C-X-C chemokine receptor 4 (CXCR4) and CXCR7 are 7-transmembrane chemokine receptors of the stroma-derived factor (SDF-1). CXCR4, but not CXCR7, has been examined in bladder cancer (BCa). This study examined the functional and clinical significance of CXCR7 in BCa.
CXCR4 and CXCR7 levels were measured in BCa cell lines, tissues (normal = 25; BCa = 44), and urine specimens (n = 186) by quantitative polymerase chain reaction and/or immunohistochemistry. CXCR7 function in BCa cells were examined by transient transfections using a CXCR7 expression vector or small interfering RNA.
In BCa cell lines, CXCR7 messenger RNA levels were 5- to 37-fold higher than those for CXCR4. Transient overexpression of CXCR7 in BCa cell lines promoted growth and chemotactic motility. CXCR7 colocalized and formed a functional complex with epidermal growth factor receptor, phosphoinositide 3-kinase/Akt, Erk, and src and induced their phosphorylation. CXCR7 also induced up-regulation of cyclin-D1 and bcl-2. Suppression of CXCR7 expression reversed these effects and induced apoptosis. CXCR7 messenger RNA levels and CXCR7 staining scores were significantly (5- to 10-fold) higher in BCa tissues than in normal tissues (P < .001). CXCR7 expression independently associated with metastasis (P = .019) and disease-specific mortality (P = .03). CXCR7 was highly expressed in endothelial cells in high-grade BCa tissues when compared to low-grade BCa and normal bladder. CXCR7 levels were elevated in exfoliated urothelial cells from high-grade BCa patients (P = .0001; 90% sensitivity; 75% specificity); CXCR4 levels were unaltered.
Tumor heterogeneity in terms of metastasis and frequent recurrence make the clinical management of bladder cancer (BCa) challenging.1 Although several determinants of growth and metastasis have been characterized in BCa, the functional and clinical significance of C-X-C chemokine receptor 7 (CXCR7) has not been evaluated. CXCR7/RDC-1 binds C-X-C chemokine ligand 11 (CXCL11/I-TAC) and CXCL12.2-5 Both of these are C-X-C chemokines that lack the glutamine-leucine-arginine (or ELR) motif.2 CXCL12 or stromal cell–derived growth factor-1 (SDF-1) promotes angiogenesis and metastasis by binding to another chemokine receptor, CXCR4.2-5
Both CXCR7 and CXCR4 are 7-span transmembrane G protein–coupled receptors.2-4 Once CXCL12 binds CXCR4, the receptor complexes with the Gαi subunit of G-protein. This results in the inhibition of cyclic adenosine monophosphate production and mobilization of intracellular calcium. The dissociation of Gαi subunit from Gβγ activates, among others, mitogen-activated protein kinase and Akt signaling.6 CXCR4 expression either alone or together with CXCL12 or CXCR7 correlates with metastasis in breast and renal carcinomas.7-9 CXCR4 expression is increased in bladder tumor tissues; however, the expression does not correlate with prognosis.10-12 A peptide antagonist of CXCR4 has been shown to aid in fluorescent imaging of exfoliated cells in the urine of patients with invasive BCa.13
CXCR7 binds to CXCL12 with high affinity, but it has low affinity for CXCL11. It does not activate the Gi pathway and consequently does not induce calcium mobilization.4 CXCR7 expression is elevated in renal and lung tumors and correlates with metastasis.14, 15 In breast, prostate, and lung xenograft models, CXCR7 expression stimulates proliferation, invasion, and motility, and promotes tumor growth and angiogenesis.16-19 In prostate cancer cells, interleukin-8 (IL-8) expression increases CXCR7 expression by 4- to 14-fold.19
In this study, we found that CXCR7 expression in BCa cells regulated proliferation, chemotactic motility, and a complex formation between the signaling molecules epidermal growth factor receptor (EGFR), phosphoinositide 3-kinase (PI3K), and src. CXCR7 expression was up-regulated in bladder tumor tissues and exfoliated urothelial cells present in the urine of patients with BCa, and associated with prognosis.
