Targeting the perivascular niche sensitizes disseminated tumour cells to chemotherapy

Life after Cell Death-Survival and Survivorship Following Chemotherapy

Simple Summary

Treatment of aggressive cancers often relies on chemotherapy. This treatment has improved survival rates, but while effective at killing cancer cells, inevitably it also kills or alters the function of others. While many of the known effects are transient and resolve after treatment, as survival rates increase, so does our understanding of the long-term health costs that accompany cancer survivors. Here we provide an overview of common long-term morbidities known to be caused by conventional chemotherapy, including the risk of relapse, but more importantly, the cost of quality of life experienced, especially by those who have cancer in early life. We aim to highlight the importance of the development of targeted therapies to replace the use of conventional chemotherapy, but also that of treating the patients along with the disease to enable not only longer but also healthier life after cancer.

Abstract

To prevent cancer cells replacing and outnumbering their functional somatic counterparts, the most effective solution is their removal. Classical treatments rely on surgical excision, chemical or physical damage to the cancer cells by conventional interventions such as chemo- and radiotherapy, to eliminate or reduce tumour burden. Cancer treatment has in the last two decades seen the advent of increasingly sophisticated therapeutic regimens aimed at selectively targeting cancer cells whilst sparing the remaining cells from severe loss of viability or function. These include small molecule inhibitors, monoclonal antibodies and a myriad of compounds that affect metabolism, angiogenesis or immunotherapy. Our increased knowledge of specific cancer types, stratified diagnoses, genetic and molecular profiling, and more refined treatment practices have improved overall survival in a significant number of patients. Increased survival, however, has also increased the incidence of associated challenges of chemotherapy-induced morbidity, with some pathologies developing several years after termination of treatment. Long-term care of cancer survivors must therefore become a focus in itself, such that along with prolonging life expectancy, treatments allow for improved quality of life.

Targeted Therapy to β3 Integrin Reduces Chemoresistance in Breast Cancer Bone Metastases

Breast cancer bone metastases are common and incurable. Tumoral integrin β3 (β3) expression is induced through interaction with the bone microenvironment. Although β3 is known to promote bone colonization, its functional role during therapy of established bone metastases is not known. We found increased numbers of β3+ tumor cells in murine bone metastases after docetaxel chemotherapy. β3+ tumor cells were present in 97% of post-neoadjuvant chemotherapy triple-negative breast cancer patient samples (n = 38). High tumoral β3 expression was associated with worse outcomes in both pre- and postchemotherapy triple-negative breast cancer groups. Genetic deletion of tumoral β3 had minimal effect in vitro, but significantly enhanced in vivo docetaxel activity, particularly in the bone. Rescue experiments confirmed that this effect required intact β3 signaling. Ultrastructural, transcriptomic, and functional analyses revealed an alternative metabolic response to chemotherapy in β3-expressing cells characterized by enhanced oxygen consumption, reactive oxygen species generation, and protein production. We identified mTORC1 as a candidate for therapeutic targeting of this β3-mediated, chemotherapy-induced metabolic response. mTORC1 inhibition in combination with docetaxel synergistically attenuated murine bone metastases. Furthermore, micelle nanoparticle delivery of mTORC1 inhibitor to cells expressing activated αvβ3 integrins enhanced docetaxel efficacy in bone metastases. Taken together, we show that β3 integrin induction by the bone microenvironment promotes resistance to chemotherapy through an altered metabolic response that can be defused by combination with αvβ3-targeted mTORC1 inhibitor nanotherapy. Our work demonstrates the importance of the metastatic microenvironment when designing treatments and presents new, bone-specific strategies for enhancing chemotherapeutic efficacy.

Bone marrow niches in the regulation of bone metastasis

The bone marrow has been widely recognised to host a unique microenvironment that facilitates tumour colonisation. Bone metastasis frequently occurs in the late stages of malignant diseases such as breast, prostate and lung cancers. The biology of bone metastasis is determined by tumour-cell-intrinsic traits as well as their interaction with the microenvironment. The bone marrow is a dynamic organ in which various stages of haematopoiesis, osteogenesis, osteolysis and different kinds of immune response are precisely regulated. These different cellular components constitute specialised tissue microenvironments-niches-that play critical roles in controlling tumour cell colonisation, including initial seeding, dormancy and outgrowth. In this review, we will dissect the dynamic nature of the interactions between tumour cells and bone niches. By targeting certain steps of tumour progression and crosstalk with the bone niches, the development of potential therapeutic approaches for the clinical treatment of bone metastasis might be feasible.

The PI3K/mTOR inhibitor Gedatolisib eliminates dormant breast cancer cells in organotypic culture, but fails to prevent metastasis in preclinical settings

Dormant, disseminated tumor cells (DTCs) are thought to be the source of breast cancer metastases several years or even decades after initial treatment. To date, a selective therapy that leads to their elimination has not been discovered. While dormant DTCs resist chemotherapy, evidence suggests that this resistance is driven not by their lack of proliferation, but by their engagement of the surrounding microenvironment, via integrin‐β1‐mediated interactions. Because integrin‐β1‐targeted agents have not been translated readily to the clinic, signaling nodes downstream of integrin‐β1 could serve as attractive therapeutic targets in order to sensitize dormant DTCs to therapy. By probing a number of kinases downstream of integrin‐β1, we determined that PI3K inhibition with either a tool compounds or a compound (PF‐05212384; aka Gedatolisib) in clinical trials robustly sensitizes quiescent breast tumor cells seeded in organotypic bone marrow cultures to chemotherapy. These results motivated the preclinical study of whether Gedatolisib—with or without genotoxic therapy—would reduce DTC burden and prevent metastases. Despite promising results in organotypic culture, Gedatolisib failed to reduce DTC burden or delay, reduce or prevent metastasis in murine models of either triple‐negative or estrogen receptor‐positive breast cancer dissemination and metastasis. This result held true whether analyzing Gedatolisib on its own (vs. vehicle‐treated animals) or in combination with dose‐dense doxorubicin and cyclophosphamide (vs. animals treated only with dose‐dense chemotherapies). These data suggest that PI3K is not the node downstream of integrin‐β1 that confers chemotherapeutic resistance to DTCs. More broadly, they cast doubt on the strategy to target PI3K in order to eliminate DTCs and prevent breast cancer metastasis.

Breast cancer dormancy: need for clinically relevant models to address current gaps in knowledge

Breast cancer is the most commonly diagnosed cancer in the USA. Although advances in treatment over the past several decades have significantly improved the outlook for this disease, most women who are diagnosed with estrogen receptor positive disease remain at risk of metastatic relapse for the remainder of their life. The cellular source of late relapse in these patients is thought to be disseminated tumor cells that reactivate after a long period of dormancy. The biology of these dormant cells and their natural history over a patient's lifetime is largely unclear. We posit that research on tumor dormancy has been significantly limited by the lack of clinically relevant models. This review will discuss existing dormancy models, gaps in biological understanding, and propose criteria for future models to enhance their clinical relevance.

