Tracing Quiescent Cancer Cells In Vivo

QCCs have long gained significant interest as potential "seeds" for recurrent cancers. Clinical evidence suggests that a subset of cancer cells exits the cell cycle and enters a quiescent state following anti-cancer treatment. These microscopic-residual QCCs are extremely challenging to trace and detect within patients. Additionally, QCCs resist conventional anti-cancer therapies due to the lack of cell activity. Notably, upon the unknown environmental cues in unknown time points, sometimes decades later, QCCs can reactivate, triggering cancer relapse at primary or secondary sites. Currently, no targeted therapies or diagnostic tools exist for QCCs, and their molecular regulatory mechanisms remain largely unknown. The major challenge in understanding QCCs lies in the limited availability of human-relevant pre-clinical models that trace and collect QCCs in vivo. This review provides an overview of existing QCC tracing systems and analyzes their limitations. It also cautiously proposes potential improvements for tracing QCCs in vivo based on recent advancements in QCC studies and lineage-tracing techniques. Developing human-relevant and easily accessible in vivo tracing systems will be a crucial step in advancing QCC diagnostics and therapeutic strategies.

Metabolic landscape of disseminated cancer dormancy

Cancer dormancy is a phenomenon defined by the entry of cancer cells into a reversible quiescent, nonproliferative state, and represents an essential part of the metastatic cascade responsible for cancer recurrence and mortality. Emerging evidence suggests that metabolic reprogramming plays a pivotal role in enabling entry, maintenance, and exit from dormancy in the face of the different environments of the metastatic cascade. Here, we review the current literature to understand the dynamics of metabolism during dormancy, highlighting its fine-tuning by the host micro- and macroenvironment, and put forward the importance of identifying metabolic vulnerabilities of the dormant state as therapeutic targets to eradicate recurrent disease.

CB-839 induces reversible dormancy in lung tumor-cells

Glutaminase inhibitors are currently being explored as potential treatments for cancer. This study aimed to elucidate the molecular mechanisms underlying the effects of CB-839 on lung tumor cell lines compared to non-tumor cell lines. Viability assays based on NADPH-dependent dehydrogenases activity, ATP energy production, or mitochondrial reductase activity were used to determine that CB-839 caused significant tumor cell specific inhibition of cellular functions. Clonogenic survival assay revealed a dose dependent reduction in clonogenic survival of various lung tumor cells presenting estimated IC50 values between 10 and 90 nM, while no effect on non-tumor cells was observed. CB-839 led to a 20% reduction in glutaminase (GLS1, a mitochondrial enzyme that catalyzes the conversion of glutamine to glutamate) activity, and a dose-dependent reduced glutamine consumption in tumor cells and had no effect on non-tumor cells. Cell cycle analysis showed the CB-839 did not lead to cell cycle arrest. Apoptosis and necrosis assays revealed an only slight increase in apoptosis in tumor cells. Furthermore, a trypan blue exclusion assay revealed about 40% growth reduction in tumor cells at 0.1-1 μM CB-839 treatment. Surprisingly, treated cells resumed normal growth when re-plated in a drug-free medium, demonstrating reversibility. In hypoxic conditions, CB-839's effect on clonogenic survival was amplified in a dose dependent manner consistent with increased role of GLS1 for energy production under hypoxic conditions. In conclusion, these results suggest CB-839 efficacy is linked to temporary and reversible reduction in glutamine utilization suggesting induction of dormancy.

Inertial effect of cell state velocity on the quiescence-proliferation fate decision

Energy landscapes can provide intuitive depictions of population heterogeneity and dynamics. However, it is unclear whether individual cell behavior, hypothesized to be determined by initial position and noise, is faithfully recapitulated. Using the p21-/Cdk2-dependent quiescence-proliferation decision in breast cancer dormancy as a testbed, we examined single-cell dynamics on the landscape when perturbed by hypoxia, a dormancy-inducing stress. Combining trajectory-based energy landscape generation with single-cell time-lapse microscopy, we found that a combination of initial position and velocity on a p21/Cdk2 landscape, but not position alone, was required to explain the observed cell fate heterogeneity under hypoxia. This is likely due to additional cell state information such as epigenetic features and/or other species encoded in velocity but missing in instantaneous position determined by p21 and Cdk2 levels alone. Here, velocity dependence manifested as inertia: cells with higher cell cycle velocities prior to hypoxia continued progressing along the cell cycle under hypoxia, resisting the change in landscape towards cell cycle exit. Such inertial effects may markedly influence cell fate trajectories in tumors and other dynamically changing microenvironments where cell state transitions are governed by coordination across several biochemical species.

