisupregulation is prevented by BCL-2 protein over-expression. BCL-2 (Chinnayian et al., 1996) wasshown to delay etoposide-induced apoptotic cell deathby preventing the activation of CED-3-like caspases.Our results suggest that this protein could also preventthe activation of one or several transcriptional factors

that regulate the expression of CASP and otherapoptosis-related genes. A role for transcriptionalfactors in apoptosis regulation has been suggestedboth in the nematode Caenorrhabditis elegans(Metzstein et al., 1996) and in mammals (Inaba etal., 1996). The process of cell death induced by the p53tumor suppressor was shown to activate eight cDNAsin the mouse M1 myloid leukemia cell line (Amson etal., 1996). We reported recently that increasedtranscription of the gadd153 gene, observed in etopo-side-treated U937 cells undergoing apoptosis, wasprevented by overexpression of BCL-2 (Eymin et al.,1997). A good correlation was found between theextent of cellular injury and treatment-induced increasein gadd153 mRNA level (Gately et al., 1996). Ourresults suggest that the ability of tumor cells toupregulate the expression of CASP genes in responseto a cytotoxic drug could be predictive of theirsensitivity to this drug.The cell death process itself is associated with a

downregulation of CASP gene expression. In etopo-side-treated U937 cells, the permeant peptides Z-VAD-CH2F and Z-DEVD-CH2F prevent apoptotic DNAfragmentation and PARP cleavage (Dubrez et al.,1996; Slee et al., 1996). These peptides suppress CASP-mRNA degradation associated with the cell deathprocess without inhibiting CASP-3 gene upregulationin etoposide-treated U937 cells. This observationsuggests that factors that increase the transcription ofsome CASP genes in etoposide-treated cells are notactivated by caspases.Multiple procaspases are present in human tumor

cells prior to the application of an apoptotic stumulus.This stimulus could either activate proteases frompreexisting proenzymes or trigger de novo synthesis ofadditional procaspases. Whether increased transcrip-tion of some CASP genes and the accumulation of therelated procaspases are necessary for the apoptoticprocess to occur remains to be demonstrated.Treatment of HL60 cells with concentrations ofcycloheximide and puromycin that inhibit mostprotein synthesis induces apoptotic cell death, suggest-ing that de novo protein synthesis is not alwaysnecessary for apoptosis to occur (Martins et al.,1977). In U937 cells treated with etoposide, weobserved an accumulation of procaspase-2S thatpreceded the appearance of apoptosis markers. Bythe use of the HT29 cell line that grows attached to theplastic ¯ask and detach when undergoing apoptosis, wewere able to separate pre-apoptotic from apoptoticcells. Etoposide-treated HT29 cells accumulated bothCASP genes and caspase proenzymes before detachingfrom the ¯ask. Interestingly, procaspase-2S, thatencodes an antiapoptotic protein, was upregulated inboth U937 and HT29 cells. One hypothesis is that, inresponse to apoptotic stimuli, cells accumulate caspasesthat are neutralized by heterodimerization of proapop-totic with antiapoptotic subunits until the start of theapoptotic process.Altogether, the present study points out the

heterogeneous expression of CASP genes in humantumor cells and suggests that cells that are sensitive toetoposide-induced cell death upregulate CASP-2 andCASP-3 gene expression and synthesize the corre-sponding procaspases before showing signi®cantapoptotic proteolysis and DNA fragmentation.

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Materials and methods

Drug and chemicals

[2-14C]thymidine (50 mCi/mmol) was obtained from Amer-sham (Les Ulis, France). The tetrapeptide inhibitors Z-Val-Ala-Asp-CH2F (Z-VAD) and Z-Asp-Glu-Val-Asp-CH2F(Z-DEVD) were synthetized by Enzyme System Products(Dublin, CA). Oligonucleotides were synthetized byEurogentec (Angers, France). Etoposide and other chemi-cals were obtained from Sigma-Aldrich laboratories (StQuentin Fallavier, France).

