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1 Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, Tokyo; Departments of 2 Urology, and 3 Tumor Pathology, Kochi Medical School, Nankoku; 4 Laboratory for Medical Informatics, SNP Research Center, RIKEN (Institute of Physical and Chemical Research), Yokohama; 5 Department of Urology, Kyoto Prefectural University of Medicine, Kyoto; and 6 Department of Urology, Iwate Medical University, Morioka, Japan.
| ABSTRACT |
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| INTRODUCTION |
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Investigations designed to clarify the molecular mechanisms that underlie the initiation and progression of PC have revealed altered expression of PTEN, p27KIPS, and Nkx3.1 tumor suppressor genes, the c-myc oncogene, and the Bcl-2 antiapoptotic gene in human and/or mouse PCs (5 , 6) . However, the overall molecular events are still largely unknown. High-grade PIN (prostatic intraepithelial neoplasia) has been considered to be a putative precursor of cancer (7) , and some clinical and scientific evidence supports a relationship between high-grade PINs and PCs. However, that hypothesis is still controversial (8 , 9) . Added to the difficulties of proving a PIN-to-PC transition is that human PCs are histopathologically very heterogeneous, and PIN is generally a very small lesion. Most previous molecular-based studies have analyzed bulk cancer tissues that were contaminated by a large proportion of noncancerous cells including fibromascular, microvasculature, and inflammatory cells, but such strategies do not yield precise molecular profiles of the respective lesions.
To overcome that limitation, we obtained purified populations of PC and PIN cells, as well as normal prostatic epithelial cells as a control, by LMM (laser microbeam microdissection), before analyzing genome-wide gene-expression profiles of 20 PCs and 10 PINs on a cDNA microarray representing 23,040 genes. This is the first report to show precise expression profiles of PC and PIN tumors, and to disclose molecular features confirming a relationship between PINs and PCs.
| MATERIALS AND METHODS |
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Laser Microbeam Microdissection and T7-based RNA Amplification.
LMM and T7-based RNA amplification were performed as described previously (11)
. Prostate tumor cells and normal prostatic epithelial cells were isolated selectively by the EZ cut system with a pulsed UV narrow beam-focus laser (SL Microtest GmbH, Germany) in accordance with the manufacturers protocols. After DNase treatment, total RNAs were subjected to two rounds of T7-based amplification, which yielded 50 to 100 µg of amplified RNA (3)
from each sample. Then 2.5-µg aliquots of amplified RNA from PC or PIN cells and from normal prostatic ductal epithelial cells were labeled by reverse transcription with Cy5-dCTP (tumor cells) or Cy3-dCTP (normal cells (Amersham Biosciences, Buckinghamshire, United Kingdom), as described previously (12)
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cDNA Microarray Analysis and Acquisition of Data.
We fabricated a genome-wide cDNA microarray with 23,040 cDNAs selected from the UniGene database (build no.131) of the National Center for Biotechnology Information (NCBI).7
Construction, hybridization, washing, and scanning were carried out according to methods described previously (11
, 12)
. Signal intensities of Cy3 and Cy5 from the 23,040 spots were quantified and analyzed by substituting backgrounds with ArrayVision software (Imaging Research, Inc., St. Catharines, Ontario, Canada). Subsequently, the fluorescent intensities of Cy5 (tumor) and Cy3 (control) for each target spot were adjusted so that the mean Cy3/Cy5 ratio of 52 housekeeping genes was equal to one. Because data derived from low-signal intensities are less reliable, we determined a cutoff value on each slide (12)
, and we excluded genes from further analysis when both the Cy3 and the Cy5 dyes yielded signal intensities lower than that of the cutoff. For other genes, we calculated the Cy5/Cy3 ratio using the raw data of each sample.
Cluster Analysis of 20 Prostate Cancers and 10 PINs According to Gene-expression Profiles.