Navigating the biophysical landscape: how physical cues steer the journey of bone metastatic tumor cells

Many tumors prefer to metastasize to bone, but the underlying mechanisms remain elusive. The human skeletal system has unique physical properties, that are distinct from other organs, which play a key role in directing the behavior of tumor cells within bone. Understanding the physical journey of tumor cells within bone is crucial. In this review we discuss bone metastasis in the context of how physical cues in the bone vasculature and bone marrow niche regulate the fate of tumor cells. Our objective is to inspire innovative diagnostic and therapeutic approaches for bone metastasis from a mechanobiological perspective.

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.

Nonepithelial cancer dissemination: specificities and challenges

Epithelial cancers have served as a paradigm to study tumor dissemination but recent data have highlighted significant differences with nonepithelial cancers. Here, we review the current knowledge on nonepithelial tumor dissemination, drawing examples from the latest developments in melanoma, glioma, and sarcoma research. We underscore the importance of the reactivation of developmental processes during cancer progression and describe the nongenetic mechanisms driving nonepithelial tumor spread. We also outline therapeutic opportunities and ongoing clinical approaches to fight disseminating cancers. Finally, we discuss remaining challenges and emerging questions in the field. Defining the core principles underlying nonepithelial cancer dissemination may uncover actionable vulnerabilities of metastatic tumors and help improve the prognosis of patients with cancer.

Revealing role of epigenetic modifiers and DNA oxidation in cell-autonomous regulation of Cancer stem cells

BACKGROUND

Breast cancer cells (BCCs) can remain undetected for decades in dormancy. These quiescent cells are similar to cancer stem cells (CSCs); hence their ability to initiate tertiary metastasis. Dormancy can be regulated by components of the tissue microenvironment such as bone marrow mesenchymal stem cells (MSCs) that release exosomes to dedifferentiate BCCs into CSCs. The exosomes cargo includes histone 3, lysine 4 (H3K4) methyltransferases - KMT2B and KMT2D. A less studied mechanism of CSC maintenance is the process of cell-autonomous regulation, leading us to examine the roles for KMT2B and KMT2D in sustaining CSCs, and their potential as drug targets.

METHODS

Use of pharmacological inhibitor of H3K4 (WDR5-0103), knockdown (KD) of KMT2B or KMT2D in BCCs, real time PCR, western blot, response to chemotherapy, RNA-seq, and flow cytometry for circulating markers of CSCs and DNA hydroxylases in BC patients. In vivo studies using a dormancy model studied the effects of KMT2B/D to chemotherapy.

RESULTS

H3K4 methyltransferases sustain cell autonomous regulation of CSCs, impart chemoresistance, maintain cycling quiescence, and reduce migration and proliferation of BCCs. In vivo studies validated KMT2's role in dormancy and identified these genes as potential drug targets. DNA methylase (DNMT), predicted within a network with KMT2 to regulate CSCs, was determined to sustain circulating CSC-like in the blood of patients.

CONCLUSION

H3K4 methyltransferases and DNA methylation mediate cell autonomous regulation to sustain CSC. The findings provide crucial insights into epigenetic regulatory mechanisms underlying BC dormancy with KMT2B and KMT2D as potential therapeutic targets, along with standard care. Stem cell and epigenetic markers in circulating BCCs could monitor treatment response and this could be significant for long BC remission to partly address health disparity.

