CCL2 Mediates Cross-talk between Cancer Cells and Stromal Fibroblasts That Regulates Breast Cancer Stem Cells Free

Recent studies indicate that a subset of cancer cells possessing stem cell properties, referred to as cancer-initiating or cancer stem cells (CSC), play crucial roles in tumor initiation, progression, and therapeutic refractoriness (1, 2). Similar to embryonic and somatic stem cells, the self-renewal and differentiation of CSCs are simultaneously regulated by intrinsic (cancer cell–endowed) and extrinsic (microenvironmental) factors. Despite the increasing number of studies on genes and pathways involved in cancer stemness, factors in the tumor microenvironment that regulate CSCs, and how cancer cells, in turn, modify the niche by influencing their neighboring cells remain largely uncharacterized. In this study, we focus on the regulation of CSCs by stromal fibroblasts, an important cellular component of the tumor-hosting niche in many human cancers, especially breast cancer. Fibroblasts release a variety of growth factors, chemokines, and components of the extracellular matrix into the microenvironment and influence the differentiation and homeostasis of adjacent epithelia (3). Reconstitution of human-specific mammary glands in cleared mouse mammary fat pads using stem cell–enriched human mammary epithelial cell organoids requires coinjection of human mammary fibroblasts (4), suggesting a critical role for fibroblasts in regulating stem cell functions. Fibroblasts interplay with cancer cells at all stages of cancer progression through complex paracrine mechanisms. Cancer-associated fibroblasts (CAF) can promote cancer progression by modulating multiple components in the cancer niche to build a permissive and supportive microenvironment for tumor growth and invasion.

In the current study, we observed a robust induction of the stem cell–like mammosphere-forming phenotype in breast cancer cells cocultured with CAFs. The CSC-stimulating effect was attributed to the increased secretion of CCL2 (monocyte chemotactic protein-1; MCP-1) by CAFs when compared with normal cell–associated fibroblasts (NAF). CCL2 signals through the G-protein–coupled receptor chemokine (C-C motif) receptors CCR2 and CCR4, and is a potent chemoattractant for monocytes and other immune cells to areas of inflammation (5). Both monocytic and nonmonocytic cells, such as fibroblasts and endothelial cells, secrete CCL2 in response to cytokine stimulation (6, 7). Cancer cells also frequently overexpress CCL2 to modify their local environment. Here, we further found that CCL2 induced the self-renewal of CSCs by inducing NOTCH1 expression at both RNA and protein levels. NOTCH signaling has been recognized as a key regulator in normal and malignant stem cells from various tissues, including the breast (8, 9). Upon ligand binding, NOTCH receptors are activated by sequential cleavages involving members of the ADAM (a disintegrin and metalloprotease domain) protease family (α-secretases) and γ-secretase. These cleavage events result in translocation of the NOTCH intracellular domain (NICD) to the nucleus, where it acts on downstream targets (10). Our identification of NOTCH-mediated activation by tumor environmental factor CCL2 therefore presents a unique mode of niche-conferred regulation of CSCs. These findings further emphasize the importance of simultaneously targeting cancer cells and events within the tumor microenvironment, such as production of CCL2 by stromal cells, in future anticancer therapies that also incorporate the latest stem cell research.

