MAF amplification licenses ERα through epigenetic remodelling to drive breast cancer metastasis

MAF amplification increases the risk of breast cancer (BCa) metastasis through mechanisms that are still poorly understood yet have important clinical implications. Oestrogen-receptor-positive (ER+) BCa requires oestrogen for both growth and metastasis, albeit by ill-known mechanisms. Here we integrate proteomics, transcriptomics, epigenomics, chromatin accessibility and functional assays from human and syngeneic mouse BCa models to show that MAF directly interacts with oestrogen receptor alpha (ERα), thereby promoting a unique chromatin landscape that favours metastatic spread. We identify metastasis-promoting genes that are de novo licensed following oestrogen exposure in a MAF-dependent manner. The histone demethylase KDM1A is key to the epigenomic remodelling that facilitates the expression of the pro-metastatic MAF/oestrogen-driven gene expression program, and loss of KDM1A activity prevents this metastasis. We have thus determined that the molecular basis underlying MAF/oestrogen-mediated metastasis requires genetic, epigenetic and hormone signals from the systemic environment, which influence the ability of BCa cells to metastasize.

The CDK4/6 Inhibitor Palbociclib Inhibits Estrogen-Positive and Triple Negative Breast Cancer Bone Metastasis In Vivo

CDK 4/6 inhibitors have demonstrated significant improved survival for patients with estrogen receptor (ER) positive breast cancer (BC). However, the ability of these promising agents to inhibit bone metastasis from either ER+ve or triple negative BC (TNBC) remains to be established. We therefore investigated the effects of the CDK 4/6 inhibitor, palbociclib, using in vivo models of breast cancer bone metastasis. In an ER+ve T47D model of spontaneous breast cancer metastasis from the mammary fat pad to bone, primary tumour growth and the number of hind limb skeletal tumours were significantly lower in palbociclib treated animals compared to vehicle controls. In the TNBC MDA-MB-231 model of metastatic outgrowth in bone (intracardiac route), continuous palbociclib treatment significantly inhibited tumour growth in bone compared to vehicle. When a 7-day break was introduced after 28 days (mimicking the clinical schedule), tumour growth resumed and was not inhibited by a second cycle of palbociclib, either alone or when combined with the bone-targeted agent, zoledronic acid (Zol), or a CDK7 inhibitor. Downstream phosphoprotein analysis of the MAPK pathway identified a number of phosphoproteins, such as p38, that may contribute to drug-insensitive tumour growth. These data encourage further investigation of targeting alternative pathways in CDK 4/6-insensitive tumour growth.

CDK4/6 inhibitors: A potential therapeutic approach for triple negative breast cancer

Triple negative breast cancer (TNBC) cells lack expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER-2). Thus, TNBC does not respond to hormone-based therapy. TNBC is also an aggressive subtype associated with poorer prognoses compared to other breast cancers. Conventional chemotherapeutics are used to manage TNBC although systemic relapse is common with limited benefits being reported as well as adverse events being documented. Here, we discuss current therapies for TNBC in the neo- and adjuvant settings, as well as recent advancements in the targeting of PD-L1-positive tumors and inclusion of PARP inhibitors for TNBC patients with BRCA mutations. The recent development of cyclin-dependent kinase (CDK) 4/6 inhibitors in ER-positive breast cancers has demonstrated significant improvements in progression free survival in patients. Here, we review preclinical data of CDK 4/6 inhibitors and describe current clinical trials assessing these in TNBC disease.

Adoption of adjuvant bisphosphonates for early breast cancer into standard clinical practice: Challenges and lessons learnt from comparison of the UK and Australian experience

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Adoption of adjuvant bisphosphonates for early breast cancer into standard clinical practice.

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UK and Australian experience of adjuvant bisphosphonates in early breast cancer.

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Pathway taken for adjuvant bisphosphonates implementation in the UK.

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Steps to increase update of adjuvant bisphosphonates in early breast cancer.

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Improve the care of women with early breast cancer.

International guidelines recommend adjuvant bisphosphonates (BPs) for post-menopausal women with early breast cancer to reduce recurrence and mortality. However, globally, wide variation exists in their adoption. In the UK, adjuvant BPs were a recommendation in the breast cancer Clinical Reference Group service specification and were included as a priority for implementation by the national oncologists group UK Breast Cancer Group in November 2015, promoting national uptake, guidance and funding arrangements. In 2018, adjuvant BPs were recommended by the UKs National Institute for Health and Care Excellence. In Australia, adjuvant BPs are still ‘off-label’ and do not receive national reimbursement or endorsement. To date there has been no research into the prescribing habits of these agents in Australia. With the aim to gather data on adjuvant BPs prescribing practices, online surveys were developed and disseminated to breast oncologists in both countries between December 2018 and June 2019. Almost all of the UK oncologists prescribed adjuvant BPs, demonstrating that education, endorsement from professional bodies, presence of national guidelines and funding decisions have been critical to implementation. In contrast, only 48% of the Australian responders prescribed adjuvant BPs, while 83% reported that they would prescribe them if funding was available. Lack of local protocol guidance was also seen as a major barrier. This study was intended to assess the pathway taken for adjuvant BP implementation in the UK and how it might inform changes in Australian practice and also guide other countries with similar issues with the ultimate aim of improving the care of women with early breast cancer globally.

Atomic force microscopy reveals the mechanical properties of breast cancer bone metastases

Mechanically dependent processes are essential in cancer metastases. However, reliable mechanical characterization of metastatic cancer remains challenging whilst maintaining the tissue complexity and an intact sample. Using atomic force microscopy, we quantified the micro-mechanical properties of relatively intact metastatic breast tumours and their surrounding bone microenvironment isolated from mice, and compared with other breast cancer models both ex vivo and in vitro. A mechanical distribution of extremely low elastic modulus and viscosity was identified on metastatic tumours, which were significantly more compliant than both 2D in vitro cultured cancer cells and subcutaneous tumour explants. The presence of mechanically distinct metastatic tumour did not result in alterations of the mechanical properties of the surrounding microenvironment at meso-scale distances (>200 μm). These findings demonstrate the utility of atomic force microscopy in studies of complex tissues and provide new insights into the mechanical properties of cancer metastases in bone.

Transcriptomic Profiling Reveals Novel Candidate Genes and Signalling Programs in Breast Cancer Quiescence and Dormancy

Metastatic recurrence, the major cause of breast cancer mortality, is driven by reactivation of dormant disseminated tumour cells that are defined by mitotic quiescence and chemoresistance. The molecular mechanisms underpinning mitotic quiescence in cancer are poorly understood, severely limiting the development of novel therapies for removal of residual, metastasis-initiating tumour cells. Here, we present a molecular portrait of the quiescent breast cancer cell transcriptome across the four main breast cancer sub-types (luminal, HER2-enriched, basal-like and claudin-low) and identify a novel quiescence-associated 22-gene signature using an established lipophilic-dye (Vybrant^® DiD) retention model and whole-transcriptomic profiling (mRNA-Seq). Using functional association network analysis, we elucidate the molecular interactors of these signature genes. We then go on to demonstrate that our novel 22-gene signature strongly correlates with low tumoural proliferative activity, and with dormant disease and late metastatic recurrence (≥5 years after primary tumour diagnosis) in metastatic breast cancer in multiple clinical cohorts. These genes may govern the formation and persistence of disseminated tumour cell populations responsible for breast cancer recurrence, and therefore represent prospective novel candidates to inform future development of therapeutic strategies to target disseminated tumour cells in breast cancer, eliminate minimal residual disease and prevent metastatic recurrence.

