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Neoplasia. Jul 2006; 8(7): 578–586.
PMCID: PMC1601934

CCL2 is a Potent Regulator of Prostate Cancer Cell Migration and Proliferation1

Abstract

Tumor cells in the bone interact with the microenvironment to promote tumor cell survival and proliferation, resulting in a lethal phenotype for patients with advanced prostate cancer. Monocyte chemoattractant protein 1 (CCL2) is a member of the CC chemokine family and is known to promote monocyte chemotaxis to sites of inflammation. Here we have shown that human bone marrow endothelial (HBME) cells secrete significantly higher levels of CCL2 compared to human aortic endothelial cells and human dermal micro-vascular endothelial cells. Furthermore, we demonstrate that CCL2 is a potent chemoattractant of prostate cancer epithelial cells, and that stimulation of PC-3 and VCaP cells resulted in a dose-dependent activation of PI3 kinase/Akt signaling pathway. Activation of the PI3 kinase/Akt pathway was found to be vital to the proliferative effects of CCL2 stimulation of both PC-3 and VCaP cells. Additionally, CCL2 stimulated the phosphorylation of p70-S6 kinase (a downstream target of Akt) and induced actin rearrangement, resulting in a dynamic morphologic change indicative of microspike formation. These data suggest that bone marrow endothelial cells are a major source of CCL2, and that an elevated secretion of CCL2 recruits prostate cancer epithelial cells to the bone microenvironment and regulates their proliferation rate.

Keywords: CCL2, prostate cancer, migration, chemokine, metastasis

Introduction

The predominance of prostate cancer metastasis to the bone has been well documented from three independent autopsy series and has been reported to occur with > 85% frequency on patients with advanced hormone-refractory prostate cancer [1]. However, the mechanisms responsible for the prevalence of osseous lesions in men with prostate cancer remain unclear. Metastasis is a process that is defined by a series of sequential steps resulting in end organ tumor metastasis through a migratory pattern that appears to be directed, specific, and predictable [1]. One mechanism that has been proposed to explain the enhanced frequency of bone metastases in prostate cancer is preferential adhesion to the bone marrow endothelium [2]. However, simply adhering to the endothelial wall is not sufficient to invade an organ; thus, some cancer cells must acquire the ability to migrate from the luminal side of the endothelial cells into the surrounding tissue in response to chemotactic molecules released by stromal cells. Several chemokines, including stromal cell-derived factor 1 (SDF-1/CXCL12) and, recently, CCL2 [3–5], have been shown to promote chemotactic migration of cancer cells (prostate for SDF-1 [4] and myeloma for CCL2 [5]). CCL2 is a member of the CCβ chemokine family and was originally known to promote monocyte and macrophage migration to sites of inflammation [6,7].

CCL2 has previously been shown to be an important determinant of macrophage and monocyte infiltration in breast, cervix, and pancreatic carcinomas [8]. Recent studies have demonstrated that CCL2 localizes to tumor epithelial cells [9], and that the levels of CCL2 expression have been correlated with the involvement of lymphocyte and macrophage localization in secondary sites of tumor formation [10]. There is growing evidence to suggest that CCL2 may act directly on the epithelial cells of several human carcinomas and may regulate the migration and invasive properties of tumor cells, resulting in enhanced metastatic potential. Youngs et al. [11] demonstrated a dose-dependent migratory response of breast cancer cells to increasing concentrations of exogenous CCL2. Additionally, CCL2 expression has been shown to correlate with progression in pancreatic cancer [12] and breast cancer [13]. Here we describe a role for CCL2 in prostate cancer migration and proliferation as a mechanism for increased bone metastases. Accordingly, we hypothesized that CCL2 is a novel potent regulator of prostate cancer migration and proliferation at the site of the bone microenvironment.

Experimental

Materials

Human recombinant CCL2 (hrCCL2) and anti-CCL2 anti-body were obtained from Chemicon International (Temecula, CA); anti-phospho AktSer473, anti-Akt, anti-GSK3β, anti-phospho GSK3α/β, anti-p70-S6 kinase, and anti-phospho p70-S6 kinase were obtained from Cell Signaling (Beverly, MA); and all other reagents were obtained from Sigma-Aldrich (St. Louis, MO).