Human breast cancer tissues were obtained from consented patients at City of Hope Medical Center (Duarte, CA) under approved Institutional Review Board protocols. Tissues were mechanically and enzymatically dissociated, and epithelial tumor cells and fibroblasts (CAF) were isolated as previously described (11). Briefly, breast cancer tissue was mechanically minced into small pieces, placed in digestion solution containing collagenase III (Worthington Biochemical Corporation) and DNase I (AppliChem), and incubated at 37°C for 2 to 3 hours with occasional pipetting. The cells were separated by differential centrifugation at 90 × g for 2 minutes. The epithelial (tumor) cells in the pellet were cultured in Iscove's Modified Dulbecco's Media (Invitrogen) containing 0.7 mmol/L l-glutamine (Mediatech/Cellgro), 5 μg/mL insulin (Lonza), 5 μg/mL transferrin (Lonza), 5 ng/mL selenium (Lonza), and 20% FBS (PAA Laboratories). The supernatant containing fibroblasts were centrifuged at 800 × g for 10 minutes, resuspended and cultured in Dulbecco's Modified Eagle's Medium (Mediatech/Cellgro) containing 10% FBS on a nontreated dish. Purity of primary tumor cells and CAFs were confirmed by expression of epithelial-specific antigen (ESA) and vimentin, respectively, in flow cytometric and immunofluorescence assays (Supplementary Fig. S1). CAF265922 (primary CAFs) and XP265922 (primary tumor cells) were isolated from a primary triple-negative breast cancer that was resistant to the chemotherapy regimen containing cisplatin, 5-fluorouracil, and docetaxel. CAF3 were isolated from a primary HER2-positive breast cancer for which the primary tumor cells were not available. Normal human mammary fibroblasts (NAF2) were purchased from ScienCell. For immunohistochemistry in primary breast tumors, pretreatment core biopsies or surgical specimens were obtained from patients with HER2-positive (31 cases) or triple-negative (ER⁻/PR⁻/HER2⁻; 20 cases) breast cancer. Specimens were collected and processed for formalin fixation and paraffin embedding in a time frame that would preserve the integrity of protein epitopes.

Human breast cancer cell lines BT474, MDA-MB-361 (MDA361), and MCF7, and the noncancerous mammary epithelial cell line MCF10A were obtained from American Type Culture Collection and cultured in the recommended media in a humidified 5% CO₂ incubator at 37°C. Recombinant human CCL2 was purchased from R&D Systems. The STAT3 inhibitor Stattic, p38 mitogen—activated protein kinase (MAPK) inhibitor SB202190, and γ-secretase inhibitor (GSI) DAPT were purchased from Sigma-Aldrich. The α-secretase inhibitor INCB3619 was provided by Incyte Corporation. For conditional knockdown of CCL2, the short hairpin RNA (shRNA) targeting the CCL2 mRNA (TRCN0000006283) was constructed into the pTIG (pHIV7-TetR-IRES-GFP) lentiviral vector (ref. 12; kindly provided by Dr. Rossi) downstream of a doxycycline-inducible U6 promoter, as described elsewhere (13). GFP-labeled CAF265922 were generated using pBABE-GFP retroviral vector. Production of viruses, as well as infection and selection of CAFs were carried out as previously described (13).

Please see Supplementary Materials for procedures.

Please see Supplementary Materials for procedures.

Please see Supplementary Materials for procedures.

Please see Supplementary Materials for procedures.

Single-cell suspensions prepared from tumors or cell culture were stained with allophycocyanin (APC)-conjugated human ESA antibody (catalog #347200; BD Biosciences) or analyzed by ALDEFLUOR assay kit (catalog #01700; Stemcell Technologies) following the manufacturer's protocol. Flow cytometric assays were conducted using a CyAn ADP cytometer (Dako) and analyzed with FlowJo software (TreeStar). Electronic cell sorting based on intensity of GFP, APC, PKH67, or ALDEFLUOR was done on a FACSAriaIII cell sorter (BD Biosciences).

All animal experiments were approved by the institutional animal care and use committee at City of Hope. Please see Supplementary Materials for procedures.

Immunohistochemical staining of formaldehyde-fixed, paraffin-embedded primary or xenograft tumor tissues was carried out as previously reported (14) using the following antibodies and dilutions: human CCL2 (catalog #ab9669; Abcam), 1:75 dilution; NOTCH1 (for primary breast cancer: catalog #1935-1, Epitomics; for xenograft tumors: catalog #3608, Cell Signaling), 1:40 dilution; and SMA (catalog #ab5694; Abcam), 1:100 dilution. For CCL2 and α-smooth muscle actin (SMA), cytoplasmic staining was evaluated and for NOTCH1 nuclear staining was evaluated. Stained slides were scored according to intensity of staining (−, 0; +, 1; ++, 2; and +++, 3) and percentage of tumor cells staining positive for each antigen (0%, 0; 1%–30%, 1; 31%–70%, 2; and >70%, 3). The score for the intensity of immunostaining was multiplied by the score for the percentage of cells staining positive to obtain a final score, which was used in the statistical correlation analysis of CCL2 and NOTCH1.