Osteoblast-Derived Paracrine and Juxtacrine Signals Protect Disseminated Breast Cancer Cells from Stress

Metastatic breast cancer in bone is incurable and there is an urgent need to develop new therapeutic approaches to improve survival. Key to this is understanding the mechanisms governing cancer cell survival and growth in bone, which involves interplay between malignant and accessory cell types. Here, we performed a cellular and molecular comparison of the bone microenvironment in mouse models representing either metastatic indolence or growth, to identify mechanisms regulating cancer cell survival and fate. In vivo, we show that regardless of their fate, breast cancer cells in bone occupy niches rich in osteoblastic cells. As the number of osteoblasts in bone declines, so does the ability to sustain large numbers of breast cancer cells and support metastatic outgrowth. In vitro, osteoblasts protected breast cancer cells from death induced by cell stress and signaling via gap junctions was found to provide important juxtacrine protective mechanisms between osteoblasts and both MDA-MB-231 (TNBC) and MCF7 (ER+) breast cancer cells. Combined with mathematical modelling, these findings indicate that the fate of DTCs is not controlled through the association with specific vessel subtypes. Instead, numbers of osteoblasts dictate availability of protective niches which breast cancer cells can colonize prior to stimulation of metastatic outgrowth.

Bone metastasis: mechanisms, therapies, and biomarkers

Skeletal metastases are frequent complications of many cancers, causing bone complications (fractures, bone pain, disability) that negatively affect the patient's quality of life. Here, we first discuss the burden of skeletal complications in cancer bone metastasis. We then describe the pathophysiology of bone metastasis. Bone metastasis is a multistage process: long before the development of clinically detectable metastases, circulating tumor cells settle and enter a dormant state in normal vascular and endosteal niches present in the bone marrow, which provide immediate attachment and shelter, and only become active years later as they proliferate and alter the functions of bone-resorbing (osteoclasts) and bone-forming (osteoblasts) cells, promoting skeletal destruction. The molecular mechanisms involved in mediating each of these steps are described, and we also explain how tumor cells interact with a myriad of interconnected cell populations in the bone marrow, including a rich vascular network, immune cells, adipocytes, and nerves. We discuss metabolic programs that tumor cells could engage with to specifically grow in bone. We also describe the progress and future directions of existing bone-targeted agents and report emerging therapies that have arisen from recent advances in our understanding of the pathophysiology of bone metastases. Finally, we discuss the value of bone turnover biomarkers in detection and monitoring of progression and therapeutic effects in patients with bone metastasis.

Mechanical Heterogeneity in the Bone Microenvironment as Characterized by Atomic Force Microscopy

Bones are structurally heterogeneous organs with diverse functions that undergo mechanical stimuli across multiple length scales. Mechanical characterization of the bone microenvironment is important for understanding how bones function in health and disease. Here, we describe the mechanical architecture of cortical bone, the growth plate, metaphysis, and marrow in fresh murine bones, probed using atomic force microscopy in physiological buffer. Both elastic and viscoelastic properties are found to be highly heterogeneous with moduli ranging over three to five orders of magnitude, both within and across regions. All regions include extremely compliant areas, with moduli of a few pascal and viscosities as low as tens of Pa·s. Aging impacts the viscoelasticity of the bone marrow strongly but has a limited effect on the other regions studied. Our approach provides the opportunity to explore the mechanical properties of complex tissues at the length scale relevant to cellular processes and how these impact aging and disease.

A role for CBFβ in maintaining the metastatic phenotype of breast cancer cells

Epithelial to mesenchymal transition (EMT) is a dynamic process that drives cancer cell plasticity and is thought to play a major role in metastasis. Here we show, using MDA-MB-231 cells as a model, that the plasticity of at least some metastatic breast cancer cells is dependent on the transcriptional co-regulator CBFβ. We demonstrate that CBFβ is essential to maintain the mesenchymal phenotype of triple-negative breast cancer cells and that CBFβ-depleted cells undergo a mesenchymal to epithelial transition (MET) and re-organise into acini-like structures, reminiscent of those formed by epithelial breast cells. We subsequently show, using an inducible CBFβ system, that the MET can be reversed, thus demonstrating the plasticity of CBFβ-mediated EMT. Moreover, the MET can be reversed by expression of the EMT transcription factor Slug whose expression is dependent on CBFβ. Finally, we demonstrate that loss of CBFβ inhibits the ability of metastatic breast cancer cells to invade bone cell cultures and suppresses their ability to form bone metastases in vivo. Together our findings demonstrate that CBFβ can determine the plasticity of the metastatic cancer cell phenotype, suggesting that its regulation in different micro-environments may play a key role in the establishment of metastatic tumours.

Abstract P2-18-08: Inclusion of adjuvant bone-modifying agents for early breast cancer into standard clinical practice: Challenges and lessons learnt from an international collaboration

Background: International guidelines recommend adjuvant bone-modifying agents (BMAs) for post-menopausal women with early breast cancer (EBC) to reduce recurrence and mortality. Despite this, wide variation exists internationally in the adoption of these recommendations. BMAs are off-patent with generic formulations being manufactured. Therefore, pharmaceutical lobbying for BMAs to gain regulatory approval for this indication are lacking. This may have negative impact on promotion and education of prescribing physicians, resulting in EBC patients not receiving this intervention. In the UK, BMAs were included as a recommendation in the breast cancer CRG (Clinical Reference Group) service specification and were endorsed as a priority for implementation by the UK Breast Cancer Group (UKBCG) in November 2015, promoting national uptake, local guidance and sharing of funding arrangements through local commissioning agreements. Following these, UKBCG and Breast Cancer Now, conducted 2 surveys in March and October 2016, showing that 24% and 44% respectively, of oncologists prescribe BMAs. In November 2017, a subsequent survey was performed at the annual UKBCG meeting, showed that 77% of the attendees prescribe BMAs. From 2018, BMAs are also part of the UKs NICE (National Institute for Health and Care Excellence) recommendations for EBC treatment, with the current survey to come after the full endorsement of BMAs into UK EBC guidelines. In Australia, BMAs are still ‘off-label’ and do not receive national reimbursement or endorsement. To date there has been no formal inquiry into the prescribing habits in Australia. The aim of this international collaboration was to further evaluate this and translate the methodology for adjuvant BMA implementation in the UK to Australian practice and potentially pave a pathway for other nations struggling with similar barriers to ultimately improve outcomes for women with EBC globally.

Methods: Brief, anonymous, online surveys were developed at each of our institutions using a similar template. The surveys consisted of a series of questions aimed to gather data on their respective local oncologists including demographics, knowledge of current guidelines, current prescribing habits and perceived barriers to prescribing BMAs to women with EBC. The results of the UK survey and experience were used in a collaborative manner to understand the health economics and promote the deliverability of BMAs to women with EBC in Australia.

Results: Between March 2019 and June 2019, the national UK survey received 67 responses from 35 centres around the UK. 98.5% of UK respondents currently prescribe adjuvant BMAs for prevention of disease recurrence. 84.6% report they follow the UKBCG guidelines on the topic and 67.7% report it is discussed in their MDTs.

Between December 2018 and April 2019, 60 responses to the Australian survey were received. 48% of Australian respondents currently prescribe adjuvant BMAs for prevention of disease recurrence. However, 83% reported that they would prescribe adjuvant BMAs if funding was available. Most respondents were aware of the international guidelines on the topic but lack of local protocol guidance was seen as a significant barrier. Only 18.3% report it is discussed in their MDTs.

Conclusions: From 2016 to 2019, the number of UK oncologists who prescribe BMAs has significantly increased, demonstrating that education, pressure from national bodies, national guidelines and funding decisions have been critical to implementation. Acquiring national data on this topic for Australian medical oncologists will help to address the vital need for the development of a national consensus and to clarify the financial and educational barriers that currently limit the prescription of adjuvant BMAs to women with EBC.

