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Experimental Therapeutics |
Cell and Molecular Biology Section, Pediatric Oncology Branch [J. J., J. W., H. H., C. K., C. J. T.] and the Experimental Therapeutics Branch [R. R., S. E. B.], Center for Cancer Research, National Cancer Institute, Bethesda, Maryland 20892, and Department of Pathology, Seoul National University of Medicine, Seoul, Korea 100-799 [C. J. K.]
| ABSTRACT |
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| INTRODUCTION |
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and PLZF-RAR
) have been found to repress transcription of target genes such as the RARß gene. Transcriptional corepressor complexes contain HDAC activity and transcriptional coactivator complexes contain histone acetyltransferase activity. Recent studies indicate that HDAC inhibitors such as trichostatin A or NaBu are able to relieve the transcriptional repression caused by the chimeric transcription factors PML-RARa and PLZF-RARa (8
, 9)
. HDAC inhibitors alone or in combination with retinoids have been shown to induce leukemia remission and prolonged survival in an animal model of acute promyelocytic leukemia without apparent side effects (10)
. Acetylation and deacetylation of histones alter higher order chromatin structure by influencing histone interaction with DNA (11 , 12) . Transcription factors may also be acetylated (13) , and the acetylated status of these proteins may influence their interaction with DNA, as well as their ability to interact with other transcriptional coregulatory proteins. For example, acetylation of p53 enhances its sequence-specific DNA binding activity (14 , 15) . Deacetylated histones are associated with cell growth, whereas hyperacetylated histones (16 , 17) are associated with cell growth arrest, differentiation, and/or apoptosis. Finally, tubulin can be acetylated, and this may affect the dynamic equilibrium between soluble and polymerized tubulin and the structure of microtubules (18) .
HDAC inhibitors differ with regards to their antitumor activity, toxicity, and stability (17 , 19) . High concentrations of NaBu are required to induce tumor cell apoptosis or differentiation (20) . Additionally, NaBu has a short half-life that limits its effectiveness as a therapeutic agent. Hybrid polar compounds such as M-carboxycinnamic acid bishydroxamide have been shown to induce apoptosis in NB cells in vitro, although the mechanism has not been fully elucidated (21) . Saito et al. (22) recently evaluated the efficacy of MS-275, a synthetic benzene derivative that inhibits HDAC. In a number of adult tumor cell lines, MS-275 inhibited tumor cell growth with an IC50 in the submicromolar range. The inhibition of cell growth was accompanied by a cell cycle arrest and an induction of the cell cycle inhibitor p21. MS-275 administered p.o. inhibited the growth of established adult tumor lines s.c. implanted in nude mice with minimal toxicities.
To date, there have been few studies (21 , 23) using HDAC inhibitors in pediatric solid tumors, although these tumors are marked by alterations in the expression and structure of transcription factors. In this study, we assessed the antitumor activity of MS-275 against several pediatric solid tumor cell lines in vitro and against selected in vivo orthotopic xenograft murine models of pediatric solid tumors. We found that MS-275 inhibited in vitro cell growth in all cell lines tested with an IC50 ranging from 50 nM to 1.3 µM. In NB, EWS, and US orthotopic murine xenograft models, the oral administration of MS-275 inhibited tumor growth with few side effects.
| MATERIALS AND METHODS |
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Ac-H3 Analysis.
For acetylated histone assessment by Western analysis, cells were trypsinized, washed twice with PBS, resuspended in lysis buffer [0.02 M Tris (pH 7.4), 0.2 mM Triton X-100, and 0.02% 2-mercaptoethanol] with 2 ng/ml aprotinin, and solubilized by sonication. Protein extracts (10 µg) were subjected to SDS-PAGE and transferred to polyvinylidene difluoride membranes. Membranes were probed with Ac-H3 (Upstate Biotechnology, Lake Placid, NY). Glyceraldehyde-3-phosphate dehydrogenase (American Research Products, Belmont, MA) was used as a control. Cytospins of trypsinized cells were fixed in 95% ethanol/5% acetic acid and blocked in 8% BSA in PBS. After an overnight incubation with anti-
Ac-H3, cells were stained with horse antirabbit FITC-conjugated secondary antibody (Vector Laboratories, Burlingame, CA) and counterstained with TO-PRO-3 iodide (Molecular Probes, Eugene, OR). Slides were scanned using a CompuCyte Laser Scanning Cytometer equipped with WinCyte software (Cambridge, MA).