All quantitative data are presented as mean ± SD. All results were confirmed in at least 3 independent experiments, and data from one representative experiment was shown. The statistical analysis was conducted with a SPSS 13.0 software package. Student t test or ANOVA test was used for comparison of quantitative data. Values of P less than 0.05 were considered significant. The differential expression of CCL2, CCR2, and NOTCH family of receptors and ligands between grade III and grade I breast cancers were analyzed using a logistic regression model after which an OR was reported. For example, an OR of 13.72 (CCL2) means that tumors with each unit increase in CCL2 expression are 13.72 times more likely to be grade III, rather than grade I [95% confidence internal (CI), 2.66–70.89]. The linear dependence between CCL2 and NOTCH1 expression was evaluated by Pearson (for the microarray data set) or Spearman correlation coefficient (for the immunohistochemical data set). The microarray gene expression data and associated clinical data were extracted from a published 295 breast cancer data set (15, 16).

The mammosphere assay has been used to functionally characterize and enrich normal and malignant stem cells from the breast, relying on the unique feature of stem cells to escape anoikis and grow into spheres in anchorage-independent conditions (17, 18). Using this approach, we examined the sphere-forming efficiency (SFE) in BT474 breast cancer cells in the presence or absence of cocultured primary human mammary NAFs or CAFs. Compared with the breast cancer cells that were cultured alone, coculture with CAFs, but not NAFs, significantly increased mammosphere formation in breast cancer cells (Fig. 1A). When NAFs and CAFs were first activated in vitro by coculturing with BT474 cells before being transferred to the mammosphere coculture, both activated NAFs and the CAFs that were continuously activated by breast cancer cells in vitro were able to induce BT474 mammosphere formation to a greater extent than their preactivation counterparts (Fig. 1A). The mammosphere-inducing effect was also observed with the conditioned medium (CM) harvested from CAFs, but not NAFs. CM from CAFs that were further activated by BT474 or MDA361 breast cancer cells exhibited an enhanced capacity for inducing mammosphere formation. In vitro activation of NAFs also resulted in mammosphere-inducing activity in the CM, which was more significant in NAFs that had been cocultured with BT474 cells for a prolonged 10-day period (Fig. 1B). These results indicate that the continuous activation of stromal fibroblasts by breast cancer cells leads to secretion of fibroblast-derived soluble factors that can induce the CSC-like phenotype.

To identify these soluble factors, we conducted a cytokine array assay using the CM of untreated and BT474-treated CAFs. Significant induction of CCL2 was observed following in vitro activation of CAFs (Fig. 1C). At the RNA level, expression of CCL2 was induced in CAFs 7- to 9-fold in response to activation by various breast cancer cells, with breast cancer–activated CAFs producing the highest amounts of CCL2 among various cell types, including NAFs and breast cancer cells. Treatment of CAFs with the noncancerous MCF10A human mammary epithelial cells, as well as short-term (3 day) treatments of NAFs with breast cancer cells only modestly induced CCL2 expression (Fig. 1D). To examine the direct effect of CCL2 on CSCs, we added increasing amounts of recombinant CCL2 to various breast cancer cells, and observed dose-dependent formation of mammospheres in BT474 and MDA361, but not MCF7 cells (Fig. 1E). Using a neutralizing antibody for CCL2, we further showed that depletion of functional CCL2 from the CM of activated CAFs abolished the CSC-inducing activity (Fig. 1F). Thus, we concluded that breast cancer–activated CAFs regulate CSCs through expression and secretion of higher levels of CCL2.

