Implantable pre-metastatic niches for the study of the microenvironmental regulation of disseminated human tumour cells

Inflammation drives tumor growth in an immunocompetent implantable metastasis model

Nearly 90% of cancer deaths are due to metastasis. Conventional cancer therapeutics including chemotherapy, surgery, and radiotherapy, are effective in treating primary tumors, but may aggravate disseminated tumor cells (DTCs) into regaining a proliferative state. Models isolating the post dissemination environment are needed to address the potential risks of these therapies, however modeling post dissemination environments is challenging. Often, host organisms become moribund due to primary tumor mass before native metastatic niches can evolve. Implantable tissue engineered niches have been used to attract circulating tumor cells independent of the primary tumor. Here, we serially transplant such tissue engineered niches with recruited DTCs in order to isolate the post dissemination environment. After transplantaion, 69% of scaffolds developed overt post-dissemination cancer growth, however 100% of scaffolds did not grow to a life-threatening critical size within twelve weeks. Adjuvant chemotherapy, while initially effective, did not prevent long-term DTC growth in scaffolds. Subjecting these transplanted niches to surgical resection via biopsy punch enhanced CD31, MMP9, Ly6G, and tumor burden compared to control scaffolds. Biopsy punching was able to rescue tumor incidence from prior chemotherapy. This model of serial transplantation of engineered DTC niches is a highly controllable and flexible method of establishing and systematically investigating the post-dissemination niche.

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.

Targeting lymph nodes for enhanced cancer vaccination: From nanotechnology to tissue engineering

Lymph nodes (LNs) occupy a critical position in initiating and augmenting immune responses, both spatially and functionally. In cancer immunotherapy, tumor-specific vaccines are blooming as a powerful tool to suppress the growth of existing tumors, as well as provide preventative efficacy against tumorigenesis. Delivering these vaccines more efficiently to LNs, where antigen-presenting cells (APCs) and T cells abundantly reside, is under extensive exploration. Formulating vaccines into nanomedicines, optimizing their physiochemical properties, and surface modification to specifically bind molecules expressed on LNs or APCs, are common routes and have brought encouraging outcomes. Alternatively, porous scaffolds can be engineered to attract APCs and provide an environment for them to mature, proliferate and migrate to LNs. A relatively new research direction is inducing the formation of LN-like organoids, which have shown positive relevance to tumor prognosis. Cutting-edge advances in these directions and discussions from a future perspective are given here, from which the up-to-date pattern of cancer vaccination will be drawn to hopefully provide basic guidance to future studies.

Fabrication of a Coculture Organoid Model in the Biomimetic Matrix of Alginate to Investigate Breast Cancer Progression in a TAMs-Leading Immune Microenvironment

The current research models of breast cancer are usually limited in their capacity to recapitulate the tumor microenvironment in vitro. The lack of an extracellular matrix (ECM) oversimplifies cell-cell or cell-ECM cross-talks. Moreover, the lack of tumor-associated macrophages (TAMs), that can comprise up to 50% of some solid neoplasms, poses a major problem for recognizing various hallmarks of cancer. To address these concerns, a type of direct breast cancer cells (BCCs)-TAMs coculture organoid model was well developed by a sequential culture method in this study. Alginate cryogels were fabricated with appropriate physical and mechanical properties to serve as an alternative ECM. Then, our previous experience was leveraged to polarize TAMs inside of the cryogels for creating an in vitro immune microenvironment. The direct coculture significantly enhanced BCCs organoid growth and cancer aggressive phenotypes, including the stemness, migration, ECM remodeling, and cytokine secretion. Furthermore, transcriptomic analysis and protein-protein interaction networks implied certain pathways (PI3K-Akt pathway, MAPK signaling pathway, etc.) and targets (TNF, PPARG, TLR2, etc.) during breast cancer progression in a TAM-leading immune microenvironment. Future studies to advance treatment strategies for BCC patients may benefit from using this facile model to reveal and target the interactions between cancer signaling and the immune microenvironment.

A synthetic metastatic niche reveals antitumor neutrophils drive breast cancer metastatic dormancy in the lungs

Biomaterial scaffolds mimicking the environment in metastatic organs can deconstruct complex signals and facilitate the study of cancer progression and metastasis. Here we report that a subcutaneous scaffold implant in mouse models of metastatic breast cancer in female mice recruits lung-tropic circulating tumor cells yet suppresses their growth through potent in situ antitumor immunity. In contrast, the lung, the endogenous metastatic organ for these models, develops lethal metastases in aggressive breast cancer, with less aggressive tumor models developing dormant lungs suppressing tumor growth. Our study reveals multifaceted roles of neutrophils in regulating metastasis. Breast cancer-educated neutrophils infiltrate the scaffold implants and lungs, secreting the same signal to attract lung-tropic circulating tumor cells. Second, antitumor and pro-tumor neutrophils are selectively recruited to the dormant scaffolds and lungs, respectively, responding to distinct groups of chemoattractants to establish activated or suppressive immune environments that direct different fates of cancer cells.