Beta-blocker adjunct therapy as a prospective anti-metastatic with cardio-oncologic regulation

The prevailing treatment stratagem in cancer therapy still challenges the dilemma of a probable metastatic spread following an initial diagnosis. Including an anti-metastatic agent demands a significant focus to overrule the incidence of treatment failures. Adrenergic stimulation underlying the metastatic spread paved the way for beta blockers as a breakthrough in repurposing as an anti-metastatic agent. However, the current treatment approach fails to fully harness the versatile potential of the drug in inhibiting probable metastasis. The beta blockers were seen to show a myriad of grip over the pro-metastatic and prognostic parameters of the patient. Novel interventions in immune therapy, onco-hypertension, surgery-induced stress, induction of apoptosis and angiogenesis inhibition have been used as evidence to interpret our objective of discussing the potential adjuvant role of the drug in the existing anti-cancer regimens. Adding weight to the relative incidence of onco-hypertension as an unavoidable side effect from chemotherapy, the slot for an anti-hypertensive agent is necessitated, and we try to suggest beta-blockers to fill this position. However, pointing out the paucity in the clinical study, we aim to review the current status of beta blockers under this interest to state how the drug should be included as a drug of choice in every patient undergoing cancer treatment.

Tumor-mesothelium HOXA11-PDGF BB/TGF β1-miR-181a-5p-Egr1 feedforward amplifier circuity propels mesothelial fibrosis and peritoneal metastasis of gastric cancer

A proportion of gastric cancer (GC) patients suffer from peritoneal metastasis (PM) in the late stage of tumor and these patients have a poor prognosis. To provide more care for GC patient with PM, a deeper exploration of the molecular characteristics of GC-PM is needed. Here we performed the in vitro and in vivo study to illustrate the effect of HOXA11 over-expressed GC cells on peritoneal mesothelial cells (HMrSV5), transcriptomics analyses of HMrSV5 cells co-cultured with HOXA11 over-expressed GC cells, counterparts or alone, cytokine array analyses of serum-free culture medium of HOXA11 over-expressed GC cells, we validated our findings through genetic manipulation of HMrSV5 cells and neutralizing antibodies targeting cytokines secreted by HOXA11 over-expressed GC cells in vitro, as well as utilized human peritoneal metastatic lesions to validate expression of potential targets. We identified that HOXA11 over-expressed GC cells strongly propelled mesothelial fibrosis in vivo and in vitro, and HOXA11 regulated paracrine and autocrine of PDGF BB and TGF β1 in GC cells to propel mesothelial fibrosis. Meanwhile, HOXA11 over-expressed GC cells drove PDGF BB and TGF β1 secretion to activate developmental-process related genes in HMrSV5 cells, including Egr1, which processes dependent on miR-181a-5p. Then, Egr1 could mediate peritoneal mesothelial fibrosis. Correspondingly, Egr1 over-expressed HMrSV5 cells supported migration and peritoneal dissemination of GC cells. Together our results suggest that a feedforward amplifier circuity governing GC cells and mesothelial cells in peritoneum contribute to peritoneal metastasis of GC cells.

Cell fusion upregulates PD-L1 expression for evasion from immunosurveillance

MSCs (mesenchymal stem cells), responsible for tissue repair, rarely undergo cell fusion with somatic cells. Here, we show that ~5% of bladder cancer cells (UMUC-3) fuses with bone marrow-derived MSC (BM-MSC) in co-culture and maintains high tumorigenicity. In eleven fusion cell clones that have been established, Mb-scale deletions carried by the bladder cancer cells are mostly absent in the fusion cells, but copy number gains contributed by the cancer cells have stayed. Fusion cells exhibit increased populations of mitotic cells with 3-polar spindles, indicative of genomic instability. They grow faster in vitro and exhibit higher colony formation in anchorage-independent growth assay in soft agar than the parent UMUC-3 does. Fusion cells develop tumors, after 4 weeks of time lag, as efficiently as the parent UMUC-3 does in xenograft experiments. 264 genes are identified whose expression is specifically altered in the fusion cells. Many of them are interferon-stimulated genes (ISG), but are activated in a manner independent of interferon. Among them, we show that PD-L1 is induced in fusion cells, and its knockout decreases tumorigenesis in a xenograft model. PD-L1 is induced in a manner independent of STAT1 known to regulate PD-L1 expression, but is regulated by histone modification, and is likely to inhibit phagocytosis by PD1-expressing macrophages, thus protecting cancer cells from immunological attacks. The fusion cells overexpress multiple cytokines including CCL2 that cause tumor progression by converting infiltrating macrophages to tumor-associated-macrophage (TAM). The results present mechanisms of how cell fusion promotes tumorigenesis, revealing a novel link between cell fusion and PD-L1, and underscore the efficacy of cancer immunotherapy.

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.