Citation Format: Sally Baron-Hay, Elisavet Theodoulou, Isobel Porter, Caroline Wilson, Ingunn Holen, Catherine Harper-Wynne, Janet Brown. Inclusion of adjuvant bone-modifying agents for early breast cancer into standard clinical practice: Challenges and lessons learnt from an international collaboration [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-18-08.

CDK4/6 inhibitors in breast cancer - from in vitro models to clinical trials

Background: Breast cancer (BC) is one of the leading causes of cancer-related deaths worldwide. Standard therapies aim to disrupt pathways that regulate the growth and survival of BC cells. Therapeutic agents such as endocrine therapy target hormone dependent cancer cells and have shown to be suitable approaches in BC treatment. However, in the case of metastatic BC, curative options are limited, thus strategies have been explored to improve survival and clinical benefit. In this review we provide an up to date overview of the development of anti-cancer agents, particularly the newly developed CDK4/6 inhibitors.Material and methods: A search of PubMed was conducted to identify preclinical data surrounding the development of endocrine therapy and CDK4/6 inhibitors in early and metastatic BC. Clinical data were also sought using PubMed and clinicaltrials.gov.Results: Agents targeting oestrogen and its receptor have demonstrated positive outcomes in clinical trial with improvements in objective responses and overall survival. However, patients do exhibit adverse effects and some will eventually fail to respond to endocrine therapy. Subsequently, the development and success of 3rd generation CDK4/6 inhibitors in preclinical studies has allowed their introduction in clinical studies. In patients with ER + BC, CDK4/6 have demonstrated dramatic improvements in progression free survival when used in combination with endocrine therapies. Similar findings were also observed in metastatic disease. Adverse effects were limited in CDK4/6 treated patients, demonstrating the safety of these agents.Conclusion: CDK4/6 inhibitors are highly specific making them a safe and viable therapeutic for BC and there is increasing evidence of their potential to improve survival, even in the metastatic setting. Although a number of trials have demonstrated this, as a lone therapy or in combination, optimisation of treatment scheduling are still required in further clinical investigations.

Development of clinically relevant in vivo metastasis models using human bone discs and breast cancer patient-derived xenografts

Background

Late-stage breast cancer preferentially metastasises to bone; despite advances in targeted therapies, this condition remains incurable. The lack of clinically relevant models for studying breast cancer metastasis to a human bone microenvironment has stunted the development of effective treatments for this condition. To address this problem, we have developed humanised mouse models in which breast cancer patient-derived xenografts (PDXs) metastasise to human bone implants with low variability and high frequency.

Methods

To model the human bone environment, bone discs from femoral heads of patients undergoing hip replacement surgery were implanted subcutaneously into NOD/SCID mice. For metastasis studies, 7 patient-derived xenograft tumours (PDX: BB3RC32, ER+ PR+ HER2−; BB2RC08, ER+ PR+ ER2−; BB6RC37, ER− PR− HER2− and BB6RC39, ER+ PR+ HER2+), MDA-MB-231-luc2, T47D-luc2 or MCF7-Luc2 cells were injected into the 4th mammary ducts and metastases monitored by luciferase imaging and confirmed on histological sections. Bone integrity, viability and vascularisation were assessed by uCT, calcein uptake and histomorphometry. Expression profiling of genes/proteins during different stages of metastasis were assessed by whole genome Affymetrix array, real-time PCR and immunohistochemistry. Importance of IL-1 was confirmed following anakinra treatment.

Results

Implantation of femoral bone provided a metabolically active, human-specific site for tumour cells to metastasise to. After 4 weeks, bone implants were re-vascularised and demonstrated active bone remodelling (as evidenced by the presence of osteoclasts, osteoblasts and calcein uptake). Restricting bone implants to the use of subchondral bone and introduction of cancer cells via intraductal injection maximised metastasis to human bone implants. MDA-MB-231 cells specifically metastasised to human bone (70% metastases) whereas T47D, MCF7, BB3RC32, BB2RC08, and BB6RC37 cells metastasised to both human bone and mouse bones. Importantly, human bone was the preferred metastatic site especially from ER+ PDX (100% metastasis human bone compared with 20–75% to mouse bone), whereas ER-ve PDX developed metastases in 20% of human and 20% of mouse bone. Breast cancer cells underwent a series of molecular changes as they progressed from primary tumours to bone metastasis including altered expression of IL-1B, IL-1R1, S100A4, CTSK, SPP1 and RANK. Inhibiting IL-1B signalling significantly reduced bone metastasis.

Conclusions

Our reliable and clinically relevant humanised mouse models provide significant advancements in modelling of breast cancer bone metastasis.

Bone marrow osteoprogenitors are depleted whereas osteoblasts are expanded independent of the osteogenic vasculature in response to zoledronic acid

Zoledronic acid (ZOL) is an antiresorptive drug used to prevent bone loss in a variety of conditions, acting mainly through suppression of osteoclast activity. There is growing evidence that ZOL can also affect cells of the mesenchymal lineage in bone. We present novel data revealing significant changes in the abundance of perivascular mesenchymal stromal cells (MSCs)/osteoprogenitors and osteoblasts following the injection of ZOL, in vivo. In young mice with high bone turnover and an abundance of perivascular osteoprogenitors, ZOL significantly (P < 0.0001) increased new bone formation. This was accompanied by a decline in osterix-positive osteoprogenitors and a corresponding increase in osteoblasts. However, these effects were not observed in mature mice with low bone turnover. Interestingly, the ZOL-induced changes in cells of the mesenchymal lineage occurred independently of effects on the osteogenic vasculature. Thus, we demonstrate that a single, clinically relevant dose of ZOL can induce new bone formation in microenvironments enriched for perivascular MSC/osteoprogenitors and high osteogenic potential. This arises from the differentiation of perivascular osterix-positive MSC/osteoprogenitors into osteoblasts at sites that are innately osteogenic. Collectively, our data demonstrate that ZOL affects multiple cell types in bone and has differential effects depending on the level of bone turnover.-Hughes, R., Chen, X., Hunter, K. D., Hobbs, J. K., Holen, I., Brown, N. J. Bone marrow osteoprogenitors are depleted whereas osteoblasts are expanded independent of the osteogenic vasculature in response to zoledronic acid.

The bone metastasis niche in breast cancer-potential overlap with the haematopoietic stem cell niche in vivo

BACKGROUND

Bone metastasis is one of the most common complications of advanced breast cancer. During dissemination to bone, breast cancer cells locate in a putative 'metastatic niche', a microenvironment that regulates the colonisation, maintenance of tumour cell dormancy and subsequent tumour growth. The precise location and composition of the bone metastatic niche is not clearly defined. We have used in vivo models of early breast cancer dissemination to provide novel evidence that demonstrates overlap between endosteal, perivascular, HSC and the metastatic niche in bone.

METHODS

Estrogen Receptor (ER) +ve and -ve breast cancer cells were labelled with membrane dyes Vybrant-DiD and Vybrant-CM-DiI and injected via different routes in BALBc/nude mice of different ages. Two-photon microscopy was used to detect and quantitate tumour cells and map their location within the bone microenvironment as well as their distance to the nearest bone surface compared to the nearest other tumour cell. To investigate whether the metastatic niche overlapped with the HSC niche, animals were pre-treated with the CXCR4 antagonist AMD3100 to mobilise hematopoietic (HSCs) prior to injection of breast cancer cells.