The roles of integrins in cancer

Integrin-mediated adhesion of cells to the extracellular matrix (ECM) is crucial for the physiological development and functioning of tissues but is pathologically disrupted in cancer. Indeed, abnormal regulation of integrin receptors and ECM ligands allows cancer cells to break down tissue borders, breach into blood and lymphatic vessels, and survive traveling in suspension through body fluids or residing in metabolically or pharmacologically hostile environments. Different molecular and cellular mechanisms responsible for the modulation of integrin adhesive function or mechanochemical signaling are altered and participate in cancer. Cancer development and progression are also bolstered by dysfunctionalities of integrin-mediated ECM adhesion occurring both in tumor cells and in elements of the surrounding tumor microenvironment, such as vascular cells, cancer-associated fibroblasts, and immune cells. Mounting evidence suggests that integrin inhibitors may be effectively exploited to overcome resistance to standard-of-care anti-cancer therapies.

Tumor Dormancy: Implications for Invasion and Metastasis

Tumor dormancy refers to a critical stage of cancer development when tumor cells are present, but cancer does not progress. It includes both the concept of cellular dormancy, indicating the reversible switch of a cancer cell to a quiescent state, and that of tumor mass dormancy, indicating the presence of neoplastic masses that have reached cell population equilibrium via balanced growth/apoptosis rates. Tumor dormancy provides the conceptual framework, potentially explaining a major challenge in clinical oncology, tumor recurrence, which may occur years after cancer diagnosis. The mechanisms by which tumors are kept dormant, and what triggers their reawakening, are fundamental questions in cancer biology. It seems that a plethora of intracellular pathways and extracellular factors are involved in this process, rewiring the cells to plastically alter their metabolic and proliferative status. This phenomenon is highly dynamic in space and time. Mechanistic insights into both cellular and tumor dormancy have provided the rationale for targeting this otherwise stable period of cancer development, in order to prevent recurrence and maximize therapeutic benefit.

Bioengineering the Bone Marrow Vascular Niche

The bone marrow (BM) tissue is the main physiological site for adult hematopoiesis. In recent years, the cellular and matrix components composing the BM have been defined with unprecedent resolution, both at the molecular and structural levels. With the expansion of this knowledge, the possibility of reproducing a BM-like structure, to ectopically support and study hematopoiesis, becomes a reality. A number of experimental systems have been implemented and have displayed the feasibility of bioengineering BM tissues, supported by cells of mesenchymal origin. Despite being known as an abundant component of the BM, the vasculature has been largely disregarded for its role in regulating tissue formation, organization and determination. Recent reports have highlighted the crucial role for vascular endothelial cells in shaping tissue development and supporting steady state, emergency and malignant hematopoiesis, both pre- and postnatally. Herein, we review the field of BM-tissue bioengineering with a particular focus on vascular system implementation and integration, starting from describing a variety of applicable in vitro models, ending up with in vivo preclinical models. Additionally, we highlight the challenges of the field and discuss the clinical perspectives in terms of adoptive transfer of vascularized BM-niche grafts in patients to support recovering hematopoiesis.

Innovative Approaches in the Battle Against Cancer Recurrence: Novel Strategies to Combat Dormant Disseminated Tumor Cells

Cancer recurrence remains a great fear for many cancer survivors following their initial, apparently successful, therapy. Despite significant improvement in the overall survival of many types of cancer, metastasis accounts for ~90% of all cancer mortality. There is a growing understanding that future therapeutic practices must accommodate this unmet medical need in preventing metastatic recurrence. Accumulating evidence supports dormant disseminated tumor cells (DTCs) as a source of cancer recurrence and recognizes the need for novel strategies to target these tumor cells. This review presents strategies to target dormant quiescent DTCs that reside at secondary sites. These strategies aim to prevent recurrence by maintaining dormant DTCs at bay, or eradicating them. Various approaches are presented, including: reinforcing the niche where dormant DTCs reside in order to keep dormant DTCs at bay; promoting cell intrinsic mechanisms to induce dormancy; preventing the engagement of dormant DTCs with their supportive niche in order to prevent their reactivation; targeting cell-intrinsic mechanisms mediating long-term survival of dormant DTCs; sensitizing dormant DTCs to chemotherapy treatments; and, inhibiting the immune evasion of dormant DTCs, leading to their demise. Various therapeutic approaches, some of which utilize drugs that are already approved, or have been tested in clinical trials and may be considered for repurposing, will be discussed. In addition, clinical evidence for the presence of dormant DTCs will be reviewed, along with potential prognostic biomarkers to enable the identification and stratification of patients who are at high risk of recurrence, and who could benefit from novel dormant DTCs targeting therapies. Finally, we will address the shortcomings of current trial designs for determining activity against dormant DTCs and provide novel approaches.

The matrix in cancer

The extracellular matrix is a fundamental, core component of all tissues and organs, and is essential for the existence of multicellular organisms. From the earliest stages of organism development until death, it regulates and fine-tunes every cellular process in the body. In cancer, the extracellular matrix is altered at the biochemical, biomechanical, architectural and topographical levels, and recent years have seen an exponential increase in the study and recognition of the importance of the matrix in solid tumours. Coupled with the advancement of new technologies to study various elements of the matrix and cell-matrix interactions, we are also beginning to see the deployment of matrix-centric, stromal targeting cancer therapies. This Review touches on many of the facets of matrix biology in solid cancers, including breast, pancreatic and lung cancer, with the aim of highlighting some of the emerging interactions of the matrix and influences that the matrix has on tumour onset, progression and metastatic dissemination, before summarizing the ongoing work in the field aimed at developing therapies to co-target the matrix in cancer and cancer metastasis.

Fluorescent Silica Nanoparticles to Label Metastatic Tumor Cells in Mineralized Bone Microenvironments

During breast cancer bone metastasis, tumor cells interact with bone microenvironment components including inorganic minerals. Bone mineralization is a dynamic process and varies spatiotemporally as a function of cancer-promoting conditions such as age and diet. The functional relationship between skeletal dissemination of tumor cells and bone mineralization, however, is unclear. Standard histological analysis of bone metastasis frequently relies on prior demineralization of bone, while methods that maintain mineral are often harsh and damage fluorophores commonly used to label tumor cells. Here, fluorescent silica nanoparticles (SNPs) are introduced as a robust and versatile labeling strategy to analyze tumor cells within mineralized bone. SNP uptake and labeling efficiency of MDA-MB-231 breast cancer cells is characterized with cryo-scanning electron microscopy and different tissue processing methods. Using a 3D in vitro model of marrow-containing, mineralized bone as well as an in vivo model of bone metastasis, SNPs are demonstrated to allow visualization of labeled tumor cells in mineralized bone using various imaging modalities including widefield, confocal, and light sheet microscopy. This work suggests that SNPs are valuable tools to analyze tumor cells within mineralized bone using a broad range of bone processing and imaging techniques with the potential to increase the understanding of bone metastasis.