MATERIALS AND METHODS
BCa cells were obtained from ATCC (Manassas, Va), except 253J-Lung cells that were provided by Dr. Colin Dinney of the MD Anderson Cancer Center, Houston, Texas. All BCa cell lines except the 253J-Lung cell line were isolated from primary high-grade (HG) muscle-invasive tumors. The 253J-Lung cells metastasize to lung when injected orthotopically in athymic mice20 (see also the ATCC Web site). All BCa cells were cultured in Roswell Park Memorial Institute 1640 medium plus 10% fetal bovine serum plus gentamicin. Cell lines were authenticated by Genetica DNA Laboratories (Cincinnati, Ohio).
All antibodies were described previously,21 except the following: CXCR7 (GeneTex, Irvine, Calif), phospho-src (Y-416), src (Cell Signaling Technology, Danvers, Mass), EGFR, CXCR4 (Sigma Aldrich, St. Louis, Mo), phospho-EGFR (Y-1173) (Epitomics, Burlingame, Calif).
All specimens were obtained based on their availability for research purpose and under a protocol approved by the University of Miami Institutional Review Board (Table 1). Normal bladder tissues (NBL; n = 25) were obtained either from organ donors or from patients who underwent cystectomy. A portion of each BCa (n = 44) and NBL tissue was paraffin-embedded and the other was flash-frozen. Total RNA was isolated from tissues (∼30 mg) using the RNeasy Mini kit.
Table 1. Patient Characteristics and Characteristics of Bladder Tissue and Urine Specimens Are shown
Metastasis (+): 65.7 ± 12.1 y Median: 68 y Metastasis (–): 64.1 ± 10.9 y Median 63 y
(+) = 8 (–) = 24 Unknown = 12
(+) = 6 (–) = 22 Unknown = 16
(+) = 6 (–) = 19 Unknown = 19
(–) = 23 (+) = 18; Bca-specific = 16 Unknown = 3
Mean follow-up For patients with stage ≥ T2 tumor
26.8 ± 4.4; median = 20.5 (1a-120 months) 19.1 ± 2.2; median = 17.5 (1a-41)
Urine specimens were collected from 186 individuals (Table 1). Clinical follow-up was collected on patients who had a history of bladder cancer (HxBCa). All urine specimens were brought to the laboratory within 2 hours of collection and processed for total RNA isolation using the ZR urine isolation kit.22
Transfection of BCa Cells With CXCR7 Plasmid and CXCR7 Small Interfering RNA
HT1376 and 253J-Lung cells were transiently transfected with a full-length CXCR7 complementary DNA construct (Origene) using Lipofectamine 2000 (Invitrogen, Carlsbad, Calif). Human 5637 cells (bladder carcinoma cell line) were transfected with CXCR7 small interfering RNA (siRNA) or control siRNA (100 nM; Dharmacon-RNAi Technologies; Thermo Scientific). At 72 hours after transfection, the transfectants were counted and apoptosis was measured by Cell Death ELISA (enzyme-linked immunosorbent assay) Plus kit. Apoptosis index was calculated as optical density at 450 nm wavelength (OD450nm) per 20,000 cells. Chemotactic motility assays was carried out using the Boyden chamber (8-μm inserts), as described.21 Percent motility was calculated as (OD bottom chamber ÷ OD (top + bottom chambers)) × 100. Cell lysates were analyzed by immunoblotting.
Vector and CXCR7 transient transfectants of 253J-Lung cells (2 × 106 cells) were immunoprecipitated using a rabbit anti-CXCR7 antibody and protein A-agarose beads.19 The immunoprecipitates were subjected to immunoblotting using anti-CXCR7, anti-src antibodies under nonreduced conditions and anti-EGFR, anti-85 PI3K antibodies under reduced conditions.
Quantitative Polymerase Chain Reaction
Total RNA isolated from transfectants, tissues, or exfoliated cells was subjected to quantitative polymerase chain reaction (Q-PCR) in a Bio-Rad iCycler iQ real-time PCR system22 using the following primers: CXCR4: forward: 5′-TCATCAAGCAAGGGTGTGAG-3′; reverse: 5′GGCT CCAAGGAAAGCATAGA-3′; CXCR7: forward: 5′-TACCCCGAGCACAGCATCAA-3′;reverse: 5′-TG GAGAAGGGAACGGCAAAG-3′. Each complementary DNA sample was simultaneously subjected to β-actin (for tissues and transfectants) or 18S (urine specimens) Q-PCR, and the normalized transcript levels for each gene were calculated as (1/2Δct × 100); ΔCt = Ct (transcript) − Ct (β-actin).