RESULTS

Breast cancer cells displayed a characteristic pattern of homing in the long bones, with the majority of tumour cells seeded in the trabecular regions, regardless of the route of injection, cell-line characteristics (ER status) or animal age. Breast cancer cells located in close proximity to the nearest bone surface and the average distance between individual tumour cells was higher than their distance to bone. Mobilisation of HSCs from the niche to the circulation prior to injection of cell lines resulted in increased numbers of tumour cells disseminated in trabecular regions.

CONCLUSION

Our data provide evidence that homing of breast cancer cells is independent of their ER status and that the breast cancer bone metastasis niche is located within the trabecular region of bone, an area rich in osteoblasts and microvessels. The increased number of breast cancer cells homing to bone after mobilisation of HSCs suggests that the HSC and the bone metastasis niche overlap.

Endogenous Production of IL1B by Breast Cancer Cells Drives Metastasis and Colonization of the Bone Microenvironment

PURPOSE

Breast cancer bone metastases are incurable, highlighting the need for new therapeutic targets. After colonizing bone, breast cancer cells remain dormant, until signals from the microenvironment stimulate outgrowth into overt metastases. Here we show that endogenous production of IL1B by tumor cells drives metastasis and growth in bone.

EXPERIMENTAL DESIGN

Tumor/stromal IL1B and IL1 receptor 1 (IL1R1) expression was assessed in patient samples and effects of the IL1R antagonist, Anakinra, or the IL1B antibody canakinumab on tumor growth and spontaneous metastasis were measured in a humanized mouse model of breast cancer bone metastasis. Effects of tumor cell-derived IL1B on bone colonization and parameters associated with metastasis were measured in MDA-MB-231, MCF7, and T47D cells transfected with IL1B/control.

RESULTS

In tissue samples from >1,300 patients with stage II/III breast cancer, IL1B in tumor cells correlated with relapse in bone (HR = 1.85; 95% CI, 1.05-3.26; P = 0.02) and other sites (HR = 2.09; 95% CI, 1.26-3.48; P = 0.0016). In a humanized model of spontaneous breast cancer metastasis to bone, Anakinra or canakinumab reduced metastasis and reduced the number of tumor cells shed into the circulation. Production of IL1B by tumor cells promoted epithelial-to-mesenchymal transition (altered E-Cadherin, N-Cadherin, and G-Catenin), invasion, migration, and bone colonization. Contact between tumor and osteoblasts or bone marrow cells increased IL1B secretion from all three cell types. IL1B alone did not stimulate tumor cell proliferation. Instead, IL1B caused expansion of the bone metastatic niche leading to tumor proliferation.

CONCLUSIONS

Pharmacologic inhibition of IL1B has potential as a novel treatment for breast cancer metastasis.

Chemotherapy resistance and stemness in mitotically quiescent human breast cancer cells identified by fluorescent dye retention

Metastatic recurrence in breast cancer is a major cause of mortality and often occurs many years after removal of the primary tumour. This process is driven by the reactivation of disseminated tumour cells that are characterised by mitotic quiescence and chemotherapeutic resistance. The ability to reliably isolate and characterise this cancer cell population is critical to enable development of novel therapeutic strategies for prevention of breast cancer recurrence. Here we describe the identification and characterisation of a sub-population of slow-cycling tumour cells in the MCF-7 and MDA-MB-231 human breast cancer cell lines based on their ability to retain the lipophilic fluorescent dye Vybrant^® DiD for up to six passages in culture. Vybrant^® DiD-retaining (DiD+) cells displayed significantly increased aldehyde dehydrogenase activity and exhibited significantly reduced sensitivity to chemotherapeutic agents compared to their rapidly dividing, Vybrant^® DiD-negative (DiD−) counterparts. In addition, DiD+ cells were exclusively capable of initiating population re-growth following withdrawal of chemotherapy. The DiD+ population displayed only partial overlap with the CD44⁺CD24^−/low cell surface protein marker signature widely used to identify breast cancer stem cells, but was enriched for CD44⁺CD24⁺ cells. Real-time qPCR profiling revealed differential expression of epithelial-to-mesenchymal transition and stemness genes between DiD+ and DiD− populations. This is the first demonstration that both MCF-7 and MDA-MB-231 human breast cancer lines contain a latent therapy-resistant population of slow-cycling cells capable of initiating population regrowth post-chemotherapy. Our data support that label-retaining cells can serve as a model for identification of molecular mechanisms driving tumour cell quiescence and de novo chemoresistance and that further characterisation of this prospective tumour-reinitiating population could yield novel therapeutic targets for elimination of the cells responsible for breast cancer recurrence.

Parathyroid Hormone (PTH) Increases Skeletal Tumour Growth and Alters Tumour Distribution in an In Vivo Model of Breast Cancer

Breast cancer cells colonize the skeleton by homing to specific niches, but the involvement of osteoblasts in tumour cell seeding, colonization, and progression is unknown. We used an in vivo model to determine how increasing the number of cells of the osteoblast lineage with parathyroid hormone (PTH) modified subsequent skeletal colonization by breast cancer cells. BALB/c nude mice were injected for five consecutive days with PBS (control) or PTH and then injected with DiD-labelled breast cancer cells via the intra-cardiac route. Effects of PTH on the bone microenvironment and tumour cell colonization and growth was analyzed using bioluminescence imaging, two-photon microscopy, and histological analysis. PTH treatment caused a significant, transient increase in osteoblast numbers compared to control, whereas bone volume/structure in the tibia was unaffected. There were no differences in the number of tumour cells seeding to the tibias, or in the number of tumours in the hind legs, between the control and PTH group. However, animals pre-treated with PTH had a significantly higher number of tumour colonies distributed throughout skeletal sites outside the hind limbs. This is the first demonstration that PTH-induced stimulation of osteoblastic cells may result in alternative skeletal sites becoming available for breast cancer cell colonization.

Abstract B13: Plasma G-CSF levels are predictive of lack of response to zoledronic acid treatment in reducing breast cancer recurrence

Some breast cancer patients have no evidence of metastatic disease at the time of original diagnosis, yet ~30-40% of patients will experience recurrent breast cancer. Over 90% of breast cancer deaths occur due to distant spread of the disease, and bone is the most common site of breast cancer metastasis. Bisphosphonates, such as zoledronic acid (ZA), are used to treat patients with osteolytic disease, including metastatic breast cancer. ZA has been demonstrated to reduce disseminated tumor cells (DTCs) and breast cancer recurrence, but not all patients see this benefit and it is not yet clear what predicts benefit from adjuvant ZA treatment; the mechanism of action for this effect of ZA in reducing breast cancer remains undetermined. Here, we establish that ZA renders osteoclast progenitor cells (OCPs) tumor suppressive, primarily by directing their lineage potential. In instances where the OCP lineage potential was modulated via systemic or tumor-derived granulocyte-colony stimulating factor (G-CSF), OCPs did not differentiate into tumor-suppressive populations but instead into mature osteoclasts. High G-CSF was sufficient to abrogate the tumor-suppressive effect of ZA. Furthermore, we determined that in a subset of patient plasma samples from the AZURE clinical trial where stage II/III breast cancer patients were randomized to receive standard systemic treatment alone with adjuvant ZA treatment, patients who had baseline plasma G-CSF levels &gt;23 pg/mL had reduced disease-free survival as assessed over a 10-year period post treatment initiation compared to patients with baseline plasma G-CSF levels &lt;23 pg/mL (p=0.02). These findings indicate that patients with higher baseline plasma G-CSF levels will not likely observe reduction in breast cancer recurrence with adjuvant ZA treatment, and in fact may have worse prognosis with adjuvant ZA treatment as compared to control treatment. Our data are the first to demonstrate that ZA mediates its tumor-suppressive function via the OCP population and implicate capitalizing on the differentiation potential of the OCPs to maximize patient response to adjuvant ZA treatment in breast cancer risk of recurrence.

