The Transcription Factor Sterile Alpha Motif (SAM) Pointed Domain-containing ETS Transcription Factor (SPDEF) Is Required for E-cadherin Expression in Prostate Cancer Cells*

Background: SPDEF functions like a tumor metastasis suppressor. However, the underlying mechanism remains unclear. Results: SPDEF is required for E-cadherin expression in prostate epithelial cells. Conclusion: SPDEF directly modulates E-cadherin expression in prostate cancer cells. Significance: The results provide a potential mechanism by which SPDEF suppresses prostate cancer progression and metastasis. Loss of E-cadherin is one of the key steps in tumor progression. Our previous studies demonstrate that SAM pointed domain-containing ETS transcription factor (SPDEF) inhibited prostate cancer metastasis in vitro and in vivo. In the present study, we evaluated the relationship between SPDEF and E-cadherin expression in an effort to better understand the mechanism of action of SPDEF in prostate tumor cell invasion and metastasis. The results presented here demonstrate a direct correlation between expression of E-cadherin and SPDEF in prostate cancer cells. Additional data demonstrate that modulation of E-cadherin and SPDEF had similar effects on cell migration/invasion. In addition, siRNA-mediated knockdown of E-cadherin was sufficient to block the effects of SPDEF on cell migration and invasion. We also show that stable forced expression of SPDEF results in increased expression of E-cadherin, whereas down-regulation of SPDEF decreased E-cadherin expression. In addition, we demonstrate that SPDEF expression is not regulated by E-cadherin. Moreover, our chromatin immunoprecipitation and luciferase reporter assay revealed that SPDEF occupies E-cadherin promoter site and acts as a direct transcriptional inducer of E-cadherin in prostate cancer cells. Taken together, to the best of our knowledge, these studies are the first demonstrating requirement of SPDEF for expression of E-cadherin, an essential epithelial cell junction protein. Given that loss of E-cadherin is a central tenant in tumor metastasis, the results of our studies, by providing a new mechanism for regulation of E-cadherin expression, could have far reaching impact.

The progression of a tumor in situ to an invasive tumor is a major prerequisite to cancer metastasis that requires the movement and invasion of cancer cells from the primary tumor into the surrounding tissue (1). These processes include the acquisition of cell motility, novel cell adhesion properties, and invasiveness, which involves a dramatic reorganization of the actin cytoskeleton, a loss of cell-cell adhesion along with a gain of cell-matrix adhesion, and the concomitant formation of membrane protrusions required for invasive growth (2). The molecular processes underlying such cellular changes are still poorly understood, and the various migratory, invasive, and metastatic phenotypes still require a better functional and molecular characterization.
ETS (E-twenty-six transformation-specific) transcription factors are involved in a multitude of normal and pathological cellular processes and play an important role in proliferation, differentiation, development, apoptosis, migration, invasion, and angiogenesis (3). Many ETS factors are deregulated and are thought to be key players in cancer (4). ETS factors have been shown to play a role in the majority of prostate cancer patients (5). SPDEF 2 was originally identified and defined as a prostatederived ETS factor, present in normal prostate luminal cells (6). SPDEF is unique among ETS factors because its expression is highly restricted to the tissues with high epithelial content, namely epithelial cells of prostate, mammary gland, endometrium, ovary, salivary gland, and colon (7). Although expression of SPDEF in cancer tissues remains debated, it is abundantly clear that SPDEF suppresses tumor metastasis in vitro and in vivo (7)(8)(9)(10). We are the first group to demonstrate that decreased SPDEF expression is associated with an increased Gleason score in clinical samples of prostate cancer (10). We also demonstrated that there is an inverse relationship between SPDEF expression and MMP9 expression in the clinical samples in tissue microarray having both normal and cancerous tumor samples of prostate cancer (10). Our results demonstrat-ing the loss of SPDEF and aggressive prostate cancer have been confirmed by at least two other independent studies (11,12); one follow-up study in fact suggests that loss of SPDEF could be a predictor not only of aggressive prostate cancer but also of prostate cancer-associated death (12). Taken together, these studies clearly provide compelling evidence of the association between loss of SPDEF and aggressive prostate cancer. Therefore, seeking an understanding of the mechanisms by which SPDEF regulates cancer progression in general and prostate cancer in particular is highly warranted.