The sphere-initiating CSCs are maintained in the primary mammospheres through self-renewal, and are able to give rise to secondary mammospheres when cells from the primary spheres are dissociated and allowed to grow in anchorage-independent conditions (17). We therefore examined the effect of CCL2 on CSC self-renewal by secondary mammosphere culture. Interestingly, compared with the control spheres that had not been treated with CCL2, the first-passage spheres that had been treated with CCL2 contained higher numbers of CSCs capable of initiating secondary spheres, even in the absence of continuous CCL2 treatment (Fig. 2A). On the basis of this result, we hypothesized that CCL2 either promoted the self-renewal (to cause a self-renewing expansion) of existing CSCs, or, alternatively, promoted the conversion of non-CSCs to CSCs. These possibilities were examined using a reported approach involving the labeling of an initial cell population with PKH fluorescent dye and tracking the serial dilution of fluorescence in single-cell–formed spheres during the cell division events in mammosphere formation (19). Because the PKH dye binds to cell membranes and segregates in daughter cells after each cell division, the PKH fluorescence intensity of each cell in a sphere reflects its proliferation history. The PKH^high cells, therefore, represent the stem cell population that has undergone a limited number of divisions during sphere formation, in contrast to the nonstem, PKH^low cells that are highly proliferative (19). Using PKH67-labeled BT474 cells, we generated primary mammospheres containing cells with various PKH67 intensities. In the absence of CCL2 treatment, about 50% of mammospheres carried 1 PKH67^high cell per sphere, and about 33% and 13% carried 2 and 3 PKH67^high cells, respectively. This distribution was altered in CCL2-treated spheres, where the percentage of spheres carrying a single PKH67^high cell decreased to 28%, and of those carrying 3 or more PKH67^high cells significantly increased (Fig. 2B). In spheres carrying multiple PKH67^high cells, the PKH67^high cells also exhibited slightly dimmer fluorescence comparing to those in spheres carrying single PKH67^high cells (Fig. 2B and Supplementary Fig. S2), likely indicating one or 2 more rounds of division (and PKH67 dilution) of the PKH67^high cells before entering the stem cell–like, slow-proliferating or quiescent state in the multi-PKH67^high cell spheres. The overall population of PKH67^high cells in dissociated sphere cells was also approximately 2-fold higher in CCL2-treated spheres than untreated spheres, as determined by flow cytometric analysis (Fig. 2C). When the PKH67^high and PKH67^low sphere cells were purified by fluorescence-activated cell sorting, the PKH67^high primary sphere cells exhibited the highest SFE in the secondary mammosphere formation assay when compared with the PKH67^low cells (Fig. 2D), consistent with their CSC property. CCL2 treatment did not alter the SFE of PKH67^low and PKH67^high cells in the secondary sphere formation (Fig. 2D), indicating that CCL2 does not cause conversion of non-CSCs (PKH67^low) to CSCs (sphere initiating). Therefore, we conclude that CCL2 regulates CSCs by inducing their self-renewing expansion. Interestingly, a similar effect has been previously described for NOTCH activation on stem cells (8). The role of NOTCH pathway in mediating CSC regulation by CCL2 was further investigated in Fig. 4.

We then set out to identify the mechanism underlying increased CCL2 production in breast cancer–activated CAFs. STAT3 has been recently reported to bind to and activate the promoter of CCL2 (20). To determine whether breast cancer cells induce CCL2 expression in fibroblasts through STAT3 activation, we first examined the effects of breast cancer–derived CM and a STAT3 inhibitor on a previously reported CCL2 promoter reporter (21) and on CCL2 expression in CAFs freshly isolated from a triple-negative (ER⁻/PR⁻/HER2⁻) primary breast cancer (CAF265922). CM from breast cancer cells, including BT474, MDA361, and the primary breast cancer cells isolated from the same breast cancer specimen (XP265922), but not from MCF10A cells, markedly induced luciferase expression driven by the CCL2 promoter (Fig. 3A), as well as endogenous expression of CCL2 (Fig. 3B). These effects were abrogated by addition of the STAT3 inhibitor (Fig. 3A and B), indicating STAT3 involvement in mediating the induction of fibroblast-derived CCL2 expression. Similar results were also obtained using CAFs from a different primary breast cancer (Supplementary Fig. S3). Indeed, high levels of STAT3 phosphorylation were observed in CAFs as early as 30 minutes following treatment with breast cancer–derived, but not MCF10A-derived, CM (Fig. 3C). The induction of CCL2 expression was sustained over a time course of 5 days in CAFs cocultured with breast cancer cells, and was abolished by inhibition of STAT3 (Supplementary Fig. S4). Although CM from BT474, MDA361, and primary breast cancer cells all induced STAT3 phosphorylation and CCL2 expression in CAFs, different cytokines were detected in the CM of BT474 and MDA361 (Fig. 3D). In either case, multiple cytokines with reported STAT3-activating effect were detected, suggesting that the STAT3 activation observed in breast cancer–treated CAFs was likely a combined effect of various breast cancer–secreted cytokines. Thus, breast cancer cells derived from different origins and secreting different cytokines activate the STAT3 core pathway in fibroblasts of the tumor microenvironment, leading to STAT3-mediated promoter activation of CCL2.