Bone Marrow Adipocytes Contribute to Tumor Microenvironment-Driven Chemoresistance via Sequestration of Doxorubicin

Chemoresistance is a significant problem in the effective treatment of bone metastasis. Adipocytes are a major stromal cell type in the bone marrow and may play a crucial role in developing microenvironment-driven chemoresistance. However, detailed investigation remains challenging due to the anatomical inaccessibility and intrinsic tissue complexity of the bone marrow microenvironment. In this study, we developed 2D and 3D in vitro models of bone marrow adipocytes to examine the mechanisms underlying adipocyte-induced chemoresistance. We first established a protocol for the rapid and robust differentiation of human bone marrow stromal cells (hBMSCs) into mature adipocytes in 2D tissue culture plastic using rosiglitazone (10 μM), a PPARγ agonist. Next, we created a 3D adipocyte culture model by inducing aggregation of hBMSCs and adipogenesis to create adipocyte spheroids in porous hydrogel scaffolds that mimic bone marrow sinusoids. Simulated chemotherapy treatment with doxorubicin (2.5 μM) demonstrated that mature adipocytes sequester doxorubicin in lipid droplets, resulting in reduced cytotoxicity. Lastly, we performed direct coculture of human multiple myeloma cells (MM1.S) with the established 3D adipocyte model in the presence of doxorubicin. This resulted in significantly accelerated multiple myeloma proliferation following doxorubicin treatment. Our findings suggest that the sequestration of hydrophobic chemotherapeutics by mature adipocytes represents a potent mechanism of bone marrow microenvironment-driven chemoresistance.

Immune determinants of the pre-metastatic niche

Primary tumors actively and specifically prime pre-metastatic niches (PMNs), the future sites of organotropic metastasis, preparing these distant microenvironments for disseminated tumor cell arrival. While initial studies of the PMN focused on extracellular matrix alterations and stromal reprogramming, it is increasingly clear that the far-reaching effects of tumors are in great part achieved through systemic and local PMN immunosuppression. Here, we discuss recent advances in our understanding of the tumor immune microenvironment and provide a comprehensive overview of the immune determinants of the PMN's spatiotemporal evolution. Moreover, we depict the PMN immune landscape, based on functional pre-clinical studies as well as mounting clinical evidence, and the dynamic, reciprocal crosstalk with systemic changes imposed by cancer progression. Finally, we outline emerging therapeutic approaches that alter the dynamics of the interactions driving PMN formation and reverse immunosuppression programs in the PMN ensuring early anti-tumor immune responses.

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.

A BMP-2-triggered in vivo osteo-organoid for cell therapy

Cell therapies and regenerative medicine interventions require an adequate source of therapeutic cells. Here, we demonstrate that constructing in vivo osteo-organoids by implanting bone morphogenetic protein-2-loaded scaffolds into the internal muscle pocket near the femur of mice supports the growth and subsequent harvest of therapeutically useful cells including hematopoietic stem/progenitor cells (HSPCs), mesenchymal stem cells (MSCs), lymphocytes, and myeloid cells. Profiling of the in vivo osteo-organoid maturation process delineated three stages-fibroproliferation, osteochondral differentiation, and marrow generation-each of which entailed obvious changes in the organoid structure and cell type distribution. The MSCs harvested from the osteochondral differentiation stage mitigated carbon tetrachloride (CCl₄)-induced chronic liver fibrosis in mice, while HSPCs and immune cells harvested during the marrow generation stage rapidly and effectively reconstituted the impaired peripheral and solid immune organs of irradiated mice. These findings demonstrate the therapeutic potentials of in vivo osteo-organoid-derived cells in cell therapies.

The Hepatic Pre-Metastatic Niche

Simple Summary

The pre-metastatic niche is a recently established concept that could lead to targeted therapies that prevent metastasis before ever occurring. Considering that 90% of cancer mortality results from metastasis, the PMN is thus a salient opportunity for intervention. The purpose of the current review is to cover what is known specifically about the hepatic pre-metastatic niche, a topic that has garnered increasing research focus within the last decade. We discuss the methods of communication between primary tumors and the liver, the involved cell populations, the key changes within liver tissue, and perspectives on the future of the field.

Abstract

Primary tumors can communicate with the liver to establish a microenvironment that favors metastatic colonization prior to dissemination, forming what is termed the “pre-metastatic niche” (PMN). Through diverse signaling mechanisms, distant malignancies can both influence hepatic cells directly as well as recruit immune cells into the PMN. The result is a set of changes within the hepatic tissue that increase susceptibility of tumor cell invasion and outgrowth upon dissemination. Thus, the PMN offers a novel step in the traditional metastatic cascade that could offer opportunities for clinical intervention. The involved signaling molecules also offer promise as biomarkers. Ultimately, while the existence of the hepatic PMN is well-established, continued research effort and use of innovative models are required to reach a functional knowledge of PMN mechanisms that can be further targeted.

Stromal Co-Cultivation for Modeling Breast Cancer Dormancy in the Bone Marrow

Cancers metastasize to the bone marrow before primary tumors can be detected. Bone marrow micrometastases are resistant to therapy, and while they are able to remain dormant for decades, they recur steadily and result in incurable metastatic disease. The bone marrow microenvironment maintains the dormancy and chemoresistance of micrometastases through interactions with multiple cell types and through structural and soluble factors. Modeling dormancy in vitro can identify the mechanisms of these interactions. Modeling also identifies mechanisms able to disrupt these interactions or define novel interactions that promote the reawakening of dormant cells. The in vitro modeling of the interactions of cancer cells with various bone marrow elements can generate hypotheses on the mechanisms that control dormancy, treatment resistance and reawakening in vivo. These hypotheses can guide in vivo murine experiments that have high probabilities of succeeding in order to verify in vitro findings while minimizing the use of animals in experiments. This review outlines the existing data on predominant stromal cell types and their use in 2D co-cultures with cancer cells.