Citation Format: Jessalyn M. Ubellacker, Ninib Baryawno, Nicolas Severe, Molly J. DeCristo, Jaclyn Sceneay, Marie-Therese Haider, Catherine S. Rhee, Yuanbo Qin, Ingunn Holen, Walter M. Gregory, Janet E. Brown, Robert E. Coleman, David T. Scadden, Sandra S. McAllister. Plasma G-CSF levels are predictive of lack of response to zoledronic acid treatment in reducing breast cancer recurrence [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B13.

Modulating Bone Marrow Hematopoietic Lineage Potential to Prevent Bone Metastasis in Breast Cancer

The presence of disseminated tumor cells in breast cancer patient bone marrow aspirates predicts decreased recurrence-free survival. Although it is appreciated that physiologic, pathologic, and therapeutic conditions impact hematopoiesis, it remains unclear whether targeting hematopoiesis presents opportunities for limiting bone metastasis. Using preclinical breast cancer models, we discovered that marrow from mice treated with the bisphosphonate zoledronic acid (ZA) are metastasis-suppressive. Specifically, ZA modulated hematopoietic myeloid/osteoclast progenitor cell (M/OCP) lineage potential to activate metastasis-suppressive activity. Granulocyte-colony stimulating factor (G-CSF) promoted ZA resistance by redirecting M/OCP differentiation. We identified M/OCP and bone marrow transcriptional programs associated with metastasis suppression and ZA resistance. Analysis of patient blood samples taken at randomization revealed that women with high-plasma G-CSF experienced significantly worse outcome with adjuvant ZA than those with lower G-CSF levels. Our findings support discovery of therapeutic strategies to direct M/OCP lineage potential and biomarkers that stratify responses in patients at risk of recurrence.Significance: Bone marrow myeloid/osteoclast progenitor cell lineage potential has a profound impact on breast cancer bone metastasis and can be modulated by G-CSF and bone-targeting agents. Cancer Res; 78(18); 5300-14. ©2018 AACR.

IL-1 drives breast cancer growth and bone metastasis in vivo

Background

We have recently identified interleukin 1B (IL-1B) as a potential biomarker for predicting breast cancer patients at increased risk for developing bone metastasis. In mouse models, IL-1B and its receptor (IL-1R1) are upregulated in breast cancer cells that metastasise to bone compared with cells that do not. We have now investigated the functional role of IL-1 by blocking IL-1R signalling with the clinically licensed antagonist, anakinra.

Methodology

6-week old female BALB/c mice received a subcutaneous or intra-venous injection of MDA-MB-231-IV or MCF7 cells. Anakinra (1mg/kg/day) or placebo was administered 3 days before (preventative) or 7 days later (treatment). Tumour volume, apoptosis (TUNEL, Caspase 3), proliferation (Ki67) and angiogenesis (CD34, VEGF and endothelin) were analysed. Effects on bone were measured by uCT, and TRAP, P1NP, IL-1B, TNF alpha and IL-6 ELISA.

Results

Anakinra significantly reduced growth of MDA-MB-231-IV tumours in bone from 6.50+/3.00mm² (placebo) to 2.56+/−1.07mm² (treatment) and 0.63+/−0.18mm² (preventative). Anakinra also reduced the number of mice that developed bone metastasis from 90% (placebo) to 40% (treatment) and 10% (preventative). Anti-tumour effects were not confined to bone, subcutaneous tumour volumes reduced from 656.68mm³ (placebo) to 160.47mm³ (treatment) and 31.08mm³ (preventative). Anakinra did not increase tumour cell apoptosis but reduced proliferation and angiogenesis in addition to exerting significant effects on the tumour environment reducing bone turnover markers, IL-1B and TNF alpha.

Conclusions

Our novel data demonstrate a functional role of IL-1 signalling in breast tumour progression and metastasis, supporting that anakinra could be repurposed for the treatment of breast cancer bone metastasis.

Introducing SEARCHBreast: a virtual resource to facilitate sharing of surplus animal material developed for breast cancer research

Animals studies have made significant contribution to expanding our knowledge of breast cancer. Often material is leftover and archived. SEARCHBreast provides a platform for collaborative sharing of archived material via a dedicated on-line database whereby users can both share and search available tissue. The SEARCHBreast database has information on over 50 different mouse models, including tissue from PDX models, available to share. With thousands of samples freely available, SEARCHBreast should be the first point of call for any researcher looking for animal material to aid their breast cancer research.

In vivo models in breast cancer research: progress, challenges and future directions

Research using animal model systems has been instrumental in delivering improved therapies for breast cancer, as well as in generating new insights into the mechanisms that underpin development of the disease. A large number of different models are now available, reflecting different types and stages of the disease; choosing which one to use depends on the specific research question(s) to be investigated. Based on presentations and discussions from leading experts who attended a recent workshop focused on in vivo models of breast cancer, this article provides a perspective on the many varied uses of these models in breast cancer research, their strengths, associated challenges and future directions. Among the questions discussed were: how well do models represent the different stages of human disease; how can we model the involvement of the human immune system and microenvironment in breast cancer; what are the appropriate models of metastatic disease; can we use models to carry out preclinical drug trials and identify pathways responsible for drug resistance; and what are the limitations of patient-derived xenograft models? We briefly outline the areas where the existing breast cancer models require improvement in light of the increased understanding of the disease process, reflecting the drive towards more personalised therapies and identification of mechanisms of drug resistance.

Zoledronic acid alters hematopoiesis and generates breast tumor-suppressive bone marrow cells

BACKGROUND

The bone-targeting agent zoledronic acid (ZOL) increases breast cancer survival in subsets of patients, but the underlying reasons for this protective effect are unknown. ZOL modulates the activity of osteoclasts and osteoblasts, which form hematopoietic stem cell niches, and therefore may affect hematopoietic cells that play a role in breast cancer progression.

METHOD

Immunocompetent and immunocompromised strains of mice commonly used for breast cancer research were injected with a single, clinically relevant dose of ZOL (100 μg/kg) or vehicle control. The effects of ZOL on the bone marrow microenvironment (bone volume, bone cell number/activity, extracellular matrix composition) were established at various time points following treatment, using micro-computed tomography (μCT) analysis, histomorphometry, ELISA and immunofluorescence. The effects on peripheral blood and bone marrow hematopoietic progenitor populations were assessed using a HEMAVET® hematology analyzer and multicolor flow cytometry, respectively. Tumor support function of bone marrow cells was determined using an in vivo functional assay developed in our laboratory.

RESULTS

Using multiple mouse strains, we observed transient changes in numbers of hematopoietic stem cells, myeloid-biased progenitor cells, and lymphoid-biased cells concurrent with changes to hematopoietic stem cell niches following ZOL administration. Importantly, bone marrow cells from mice treated with a single, clinically relevant dose of ZOL inhibited breast tumor outgrowth in vivo. The ZOL-induced tumor suppressive function of the bone marrow persisted beyond the time point at which numbers of hematopoietic progenitor cells had returned to baseline.

CONCLUSIONS

These findings provide novel evidence that alterations to the bone marrow play a role in the anti-tumor activity of ZOL and suggest possibilities for capitalizing on the beneficial effects of ZOL in reducing breast cancer development and progression.

Abstract P3-07-14: Targeting ERR-α regulated lactate metabolism eliminates drug-resistant breast cancer cells

Novel therapeutic strategies to eliminate chemo-resistant tumour cells responsible for the development of secondary lesions are essential in order to prevent breast cancer relapse. We have developed an in vitro model system enabling isolation of a putative metastasis-initiating breast cancer cell sub-population with a mitotically quiescent, drug-resistant phenotype. We hypothesise that this population is able to utilise oestrogen-related receptor-alpha (ERR-α)-regulated oxidative lactate metabolism and that inhibition of this critical survival pathway will result in elimination of tumour cells that have survived anti-cancer therapies.