Transient transfectants of 253J-Lung cells were fixed in 4% paraformaldehyde at 4°C for 15 minutes and then subjected to the “In-cell co-IP,” a fluorescence PCR-based antibody binding technique, using rabbit anti-CXCR7 (1:500), mouse anti-EGFR (1:500) antibodies and the Duolink II kit, as per the manufacturer's instructions (Olink-Bioscience, Uppsala, Sweden). The slides were observed under a Zeiss LSM700 Confocal microscope equipped with multivariant fluorescence filters in 2 channels (red and blue) under a 40× oil-immersion lens.
Sections (5 μm) of paraffin-fixed bladder tissues on positively charged slides were sequentially deparaffinized, rehydrated, and subjected to antigen retrieval.22 The slides were incubated at 37°C and room temperature for 1 hour each, with a rabbit anti-CXCR7 (1:4000) or a rabbit anti-CXCR4 (1:1750) antibody. Immunohistochemistry (IHC) procedure and all the controls were performed as described.22 The same batch of antibodies and commercial reagents were used in all experiments. The specificity of the antibodies was determined by down-regulation of CXCR7 and CXCR4 by siRNA, followed by immunoblotting.
Stained slides were graded by 2 individuals in a blinded fashion. To account for the heterogeneity in staining, each specimen was graded for staining intensity (0 to 3+) and then multiplied by the area in the specimen showing that staining intensity. The intensity scores in all areas within a specimen were added to obtain the staining score for the specimen. Therefore, each specimen received a staining score between 0 and 300.22 The intensity scores of the 2 readers were then averaged to obtain the final score.
The in-cell coimmunoprecipitation was performed on paraffin-fixed bladder tissues using the same procedure as the IHC, up to the step of antigen retrieval. The specimens were stained using the rabbit anti-CXCR7 (1:300) and mouse anti-CD31 (GeneTex; 1:300) antibodies at 37°C for 1 hour and the Duolink II kit, as per the manufacturer's instructions (Olink-Bioscience, Uppsala, Sweden). The slides were observed by confocal microscopy.
Differences in proliferation, apoptosis, and motility among transfectants were compared, using the unpaired t test. CXCR7 and CXCR4 levels among bladder tissues (eg, NBL versus low-grade [LG], NBL versus HG) and among various categories of urine specimens were compared using the Mann-Whitney U test, because the data showed a non-normal distribution. All of the P values reported in this study are 2-tailed. Logistic regression model (univariate analysis) was used to determine: 1) the association of clinical parameters, and CXCR7 levels (ie, transcript levels or staining scores) with metastasis and disease-specific survival; 2) the association of urinary CXCR7 levels with BCa. The Cox proportional hazards model (multivariate analysis) was used to determine which of the pre- and postoperative parameters and/or tissue CXCR levels predict metastasis and disease-specific mortality.
Cutoff values were selected from the receiver operating characteristic curve for calculating sensitivity and specificity of CXCR7 to predict metastasis, disease-specific survival (tissue specimens) and the presence of BCa (urine specimens). A biomarker level that yielded the highest efficacy, ie, sensitivity − (1-specificity), was selected by the statistical program as the cutoff limit. JMP Software Program (SAS Institute, Cary, NC) was used for statistical analyses.
Expression of CXCR7 and CXCR4 in BCa Cell Lines
In all BCa cell lines, except for 253J-Lung, CXCR7 messenger RNA (mRNA) levels were 5- to 37-fold higher than CXCR4 levels (Fig. 1A). Furthermore, the expression of CXCR7 in 5637 and HT1197 cell lines was 3- to 10-fold higher than in other cell lines.
CXCR7 Enhances Proliferation and Motility in BCa Cells
To determine the functional significance of CXCR7, we either transiently overexpressed CXCR7 in 253J-Lung and HT1376 cells or down-regulated CXCR7 expression in 5637 cells. CXCR7 overexpression or down-regulation was confirmed by Q-PCR (data not shown). CXCR7 overexpression increased, whereas CXCR7 down-regulation decreased the growth of respective transfectants by 1.5-fold (P < .0001; Fig. 1B). Down-regulation of CXCR7 also significantly decreased the growth of HT1197 cells (Cell number: control: 6.5 × 104 ± 0.45 × 104; CXCR7 siRNA: 3.4 × 104 ± 0.25 × 104; P < .001).