Brain Vascular Microenvironments in Cancer Metastasis

Primary tumours, particularly from major solid organs, are able to disseminate into the blood and lymphatic system and spread to distant sites. These secondary metastases to other major organs are the most lethal aspect of cancer, accounting for the majority of cancer deaths. The brain is a frequent site of metastasis, and brain metastases are often fatal due to the critical role of the nervous system and the limited options for treatment, including surgery. This creates a need to further understand the complex cell and molecular biology associated with the establishment of brain metastasis, including the changes to the environment of the brain to enable the arrival and growth of tumour cells. Local changes in the vascular network, immune system and stromal components all have the potential to recruit and foster metastatic tumour cells. This review summarises our current understanding of brain vascular microenvironments, fluid circulation and drainage in the context of brain metastases, as well as commenting on current cutting-edge experimental approaches used to investigate changes in vascular environments and alterations in specialised subsets of blood and lymphatic vessel cells during cancer spread to the brain.

Localized delivery of immunotherapeutics: A rising trend in the field

Immunotherapy is becoming a new standard of care for multiple cancers, while several limitations are impending its further clinical success. Immunotherapeutic agents often have inappropriate pharmacokinetics on their own and/or exhibit limited specificity to tumor cells, leading to severe immuno-related adverse effects and limited efficacy. Suitable formulating strategies that confer prolonged contact with or efficient proliferation in tumors while reducing exposure to normal tissues are highly worthy to explore. With the assistance of biomaterial carriers, targeted therapy can be achieved artificially by implanting or injecting drug depots into desired sites, about which the wisdoms in literature have been rich. The relevant results have suggested a "local but systemic" effect, that is, local replenishment of immune modulators achieves a high treatment efficacy that also governs distant metastases, thereby building another rationale for localized delivery. Particularly, implantable scaffolds have been further engineered to recruit disseminated tumor cells with an efficiency high enough to reduce tumor burdens at typical metastatic organs, and simultaneously provide diagnostic signals. This review introduces recent advances in this emerging area along with a perspective on the opportunities and challenges in the way to clinical application.

Rebuilding the hematopoietic stem cell niche: Recent developments and future prospects

Hematopoietic stem cells (HSCs) have proven their clinical relevance in stem cell transplantation to cure patients with hematological disorders. Key to their regenerative potential is their natural microenvironment - their niche - in the bone marrow (BM). Developments in the field of biomaterials enable the recreation of such environments with increasing preciseness in the laboratory. Such artificial niches help to gain a fundamental understanding of the biophysical and biochemical processes underlying the interaction of HSCs with the materials in their environment and the disturbance of this interplay during diseases affecting the BM. Artificial niches also have the potential to multiply HSCs in vitro, to enable the targeted differentiation of HSCs into mature blood cells or to serve as drug-testing platforms. In this review, we will introduce the importance of artificial niches followed by the biology and biophysics of the natural archetype. We will outline how 2D biomaterials can be used to dissect the complexity of the natural niche into individual parameters for fundamental research and how 3D systems evolved from them. We will present commonly used biomaterials for HSC research and their applications. Finally, we will highlight two areas in the field of HSC research, which just started to unlock the possibilities provided by novel biomaterials, in vitro blood production and studying the pathophysiology of the niche in vitro. With these contents, the review aims to give a broad overview of the different biomaterials applied for HSC research and to discuss their potentials, challenges and future directions in the field. STATEMENT OF SIGNIFICANCE: Hematopoietic stem cells (HSCs) are multipotent cells responsible for maintaining the turnover of all blood cells. They are routinely applied to treat patients with hematological diseases. This high clinical relevance explains the necessity of multiplication or differentiation of HSCs in the laboratory, which is hampered by the missing natural microenvironment - the so called niche. Biomaterials offer the possibility to mimic the niche and thus overcome this hurdle. The review introduces the HSC niche in the bone marrow and discusses the utility of biomaterials in creating artificial niches. It outlines how 2D systems evolved into sophisticated 3D platforms, which opened the gateway to applications such as, expansion of clinically relevant HSCs, in vitro blood production, studying niche pathologies and drug testing.

A humanized orthotopic tumor microenvironment alters the bone metastatic tropism of prostate cancer cells

Prostate cancer (PCa) is the second most commonly diagnosed cancer in men, and bone is the most frequent site of metastasis. The tumor microenvironment (TME) impacts tumor growth and metastasis, yet the role of the TME in PCa metastasis to bone is not fully understood. We used a tissue-engineered xenograft approach in NOD-scid IL2Rγ^null (NSG) mice to incorporate two levels of humanization; the primary tumor and TME, and the secondary metastatic bone organ. Bioluminescent imaging, histology, and immunohistochemistry were used to study metastasis of human PC-3 and LNCaP PCa cells from the prostate to tissue-engineered bone. Here we show pre-seeding scaffolds with human osteoblasts increases the human cellular and extracellular matrix content of bone constructs, compared to unseeded scaffolds. The humanized prostate TME showed a trend to decrease metastasis of PC-3 PCa cells to the tissue-engineered bone, but did not affect the metastatic potential of PCa cells to the endogenous murine bones or organs. On the other hand, the humanized TME enhanced LNCaP tumor growth and metastasis to humanized and murine bone. Together this demonstrates the importance of the TME in PCa bone tropism, although further investigations are needed to delineate specific roles of the TME components in this context.

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.

Fibroblasts in cancer dormancy: foe or friend?

Cancer dormancy is defined that the residual cancer cells could enter into a state of quiescence and patients remain asymptomatic for years or even decades after anti-tumor therapies. Fibroblasts, which represent a predominant cell type in tumor microenvironment, play a pivotal role in determining the ultimate fate of tumor cells. This review recapitulates the pleiotropic roles of fibroblasts which are divided into normal, senescent, cancer-associated fibroblasts (CAFs) and circulation CAFs in tumor dormancy, relapse, metastasis and resistance to therapy to help the treatment of cancer metastasis.