E-cadherin belongs to the cadherin family of calcium-dependent adhesion molecules and is highly expressed in normal epithelial cells and well differentiated cancer cells, but its expression is largely reduced in undifferentiated cancers (13). E-cadherin plays an important role in the maintenance of the structural integrity of epithelial sheets (14) and is regulated at both the transcriptional and post-transcriptional levels (15). Loss of E-cadherin expression has been regarded as a central event in tumor metastasis, because loss of adhesion between tumor cells facilitates their ability to invade locally and to spread to distant organs (16,17). Many studies have focused on the relationship between loss of E-cadherin expression and the invasive and metastatic process. Recent studies have demonstrated that the loss of E-cadherin expression is frequently associated with parameters of enhanced biological aggressiveness such as poor histological differentiation, increased invasiveness, metastatic disease, and a poorer survival rate in patients with prostate (18), breast (19), bladder (20), renal (21), oral (22), hepatocellular (23), pancreatic (24), esophageal (25), thyroid (26), head and neck (27), and gastric carcinomas (28). Experimental studies in vitro and in vivo have suggested that E-cadherin may be a useful prognostic marker for prostate cancer progression (29). Therefore, understanding the molecular mechanisms that regulate the expression of E-cadherin is essential to our understanding of tumor progression.
Because loss of SPDEF and E-cadherin has been observed in cancer progression in several independent studies as described above, we set out to determine whether or not there existed any association between expression of SPDEF and E-cadherin in prostate cancer cells. In the present study, we observed a direct correlation between expression of SPDEF and E-cadherin in prostate cancer. We also show for the first time that stable forced expression of SPDEF in prostate cancer cells up-regulates E-cadherin expression, whereas knockdown of SPDEF down-regulates E-cadherin expression. Moreover, modulation of E-cadherin expression had no effect on SPDEF levels, indicating that SPDEF is upstream of E-cadherin. Moreover, SPDEF and E-cadherin expression decreased cell migration and invasion. Finally, we show that siRNA-mediated knockdown of E-cadherin impairs the ability of SPDEF to modulate cell migration and invasion. Most importantly, we show that SPDEF binds to the E-cadherin locus, suggesting a direct role for SPDEF in the regulation of E-cadherin expression. Taken together, our results provide the first direct demonstration of regulation of E-cadherin expression and a critical role for E-cadherin in modulating the function of SPDEF with respect to cell migration and invasion in prostate cancer.

EXPERIMENTAL PROCEDURES
Reagents and Antibodies-The reagents and antibodies were as follows: ␤-tubulin, and E-cadherin (Cell Signaling, Danvers, MA); Alexa Fluor 488 goat anti-rabbit IgG and Alexa Fluor 594 goat anti-mouse IgG (Molecular Probes/Invitrogen); HRP-conjugated secondary antibodies (Santa Cruz Biotechnology, Santa Cruz, CA); and cell culture medium (Cellgro; Mediatech Inc., Manassas, VA). E-cadherin GFP plasmid and E-cadherin promoter luciferase reporter plasmid (pGL2Basic-EcadK1) were from Addgene (Cambridge, MA). Oligonucleotide primers for PCR were purchased from Integrated DNA Technologies. Except where otherwise stated, all other chemicals were from Sigma-Aldrich.
Constructs and Cell Lines-All cell lines (RWPE-1, PC3, LNCaP, LNCaP C4-2, and LNCaP C4-2B) were purchased from ATCC and maintained according to ATCC guidelines. Phoenix cells were grown in DMEM containing 10% fetal bovine serum. Cloning of SPDEF from PC3 cDNA with an amino-terminal FLAG tag was performed into retroviral vector pBABE and the bicistronic vector QCXIX (Clontech). Creation of stable SPDEF expressing and vector control PC3 cell lines was described previously (10). PC3 cells were transfected with E-cadherin GFP plasmid using Effectene transfection reagents (Qiagen) according to the manufacturer's instructions, yielding transfection efficiencies of 70 -80% for PC3 cells, as estimated by the number of cells expressing GFP-tagged proteins.