To explore the molecular basis for CCL2-mediated regulation of CSCs, we surveyed the CCL2 responsiveness of genes involved in NOTCH, Wnt/β-catenin and Hedgehog pathways, which are known to regulate stem cells (8, 22, 23). We found that expression of NOTCH1 and a target gene of NOTCH signaling, HES1, were induced by CCL2 in various breast cancer cells, and that HES1 expression occurred subsequent to NOTCH1 expression (Fig. 4A and B). Interestingly, the NOTCH-activating effect of CCL2 was only subtle in MCF7 cells, which also failed to respond to CCL2-induced sphere formation (Fig. 1E). This may be related to the lower expression level of CCR2 in MCF7 than in other cells (Supplementary Fig. S5), making MCF7 less sensitive to CCL2-triggered effects. Transfected HES1 and HEY1 reporters were significantly activated by CCL2 in primary breast cancer cells and BT474 cells, and these effects were abolished by inhibitors of the NOTCH-activating α- and γ-secretases (Fig. 4C and Supplementary Fig. S6). Activation of NOTCH signaling by CCL2 was thus attributed to increased NOTCH1 expression, as CCL2 showed no effect on the activities of α- and γ-secretases as assessed by in vitro substrate cleavage assays (Supplementary Fig. S7). Upon CCL2 treatment, a rapid and dramatic induction of p38 MAPK phosphorylation was observed, followed by the induction of NOTCH1 and NICD1 (cleaved NOTCH1) proteins at a later time point (Fig. 4D). Both p38 phosphorylation and NOTCH1/NICD1 induction were abolished by an inhibitor of p38 MAPK (Fig. 4D and E). Using a 6-kb NOTCH1 promoter reporter, we observed an approximately 4-fold induction of NOTCH1 promoter activity by CCL2 in primary breast cancer cells, and this effect was also completely suppressed by the p38 MAPK inhibitor, but only partially suppressed by inhibitors of phosphoinositide 3-kinase (PI3K) and MAP–ERK kinase (MEK)1/2 MAPK (Fig. 4F). This suggests that p38 MAPK plays an essential role in activating NOTCH1 promoter, whereas PI3K and MEK1/2 MAPK may play an accessory role. Induction of p38 MAPK phosphorylation and NOTCH1 proteins (full-length and cleaved) was also observed in XP265922 breast cancer cells treated with CM from CAF265922 previously activated by coculturing with XP265922. This effect was abolished by treatment with a p38 MAPK inhibitor or CCL2 neutralizing antibody, and also by using CM from CAF265922 previously cocultured with XP265922 cells but in the presence of a STAT3 inhibitor (Fig. 4G). We therefore concluded that in breast cancer cells, CCL2 induces NOTCH1 expression and its downstream signaling mainly through p38-dependent activation of the NOTCH1 promoter. In BT474 and MDA361 breast cancer cells, CCL2-induced mammosphere formation was efficiently blocked by inhibitors of the α- and γ-secretases and p38 MAPK, and by RNA interference (RNAi) of NOTCH1 (Fig. 4H), indicating that activation of NOTCH1 mediates the effect of CCL2 on CSCs.