We applied a hierarchical clustering method to both genes and tumors, excluding genes whose Cy3- and Cy5-fluorescence intensities were both below the cutoff value. To obtain reproducible clusters for classification of the 30 tumors, we selected 63 genes for which valid data were obtained in 80% of the experiments and whose expression ratios varied by SDs of more than 1.5. The analysis was performed with web-available software (Cluster and TreeView) written by Eisen.8
Before applying the clustering algorithm, we log-transformed the fluorescence ratio for each spot and then median-centered the data for each sample to remove experimental biases.
Identification of Genes That Were Up- or Down-regulated Commonly from Normal Epithelium to PINs and PCs.
We identified genes the expression of which was altered from normal epithelium to 10 PINs and 20 PCs according to the following criteria: (a) genes for which we were able to obtain expression data in more than 50% of the cases examined; and (b) genes the expression ratio of which was more than 3.0 or less than 0.33 in more than 50% of informative cases.
Identification of Genes That Were Up- or Down-regulated from PINs to PCs.
We identified genes with changed expression in 20 PCs and 10 PINs according to the following criteria: (a) genes for which we were able to obtain expression data in more than 50% of the cases examined; and (b) genes the expression ratio of which was more than 3.0 in PCs and between 0.5 and 2.0 in PINs (defined as up-regulated genes) or genes whose expression ratio was less than 0.33 in cancers and between 0.5 and 2.0 in PINs (defined as down-regulated genes) in more than 50% of informative cases.
Immunohistochemistry.
Formalin-fixed and paraffin-embedded prostatic tumor sections were immunostained with a mouse anti-APOD (apolipoprotein D) monoclonal antibody (NeoMarkers, Fremont, CA) or a rabbit anti-EPHA4 (EphA4) polyclonal antibody (Santa Cruz Biotechnology Inc., Santa Cruz, CA) for APOD or EPHA4 expression. PC tissues included PC cells, PIN cells, and normal prostatic epithelium heterogeneously. Deparaffinized tissue sections were placed in 10 mmol/L citrate buffer (pH 6.0) and were heated to 108°C in an autoclave for 15 minutes for antigen retrieval. Sections were incubated with a 1:10 dilution or a 1:100 dilution of primary antibody for APOD or EPHA4, respectively, in a humidity chamber for 1 hour at room temperature and were developed with peroxidase labeled-dextran polymer followed by diaminobenzidine (DAKO Envision Plus System; DAKO Corporation, Carpinteria, CA). Sections were counterstained with hematoxylin. For negative controls, primary antibody was omitted.
Small Interfering RNA-expressing Constructs and Colony Formation/MTT Assay.
We used small interfering RNA (siRNA)-expression vector (psiU6BX) for RNA interference effect to the target genes. The U6 promoter was cloned upstream of the gene-specific sequence (19-nucleotide sequence from the target transcript, separated by a short spacer TTCAAGAGA from the reverse complement of the same sequence) and five thymidines as a termination signal as well as neocassette for selection with Geneticin (Sigma). The target sequences for EphA4 are 5'-TCCGAACCTACCAAGTGTG-3' (198si), 5'-TCATGAAGCTGAACACCGA-3' (468si) and 5'-GCAGCACCATCATCCATTG-3' (1313si). The sequence of 5'-GAAGCAGCACGACTTCTTC-3' (EGFPsi) corresponding to EGFP was as a negative control. PC3 PC cells were plated onto 10-cm dishes (5 x 105 cells/dish) and transfected with psiU6BX containing EGFP target sequence (EGFP) or psiU6BX containing EPHA4 target sequence with LipofectAMINE 2000 (Invitrogen) according to the manufacturers instruction. Cells were selected by 500 µg/mL Geneticin for 1 week, harvested 48 hours after transfection, and analyzed by reverse transcription (RT)-PCR to validate knockdown effect on EPHA4. The primers of RT-PCR were 5'-GAAGGCGTGGTCACTAAATGTAA-3' and 5'- TTTAATTTCAGAGGGCGAAGAC-3' for EPHA4, and 5'-GAGAGAGAATGAAAAGTGGAGCA-3' and 5'-GATTAACCACAACCATGCCTTAC-3' for ß2-MG, used to quantify the amount of cDNA input. These cells were also stained by Giemsa solution and applied for 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay to evaluate the colony formation and the cell number, respectively.
| RESULTS |
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Growth Suppression Mediated by EPHA4-specific siRNA in Prostate Cancer Cells.