RNA Preparation and Northern Blot Analysis.
Control and MS-275-treated cells were detached mechanically from the 150-cm2 plate, washed twice with ice-cold PBS, and processed for RNA extraction with the RNAeasy kit (Qiagen, Inc., Chatsworth, CA) according to the manufacturers instructions and analyzed by Northern blot analysis as described previously (24)
. Probes were labeled with [32P]dCTP using RediPrime II (Amersham Pharmacia Biotech) according to manufacturers recommendations.
In Vivo Animal Model.
The US (2 million) cells and the EWS cell line (TC71; 2 million cells) were injected orthotopically into the gastrocnemous muscle of SCID-beige mice. The NB cell line (KCNR; 2 million cells) was injected into the periadrenal fat pad of SCID-beige mice as described previously (28)
. Treatment was initiated 1 week after injection for KCNR cells when US tumors had reached 0.3 cm3 and when the TC71 cells reached 0.6 cm3. MS-275 was dissolved in 0.05 N HCl and 0.1% Tween in PBS and diluent served as placebo. Drug doses and scheduling were based on a maximally tolerated dose from murine studies on adult tumor cell lines (22)
. MS-275 and placebo were administered p.o. once daily 5 days/week for 4 weeks for US (58 animals in each treatment group) and KCNR (12 animals in each treatment group) and daily for 2 weeks for TC71 (10 animals in each treatment group). KCNR tumors were palpated daily for size, whereas the US and TC71 tumors were measured using a vernier caliper, and tumor volume was determined as described previously (28)
. A post test-Bonferronis multiple comparison test was used to assess the differences in the US and TC71 experiment, and an unpaired t test was used to assess the differences in the KCNR experiment.
| RESULTS |
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MS-275 Induced Changes in Gene Expression in Pediatric Tumors.
Other HDAC inhibitors (30)
as well as MS-275 (22)
have been shown to induce p21. We evaluated induction of p21 in selected pediatric tumor cell lines and found that in six of seven cell lines, there was an increase in p21 that occurred from 418 h after treatment with 1 µM MS-275 (Fig. 5A)
. Induction of p21 occurred regardless of the p53 status. Rh30 has an Arg to Ser mutation in codon 280 of p53 and shows a marked induction of p21 within hours of treatment with MS-275. The US cell line, which was the least sensitive to MS-275, did not show an increase in or induction of p21 under these conditions, but p21 was induced using 3 µM MS-275 (data not shown).
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Patients with NB tumors that contain amplification and overexpression of N-myc have a poor prognosis (33)
. We evaluated the expression of N-myc in NB cells treated with 1 µM MS-275 for various times. Within 4 h of MS-275 treatment, there was a decrease in N-myc levels, and expression was barely detected at 48 h (Fig. 5C)
. Similar results have been detected in an additional 4 of 4 N-myc-amplified NB cell lines examined (data not shown). In contrast, c-myc levels were increased in the EWS cells after treatment with MS-275. Thus, there is differential regulation of members of the myc gene family after treatment with MS-275.
Antitumor Effect of MS-275 in Tumor-bearing SCID-Beige Mice.