To obtain in vivo evidence of the CSC-regulating function of fibroblast-derived CCL2, we established an orthotopic xenograft model of cotransplanted primary CAFs and breast cancer cells. CAF265922 were stably transduced to express doxycycline-inducible shRNA of CCL2, which suppressed CCL2 expression in vitro by approximately 60% in the presence of doxycycline (Fig. 5A). The modified CAFs were then cotransplanted with breast cancer cells from the same primary tumor (XP265922) into the mammary fat pads of female NOD/SCID/IL-2Rγ-null (NSG) mice. Mammary tumor formation was monitored in transplanted mice with or without doxycycline treatment. Fibroblast-specific knockdown of CCL2 in the doxycycline (Dox)⁺ group resulted in significantly delayed tumor formation and reduced tumor volume, compared with the Dox⁻ control group (Fig. 5B and C). Xenograft tumors from both groups were harvested, and the fibroblast (labeled by lentiviral-encoded GFP) and tumor cell (positive for human ESA) components were separated for gene expression analyses. In Dox⁺ tumors, both fibroblast-derived CCL2 expression and tumor cell–derived NOTCH1 expression were significantly lower than their counterparts in Dox⁻ tumors (Fig. 5D). Immunohistochemical staining also indicated lower levels of CCL2 and NOTCH1 proteins in the Dox⁺ tumors than in the Dox⁻ tumors (Fig. 5E). We further analyzed the CSC population within all tumor cells in each xenograft tumor using ALDEFLUOR flow cytometric analysis. The ALDEFLUOR^bright CSC populations in Dox⁺ tumors were significantly smaller than those in Dox⁻ tumors (Fig. 5F; ∼0.45% vs. ∼1.3%), indicating a decrease in the number of CSCs. Thus, fibroblast-derived CCL2 secretion appears to play an important role in tumorigenesis and in regulating NOTCH1 expression and the CSC population in breast cancer cells. Consistent with these results, the CCL2 neutralizing antibody, but not a control IgG or PBS, significantly suppressed tumorigenesis and decreased NOTCH1 expression in tumor cells when XP265922 and GFP-labeled CAF265922 were cotransplanted into NSG mice (Fig. 5G and H and Supplementary Fig. S8). Thus, based on the studies that have been described to this point, we propose a model of CSC generation that incorporates a cross-talk circuit involving STAT3, CCL2, and NOTCH pathways (Fig. 5I).

To extend our findings to a larger number of primary breast cancers, OR and 95% CI were calculated using unconditional logistic regression to determine whether the microarray-determined expression levels of CCL2, CCR2, and NOTCH family of receptors and ligands in a previously reported breast cancer data set (15, 16) were associated with tumor grade (Table 1). Patients with grade III (poorly differentiated) and grade I (well-differentiated) tumors were included in the analysis. We did not include the grade II tumors because they fell between grade I and grade III and exhibited highly diverse levels of differentiation than grades I and III. Patients with poor differentiation (grade III) were significantly associated with higher levels of CCL2 (OR, 13.72; 95% CI, 2.66–70.89), NOTCH1 (OR, 9.56; 95% CI, 1.54–59.27), and delta-like 3 (OR, 20.93; 95% CI, 1.53–289.94), as well as lower level of Jagged 1 (OR, 0.14; 95% CI, 0.03–0.57).

In addition, correlation coefficients were calculated between microarray-determined CCL2 and NOTCH1 expression and stratified by tumor grade and stage (Table 2, microarray-based expression). A significant linear correlation was observed between CCL2 and NOTCH1 expression among all breast cancers (R = 0.18, P < 0.01). This association was especially pronounced in grade III tumors (R = 0.19, P = 0.03), and in Stage I (R = 0.33, P < 0.01) and Stage IIa disease (R = 0.19, P = 0.02). We further evaluated the expression of CCL2 and NOTCH1 at the protein level by immunohistochemistry in 51 HER2⁺ or triple-negative primary breast cancers. The correlation between 2 proteins was approaching statistical significance (R = 0.26, P = 0.06) among all tumors, and was indeed significant within HER2⁺ tumors (R = 0.40, P = 0.03), but not triple-negative breast cancers (Table 2, immunohistochemistry-based expression). Although the previously reported association of CCL2 with poor clinical outcome (24) was not detected in the current cohort, our results nevertheless support a relationship between CCL2 and NOTCH1 with effects on CSCs in primary breast cancers.

