Targeting the perivascular niche sensitizes disseminated tumour cells to chemotherapy

Advances in the molecular regulation mechanism of tumor dormancy and its therapeutic strategy

Tumor dormancy is a stage in the growth and development of malignant cells and is one of the biological characteristics of malignant cells. Complex transitions involving dormant tumor cells between quiescent and proliferative states pose challenges for tumor eradication. This paper explores the biological features and molecular mechanisms of tumor dormancy and highlights emerging therapies. The strategies discussed promise innovative clinical potential against malignant tumors. Understanding the mechanisms of dormancy can help provide valuable insights into the diagnosis and treatment of malignant tumors to advance the fight against this world problem.

Tumor-initiating cells establish a niche to overcome isolation stress

While the tumor microenvironment is a critical contributor to cancer progression, early steps of tumor initiation and metastasis also rely on the ability of individual tumor cells to survive and thrive at locations where tumor stroma or immune infiltration has yet to be established. In this opinion article, we use the term 'isolation stress' to broadly describe the challenges that individual tumor cells must overcome during the initiation and expansion of the primary tumor beyond permissive boundaries and metastatic spread into distant sites, including a lack of cell-cell contact, adhesion to protumor extracellular matrix proteins, and access to nutrients, oxygen, and soluble factors that support growth. In particular, we highlight the ability of solitary tumor cells to autonomously generate a specialized fibronectin-enriched extracellular matrix to create their own pericellular niche that supports tumor initiation. Cancer cells that can creatively evade the effects of isolation stress not only become more broadly stress tolerant, they also tend to show enhanced stemness, drug resistance, tumor initiation, and metastasis.

Cell-intrinsic and microenvironmental determinants of metastatic colonization

Cancer metastasis is a biologically complex process that remains a major challenge in the oncology clinic, accounting for nearly all of the mortality associated with malignant neoplasms. To establish metastatic growths, carcinoma cells must disseminate from the primary tumour, survive in unfamiliar tissue microenvironments, re-activate programs of proliferation, and escape innate and adaptive immunosurveillance. The entire process is extremely inefficient and can occur over protracted timescales, yielding only a vanishingly small number of carcinoma cells that are able to complete all of the required steps. Here we review both the cancer-cell-intrinsic mechanisms and microenvironmental interactions that enable metastatic colonization. In particular, we highlight recent work on the behaviour of already-disseminated tumour cells, since meaningful progress in treating metastatic disease will clearly require a better understanding of the cells that spawn metastases, which generally have disseminated by the time of initial diagnosis.

Educate, not kill: treating cancer without triggering its defenses

Traditionally, anticancer therapies focus on restraining uncontrolled proliferation. However, these cytotoxic therapies expose cancer cells to direct killing, instigating the process of natural selection favoring survival of resistant cells that become the foundation for tumor progression and therapy failure. Recognizing this phenomenon has prompted the development of alternative therapeutic strategies. Here we propose strategies targeting cancer hallmarks beyond proliferation, aiming at re-educating cancer cells towards a less malignant phenotype. These strategies include controlling cell dormancy, transdifferentiation therapy, normalizing the cancer microenvironment, and using migrastatic therapy. Adaptive resistance to these educative strategies does not confer a direct proliferative advantage to resistant cells, as non-resistant cells are not subject to eradication, thereby delaying or preventing the development of therapy-resistant tumors.

Unveiling cancer dormancy: Intrinsic mechanisms and extrinsic forces

Tumor cells disseminate in various distant organs at early stages of cancer progression. These disseminated tumor cells (DTCs) can stay dormant/quiescent without causing patient symptoms for years or decades. These dormant tumor cells survive despite curative treatments by entering growth arrest, escaping immune surveillance, and/or developing drug resistance. However, these dormant cells can reactivate to proliferate, causing metastatic progression and/or relapse, posing a threat to patients' survival. It's unclear how cancer cells maintain dormancy and what triggers their reactivation. What are better approaches to prevent metastatic progression and relapse through harnessing cancer dormancy? To answer these remaining questions, we reviewed the studies of tumor dormancy and reactivation in various types of cancer using different model systems, including the brief history of dormancy studies, the intrinsic characteristics of dormant cells, and the external cues at the cellular and molecular levels. Furthermore, we discussed future directions in the field and the strategies for manipulating dormancy to prevent metastatic progression and recurrence.

How much do we know about the metastatic process?

Cancer cells can leave their primary sites and travel through the circulation to distant sites, where they lodge as disseminated cancer cells (DCCs), even during the early and asymptomatic stages of tumor progression. In experimental models and clinical samples, DCCs can be detected in a non-proliferative state, defined as cellular dormancy. This state can persist for extended periods until DCCs reawaken, usually in response to niche-derived reactivation signals. Therefore, their clinical detection in sites like lymph nodes and bone marrow is linked to poor survival. Current cancer therapy designs are based on the biology of the primary tumor and do not target the biology of the dormant DCC population and thus fail to eradicate the initial or subsequent waves of metastasis. In this brief review, we discuss the current methods for detecting DCCs and highlight new strategies that aim to target DCCs that constitute minimal residual disease to reduce or prevent metastasis formation. Furthermore, we present current evidence on the relevance of DCCs derived from early stages of tumor progression in metastatic disease and describe the animal models available for their study. We also discuss our current understanding of the dissemination mechanisms utilized by genetically less- and more-advanced cancer cells, which include the functional analysis of intermediate or hybrid states of epithelial-mesenchymal transition (EMT). Finally, we raise some intriguing questions regarding the clinical impact of studying the crosstalk between evolutionary waves of DCCs and the initiation of metastatic disease.

Outlook and opportunities for engineered environments of breast cancer dormancy

Dormant, disseminated breast cancer cells resist treatment and may relapse into malignant metastases after decades of quiescence. Identifying how and why these dormant breast cancer cells are triggered into outgrowth is a key unsolved step in treating latent, metastatic breast cancer. However, our understanding of breast cancer dormancy in vivo is limited by technical challenges and ethical concerns with triggering the activation of dormant breast cancer. In vitro models avoid many of these challenges by simulating breast cancer dormancy and activation in well-controlled, bench-top conditions, creating opportunities for fundamental insights into breast cancer biology that complement what can be achieved through animal and clinical studies. In this review, we address clinical and preclinical approaches to treating breast cancer dormancy, how precisely controlled artificial environments reveal key interactions that regulate breast cancer dormancy, and how future generations of biomaterials could answer further questions about breast cancer dormancy.