CXCR7 expression regulates apoptosis.23,24 CXCR7 down-regulation in 5637 cells induced apoptosis (Fig. 1B, right panel; apoptosis index: control: 0.16 ± 0.02; CXCR7 siRNA: 0.26 ± 0.03). The down-regulation of CXCR7 in HT1197 also induced apoptosis (control: 0.1 ± 0.015; CXCR7 siRNA: 0.21 ± 0.025; P = .0027). Low basal level of apoptosis was observed in HT1376 cells (0.09 ± 0.02) and CXCR7 overexpression caused a small decrease (0.07 ± 0.006) in this basal level of apoptosis (Fig. 1B, right panel).
CXCR7 overexpression also increased the chemotactic motility in HT1376 and 253J-Lung cells by 1.7- and 3.1-fold, respectively (Fig. 1C). Conversely, CXCR7 down-regulation decreased the motility by 2.7-fold in 5637 cells.
CXCR7 Expression Regulates EGFR, Erk, and Akt Activation
Figure 1D shows CXCR7 overexpression and down-regulation in various transfectants. CXCR7 expression has been shown to regulate cell cycle progression by modulating cyclin D1 levels.19 Consistent with the effect of CXCR7 on cell growth, CXCR7 overexpression increased (>2-fold) and CXCR7 down-regulation decreased (≈2-fold) cyclin D1 levels in HT1376 and 5637 cells, respectively (Fig. 1D). Consistent with the increase in apoptosis, CXCR7 down-regulation induced PARP cleavage in 5637 cells (Fig. 1D).
CXCR7 has been shown to form a functional complex with EGFR.19 CXCR7 overexpression increased EGFR (Tyr1173) phosphorylation by >2-fold and CXCR7 down-regulation, inhibited EGFR phosphorylation (Fig. 1D). CXCR7 overexpression or down-regulation did not affect EGFR mRNA levels (data not shown). In-cell coimmunoprecipitation coupled with confocal microscopy showed that both EGFR and CXCR7 colocalize on the plasma membrane of 253J-Lung cells transfected with a CXCR7 construct (Fig. 2A).
Erk- and PI3K/Akt signaling is downstream of EGFR activation, and CXCR7 induces Erk and Akt activation.19, 24, 25 Overexpression of CXCR7 increased phospho-Erk, phospho-Akt levels ≈2-fold (Fig. 1D). Consistent with the cross-talk between EGFR and src,26, 27 CXCR7 overexpression increased and its down-regulation decreased phospho-src levels by ≥2-fold (Fig. 1D). Consistent with the modulation of Akt signaling, CXCR7 overexpression increased and its down-regulation decreased bcl-2 levels.
To determine how CXCR7 expression regulates phosphorylation of EGFR, Akt, or src, we examined whether CXCR7 forms a complex with EGFR, src, and PI3K. EGFR, p85 subunit of PI3K, and src were coimmunoprecipitated with CXCR7 (Fig. 1D). Furthermore, overexpression of CXCR7 significantly increased the coimmunoprecipitation of these proteins together with CXCR7. Therefore, CXCR7 may promote BCa cell growth and motility by activating EGFR and downstream signaling. Contrary to previous reports,23, 28 CXCR7 overexpression or its down-regulation did not significantly alter vascular endothelial growth factor and IL-8 transcript levels (data not shown).
CXCR7 Expression Is Increased in BCa Tissues
Figure 2B shows that when compared to NBL (NBL-O: 1.2 ± 1.4; NBL-T: 0.94 ± 1.0) tissues, CXCR4 mRNA levels were 3- to 4-fold elevated in LG (3.5 ± 2.8; P ≤ .04) and HG (4.2 ± 4.6; P ≤ .03) BCa tissues. CXCR7 levels were 5- to 9-fold elevated in both LG (2.0 ± 0.4; P ≤ .009) and HG (3.1 ± 2.3; P ≤ .001) BCa tissues when compared to NBL tissues (NBL-O: 0.33 ± 0.4; NBL-T: 0.4 ± 0.8). No significant correlation was observed between tumor grade and CXCR4 (P = .39) or CXCR7 (P = .13) levels. CXCR4 (P = .32) and CXCR7 (P = .25) levels also did not correlate with stage. Furthermore, CXCR4 or CXCR7 levels were not significantly different among patients with non–muscle invasive (stages Ta, T1) or muscle invasive (stages ≥ T2; P > .05) disease.