Flow cytofluormetric monitoring of Vybrant® DiD retention identified a mitotically quiescent sub-population (~0.05%) in MDA-MB-231 human breast cancer cells grown under high glucose (11mM) conditions. DiD-retaining cells accumulated in the G2/M phase of the cell cycle and were shown to be non-senescent following cytochemical analysis of β-galactosidase activity. Cytotoxicity assays with doxorubicin (0.40μM for 72 hours) demonstrated increased survival in the quiescent fraction (40.76%) compared with the rapidly dividing bulk cell population (1.63%, P ≤ 0.0001). Isolated drug-resistant cells contained a sub-fraction (~1%) that was able to form new clonal populations following cessation of treatment. Mitotically quiescent cells were cultured under high glucose (11mM) or glucose-depleted conditions with high lactate (22mM) in the presence of ERR-α inhibitors Cpd29 (10μM) or XCT790 (10μM). Colony formation was completely eliminated in Cpd29- and XCT790-treated samples compared to untreated controls. Drug-resistant sub-clones were isolated from the quiescent population following 72 hours treatment with doxorubicin (0.40μM) and were cultured under glucose-depleted conditions with high lactate (22mM) in the presence of ERR-α inhibitors Cpd29 (10μM) or XCT790 (10μM). In both instances, colony formation was completely prevented.

We provide the first evidence that blocking cellular energy production through inhibition of ERR-α regulated oxidative lactate metabolism can eradicate a chemo-resistant, quiescent breast cancer cell population. Our data suggest that ERR-α inhibitors may be used in combination with chemotherapy to eliminate minimal residual disease and reduce breast cancer recurrence.

Citation Format: Quayle L, Park S, McDonnell DP, Ottewell PD, Holen I. Targeting ERR-α regulated lactate metabolism eliminates drug-resistant breast cancer cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-07-14.

The Sharing Experimental Animal Resources, Coordinating Holdings (SEARCH) Framework: Encouraging Reduction, Replacement, and Refinement in Animal Research

While significant medical breakthroughs have been achieved through using animal models, our experience shows that often there is surplus material remaining that is frequently never revisited but could be put to good use by other scientists. Recognising that most scientists are willing to share this material on a collaborative basis, it makes economic, ethical, and academic sense to explore the option to utilise this precious resource before generating new/additional animal models and associated samples. To bring together those requiring animal tissue and those holding this type of archival material, we have devised a framework called Sharing Experimental Animal Resources, Coordinating Holdings (SEARCH) with the aim of making remaining material derived from animal studies in biomedical research more visible and accessible to the scientific community. We encourage journals, funding bodies, and scientists to unite in promoting a new way of approaching animal research by adopting the SEARCH framework.

The endocrine influence on the bone microenvironment in early breast cancer

Multiple factors influence the survival of disseminated breast tumour cells (DTCs) in bone. Whereas gene signature studies have identified genes that predict a propensity of tumours to metastasise to bone, the bone environment is key in determining the fate of these tumour cells. Breast cancer cells locate to specific niches within the bone that support their survival, regulated by host factors within the bone microenvironment including bone cells, cells of the bone micro vasculature, immune cells and the extracellular matrix. Reproductive endocrine hormones that affect bone and clinical studies across the menopausal transition have provided comprehensive understanding of the changes in the bone microenvironment during this time. Menopause is characterized by a decrease in ovarian oestradiol and inhibins, with an increase in pituitary follicle-stimulating hormone and this review will focus on the role of these three hormones in determining the fate of DTCs in bone. Both in vivo and clinical data suggest that premenopausal bone is a conducive environment for growth of breast cancer cells in bone. Adjuvant cancer treatment aims to reduce the risk of tumour recurrence by affecting DTCs. Drugs targeting the bone resorbing osteoclasts, such as bisphosphonates, have therefore been evaluated in this setting. Both preclinical and adjuvant clinical studies have shown that bisphosphonates' ability to decrease tumour growth in bone is influenced by the levels of endocrine hormones, with enhanced effects in a postmenopausal bone microenvironment. The challenge is to understand the molecular mechanisms behind this phenomenon and to evaluate if alternative adjuvant bone-targeted therapies may be effective in premenopausal women.

Bone Metastasis: Molecular Mechanisms Implicated in Tumour Cell Dormancy in Breast and Prostate Cancer

Metastasis to the bone is most frequently observed in advanced cases of breast and prostate cancer. The latent development of overt metastatic lesions is associated with debilitating skeletal morbidity and eventual patient mortality. Secondary tumours in bone are derived from disseminated tumour cells (DTCs) that enter into a state of cellular dormancy. The dormant state confers resistance to conventional chemotherapeutic agents and prevents elimination of DTCs from the bone using current drug therapies. Expansion of our presently limited understanding of the molecular mechanisms underpinning disseminated breast and prostate tumour cell dormancy is critical to the future development of novel drug therapies aimed at the removal of DTCs, and thereby, the prevention of bone metastasis. This review provides an overview of the main putative molecular mechanisms underlying cellular dormancy in breast and prostate cancer bone metastasis reported from multiple experimental in vitro and in vivo models.

Effects of Src-kinase inhibition in cancer-induced bone pain

Background

Bone metastases occur frequently in advanced breast, lung, and prostate cancer, with approximately 70% of patients affected. Pain is a major symptom of bone metastases, and current treatments may be inadequate or have unacceptable side effects. The mechanisms that drive cancer-induced bone pain are not fully understood; however, it is known that there is sensitization of both peripheral bone afferents and central spinal circuits. It is well established that the N-methyl-D-aspartate receptor plays a major role in the pathophysiology of pain hypersensitivity. Inhibition of the non-receptor tyrosine kinase Src controls N-methyl-D-aspartate receptor activity and inhibiting Src reduces the hypersensitivity associated with neuropathic and inflammatory pains. As Src is also implicated in osteoclastic bone resorption, we have investigated if inhibiting Src ameliorates cancer-induced bone pain. We have tested this hypothesis using an orally bioavailable Src inhibitor (saracatinib) in a rat model of cancer-induced bone pain.

Results

Intra-tibial injection of rat mammary cancer cells (Mammary rat metastasis tumor cells -1), but not vehicle, in rats produced hindpaw hypersensitivity to thermal and mechanical stimuli that was maximal after six days and persisted for at least 13 days postinjection. Daily oral gavage with saracatinib (20 mg/kg) beginning seven days after intra-tibial injection reversed the thermal hyperalgesia but not the mechanical allodynia. The analgesic mechanisms of saracatinib appear to be due to an effect on the nervous system as immunoblotting of L2-5 spinal segments showed that mammary rat metastasis tumor cells-1 injection induced phosphorylation of the GluN1 subunit of the N-methyl-D-aspartate receptor, indicative of receptor activation, and this was reduced by saracatinib. Additionally, histology showed no anti-tumor effect of saracatinib at any dose and no significant effect on bone preservation.

Conclusions

This is the first demonstration that Src plays a role in the development of cancer-induced bone pain and that Src inhibition represents a possible new analgesic strategy for patients with bone metastases.