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.

Printing the Pathway Forward in Bone Metastatic Cancer Research: Applications of 3D Engineered Models and Bioprinted Scaffolds to Recapitulate the Bone-Tumor Niche

Breast cancer commonly metastasizes to bone, resulting in osteolytic lesions and poor patient quality of life. The bone extracellular matrix (ECM) plays a critical role in cancer cell metastasis by means of the physical and biochemical cues it provides to support cellular crosstalk. Current two-dimensional in-vitro models lack the spatial and biochemical complexities of the native ECM and do not fully recapitulate crosstalk that occurs between the tumor and endogenous stromal cells. Engineered models such as bone-on-a-chip, extramedullary bone, and bioreactors are presently used to model cellular crosstalk and bone-tumor cell interactions, but fall short of providing a bone-biomimetic microenvironment. Three-dimensional bioprinting allows for the deposition of biocompatible materials and living cells in complex architectures, as well as provides a means to better replicate biological tissue niches in-vitro. In cancer research specifically, 3D constructs have been instrumental in seminal work modeling cancer cell dissemination to bone and bone-tumor cell crosstalk in the skeleton. Furthermore, the use of biocompatible materials, such as hydroxyapatite, allows for printing of bone-like microenvironments with the ability to be implanted and studied in in-vivo animal models. Moreover, the use of bioprinted models could drive the development of novel cancer therapies and drug delivery vehicles.

Black Bioceramics: Combining Regeneration with Therapy

Bioceramics have been developed from bioinert to bioactive or biodegradable materials in the past few decades. However, at present, traditional bioceramics are still mainly used in bone tissue regeneration and dental restoration. In this work, a new generation of "black bioceramics," extending the applications from tissue regeneration to disease therapy, is presented. Black bioceramics, through magnesium thermal reduction of traditional white ceramics, including silicate-based (e.g., CaSiO₃ , MgSiO₃ ) and phosphate-based (e.g., Ca₃ (PO₄ )₂ , Ca₅ (PO₄ )₃ (OH)), are successfully synthesized. Due to the presence of oxygen vacancies and structural defects, the black bioceramics possess photothermal functionality while maintaining their initial high bioactivity and regenerative capacity. These black bioceramics show excellent photothermal antitumor effects for both skin and bone tumors. At the same time, they have significantly improved bioactivity for skin/bone tissue repair in vitro and in vivo. These fascinating properties award the black bioceramics with profound applications in both tumor therapy and tissue regeneration, which should greatly promote the scientific relevance and clinical application of bioceramics, representing a promising new direction of cell-instructive biomaterials.

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.

Exosome-Derived LncRNAs in Lung Cancer

As extracellular vesicles, exosomes are released from most cells to perform cell–cell communication. Recent studies have shown that exosomes could be released into tumor microenvironment and blood to promote tumor progression through packaging and transmitting various bioactive molecules, such as cholesterol, proteins, lipids, miRNAs, mRNAs, and long non-coding RNAs (lncRNAs) to distant cells. LncRNAs have emerged as a major class of non-coding transcripts. A lot of LncRNAs have been discovered during the past few years of research on genomics. They have been proven to participate in various biological functions and disease processes through multiple mechanisms. In this review, we analyzed the role of exosome-derived lncRNAs in lung carcinogenesis and metastasis. We also highlight opportunities for the clinical potential of exosomes with specific lncRNAs as biomarkers and therapeutic intervention in lung cancer.

Engineered Niches to Analyze Mechanisms of Metastasis and Guide Precision Medicine

Cancer metastasis poses a challenging problem both clinically and scientifically, as the stochastic nature of metastatic lesion formation introduces complexity for both early detection and the study of metastasis in preclinical models. Engineered metastatic niches represent an emerging approach to address this stochasticity by creating bioengineered sites where cancer can preferentially metastasize. As the engineered niche captures the earliest metastatic cells at a nonvital location, both noninvasive and biopsy-based monitoring of these sites can be performed routinely to detect metastasis early and monitor alterations in the forming metastatic niche. The engineered metastatic niche also provides a new platform technology that serves as a tunable site to molecularly dissect metastatic disease mechanisms. Ultimately, linking the engineered niches with advances in sensor development and synthetic biology can provide enabling tools for preclinical cancer models and fosters the potential to impact the future of clinical cancer care.

Combined Targeting of Estrogen Receptor Alpha and Exportin 1 in Metastatic Breast Cancers

The majority of breast cancer specific deaths in women with estrogen receptor positive (ER+) tumors occur due to metastases that are resistant to therapy. There is a critical need for novel therapeutic approaches to achieve tumor regression and/or maintain therapy responsiveness in metastatic ER+ tumors. The objective of this study was to elucidate the role of metabolic pathways that undermine therapy efficacy in ER+ breast cancers. Our previous studies identified Exportin 1 (XPO1), a nuclear export protein, as an important player in endocrine resistance progression and showed that combining selinexor (SEL), an FDA-approved XPO1 antagonist, synergized with endocrine agents and provided sustained tumor regression. In the current study, using a combination of transcriptomics, metabolomics and metabolic flux experiments, we identified certain mitochondrial pathways to be upregulated during endocrine resistance. When endocrine resistant cells were treated with single agents in media conditions that mimic a nutrient deprived tumor microenvironment, their glutamine dependence for continuation of mitochondrial respiration increased. The effect of glutamine was dependent on conversion of the glutamine to glutamate, and generation of NAD+. PGC1α, a key regulator of metabolism, was the main driver of the rewired metabolic phenotype. Remodeling metabolic pathways to regenerate new vulnerabilities in endocrine resistant breast tumors is novel, and our findings reveal a critical role that ERα-XPO1 crosstalk plays in reducing cancer recurrences. Combining SEL with current therapies used in clinical management of ER+ metastatic breast cancer shows promise for treating and keeping these cancers responsive to therapies in already metastasized patients.