In this study, 8 patients received neoadjuvant chemotherapy. CXCR4 or CXCR7 transcript levels were not significantly different in these patients when compared to those who did not received neoadjuvant chemotherapy (P > .5). Six patients received radiation therapy following cystectomy due to advanced disease. Both CXCR4 (P = .093) and CXCR7 (P = .055) levels were not significantly different among patients who received radiation versus those who did not.
Expression of CXCR4 and CXCR7 proteins was evaluated by IHC in the same set of BCa tissues. Figure 2C shows that in both NBL and BCa tissues, CXCR4 and CXCR7 were expressed only in urothelial cells. CXCR4 levels were similar in both NBL (NBL-O: 76 ± 95; NBL-T: 38 ± 45) and BCa (LG: 88 ± 86; HG: 77 ± 93) tissues. However, CXCR7 expression was elevated in BCa specimens (LG: 136 ± 114; HG: 183 ± 99), when compared to NBL tissues (NBL-O: 6.3 ± 9.2; NBL-T: 32 ± 47; Fig. 2C). CXCR4 expression was not significantly different between NBL and BCa tissues, regardless of the tumor grade (P = .72; Fig. 2D). However, CXCR7 staining was significantly higher in both LG and HG tissues when compared to NBL tissues (P = .002; Fig. 2D). The staining inferences for CXCR4 (P = .57) and CXCR7 (P = .22) did not significantly correlate with tumor grade (CXCR4, P = .57; CXCR7: P = .22) or stage (CXCR4, P = .0114; CXCR7 (P = .78).
Association of CXCR4 and CXCR7 Expression With Metastasis
In this study, the majority of the patients had HG (n = 37) and muscle-invasive (n = 33) BCa. In univariate analysis stage, lymph node, CXCR7 transcript levels, and CXCR7 staining score significantly associated with metastasis (Table 2, univariate analysis). In the multivariate model, stage and CXCR7 mRNA levels associated with metastasis (Table 2, multivariate analysis). For disease-specific mortality, only sex and CXCR7 staining scores were significant predictors (Table 2, multivariate analysis).
Table 2. Determination of the Association Between Metastasis and Pre- and Postoperative Parameters and CXCR4 and CXCR7 Expression
The pre- and postoperative parameters included age, sex, tumor grade, stage, lymph node status, and concomitant presence of CIS. Only significant parameters (P < .05) are shown for both univariate and multivariate analyses. For univariate analysis, ND indicates odds ratios were not determined for parameters that did not reach significance. For multivariate analysis, Cox proportional hazards analysis was performed by including pre- and postoperative parameters and the transcript levels/staining scores of CXCR4 and CXCR7.
5640; Not defined
Although the number of specimens was limited, CXCR7 transcript levels (cutoff of 2.4) had 81.2% sensitivity and 70% specificity to associate with metastasis, respectively. For both CXCR7 mRNA levels and CXCR7 staining scores, respectively, the sensitivity (61%, 100%) and specificity (55%, 39%) were low for predicting disease-specific mortality.
In hepatocellular carcinoma and meningioma, CXCR7 is expressed in tumor endothelial cells.29, 30 Visualization of CXCR7 expression in endothelial cells by the “In-cell co-IP” technique showed no CXCR7 expression in endothelial cells in NBL tissue and weak expression in LG BCa tissue (Fig. 2E). However, there was high CXCR7 expression in endothelial cells in the HG BCa tissue.
CXCR7 mRNA Expression Is Increased in Exfoliated Urothelial Cells From BCa Patients
We used the Q-PCR assay to measure the transcript levels of CXCR4 and CXCR7 exfoliated urothelial cells present in urine specimens. Normalized CXCR4 mRNA levels were not significantly different among BCa patients and the other categories of individuals (ie, normal, benign genitourinary [BGU], HxBCa, history [Hx] of other cancer [Ca], other Ca) and ranged between 1.4 and 3.7 (Fig. 3A). Contrarily, CXCR7 levels were significantly elevated in patients with HG BCa (Fig. 3B, Table 3). CXCR7 levels in various categories were as follows: normal: 0.4 ± 0.2; BGU: 0.5 ± 0.4; HxBCa: 0.5 ± 0.4; Hx other Ca: 0.4 ± 0.1; LG: 0.6 ± 0.3; HG: 1.3 ± 1.5; other Ca: 0.7 ± 0.5. The differences in CXCR7 levels between patients with HG BCa and normal (P = .0002), BGU (P = .0002), or HxBCa or other cancers (0.002) categories were significant. The differences in CXCR7 levels among patients with LG BCa and the control categories were not significant (P > .05).