Cells, cell culture and transfection

The human leukemic cell lines BV173, HL60, U937, K562and KCL22 were grown in suspension in RPMI 1640medium (BioWhittaker, Fontenay-sous-bois, France) sup-plemented with 10% (vol/vol) heat-inactivated fetal bovineserum and 2 mM L-glutamine in an atmosphere of 95% airand 5% CO2. Cell viability was determined using thetrypan blue exclusion assay. Etoposide stock solution(20 mM), prepared by diluting the drug in dimethylsulphoxide (DMSO), was conserved at 7208C for lessthan 1 month. The ®nal concentration of DMSO in theculture medium never exceed 1% (vol/vol), which was nontoxic to the cells. Cells were resuspended at a density of1.56106/ml in fresh medium before treatment. Bonemarrow leukemic cells were obtained with informedconsent from patients with de novo acute myelogenousleukemia at diagnosis, before initiation of any treatment.Samples were sent by overnight mail for immediate RNAextraction and cDNA preparation. Normal bone marrowwas obtained from healthy donors with informed consent.Blast cells and normal bone marrow mononuclear cellswere isolated on ®coll-hypaque (Eurobio, France).

Reverse transcriptase-polymerase chain reaction

RT ± PCR was performed as previously described (Koosand Seidel, 1989). Total RNA was isolated with RNAPlusTM (Bioprobe Systems, Montreuil-sous-bois, France)from either di�erent leukemic cell lines treated or not withetoposide or bone marrow samples from patients withAML. RNA was reverse transcribed by M-MLV reversetranscriptase (Gibco, Cergy Pontoise, France) with randomprimers (pdN6, Boerhinger-Manheim, Germany). Senseand anti-sense primers used to amplify human CASP andb2 microglobulin genes as well as the size of ampli®edproducts are indicated in Table 1. For each studied gene,primers were located in separate exons in genomic DNA sothat we could eliminate the possibility of genomic DNAcontamination. The following conditions were used for thePCR reactions: 16reaction bu�er (Eurobio, Les Ulis,France), 1.5 ± 2.5 mM MgCl2 optimized according to eachpair of primers, 200 mM dNTP, 1 mM of each primer and2.5 U of Taq DNA polymerase (Eurobio, Les Ulis, France)in a total volume of 50 ml. DNA was denatured for 1 minat 948C prior to 29 ± 38 PCR cycles, depending of theprimer used. Ampli®cation was carried out on a PTC 150(Prolabo, Fontenay-sous-bois, France). The PCR productswere analysed on 1.2% agarose gels. The ampli®ed DNAwas stained with ethidium bromide and the ¯uorescenceintensity was detected by video camera and measured usingPhoretix 1D software (Phoretix-International, Newcastle,England). All the results were normalized with theexpression of the b2 microglobulin gene.

Nuclear run-on assay

The relative transcription rates of CASP-2, CASP-3 andCASP-6 genes was studied in etoposide-treated U937 cells

by the use of a previously described method (Srivastava etal., 1993). Brie¯y, untreated and etoposide-treated (50 mMfor 3 h) U937 cells were lysed in a lysis bu�er (Tris-HCl,10 mM, pH 7.4, NaCl 10 mM, MgCl2 3 mM NP-40 0.5%)and nuclei were harvested by centrifugation. For run-onreactions, 200 ml of a 26 reaction mixture (Tris-HCl,10 mM, pH 8.0, KCl 300 mM, MgCl2 5 mM, ATP 1 mM,CTP 1 mM, GTP 1 mM, dithiothreitol 5 mM) and 100 mCiof [32P]UTP were added to 16107 nuclei and incubated at258C for 25 min. RNA was extracted by mixing the nuclearsuspension with 400 ml of RNAzol and equal counts perminute were hybridized with appropriate cDNA onnitrocellulose at 658C for 36 h before autoradiography.