Inhibition of E-cadherin Expression by RNA Interference-Three siRNA targeting E-cadherin (GenBank TM accession number NM_004360) were purchased: SASI_Hs01_00086310, SASI_Hs01_00086311, and SASI_Hs01_00086312 from Sigma-Aldrich. As a control siRNA, we used MISSION siRNA Universal Negative Control 1 (Sigma-Aldrich). siRNA transfection of E-cadherin-specific siRNAs was performed in 6-well tissue culture plates according to the protocol recommended for HiPerFect reagent (Qiagen). The final concentration of the siRNAs was 30 nM.
Immunofluorescence-Staining was performed as described previously (9), and images were captured by LEICA DM RXA microscope using PL-APO 40ϫ/1.25-0.75 oil objective over 15-20 high power fields.
Total RNA Isolation and Real Time Quantitative PCR-Total RNA was purified from homogenized cells using RNAeasy kit (Qiagen). One g of total RNA was reverse transcribed using a cDNA synthesis kit (Bio-Rad), and quantitative PCR was performed as described previously (30). Expression was related to the control gene (GAPDH), which did not change under the experimental conditions studied.
Chromatin Immunoprecipitation Assay-ChIP was performed as described previously (30), except that anti-SPDEF (N-14; Santa Cruz Biotechnology) and anti-FLAG M2 (Sigma-Aldrich) antibodies were used. The sequence of the ChIP primers was as given in Table 1.
Reporter Assay-Relative luciferase activity was performed as described previously (31). In brief, the cell lysate was harvested 24 h post-transfection of E-cadherin reporter plasmid in stable pBABE Vec/pBABE SPDEF PC3 and Scr/SPDEF KD LNCaP cells, and luciferase activity was measured and normalized relative to total cell lysate protein for variations in transfection efficiency.
Cell Migration and Invasion-In vitro scratch wound healing assay was performed as described previously (32) using 80 -90% confluent cultures in serum-free medium. In vitro invasion assay was carried out in BD BioCoat Matrigel chambers (Transwell) as described previously (9).
Data Analysis-The experiments presented in the figures are representative of three different repetitions. The data were analyzed statistically by two-tailed Student's t tests using SPSS software. The values are expressed as the means Ϯ S.E., and p Ͻ 0.05 was considered statistically significant.

Loss of SPDEF Is Associated with Loss of E-cadherin in Pros-
tate Cancer-First, we investigated the correlation between SPDEF and E-cadherin mRNA expression levels in different prostate cancer cell lines by quantitative real time RT-PCR (qRT-PCR) on a panel of immortalized prostate cancer cell lines (LNCaP, LNCaP C4-2, LNCaP C4-2B, and PC3) with varying degrees of tumorigenic and metastatic potential to compare expression levels of SPDEF and E-cadherin with respect to nontumorigenic human prostate epithelial cells, RWPE-1. The results of these studies show for the first time a direct correlation between expression of E-cadherin and SPDEF in four prostate cancer cell lines and one normal prostate cell line (Fig. 1, A  and B). All the indicated cell lines showed significant (p Ͻ 0.05) decreases in E-cadherin, as well as SPDEF expression, when compared with normal prostate epithelial cells, RWPE-1. Importantly, the expression of E-cadherin in normal prostate epithelial cell line RWPE-1 was severalfold greater than in the metastatic prostate cancer cells (PC3 and LNCaP C4 2 cells) (Fig. 1A). We also verified alteration in the expression of SPDEF, and E-cadherin, at the mRNA level in different prostate cancer and normal cell lines by semiquantitative reverse transcription-polymerase chain reaction (Fig. 1C). Taken together, these studies revealed a graded decrease of SPDEF and E-cadherin in prostate cancer cells with increasing aggressiveness.
To further confirm the observed correlation between SPDEF and E-cadherin mRNA levels, immunoblot analysis was performed on a similar panel of different prostate cancer cell lines. The results shown in Fig. 1D demonstrate a negligible expression of E-cadherin, as well as SPDEF in aggressive prostate cancer cells (PC3 cells) as compared with RWPE-1 cells. These results show an inverse correlation between expression of E-cadherin, as well as SPDEF and aggressive behavior of cancer cells. A direct correlation between E-cadherin and SPDEF expression led us to further probe into the relationship between E-cadherin and SPDEF.

