Background
Ductal carcinoma in situ (DCIS) of the breast is a pre-invasive lesion and a risk factor for subsequent invasive ductal carcinoma (IDC) [
1]. DCIS represents about 20 % of newly diagnosed breast cancers in the United States [
2]. If left untreated, approximately half of DCIS tumors will progress to IDC while half will remain indolent [
3,
4]. Although there are many subtypes of DCIS, it is not currently possible to identify which will progress. This has led to aggressive treatments, specifically radiation with either lumpectomy or mastectomy [
5].
Components of the tumor microenvironment are increasingly implicated in the progression of many cancers. Early morphological and physiological changes in breast epithelium are minimal, and compounding factors such as tumor-suppressive paracrine signaling from myoepithelial cells [
6] or the extracellular matrix [
7] may hide early indicators of ductal cell aberration. Such changes may include, but are not limited to: gene expression modulation, epigenetic alterations, and loss of genomic stability in both the epithelial and stromal compartments. In the tumor microenvironment, carcinoma-associated fibroblasts (CAFs) represent a fibroblast population or mixture of sub-populations that can promote tumor progression [
8‐
13]. Although this mechanism is not fully understood, it is known that CAFs secrete numerous cytokines and growth factors [
14].
Interleukin 6 (IL-6) is a pro-inflammatory cytokine shown to alter cell morphology, modulate cell migration and the epithelial to mesenchymal transition [
15‐
17]. Many of these processes occur upon IL-6 activation of the transmembrane IL-6 receptor (IL-6R), which heterodimerizes with the ubiquitously expressed cell surface receptor glycoprotein 130 (gp130). Downstream activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway initiates IL-6 target gene transcription [
18,
19]. Alternatively, secreted IL-6 can bind to the soluble IL-6 receptor (sIL-6R), which then binds gp130 at the cell surface and initiates intracellular signaling. This form of IL-6 signaling has been coined “IL-6 trans-signaling” (IL-6TS) [
20]. IL-6 has been linked to the upregulation of proteases such as cysteine cathepsins and matrix metalloproteinases that are known to play a role in cancer progression [
21,
22]. The IL-6 signaling axis is commonly upregulated in invasive cancers, suggesting that IL-6 may be an important mediator of events involved in tumor cell invasion [
23‐
25].
The effects of IL-6 signaling inhibitors on breast cancer cell morphology and proliferation have been evaluated in monotypic cultures. Two studies have shown that inhibiting autocrine IL-6 signaling in either triple negative breast cancer cell lines [
26], or the ER-positive MCF7 cell line [
27] significantly inhibited cell growth. Additionally, Leslie et al. show that knockdown of IL-6 in an invasive variant of the MCF10A cells, MCF10A-H-RasV12, inhibited cell migration in a transwell assay, and inhibited growth in a xenograft mouse model [
28]. Although these studies evaluated paracrine signaling, cells were treated with exogenous recombinant protein rather than co-culturing different cell types. Therefore the authors were unable to evaluate in 3D the dynamic cell:cell interactions between two separate human cell types or the cell:microenvironment interactions.
Our 3D mammary architecture and microenvironment engineering (MAME) culture model mimics in vivo architecture, providing a suitable setting to study cell:cell interactions and, notably, physiologically relevant cell:cell signaling over time. Additionally, 3D in vitro cell culture models better represent in vivo tumor drug response, which would facilitate efficacious therapy development at the preclinical stage [
29]. Here we examine the role of IL-6 in progression of pre-invasive breast DCIS to an invasive phenotype, and show how co-culture of DCIS cells with CAFs changes DCIS growth and invasive potential.