MS-275 has been shown to be effective in inhibiting the in vivo growth of most of the adult tumor cell lines grown s.c. in nude mice (22)
. To evaluate effects of MS-275 on the in vivo growth of the US cell line, which was most resistant to the effects of MS-275 in vitro, 2 x 106 US cells were injected into the gastrocnemious muscle and allowed to grow until the tumor reached 0.3 cm3. At that time (
5 days), the animals received various doses of MS-275 ranging from 8.25 to 24.5 mg/kg based on doses in the murine study of adult tumor cell lines that did not have significant toxicity. Both control and MS-275 tumors grew during the course of therapy. However, at the termination of therapy (4 weeks), the volume of the tumors in the MS-275 animals was decreased compared with controls. There was a
60% decrease in tumor volume in the animals treated with the maximal dose (24.5 mg/kg/day every 5 days) of MS-275 (Fig. 6A)
Using a Bonferronis multiple comparison test, (34)
only the US tumor size in the 24.5 mg/kg was significantly different (P < 0.001) from the size of the US tumors in the untreated group.
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The NB cell line KCNR (2 x 106cells) was injected orthotopically to the adrenal gland of 18 mice. After 1 week, 2 animals were sacrificed, and small periadrenal masses could be visualized (28)
. Remaining animals were randomly placed into two groups, and 24.5 mg/kg MS-275 (12 mice) or diluent (10 mice) were administered by oral gavage five times/week. After 1 month, when the tumor volume in the placebo cohort had reached
1.5 cm3 (as determined by external palpation), the animals were sacrificed, and tumors were excised, measured, and weighed. In the placebo group, all animals had tumors, and the tumor volume ranged from 1 to 7 cm3 with a mean tumor volume of 3.5 cm3. In contrast, only 50% of animals had a visually detectable tumor in the MS-275 group, and the volume of these tumors ranged from 0.6 to 2.7 cm3, which was significantly different from the controls (P = 0.0009 using an unpaired t test; Fig. 6C
). NB tumors and xenografts growth in orthotopic sites in SCID-beige mice are highly vascular; KCNR tumors in the placebo group are also well vascularized (Fig. 7A)
. In the MS-275-treated group, there was a marked decrease in vascularization (Fig. 7B)
. Consistent with in vitro studies, there was a decrease in N-myc mRNA and an increase in p21mRNA in tumors from mice treated with MS-275 compared with control (inset panel on Fig. 7
).
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| DISCUSSION |
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In contrast to adult studies in which MS-275 induced a G1 arrest in all cell lines examined, the effects of MS-275 on cell growth and induction of apoptosis were varied among the different pediatric tumor cell lines evaluated. In some cell lines, apoptosis was induced, whereas in others, cell cycle arrest occurred in either the G1 or G2 phase of the cell cycle. The variability in response of the cell lines to the drug may reflect the nature of the genetic alterations and the signal transduction paths altered in the different cell lines. As has been shown for a number of HDAC inhibitors, p21WAF/CIP1 was induced in most of the pediatric solid tumor cell lines evaluated irrespective of the p53 status of the cells. This suggests that HDAC inhibitors as a class may induce cell death irrespective of the status of p53.
Although we cannot exclude that there is a common mechanism by which MS-275 affects the growth and survival of different tumor cell lines, it is equally possible that the effect of MS-275 on the different pediatric solid tumor cell lines may be dependent on the nature of the genetic alteration in the different tumor cell lines. We evaluated changes in TGF-ßRII mRNA, which is known to be transcriptionally repressed by the EWS/FLI transcription factor in EWS and the N-myc gene whose amplification is associated with a poor prognosis in NB tumors. Studies on MS-275 indicate that it induces the expression of TGF-ßRII mRNA and restores TGF-ß signaling in breast cancer cell lines (35) . We have found that in EWS in which the chimeric transcription factor EWS/FLI transcriptionally represses the TGF-ßRII gene (31) , MS-275 is able to stimulate increases in TGF-ßRII mRNA despite the presence of the EWS/FLI transcriptional repressor. Our finding that there is an increase in PAI mRNA, a downstream target of the TGF-ß signal transduction path coincident or subsequent to the MS-275-induced increase in TGF-ßRII mRNA, is consistent with the restoration of TGF-ß signaling in EWS. Previous studies indicate that restoration of the TGF-ß path in EWS cell lines inhibits their growth in animal models (31) . Additional studies will be needed to assess whether the effects of MS-275 on the growth of TC71 cells in vivo are dependent on the induction of TGF-ßRII and restoration of the TGF-ß signal transduction path.