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.

Progress on Modulating Tumor-Associated Macrophages with Biomaterials

Tumor-associated macrophages (TAMs) are a complex and heterogeneous population of cells within the tumor microenvironment. In many tumor types, TAMs contribute toward tumor malignancy and are therefore a therapeutic target of interest. Here, three major strategies for regulating TAMs are highlighted, emphasizing the role of biomaterials in these approaches. First, systemic methods for targeting tumor-associated macrophage are summarized and limitations to both passive and active targeting approaches considered. Second, lessons learned from the significant literature on wound healing and macrophage response to implanted biomaterials are discussed with the vision of applying these principles to localized, biomaterial-based modulation of tumor-associated macrophage. Finally, the developing field of engineered macrophages, including genetic engineering and integration with biomaterials or drug delivery systems, is examined. Analysis of major challenges in the field along with exciting opportunities for the future of macrophage-based therapies in oncology are included.

ZEB1: A Critical Regulator of Cell Plasticity, DNA Damage Response, and Therapy Resistance

The predominant way in which conventional chemotherapy kills rapidly proliferating cancer cells is the induction of DNA damage. However, chemoresistance remains the main obstacle to therapy effectivity. An increasing number of studies suggest that epithelial-to-mesenchymal transition (EMT) represents a critical process affecting the sensitivity of cancer cells to chemotherapy. Zinc finger E-box binding homeobox 1 (ZEB1) is a prime element of a network of transcription factors controlling EMT and has been identified as an important molecule in the regulation of DNA damage, cancer cell differentiation, and metastasis. Recent studies have considered upregulation of ZEB1 as a potential modulator of chemoresistance. It has been hypothesized that cancer cells undergoing EMT acquire unique properties that resemble those of cancer stem cells (CSCs). These stem-like cells manifest enhanced DNA damage response (DDR) and DNA repair capacity, self-renewal, or chemoresistance. In contrast, functional experiments have shown that ZEB1 induces chemoresistance regardless of whether other EMT-related changes occur. ZEB1 has also been identified as an important regulator of DDR by the formation of a ZEB1/p300/PCAF complex and direct interaction with ATM kinase, which has been linked to radioresistance. Moreover, ATM can directly phosphorylate ZEB1 and enhance its stability. Downregulation of ZEB1 has also been shown to reduce the abundance of CHK1, an effector kinase of DDR activated by ATR, and to induce its ubiquitin-dependent degradation. In this perspective, we focus on the role of ZEB1 in the regulation of DDR and describe the mechanisms of ZEB1-dependent chemoresistance.

Chemokine releasing particle implants for trapping circulating prostate cancer cells

Prostate cancer (PCa) is the most prevalent cancer in U.S. men and many other countries. Although primary PCa can be controlled with surgery or radiation, treatment options of preventing metastatic PCa are still limited. To develop a new treatment of eradicating metastatic PCa, we have created an injectable cancer trap that can actively recruit cancer cells in bloodstream. The cancer trap is composed of hyaluronic acid microparticles that have good cell and tissue compatibility and can extend the release of chemokines to 4 days in vitro. We find that erythropoietin (EPO) and stromal derived factor-1α can attract PCa in vitro. Animal results show that EPO-releasing cancer trap attracted large number of circulating PCa and significantly reduced cancer spreading to other organs compared with controls. These results support that cancer trap may serve as a unique device to sequester circulating PCa cells and subsequently reduce distant metastasis.

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.

Fabrication of Bioactive Inverted Colloidal Crystal Scaffolds Using Expanded Polystyrene Beads

Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity to reproduce lymphoid tissue microenvironments. ICC geometry promotes the formation of stromal cell networks and their interaction with hematopoietic cells, a core cellular process in lymphoid tissues. When subdermally implanted, ICC hydrogel scaffolds direct unique foreign body responses to form a vascularized stromal tissue with prolonged attraction of hematopoietic cells, which together resemble lymphoid tissue microenvironments. While conceptually simple, fabrication of ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials. Here, we introduce a facile route for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads. EPS beads shrink and fuse in a tunable manner under pressurized thermal conditions, which serves as colloidal crystal templates for ICC scaffold fabrication. Inclusion of collagen in the precursor solution greatly simplified preparation of bioactive hydrogel scaffolds. The resultant EPS-templated bioactive ICC hydrogel scaffolds demonstrate characteristic features required for lymphoid tissue modeling in both in vitro and in vivo settings. We envision that the presented method will facilitate widespread implementation of ICC hydrogel scaffolds for lymphoid tissue engineering and other emerging applications. Impact statement Inverted colloidal crystal (ICC) hydrogel scaffolds have emerged as a new class of three-dimensional cell culture matrix that represents a unique opportunity for lymphoid tissue modeling and other emerging novel bioengineering applications. While conceptually simple, fabrication of the ICC hydrogel scaffold requires multiple steps and laborious handling of delicate materials with highly toxic chemicals. The presented method for ICC hydrogel scaffold fabrication using expanded polystyrene (EPS) beads is simple, cost-effective, and involves less toxic chemicals than conventional methods, while retaining comparable biological significance. We envision that EPS bead-based hydrogel scaffold fabrication will greatly facilitate the widespread implementation of ICC hydrogel scaffolds and their practical applications.