Methods
Cell lines
MCF10A human breast non-transformed epithelial cells, MCF10.DCIS and SUM102 human breast DCIS cells, and WS-12Ti human breast carcinoma-associated fibroblasts were provided by Dr. Bonnie F. Sloane. All primary fibroblasts were derived from human breast tissue. CAF40T were derived from biopsy tissue diagnosed as invasive carcinoma. NAF98 were derived from benign tissue. Both CAF40T and NAF98 fibroblast cell lines were provided by Dr. Simon W. Hayward. These fibroblasts were immortalized in Dr. Sloane’s lab and are designated CAF40TKi and NAF98i, respectively. The FB-NF, NAF-FB, and FB-CAF primary fibroblasts were derived from patient biopsies diagnosed as: benign (FB-NF, NAF-FB), or invasive carcinoma with accompanying DCIS (FB-CAF) and provided by Dr. Fariba Behbod. The FB-NF fibroblasts were immortalized in Dr. Sloane’s lab and designated FB-NF-i. The FB-CAF and NAF-FB fibroblasts were not immortalized. All patient derived cells were received de-identified and therefore are exempt from IRB oversight.
Cell culture
In this study we utilized non-tumor forming MCF10A human breast epithelial cells [
30] and the human DCIS cell lines MCF10.DCIS and SUM102, which were maintained as previously described [
31]. See supplemental methods for more detailed information (Additional file
1). All 3D MAME cultures were performed using Cultrex (3433-005-01, Trevigen, Gaithersburg, MD) similar to previously described [
32]. Briefly, cell culture dishes were coated with 100 % Cultrex. Cells were added on top of solidified Cultrex and allowed to adhere for 30–45 min before being overlaid with 2 % Cultrex in phenol red-free DMEM F12 media containing 2.5 % fetal bovine serum (Additional file
2: Figure S1). In co-culture experiments, fibroblasts were added first and allowed to adhere before adding tumor cells. Once tumor cells had adhered to the matrix, an overlay of 2 % Cultrex was added. Media were changed every 4 days.
Measurement of multicellular structures
Differential interference contrast (DIC) images of three random fields at 20X magnification were used to measure multicellular structures. Three individuals of whom two were study-blinded measured the diameter and perimeter of structures and the number and length of interconnections between structures. All three data sets were used in the quantification. This analysis was performed using Zen imaging software (Zeiss, Thornwood, NY). Volume measurements were obtained using 3D fluorescent images and quantified using Volocity software (Perkin-Elmer, Waltham, Mass).
Gene expression
RNA was isolated from cells grown in either 2D monolayer or 3D MAME cultures. For 2D culture, cells were washed in phosphate buffered saline and harvested using 0.5 % Trypsin/EDTA (Life Technologies, Foster City, CA) and pelleted. The cell pellets were resuspended in TRIzol® Reagent (Life Technologies, Foster City, CA) for RNA extraction. All qRT-PCR reactions were performed using Taqman Assays (Life Technologies, Foster City, CA). See supplemental list of Taqman Assays (Additional file
3: Table S1).
ELISA
ELISA kits (human IL6R-ab46029, human IL6-ab46044, and human GAPDH-ab119627) were purchased from Abcam® (San Francisco, CA). Aliquots of lysates were collected for ELISA assays and measurement of total DNA.
Immunohistochemistry
A breast tissue microarray (BR8011) was purchased from US Biomax® (Rockville, MD). Dr. Fariba Behbod provided patient tissue microarrays, and biopsy sections were purchased from ProSci Incorporated (cat # 10–010 and 10–003, Poway, CA). The thickness of all tissue sections immunostained were 10-microns.
Immunofluorescence
Nuclei were labeled with Hoechst (33342, Thermo Scientific) or EDU (Life Technologies, Foster City, CA). Polyclonal antibodies to human IL-6 (AF-206-NA, R&D Systems, Minneapolis, MN) were used at a concentration of 1 μg/ml. Mono-specific antibodies to human cathepsin B have been previously isolated and characterized [
33]. Cathepsin B immunostaining was performed as previously described with the exception that 1 % Tween 20 replaced the 0.01 % saponin [
34]. For some studies, CAF40TKi were pre-labeled, prior to seeding in MAME co-cultures, utilizing CellTrace CFSE (carboxyfluorescein diacetate succinimidyl ester; Life Technologies, Foster City, CA) according to the manufacturer’s protocol.