SEARCHBreast: a new resource to locate and share surplus archival material from breast cancer animal models to help address the 3Rs

Animal models have contributed to our understanding of breast cancer, with publication of results in high-impact journals almost invariably requiring extensive in vivo experimentation. As such, many laboratories hold large collections of surplus animal material, with only a fraction being used in publications relating to the original projects. Despite being developed at considerable cost, this material is an invisible and hence an underutilised resource, which often ends up being discarded. Within the breast cancer research community there is both a need and desire to make this valuable material available for researchers. Lack of a coordinated system for visualisation and localisation of this has prevented progress. To fulfil this unmet need, we have developed a novel initiative called Sharing Experimental Animal Resources: Coordinating Holdings-Breast (SEARCHBreast) which facilitates sharing of archival tissue between researchers on a collaborative basis and, de facto will reduce overall usage of animal models in breast cancer research. A secure searchable database has been developed where researchers can find, share, or upload materials related to animal models of breast cancer, including genetic and transplant models. SEARCHBreast is a virtual compendium where the physical material remains with the original laboratory. A bioanalysis pipeline is being developed for the analysis of transcriptomics data associated with mouse models, allowing comparative study with human and cell line data. Additionally, SEARCHBreast is committed to promoting the use of humanised breast tissue models as replacement alternatives to animals. Access to this unique resource is freely available to all academic researchers following registration at https://searchbreast.org.

SEARCHBreast Workshop Proceedings: 3D Modelling of Breast Cancer

SEARCHBreast, a UK initiative supported by the NC3Rs, organised a workshop entitled 3D Modelling of Breast Cancer. The workshop focused on providing researchers with solutions to overcome some of the perceived barriers to working with human-derived tumour cells, cell lines and tissues, namely: a) the limited access to human-derived material; and b) the difficulty in working with these samples. The workshop presentations provided constructive advice and information on how to best prepare human cells or tissues for further downstream applications. Techniques in developing primary cultures from patient samples, and considerations when preserving tissue slices, were discussed. A common theme throughout the workshop was the importance of ensuring that the cells are grown in conditions as similar to the in vivo microenvironment as possible. Comparisons of the advantages of several in vitro options, such as primary cell cultures, cell line cultures, explants or tissue slices, suggest that all offer great potential applications for breast cancer research, and highlight that it need not be a case of choosing one over the other. The workshop also offered cutting-edge examples of on-chip technologies and 3-D tumour modelling by using virtual pathology, which can contribute to clinically relevant studies and provide insights into breast cancer metastatic mechanisms.

CAPG and GIPC1: Breast Cancer Biomarkers for Bone Metastasis Development and Treatment

Background:

Bone is the predominant site of metastasis from breast cancer, and recent trials have demonstrated that adjuvant bisphosphonate therapy can reduce bone metastasis development and improve survival. There is an unmet need for prognostic and predictive biomarkers so that therapy can be appropriately targeted.

Methods:

Potential biomarkers for bone metastasis were identified using proteomic comparison of bone-metastatic, lung-metastatic, and nonmetastatic variants of human breast cancer MDA-MB-231 cells. Clinical validation was performed using immunohistochemical staining of tumor tissue microarrays from patients in a large randomized trial of adjuvant zoledronic acid (zoledronate) (AZURE-ISRCTN79831382). We used Cox proportional hazards regression, the Kaplan-Meier estimate of the survival function, and the log-rank test to investigate associations between protein expression, clinical variables, and time to distant recurrence events. All statistical tests were two-sided.

Results:

Two novel biomarker candidates, macrophage-capping protein (CAPG) and PDZ domain–containing protein GIPC1 (GIPC1), were identified for clinical validation. Cox regression analysis of AZURE training and validation sets showed that control patients (no zoledronate) were more likely to develop first distant recurrence in bone (hazard ratio [HR] = 4.5, 95% confidence interval [CI] = 2.1 to 9.8, P < .001) and die (HR for overall survival = 1.8, 95% CI = 1.01 to 3.24, P = .045) if both proteins were highly expressed in the primary tumor. In patients with high expression of both proteins, zoledronate had a substantial effect, leading to 10-fold hazard ratio reduction (compared with control) for first distant recurrence in bone (P = .008).

Conclusions:

The composite biomarker, CAPG and GIPC1 in primary breast tumors, predicted disease outcomes and benefit from zoledronate and may facilitate patient selection for adjuvant bisphosphonate treatment.

Adjuvant bisphosphonates in early breast cancer: consensus guidance for clinical practice from a European Panel

Bisphosphonates have been studied in randomised trials in early breast cancer to investigate their ability to prevent cancer treatment-induced bone loss (CTIBL) and reduce the risk of disease recurrence and metastasis. Treatment benefits have been reported but bisphosphonates do not currently have regulatory approval for either of these potential indications. This consensus paper provides a review of the evidence and offers guidance to breast cancer clinicians on the use of bisphosphonates in early breast cancer. Using the nominal group methodology for consensus, a systematic review of the literature was augmented by a workshop held in October 2014 for breast cancer and bone specialists to present and debate the available pre-clinical and clinical evidence for the use of adjuvant bisphosphonates. This was followed by a questionnaire to all members of the writing committee to identify areas of consensus. The panel recommended that bisphosphonates should be considered as part of routine clinical practice for the prevention of CTIBL in all patients with a T score of <-2.0 or ≥2 clinical risk factors for fracture. Compelling evidence from a meta-analysis of trial data of >18,000 patients supports clinically significant benefits of bisphosphonates on the development of bone metastases and breast cancer mortality in post-menopausal women or those receiving ovarian suppression therapy. Therefore, the panel recommends that bisphosphonates (either intravenous zoledronic acid or oral clodronate) are considered as part of the adjuvant breast cancer treatment in this population and the potential benefits and risks discussed with relevant patients.

Rapid modification of the bone microenvironment following short-term treatment with Cabozantinib in vivo

INTRODUCTION

Bone metastasis remains incurable with treatment restricted to palliative care. Cabozantinib (CBZ) is targeted against multiple receptor tyrosine kinases involved in tumour pathobiology, including hepatocyte growth factor receptor (MET) and vascular endothelial growth factor receptor 2 (VEGFR-2). CBZ has demonstrated clinical activity in advanced prostate cancer with resolution of lesions visible on bone scans, implicating a potential role of the bone microenvironment as a mediator of CBZ effects. We characterised the effects of short-term administration of CBZ on bone in a range of in vivo models to determine how CBZ affects bone in the absence of tumour.

METHODS

Studies were performed in a variety of in vivo models including male and female BALB/c nude mice (age 6-17-weeks). Animals received CBZ (30 mg/kg, 5× weekly) or sterile H2O control for 5 or 10 days. Effects on bone integrity (μCT), bone cell activity (PINP, TRAP ELISA), osteoblast and osteoclast number/mm trabecular bone surface, area of epiphyseal growth plate cartilage, megakaryocyte numbers and bone marrow composition were assessed. Effects of longer-term treatment (15-day & 6-week administration) were assessed in male NOD/SCID and beige SCID mice.

RESULTS

CBZ treatment had significant effects on the bone microenvironment, including reduced osteoclast and increased osteoblast numbers compared to control. Trabecular bone structure was altered after 8 administrations. A significant elongation of the epiphyseal growth plate, in particular the hypertrophic chondrocyte zone, was observed in all CBZ treated animals irrespective of administration schedule. Both male and female BALB/c nude mice had increased megakaryocyte numbers/mm(2) tissue after 10-day CBZ treatment, in addition to vascular ectasia, reduced bone marrow cellularity and extravasation of red blood cells into the extra-vascular bone marrow. All CBZ-induced effects were transient and rapidly lost following cessation of treatment.

CONCLUSION

Short-term administration of CBZ induces rapid, reversible effects on the bone microenvironment in vivo highlighting a potential role in mediating treatment responses.