CAFs vs. TECs: when blood feuds fuel cancer progression, dissemination and therapeutic resistance

Neoplastic progression involves complex interactions between cancer cells and the surrounding stromal milieu, fostering microenvironments that crucially drive tumor progression and dissemination. Of these stromal constituents, cancer-associated fibroblasts (CAFs) emerge as predominant inhabitants within the tumor microenvironment (TME), actively shaping multiple facets of tumorigenesis, including cancer cell proliferation, invasiveness, and immune evasion. Notably, CAFs also orchestrate the production of pro-angiogenic factors, fueling neovascularization to sustain the metabolic demands of proliferating cancer cells. Moreover, CAFs may also directly or indirectly affect endothelial cell behavior and vascular architecture, which may impact in tumor progression and responses to anti-cancer interventions. Conversely, tumor endothelial cells (TECs) exhibit a corrupted state that has been shown to affect cancer cell growth and inflammation. Both CAFs and TECs are emerging as pivotal regulators of the TME, engaging in multifaceted biological processes that significantly impact cancer progression, dissemination, and therapeutic responses. Yet, the intricate interplay between these stromal components and the orchestrated functions of each cell type remains incompletely elucidated. In this review, we summarize the current understanding of the dynamic interrelationships between CAFs and TECs, discussing the challenges and prospects for leveraging their interactions towards therapeutic advancements in cancer.

It's all about the base: stromal cells are central orchestrators of metastasis

The tumor microenvironment (TME) is an integral part of tumors and plays a central role in all stages of carcinogenesis and progression. Each organ has a unique and heterogeneous microenvironment, which affects the ability of disseminated cells to grow in the new and sometimes hostile metastatic niche. Resident stromal cells, such as fibroblasts, osteoblasts, and astrocytes, are essential culprits in the modulation of metastatic progression: they transition from being sentinels of tissue integrity to being dysfunctional perpetrators that support metastatic outgrowth. Therefore, better understanding of the complexity of their reciprocal interactions with cancer cells and with other components of the TME is essential to enable the design of novel therapeutic approaches to prevent metastatic relapse.

Unveiling the role of cellular dormancy in cancer progression and recurrence

PURPOSE OF REVIEW

Cellular dormancy is a major contributor to cancer progression and recurrence. This review explores recent findings on the molecular mechanisms implicated in cancer dormancy and investigates potential strategies to improve therapeutic interventions.

RECENT FINDINGS

Research on cancer dormancy reveals a complex and multifaceted phenomenon. Providing a latent reservoir of tumor cells with reduced proliferation and enhanced drug-tolerance, dormant cancer cells emerge from a clonally diverse population after therapy or at metastatic sites. These cells exhibit distinct transcriptional and epigenetic profiles, involving the downregulation of Myc and mechanistic target of rapamycin (mTOR) pathways, and the induction of autophagy. Senescence traits, under the control of factors such as p53, also contribute significantly. The tumor microenvironment can either promote or prevent dormancy establishment, notably through the involvement of T and NK cells within the dormant tumor niche. Strategies to combat dormancy-related relapse include direct elimination of dormant tumor cells, sustaining dormancy to prolong survival, or awakening dormant cells to re-sensitize them to antiproliferative drugs.

SUMMARY

Improving our understanding of cancer dormancy at primary and secondary sites provides valuable insights into patient care and relapse prevention.

Stress in the metastatic journey - the role of cell communication and clustering in breast cancer progression and treatment resistance

Breast cancer stands as the most prevalent malignancy afflicting women. Despite significant advancements in its diagnosis and treatment, breast cancer metastasis continues to be a leading cause of mortality among women. To metastasize, cancer cells face numerous challenges: breaking away from the primary tumor, surviving in the circulation, establishing in a distant location, evading immune detection and, finally, thriving to initiate a new tumor. Each of these sequential steps requires cancer cells to adapt to a myriad of stressors and develop survival mechanisms. In addition, most patients with breast cancer undergo surgical removal of their primary tumor and have various therapeutic interventions designed to eradicate cancer cells. Despite this plethora of attacks and stresses, certain cancer cells not only manage to persist but also proliferate robustly, giving rise to substantial tumors that frequently culminate in the patient's demise. To enhance patient outcomes, there is an imperative need for a deeper understanding of the molecular and cellular mechanisms that empower cancer cells to not only survive but also expand. Herein, we delve into the intrinsic stresses that cancer cells encounter throughout the metastatic journey and the additional stresses induced by therapeutic interventions. We focus on elucidating the remarkable strategies adopted by cancer cells, such as cell-cell clustering and intricate cell-cell communication mechanisms, to ensure their survival.

From breast cancer cell homing to the onset of early bone metastasis: The role of bone (re)modeling in early lesion formation

Breast cancer often metastasizes to bone, causing osteolytic lesions. Structural and biophysical changes are rarely studied yet are hypothesized to influence metastasis. We developed a mouse model of early bone metastasis and multimodal imaging to quantify cancer cell homing, bone (re)modeling, and onset of metastasis. Using tissue clearing and three-dimensional (3D) light sheet fluorescence microscopy, we located enhanced green fluorescent protein-positive cancer cells and small clusters in intact bones and quantified their size and spatial distribution. We detected early bone lesions using in vivo microcomputed tomography (microCT)-based time-lapse morphometry and revealed altered bone (re)modeling in the absence of detectable lesions. With a new microCT image analysis tool, we tracked the growth of early lesions over time. We showed that cancer cells home in all bone compartments, while osteolytic lesions are only detected in the metaphysis, a region of high (re)modeling. Our study suggests that higher rates of (re)modeling act as a driver of lesion formation during early metastasis.

Dissemination of Circulating Tumor Cells in Breast and Prostate Cancer: Implications for Early Detection

Burgeoning evidence suggests that circulating tumor cells (CTCs) may disseminate into blood vessels at an early stage, seeding metastases in various cancers such as breast and prostate cancer. Simultaneously, the early-stage CTCs that settle in metastatic sites [termed disseminated tumor cells (DTCs)] can enter dormancy, marking a potential source of late recurrence and therapy resistance. Thus, the presence of these early CTCs poses risks to patients but also holds potential benefits for early detection and treatment and opportunities for possibly curative interventions. This review delves into the role of early DTCs in driving latent metastasis within breast and prostate cancer, emphasizing the importance of early CTC detection in these diseases. We further explore the correlation between early CTC detection and poor prognoses, which contribute significantly to increased cancer mortality. Consequently, the detection of CTCs at an early stage emerges as a critical imperative for enhancing clinical diagnostics and allowing for early interventions.