Drug treatments
For treatment of MAME cultures with IL-6 nAb (R&D Systems, AF-206-NA), we added 1 μg/ml of IL-6 nAb in the 2 % Cultrex overlay to 3D cultures on the first day of culture and refreshed with IL-6 nAb and 2 % overlay every 4 days. The antibody concentration was selected based on preliminary studies in which we determined the lowest concentration needed to inhibit growth of tumor structures. Oxymatrine (Sigma, St. Louis, MO) at 1-mg/ml (3.7 mM) was added 24 hours after cell seeding and was replaced with fresh drug every 4 days. The oxymatrine concentration was determined empirically based on the concentration at which proliferation was inhibited to 50 % of control. The protease inhibitors CA074Me and E64d (Sigma, St. Louis, MO) were used at a concentration of 10 μM [
35].
Live cell proteolysis assay
Dye-quenched collagen IV (DQ-collagen IV, Life Technologies, Foster City, CA) was used admixed in Cultrex as previously described [
32]. MAME cultures in optical glass bottom cell culture dishes were imaged live for a period of 10 to 60 min under 5 % CO
2 at 37 °C.
Confocal microscopy
Confocal microscopy was performed on either a Zeiss LSM 510 or LSM 780 upright confocal microscope (Zeiss, Thornwood, NY). All cell cultures used for imaging were seeded on 40 mm optical glass bottom culture dishes at a density of 45 cells/mm2 (~5000 cells/dish).
Statistics
Data were statistically analyzed using Student’s t-test on GraphPad Prism 6.0 (GraphPad Software Inc., La Jolla, CA).
Ethics statement
All human subject materials and experiments in this study have been reviewed by the Wayne State University Institutional Review Board and deemed exempt according to the definition codified in the common rule at 45 CFR 46.102(d)(f).
Discussion
A number of factors produced by CAFs have been shown to be involved in promoting malignant transformation in epithelial cells, these include TGFß [
59‐
61] and CXCL12 (SDF-1) [
59,
61,
62]. These factors are involved in eliciting a range of responses that are context-dependent and that benefit the tumor in various ways. The present study describes a novel addition to these known interactions and a new mechanism by which CAF can influence tumor progression. There is a direct correlation between serum IL-6 levels and poor prognosis in breast cancer patients [
36,
63‐
65]. Studies have shown that CAFs [
66], and various immune/inflammatory cells secrete pro-inflammatory cytokines including IL-6 and contribute to tumorigenicity [
67‐
73].
In the current study, we show that IL-6 expression can be found in both the tumor and stromal compartments. In our IHC data we found approximately 65 % of patient samples had positive IL-6 staining; however, when we examined IL-6 expression in our DCIS tumor cell lines, we found the expression to be near the lower threshold of our assay. This discrepancy may be due to differences in gene expression between tissue and cells [
74], differences in IL-6 expression with tumor grade/invasiveness [
65], degree of “stemness” in cell lines vs. tissue [
75], and differences between the assays and/or sample collection.
We next examined the interaction between human breast DCIS epithelial cells and human breast CAFs in the context of an in vitro 3D microenvironment. We hypothesized that CAFs promoted the migration of breast DCIS cells via paracrine signaling within the tumor microenvironment. Here we observed that DCIS cells migrated towards CAFs and upon attachment to CAFs, DCIS cells remained attached and migrated through the matrix following the lead of the CAFs. Similar findings of fibroblast-led migration have been reported for co-cultures of invasive squamous carcinoma cells and CAFs [
76]. It is also likely that CAFs migrated to DCIS cells, as some fibroblasts moved more easily and quickly through the matrix as seen in time-lapse videos; therefore it is possible that some fibroblasts migrated to and attached to DCIS cells thereupon leading migration of the DCIS cells.