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.

Exploiting bone niches: progression of disseminated tumor cells to metastasis

Many solid cancers metastasize to the bone and bone marrow (BM). This process may occur even before the diagnosis of primary tumors, as evidenced by the discovery of disseminated tumor cells (DTCs) in patients without occult malignancies. The cellular fates and metastatic progression of DTCs are determined by complicated interactions between cancer cells and BM niches. Not surprisingly, these niches also play important roles in normal biology, including homeostasis and turnover of skeletal and hematopoiesis systems. In this Review, we summarize recent findings on functions of BM niches in bone metastasis (BoMet), particularly during the early stage of colonization. In light of the rich knowledge of hematopoiesis and osteogenesis, we highlight how DTCs may progress into overt BoMet by taking advantage of niche cells and their activities in tissue turnover, especially those related to immunomodulation and bone repair.

Laboratory Models for Investigating Breast Cancer Therapy Resistance and Metastasis

While numerous therapies are highly efficacious in early-stage breast cancers and in particular subsets of breast cancers, therapeutic resistance and metastasis unfortunately arise in many patients. In many cases, tumors that are resistant to standard of care therapies, as well as tumors that have metastasized, are treatable but incurable with existing clinical strategies. Both therapy resistance and metastasis are multi-step processes during which tumor cells must overcome diverse environmental and selective hurdles. Mechanisms by which tumor cells achieve this are numerous and include acquisition of invasive and migratory capabilities, cell-intrinsic genetic and/or epigenetic adaptations, clonal selection, immune evasion, interactions with stromal cells, entering a state of dormancy or senescence, and maintaining self-renewal capacity. To overcome therapy resistance and metastasis in breast cancer, the ability to effectively model each of these mechanisms in the laboratory is essential. Herein we review historic and the current state-of-the-art laboratory model systems and experimental approaches used to investigate breast cancer metastasis and resistance to standard of care therapeutics. While each model system has inherent limitations, they have provided invaluable insights, many of which have translated into regimens undergoing clinical evaluation. We will discuss the limitations and advantages of a variety of model systems that have been used to investigate breast cancer metastasis and therapy resistance and outline potential strategies to improve experimental modeling to further our knowledge of these processes, which will be crucial for the continued development of effective breast cancer treatments.

EphB6 Regulates TFEB-Lysosomal Pathway and Survival of Disseminated Indolent Breast Cancer Cells

Late relapse of disseminated cancer cells is a common feature of breast and prostate tumors. Several intrinsic and extrinsic factors have been shown to affect quiescence and reawakening of disseminated dormant cancer cells (DDCCs); however, the signals and processes sustaining the survival of DDCCs in a foreign environment are still poorly understood. We have recently shown that crosstalk with lung epithelial cells promotes survival of DDCCs of estrogen receptor-positive (ER+) breast tumors. By using a lung organotypic system and in vivo dissemination assays, here we show that the TFEB-lysosomal axis is activated in DDCCs and that it is modulated by the pro-survival ephrin receptor EphB6. TFEB lysosomal direct targets are enriched in DDCCs in vivo and correlate with relapse in ER+ breast cancer patients. Direct coculture of DDCCs with alveolar type I-like lung epithelial cells and dissemination in the lung drive lysosomal accumulation and EphB6 induction. EphB6 contributes to survival, TFEB transcriptional activity, and lysosome formation in DDCCs in vitro and in vivo. Furthermore, signaling from EphB6 promotes the proliferation of surrounding lung parenchymal cells in vivo. Our data provide evidence that EphB6 is a key factor in the crosstalk between disseminated dormant cancer cells and the lung parenchyma and that the TFEB-lysosomal pathway plays an important role in the persistence of DDCCs.

High VCAM-1 Predicts Poor Prognosis and is Associated with Chemotherapy Resistance in Nasopharyngeal Carcinoma

Purpose

Nasopharyngeal carcinoma (NPC) is a malignant tumor endemic in southern China and Southeast Asia with a poor prognosis. Vascular cell adhesion protein 1 (VCAM-1) is highly expressed in NPC; however, it is unclear whether VCAM-1 is correlated with chemotherapy resistance and prognosis in NPC.

Patients and Methods

To further explore the role of VCAM-1 in chemotherapy resistance and prognosis in NPC, we examined the expression of VCAM-1, the sensitivity of chemotherapy drugs, and clinical follow-up data from 73 patients with NPC. Then, the results of VCAM-1 expression were analyzed in response to chemotherapy drugs, progression-free survival (PFS), and overall survival (OS).

Results

The expression of VCAM-1 protein in NPC was significantly higher than that in chronic inflammatory tissue. No significant differences in the expression of VCAM-1 among gender, age, pathologic classification, tumor classification, lymph node status, metastasis status, and overall clinical stage were found. The periods of PFS and OS in patients with high VCAM-1 expression were significantly shorter than those in patients with low VCAM-1 expression. The sensitivity rates of NPC to eight chemotherapy drugs were different; carboplatin and docetaxel showed the highest chemotherapy sensitivity and resistance rates, respectively. The resistance rates to paclitaxel were different between the patients with high VCAM-1 expression and those with low VCAM-1 expression.

Conclusion

Our data indicated that VCAM-1 was highly expressed in NPC. Patients with high VCAM-1 expression were more prone to shorter periods of PFS and OS. VCAM-1 could be a prognostic marker of NPC patients. The detection of VCAM-1 expression in NPC may be valuable for chemotherapy drug evaluation and management of patients with NPC in the clinic.

Could Extracellular Vesicles Contribute to Generation or Awakening of "Sleepy" Metastatic Niches?

Pre-metastatic niches provide favorable conditions for tumor cells to disseminate, home to and grow in otherwise unfamiliar and distal microenvironments. Tumor-derived extracellular vesicles are now recognized as carriers of key messengers secreted by primary tumors, signals that induce the formation of pre-metastatic niches. Recent evidence suggests that tumor cells can disseminate from the very earliest stages of primary tumor development. However, once they reach distal sites, tumor cells can persist in a dormant state for long periods of time until their growth is reactivated and they produce metastatic lesions. In this new scenario, the question arises as to whether extracellular vesicles could influence the formation of these metastatic niches with dormant tumor cells? (here defined as "sleepy niches"). If so, what are the molecular mechanisms involved? In this perspective-review article, we discuss the possible influence of extracellular vesicles in early metastatic dissemination and whether they might play a role in tumor cell dormancy. In addition, we comment whether extracellular vesicle-mediated signals may be involved in tumor cell awakening, considering the possibility that extracellular vesicles might serve as biomarkers to detect early metastasis and/or minimal residual disease (MRD) monitoring.