Stable Forced Expression of SPDEF Results in Increased Expression of E-cadherin in PC3
Cells-To understand the relationship between SPDEF and E-cadherin expression, we examined the role of stable forced expression of SPDEF in regulating E-cadherin expression. For these studies, we generated PC3 cells with stable expression of SPDEF (pBABE SPDEF) or PC3 vector control (pBABE Vec) cells. The results presented in Fig.  2 (A and B) show that stable expression of SPDEF resulted in 12-fold increase in SPDEF mRNA levels and a 40-fold increase in the E-cadherin mRNA levels by qRT-PCR analysis. These results were further confirmed in semiquantitative reverse transcription-polymerase chain reaction (Fig. 2C). To further evaluate whether changes in mRNA levels of SPDEF correlated with the increased protein levels of these two factors, we performed Western blot analysis using total cell lysate of these cell lines. The results of these studies confirmed that the mRNA expression and protein expression patterns of SPDEF and E-cadherin (Fig. 2, A-D) in our cell system were identical. Dual immunofluorescence staining with anti-E-cadherin and anti-SPDEF antibodies provided visual images (Fig. 2E) showing that the lateral membrane staining of E-cadherin was significantly increased in pBABE SPDEF PC3 cells, whereas it was almost undetectable in pBABE Vec cells, supporting the findings from qRT-PCR and immunoblot analysis.

Down-regulation of SPDEF Decreases E-cadherin Expression, but siRNA-mediated Knockdown of E-cadherin Does Not Effect on SPDEF Expression in LNCaP
Cells-To further examine the relationship between SPDEF and E-cadherin expression, we knocked down SPDEF in LNCaP cells using SPDEF-specific shRNA. Next we performed qRT-PCR and Western blot analysis of SPDEF and E-cadherin in these cells. The results of these studies, presented in Fig. 3A, show that shRNA-mediated knockdown of SPDEF in LNCaP (LNCaP SPDEF KD) cells significantly decreased E-cadherin mRNA expression (p Ͻ 0.05), as well as E-cadherin protein levels (Fig. 3B). These results further support the notion of modulation of E-cadherin by SPDEF.
To further test whether or not E-cadherin expression had any role in regulating SPDEF expression, we performed siRNA-mediated knockdown of E-cadherin in LNCaP cells and was no significant change in SPDEF expression following E-cadherin knockdown (Fig. 3, C and D), suggesting a common underlying mechanism for the regulatory roles of SPDEF on   In Silico Analysis of the Promoter Regions of E-cadherin-The ETS family of transcription factors regulates a diverse set of genes through its interaction with specific consensus sequences upstream of target genes (33). SPDEF specifically binds to the core GGAT sequence present in downstream target genes (34). To further address the mechanism by which SPDEF positively regulated E-cadherin expression and to evaluate whether or not SPDEF directly regulated E-cadherin expression, we performed in silico analysis of predicted putative transcription factor, ETS-1 core binding sites present in the E-cadherin promoter ϳ1.5 kb upstream of the E-cadherin start codon (Fig. 5A) with the TFSEARCH program. Using these computational predictions, we identified six ETS putative common transcription factor-binding sites on human E-cadherin promoter as shown in Fig. 5A.
SPDEF Binds to E-cadherin Promoter in Vivo-To determine whether SPDEF interacts with the endogenous E-cadherin promoter at the chromatin level, we performed ChIP experiments in PC3 cells, which lack E-cadherin, and pBABE SPDEF PC3 cells, which express high amounts of E-cadherin. Anti-SPDEF directed against the amino terminus and anti-FLAG tag antibody efficiently pulled down SPDEF protein complexes together with chromatin fragments comprising the Ϫ155 to ϩ 139 E-cadherin proximal regulatory promoter region encompassing of two putative ETS-1 binding sites (Ϫ117/Ϫ111 and Ϫ90/Ϫ84) from transcriptional initiation site (Fig. 5B) relative to IgG control, whereas untreated beads alone did not precipitate E-cadherin promoter fragments. Prostate-specific antigen promoter was used as a positive control (34). The PCR primers flanking the SPDEF binding sites in the proximal promoter produce a 294-bp chromatin fragment detected by ethidium bromide staining (Fig. 5B). To confirm that SPDEF could regulate E-cadherin promoter activity, we performed luciferase reporter assay with an E-cadherin promoter-luciferase reporter construct that contained an endogenous 233-bp (Ϫ108/125 fragment) regulatory region of E-cadherin using stable pBABE Vec/ pBABE SPDEF PC3 and LNCaP Scr/LNCaP SPDEF KD cells. Consistent with our ChIP assays, we found 13-fold higher promoter activity in pBABE SPDEF compared with pBABE Vec PC3 cells (Fig. 5C), whereas SPDEF KD LNCaP had markedly 8-fold lower activity compared with its respective vector control (Fig. 5D). Therefore, the modulation of E-cadherin promoter activity by stable overexpression and knockdown of SPDEF suggested that endogenous SPDEF could regulate E-cadherin promoter activity. Thus, SPDEF physically associated with the E-cadherin proximal promoter in vivo and could therefore function as a direct transcriptional inducer of E-cadherin.