Table 3. Determination of the Association Between Presence of BCa and CXCR4 or CXCR7 Transcript Levels in Exfoliated Urothelial Cellsa
Odds Ratio; 95% CI
aResults of univariate analysis. The cutoff limit generated by the receiver operating characteristic curve was used to determine the sensitivity and specificity and accuracy of CXCR7 to detect BCa; in each column, mean ± standard deviation and 95% confidence interval (CI) values obtained by cross-validation (bootstrap modeling; specific sampling rate = 0.5; resampling = 104) are shown.
Abbreviations: AUC, area under the receiver operating characteristic curve; ND, not determined.
79.5 ± 5.0;
75.5 ± 5.0
Efficacy of CXCR7 to Detect BCa
Based on the cutoff values generated by ROC curves, CXCR7 transcript levels had 80.7% sensitivity to detect BCa. This reasonably high sensitivity was due to the high sensitivity of CXCR7 to detect HG BCa (Table 4). The specificity of CXCR7 levels among normal individuals was high (Table 4). However, the specificity of CXCR7 levels among patients with BGU conditions was low. This low specificity was because 10 of 13 patients with nephrolithiasis had CXCR7 transcript levels above the cutoff limit (ie, 0.52). Among the 30 patients with HxBCa, 6 recurred within 6 months. Although 4 of these 6 patients had CXCR7 levels above the cutoff limit CXCR7 marker, only 2 of the 24 HxBCa patients who did not have recurrence had levels above the cutoff limit (chi-square: 10.2; P = .0014; relative risk = 8.0; 95% CI = 1.9-33.9).
Table 4. Analysis of Sensitivity by Tumor Grade and of Specificity by Non-BCa Conditionsa
The benign genitourinary (BGU) category also included 8 patients with history of other urologic cancers.
This is the first study that establishes the functional and clinical significance of CXCR7 expression in BCa. The key findings of our study are: 1) Most BCa cell lines express higher levels of CXCR7 when compared to CXCR4; 2) CXCR7 expression promotes BCa cell growth, survival and motility; 3) CXCR7 forms a complex with EGFR, src, and PI3K. Such a signaling complex may be responsible for the observed increase in the phosphorylation of EGFR, Akt, and src; 4) CXCR7 expression (both mRNA and protein) is elevated in bladder tumor tissues and associates with prognosis; and 5) in exfoliated urothelial cells, CXCR7 mRNA expression is up-regulated in HG BCa patients.
Treatment of T24 and J82 BCa-cells with SDF-1 has been shown to increase cell proliferation and motility11 and it is partially blocked by coincubation with an anti-CXCR4 antibody.11 Our data show that in T24 and J82 cells, CXCR7 expression is 6- to 10-fold higher than CXCR4. Because CXCR7 has higher affinity for SDF-1, it can explain why anti-CXCR4 antibody only partially blocked the biological activity of SDF-1. In our study, CXCR7-stimulated cell proliferation, survival and chemotactic motility was observed in the absence of any externally added SDF-1. Furthermore, fetal bovine serum contains little SDF-1.19 BCa cell lines also expressed low levels of SDF-1 and the expression did not correlate with either CXCR7 or CXCR4 expression (unpublished results). Therefore, due to its high affinity for SDF-1, CXCR7-mediated signaling may occur at very low concentrations of SDF-1 or CXCR7 may function in a ligand-independent manner.19
The colocalization and coimmunoprecipitation experiments show that CXCR7 and EGFR form a complex that recruits downstream signaling molecules such as PI3K and src. This signaling complex also explains, why unlike CXCR4, although CXCR7 does not induce Gβγ activation,2-4 it still activates Akt (phosphorylation at Ser473) and Erk. The connection between CXCR7 and mitotic signaling is further strengthened by the observation that CXCR7 expression modulates cyclin D1 levels. The effect of CXCR7 expression on apoptosis is very likely due to Akt signaling, because CXCR7 expression also regulates bcl-2 levels. These results suggest that CXCR7 expression regulates proliferation, survival, and motility in BCa cells through signaling events such as EGFR activation and Akt signaling.