Thermoresponsive Inverted Colloidal Crystal Hydrogel Scaffolds for Lymphoid Tissue Engineering

Inverted colloidal crystal (ICC) hydrogel scaffolds represent unique opportunities in modeling lymphoid tissues and expanding hematopoietic-lymphoid cells. Fully interconnected spherical pore arrays direct the formation of stromal networks and facilitate interactions between stroma and hematopoietic-lymphoid cells. However, due to the intricate architecture of these materials, release of expanded cells is restricted and requires mechanical disruption or chemical dissolution of the hydrogel scaffold. One potent biomaterials strategy to release pore-entrapped hematopoietic-lymphoid cells without breaking the scaffolds apart is to transiently increase the dimensions of these materials using stimuli-responsive polymers. Having this mindset, thermoresponsive ICC scaffolds that undergo rapid (<1 min) and substantial (>300%) diameter change over a physiological temperature range (4-37 °C) by using poly(N-isopropylacrylamide) (PNIPAM) with nanogel crosslinkers is developed. For a proof-of-concept study, the stromal niche by creating osteospheroids, aggregates of osteoblasts, and bone chips is first replicated, and subsequently Nalm-6 model hematopoietic-lymphoid cells are introduced. A sixfold increase in cell count is harvested when ICC hydrogel scaffolds are expanded without termination of the established 3D stromal cell culture. It is envisioned that thermoresponsive ICC hydrogel scaffolds will enable for scalable and sustainable ex vivo expansion of hematopoietic-lymphoid cells.

Design and Fabrication of Three-Dimensional Printed Scaffolds for Cancer Precision Medicine

Three-dimensional (3D)-engineered scaffolds have been widely investigated as drug delivery systems (DDS) or cancer models with the aim to develop effective cancer therapies. The in vitro and in vivo models developed via 3D printing (3DP) and tissue engineering concepts have significantly contributed to our understanding of cell-cell and cell-extracellular matrix interactions in the cancer microenvironment. Moreover, 3D tumor models were used to study the therapeutic efficiency of anticancer drugs. The present study aims to provide an overview of applying the 3DP and tissue engineering concepts for cancer studies with suggestions for future research directions. The 3DP technologies being used for the fabrication of personalized DDS have been highlighted and the potential technical approaches and challenges associated with the fused deposition modeling, the inkjet-powder bed, and stereolithography as the most promising 3DP techniques for drug delivery purposes are briefly described. Then, the advances, challenges, and future perspectives in tissue-engineered cancer models for precision medicine are discussed. Overall, future advances in this arena depend on the continuous integration of knowledge from cancer biology, biofabrication techniques, multiomics and patient data, and medical needs to develop effective treatments ultimately leading to improved clinical outcomes. Impact statement Three-dimensional printing (3DP) enables the fabrication of personalized medicines and drug delivery systems. The convergence of 3DP, tissue engineering concepts, and cancer biology could significantly improve our understanding of cancer biology and contribute to the development of new cancer therapies.

Premetastatic niches, exosomes and circulating tumor cells: Early mechanisms of tumor dissemination and the relation to surgery

The physiological stress response to surgery promotes wound healing and functional recovery and includes the activation of neural, inflammatory and proangiogenic signaling pathways. Paradoxically, the same pathways also promote metastatic spread and growth of residual cancer. Human and animal studies show that cancer surgery can increase survival, migration and proliferation of residual tumor cells. To secure the survival and growth of disseminated tumor cells, the formation of premetastatic niches in target organs involves a complex interplay between microenvironment, immune system, circulating tumor cells, as well as chemical mediators and exosomes secreted by the primary tumor. This review describes the current understanding of the early mechanisms of dissemination, as well as how surgery may facilitate disease progression.

Novel Techniques to Study the Bone-Tumor Microenvironment

Many cancers commonly metastasize to bone. After entering the bone, cancer cells can interact with surrounding stromal cells, which ultimately influences metastasis progression. Extracellular vesicles, direct cell contact and gap junctions, and cytokines are all mechanisms of intercellular communication that have been observed to occur in the bone microenvironment. These methods of cellular crosstalk can occur between cancer cells and a variety of stromal cells, with each interaction having a different impact on cancer progression. Communication between cancer cells and bone-resident cells has previously been implicated in processes such as cancer cell trafficking and arrest in bone, cancer cell dormancy, cancer cell reactivation, and proliferation. In this chapter we review innovative techniques and model systems that can be used to study bidirectional crosstalk between cancer cells and stromal cells in the bone, with an emphasis specifically on bone-metastatic breast cancer. Investigating how metastatic cancer cells interact with, and are influenced by, the bone microenvironment is crucial to better understanding of the progression of bone metastasis.

Microporous scaffolds loaded with immunomodulatory lentivirus to study the contribution of immune cell populations to tumor cell recruitment in vivo

Metastases are preceded by stochastic formation of a hospitable microenvironment known as the premetastatic niche, which has been difficult to study. Herein, we employ implantable polycaprolactone scaffolds as an engineered premetastatic niche to independently investigate the role of interleukin-10 (IL10), CXCL12, and CCL2 in recruiting immune and tumor cells and impacting breast cancer cell phenotype via lentiviral overexpression. Lentivirus delivered from scaffolds in vivo achieved sustained transgene expression for 56 days. IL10 lentiviral expression, but not CXCL12 or CCL2, significantly decreased tumor cell recruitment to scaffolds in vivo. Delivery of CXCL12 enhanced CD45+ immune cell recruitment to scaffolds while delivery of IL10 reduced immune cell recruitment. CCL2 did not alter immune cell recruitment. Tumor cell phenotype was investigated using conditioned media from immunomodulated scaffolds, with CXCL12 microenvironments reducing proliferation, and IL10 microenvironments enhancing proliferation. Migration was enhanced with CCL2 and reduced with IL10-driven microenvironments. Multiple linear regression identified populations of immune cells associated with tumor cell abundance. CD45+ immune and CD8+ T cells were associated with reduced tumor cell abundance, while CD11b+Gr1+ neutrophils and CD4+ T cells were associated with enhanced tumor cell abundance. Collectively, biomaterial scaffolds provide a tool to probe the formation and function of the premetastatic niche.