SPDEF Delayed the Migration and Invasion of Prostate Cancer PC3 and LNCaP Cells by Modulating E-cadherin Expression-
Our previous studies have shown that SPDEF stable expression suppresses cell migration. Therefore, we set to further test the role of E-cadherin in mediating the effects of SPDEF on cell migration. For these studies, the cells were seeded 24 h before the wound was generated, and the uncovered area of wound was subsequently measured at 0, 24, and 48 h. The results showed that the migration rate of SPDEF-transfected (Fig. 6A,  left panel, and B, top panel) or E-cadherin-transfected (Fig. 6C,  left panel, and D, top panel) PC3 cells decreased as compared with respective control cells at the same time point. Cells expressing SPDEF or E-cadherin failed to close the wound scratch at 48 h after wounding, whereas control cells were able to move to an almost complete closure. These results suggest that up-regulation of SPDEF/E-cadherin decreased cell migration. In SPDEF-and E-cadherin-transfected PC3 cells, cell-free area was increased by more than 6-fold compared with respective Vec-treated cells at 48 h (p Ͻ 0.005). During the incubation period, the cells moved forward and closed the gap independently of cell division. There was no significant increase in cell growth within 48 h under the same treatment conditions as revealed by cell proliferation assay (data not shown). Similar effects of SPDEF and E-cadherin expression were observed on cell invasion (Fig. 6, A and C, right panels, and B and D, bottom  panels).
To further confirm the role of SPDEF on cell migration and invasion in other prostate cancer cell lines, we performed in vitro cell migration and invasion assay using LNCaP SPDEF KD cells. The closure of the wounded gap was significantly accelerated in LNCaP SPDEF KD monolayer cells compared with respective controls (LNCaP Scr) at 48 h (Fig. 7, A and B, left  panels). Cell invasion assay confirmed a similar migratory prototype (Fig. 7, A and B, right panels) as in cell migration assay.
In complementary experiments, we performed siRNAmediated E-cadherin knockdown of pBABE SPDEF PC3 and LNCaP cells. Interestingly, consistent with the above observation, our live imaging results showed that E-cadherin knockdown into both cell lines closed the wound by 72 h ahead as compared with its respective control, which failed to close the wound by the end of the 48-h experimental period (Fig. 8, A and  C, left panels, and B and D, top panels). The uncovered area at the indicated time points relative to control at 0 h was deter-

FIGURE 5. SPDEF occupies E-cadherin promoter site and acts as a direct transcriptional inducer of E-cadherin in prostate cancer cells. A, schematic
representations showing six c-Ets-1 transcription factor binding sites within Ϫ1.5-kb human E-cadherin promoter regions. B, association of SPDEF with the proximal E-cadherin promoter was analyzed by ChIP analyses in pBABE SPDEF transfected PC3 cells. The amplified human E-cadherin promoter fragment (Ϫ155 to ϩ139) is shown. A control region upstream of the SPDEF-binding site served as negative control. Independent ChIP experiments were performed two times. Lane M, 100-bp DNA marker. C and D, stable prostate cancer cells, pBABE Vec/pBABE SPDEF PC3 (C) and LNCaP Scr/LNCaP SPDEF KD (D), were transfected with E-cadherin promoter luciferase reporter constructs. Twenty-four hours post-transfection, the cells were harvested, and luciferase activity was measured and normalized to total cellular protein. The results are shown from two independent experiments. **, p Ͻ 0.005. mined (Fig. 8, B and D, top panels). The results demonstrated that the rates of cell migration of E-cadherin knockdown in pBABE SPDEF PC3 and LNCaP cells were significantly (p Ͻ 0.05) higher than their respective control cells (Fig. 8, B and D,  top panels). We also found that the numbers of invasive cells in siRNA-mediated E-cadherin knockdown pBABE SPDEF PC3 and LNCaP cells were 3.5-and 6.0-fold higher than their respective controls (Fig. 8, A and C, right panels, and B and D, bottom panels) in in vitro invasion assay.