To investigate the growth or survival effect of EPHA4 on PC cells, we knocked down their endogenous expression by mammalian vector-based RNA interference technique by using the PC cell line. The transfection of one of the siRNA-expressing vectors, 1313si, clearly reduced the endogenous expression of EPHA4 (Fig. 4A)
. This knocking-down effect by the siRNA on EPHA4 mRNA resulted in drastic growth suppression in colony formation assay as well as in MTT assay (Fig. 4B and C)
. These findings strongly suggested that overexpression of EPHA4 in PC cells was associated with the enhanced growth of cancer cells.
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| DISCUSSION |
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17 versus 60%; ref. 15
). SDF1 (stromal cell-derived factor 1), which is highly expressed in stroma, showed low signal (under the cutoff) in our experiments (data not shown). However, Stamey et al. (15)
reported that gene as down-regulated in PCs, probably because stromal components were present in the normal control. Similarly, genes that are expressed in normal epithelial cells and cancer cells, but not in stromal cells, may be selected as up-regulated elements during microarray analysis of bulk tissue if noncancerous epithelial cells are enriched in the tumor tissue. For instance, PRSS8 (prostasin) was reportedly up-regulated in many microarray studies that used bulk tissues (17)
, but in our experiments, expression of this gene was either unchanged or down-regulated in some PCs as compared with normal epithelium. PRSS8 is predominantly expressed in normal prostatic epithelial cells; several studies using in situ techniques have revealed its down-regulation in PC cells (18
, 19)
. These observations make clear that contamination of cancer tissues with stromal cells or normal prostate epithelium can mislead efforts to understand the molecular pathology of PC; therefore, microdissection is definitely required for precision in microarray analyses of PCs. In this study, we applied the LMM system (11)
to purify populations of cancer cells, PIN cells, and normal prostatic epithelial cells from surgical specimens, in an effort to obviate influences from stromal components and normal prostate. Several lines of clinical and pathological evidence have linked PINs to PCs, and high-grade PIN has been widely considered the precursor of invasive PC (8 , 10) . PINs and PCs can show certain genetic alterations in common (20) . However, some data have argued against this hypothesis; for example, early-stage PCs are not always accompanied by PIN (9 , 10) and some genetic markers of PINs are not always observed in PCs (21) . Hence, considerable controversy has arisen about the natural history of high-grade PIN and the mechanism of PIN development, and the putative transition from PIN to invasive PC has remained unsettled.
We analyzed the precise molecular profiles of purified populations of PC and PIN cells to investigate the molecular mechanism of prostate carcinogenesis and to examine the molecular linkage between PINs and PCs. Our unsupervised hierarchical clustering analysis was able to distinguish most PCs from PINs on the basis of their expression profiles, although two low-grade PCs with Gleason scores of 5 fell in the PIN cluster and one high-grade PIN was classified into the PC group. The latter turned out to be a very-high-grade PIN in a retrospective histological review and probably represented a transitional lesion to invasive PC. These results implied that the gene-expression profile of very-high-grade PIN is very similar to that of PC, supporting the concept that at least some high-grade PINs are precursors of PC. It should also be added that prostate pathologists have long recognized a phenomenon in which PC spreads along prostatic ducts to mimic PIN (22) . In fact, an early controversy that arose when the concept of PIN was first introduced was that the lesions merely reflected intraductal spread of invasive cancer. Except for the one PIN case, our microarray data comparing PIN cells with PC cells rejects this criticism and clearly supports the concept that PINs are precursor lesions of PC, not ductal spread of PC cells.