Cell Culture

PC-3 prostate cancer, human aortic endothelial cells (HAECs), and human dermal microvascular endothelial cells (HDMVECs) were obtained from ATCC (Manassas, VA) and passaged under appropriate growth conditions. VCaP prostate cancer and human bone marrow endothelial (HBME) cells were a kind gift from Dr. Kenneth J. Pienta (University of Michigan, Ann Arbor, MI). PC-3 cells were maintained in RPMI 1640 + 10% fetal calf serum (FCS) (Invitrogen Corp., Carlsbad, CA). HAECs and HDMVECs were maintained in EGM + 5% FCS, whereas VCaP and HBME cells were maintained in DMEM + 10% FCS (Invitrogen Corp.). Cells were passaged by trypsinization using 1 x trypsin + EDTA (Invitrogen Corp.) and resuspended in appropriate growth media.

Cytokine Antibody Array

Normal vertebral tissue and tumor vertebral tissue were collected from a patient with advanced hormone-refractory prostate cancer, in accordance with the rapid autopsy series conducted at the University of Michigan. Tissue specimens were snap frozen in liquid nitrogen and pulverized with a mortar and pestle. Crushed tissue was resuspended in RIPA buffer (50 mM Tris-HCl, pH 7.4; 1 % NP-40; 150 mM NaCl; and 1 mM EDTA) containing protease inhibitors (1 µg/ml aprotinin, leupeptin, and pepstatin A; 1 mM PMSF; 1 mM NaF; and 1 mM Na3VO4). Lysates were pulse-sonicated at 40% duty cycle for 5 seconds, and protein lysates were collected by centrifugation at 13,000 rpm for 15 minutes at 4°C. Protein lysates were diluted, and cytokine arrays were performed according to the manufacturer's instructions (RayBiotech, Inc., Norcross, GA).

CCL2 Enzyme-Linked Immunosorbent Assay (ELISA)

Cells were plated in six-well plates and grown to 80% confluency in appropriate growth media. Cells were then washed with serum-free RPMI1640 or DMEM supplemented with 1% penicillin and streptomycin. Cells were incubated in serum-free media for 24 hours, and the conditioned medium from each well was collected and stored at -80°C until use. The level of CCL2 in cell culture supernatants was determined with Quantikine human CCL2 sandwich ELISA kit (R&D Systems, Minneapolis, MN), according to the protocol supplied by the manufacturer.

Fluorescence-Based Migration Assay

Cell migration was assessed using Innocyte Cell Migration Assay (Calbiochem, Inc.), following the manufacturer's instructions. Briefly, increasing concentrations of CCL2 (1–100 ng/ml) in the presence and absence of CCR2 inhibitors or neutralizing antibodies were added to the lower chamber of a 96-well plate. Cells were harvested by 0.5 µM EDTA release and resuspended at 2.5 x 105 cells/ml in serum-free media. A total of 2.5 x 104 cells was added to the upper chamber and allowed to migrate through the membrane with 8-µm pores for 24 hours at 37°C and 5% CO2 atmosphere. Cells that migrated through the membrane were detached and labeled with calcein AM, and fluorescence was measured using a fluorescent plate reader with an excitation wavelength of 485 nm and an emission wavelength of 520 nm. The experiment was repeated in triplicate, and each conditioned experiment was performed in quadruplicate.

Western Blot Analysis

Cells were lysed in RIPA buffer (50 mM Tris-HCl, pH 7.4; 1% NP-40; 150 mM NaCl; 1 mM EDTA; 1 mM PMSF; 1 mM Na3VO4; 1 mM NaF; 1 µM okadaicacid; and 1 µg/ml aprotinin, leupeptin, and pepstatin). Proteins were separated under reducing conditions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred onto nitrocellulose membrane. The membranes were blocked with 5% milk in 0.1% Tween in Tris-buffered saline for 1 hour at room temperature. They were incubated overnight at 4°C with primary antibodies. Membranes were washed thrice before incubation with horseradish peroxidase-conjugated secondary antibodies (Cell Signaling) for 1 hour at room temperature. Protein expression was visualized by ECL chemiluminescence (Promega, Madison, WI) and quantitated using Image J software (NCI, Bethesda, MD).