Integration of immunogenic activation and immunosuppressive reversion using mitochondrial-respiration-inhibited platelet-mimicking nanoparticles

Here, we developed platelet membranes (PM) as nano-carriers to co-encapsulate metformin (Met) and IR780 (PM-IR780-Met NPs). The resulting nano-carrier ensured a longer circulation lifetime and facilitated the greater accumulation of IR780 and Met in tumors owing to the active adhesion between PM and tumor cells. As a photodynamic therapy (PDT) agent, IR780 could effectively kill the tumor by producing toxic reactive singlet oxygen species (ROS), while the introduction of Met inhibited mitochondrial respiration and reduced tumor oxygen consumption, thereby evoking an oxygen-boosted PDT and propelling the immunogenic cell death (ICD)-based immunogenic pathway. Meanwhile, the reversed tumor hypoxia also impeded the myeloid derived suppressor cell (MDSC)-regulated immunosuppressive pathway. Finally, tremendous T cells were recruited and activated, providing a promising platform to eliminate the primary tumors and synchronously opening a new avenue for the effective ablation of tumor metastasis.

Scaffold-Assisted Ectopic Transplantation of Internal Organs and Patient-Derived Tumors

Xenotransplantation of human tissues into immunodeficient mice has emerged as an invaluable preclinical model to study human biology and disease progression and predict clinical response. The most common anatomical site for tissue transplantation is the subcutaneous pocket due to simple surgical procedures and accessibility for gross monitoring and advanced imaging modalities. However, subcutaneously implanted tissues initially experience a sharp change in oxygen and nutrient supply and increased mechanical deformation. During this acute phase of tissue integration to the host vasculature, substantial cell death and tissue fibrosis occur limiting engraftment efficiency. Previously, we demonstrated that the implantation of inverted colloidal crystal hydrogel scaffolds triggers proangiogenic and immunomodulatory functions without characteristic foreign body encapsulation. In this study, we examine the use of this unique host response to improve the ectopic transplantation of tissues to the subcutaneous site. Scaffold-assisted tissues preserved morphological features and blood vessel density compared to native tissues, whereas scaffold-free tissues collapsed and were less vascularized. Notably, the supporting biomaterial scaffold modulated the foreign body response to reduce the localization of Ly6G⁺ cells within the transplanted tissues. Cotransplantation of patient-derived gastric cancer with a scaffold resulted in a comparable level of engraftment to conventional methods; however, detailed immunohistological characterization revealed significantly better retention of proliferative cells (Ki67⁺) and human immune cells (CD45⁺) by the end of the study. We envision that leveraging the immunomodulatory properties of biomaterial interfaces can be an attractive strategy to improve the functional engraftment of xenotransplants and accelerate individualized diagnostics and the development of novel therapeutic strategies.

Tunable hydrogels for controlling phenotypic cancer cell states to model breast cancer dormancy and reactivation

During metastasis, disseminated tumor cells (DTCs) from the primary tumor infiltrate secondary organs and reside there for varying lengths of time prior to forming new tumors. The time delay between infiltration and active proliferation, known as dormancy, mediates the length of the latency period. DTCs may undergo one of four fates post-infiltration: death, cellular dormancy, dormant micrometastasis, or invasive growth which, is in part, mediated by extracellular matrix (ECM) properties. Recapitulation of these cell states using engineered hydrogels could facilitate the systematic and controlled investigation of the mechanisms by which ECM properties influence DTC fate. Toward this goal, we implemented a set of sixteen hydrogels with systematic variations in chemical (ligand (RGDS) density and enzymatic degradability) and mechanical (elasticity, swelling, mesh size) properties to investigate their influence on the fate of encapsulated metastatic breast cancer cells, MDA-MB-231. Cell viability, apoptosis, proliferation, metabolic activity, and morphological measurements were acquired at five-day intervals over fifteen days in culture. Analysis of the phenotypic metrics indicated the presence of four different cell states that were classified as: (1) high growth, (2) moderate growth, (3) single cell, restricted survival, dormancy, or (4) balanced dormancy. Correlating hydrogel properties with the resultant cancer cell state indicated that ligand (RGDS) density and enzymatic degradability likely had the most influence on cell fate. Furthermore, we demonstrate the ability to reactivate cells from the single cell, dormant state to the high growth state through a dynamic increase in ligand (RGDS) density after forty days in culture. This tunable engineered hydrogel platform offers insight into matrix properties regulating tumor dormancy, and the dormancy-proliferation switch, and may provide future translational benefits toward development of anti-dormancy therapeutic strategies.

Metastatic Latency, a Veiled Threat

Metastatic relapse is observed in cancer patients with no clinical evidence of disease for months to decades after initial diagnosis and treatment. Disseminated cancer cells that are capable of entering reversible cell cycle arrest are believed to be responsible for these late metastatic relapses. Dynamic interactions between the latent disseminated tumor cells and their surrounding microenvironment aid cancer cell survival and facilitate escape from immune surveillance. Here, we highlight findings from preclinical models that provide a conceptual framework to define and target the latent metastatic phase of tumor progression. The hope is by identifying patients harboring latent metastatic cells and providing therapeutic options to eliminate metastatic seeds prior to their emergence will result in long lasting cures.