Taken together, these results demonstrate an inverse relationship between E-cadherin as well as SPDEF expression with cell migration and invasion in prostate cancer cells. Moreover, these results clearly show a critical role for E-cadherin expression in mediating the effects of SPDEF on cell migration and invasion in human prostate cancer cell lines.

DISCUSSION
In the current study we have discovered that, in prostate cancer cells, the loss of SPDEF is associated with loss of E-cadherin. We also show that SPDEF-mediated suppression of cell migration and invasion is dependent in part on E-cadherin expression. Using complementary experiments, we demonstrate that SPDEF is a transcriptional regulator of E-cadherin. To our knowledge, this is the first study to demonstrate that SPDEF suppresses tumor metastasis, in part, by modulating E-cadherin expression in cancer cells. Given the important role of E-cadherin in tumor metastasis (35), coupled with the regulation of E-cadherin by SPDEF as shown here, SPDEF expression may play a critical role in cancer progression in general and in prostate cancer progression in particular.
The results of our studies show that decreased expression of SPDEF is associated with decreased E-cadherin levels in prostate cancer cells. Loss of E-cadherin has been associated with cancer progression in several tissues including prostate cancer (18 -21, 36). It is important to mention that current markers in prostate cancer fail to distinguish between indolent disease and an aggressive and often lethal cancer. There is at present an unmet need for new markers that can separate indolent prostate cancer from lethal disease. Our results, coupled with previous findings by others and us (10 -12) that loss of SPDEF  might serve as a prognostic indicator of aggressive prostate cancer, offer a potential use for SPDEF and E-cadherin as markers for distinguishing indolent prostate cancer from aggressive disease.
Moreover, these results, of decreased SPDEF and E-cadherin expression in invasive prostate cancer cell lines and in high grade prostate tumors, prompted us to investigate the relationship between SPDEF and E-cadherin expressions. We observed that although knockdown of SPDEF resulted in loss of E-cadherin expression, knocking down of E-cadherin had no effect on the levels of SPDEF (Fig. 3). Similarly, in complementary studies, we observed that overexpression of SPDEF increased E-cadherin expression (Fig. 2), whereas E-cadherin overexpression did not alter SPDEF expression (Fig. 4, A and B). These findings are consistent with regulatory role of SPDEF in modulating E-cadherin expression. These studies also rule out potential for E-cadherin expression in modulating SPDEF expression. Tumor metastasis is a multistage process involving changes in tumor cell migration and invasiveness. These properties, in turn, depend on alterations in cell-cell and cell-extracellular matrix adhesion that are highly dynamic and regulated processes. E-cadherin is the primary adhesive molecule in epithelial cell-cell adhesion (37). Loss of E-cadherin has been demonstrated to induce anoikis resistance and promote metastasis (38). Loss of E-cadherin, as is seen in prostate cancer cells following loss of SPDEF, may thus promote resistance to anoikis and help in tumor metastasis. The significance of E-cadherin loss in metastasis has been shown in a variety of in vitro and in vivo models (39,40), and loss of E-cadherin is the major hallmark of epithelial-mesenchymal transition (EMT), which appears to be a critical event for tumor malignancy (41,42). We have previously shown that SPDEF suppresses MMP9 expression, but there were no significant changes of expression of two EMT-related transcription factors Snail and Slug in SPDEF transfected PC3 cells (9). However, we observed an inverse correlation between SPDEF and Twist expression in prostate cancer cells (data not shown). It is interesting that the loss of E-cadherin induces Twist expression, indicating a feed forward loop to maintain EMT (43). Twist is up-regulated in several malignancies and promotes EMT (15,44); however, the precise mechanism of regulation of E-cadherin expression is not understood (45)(46)(47)(48)(49)(50). Given that both EMT and anoikis resistance are key processes for metastasis and that they share common regulators, including Twist, Snail, Zeb1, E-cadherin, and N-cadherin, our results presented herein demonstrate that SPDEF-forced expression in aggressive prostate cancer cells results in re-expression of E-cadherin (Fig. 2), which would imply phenotypic reversal from mesenchymal-like to epithelial-like. Our findings demonstrate that the critical requirement of SPDEF for E-cadherin expression is thus highly relevant to tumor progression. The movement of cells that characterizes this process is brought about by a switch in E-cadherin expression, which causes them to lose their ordered epithelial phenotype and become plastic. This is analogous to late stage tumorigenesis, when E-cadherin silencing causes loss of cell-cell adhesion and results in gain of the invasive phenotype (51). Thus, our results are consistent with regulation of metastasis by SPDEF. Re-expression of E-cadherin following SPDEF expression in prostate cancer cells may also explain phenotypic alterations seen in PC3 cells in three-dimensional culture in our previous studies (10).