The Role of Breast Cancer Cells in Bone Metastasis: Suitable Seeds for Nourishing Soil

PURPOSE OF REVIEW

The purpose of this review was to describe the characteristics of breast cancer cells prone to developing bone metastasis and determine how they are regulated by the bone microenvironment.

RECENT FINDINGS

The bone is a site of frequent breast cancer metastasis. Bone metastasis accounts for 70% of advanced breast cancer cases and remains incurable. It can lead to skeletal-related events, such as bone fracture and pain, and seriously affect the quality of life of patients. Breast cancer cells escape from the primary lesion and spread to the bone marrow in the early stages. They can then enter the dormant state and restore tumourigenicity after several years to develop overt metastasis. In the last few years, an increasing number of studies have reported on the factors promoting bone metastasis of breast cancer cells, both at the primary and metastatic sites. Identifying factors associated with bone metastasis aids in the early recognition of bone metastasis tendency. How to target these factors and minimize the side effects on the bone remains to be further explored.

Immune evasion of dormant disseminated tumor cells is due to their scarcity and can be overcome by T cell immunotherapies

The period between "successful" treatment of localized breast cancer and the onset of distant metastasis can last many years, representing an unexploited window to eradicate disseminated disease and prevent metastases. We find that the source of recurrence-disseminated tumor cells (DTCs) -evade endogenous immunity directed against tumor neoantigens. Although DTCs downregulate major histocompatibility complex I, this does not preclude recognition by conventional T cells. Instead, the scarcity of interactions between two relatively rare populations-DTCs and endogenous antigen-specific T cells-underlies DTC persistence. This scarcity is overcome by any one of three immunotherapies that increase the number of tumor-specific T cells: T cell-based vaccination, or adoptive transfer of T cell receptor or chimeric antigen receptor T cells. Each approach achieves robust DTC elimination, motivating discovery of MHC-restricted and -unrestricted DTC antigens that can be targeted with T cell-based immunotherapies to eliminate the reservoir of metastasis-initiating cells in patients.

Cancer cell cycle heterogeneity as a critical determinant of therapeutic resistance

Intra-tumor heterogeneity is now arguably one of the most-studied topics in tumor biology, as it represents a major obstacle to effective cancer treatment. Since tumor cells are highly diverse at genetic, epigenetic, and phenotypic levels, intra-tumor heterogeneity can be assumed as an important contributing factor to the nullification of chemotherapeutic effects, and recurrence of the tumor. Based on the role of heterogeneous subpopulations of cancer cells with varying cell-cycle dynamics and behavior during cancer progression and treatment; herein, we aim to establish a comprehensive definition for adaptation of neoplastic cells against therapy. We discuss two parallel and yet distinct subpopulations of tumor cells that play pivotal roles in reducing the effects of chemotherapy: "resistant" and "tolerant" populations. Furthermore, this review also highlights the impact of the quiescent phase of the cell cycle as a survival mechanism for cancer cells. Beyond understanding the mechanisms underlying the quiescence, it provides an insightful perspective on cancer stem cells (CSCs) and their dual and intertwined functions based on their cell cycle state in response to treatment. Moreover, CSCs, epithelial-mesenchymal transformed cells, circulating tumor cells (CTCs), and disseminated tumor cells (DTCs), which are mostly in a quiescent state of the cell cycle are proved to have multiple biological links and can be implicated in our viewpoint of cell cycle heterogeneity in tumors. Overall, increasing our knowledge of cell cycle heterogeneity is a key to identifying new therapeutic solutions, and this emerging concept may provide us with new opportunities to prevent the dreadful cancer recurrence.

A PERK-Specific Inhibitor Blocks Metastatic Progression by Limiting Integrated Stress Response-Dependent Survival of Quiescent Cancer Cells

PURPOSE

The integrated stress response (ISR) kinase PERK serves as a survival factor for both proliferative and dormant cancer cells. We aim to validate PERK inhibition as a new strategy to specifically eliminate solitary disseminated cancer cells (DCC) in secondary sites that eventually reawake and originate metastasis.

EXPERIMENTAL DESIGN

A novel clinical-grade PERK inhibitor (HC4) was tested in mouse syngeneic and PDX models that present quiescent/dormant DCCs or growth-arrested cancer cells in micro-metastatic lesions that upregulate ISR.

RESULTS

HC4 significantly blocks metastasis, by killing quiescent/slow-cycling ISRhigh, but not proliferative ISRlow DCCs. HC4 blocked expansion of established micro-metastasis that contained ISRhigh slow-cycling cells. Single-cell gene expression profiling and imaging revealed that a significant proportion of solitary DCCs in lungs were indeed dormant and displayed an unresolved ER stress as revealed by high expression of a PERK-regulated signature. In human breast cancer metastasis biopsies, GADD34 expression (PERK-regulated gene) and quiescence were positively correlated. HC4 effectively eradicated dormant bone marrow DCCs, which usually persist after rounds of therapies. Importantly, treatment with CDK4/6 inhibitors (to force a quiescent state) followed by HC4 further reduced metastatic burden. In HNSCC and HER2+ cancers HC4 caused cell death in dormant DCCs. In HER2+ tumors, PERK inhibition caused killing by reducing HER2 activity because of sub-optimal HER2 trafficking and phosphorylation in response to EGF.

CONCLUSIONS

Our data identify PERK as a unique vulnerability in quiescent or slow-cycling ISRhigh DCCs. The use of PERK inhibitors may allow targeting of pre-existing or therapy-induced growth arrested "persister" cells that escape anti-proliferative therapies.

A cell cycle centric view of tumour dormancy

Tumour dormancy and recurrent metastatic cancer remain the greatest clinical challenge for cancer patients. Dormant tumour cells can evade treatment and detection, while retaining proliferative potential, often for years, before relapsing to tumour outgrowth. Cellular quiescence is one mechanism that promotes and maintains tumour dormancy due to its central role in reducing proliferation, elevating cyto-protective mechanisms, and retaining proliferative potential. Quiescence/proliferation decisions are dictated by intrinsic and extrinsic signals, which regulate the activity of cyclin-dependent kinases (CDKs) to modulate cell cycle gene expression. By clarifying the pathways regulating CDK activity and the signals which activate them, we can better understand how cancer cells enter, maintain, and escape from quiescence throughout the progression of dormancy and metastatic disease. Here we review how CDK activity is regulated to modulate cellular quiescence in the context of tumour dormancy and highlight the therapeutic challenges and opportunities it presents.