Fibroblasts are altered by cancer cells through nongenetic modifications, and in turn, effect direct changes on cancer cells or indirect changes on the tumor microenvironment to facilitate cancer growth and invasion. The resulting coevolution of cancer and the hosting niche critically influences disease progression. Our attention to the influence of fibroblasts on CSCs, the seeds of cancer, showed that, upon cancer-mediated activation, human mammary fibroblasts secreted CCL2 to induce CSC generation through activation of NOTCH signaling. In the cancer niche, CCL2 may be produced and secreted into the extracellular environment by almost all cell types, including cancer cells, stromal fibroblasts, tumor-infiltrated monocytes, and endothelial cells. Nevertheless, our study indicated that (i) CCL2 expression was 4- to 9-fold higher in activated CAFs than in breast cancer cells (Fig. 1D); (ii) fibroblast-specific knockdown of CCL2 significantly suppressed tumorigenesis and the CSC population in xenograft tumors (Fig. 5A–F); and (iii) CCL2 is detected by immunohistochemistry in both tumor cells and stromal fibroblasts in primary breast cancer (Supplementary Fig. S9). These data strongly suggest that CAFs are an important source of CCL2 in the cancer niche and a major environmental regulator of CSCs.

The CAF populations in tumor-associated stroma are known to include both fibroblasts and myofibroblasts. Myofibroblasts are endowed with the ability to promote tumor growth and associated with higher grade malignancy and poor prognosis in patients with cancer (25, 26). These cells express SMA to be distinguished from fibroblasts. Kojima and colleagues recently show that through self-sustaining autocrine signaling of TGF-β and stromal cell-derived factor-1, fibroblasts can transdifferentiate into myofibroblasts during tumor progression (26). The CAFs prepared and examined in our study indeed contained a subpopulation of SMA-expressing myofibroblasts, as indicated by immunofluorescent assay and immunohistochemistry (Supplementary Figs. S1B and S8). However, the SMA⁺ myofibroblasts and SMA⁻ fibroblasts produced CCL2 at comparable levels in the cancer niche (Supplementary Fig. S8), suggesting that the increased CCL2 production and enhanced CSC-promoting capacity of the herein examined CAFs are unlikely related to the myofibroblast phenotype, but rather a general alteration in fibroblasts in response to cancer-derived stimulation. Our findings also support further studies on the regulation of CSCs and their noncancerous counterparts by other physiologic and therapeutic conditions that locally elevate CCL2 levels. Such conditions, including wound healing, inflammation, and chemotherapy (7, 27), are becoming increasingly appreciated for their relevance to the biology of normal and cancerous stem cells.

Our data indicate that the CCL2-producing and CSC-promoting ability of CAFs is conferred by breast cancer–secreted soluble factors present in the CM, such as the cytokines listed in Fig. 3D. Although different breast cancer cells appeared to secrete distinct sets of cytokines to induce CCL2 production in CAFs, these cytokines ultimately functioned through STAT3, and inhibition of STAT3 completely abrogated the induction of CCL2 by breast cancer paracrine signaling (Fig. 3). Therefore, compared with the diverse signals released by cancer cells, the common effector STAT3 serves as a superior target to therapeutically block cancer-induced activation of stromal fibroblasts. STAT3 has been identified as an important effector and target in cancer cells and tumor-infiltrated immune cells (28). Our study now identifies STAT3-mediated fibroblast activation as a potential therapeutic target, further supporting the idea that anti-STAT3 therapies may exert dual effects on both cancer and host cells, halting their dynamic and mutual activation during cancer progression.

CCL2 has been implicated in breast cancer progression and metastasis (29). In primary breast tumors, CCL2 expression is correlated with the accumulation of tumor-associated macrophages and is a significant indicator of early relapse (24, 30). Overexpression of CCL2 in breast cancer cells promotes metastasis formation in lungs and bone through increasing macrophage infiltration and osteoclast differentiation, respectively (31). A recent report shows that CCL2 produced by both tumor and stromal cells recruits the CCR2-expressing inflammatory monocytes to the pulmonary metastases of mammary tumors, where monocyte-derived factors promote endothelial permeability and extravasation of tumor cells (32). CCL2 expression is interactively regulated in the cross-talk between tumor and niche cells. Increased expression of CCL2 is detected in the bone marrow mesenchymal stem cells (MSC) following stimulation by leukemia cells, resulting in enhanced cancer-promoting capacity of MSCs (33). Coculture with MSCs, in turn, induces CCL2 expression in cancer cells (34). These previous studies have therefore established an important role of CCL2 in cancer–host cross-talk through the regulation of tumor cell homing and metastasis, angiogenesis, and the immune system.

