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Molecular Biology and Genetics |
Sagres Discovery, Davis, California 95616 [C. T.], and Department of Cellular and Structural Biology, The University of Texas Health Science Center, San Antonio, Texas 78229-3900 [R. H. X., T. B., S. L. N.]
| ABSTRACT |
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| INTRODUCTION |
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Ovarian cancer is one of the leading causes of death in women. Given the high rate of LOH in 3p, several studies have been conducted to identify any ovarian cancer tumor suppressor genes within this region. Monochromosomal transfer studies of 3p by Rimessi et al. (15)
have identified three ovarian cancer suppression regions. One region, OCSR-C (8)
, overlaps with 3p21.3, a region implicated in lung cancer (6
, 16, 17, 18)
. Interestingly, two class III semaphorin genes (SEMA3B and SEMA3F) reside in this region and have been proposed to play a role in tumorigenesis (6
, 18, 19, 20)
. In normal ovarian tissue, SEMA3B is the predominant semaphorin expressed (20)
. Semaphorins are a family of signaling molecules initially identified to play a role in axonal guidance and can be classified as either membrane bound (classes 1, 4, 5, and 6) or secreted (classes 2 and 3) (Refs. 21, 22, 23
). All semaphorins contain
500 amino acid NH2-terminal sema domains, an immunoglobulin-like domain (with the exception of class 1 semaphorins), and a basic COOH-terminal domain (24)
. The role of the secreted class 3 semaphorins in axonal guidance has been clearly demonstrated; however, their role(s) in nonneuronal tissue remains to be elucidated (25)
. The receptors for the class 3 semaphorins are the neuropilin receptors (21
, 26
, 27) . In neuronal tissue, signaling through the neuropilin receptor has been shown to involve a heteromeric complex with the plexins (25
, 28)
. The intracellular signal transduction pathway is less clear but has been shown to involve the Rho family GTPases (29)
.
Both semaphorin 3B and 3F reside in 3p21.3 (Fig. 1)
and exhibit overlapping yet distinct tissue expression patterns (20)
. Recently, immunohistochemical studies found SEMA3F localization and expression to be altered in lung tumors compared with normal lung tissue (19)
. These results suggest a role of SEMA3F in tumorigenesis. However, The potential role of SEMA3B in tumorigenesis is less clear. SEMA3B has been found at reduced levels or not expressed in both small cell and non-small cell lung carcinomas, suggesting a role in tumorigenesis (6)
. Furthermore, mutations resulting in amino acid changes were found; however, the ramifications of these mutations remain to be elucidated (6
, 20)
. On the basis of these observations, we propose that SEMA3B plays a suppressive role in tumorigenesis. To address this hypothesis, HEY cells, a tumorigenic adenocarcinoma cell line, were stably transfected with SEMA3B and examined for tumor suppression abilities. SEMA3B transfectants exhibited decreased tumor formation. Thus, in addition to its role in axonal development, we have identified a tumor suppressive role of SEMA3B.
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| MATERIALS AND METHODS |
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-MEM and the Thermoscript RT-PCR kit were obtained from Life Technologies, Inc. Fetal bovine serum was obtained from Hyclone. The mammalian expression vector, pTracer-SV40, and the selection agent, zeocin, were obtained from Invitrogen. Total ovary RNA was obtained from Clontech. All other chemicals were of reagent grade.
Quantitative Real-Time RT-PCR.
SEMA3B gene expression was quantitated using Taqman chemistry on an ABI 7700 SDS machine (Perkin-Elmer) as described by manufacturer. The primers and probes used for detection of SEMA3B were: 5'-GCGACACCCACTTCGATCA-3'/5'-CGTGGAGAAGACGGCATAGAG-3', and the probe was 5'-6-carboxyfluorescein-CTCCAGGATGTGTTTCTGTTGTCCTCGC-6-carboxytetramethylrhodamine-3'. Briefly, reverse transcription was performed at 48°C for 30 min using the Multiscribe reverse transcriptase (Perkin-Elmer). Samples were then denatured at 95° for 10 min, followed by 40 cycles of 95°C for 15 s and 60°C for 1 min. Results were analyzed on the SDS software version 1.7.
SEMA3B cDNA Isolation and Cloning into Expression Vector.