Inertial effect of cell state velocity on the quiescence-proliferation fate decision in breast cancer

Energy landscapes can provide intuitive depictions of population heterogeneity and dynamics. However, it is unclear whether individual cell behavior, hypothesized to be determined by initial position and noise, is faithfully recapitulated. Using the p21-/Cdk2-dependent quiescence-proliferation decision in breast cancer dormancy as a testbed, we examined single-cell dynamics on the landscape when perturbed by hypoxia, a dormancy-inducing stress. Combining trajectory-based energy landscape generation with single-cell time-lapse microscopy, we found that initial position on a p21/Cdk2 landscape did not fully explain the observed cell-fate heterogeneity under hypoxia. Instead, cells with higher cell state velocities prior to hypoxia, influenced by epigenetic parameters, tended to remain proliferative under hypoxia. Thus, the fate decision on this landscape is significantly influenced by "inertia", a velocity-dependent ability to resist directional changes despite reshaping of the underlying landscape, superseding positional effects. Such inertial effects may markedly influence cell-fate trajectories in tumors and other dynamically changing microenvironments.

Genomic hallmarks and therapeutic implications of G0 cell cycle arrest in cancer

BACKGROUND

Therapy resistance in cancer is often driven by a subpopulation of cells that are temporarily arrested in a non-proliferative G0 state, which is difficult to capture and whose mutational drivers remain largely unknown.

RESULTS

We develop methodology to robustly identify this state from transcriptomic signals and characterise its prevalence and genomic constraints in solid primary tumours. We show that G0 arrest preferentially emerges in the context of more stable, less mutated genomes which maintain TP53 integrity and lack the hallmarks of DNA damage repair deficiency, while presenting increased APOBEC mutagenesis. We employ machine learning to uncover novel genomic dependencies of this process and validate the role of the centrosomal gene CEP89 as a modulator of proliferation and G0 arrest capacity. Lastly, we demonstrate that G0 arrest underlies unfavourable responses to various therapies exploiting cell cycle, kinase signalling and epigenetic mechanisms in single-cell data.

CONCLUSIONS

We propose a G0 arrest transcriptional signature that is linked with therapeutic resistance and can be used to further study and clinically track this state.

Outcomes and clinicopathologic characteristics associated with disseminated tumor cells in bone marrow after neoadjuvant chemotherapy in high-risk early stage breast cancer: the I-SPY SURMOUNT study

PURPOSE

Disseminated tumor cells (DTCs) expressing epithelial markers in the bone marrow are associated with recurrence and death, but little is known about risk factors predicting their occurrence. We detected EPCAM+/CD45- cells in bone marrow from early stage breast cancer patients after neoadjuvant chemotherapy (NAC) in the I-SPY 2 Trial and examined clinicopathologic factors and outcomes.

METHODS

Patients who signed consent for SURMOUNT, a sub-study of the I-SPY 2 Trial (NCT01042379), had bone marrow collected after NAC at the time of surgery. EPCAM+CD45- cells in 4 mLs of bone marrow aspirate were enumerated using immunomagnetic enrichment/flow cytometry (IE/FC). Patients with > 4.16 EPCAM+CD45- cells per mL of bone marrow were classified as DTC-positive. Tumor response was assessed using the residual cancer burden (RCB), a standardized approach to quantitate the extent of residual invasive cancer present in the breast and the axillary lymph nodes after NAC. Association of DTC-positivity with clinicopathologic variables and survival was examined.

RESULTS

A total of 73 patients were enrolled, 51 of whom had successful EPCAM+CD45- cell enumeration. Twenty-four of 51 (47.1%) were DTC-positive. The DTC-positivity rate was similar across receptor subtypes, but DTC-positive patients were significantly younger (p = 0.0239) and had larger pretreatment tumors compared to DTC-negative patients (p = 0.0319). Twenty of 51 (39.2%) achieved a pathologic complete response (pCR). While DTC-positivity was not associated with achieving pCR, it was significantly associated with higher RCB class (RCB-II/III, 62.5% vs. RCB-0/I; 33.3%; Chi-squared p = 0.0373). No significant correlation was observed between DTC-positivity and distant recurrence-free survival (p = 0.38, median follow-up = 3.2 years).

CONCLUSION

DTC-positivity at surgery after NAC was higher in younger patients, those with larger tumors, and those with residual disease at surgery.