Quanti®cation of DNA fragmentation

DNA fragmentation was measured using a previouslyreported ®lter elution assay (Solary et al., 1993). Exponen-tially growing cells were pre-labelled by adding 0.02 mCi/mlof [2-14C]thymidine in the culture medium for 2 days. Then,cells were chased in isotope-free medium overnight.Approximately 1.06106 14C-labelled cells or 14C-labellednuclei were loaded onto a protein absorbing ®lter(Polyvinylidene ¯uorure ®lters, 0.65 mm pore size, 25 mmdiameter; Durapore membrane, Millipore, St Quentin,France). Cells or nuclei were then washed once with 5 mlof ice-cold PBS. Lysis was subsequently performed with5 ml of LS10 bu�er (0.2% sodium sarkosyl, 2 M NaCl,0.04 M EDTA, pH 10.0). Filters were washed with 7 ml of0.02 M EDTA, pH 10. DNA was depurinated by adding0.4 ml of 1N HCl at 658C for 45 min, then released from the®lters by adding 2.5 ml of 0.4 N NaOH for 45 min at roomtemperature. Radioactivity was counted by liquid scintilla-tion spectrometry in each fraction (wash, lysis, EDTA washand ®lter). DNA fragmentation was measured as thefraction of d.p.m. in the lysis fraction plus EDTA washrelative to the total intracellular d.p.m.

Analysis of DNA fragmentation by agarose gel electrophoresis

Cellular DNA from whole cells or isolated nuclei wasextracted by a salting-out procedure as described pre-viously (Miller et al., 1988). Electrophoresis was performedin a 1.8% agarose gel in Tris-borate-EDTA bu�er (pH 8)at 20 Volts for 15 h. After electrophoresis, DNA wasvisualised by ethidium bromide staining.

Western blot analysis

After treatment, cells were washed twice in PBS, lysed in lysisbu�er (150 mM NaCl, 1 mM KH2PO4, 1 mM EGTA, 1 mM

Na3VO4, 5 mM MgCl2 and 10% glycerol containing PmsF0.1 mM, Aprotinin 0.15 U/ml, Pepstatin 1 mg/ml) andcentrifugated for 15 min at 15 000 g. Fifty mg proteins ofsupernatants (Smith et al., 1985) were separated by SDS ±PAGE using a 12% polyacrylamide gel and electroblotted toPVDF membrane (BioRad, Ivry-sur-Seine, France). Afterblocking non-speci®c binding sites overnight by 5% nonfatmilk in TPBS (PBS, Tween 20 0.1%), the membrane wasincubated for 2 h at room temperature with anti-humanprocaspase-2 monoclonal antibody that recognizes both thelong and the short isoforms of the protein (Transductionlaboratories, Lexington, KY), washed twice with TPBS,further incubated with horseradish peroxidase-conjugatedgoat anti-mouse antibody (Jackson ImmunResearch Labora-tories, West Grove, PE) for 30 min at room temperature andwashed twice with TBPS. Immunoblot was revealed usingenhanced chemioluminescence detection kit (Amersham, LesUlis, France) by autoradiography. For PARP and procas-pase-3 cleavage analyses, cells were lysed in SDS ± PAGEsample bu�er (125 mM Tris-HCl, pH 6.8, 10% b-mercapto-ethanol, 4.6% SDS, 20% glycerol and 0.003% bromophenol

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blue) and cell lysates were subjected to SDS ± PAGE on a 8%polyacrylamide gel. The 116 kDa native PARPprotein and its85 kDa cleavage product were detected by immunoblottingwith anti-human PARP polyclonal antibody (Vic. 5, kindlygiven by Dr G De Murcia) and horseradish-peroxidaseconjugated anti-rabbit antibody (Amersham, Les Ulis,France) as described above. A rabbit polyclonal anti-caspase-3-p17 antibody (kindly provided by Dr D Nichol-son) that recognizes both procaspase-3 and its p19 and p17subunits (Schlegel et al., 1996) was used to detect procaspase-3 activation in apoptotic U937 cells.

AcknowledgementsThe authors wish to thank Sylvie Chevillard, Patricia deCre moux and the GOELAMS group for providing cDNAfrom clinical samples, David Nicholson and Gilbert DeMurcia for kindly providing anti-CPP32 and anti-PARPantibodies. We thank also FrancË ois Martin and CarmenGarrido for their critical reading of the manuscript. Thiswork was supported in part by grants by ARC (Grant4075, ES), the Burgundy, Saoà ne et Loire and YonneComittees of the Ligue Nationale Contre le Cancer and theConseil Re gional de Bourgogne.

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