The full-length cDNA of SEMA3B was obtained by RT-PCR using human ovary total RNA. The Thermoscript RT-PCR kit was used for this procedure. Briefly, cDNA corresponding to the total mRNA was reverse transcribed using an oligo-dT primer as directed by the manufacturer. The SEMA3B cDNA was amplified by PCR using the following forward/reverse primers: 5'-GAACCCTGAGCACCCTGAG-3'/5'-CTGTCGTCTCCTGCTTGGTT-3'. 2.5 µl of the cDNA synthesis reaction were added to a 50-µl final reaction containing 1 µM of each primer using Platinum Taq high fidelity DNA polymerase. The thermocycle conditions were as follows: 95°C for 2.5 min; 95°C for 45 s, 60°C for 1.5 min, and 72°C for 3 min for 35 cycles; and 72°C for 10 min. The product (2318 bp corresponding to the entire coding region of SEMA3B) was cloned into the pCRII-TOPO vector (TOPO-TA Cloning kit; Invitrogen). The amplification product was verified by DNA sequence analysis (UTHSCSA Sequencing Core). The SEMA3B cDNA was excised from the pCRII-TOPO vector by digestion with EcoRI (one site flanking the multiple cloning site) and subcloned into the pTracer-SV40 mammalian expression vector (Invitrogen).
Transfection and Isolation of Stable Clones.
pTracer-SV40-SEMA3B (SEMA3B) constructs were stably integrated into HEY cells using Lipofectamine reagent (Life Technologies, Inc.) as described by manufacturer. The cells were diluted 1:10 48-h after transfection into
-MEM containing 75-µg/ml zeocin. Single clones were isolated after 10 days, and a PCR screen was used to identify clones containing SEMA3B. RNA was then isolated from the remaining clones to verify SEMA3B expression via RT-PCR. Stables were maintained in 75-µg/ml zeocin to ensure retention of the SEMA3B construct.
In Vivo Assay for Tumorigenesis.
Single SEMA3B transfectants were injected s.c. into the shoulders of five BALB/c nu/nu mice (56 weeks of age) at 2.0 x 106 cells/mouse in a volume of 0.2 ml. Control HEY cells were also injected at 2.0 x 106 cells/mouse. Tumor volume (4/3 x
x width x width x length) was assessed at 23-day intervals. Tumors were excised at day 38, wet weight was determined, and tumors were placed into culture for 3 days prior to DNA and RNA extraction.
In Vitro Assay for Anchorage Dependence.
Sixty-mm soft agar plates consisted of
-MEM containing 10% FBS in a 0.4% agar medium. Cells (103) cells of either control HEY or SEMA3B transfectants were resuspended in
-MEM containing 10% FBS in a 0.4% agar medium prewarmed to 37°C, and 2.5 ml were added to each plate. Agar plates containing cells were carefully placed into the incubator and were supplemented with medium once a week. The number of cell colonies formed was determined by staining the agar plates with p-iodonitrotetrazolium violet (Sigma Chemical Co.). Colonies were quantitated using a NucleoTech Imaging Workstation equipped with a colony counting program.
Cell Proliferation and Cytotoxicity Assays.
The Cell Proliferation Kit II (XTT; Roche) was used to assess both cell proliferation and cytotoxicity. This assay is based on the cleavage of XTT by metabolic active cells, resulting in an orange formazan dye. The amount of orange formazan dye produced is quantitated using a spectrophotometric plate reader to measure the absorbance at 450 nm. Both assays were carried out essentially as described by manufacturer. Briefly, the cell proliferation assay involved plating the cells at a low density (4 x 103 cells/well) in a 96-well plate. Cells were plated in either
-MEM containing 0.5% FBS or 10% FBS (100 µl final volume) and incubated at 37°C and 5% CO2. Twenty-four h later, 50 µl of XTT labeling mixture were added per well and incubated for an additional 6 h at 37°C and 5% CO2. For the cytotoxicity assay, cells were plated at high density (5 x 104 cells/well) in a 96-well plate. Cells were plated in either
-MEM containing either 0.5% FBS + 0.25 µM Taxol or 1.0 µM Adriamycin or serum-free (100-µl final volume) and incubated at 37°C and 5% CO2. Twenty-four h later, 50 µl of XTT labeling mixture were added per well and incubated for an additional 6 h at 37°C and 5% CO2. The absorbance was then measured on a Molecular Device ELISA plate reader.