The Bone Extracellular Matrix as an Ideal Milieu for Cancer Cell Metastases

Bone is a preferential site for cancer metastases, including multiple myeloma, prostate, and breast cancers.The composition of bone, especially the extracellular matrix (ECM), make it an attractive site for cancer cell colonization and survival. The bone ECM is composed of living cells embedded within a matrix composed of both organic and inorganic components. Among the organic components, type I collagen provides the tensile strength of bone. Inorganic components, including hydroxyapatite crystals, are an integral component of bone and provide bone with its rigidity. Under normal circumstances, two of the main cell types in bone, the osteoblasts and osteoclasts, help to maintain bone homeostasis and remodeling through cellular communication and response to biophysical signals from the ECM. However, under pathological conditions, including osteoporosis and cancer, bone remodeling is dysregulated. Once in the bone matrix, disseminated tumor cells utilize normal products of bone remodeling, such as collagen type I, to fuel cancer cell proliferation and lesion outgrowth. Models to study the complex interactions between the bone matrix and metastatic cancer cells are limited. Advances in understanding the interactions between the bone ECM and bone metastatic cancer cells are necessary in order to both regulate and prevent metastatic cancer cell growth in bone.

Combinational phototherapy and hypoxia-activated chemotherapy favoring antitumor immune responses

Background: Tumor metastasis is responsible for most cancer death worldwide, which lacks curative treatment.

Purpose: The objective of this study was to eliminate tumor and control the development of tumor metastasis.

Methods: Herein, we demonstrated a smart nano-enabled platform, in which 2-[2-[2-chloro-3-[(1,3-dihydro-3,3-dimethyl-1-propyl-2h-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindolium iodide (IR780) and tirapazamine (TPZ) were co-loaded in poly(ε-caprolactone)-poly(ethylene glycol) (PEG-PCL) to form versatile nanoparticles (PEG-PCL-IR780-TPZ NPs).

Results: The intelligence of the system was reflected in the triggered and controlled engineering. Specially, PEG-PCL not only prolonged the circulation time of IR780 and TPZ but also promoted tumor accumulation of nanodrugs through enhanced permeability and retention (EPR) effect. Moreover, reactive oxygen species (ROS) generated by IR780 armed by an 808 nm laser irradiation evoked a cargo release. Meanwhile, IR780, as a mitochondria-targeting phototherapy agent exacerbated tumor hypoxic microenvironment and activated TPZ for accomplishing hypoxia-activated chemotherapy. Most significantly, IR780 was capable of triggering immunogenic cell death (ICD) during the synergic treatment. ICD biomarkers as a “danger signal” accelerated dendritic cells (DCs) maturation, and subsequently activated toxic T lymphocytes.

Conclusion: Eventually, antitumor immune responses stimulated by combinational phototherapy and hypoxia-activated chemotherapy revolutionized the current landscape of cancer treatment, strikingly inhibiting tumor metastasis and providing a promising prospect in the clinical application.

Biomaterial Platform To Establish a Hypoxic Metastatic Niche in Vivo

Hypoxia is a hallmark of tumor microenvironments, exerting wide-ranging impacts on key processes of tumor progression and metastasis. However, our understanding of how hypoxia regulates these processes has been based primarily on studying the effects of hypoxia within the primary tumor. Recently, an increasing number of studies have suggested the importance of hypoxic regulation within metastatic target organs, but hypoxic metastatic niches in the body are difficult to access with current imaging techniques, hampering detailed in vivo investigation of hypoxia at metastatic sites. Here, we report an engineered biomaterial scaffold that is able to establish an in vivo hypoxic metastatic niche in a readily accessible area, enabling the investigation of hypoxic regulation at a metastatic site. We engineered hypoxic environments within microporous poly(lactide-co-glycolide) (PLG) scaffolds, which have previously been shown to act as premetastatic niche mimics, via the addition of CoCl2, a hypoxia-mimetic agent. When implanted into the subcutaneous region of mice, CoCl2-containing PLG (Co-PLG) scaffolds established hypoxic microenvironments, as evidenced by the stabilization of hypoxia-inducible factor 1α (HIF1α) and increased blood vessel formation in vitro and in vivo. Furthermore, implanted Co-PLG scaffolds were able to recruit 4T1 metastatic breast cancer cells. These results demonstrate that Co-PLG scaffolds can establish an in vivo hypoxic metastatic niche, providing a novel platform to investigate hypoxic regulation of disseminated tumor cells (DTCs) at target organs.

Engineered In Vitro Models of Tumor Dormancy and Reactivation

Metastatic recurrence is a major hurdle to overcome for successful control of cancer-associated death. Residual tumor cells in the primary site, or disseminated tumor cells in secondary sites, can lie in a dormant state for long time periods, years to decades, before being reactivated into a proliferative growth state. The microenvironmental signals and biological mechanisms that mediate the fate of disseminated cancer cells with respect to cell death, single cell dormancy, tumor mass dormancy and metastatic growth, as well as the factors that induce reactivation, are discussed in this review. Emphasis is placed on engineered, in vitro, biomaterial-based approaches to model tumor dormancy and subsequent reactivation, with a focus on the roles of extracellular matrix, secondary cell types, biochemical signaling and drug treatment. A brief perspective of molecular targets and treatment approaches for dormant tumors is also presented. Advances in tissue-engineered platforms to induce, model, and monitor tumor dormancy and reactivation may provide much needed insight into the regulation of these processes and serve as drug discovery and testing platforms.

Charting the course of metastatic cells

An implantable biomaterial recruits rare tumor cells and enables long-term observation of microenvironmental evolution.