Oestrogen receptor positive breast cancer metastasis to bone: inhibition by targeting the bone microenvironment in vivo

Clinical trials have shown that adjuvant Zoledronic acid (ZOL) reduces the development of bone metastases irrespective of ER status. However, post-menopausal patients show anti-tumour benefit with ZOL whereas pre-menopausal patients do not. Here we have developed in vivo models of spontaneous ER+ve breast cancer metastasis to bone and investigated the effects of ZOL and oestrogen on tumour cell dissemination and growth. ER+ve (MCF7, T47D) or ER-ve (MDA-MB-231) cells were administered by inter-mammary or inter-cardiac injection into female nude mice ± estradiol. Mice were administered saline or 100 μg/kg ZOL weekly. Tumour growth, dissemination of tumour cells in blood, bone and bone turnover were monitored by luciferase imaging, histology, flow cytometry, two-photon microscopy, micro-CT and TRAP/P1NP ELISA. Estradiol induced metastasis of ER+ve cells to bone in 80-100 % of animals whereas bone metastases from ER-ve cells were unaffected. Administration of ZOL had no effect on tumour growth in the fat pad but significantly inhibited dissemination of ER+ve tumour cells to bone and frequency of bone metastasis. Estradiol and ZOL increased bone volume via different mechanisms: Estradiol increased activity of bone forming osteoblasts whereas administration of ZOL to estradiol supplemented mice decreased osteoclast activity and returned osteoblast activity to levels comparable to that of saline treated mice. ER-ve cells require increased osteoclast activity to grow in bone whereas ER+ve cells do not. Zol does not affect ER+ve tumour growth in soft tissue, however, inhibition of bone turnover by ZOL reduced dissemination and growth of ER+ve breast cancer cells in bone.

The frequency of osteolytic bone metastasis is determined by conditions of the soil, not the number of seeds; evidence from in vivo models of breast and prostate cancer

BACKGROUND

While both preclinical and clinical studies suggest that the frequency of growing skeletal metastases is elevated in individuals with higher bone turnover, it is unclear whether this is a result of increased numbers of tumour cells arriving in active sites or of higher numbers of tumour cells being induced to divide by the bone micro-environment. Here we have investigated how the differences in bone turnover affect seeding of tumour cells and/or development of overt osteolytic bone metastasis using in vivo models of hormone-independent breast and prostate cancer.

METHODS

Cohorts of 6 (young) and 16 (mature)-week old BALB/c nude mice were culled 1, 7 and 21 days after received intracardiac injection of luciferase expressing human prostate (PC3) or breast cancer (MDA-MB-231) cell lines labelled with a fluorescent cell membrane dye (Vybrant DiD). The presence of growing bone metastases was determined by bioluminescence using an in vivo imaging system (IVIS) and followed by anatomical confirmation of tumour metastatic sites post mortem, while the presence of individual fluorescently labelled tumour cells was evaluated using two-photon microscopy ex vivo. The bone remodelling activities were compared between young and mature naïve mice (both male and female) using micro-CT analysis, ELISA and bone histomorphometry.

RESULTS

Both prostate and breast cancer cells generated higher numbers of overt skeletal lesions in young mice (~80%) than in mature mice (~20%). Although mature mice presented with fewer overt bone metastases, the number of tumour cells arriving/colonizing in the tibias was comparable between young and mature animals. Young naïve mice had lower bone volume but higher bone formation and resorption activities compared to mature animals.

CONCLUSIONS

Our studies suggest that higher frequencies of growing osteolytic skeletal metastases in these models are linked to increased bone turnover and not to the initial number of tumour cells entering the bone microenvironment.

Human breast cancer bone metastasis in vitro and in vivo: a novel 3D model system for studies of tumour cell-bone cell interactions

Bone is established as the preferred site of breast cancer metastasis. However, the precise mechanisms responsible for this preference remain unidentified. In order to improve outcome for patients with advanced breast cancer and skeletal involvement, we need to better understand how this process is initiated and regulated. As bone metastasis cannot be easily studied in patients, researchers have to date mainly relied on in vivo xenograft models. A major limitation of these is that they do not contain a human bone microenvironment, increasingly considered to be an important component of metastases. In order to address this shortcoming, we have developed a novel humanised bone model, where 1 × 10(5) luciferase-expressing MDA-MB-231 or T47D human breast tumour cells are seeded on viable human subchaodral bone discs in vitro. These discs contain functional osteoclasts 2-weeks after in vitro culture and positive staining for calcine 1-week after culture demonstrating active bone resorption/formation. In vitro inoculation of MDA-MB-231 or T47D cells colonised human bone cores and remained viable for <4 weeks, however, use of matrigel to enhance adhesion or a moving platform to increase diffusion of nutrients provided no additional advantage. Following colonisation by the tumour cells, bone discs pre-seeded with MDA-MB-231 cells were implanted subcutaneously into NOD SCID mice, and tumour growth monitored using in vivo imaging for up to 6 weeks. Tumour growth progressed in human bone discs in 80 % of the animals mimicking the later stages of human bone metastasis. Immunohistochemical and PCR analysis revealed that growing MDA-MB-231 cells in human bone resulted in these cells acquiring a molecular phenotype previously associated with breast cancer bone metastases. MDA-MB-231 cells grown in human bone discs showed increased expression of IL-1B, HRAS and MMP9 and decreased expression of S100A4, whereas, DKK2 and FN1 were unaltered compared with the same cells grown in mammary fat pads of mice not implanted with human bone discs.

Mitotic quiescence, but not unique "stemness," marks the phenotype of bone metastasis-initiating cells in prostate cancer

This study aimed to identify subpopulations of prostate cancer cells that are responsible for the initiation of bone metastases. Using rapidly dividing human prostate cancer cell lines, we identified mitotically quiescent subpopulations (<1%), which we compared with the rapidly dividing populations for patterns of gene expression and for their ability to migrate to the skeletons of athymic mice. The study used 2-photon microscopy to map the presence/distribution of fluorescently labeled, quiescent cells and luciferase expression to determine the presence of growing bone metastases. We showed that the mitotically quiescent cells were very significantly more tumorigenic in forming bone metastases than fast-growing cells (55 vs. 15%) and had a unique gene expression profile. The quiescent cells were not uniquely stem cell like, with no expression of CD133 but had the same level expression of other putative prostate stem cell markers (CD44 and integrins α2/β1), when compared to the rapidly proliferating population. In addition, mitotic quiescence was associated with very high levels of C-X-C chemokine receptor type 4 (CXCR4) production. Inhibition of CXCR4 activity altered the homing of quiescent tumor cells to bone. Our studies suggest that mitotic dormancy is a unique phenotype that facilitates tumor cell colonization of the skeleton in prostate cancer.

Molecular alterations that drive breast cancer metastasis to bone

Epithelial cancers including breast and prostate commonly progress to form incurable bone metastases. For this to occur, cancer cells must adapt their phenotype and behaviour to enable detachment from the primary tumour, invasion into the vasculature, and homing to and subsequent colonisation of bone. It is widely accepted that the metastatic process is driven by the transformation of cancer cells from a sessile epithelial to a motile mesenchymal phenotype through epithelial-mesenchymal transition (EMT). Dissemination of these motile cells into the circulation provides the conduit for cells to metastasise to distant organs. However, accumulating evidence suggests that EMT is not sufficient for metastasis to occur and that specific tissue-homing factors are required for tumour cells to lodge and grow in bone. Once tumour cells are disseminated in the bone environment, they can revert into an epithelial phenotype through the reverse process of mesenchymal-epithelial transition (MET) and form secondary tumours. In this review, we describe the molecular alterations undertaken by breast cancer cells at each stage of the metastatic cascade and discuss how these changes facilitate bone metastasis.