Involvement of redox signalling in tumour cell dormancy and metastasis

Decades of research on oncogene-driven carcinogenesis and gene-expression regulatory networks only started to unveil the complexity of tumour cellular and molecular biology. This knowledge has been successfully implemented in the clinical practice to treat primary tumours. In contrast, much less progress has been made in the development of new therapies against metastasis, which are the main cause of cancer-related deaths. More recently, the role of epigenetic and microenviromental factors has been shown to play a key role in tumour progression. Free radicals are known to communicate the intracellular and extracellular compartments, acting as second messengers and exerting a decisive modulatory effect on tumour cell signalling. Depending on the cellular and molecular context, as well as the intracellular concentration of free radicals and the activation status of the antioxidant system of the cell, the signalling equilibrium can be tilted either towards tumour cell survival and progression or cell death. In this regard, recent advances in tumour cell biology and metastasis indicate that redox signalling is at the base of many cell-intrinsic and microenvironmental mechanisms that control disseminated tumour cell fate and metastasis. In this manuscript, we will review the current knowledge about redox signalling along the different phases of the metastatic cascade, including tumour cell dormancy, making emphasis on metabolism and the establishment of supportive microenvironmental connections, from a redox perspective.

KISS1 metastasis suppressor in tumor dormancy: a potential therapeutic target for metastatic cancers?

Present therapeutic approaches do not effectively target metastatic cancers, often limited by their inability to eliminate already-seeded non-proliferative, growth-arrested, or therapy-resistant tumor cells. Devising effective approaches targeting dormant tumor cells has been a focus of cancer clinicians for decades. However, progress has been limited due to limited understanding of the tumor dormancy process. Studies on tumor dormancy have picked up pace and have resulted in the identification of several regulators. This review focuses on KISS1, a metastasis suppressor gene that suppresses metastasis by keeping tumor cells in a state of dormancy at ectopic sites. The review explores mechanistic insights of KISS1 and discusses its potential application as a therapeutic against metastatic cancers by eliminating quiescent cells or inducing long-term dormancy in tumor cells.

Prognostic Value of Circulating Tumor DNA (ctDNA) in Oncogene-Driven NSCLC: Current Knowledge and Future Perspectives

As we enter an unprecedented era of personalized medicine, molecular targeted therapies have the potential to induce improved survival outcome in patients with non-small cell lung cancer (NSCLC). However, a significant percentage of oncogene-driven NSCLC patients will relapse even after definitive treatment, whereas chronic and durable response to targeted therapies is a less common event in advanced-stage lung cancer. This phenomenon could be attributed to minimal residual disease (MRD), defined as a population of disseminated tumor cells that survive during the course or after treatment, eventually leading to recurrence and limiting patient survival. Circulating tumor DNA (ctDNA) is a powerful biomarker for MRD detection and monitoring and is a non-invasive approach of treating cancer, and especially NSCLC, based on a real-time assessment of the tumor genomic landscape. In this review, we present the key findings of studies that have used ctDNA with regard to its prognostic value and in respect to the most common druggable driver mutations of genes in NSCLC, such as epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), c-ros oncogene 1 (ROS1), rearranged during transfection (RET), Kirsten rat sarcoma virus (KRAS), B-Raf proto-oncogene (BRAF), and mesenchymal epithelial transition factor receptor (MET).

Cancer bone metastases and nanotechnology-based treatment strategies

INTRODUCTION

Bone metastases have gradually been recognized as common metastases that affect patient quality of life and survival due to the increased incidence of primary tumors. However, there is still a lack of effective clinical treatment methods for bone metastases because of their particularity and complexity. Nanomedicine provides a new strategy for the treatment of bone metastases and shows great therapeutic potential. Thus, it is important to review the latest nanomedicine treatments for bone metastases.

AREAS COVERED

This review introduces the mechanistic relationships of bone metastases and summarizes nanotechnology-based treatments of bone metastases according to targeting strategies.

EXPERT OPINION

As we start to understand the mechanisms that enable bone metastases, we can better develop nanomedicine treatments. However, many of the mechanisms behind bone metastasis remain unclear. The application of nanomedicine shows promising anti-bone metastasis efficacy and helps to explore the pathogenesis of bone metastases. The optimized construction of nanomedicine according to bone metastatic properties is crucial to ensure the desired anti-bone metastasis efficacy and good biosafety. Therefore, the transition from bench to bedside still requires continued exploration.