Immunofluorescence

PC-3 cells were plated on glass coverslips and serumstarved for 2 hours before stimulation. Cells were stimulated with CCL2 (100 ng/ml) for 30 minutes in the presence or absence of identified inhibitors. Stimulation with SDF-1 (200 ng/ml) for 30 minutes was used as a positive control. Cells were fixed in methanol-free 3.7% paraformaldehyde for 10 minutes at room temperature then permeabilized with 0.5% Triton X-100 for 5 minutes at room temperature. Cells were rinsed twice with PBS and incubated for 30 minutes at room temperature with 3% bovine serum albumin (BSA) in PBS + 0.05% Tween 20 to prepare cells for staining. Cells were incubated with Phalloidin AlexaFluor568 (Molecular Probes, Inc., Carlsbad, CA) at a 1:40 dilution in BSA solution to label actin. Cells were washed and mounted on coverslips with Pro-Long Antifade containing DAPI (Molecular Probes, Inc.), following the manufacturer's instructions. Immunofluorescence was visualized using a multiphoton laser scanning microscope, consisting of a Mai Tai Broadband Ti:Sapphire laser (Spectra-Physics, Mountain View, CA) (tunable between 710 and 990 nm) and a modified Olympus Fluoview 300 confocal microscope (Olympus, Center Valley, PA) with long working-distance water-immersion infrared objectives. Images were captured with a x60 objective (zoom, x2), 800-nm excitation wavelength, and 605- and 520-nm centered emission filters.

Proliferation Assay

Cells were seeded at a density of 4 x 103 cells/well in a six-well plate with complete media. Twenty-four hours after seeding, the medium was replaced with serum-free medium supplemented with increasing concentrations of CCL2 (1–100 ng/ml) in the presence or absence of LY294002 (1 µM). Cell growth was determined at 24, 48, 72, and 96 hours after seeding using the WST-1 assay (Pierce Biotech, Rockford, IL), following the manufacturer's instructions.

Real-Time RT-PCR

Cells were serum-starved for 24 hours before RNA isolation. Total RNA was isolated from cell lines using Trizol (Invitrogen Corp.), following the manufacturer's specifications. Purified RNA (5 µg) was converted to cDNA using Super Script II reverse transcriptase (Invitrogen Corp.), following the manufacturer's instructions, and used for gene expression analysis by real-time PCR using an ABI Prism 7900HT (Applied Biosystems, Foster City, CA) thermocycler. Primers and probes were purchased from Applied Biosystems, Inc., and used with TaqMan Universal PCR Master Mix (Applied Biosystems), No AmpErase UNG. GAPDH was used as an internal control to normalize and compare each sample. Cycle conditions for real-time PCR were 95°C (15 seconds), 60°C (1 minute), and 72°C (1 minute) for 40 cycles. The threshold cycle number for each sample was normalized to GAPDH for that sample and expressed on a log scale relative to GAPDH expression.

Statistics

Data were analyzed with GraphPad Prizm software. Oneway analysis of variance was used with Bonferroni's post hoc analysis for comparison between multiple groups. Student's t test was used for comparison between two groups. Significance was defined as P < .05.

Results

Identification of CCL2 Expression in the Tumor-Bone Microenvironment

Identification of the prominent cytokines and growth factors involved in the tumor-bone microenvironment is essential to the understanding of prostate cancer metastasis. We collected specimens from three patients diagnosed with prostatic adenocarcinoma (Table 1). Sites of metastasis were identified by gross examination, and tissue samples were collected and snap frozen for analysis. Tumor and normal (adjacent to tumor) bone specimens were collected from vertebral lesions and processed for total protein lysates. Analysis of cytokine and growth factor expression was performed using cytokine antibody arrays from RayBiotech, Inc. A representative cytokine array demonstrates that several cytokines were upregulated in the tumor-bone microenvironment, compared to the normal (adjacent to tumor) bone microenvironment (Figure 1A). In particular, monocyte chemoattractant protein 1 (CCL2) was upregulated four-fold in the tumor microenvironment compared to that in the normal microenvironment (Figure 1B).