Breast cancer-secreted factors perturb murine bone growth in regions prone to metastasis

Breast cancer frequently metastasizes to bone, causing osteolytic lesions. However, how factors secreted by primary tumors affect the bone microenvironment before the osteolytic phase of metastatic tumor growth remains unclear. Understanding these changes is critical as they may regulate metastatic dissemination and progression. To mimic premetastatic bone adaptation, immunocompromised mice were injected with MDA-MB-231-conditioned medium [tumor-conditioned media (TCM)]. Subsequently, the bones of these mice were subjected to multiscale, correlative analysis including RNA sequencing, histology, micro-computed tomography, x-ray scattering analysis, and Raman imaging. In contrast to overt metastasis causing osteolysis, TCM treatment induced new bone formation that was characterized by increased mineral apposition rate relative to control bones, altered bone quality with less matrix and more carbonate substitution, and the deposition of disoriented mineral near the growth plate. Our study suggests that breast cancer-secreted factors may promote perturbed bone growth before metastasis, which could affect initial seeding of tumor cells.

Disseminated cancer cells in breast cancer: Mechanism of dissemination and dormancy and emerging insights on therapeutic opportunities

Metastatic spread in breast cancer patients is the major driver of cancer-related deaths. A unique subset of cells disseminated from pre-invasive or primary tumor lesions are recognized as the main seeds for metastatic outgrowth. Disseminated cancer cells (DCCs) can migrate to distant organs and settle in a dormant state for a prolonged period until they emerge to overt metastases. Understanding the biology of breast cancer cells dissemination, dormancy and reactivation to form overt metastases has become an important focus. In this review, we discuss the recent advancements of molecular pathways involving breast cancer cell dissemination, role of chemokine-chemokine receptor networks in DCCs migration, DCCs phenotypic heterogeneity and unique genes signatures in tumor dormancy, microenvironmental regulation and specific niches that favors DCCs homing and dormancy. In addition, we also discuss recent findings relating to the role of immune response on DCC dissemination and dormancy. With recent advances in the field of immunotherapy/targeted therapy and its beneficial effects in cancer treatment, this review will focus on their impact on DCCs, reversal of stemness, tumor dormancy and metastatic relapse.

The role of von Willebrand factor in breast cancer metastasis

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VWF plays an important role in breast tumour progression and metastasis.

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Patients with metastatic breast cancer have significantly elevated plasma VWF.

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Increased levels of highly adhesive VWF may regulate platelet-tumour interactions.

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VWF may protect disseminated tumour cells from chemotherapy.

Breast cancer is the most common female cancer globally, with approximately 12% of patients eventually developing metastatic disease. Critically, limited effective treatment options exist for metastatic breast cancer. Recently, von Willebrand factor (VWF), a haemostatic plasma glycoprotein, has been shown to play an important role in tumour progression and metastasis. In breast cancer, a significant rise in the plasma levels of VWF has been reported in patients with malignant disease compared to benign conditions and healthy controls, with an even greater increase seen in patients with disseminated disease. Direct interactions between VWF, tumour cells, platelets and endothelial cells may promote haematogenous dissemination and thus the formation of metastatic foci. Intriguingly, patients with metastatic disease have unusually large VWF multimers. This observation has been proposed to be a result of a dysfunctional or deficiency of VWF-cleaving protease activity, ADAMTS-13 activity, which may then regulate the platelet-tumour adhesive interactions in the metastatic process. In this review, we provide an overview of VWF in orchestrating the pathological process of breast cancer dissemination, and provide supporting evidence of the role of VWF in mediating metastatic breast cancer.

CD39+PD-1+CD8+ T cells mediate metastatic dormancy in breast cancer

Some breast tumors metastasize aggressively whereas others remain dormant for years. The mechanism governing metastatic dormancy remains largely unknown. Through high-parametric single-cell mapping in mice, we identify a discrete population of CD39+PD-1+CD8+ T cells in primary tumors and in dormant metastasis, which is hardly found in aggressively metastasizing tumors. Using blocking antibodies, we find that dormancy depends on TNFα and IFNγ. Immunotherapy reduces the number of dormant cancer cells in the lungs. Adoptive transfer of purified CD39+PD-1+CD8+ T cells prevents metastatic outgrowth. In human breast cancer, the frequency of CD39+PD-1+CD8+ but not total CD8+ T cells correlates with delayed metastatic relapse after resection (disease-free survival), thus underlining the biological relevance of CD39+PD-1+CD8+ T cells for controlling experimental and human breast cancer. Thus, we suggest that a primary breast tumor could prime a systemic, CD39+PD-1+CD8+ T cell response that favors metastatic dormancy in the lungs.

Perivascular Secretome Influences Hematopoietic Stem Cell Maintenance in a Gelatin Hydrogel

Adult hematopoietic stem cells (HSCs) produce the body's full complement of blood and immune cells. They reside in specialized microenvironments, or niches, within the bone marrow. The perivascular niche near blood vessels is believed to help maintain primitive HSCs in an undifferentiated state but demonstration of this effect is difficult. In vivo studies make it challenging to determine the direct effect of the endosteal and perivascular niches as they can be in close proximity, and two-dimensional in vitro cultures often lack an instructive extracellular matrix environment. We describe a tissue engineering approach to develop and characterize a three-dimensional perivascular tissue model to investigate the influence of the perivascular secretome on HSC behavior. We generate 3D endothelial networks in methacrylamide-functionalized gelatin hydrogels using human umbilical vein endothelial cells (HUVECs) and mesenchymal stromal cells (MSCs). We identify a subset of secreted factors important for HSC function, and examine the response of primary murine HSCs in hydrogels to the perivascular secretome. Within 4 days of culture, perivascular conditioned media promoted maintenance of a greater fraction of hematopoietic stem and progenitor cells. This work represents an important first-generation perivascular model to investigate the role of niche secreted factors on the maintenance of primary HSCs.

Characteristics of pre-metastatic niche: the landscape of molecular and cellular pathways

Metastasis is a major contributor to cancer-associated deaths. It involves complex interactions between primary tumorigenic sites and future metastatic sites. Accumulation studies have revealed that tumour metastasis is not a disorderly spontaneous incident but the climax of a series of sequential and dynamic events including the development of a pre-metastatic niche (PMN) suitable for a subpopulation of tumour cells to colonize and develop into metastases. A deep understanding of the formation, characteristics and function of the PMN is required for developing new therapeutic strategies to treat tumour patients. It is rapidly becoming evident that therapies targeting PMN may be successful in averting tumour metastasis at an early stage. This review highlights the key components and main characteristics of the PMN and describes potential therapeutic strategies, providing a promising foundation for future studies.

The Role of the Microenvironment and Immune System in Regulating Stem Cell Fate in Cancer

Despite gains in knowledge of the intrinsic signals governing cancer progression, effective clinical management of cancer remains a challenge. Drug resistance and relapse, pose the greatest barriers to cancer care, and are often driven by the co-option of stem cell programs by subpopulations of aggressive cancer cells. Here, we focus on the role of the microenvironment in the acquisition and/ or maintenance of stem cell states in cancer in the context of resistance and metastasis. We further discuss the role of cancer stem cells in immune evasion through the course of metastasis, dormancy, and relapse. Understanding the niche in which cancer stem cells live and the signals that sustain them may lead to new strategies that target them by disrupting microenvironmental support.