| RESULTS |
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25-fold less than that observed in the total human ovarian RNA. The substantial decrease in the expression of SEMA3B in the ovarian adenocarcinoma cell line is analogous to the observations in lung carcinomas and suggests a role for SEMA3B in ovarian tumorigenesis. To examine whether SEMA3B has a role in tumorigenesis, HEY cells were stably transfected with a SEMA3B cDNA clone. The SEMA3B cDNA was isolated by amplification from total human placenta RNA (Clontech) and cloned into the pTracer-SV40 mammalian expression vector (Invitrogen). Stable transfectants expressing SEMA3B from the introduced cDNA were identified by RT-PCR using a forward primer specific to the vector backbone and a reverse primer specific for SEMA3B (Fig. 2A)
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-MEM containing either 0.5 or 10% FBS (Fig. 4)
36% compared with controls (Fig. 4B)
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22% decrease in metabolic activity after a 24-h exposure to Taxol. The metabolic activity decreases by
40% after 48 h. In contrast, a
3559% decrease in metabolic activity is observed in the SEMA3B transfectants after 2448 h, respectively. In the presence of Adriamycin, another genotoxic agent that inhibits topoisomerase activity, both controls and SEMA3B transfectants exhibited a similar behavior (Fig. 5B)
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| DISCUSSION |
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25-fold less than the expression in total RNA from human ovarian tissue. Moreover, mutations in SEMA3B have been identified in tumor cell lines (6
, 20)
. In this study, we demonstrate that expression of SEMA3B decreases both tumorigenicity and cell proliferation rates. Furthermore, the characteristic anchorage independence of cancer cells is severely diminished by SEMA3B expression.
Suppression of Tumorigenesis by SEMA3B May Occur through Multiple Mechanisms.
Our data indicate that there is an intrinsic change in the cells that have been transfected with semaphorin 3B. Perhaps the most significant difference is that transfected cells no longer exhibit anchorage-independent growth. This coupled with the decrease in metabolic rate would suggest that semaphorin 3B expression brings the cells more in line with normal cell activity.
Class 3 semaphorins are secreted proteins initially identified to play a role in axonal migration (33) . However, the widespread expression profile in adult tissues suggests that other functions exist. The most extensively studied semaphorin is SEMA3A. SEMA3A has been shown to be a repellent, causing the collapse of axons. This repellent nature appears to be through both alterations in cell migration as well as the induction of apoptosis in progenitor cells migrating toward the SEMA3A gradient (33, 34, 35, 36) . The effects of the class 3 semaphorins are mediated through the Np receptors, Np-1 and Np-2. For SEMA3A, Np-1 seems to be a major component of its signaling pathway. Interestingly, the Np receptors also serve as coreceptors for several isoforms of VEGFs (37, 38, 39, 40) . VEGF is known to bind two receptor tyrosine kinases, the kinase domain region (KDR) and fms-like tyrosine kinase (Flt-1). When the Np receptors are coexpressed with KDR, VEGF affinity and mitogenic activity are enhanced (37 , 40) . VEGF is known to be a potent angiogenic factor as well as a mitogenic factor and has been found to be an essential initiator of tumor angiogenesis. Thus, SEMA3A functions, in part, to competitively inhibit VEGF. This has been well demonstrated by several laboratories (37 , 38 , 40) .
Similar to SEMA3A, SEMA3B also binds to Np-1 and Np-2 with high affinity (41) . HEY cells express Np-2.4 Thus, SEMA3B action in tumorigenesis may also involve inhibition of tumor angiogenesis. On one level, SEMA3B works in an autocrine fashion to decrease cell proliferation and promote cell anchorage dependence. Concomitantly, VEGF action may be down-regulated through direct sequestration of both Np-1 and Np-2 receptors, preventing vascularization of the tumor tissue. In sum, our demonstration of the tumor suppressive role of SEMA3B combined with its wide range of expression suggests that SEMA3B is involved in homeostasis in nonneuronal tissue, and misregulation may lead to uncontrolled proliferation and tumorigenesis.
| FOOTNOTES |
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1 This research was supported by National Cancer Institute Grants CA56266 (to S. L. N.) and CA084643 (to C. T.). ![]()
2 To whom requests for reprints should be addressed, at Department of Cellular and Structural Biology, University of Texas Health Sciences Center, MSC 7762, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900. Phone: (210) 567-3842; Fax: (210) 567-6781; E-mail: Naylor{at}uthscsa.edu ![]()
3 The abbreviations used are: LOH, loss of heterozygosity; RT-PCR, reverse transcription-PCR; FBS, fetal bovine serum; XTT, 2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolum-5-carboxanilide inner salt; Np, neuropilin; VEGF, vascular endothelial growth factor. ![]()
Received 7/ 2/01. Accepted 11/13/01.
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