Figure 1
Representative cytokine antibody arrays comparing normal (A) versus tumor (B) microenvironments. Arrays demonstrate the upregulation of CCL2 (boxed in area; replicates of two) in the normal bone (A) compared to that in the tumor-bone (B) microenvironments. ...
Table 1
Patient Information for Specimens Retrieved and Analyzed by Cytokine Antibody Array.

Identification of the Source of CCL2 from Constituents of the Bone Microenvironment

CCL2 is known to be a potent stimulator of monocyte and macrophage migration to sites of inflammation [7]. To identify a role of CCL2 in prostate cancer metastasis, CCL2 secretion was determined by ELISA from PC-3, VCaP, HBME, osteoblasts, and NIH 3T3 L1 adipocytes. Interestingly, prostate cancer cells were not a significant source of CCL2; however, HBME cells secreted significantly more CCL2 compared to the other cell lines analyzed (PC-3, 2.435 ± 0.123 pg/ml; VCaP, 21.037 ± 3.213 pg/ml; HBME, 1269.083 ± 26.281 pg/ ml; OB, 4.32 ± 1.85 pg/ml; adipocytes, 18.398 ± 3.874 pg/ml; mean ± SD) (Figure 2A). Next, the secretion of CCL2 from different endothelial cell lines was compared to assess the specificity of HBME cells as a significant source of CCL2. HAECs and HDMVECs were used for comparison. HBME cells secreted significantly higher levels of CCL2 compared to HAECs and HDMVECs (HBME, 805.26 ± 29.81 pg/ml; HAECs, 10.12 ± 3.70 pg/ml; HDMVECs, 21.86 ± 8.61 pg/ml; mean ± SD) (Figure 2B). To identify CCL2 as an important chemotactic factor secreted by HBME cells that induces prostate cancer cell migration, cells were placed in a modified Boyden chamber. The conditioned medium (24 and 48 hours) from HBME cells was used as chemoattractant, and PC-3 cell migration was measured after a 24-hour period. The conditioned medium from HBME cells stimulated the migration of PC-3 cells, and the migration was inhibited by the presence of an anti-CCL2 neutralizing antibody (PC-3: SFM, 164.6 ± 8.92; CM24, 308.4 ± 54.46; CM48, 481.2 ± 31.36; CM24 + anti-CCL2, 155.6 ± 22.35; CM48 + anti-CCL2, 229.3 ± 22.35; mean ± SD) (Figure 2C).

Figure 2
ELISA of CCL2 release. (A) Two prostate cancer cell lines (PC-3 and VCaP), bone marrow endothelial cells (HBME), osteoblasts (OB), and adipocytes (NIH 3T3 L1) cells were plated in six-well plates. Conditioned medium was collected after 24 hours, and CCL2 ...

Effects of CCL2 on Prostate Cancer Cell Migration

Migration is an essential step in metastatic cascade and is dependent on the reorganization of the actin cytoskeleton. The majority of data suggests that migration is regulated, in part, by chemotactic gradients that stimulate the recruitment of tumor cells to sites of metastases. In prostate cancer, SDF-1 has been postulated as an important chemotactic factor that stimulates prostate cancer cell migration through activation of the CXCR4 receptor [4]. To further our understanding of the role of CCL2 in prostate cancer cell migration, we used a 96-well migration assay with increasing concentrations of hrCCL2 as chemoattractant. PC-3 and VCaP cells migrated in a dose-dependent manner toward hrCCL2 (PC-3: control, 44.62 ± 3.83; 1 ng/ml, 83.53 ± 2.981; 10 ng/ml, 142.2 ± 2.678; 100 ng/ml, 248.1 ± 0.761; VCaP: control, 116.4 ± 2.529; 1 ng/ml, 130.6 ±2.145; 10 ng/ml, 176.1 ± 9.051; 100 ng/ml, 296.5 ± 2.681; mean fluorescence ± SD) (Figure 3, A and C, respectively). The dose-dependent migration of both PC-3 (Figure 3A) and VCaP (Figure 3C) was attenuated by RS-102895, a CCR2b receptor antagonist (Figure 3, A and C, respectively). CCL2-induced migration was partially attenuated with the presence of an anti-CCR5 neutralizing antibody (Figure 3, A and C). Furthermore, the migration of both PC-3 and VCaP cells to CCL2 at all concentrations was attenuated by the administration of anti-human CCL2 and anti-mouse CCL2/JE neutralizing antibodies (Figure 3, B and D).