Context Matters: NOTCH Signatures and Pathway in Cancer Progression and Metastasis

The Notch signaling pathway is a critical player in embryogenesis but also plays various roles in tumorigenesis, with both tumor suppressor and oncogenic activities. Mutations, deletions, amplifications, or over-expression of Notch receptors, ligands, and a growing list of downstream Notch-activated genes have by now been described for most human cancer types. Yet, it often remains unclear what may be the functional impact of these changes for tumor biology, initiation, and progression, for cancer therapy, and for personalized medicine. Emerging data indicate that Notch signaling can also contribute to increased aggressive properties such as invasion, tumor heterogeneity, angiogenesis, or tumor cell dormancy within solid cancer tissues; especially in epithelial cancers, which are in the center of this review. Notch further supports the “stemness” of cancer cells and helps define the stem cell niche for their long-term survival, by integrating the interaction between cancer cells and the cells of the tumor microenvironment (TME). The complexity of Notch crosstalk with other signaling pathways and its roles in cell fate and trans-differentiation processes such as epithelial-to-mesenchymal transition (EMT) point to this pathway as a decisive player that may tip the balance between tumor suppression and promotion, differentiation and invasion. Here we not only review the literature, but also explore genomic databases with a specific focus on Notch signatures, and how they relate to different stages in tumor development. Altered Notch signaling hereby plays a key role for tumor cell survival and coping with a broad spectrum of vital issues, contributing to failed therapies, poor patient outcome, and loss of lives.

Graphene-modified CePO4 nanorods effectively treat breast cancer-induced bone metastases and regulate macrophage polarization to improve osteo-inductive ability

BACKGROUND

Breast cancer bone metastasis has become one of the most common complications; however, it may cause cancer recurrence and bone nonunion, as well as local bone defects.

METHODS

Herein, In vitro, we verified the effect of bioscaffold materials on cell proliferation and apoptosis through a CCK8 trial, staining of live/dead cells, and flow cytometry. We used immunofluorescence technology and flow cytometry to verify whether bioscaffold materials regulate macrophage polarization, and we used ALP staining, alizarin red staining and PCR to verify whether bioscaffold material promotes bone regeneration. In vivo, we once again studied the effect of bioscaffold materials on tumors by measuring tumor volume in mice, Tunel staining, and caspase-3 immunofluorescence. We also constructed a mouse skull ultimate defect model to verify the effect on bone regeneration.

RESULTS

Graphene oxide (GO) nanoparticles, hydrated CePO₄ nanorods and bioactive chitosan (CS) are combined to form a bioactive multifunctional CePO₄/CS/GO scaffold, with characteristics such as photothermal therapy to kill tumors, macrophage polarization to promote blood vessel formation, and induction of bone formation. CePO₄/CS/GO scaffold activates the caspase-3 proteasein local tumor cells, thereby lysing the DNA between nucleosomes and causing apoptosis. On the one hand, the as-released Ce³⁺ ions promote M2 polarization of macrophages, which secretes vascular endothelial growth factor (VEGF) and Arginase-1 (Arg-1), which promotes angiogenesis. On the other hand, the as-released Ce³⁺ ions also activated the BMP-2/Smad signaling pathway which facilitated bone tissue regeneration.

CONCLUSION

The multifunctional CePO₄/CS/GO scaffolds may become a promising platform for therapy of breast cancer bone metastases.

Targeting metastatic cancer

Despite recent therapeutic advances in cancer treatment, metastasis remains the principal cause of cancer death. Recent work has uncovered the unique biology of metastasis-initiating cells that results in tumor growth in distant organs, evasion of immune surveillance and co-option of metastatic microenvironments. Here we review recent progress that is enabling therapeutic advances in treating both micro- and macrometastases. Such insights were gained from cancer sequencing, mechanistic studies and clinical trials, including of immunotherapy. These studies reveal both the origins and nature of metastases and identify new opportunities for developing more effective strategies to target metastatic relapse and improve patient outcomes.

Tumour-directed microenvironment remodelling at a glance

The tissue microenvironment supports normal tissue function and regulates the behaviour of parenchymal cells. Tumour cell behaviour, on the other hand, diverges significantly from that of their normal counterparts, rendering the microenvironment hostile to tumour cells. To overcome this problem, tumours can co-opt and remodel the microenvironment to facilitate their growth and spread. This involves modifying both the biochemistry and the biophysics of the normal microenvironment to produce a tumour microenvironment. In this Cell Science at a Glance article and accompanying poster, we outline the key processes by which epithelial tumours influence the establishment of the tumour microenvironment. As the microenvironment is populated by genetically normal cells, we discuss how controlling the microenvironment is both a significant challenge and a key vulnerability for tumours. Finally, we review how new insights into tumour-microenvironment interactions has led to the current consensus on how these processes may be targeted as novel anti-cancer therapies.

Targetable Intercellular Signaling Pathways Facilitate Lung Colonization in Osteosarcoma

Outcomes for young people diagnosed with osteosarcoma hinge almost exclusively on whether they develop lung metastasis. The striking predilection that osteosarcoma shows for metastatic spread to lung suggests properties and/or lung interactions that generate tissue-specific survival and proliferation advantages. While these mechanisms remain overall poorly defined, studies have begun to describe biological elements important to metastasis. Mechanisms described to date include both cell-autonomous adaptations that allow disseminated tumor cells to survive the stressors imposed by metastasis and intercellular signaling networks that tumor cells exploit to pirate needed signals from surrounding tissues or to recruit other cells that create a more favorable niche. Evidence suggests that cell-autonomous changes are largely driven by epigenetic reprogramming of disseminated tumor cells that facilitates resistance to late apoptosis, manages endoplasmic reticulum (ER) stressors, promotes translation of complex transcripts, and activates clotting pathways. Tumor-host signaling pathways important for lung colonization drive interactions with lung epithelium, mesenchymal stem cells, and mediators of innate and adaptive immunity. In this chapter, we highlight one particular pathway that integrates cell-autonomous adaptations with lung-specific tumor-host interactions. In this mechanism, aberrant ΔNp63 expression primes tumor cells to produce IL6 and CXCL8 upon interaction with lung epithelial cells. This tumor-derived IL6 and CXCL8 then initiates autocrine, osteosarcoma-lung paracrine, and osteosarcoma-immune paracrine interactions that facilitate metastasis. Importantly, many of these pathways appear targetable with clinically feasible therapeutics. Ongoing work to better understand metastasis is driving efforts to improve outcomes by targeting the most devastating complication of this disease.