Dormancy in Breast Cancer

The pattern of delayed recurrence in a subset of breast cancer patients has long been explained by a model that incorporates a variable period of cellular or tumor mass dormancy prior to disease relapse. In this review, we critically evaluate existing data to develop a framework for inferring the existence of dormancy in clinical contexts of breast cancer. We integrate these clinical data with rapidly evolving mechanistic insights into breast cancer dormancy derived from a broad array of genetically engineered mouse models as well as experimental models of metastasis. Finally, we propose actionable interventions and discuss ongoing clinical trials that translate the wealth of knowledge gained in the laboratory to the long-term clinical management of patients at a high risk of developing recurrence.

Bone-matrix mineralization dampens integrin-mediated mechanosignalling and metastatic progression in breast cancer

In patients with breast cancer, lower bone mineral density increases the risk of bone metastasis. Although the relationship between bone-matrix mineralization and tumour-cell phenotype in breast cancer is not well understood, mineralization-induced rigidity is thought to drive metastatic progression via increased cell-adhesion forces. Here, by using collagen-based matrices with adjustable intrafibrillar mineralization, we show that, unexpectedly, matrix mineralization dampens integrin-mediated mechanosignalling and induces a less proliferative stem-cell-like phenotype in breast cancer cells. In mice with xenografted decellularized physiological bone matrices seeded with human breast tumour cells, the presence of bone mineral reduced tumour growth and upregulated a gene-expression signature that is associated with longer metastasis-free survival in patients with breast cancer. Our findings suggest that bone-matrix changes in osteogenic niches regulate metastatic progression in breast cancer and that in vitro models of bone metastasis should integrate organic and inorganic matrix components to mimic physiological and pathologic mineralization.

Perivascular niches: critical hubs in cancer evolution

Tumors are heterogeneous ecosystems in which cancer cells coexist within a complex tumor immune microenvironment (TIME). The malignant, stromal, and immune cell compartments establish a plethora of bidirectional cell-cell communication crosstalks that influence tumor growth and metastatic dissemination, which we are only beginning to understand. Cancer cells either co-opt or promote the formation of new blood and lymphatic vessels to cope with their need for nutrients and oxygen. Recent studies have highlighted additional key roles for the tumor vasculature and have identified the perivascular niche as a cellular hub, where intricate and dynamic cellular interactions promote cancer stemness, immune evasion, dormancy, and metastatic spreading. Here, we review these findings, and discuss how they may be exploited therapeutically.

Mouse Models of Metastasis and Dormancy

Metastasis is the ultimate and often lethal stage of cancer. Metastasis occurs in three phases that may vary across individuals: First, dissemination from the primary tumor. Second, tumor dormancy at the metastatic site where micrometastatic cancer cells remain quiescent or, in dynamic cycles of proliferation and elimination, remaining clinically undetectable. Finally, cancer cells are able to overcome microenvironmental constraints for outgrowth, or the formation of clinically detectable macrometastases that colonize distant organs and are largely incurable. A variety of approaches have been used to model metastasis to elucidate molecular mechanisms and identify putative therapeutic targets. In particular, metastatic dormancy has been challenging to model in vivo due to the sparse numbers of cancer cells in micrometastasis nodules and the long latency times required for tumor outgrowth. Here, we review state-of-the art genetically engineered mouse, syngeneic, and patient-derived xenograft approaches for modeling metastasis and dormancy. We describe the advantages and limitations of various metastasis models, novel findings enabled by such approaches, and highlight opportunities for future improvement.

3D vascularized microphysiological system for investigation of tumor-endothelial crosstalk in anti-cancer drug resistance

Despite the advantages of microfluidic system in drug screening, vascular systems responsible for the transport of drugs and nutrients have been hardly considered in the microfluidic-based chemotherapeutic screening. Considering the physiological characteristics of highly vascularized urinary tumors, we here investigated the chemotherapeutic response of bladder tumor cells using a vascularized tumor on a chip. The microfluidic chip was designed to have open-top region for tumor sample introduction and hydrophilic rail for spontaneous hydrogel patterning, which contributed to the construction of tumor-hydrogel-endothelium interfaces in a spatiotemporal on-demand manner. Utilizing the chip where intravascularly injected cisplatin diffuse across the endothelium and transport into tumor samples, chemotherapeutic responses of cisplatin-resistant or -susceptible bladder tumor cells were evaluated, showing the preservation of cellular drug resistance even within the chip. The open-top structure also enabled the direct harvest of tumor samples and post analysis in terms of secretome and gene expressions. Comparing the cisplatin efficacy of the cisplatin-resistant tumor cells in the presence or absence of endothelium, we found that the proliferation rates of tumor cells were increased in the vasculature-incorporated chip. These have suggested that our vascularized tumor chip allows the establishment of vascular-gel-tumor interfaces in spatiotemporal manners and further enables investigations of chemotherapeutic screening.

Glioblastoma Biology, Genetics and Possible Therapies

Glioblastoma is the most aggressive intracranial tumor [...].

Breast cancers as ecosystems: a metabolic perspective

Breast cancer (BC) is the most frequently diagnosed cancer and one of the major causes of cancer death. Despite enormous progress in its management, both from the therapeutic and early diagnosis viewpoints, still around 700,000 patients succumb to the disease each year, worldwide. Late recurrency is the major problem in BC, with many patients developing distant metastases several years after the successful eradication of the primary tumor. This is linked to the phenomenon of metastatic dormancy, a still mysterious trait of the natural history of BC, and of several other types of cancer, by which metastatic cells remain dormant for long periods of time before becoming reactivated to initiate the clinical metastatic disease. In recent years, it has become clear that cancers are best understood if studied as ecosystems in which the impact of non-cancer-cell-autonomous events-dependent on complex interaction between the cancer and its environment, both local and systemic-plays a paramount role, probably as significant as the cell-autonomous alterations occurring in the cancer cell. In adopting this perspective, a metabolic vision of the cancer ecosystem is bound to improve our understanding of the natural history of cancer, across space and time. In BC, many metabolic pathways are coopted into the cancer ecosystem, to serve the anabolic and energy demands of the cancer. Their study is shedding new light on the most critical aspect of BC management, of metastatic dissemination, and that of the related phenomenon of dormancy and fostering the application of the knowledge to the development of metabolic therapies.