Figure 3
CCL2 is a chemoattractant for prostate cancer cells and stimulates cell migration. Prostate cancer cell migration in response to CCL2 was also assessed with recombinant human CCL2 (1–100 ng/ml) in (A) PC-3 and (C) VCaP cells. CCR2 receptor inhibitor ...

CCL2 Induces Akt Activation in PC-3 and VCaP Cells

CCL2 has been shown to induce the activation of the PI3 kinase/Akt signaling pathway [14]. To determine whether the CCL2 stimulation of PC-3 and VCaP cells induces similar signaling pathways, PC-3 and VCaP cells were stimulated with a supraphysiological dose of CCL2 (100 ng/ml) for various time points indicated. CCL2 induced Akt phosphorylation, as measured by immunoblot analysis in a time-dependent fashion, in both VCaP and PC-3 cells, with a maximal activation at 30 minutes (Figure 4, A and C). Furthermore, PC-3 and VCaP cells were stimulated with increasing concentrations of CCL2 (0.1–100 ng/ml) for 30 minutes. CCL2 stimulated Akt phosphorylation in a dose-dependent fashion in PC-3 and VCaP cells (Figure 4, B and D). Furthermore, stimulation of PC-3 cells with CCL2 at 100 ng/ml induced p70-S6 kinase phosphorylation, but had no effect on GSK3α/β phosphorylation, both of which are downstream targets of Akt (Figure 4E).

Figure 4
CCL2 induces Akt phosphorylation in prostate cancer cells. PC-3 (A) and VCaP (C) were treated with CCL2 (100 ng/ml) for 0 to 30 minutes, and Akt phosphorylation was determined by Western blot analysis. PC-3 (B) and VCaP (D) were treated with increasing ...

Effects of CCL2 on Prostate Cancer Cell Proliferation through Activation of PI3 Kinase/Akt

Activation of PI3 kinase/Akt is known to be in favor of the proliferative signaling pathway [15], and previous evidence has shown that CCL2 stimulates the proliferation of macrophages through a PI3 kinase/Akt-dependent mechanism [16]. To assess the effects of CCL2 on prostate cancer cell proliferation, PC-3 and VCaP cells were stimulated with increasing concentrations of CCL2 for 24,48, 72, and 96 hours in the presence of LY294002 (1 µM), a PI3 kinase inhibitor. Both PC-3 and VCaP cells demonstrated enhanced proliferation in a dose-dependent fashion in response to CCL2 over the 96-hour proliferation assay (solid lines, Figure 5, A and B). The effects of CCL2 on PC-3 and VCaP cell proliferation were attenuated by the addition of LY294002 (1 µM) during the 96-hour assay (dashed lines, Figure 5, A and B).

Figure 5
CCL2 induces PC-3 and VCaP cell proliferation through a PI3 kinase/Akt-dependent mechanism. PC-3 (A) and VCaP (B) cells were plated in a 96-well plate and stimulated with increasing concentrations of hrCCL2 (1–100 ng/ml) for 24 to 96 hours (solidlines, ...

Differential Expression of CCL2 Receptors in Prostate Cancer Cell Lines

To determine the mechanism of action of CCL2 on prostate cancer cells, the differential mRNA expression of CCR2, the high-affinity receptor for CCL2, was quantified by realtime PCR and normalized to GAPDH levels expressed in a panel of prostate cancer cell lines. The results are displayed using the cycle threshold method previously described [17]. CCR2 was variably expressed in RWPE-1, PC-3, VCaP, DU145, LNCaP, C4-2B, and DUCaP. PC-3 and VCaP had the highest levels of expression (Table 2).