When a House Is Not a Home: A Survey of Antimetastatic Niches and Potential Mechanisms of Disseminated Tumor Cell Suppression

Over the last four decades, the cancer biology field has concentrated on cellular and microenvironmental drivers of metastasis. Despite this focus, mortality rates upon diagnosis of metastatic disease remain essentially unchanged. Would a small change in perspective help? Knowing what constitutes an inhospitable, rather than hospitable, microenvironment could provide the inspiration necessary to develop better therapies and preventative strategies. In this review, we canvas the literature for hints about what characteristics four common antimetastatic niches-skeletal muscle, spleen, thyroid, and yellow bone marrow-have in common. We posit that thorough molecular and mechanistic characterization of antimetastatic tissues may inspire reimagined therapies that inhibit metastatic development and/or progression in an enduring manner.

Bone Angiogenesis and Vascular Niche Remodeling in Stress, Aging, and Diseases

The bone marrow (BM) vascular niche microenvironments harbor stem and progenitor cells of various lineages. Bone angiogenesis is distinct and involves tissue-specific signals. The nurturing vascular niches in the BM are complex and heterogenous consisting of distinct vascular and perivascular cell types that provide crucial signals for the maintenance of stem and progenitor cells. Growing evidence suggests that the BM niche is highly sensitive to stress. Aging, inflammation and other stress factors induce changes in BM niche cells and their crosstalk with tissue cells leading to perturbed hematopoiesis, bone angiogenesis and bone formation. Defining vascular niche remodeling under stress conditions will improve our understanding of the BM vascular niche and its role in homeostasis and disease. Therefore, this review provides an overview of the current understanding of the BM vascular niches for hematopoietic stem cells and their malfunction during aging, bone loss diseases, arthritis and metastasis.

Targeting dormant tumor cells to prevent cancer recurrence

Over the years, developments in oncology led to significantly improved clinical outcome for cancer patients. However, cancer recurrence after initial treatment response still poses a major challenge, as it often involves more aggressive, metastatic disease. The presence of dormant cancer cells is associated with recurrence, metastasis, and poor clinical outcome, suggesting that these cells may play a crucial role in the process of disease relapse. Cancer cell dormancy typically presents as growth arrest while retaining proliferative capacity and can be induced or reversed by a wide array of cell-intrinsic and cell-extrinsic factors. Conventional therapies preferentially target fast-dividing cells, leaving dormant cancer cells largely insensitive to these treatments. In this review, we discuss the role of dormant cancer cells in cancer recurrence and highlight how novel therapy strategies based on cell-cycle modulation, modifications of existing drugs, or enhanced drug-delivery vehicles may be used to specifically target this subpopulation of tumor cells, and thereby have the potential to prevent disease recurrence.

Technical Advancements for Studying Immune Regulation of Disseminated Dormant Cancer Cells

Metastases are a major cause of cancer-related death and despite the fact that they have been focus of intense research over the last two decades, effective therapies for patients with distant secondary lesions are still very limited. In addition, in some tumor types metastases can grow years after the patients have been declared clinically cured, indicating that disseminated cancer cells (DCCs) persist undetected for years, even decades in a quiescent state. Clinical and experimental data highlight the importance of the immune system in shaping the fitness and behaviour of DCCs. Here, we review mechanisms of survival, quiescence and outgrowth of DCCs with a special focus on immune-regulation and we highlight the latest cutting-edge techniques for modelling the biology of DCCs in vitro and for studying the metastatic niche in vivo. We believe that a wide dissemination of those techniques will boost scientific findings towards new therapies to defeat metastatic relapses in cancer patients.

Bone Vasculature and Bone Marrow Vascular Niches in Health and Disease

Bone vasculature and bone marrow vascular niches supply oxygen, nutrients, and secrete angiocrine factors required for the survival, maintenance, and self-renewal of stem and progenitor cells. In the skeletal system, vasculature creates nurturing niches for bone and blood-forming stem cells. Blood vessels regulate hematopoiesis and drive bone formation during development, repair, and regeneration. Dysfunctional vascular niches induce skeletal aging, bone diseases, and hematological disorders. Recent cellular and molecular characterization of the bone marrow microenvironment has provided unprecedented insights into the complexity, heterogeneity, and functions of the bone vasculature and vascular niches. The bone vasculature is composed of distinct vessel subtypes that differentially regulate osteogenesis, hematopoiesis, and disease conditions in bones. Further, bone marrow vascular niches supporting stem cells are often complex microenvironments involving multiple different cell populations and vessel subtypes. This review provides an overview of the emerging vascular cell heterogeneity in bone and the new roles of the bone vasculature and associated vascular niches in health and disease. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Cancer progression and the invisible phase of metastatic colonization

Metastatic dissemination occurs very early in the malignant progression of a cancer but the clinical manifestation of metastases often takes years. In recent decades, 5-year survival of patients with many solid cancers has increased due to earlier detection, local disease control and adjuvant therapies. As a consequence, we are confronted with an increase in late relapses as more antiproliferative cancer therapies prolong disease courses, raising questions about how cancer cells survive, evolve or stop growing and finally expand during periods of clinical latency. I argue here that the understanding of early metastasis formation, particularly of the currently invisible phase of metastatic colonization, will be essential for the next stage in adjuvant therapy development that reliably prevents metachronous metastasis.

Deconstructing tumor heterogeneity: the stromal perspective

Significant advances have been made towards understanding the role of immune cell-tumor interplay in either suppressing or promoting tumor growth, progression, and recurrence, however, the roles of additional stromal elements, cell types and/or cell states remain ill-defined. The overarching goal of this NCI-sponsored workshop was to highlight and integrate the critical functions of non-immune stromal components in regulating tumor heterogeneity and its impact on tumor initiation, progression, and resistance to therapy. The workshop explored the opposing roles of tumor supportive versus suppressive stroma and how cellular composition and function may be altered during disease progression. It also highlighted microenvironment-centered mechanisms dictating indolence or aggressiveness of early lesions and how spatial geography impacts stromal attributes and function. The prognostic and therapeutic implications as well as potential vulnerabilities within the heterogeneous tumor microenvironment were also discussed. These broad topics were included in this workshop as an effort to identify current challenges and knowledge gaps in the field.

A Perspective on Therapeutic Pan-Resistance in Metastatic Cancer

Metastatic spread represents the leading cause of disease-related mortality among cancer patients. Many cancer patients suffer from metastatic relapse years or even decades after radical surgery for the primary tumor. This clinical phenomenon is explained by the early dissemination of cancer cells followed by a long period of dormancy. Although dormancy could be viewed as a window of opportunity for therapeutic interventions, dormant disseminated cancer cells and micrometastases, as well as emergent outgrowing macrometastases, exhibit a generalized, innate resistance to chemotherapy and even immunotherapy. This therapeutic pan-resistance, on top of other adaptive responses to targeted agents such as acquired mutations and lineage plasticity, underpins the current difficulties in eradicating cancer. In the present review, we attempt to provide a framework to understand the underlying biology of this major issue.