Here, we show that CCL2 induces CSCs both in vitro and in vivo through activation of NOTCH (Figs. 4 and 5). NOTCH activation has been shown to promote the self-renewal of mammary stem cells (8). An important role for NOTCH signaling in human cancers has been long established (35), and several GSIs are currently in early clinical development as potential NOTCH-targeting therapeutics. It is proposed that single-agent GSI therapy may be effective in triple-negative breast cancers, which are known to harbor CSC-like characteristics (36). Here, our data further support the use of NOTCH-targeting agents in efficiently blocking the stimulatory effect of stromal fibroblasts on CSCs. Our data also indicate that activation of p38 MAPK is required for CCL2-induced NOTCH1 expression (Fig. 4). The E2A-encoded transcription factors E12 and E47 have been shown to activate NOTCH1 expression through binding to multiple E-box sites in the 6-kb NOTCH1 promoter region (37). Phosphorylation of E47 by p38 MAPK and by MAPK-activated protein kinase 2 (MAPKAPK2), a kinase activated by p38, has been reported (38, 39). The function of p38-mediated E47 phosphorylation in regulating NOTCH1 promoter activity is still unclear, and may underlie the induction of NOTCH1 by CCL2, which induces potent p38 activation in primary breast cancer cells (Fig. 4D). In addition, the NOTCH-activating effect of CCL2 was only subtle in the ER⁺/PR⁺/HER2⁻ MCF7 cells (Fig. 4A and B), which also failed to respond to CCL2-induced sphere formation (Fig. 1E). Whether the lower CCR2 level in MCF7 (Supplementary Fig. S5) causes their low sensitivity to CCL2 effect, and whether levels of CCL2 receptors are associated with breast cancer subtypes need to be further investigated. Nevertheless, immunohistochemical staining of primary breast cancers indicated a significant correlation between CCL2 and NOTCH1 in HER2⁺ tumors (Table 2, immunohistochemistry-based expression), suggesting that at least in these tumors, as observed in the HER2⁺ BT474 and MDA361 breast cancer cells, the regulation of NOTCH signaling by CCL2 may indeed occur in vivo.

In summary, our study provides a model in which paracrine signaling initiated by breast cancer cells induces CCL2 production by stromal fibroblasts through STAT3 activation. The fibroblast-derived CCL2, in turn, promotes cancer progression by regulating CSCs through NOTCH activation (Fig. 5I). The results described herein provide novel insights into understanding how CSCs are influenced by the tumor microenvironment during the coevolution of cancer and the hosting niche, and identify CCL2, STAT3, and NOTCH1 as future therapeutic targets to efficiently block the CSC-stimulating cancer–host cross-talk to overcome CSC-mediated disease progression and treatment resistance.

T. Luu has commercial research support from NCI/CTEP, Bayer/Onyx, Wyth, Merk, BMS, GSK, and Pfizer and is the consultant/advisory board member for Novartis and Genomic Health. No potential conflicts of interest were disclosed by the other authors.

The authors thank Drs. Leonid S. Metelitsa and Warren S. Pear for kindly providing the plasmid constructs of CCL2 and NOTCH1 promoter reporters; Drs. John J. Rossi and Hua Yu for providing reagents, Drs. Susan Kane, Mei Kong, Toshifumi Tomoda, and Takahiro Maeda and members of the Division of Tumor Cell Biology for valuable comments, as well as the Analytical Cytometry Core, Light Microscopy Digital Imaging Core, Bioinformatics Core, Biostatistics Core, Pathology Core, and Animal Facility for professional services.

The project described was supported by the National Cancer Institute (NCI) grant number R00 CA125892 (S.E. Wang) and P30 CA033572, and by the National Natural Science Foundation of China grant number 30872986 (X. Ren) and 81171983 (H. Li).

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