An increase in DCIS cell proliferation and a change in multicellular structure morphology was observed in all of our co-culture experiments. DCIS multicellular structures showed invasive characteristics, having lost their uniform circular structure and had developed single or multiple protruding edges. An underlying mechanism for DCIS:CAF interaction, enhanced tumor cell proliferation, and migration was IL-6 signaling in the tumor microenvironment. We [
31] as well as Sung et al. [
77] have shown in 3D co-culture systems that paracrine HGF and its receptor cMet can drive the invasive potential of the MCF10.DCIS cells. We confirmed this in xenografts formed by orthotopic implantation of HGF-secreting fibroblasts and MCF10.DCIS cells in SCID mice [
31]. HGF and IL-6 have been shown to cooperatively enhance lung cancer cell invasion by upregulation of their corresponding cell surface receptors [
78]. Also of interest is the stem cell-like properties of the proliferating DCIS cell population as Krishnamurthy et al. have shown that endothelial IL-6 enhances self-renewal of cancer stem-like cells. Whether or not this is the case for CAF IL-6 in regard to proliferation of DCIS cells has yet to be determined [
79].
Treatment of DCIS:CAF co-cultures with an IL-6 nAb abrogated the proliferation and migratory phenotype acquired by DCIS cells. This phenotype was primarily produced by the inhibition of CAF secreted IL-6 as shRNA knockdown of IL-6 in CAFs, but not in DCIS cells was able to replicate the phenotype. We also showed that MCF10.DCIS cells treated with IL-6 nAb had a down regulation of genes associated with EMT. A recent study suggests that tumor cell EMT is mediated through factors secreted from CAFs and that selective inhibition of TGFβ1 is sufficient to reverse EMT associated gene expression [
80]. Other studies show cross talk between IL-6 and TGFβ signaling consistent with IL-6 and TGFβ both acting as drivers of EMT [
81].
A Federal Drug Administration (FDA) approved humanized anti-IL-6R antibody, tocilizumab (Actemra), has shown promise in the treatment of inflammatory diseases particularly rheumatoid arthritis, Crohn’s disease, and Castleman’s disease. A major caveat of Actemra is that serum IL-6 levels are increased in patients after drug administration [
82]. Since breast cancer patients with elevated serum IL-6 have poorer prognosis, Actemra may not be a practical therapy in these patients, however alternative approaches to reduce IL-6 signaling may prove efficacious.
Siltuximab, a monoclonal antibody against IL-6, is in clinical trials for therapies including; combinatorial treatment of metastatic renal cell carcinoma, multiple myeloma, and prostate cancer [
83‐
86]. The FDA recently approved siltuximab for the treatment of multicentric Castleman’s disease [
87]. Studies have shown that siltuximab significantly inhibits the growth of non-small-cell lung cancer in primary xenografts [
88], and ovarian cancer cell xenografts [
89]. Therapeutic use of siltuximab for the treatment of breast cancer has not been fully evaluated, although preliminary studies with the estrogen receptor alpha positive MCF-7 breast cancer cell line suggests potential efficacy [
90].
A limitation of the current study is that we did not have complete histories or demographic data to perform correlation studies to determine how IL-6 expression in DCIS relates to tumor prognosis. Another limitation is there is a limited number of commercially available DCIS cell lines. Here we used two cell lines that are commercially available. A third DCIS cell line that is commercially available is the SUM-225 DCIS cell line; however, this line comes from a metastatic recurrence so we chose not to use it. Another limitation is that our study only followed the progression and interaction of DCIS cells with CAFs, but not inflammatory cells associated with breast tumors. Additionally, we are investigating the probability of increasing the number of cell types grown together in our 3D MAME culture, for example: macrophages, adipocytes and lymphocytes.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
KOO and BFS designed research plan; KOO made substantial contributions to conception, acquisition, analysis and interpretation of data; MS conducted immunohistochemistry and made substantial contribution to data analysis; NA, and SS made substantial contributions to data analysis; MLS performed video editing and contributed to data analysis; KOO, BFS, RRM, SWH, OEF, YH, and FB drafted the manuscript and participated in revising it for intellectual content. All authors read and approved the final manuscript.