TGF-β-induced IGFBP-3 is a key paracrine factor from activated pericytes that promotes colorectal cancer cell migration and invasion

The crosstalk between cancer cells and the tumor microenvironment has been implicated in cancer progression and metastasis. Fibroblasts and immune cells are widely known to be attracted to and modified by cancer cells. However, the role of pericytes in the tumor microenvironment beyond endothelium stabilization is poorly understood. Here, we report that pericytes promoted colorectal cancer (CRC) cell proliferation, migration, invasion, stemness, and chemoresistance in vitro, as well as tumor growth in a xenograft CRC model. We demonstrate that coculture with human CRC cells induced broad transcriptomic changes in pericytes, mostly associated with TGF-β receptor activation. The prognostic value of a TGF-β response signature in pericytes was analyzed in CRC patient data sets. This signature was found to be a good predictor of CRC relapse. Moreover, in response to stimulation by CRC cells, pericytes expressed high levels of TGF-β1, initiating an autocrine activation loop. Investigation of secreted mediators and underlying molecular mechanisms revealed that IGFBP-3 is a key paracrine factor from activated pericytes affecting CRC cell migration and invasion. In summary, we demonstrate that the interplay between pericytes and CRC cells triggers a vicious cycle that stimulates pericyte cytokine secretion, in turn increasing CRC cell tumorigenic properties. Overall, we provide another example of how cancer cells can manipulate the tumor microenvironment.

Angiogenic biomaterials to promote therapeutic regeneration and investigate disease progression

The vasculature is a key component of the tissue microenvironment. Traditionally known for its role in providing nutrients and oxygen to surrounding cells, the vasculature is now also acknowledged to provide signaling cues that influence biological outcomes in regeneration and disease. These cues come from the cells that comprise vasculature, as well as the dynamic biophysical and biochemical properties of the surrounding extracellular matrix that accompany vascular development and remodeling. In this review, we illustrate the larger role of the vasculature in the context of regenerative biology and cancer progression. We describe cellular, biophysical, biochemical, and metabolic components of vascularized microenvironments. Moreover, we provide an overview of multidimensional angiogenic biomaterials that have been developed to promote therapeutic vascularization and regeneration, as well as to mimic elements of vascularized microenvironments as a means to uncover mechanisms by which vasculature influences cancer progression and therapy.

Understanding ER+ Breast Cancer Dormancy Using Bioinspired Synthetic Matrices for Long-Term 3D Culture and Insights into Late Recurrence

Late recurrences of breast cancer are hypothesized to originate from disseminated tumor cells that re-activate after a long period of dormancy, ≥5 years for estrogen-receptor positive (ER+) tumors. An outstanding question remains as to what the key microenvironment interactions are that regulate this complex process, and well-defined human model systems are needed for probing this. Here, a robust, bioinspired 3D ER+ dormancy culture model is established and utilized to probe the effects of matrix properties for common sites of late recurrence on breast cancer cell dormancy. Formation of dormant micrometastases over several weeks is examined for ER+ cells (T47D, BT474), where the timing of entry into dormancy versus persistent growth depends on matrix composition and cell type. In contrast, triple negative cells (MDA-MB-231), associated with early recurrence, are not observed to undergo long-term dormancy. Bioinformatic analyses quantitatively support an increased "dormancy score" gene signature for ER+ cells (T47D) and reveal differential expression of genes associated with different biological processes based on matrix composition. Further, these analyses support a link between dormancy and autophagy, a potential survival mechanism. This robust model system will allow systematic investigations of other cell-microenvironment interactions in dormancy and evaluation of therapeutics for preventing late recurrence.

Collagen and fibronectin promote an aggressive cancer phenotype in breast cancer cells but drive autonomous gene expression patterns

Understanding how various pathologies of breast cancer respond to their environment may be imperative in the creation of novel therapeutic targets. Central to the organisation and behaviour of cells within the tumour microenvironment is the extracellular matrix (ECM), a meshwork of fibrous proteins and glycoproteins that directly influences cell behaviour and the bioavailability of signalling molecules. Our appreciation on how the composition of the ECM can influence cancer behaviour has evolved significantly and although we are highly cognisant of the dramatic impact the ECM can have on cancer cell behaviour, we continue to neglect this during diagnosis and treatment. In the following study, we aimed to identify how three breast cancer cell lines respond functionally and genetically to common components of the ECM. Using real time and end point assays we have identified similar patterns of behaviour among the three breast cancer cell lines in response to commonly found ECM components of the breast. Using a selected gene panel, we have been able to identify cell line specific changes in gene differentiation when breast cancer cells are in contact with these elements. Although the response of our cells to these elements differ at the genetic level, their functional responses are consistent. This work adds to the growing arguments that highlight a need for histologically assessing ECM composition of breast tumours. In particular monitoring of fibrous protein deposition at the site of malignancy could provide critical information during clinical assessment influencing disease prognosis and treatment decisions for breast cancer patients.

Integrin Crosstalk Contributes to the Complexity of Signalling and Unpredictable Cancer Cell Fates

Integrins are heterodimeric cell surface receptors composed of α and β subunits that control adhesion, proliferation and gene expression. The integrin heterodimer binding to ligand reorganises the cytoskeletal networks and triggers multiple signalling pathways that can cause changes in cell cycle, proliferation, differentiation, survival and motility. In addition, integrins have been identified as targets for many different diseases, including cancer. Integrin crosstalk is a mechanism by which a change in the expression of a certain integrin subunit or the activation of an integrin heterodimer may interfere with the expression and/or activation of other integrin subunit(s) in the very same cell. Here, we review the evidence for integrin crosstalk in a range of cellular systems, with a particular emphasis on cancer. We describe the molecular mechanisms of integrin crosstalk, the effects of cell fate determination, and the contribution of crosstalk to therapeutic outcomes. Our intention is to raise awareness of integrin crosstalk events such that the contribution of the phenomenon can be taken into account when researching the biological or pathophysiological roles of integrins.

Endothelial Progenitors in the Tumor Microenvironment

Tumor vascularization refers to the formation of new blood vessels within a tumor and is considered one of the hallmarks of cancer. Tumor vessels supply the tumor with oxygen and nutrients, required to sustain tumor growth and progression, and provide a gateway for tumor metastasis through the blood or lymphatic vasculature. Blood vessels display an angiocrine capacity of supporting the survival and proliferation of tumor cells through the production of growth factors and cytokines. Although tumor vasculature plays an essential role in sustaining tumor growth, it represents at the same time an essential way to deliver drugs and immune cells to the tumor. However, tumor vasculature exhibits many morphological and functional abnormalities, thus resulting in the formation of hypoxic areas within tumors, believed to represent a mechanism to maintain tumor cells in an invasive state.Tumors are vascularized through a variety of modalities, mainly represented by angiogenesis, where VEGF and other members of the VEGF family play a key role. This has represented the basis for the development of anti-VEGF blocking agents and their use in cancer therapy: however, these agents failed to induce significant therapeutic effects.Much less is known about the cellular origin of vessel network in tumors. Various cell types may contribute to tumor vasculature in different tumors or in the same tumor, such as mature endothelial cells, endothelial progenitor cells (EPCs), or the same tumor cells through a process of transdifferentiation. Early studies have suggested a role for bone marrow-derived EPCs; these cells do not are true EPCs but myeloid progenitors differentiating into monocytic cells, exerting a proangiogenic effect through a paracrine mechanism. More recent studies have shown the existence of tissue-resident endothelial vascular progenitors (EVPs) present at the level of vessel endothelium and their possible involvement as cells of origin of tumor vasculature.