In our study, CXCR7 expression did not alter IL-8 or vascular endothelial growth factor expression. However, in hepatocellular carcinoma and meningioma, CXCR7 has been shown to localize in tumor endothelial cells.29, 30 Our results show that CXCR7 expression in bladder endothelial cells increases with tumor grade; HG bladder tumors show high expression of CXCR7 in endothelial cells.
In most BCa cell lines, the ratio of CXCR7:CXCR4 expression was between 5 and 37, ie, CXCR7 expression was significantly higher than CXCR4 expression in BCa cell lines. Contrarily in NBL tissues, CXCR7 expression was 2.5- to 3-fold lower than CXCR4 expression (CXCR7:CXCR4 ratio: NBL-O: 0.35; NBL-T: 0.36). The functional significance of this switch with respect to the CXCR7:CXCR4 ratio in BCa cell lines when compared to normal bladder is currently unknown. It is also noteworthy that the expression of both CXCR4 and CXCR7 was lower in BCa cell lines than in BCa tissues. It is possible that in BCa tissues, both tumor cells and endothelial cells contribute to the expression of these chemokine receptors. Nevertheless, in exfoliated bladder tumor cells, CXCR7 mRNA expression was higher (1.3 ± 1.5) in patients with HG bladder tumors than in healthy individuals, patients with benign conditions, or in patients with a history of BCa (∼ 0.5 ± 0.3). Therefore, the differences in BCa cell lines and tumor cells in BCa tissues, with respect to the chemokine receptor expression, may be due to cell culture conditions, and established cell lines versus the actual tissue microenvironment.
The detection of CXCR4 by a peptide antagonist has been suggested as a diagnostic tool to detect invasive BCa cells in urine.13 However, in our study, CXCR4 mRNA levels were similar in BCa patients and the individuals in various control categories. Contrarily, CXCR7 mRNA levels were significantly elevated in the exfoliated urothelial cells from patients with HG BCa; 90% sensitivity. Based on a recent study which showed that bind of bacterial lipopolysaccharide to the Toll-like receptor 4 (TLR4) increased CXCR7 expression in a colon carcinoma cell line,31 one would have expected a consistent increase in CXCR7 expression among patients with urinary tract infection. However, CXCR7 mRNA levels were consistently increased among patients with nephrolithiasis but not among patients with other inflammatory conditions such as cystitis or urinary tract infections.
In this study, although CXCR7 mRNA levels independently correlated with metastasis, CXCR7 staining associated with disease specific mortality. This variability among mRNA levels and staining scores could be due to small cohort size and/or insufficient follow-up. Some patients with HG BCa did not metastasize and/or experience mortality as the endpoint within the available follow-up. These reasons also bring to light certain limitations of this study. First, to validate the findings of the study, a multi-institutional study will be needed to address the inherent limitation of a single-center study. The plausibility of tumor heterogeneity resulting in higher levels of CXCR7 mRNA as a limitation may be related to sampling during tissue analysis. This limitation however is self-limiting, because stage ultimately correlated to metastasis in multivariate analysis.
Taken together, CXCR7 has multiple functions in BCa cells, including promoting proliferation, motility and survival. This study also suggests that a reexamination of the functions attributed to the binding of SDF-1 to CXCR4 in BCa cell lines may be required, because we found higher expression of CXCR7 than CXCR4 in those very BCa cell lines. Furthermore in BCa, CXCR7 expression associates with invasion and metastasis. On the basis of these results, a multicenter study to prospectively evaluate the prognostic potential of CXCR7 is warranted.
Grant support provided by: Women's Cancer Association of University of Miami pilot award; Florida Department of Health - James and Esther King Biomedical Research Program (10KT-01; to V.B. Lokeshwar, University of Miami site; Team Science Project Principal Investigator: Dr. Charles Rosser, MD Anderson Cancer Center Orlando); 2R01CA072821-11 (to V.B. Lokeshwar). Judith Knapp was a fellow of the International Academy of Life Sciences, Biomedical Science Exchange Program.
We thank Mr. Gabriel Gaidosh for his help with the confocal microscopy. We are grateful to Dr. Mark Soloway, Chairman Emeritus, Department of Urology, University of Miami, Miller School of Medicine, for critically reviewing the manuscript.