Skeletal-Vascular Interactions in Bone Development, Homeostasis, and Pathological Destruction

Bone is a highly vascularized organ that not only plays multiple roles in supporting the body and organs but also endows the microstructure, enabling distinct cell lineages to reciprocally interact. Recent studies have uncovered relevant roles of the bone vasculature in bone patterning, morphogenesis, homeostasis, and pathological bone destruction, including osteoporosis and tumor metastasis. This review provides an overview of current topics in the interactive molecular events between endothelial cells and bone cells during bone ontogeny and discusses the future direction of this research area to find novel ways to treat bone diseases.

The covert symphony: cellular and molecular accomplices in breast cancer metastasis

Breast cancer has emerged as the most commonly diagnosed cancer and primary cause of cancer-related deaths among women worldwide. Although significant progress has been made in targeting the primary tumor, the effectiveness of systemic treatments to prevent metastasis remains limited. Metastatic disease continues to be the predominant factor leading to fatality in the majority of breast cancer patients. The existence of a prolonged latency period between initial treatment and eventual recurrence in certain patients indicates that tumors can both adapt to and interact with the systemic environment of the host, facilitating and sustaining the progression of the disease. In order to identify potential therapeutic interventions for metastasis, it will be crucial to gain a comprehensive framework surrounding the mechanisms driving the growth, survival, and spread of tumor cells, as well as their interaction with supporting cells of the microenvironment. This review aims to consolidate recent discoveries concerning critical aspects of breast cancer metastasis, encompassing the intricate network of cells, molecules, and physical factors that contribute to metastasis, as well as the molecular mechanisms governing cancer dormancy.

Awakening of Dormant Breast Cancer Cells in the Bone Marrow

Up to 40% of patients with breast cancer (BC) have metastatic cells in the bone marrow (BM) at the initial diagnosis of localized disease. Despite definitive systemic adjuvant therapy, these cells survive in the BM microenvironment, enter a dormant state and recur stochastically for more than 20 years. Once they begin to proliferate, recurrent macrometastases are not curable, and patients generally succumb to their disease. Many potential mechanisms for initiating recurrence have been proposed, but no definitive predictive data have been generated. This manuscript reviews the proposed mechanisms that maintain BC cell dormancy in the BM microenvironment and discusses the data supporting specific mechanisms for recurrence. It addresses the well-described mechanisms of secretory senescence, inflammation, aging, adipogenic BM conversion, autophagy, systemic effects of trauma and surgery, sympathetic signaling, transient angiogenic bursts, hypercoagulable states, osteoclast activation, and epigenetic modifications of dormant cells. This review addresses proposed approaches for either eliminating micrometastases or maintaining a dormant state.

Live Cell Lineage Tracing of Dormant Cancer Cells

Breast cancer is a leading cause of global cancer-related deaths, and metastasis is the overwhelming culprit of poor patient prognosis. The most nefarious aspect of metastasis is dormancy, a prolonged period between primary tumor resection and relapse. Current therapies are insufficient at killing dormant cells; thus, they can remain quiescent in the body for decades until eventually undergoing a phenotypic switch, resulting in metastases that are more adaptable and drug resistant. Unfortunately, dormancy has few in vitro models, largely because lab-derived cell lines are highly proliferative. Existing models address tumor dormancy, not cellular dormancy, because tracking individual cells is technically challenging. To combat this problem, a live cell lineage approach to find and track individual dormant cells, distinguishing them from proliferative and dying cells over multiple days, is adapted. This approach is applied across a range of different in vitro microenvironments. This approach reveals that the proportion of cells that exhibit long-term quiescence is regulated by both cell intrinsic and extrinsic factors, with the most dormant cells found in 3D collagen gels. This paper envisions that this approach will prove useful to biologists and bioengineers in the dormancy community to identify, quantify, and study dormant tumor cells.

Osteoprogenitor-GMP crosstalk underpins solid tumor-induced systemic immunosuppression and persists after tumor removal

Remote tumors disrupt the bone marrow (BM) ecosystem (BME), eliciting the overproduction of BM-derived immunosuppressive cells. However, the underlying mechanisms remain poorly understood. Herein, we characterized breast and lung cancer-induced BME shifts pre- and post-tumor removal. Remote tumors progressively lead to osteoprogenitor (OP) expansion, hematopoietic stem cell dislocation, and CD41- granulocyte-monocyte progenitor (GMP) aggregation. The tumor-entrained BME is characterized by co-localization between CD41- GMPs and OPs. OP ablation abolishes this effect and diminishes abnormal myeloid overproduction. Mechanistically, HTRA1 carried by tumor-derived small extracellular vesicles upregulates MMP-13 in OPs, which in turn induces the alterations in the hematopoietic program. Importantly, these effects persist post-surgery and continue to impair anti-tumor immunity. Conditional knockout or inhibition of MMP-13 accelerates immune reinstatement and restores the efficacies of immunotherapies. Therefore, tumor-induced systemic effects are initiated by OP-GMP crosstalk that outlasts tumor burden, and additional treatment is required to reverse these effects for optimal therapeutic efficacy.

Reciprocal Regulation of Cancer-Associated Fibroblasts and Tumor Microenvironment in Gastrointestinal Cancer: Implications for Cancer Dormancy

Gastrointestinal (GI) cancers remain a major cause of cancer-related deaths worldwide. Despite the progress made in current treatments, patients with GI cancers still have high recurrence rates after initial treatment. Cancer dormancy, which involves the entry and escape of cancer cells from dormancy, is linked to treatment resistance, metastasis, and disease relapse. Recently, the role of the tumor microenvironment (TME) in disease progression and treatment has received increasing attention. The crosstalk between cancer-associated fibroblasts (CAF)-secreted cytokines/chemokines and other TME components, for example, extracellular matrix remodeling and immunomodulatory functions, play crucial roles in tumorigenesis. While there is limited direct evidence of a relationship between CAFs and cancer cell dormancy, this review explores the potential of CAF-secreted cytokines/chemokines to either promote cancer cell dormancy or awaken dormant cancer cells under different conditions, and the therapeutic strategies that may be applicable. By understanding the interactions between cytokines/chemokines released by CAFs and the TME, and their impact on the entry/escape of cancer dormancy, researchers may develop new strategies to reduce the risk of therapeutic relapse in patients with GI cancers.