The current paradigm and challenges ahead for the dormancy of disseminated tumor cells

Disseminated tumor cells (DTCs) are known to enter a state of dormancy that is achieved via growth arrest of DTCs and/or a form of population equilibrium state, strongly influenced by the organ microenvironment. During this time, expansion of residual disseminated cancer is paused and DTCs survive to fuel relapse, sometimes decades later. This notion has opened a new window of opportunity for intervening and preventing relapse. Here we review recent data that have further augmented the understanding of cancer dormancy and discuss how this is leading to new strategies for monitoring and targeting dormant cancer.

The dormant cancer cell life cycle

The success of targeted therapies and immunotherapies has created optimism that cancers may be curable. However, not all patients respond, drug resistance is common and many patients relapse owing to dormant cancer cells. These rare and elusive cells can disseminate early and hide in specialized niches in distant organs before being reactivated to cause disease relapse after successful treatment of the primary tumour. Despite their importance, we are yet to leverage knowledge generated from experimental models and translate the potential of targeting dormant cancer cells to prevent disease relapse in the clinic. This is due, at least in part, to the lack of adherence to consensus definitions by researchers, limited models that faithfully recapitulate this stage of metastatic spread and an absence of interdisciplinary approaches. However, the application of new high-resolution, single-cell technologies is starting to revolutionize the field and transcend classical reductionist models of studying individual cell types or genes in isolation to provide a global view of the complex underlying cellular ecosystem and transcriptional landscape that controls dormancy. In this Perspective, we synthesize some of these recent advances to describe the hallmarks of cancer cell dormancy and how the dormant cancer cell life cycle offers opportunities to target not only the cancer but also its environment to achieve a durable cure for seemingly incurable cancers.

Advances in quantitative immunohistochemistry and their contribution to breast cancer

Introduction: Automated image analysis provides an objective, quantitative, and reproducible method of measurement of biomarkers. Image quantification is particularly well suited for the analysis of tissue microarrays which has played a major pivotal role in the rapid assessment of molecular biomarkers. Data acquired from grinding up bulk tissue samples miss spatial information regarding cellular localization; therefore, methods that allow for spatial cell phenotyping at high resolution have proven to be valuable in many biomarker discovery assays. Here, we focus our attention on breast cancer as an example of a tumor type that has benefited from quantitative biomarker studies using tissue microarray format.Areas covered: The history of immunofluorescence and immunohistochemistry and the current status of these techniques, including multiplexing technologies (spectral and non-spectral) and image analysis software will be addressed. Finally, we will turn our attention to studies that have provided proof-of-principle evidence that have been impacted from the use of these techniques.Expert opinion: Assessment of prognostic and predictive biomarkers on tissue sections and TMA using Quantitative immunohistochemistry is an important advancement in the investigation of biologic markers. The challenges in standardization of quantitative technologies for accurate assessment are required for adoption into routine clinical practice.

In vitro Models of Breast Cancer Metastatic Dormancy

Delayed relapses at distant sites are a common clinical observation for certain types of cancers after removal of primary tumor, such as breast and prostate cancer. This evidence has been explained by postulating a long period during which disseminated cancer cells (DCCs) survive in a foreign environment without developing into overt metastasis. Because of the asymptomatic nature of this phenomenon, isolation, and analysis of disseminated dormant cancer cells from clinically disease-free patients is ethically and technically highly problematic and currently these data are largely limited to the bone marrow. That said, detecting, profiling and treating indolent metastatic lesions before the onset of relapse is the imperative. To overcome this major limitation many laboratories developed in vitro models of the metastatic niche for different organs and different types of cancers. In this review we focus specifically on in vitro models designed to study metastatic dormancy of breast cancer cells (BCCs). We provide an overview of the BCCs employed in the different organotypic systems and address the components of the metastatic microenvironment that have been shown to impact on the dormant phenotype: tissue architecture, stromal cells, biochemical environment, oxygen levels, cell density. A brief description of the organ-specific in vitro models for bone, liver, and lung is provided. Finally, we discuss the strategies employed so far for the validation of the different systems.

Tumor cell-organized fibronectin maintenance of a dormant breast cancer population

Tumors can undergo long periods of dormancy, with cancer cells entering a largely quiescent, nonproliferative state before reactivation and outgrowth. To understand the role of the extracellular matrix (ECM) in regulating tumor dormancy, we created an in vitro cell culture system with carefully controlled ECM substrates to observe entrance into and exit from dormancy with live imaging. We saw that cell populations capable of surviving entrance into long-term dormancy were heterogeneous, containing quiescent, cell cycle-arrested, and actively proliferating cells. Cell populations capable of entering dormancy formed an organized, fibrillar fibronectin matrix via αvβ3 and α5β1 integrin adhesion, ROCK-generated tension, and TGFβ2 stimulation, and cancer cell outgrowth after dormancy required MMP-2-mediated fibronectin degradation. We propose this approach as a useful, in vitro method to study factors important in regulating dormancy, and we used it here to elucidate a role for fibronectin deposition and MMP activation.

Fluids and their mechanics in tumour transit: shaping metastasis

Metastasis is a dynamic succession of events involving the dissemination of tumour cells to distant sites within the body, ultimately reducing the survival of patients with cancer. To colonize distant organs and, therefore, systemically disseminate within the organism, cancer cells and associated factors exploit several bodily fluid systems, which provide a natural transportation route. Indeed, the flow mechanics of the blood and lymphatic circulatory systems can be co-opted to improve the efficiency of cancer cell transit from the primary tumour, extravasation and metastatic seeding. Flow rates, vessel size and shear stress can all influence the survival of cancer cells in the circulation and control organotropic seeding patterns. Thus, in addition to using these fluids as a means to travel throughout the body, cancer cells exploit the underlying physical forces within these fluids to successfully seed distant metastases. In this Review, we describe how circulating tumour cells and tumour-associated factors leverage bodily fluids, their underlying forces and imposed stresses during metastasis. As the contribution of bodily fluids and their mechanics raises interesting questions about the biology of the metastatic cascade, an improved understanding of this process might provide a new avenue for targeting cancer cells in transit.