Previous studies using the HDAC inhibitor CHBA have indicated induction of p21 in NB cell lines and inhibition of NB tumor growth in vitro and in vivo (21
, 23)
. Our results using MS-275 are similar. Typically, the use of HDAC inhibitors has been found to increase gene expression, whereas repression of gene expression by HDAC inhibitors is not common. In N-myc-amplified NB cells, we found that MS-275 induced a decrease in N-myc mRNA expression (Fig. 5
and data not shown), and similar findings were found in tumors taken from animals treated with MS-275 (Fig. 7)
. Decreases in N-myc are known to affect the survival and growth of NB cells (36
, 37)
. Although this may affect the biology of N-myc-expressing tumor cell lines, we also found that MS-275 was able to inhibit the growth of a non-N-myc-amplified cell line, SK-N-AS. Thus the mechanism by which MS-275 affects NB cell growth and survival may not require N-myc down-regulation or may be distinct in different NB cell types. However, both KCNR and AS contain 1pLOH and increases in 17q, common although uncharacterized genetic lesions in NB, and it is possible that MS-275 may complement these genetic alterations to alter NB cell growth.
In our animal studies, we found that the growth of established tumors in vivo was markedly affected by treatment with MS-275. Toxicities were minimal with ruffled fur being the consistent alteration noted in all of the experiments. Initial studies indicated that 24.5 mg/kg every 57 days by oral gavage was the most effective dose even in the US cell line. In the NB and EWS models, tumor growth was markedly altered and in some cases there was no detectable tumor at the highest drug concentrations. The mechanism by which the drug affects tumor growth in vivo remains to be elucidated. Recent studies indicate that HDAC inhibitors may affect angiogenesis (38
, 39)
and, in the NB model preliminary analysis indicated, a decrease in tumor angiogenesis (Fig. 7)
. Current studies are aimed at more precisely detailing the changes in angiogenesis and metastasis in the xenograft tumor models.
In this report we have evaluated the in vitro and in vivo effects of the HDAC inhibitor MS-275 in a number of pediatric solid tumor cell lines. Future studies on the changes in gene expression induced by MS-275 has the potential to reveal genes with expressions that are affected by the altered transcription factors in these different pediatric solid tumors. As in vitro studies indicated that MS-275 induced apoptosis in many cases, histological and molecular genetic analysis of the xenograft tumors will enable us to determine whether induction of apoptosis and/or inhibition of angiogenesis affects tumor growth in vivo. Currently, MS-275 is in Phase I clinical trials in adults and these studies indicate that it should be considered for the treatment of pediatric malignancies as well.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 These individuals contributed equally to this work. ![]()
2 J. J. is a M.D./Ph.D. student from Howard University, College of Medicine doing his thesis work in the Cell and Molecular Biology Section/Pediatric Oncology Branch/Center for Cancer Research/National Cancer Institute. ![]()
3 To whom requests for reprints should be addressed, at National Cancer Institute, Building 10, Room 13N240, 10 Center Drive, MSC 1928, Bethesda, MD 20892. Phone: (301) 496-1543; Fax: (301) 402-0575; E-mail: ct47a{at}nih.gov ![]()
4 The abbreviations used are: NB, neuroblastoma; EWS, Ewings sarcoma; RH, rhabdomyosarcoma; HDAC, histone deacetylase; NaBu, sodium n-butyrate; MS-275, MS-27-275; US, undifferentiated sarcoma; TdT, terminal deoxytransferase; TUNEL, Tdt-mediated nick end labeling; Ac-H3, acetylated histone H3; PAI, plasminogen activator inhibitor. ![]()
Received 3/28/02. Accepted 9/ 5/02.
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