Bone serves as a transfer station for secondary dissemination of breast cancer

Metastasis is responsible for the majority of deaths among breast cancer patients. Although parallel polyclonal seeding has been shown to contribute to organ-specific metastasis, in the past decade, horizontal cross-metastatic seeding (metastasis-to-metastasis spreading) has also been demonstrated as a pattern of distant metastasis to multiple sites. Bone, as the most frequent first destination of breast cancer metastasis, has been demonstrated to facilitate the secondary dissemination of breast cancer cells. In this review, we summarize the clinical and experimental evidence that bone is a transfer station for the secondary dissemination of breast cancer. We also discuss the regulatory mechanisms of the bone microenvironment in secondary seeding of breast cancer, focusing on stemness regulation, quiescence-proliferation equilibrium regulation, epigenetic reprogramming and immune escape of cancer cells. Furthermore, we highlight future research perspectives and strategies for preventing secondary dissemination from bone.

In vitro models of breast cancer bone metastasis: analyzing drug resistance through the lens of the microenvironment

Even though breast cancers usually have a good outcome compared to other tumors, the cancer can progress and create metastases in different parts of the organism, the bone being a predilection locus. These metastases are usually the cause of death, as they are mostly resistant to treatments. This resistance can be caused by intrinsic properties of the tumor, such as its heterogeneity, but it can also be due to the protective role of the microenvironment. By activating signaling pathways protecting cancer cells when exposed to chemotherapy, contributing to their ability to reach dormancy, or even reducing the amount of drug able to reach the metastases, among other mechanisms, the specificities of the bone tissue are being investigated as important players of drug resistance. To this date, most mechanisms of this resistance are yet to be discovered, and many researchers are implementing in vitro models to study the interaction between the tumor cells and their microenvironment. Here, we will review what is known about breast cancer drug resistance in bone metastasis due to the microenvironment and we will use those observations to highlight which features in vitro models should include to properly recapitulate these biological aspects in vitro. We will also detail which elements advanced in vitro models should implement in order to better recapitulate in vivo physiopathology and drug resistance.

The evolving tumor microenvironment: From cancer initiation to metastatic outgrowth

Cancers represent complex ecosystems comprising tumor cells and a multitude of non-cancerous cells, embedded in an altered extracellular matrix. The tumor microenvironment (TME) includes diverse immune cell types, cancer-associated fibroblasts, endothelial cells, pericytes, and various additional tissue-resident cell types. These host cells were once considered bystanders of tumorigenesis but are now known to play critical roles in the pathogenesis of cancer. The cellular composition and functional state of the TME can differ extensively depending on the organ in which the tumor arises, the intrinsic features of cancer cells, the tumor stage, and patient characteristics. Here, we review the importance of the TME in each stage of cancer progression, from tumor initiation, progression, invasion, and intravasation to metastatic dissemination and outgrowth. Understanding the complex interplay between tumor cell-intrinsic, cell-extrinsic, and systemic mediators of disease progression is critical for the rational development of effective anti-cancer treatments.

Dynamic bioinspired coculture model for probing ER+ breast cancer dormancy in the bone marrow niche

Late recurrences of breast cancer are hypothesized to arise from disseminated tumor cells (DTCs) that reactivate after dormancy and occur most frequently with estrogen receptor-positive (ER⁺) breast cancer cells (BCCs) in bone marrow (BM). Interactions between the BM niche and BCCs are thought to play a pivotal role in recurrence, and relevant model systems are needed for mechanistic insights and improved treatments. We examined dormant DTCs in vivo and observed DTCs near bone lining cells and exhibiting autophagy. To study underlying cell-cell interactions, we established a well-defined, bioinspired dynamic indirect coculture model of ER⁺ BCCs with BM niche cells, human mesenchymal stem cells (hMSCs) and fetal osteoblasts (hFOBs). hMSCs promoted BCC growth, whereas hFOBs promoted dormancy and autophagy, regulated in part by tumor necrosis factor-α and monocyte chemoattractant protein 1 receptor signaling. This dormancy was reversible by dynamically changing the microenvironment or inhibiting autophagy, presenting further opportunities for mechanistic and targeting studies to prevent late recurrence.

Host-Related Factors in the Interplay among Inflammation, Immunity and Dormancy in Breast Cancer Recurrence and Prognosis: An Overview for Clinicians

A significant proportion of patients treated for early breast cancer develop medium-term and late distant recurrence. The delayed manifestation of metastatic disease is defined as "dormancy". This model describes the aspects of the clinical latency of isolated metastatic cancer cells. Dormancy is regulated by extremely complex interactions between disseminated cancer cells and the microenvironment where they reside, the latter in turn influenced directly by the host. Among these entangled mechanisms, inflammation and immunity may play leading roles. This review is divided into two parts: the first describes the biological underpinnings of cancer dormancy and the role of the immune response, in particular, for breast cancer; the second provides an overview of the host-related factors that may influence systemic inflammation and immune response, subsequently impacting the dynamics of breast cancer dormancy. The aim of this review is to provide physicians and medical oncologists a useful tool to understand the clinical implications of this relevant topic.

Dormant cancer cells: programmed quiescence, senescence, or both?

Metastasis is the overwhelming driver of cancer mortality, accounting for the majority of cancer deaths. Many patients present with metastatic relapse years after eradication of the primary lesion. Disseminated cancer cells can undergo a durable proliferative arrest and lie dormant in secondary tissues before reentering the cell cycle to seed these lethal relapses. This process of cancer cell dormancy remains poorly understood, largely due to difficulties in studying these dormant cells. In the face of these challenges, the application of knowledge from the cellular senescence and quiescence fields may help to guide future thinking on the study of dormant cancer cells. Both senescence and quiescence are common programs of proliferative arrest that are integral to tissue development and homeostasis. Despite phenotypic differences, these two states also share common characteristics, and both likely play a role in cancer dormancy and delayed metastatic relapse. Understanding the cell biology behind these states, their overlaps and unique characteristics is critical to our future understanding of dormant cancer cells, as these cells likely employ some of the same molecular programs to promote survival and dissemination. In this review, we highlight the biology underlying these non-proliferative states, relate this knowledge to what we currently know about dormant cancer cells, and discuss implications for future work toward targeting these elusive metastatic seeds.

The genomic regulation of metastatic dormancy

The genomics and pathways governing metastatic dormancy are critically important drivers of long-term patient survival given the considerable portion of cancers that recur aggressively months to years after initial treatments. Our understanding of dormancy has expanded greatly in the last two decades, with studies elucidating that the dormant state is regulated by multiple genes, microenvironmental (ME) interactions, and immune components. These forces are exerted through mechanisms that are intrinsic to the tumor cell, manifested through cross-talk between tumor and ME cells including those from the immune system, and regulated by angiogenic processes in the nascent micrometastatic niche. The development of new in vivo and 3D ME models, as well as enhancements to decades-old tumor cell pedigree models that span the development of metastatic dormancy to aggressive growth, has helped fuel what arguably is one of the least understood areas of cancer biology that nonetheless contributes immensely to patient mortality. The current review focuses on the genes and molecular pathways that regulate dormancy via tumor-intrinsic and ME cells, and how groups have envisioned harnessing these therapeutically to benefit patient survival.