The precise mechanisms responsible for E-cadherin inactivation in cancer cells are not completely understood. Alterations at the transcriptional level have been proposed to be one of the major mechanisms responsible for decreased expression of E-cadherin in several cancer types (45)(46)(47). Inactivation of cell adhesion in cancer progression has been linked to modulation of E-cadherin by multiple mechanisms, such as a gene mutation, promoter hypermethylation, chromatin rearrangement, post-translational truncation or modification, and the recently characterized transcriptional repression (45, 46, 48 -50). Our results suggest that SPDEF serves as a transcriptional activator of E-cadherin. This finding was verified by qRT-PCR and immunoblotting on a series of immortalized prostate cancer cell lines that reflect many features of cancer cells in vivo (10). E-cadherin has a tissue-specific distribution in both mice and humans (39), implying the existence of a finely tuned regulatory mechanism for E-cadherin gene expression. Our in silico computational analysis of human E-cadherin promoter sequence revealed the presence of five specific SPDEF-binding (GGAT) sequences in the short stretch of 1.5 kb upstream of the transcription start site. Indeed, using E-cadherin promoter-driven luciferase assay, we demonstrate that SPDEF is required for E-cadherin promoter activity. Moreover, our results demonstrate that SPDEF occupies the E-cadherin promoter in ChIP analysis (Fig. 5B). These results further strengthen the role of SPDEF in transcriptional regulation of E-cadherin.
We observed that E-cadherin expression could reverse the migratory and invasive phenotype that is driven by loss of SPDEF in prostate cancer cells (Fig. 6). In complementary studies, we observed that suppression of E-cadherin prevented affects of SPDEF on cell migration and invasion (Fig. 8). Taken together, these results indicate that the affects of SPDEF, at least with respect to its role in modulating cell migration and cell invasion, are in part mediated by E-cadherin. It is interesting to point out that SPDEF has been shown to regulate several genes that are associated with metastasis in various cancer cells (7). In our previous studies, we observed that among these genes known to be regulated by SPDEF in other cells, only MMP9 and MMP13 are modulated by SPDEF in vitro and in vivo (9). Although MMP9 has been shown to promote cell invasion by degrading the extracellular matrix (52), MMP13 has been shown to function in activation of MMP9 (53). Taken together with our current findings of regulation of E-cadherin by SPDEF, it is obvious that SPDEF modulates at least two important steps in tumor metastasis: cell migration via regulation of E-cadherin expression and cell invasion via regulation of MMP9. These considerations have further brought into focus the multifaceted roles of SPDEF in neoplastic transformation. Moreover, given that cell migration and cell invasion are two essential determinants in tumor cell metastasis, our studies further strengthen the role of SPDEF as a tumor metastasis suppressor in general and prostate tumor metastasis suppressor in particular.
In summary, our studies highlight the role of SPDEF as a transcriptional regulator of E-cadherin and offer a new potential mechanism by which SPDEF could suppress cancer progression and metastasis. Thus, decreased expression of SPFEF and E-cadherin may serve not only as indicators of aggressive prostate cancer but may also serve as novel targets in management of aggressive prostate cancer and perhaps other malignancies. However, additional studies are necessary to cement these conclusions.