We identified genes that were up- or down-regulated (Tables 1
and 2
) commonly in PINs and PCs comparing with normal epithelium; those genes may be involved in an early step of prostatic carcinogenesis. The up-regulated elements in PINs and PCs included AMACR (
-methyl acyl-CoA racemase), OR51E2 (olfactory receptor, family 51, subfamily E, member 2), RDOH (3-hydroxysteroid epimerase), and SMS (spermine synthase). AMACR has a key role in ß-oxidation of dietary branched-chain fatty acids and is often overexpressed in PCs (23
, 24)
. OR51E2 is also overexpressed in PCs (25)
, but its function is unknown. RDOH functions in androgen catabolism and is likely to be associated with androgen-dependent prostate tumorigenesis. SMS is a polyamine-biosynthesis enzyme; polyamines such as spermine have been implicated in the growth of PC cells and protection from apoptosis (26)
. On the other hand, SERPIN-B1 [serine (or cysteine) proteinase inhibitor, clade B (ovalbumin) member 1] and SERPIN-G1 [serine (or cysteine) proteinase inhibitor, clade G (ovalbumin), member 1] were down-regulated from normal prostate epithelium to PIN. The pigment epithelium-derived factor encoded by SERPIN-F1 was recently reported to be a key inhibitor of stromal vasculature and growth of epithelial tissue in mouse prostate (27)
; in pigment epithelium-derived factor-deficient mice, stromal vessels were increased and associated with epithelial-cell hyperplasia. Those observations suggest to us that SERPIN-B1 and SERPIN-G1 may also be key regulators of prostate growth. Almost all genes that were up- or down-regulated in PINs were also up- or down-regulated, respectively, in PCs compared with normal prostate epithelium, implying that PCs retain the molecular features of their putative precursor PINs. These data supported the notion of PIN-to-PC transition.
Next, we focused on genes showing differential expression after the transition from PINs to PCs (see Fig. 3B
; Tables 3
and 4
). The list included POV1, CDKN2C, EPHA4, APOD, and FAM as up-regulated genes and LAMB2, ITGB2, PLAU, and TIMP1 as down-regulated genes. EPHA4 is one of the receptor tyrosine kinase receptors and is likely to play a critical role in neuronal circuit development and angiogenesis by regulating cell shape and motility (28)
. However, EPHA4 overexpression in PCs is novel and may involve PC cell viability and motility. Some of the latter are associated with cell adhesion and proteinase activity, suggesting that their expression changes may contribute to the invasive phenotype by abolishing ductal structures during the transition from PIN to PC. We validated the differential expression of APOD and EPHA4 by the immunohistochemical analysis on several PC tissues (Fig. 3C)
, which was consistent with our precise RNA profiles and indicated that our profile analysis was highly reliable.
Furthermore, we investigated by the small interfering RNA strategy whether overexpression of EPHA4 was associated with enhanced growth of PC cells. As shown in Fig. 4
, down-regulation of EPHA4 in PC cell line resulted in a drastic reduction in PC cell viability, suggesting that the overexpression of EPHA4 in the transition of PIN to PC cells is likely to be essential in PC cell growth, indicating that targeting to EPHA4 may be a promising approach to develop novel PC treatments.
Overall, it is interesting that our lists of differentially expressed genes in prostate carcinogenesis via PINs to invasive PCs included many genes associated with lipid metabolism, including AMACR, FASN (fatty acid synthase), MFGE8 (milk fat globule-EGF factor 8 protein), APOD (apolipoprotein D), APOL1 (apolipoprotein L1), PLA2G2A (phospholipase A2, group IIA), and SORL1 (sortilin-related receptor L). Epidemiological aspects suggest that the development of PC is strongly associated with high fat intake (1) ; our data concerning these genes may help to explain the apparent association between fat intake and prostate carcinogenesis.
In conclusion, our extensive list of genes derived from precise expression profiles of PCs and PINs should provide useful information for identifying molecular targets for the prevention and treatment of PC, as well as for understanding the molecular mechanism of prostatic carcinogenesis, especially the transition from PIN to PC.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Y. Nakamura. Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108--8639, Japan. Phone: 81-3-5449-5372; Fax: 81-3-5449-5433; E-mail: yusuke{at}ims.u-tokyo.ac.jp
7 Internet address of database: http://www.ncbi.nlm.nih.gov. ![]()
8 Internet address: http://genome-www5.stanford.edu/MicroArray/SMD/restech.html. ![]()
Received 1/ 5/04. Revised 6/ 8/04. Accepted 6/21/04.
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