Dormancy, stemness, and therapy resistance: interconnected players in cancer evolution

The biological complexity of cancer represents a tremendous clinical challenge, resulting in the frequent failure of current treatment protocols. In the rapidly evolving scenario of a growing tumor, anticancer treatments impose a drastic perturbation not only to cancer cells but also to the tumor microenvironment, killing a portion of the cells and inducing a massive stress response in the survivors. Consequently, treatments can act as a double-edged sword by inducing a temporary response while laying the ground for therapy resistance and subsequent disease progression. Cancer cell dormancy (or quiescence) is a central theme in tumor evolution, being tightly linked to the tumor's ability to survive cytotoxic challenges, metastasize, and resist immune-mediated attack. Accordingly, quiescent cancer cells (QCCs) have been detected in virtually all the stages of tumor development. In recent years, an increasing number of studies have focused on the characterization of quiescent/therapy resistant cancer cells, unveiling QCCs core transcriptional programs, metabolic plasticity, and mechanisms of immune escape. At the same time, our partial understanding of tumor quiescence reflects the difficulty to identify stable QCCs biomarkers/therapeutic targets and to control cancer dormancy in clinical settings. This review focuses on recent discoveries in the interrelated fields of dormancy, stemness, and therapy resistance, discussing experimental evidences in the frame of a nonlinear dynamics approach, and exploring the possibility that tumor quiescence may represent not only a peril but also a potential therapeutic resource.

Physical and biological advances in endothelial cell-based engineered co-culture model systems

Scientific knowledge in the field of cell biology and mechanobiology heavily leans on cell-based in vitro experiments and models that favor the examination and comprehension of certain biological processes and occurrences across a variety of environments. Cell culture assays are an invaluable instrument for a vast spectrum of biomedical and biophysical investigations. The quality of experimental models in terms of simplicity, reproducibility, and combinability with other methods, and in particular the scale at which they depict cell fate in native tissues, is critical to advancing the knowledge of the comprehension of cell-cell and cell-matrix interactions in tissues and organs. Typically, in vitro models are centered on the experimental tinkering of mammalian cells, most often cultured as monolayers on planar, two-dimensional (2D) materials. Notwithstanding the significant advances and numerous findings that have been accomplished with flat biology models, their usefulness for generating further new biological understanding is constrained because the simple 2D setting does not reproduce the physiological response of cells in natural living tissues. In addition, the co-culture systems in a 2D stetting weakly mirror their natural environment of tissues and organs. Significant advances in 3D cell biology and matrix engineering have resulted in the creation and establishment of a new type of cell culture shapes that more accurately represents the in vivo microenvironment and allows cells and their interactions to be analyzed in a biomimetic approach. Contemporary biomedical and biophysical science has novel advances in technology that permit the design of more challenging and resilient in vitro models for tissue engineering, with a particular focus on scaffold- or hydrogel-based formats, organotypic cultures, and organs-on-chips, which cover the purposes of co-cultures. Even these complex systems must be kept as simplified as possible in order to grasp a particular section of physiology too very precisely. In particular, it is highly appreciated that they bridge the space between conventional animal research and human (patho)physiology. In this review, the recent progress in 3D biomimetic culturation is presented with a special focus on co-cultures, with an emphasis on the technological building blocks and endothelium-based co-culture models in cancer research that are available for the development of more physiologically relevant in vitro models of human tissues under normal and diseased conditions. Through applications and samples of various physiological and disease models, it is possible to identify the frontiers and future engagement issues that will have to be tackled to integrate synthetic biomimetic culture systems far more successfully into biomedical and biophysical investigations.

The metastatic niche formation: focus on extracellular vesicle-mediated dialogue between lung cancer cells and the microenvironment

Lung cancer is the deadliest cancer in the world, with the majority of patients presenting with advanced or metastatic disease at first diagnosis. The lungs are also one of the most common sites of metastasis from lung cancer and other tumors. Understanding the mechanisms that regulate metastasis formation from primary lung cancer and in the lungs is therefore fundamental unmet clinical need. One of the first steps during the establishment of lung cancer metastases includes the formation of the pre-metastatic niche (PMN) at distant organs, which may occur even during the early phases of cancer development. The PMN is established through intricate cross-talk between primary tumor-secreted factors and stromal components at distant sites. Mechanisms controlling primary tumor escape and seeding of distant organs rely on specific properties of tumor cells but are also tightly regulated by interactions with stromal cells at the metastatic niche that finally dictate the success of metastasis establishment. Here, we summarize the mechanisms underlying pre-metastatic niche formation starting from how lung primary tumor cells modulate distant sites through the release of several factors, focusing on Extracellular Vesicles (EVs). In this context, we highlight the role of lung cancer-derived EVs in the modulation of tumor immune escape. Then, we illustrate the complexity of Circulating Tumor Cells (CTCs) that represent the seeds of metastasis and how interactions with stromal and immune cells can help their metastatic dissemination. Finally, we evaluate the contribution of EVs in dictating metastasis development at the PMN through stimulation of proliferation and control of disseminated tumor cell dormancy. Overall, we present an overview of different steps in the lung cancer metastatic cascade, focusing on the EV-mediated interactions between tumor cells and stromal/immune cells.

Breast Cancer Metastatic Dormancy and Relapse: An Enigma of Microenvironment(s)

Multiple factors act in concert to define the fate of disseminated tumor cells (DTC) to enter dormancy or develop overt metastases. Here, we review these factors in the context of three stages of the metastatic cascade that impact DTCs. First, cells can be programmed within the primary tumor microenvironment to promote or inhibit dissemination, and the primary tumor can condition a premetastatic niche. Then, cancer cells from the primary tumor spread through hematogenous and lymphatic routes, and the primary tumor sends cues systematically to regulate the fate of DTCs. Finally, DTCs home to their metastatic site, where they are influenced by various organ-specific aspects of the new microenvironment. We discuss these factors in the context of breast cancer, where about one-third of patients develop metastatic relapse. Finally, we discuss how the standard-of-care options for breast cancer might affect the fate of DTCs.