Table 2
Differential Expression of the CCL2 Receptor CCR2 in a Panel of Prostate Cancer Cell Lines.

CCL2 Induces Actin Reorganization in PC-3 Cells

Change in the organization of the actin cytoskeleton is an essential step in the migratory and proliferative phenotype of most cells and is known to be linked to G protein-coupled receptors [11]. CCR2 is a G protein-coupled receptor and has been shown to regulate the actin cytoskeleton, resulting in a phenotypic change in migration of B cells [18]. Additionally, p70-S6 kinase has been shown to regulate actin polymerization and to colocalize with actin at the leading edge during filapodial extension [19]. We assessed the ability of CCL2 to stimulate alteration in the actin cytoskeleton in PC-3 cells. Immunofluorescence revealed morphologic changes consistent with actin rearrangement and the formation of “finger-like” projections or microspikes after 30-minute stimulation with CCL2 (10 ng/ml) compared to control cells (Figure 6, A and B). Furthermore, coincubation of CCL2 (10 ng/ml) with an anti-CCL2 (2 µg/ml) neutralizing antibody attenuated the actin reorganization and formation of finger-like projections (Figure 6C).

Figure 6
CCL2 induces actin rearrangement and lamellipodial formation in PC-3 cells. Unstimulated PC-3 cells (A) were compared to PC-3 cells that were stimulated with CCL2 (100 ng/ml) (B) in the presence of a neutralizing CCL2 antibody (4 µg/ml) (C) or ...

Discussion

Recognition of prostate cancer metastasis to the bone as a lethal phenotype leads to the design of new targeted therapies directed at both the cancer cells and the bone microenvironment. Tumor cells in the bone interact with the extracellular matrix, stromal cells, osteoblasts, osteoclasts, and endothelial cells to coordinate a sophisticated series of interactions to promote tumor cell survival and proliferation, leading to morbidity and mortality for patients with advanced prostate cancer [20,21]. There is growing evidence supporting the hypothesis that cytokines and chemokines released in the local microenvironment promote metastasis, tumor cell proliferation, and tumor growth in a specific coordinated mechanism [4]. Here we demonstrate an important role of CCL2, a member of the CC chemokine family, in the pathogenesis of prostate cancer skeletal metastases.

Utilizing tissue procured during the rapid autopsy program at the University of Michigan, a comparison was made between cytokine and chemokine expression in the microenvironment of bone metastatic lesions and normal/adjacent bone from three patients with advanced metastatic disease using a cytokine/growth factor antibody array (Figure 1). The most upregulated chemokine (more than a four-fold increase) in the tumor-bone microenvironment was identified as CCL2 (Figure 1C). A number of other cytokines were elevated by more than two-fold in the tumor-bone microenvironment when compared to the normal/adjacent bone (EGF, FGF-6, IGFBP-1, IGFBP-2, IL-3, IL-4, IL-6, MDC, MIG, TGFβ1, TGFβ3, TNF-β, and angiogenin). Furthermore, CCL2 expression in the tumor microenvironment was specifically elevated in bone lesions compared to its expression in soft tissue lesions, suggesting that CCL2 may be an important chemokine in prostate cancer bone biology. Current studies collecting multiple patient samples to more closely investigate the correlation of CCL2 upregulation in prostate cancer bone metastases are underway. Analysis of CCR2 receptor expression using the ONCOMINE database, which contains publicly available gene expression sets comparing expression data between benign prostatic hyperplasia (n = 23), localized prostate cancer (n = 64), and metastatic prostate cancer (n = 25), identified a statistically significant (P < .01) increase in CCR2 mRNA expression, which correlates with disease progression [22]. Taken together, these data suggest that CCL2 produced by the bone microenvironment induces CCR2 receptor activation on prostate cancer epithelial cells and may promote prostate cancer growth and metastasis.