Role of extracellular vesicles in cancer-specific interactions between tumour cells and the vasculature

Cancer progression impacts and exploits the vascular system in several highly consequential ways. Among different types of vascular cells, blood cells and mediators that are engaged in these processes, endothelial cells are at the centre of the underlying circuitry, as crucial constituents of angiogenesis, angiocrine stimulation, non-angiogenic vascular growth, interactions with the coagulation system and other responses. Tumour-vascular interactions involve soluble factors, extracellular matrix molecules, cell-cell contacts, as well as extracellular vesicles (EVs) carrying assemblies of molecular effectors. Oncogenic mutations and transforming changes in the cancer cell genome, epigenome and signalling circuitry exert important and often cancer-specific influences upon pathways of tumour-vascular interactions, including the biogenesis, content, and biological activity of EVs and responses of cancer cells to them. Notably, EVs may carry and transfer bioactive, oncogenic macromolecules (oncoproteins, RNA, DNA) between tumour and vascular cells and thereby elicit unique functional changes and forms of vascular growth and remodeling. Cancer EVs influence the state of the vasculature both locally and systemically, as exemplified by cancer-associated thrombosis. EV-mediated communication pathways represent attractive targets for therapies aiming at modulation of the tumour-vascular interface (beyond angiogenesis) and could also be exploited for diagnostic purposes in cancer.

Viscoelastic Liquid Matrix with Faster Bulk Relaxation Time Reinforces the Cell Cycle Arrest Induction of the Breast Cancer Cells via Oxidative Stress

The reactivating of disseminated dormant breast cancer cells in a soft viscoelastic matrix is mostly correlated with metastasis. Metastasis occurs due to rapid stress relaxation owing to matrix remodeling. Here, we demonstrate the possibility of promoting the permanent cell cycle arrest of breast cancer cells on a viscoelastic liquid substrate. By controlling the molecular weight of the hydrophobic molten polymer, poly(ε-caprolactone-co-D,L-lactide) within 35-63 g/mol, this study highlights that MCF7 cells can sense a 1000 times narrower relaxation time range (80-290 ms) compared to other studies by using a crosslinked hydrogel system. We propose that the rapid bulk relaxation response of the substrate promotes more reactive oxygen species generation in the formed semi-3D multicellular aggregates of breast cancer cells. Our finding sheds light on the potential role of bulk stress relaxation in a viscous-dominant viscoelastic matrix in controlling the cell cycle arrest depth of breast cancer cells.

Regulation of Metastatic Tumor Dormancy and Emerging Opportunities for Therapeutic Intervention

Cancer recurrence and metastasis, following successful treatment, constitutes a critical threat in clinical oncology and are the leading causes of death amongst cancer patients. This phenomenon is largely attributed to metastatic tumor dormancy, a rate-limiting stage during cancer progression, in which disseminated cancer cells remain in a viable, yet not proliferating state for a prolonged period. Dormant cancer cells are characterized by their entry into cell cycle arrest and survival in a quiescence state to adapt to their new microenvironment through the acquisition of mutations and epigenetic modifications, rendering them resistant to anti-cancer treatment and immune surveillance. Under favorable conditions, disseminated dormant tumor cells 're-awake', resume their proliferation and thus colonize distant sites. Due to their rarity, detection of dormant cells using current diagnostic tools is challenging and, thus, therapeutic targets are hard to be identified. Therefore, unraveling the underlying mechanisms required for keeping disseminating tumor cells dormant, along with signals that stimulate their "re-awakening" are crucial for the discovery of novel pharmacological treatments. In this review, we shed light into the main mechanisms that control dormancy induction and escape as well as emerging therapeutic strategies for the eradication of metastatic dormant cells, including dormancy maintenance, direct targeting of dormant cells and re-awakening dormant cells. Studies on the ability of the metastatic cancer cells to cease proliferation and survive in a quiescent state before re-initiating proliferation and colonization years after successful treatment, will pave the way toward developing innovative therapeutic strategies against dormancy-mediated metastatic outgrowth.

Perivascular Stromal Cells Instruct Glioblastoma Invasion, Proliferation, and Therapeutic Response within an Engineered Brain Perivascular Niche Model

Glioblastoma (GBM) tumor cells are found in the perivascular niche microenvironment and are believed to associate closely with the brain microvasculature. However, it is largely unknown how the resident cells of the perivascular niche, such as endothelial cells, pericytes, and astrocytes, influence GBM tumor cell behavior and disease progression. A 3D in vitro model of the brain perivascular niche developed by encapsulating brain-derived endothelial cells, pericytes, and astrocytes in a gelatin hydrogel is described. It is shown that brain perivascular stromal cells, namely pericytes and astrocytes, contribute to vascular architecture and maturation. Cocultures of patient-derived GBM tumor cells with brain microvascular cells are used to identify a role for pericytes and astrocytes in establishing a perivascular niche environment that modulates GBM cell invasion, proliferation, and therapeutic response. Engineered models provide unique insight regarding the spatial patterning of GBM cell phenotypes in response to a multicellular model of the perivascular niche. Critically, it is shown that engineered perivascular models provide an important resource to evaluate mechanisms by which intercellular interactions modulate GBM tumor cell behavior, drug response, and provide a framework to consider patient-specific disease phenotypes.

Chemotherapy-induced complement signaling modulates immunosuppression and metastatic relapse in breast cancer

Mortality from breast cancer is almost exclusively a result of tumor metastasis and resistance to therapy and therefore understanding the underlying mechanisms is an urgent challenge. Chemotherapy, routinely used to treat breast cancer, induces extensive tissue damage, eliciting an inflammatory response that may hinder efficacy and promote metastatic relapse. Here we show that systemic treatment with doxorubicin, but not cisplatin, following resection of a triple-negative breast tumor induces the expression of complement factors in lung fibroblasts and modulates an immunosuppressive metastatic niche that supports lung metastasis. Complement signaling derived from cancer-associated fibroblasts (CAFs) mediates the recruitment of myeloid-derived suppressor cells (MDSCs) to the metastatic niche, thus promoting T cell dysfunction. Pharmacological targeting of complement signaling in combination with chemotherapy alleviates immune dysregulation and attenuates lung metastasis. Our findings suggest that combining cytotoxic treatment with blockade of complement signaling in triple-negative breast cancer patients may attenuate the adverse effects of chemotherapy, thus offering a promising approach for clinical use.

Accumulating evidence suggest that chemotherapy could paradoxically promote cancer metastasis. Here the authors report that, in preclinical breast cancer models, adjuvant treatment with doxorubicin induces the formation of an immunosuppressive metastatic niche that promotes relapse but that can be reverted with pharmacological blockade of complement signaling.