CCL2 belongs to a family of cytokines that is known to promote the migration of monocytes and macrophages to sites of inflammation. Recently, a role of CCL2 in regulating the migration and proliferation of cancer epithelial cells has been shown in breast cancer and multiple myeloma [5,7,23,24]. Upregulation of cytokines and chemokines at the site of a secondary lesion has been postulated to play an important role in “homing” and tumor formation. For example, SDF-1/CXCR4 has recently been shown to exert a predominant role in regulating prostate cancer cell metastasis to the bone [4]. In the data presented here, we demonstrate the ability of CCL2 to stimulate prostate cancer cell migration and proliferation in a dose-dependent manner. Additionally, the predominant source of CCL2 in the bone microenvironment appears to be the bone marrow endothelial cells (Figure 2). CCL2 is known to be synthesized by the vascular endothelium as a mechanism to recruit monocytes and macrophages to sites of vascular injury. Our initial data have demonstrated that bone marrow endothelial cells secrete significantly higher levels of CCL2 compared to two vascular endothelial cell lines in culture (HAECs and HDMVECs; Figure 2B). The significance of the elevated secretion of CCL2 by bone marrow endothelial cells is unclear; however, we hypothesize that the elevated CCL2 levels released by the bone marrow endothelium may be an important component of the bone microenvironment that influences the predominance of prostate cancer bone metastases.

Currently, the only study to have addressed the role of CCL2 in prostate cancer focused on CCL2 expression in prostate epithelial cells and stromal cells during benign prostatic hyperplasia and localized prostatic adenocarcinoma. CCL2 was shown to be expressed by smooth muscle cells in the prostate gland surrounding the epithelial cells and in benign epithelial cells. CCL2 expression was reported to be less in the cancerous epithelial cells of localized prostate cancer [25]. The data presented here suggest that the function of CCL2 in prostate cancer pathogenesis may be localized to the metastatic process and may be an important mechanism in the development of bone metastases of prostate cancer cells. Here we demonstrate the ability of HBME cells to secrete significantly higher levels of CCL2 compared to HAECs and HDMVECs (Figure 2B). We recognize that monocytes/macrophages and osteoclasts are known to secrete CCL2; to date, it remains unclear how these cells contribute to the CCL2-rich bone microenvironment. The data presented here suggest that the bone marrow endo-thelium contributes to the overall elevated levels of CCL2 in the tumor-bone microenvironment and may promote a variety of functions, including tumor cell migration and extravasation, tumor cell growth, macrophage infiltration and differentiation, and osteoclast activation. Furthermore, we have demonstrated CCL2 to act as a chemoattractant for bone-derived prostate cancer epithelial cells and to regulate their migratory properties in a dose-dependent fashion (Figure 3). Additionally, we have demonstrated that CCL2 stimulates the proliferation of prostate cancer cells by a LY294002-sensitive mechanism that is consistent with PI3 kinase-mediated mechanism. Furthermore, stimulation with CCL2 induces Akt phosphorylation with further downstream activation of the p70-S6 kinase (Figure 4). Activation of p70-S6 kinase has been shown to regulate changes in the actin cytoskeleton [19] and, thus, may play a role in the enhanced migratory phenotype of prostate cancer cells when stimulated with CCL2. Furthermore, the p85-S6 kinase is a second iso-form of the p70-S6 kinase, which contains a nuclear localization signal in the N-terminus. Stimulation of PC-3 cells resulted in the activation of p85-S6 kinase, which may be important in regulating portions of the nuclear matrix. The chemokine family has been postulated to play a significant role in the tumorigenesis and metastasis of several human cancers [6,26]. Recently, evidence has suggested that CCR2, the high-affinity receptor for CCL2, is linked to the actin cytoskeleton through interactions with FROUNT [27,28]. It will be important to determine if alternative CCL2 receptors interact with the cytoskeleton to regulate the migration and proliferation of prostate cancer cells. Further analysis of CCL2 is essential to fully understand the involvement of CCL2 in the pathogenesis of prostate cancer.

Footnotes

1This work was supported, in part, by grants from the National Cancer Institute and the National Institutes of Health (SPORE, PO1, and RO1; to K.J.P.). R.D.L was a scholar of the American Foundation of Urologic Diseases.

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