Ontogeny and Vulnerabilities of Drug-Tolerant Persisters in HER2+ Breast Cancer

Resistance to targeted therapies is an important clinical problem in HER2-positive (HER2+) breast cancer. "Drug-tolerant persisters" (DTP), a subpopulation of cancer cells that survive via reversible, nongenetic mechanisms, are implicated in resistance to tyrosine kinase inhibitors (TKI) in other malignancies, but DTPs following HER2 TKI exposure have not been well characterized. We found that HER2 TKIs evoke DTPs with a luminal-like or a mesenchymal-like transcriptome. Lentiviral barcoding/single-cell RNA sequencing reveals that HER2+ breast cancer cells cycle stochastically through a "pre-DTP" state, characterized by a G0-like expression signature and enriched for diapause and/or senescence genes. Trajectory analysis/cell sorting shows that pre-DTPs preferentially yield DTPs upon HER2 TKI exposure. Cells with similar transcriptomes are present in HER2+ breast tumors and are associated with poor TKI response. Finally, biochemical experiments indicate that luminal-like DTPs survive via estrogen receptor-dependent induction of SGK3, leading to rewiring of the PI3K/AKT/mTORC1 pathway to enable AKT-independent mTORC1 activation.

SIGNIFICANCE

DTPs are implicated in resistance to anticancer therapies, but their ontogeny and vulnerabilities remain unclear. We find that HER2 TKI-DTPs emerge from stochastically arising primed cells ("pre-DTPs") that engage either of two distinct transcriptional programs upon TKI exposure. Our results provide new insights into DTP ontogeny and potential therapeutic vulnerabilities. This article is highlighted in the In This Issue feature, p. 873.

Dormancy: There and Back Again

Many cells are capable of maintaining viability in a non-dividing state with minimal metabolism under unfavorable conditions. These are germ cells, adult stem cells, and microorganisms. Unfortunately, a resting state, or dormancy, is possible for tuberculosis bacilli in a latent form of the disease and cancer cells, which may later form secondary tumors (metastases) in different parts of the body. These cells are resistant to therapy that can destroy intensely dividing cells and to the host immune system. A cascade of reactions that allows cells to enter and exit dormancy is triggered by regulatory factors from the microenvironment in niches that harbor the cells. A ratio of forbidding and permitting signals dictates whether the cells become dormant or start proliferation. The only difference between the cell dormancy regulation in normal and pathological conditions is that pathogens, mycobacteria, and cancer cells can influence their own fate by changing their microenvironment. Certain mechanisms of these processes are considered in the review.

Diversity of Vascular Niches in Bones and Joints During Homeostasis, Ageing, and Diseases

The bones and joints in the skeletal system are composed of diverse cell types, including vascular niches, bone cells, connective tissue cells and mineral deposits and regulate whole-body homeostasis. The capacity of maintaining strength and generation of blood lineages lies within the skeletal system. Bone harbours blood and immune cells and their progenitors, and vascular cells provide several immune cell type niches. Blood vessels in bone are phenotypically and functionally diverse, with distinct capillary subtypes exhibiting striking changes with age. The bone vasculature has a special impact on osteogenesis and haematopoiesis, and dysregulation of the vasculature is associated with diverse blood and bone diseases. Ageing is associated with perturbed haematopoiesis, loss of osteogenesis, increased adipogenesis and diminished immune response and immune cell production. Endothelial and perivascular cells impact immune cell production and play a crucial role during inflammation. Here, we discuss normal and maladapted vascular niches in bone during development, homeostasis, ageing and bone diseases such as rheumatoid arthritis and osteoarthritis. Further, we discuss the role of vascular niches during bone malignancy.

Engineering strategies to capture the biological and biophysical tumor microenvironment in vitro

Despite decades of research and advancements in diagnostic and treatment modalities, cancer remains a major global healthcare challenge. This is due in part to a lack of model systems that allow investigating the mechanisms underlying tumor development, progression, and therapy resistance under relevant conditions in vitro. Tumor cell interactions with their surroundings influence all stages of tumorigenesis and are shaped by both biological and biophysical cues including cell-cell and cell-extracellular matrix (ECM) interactions, tissue architecture and mechanics, and mass transport. Engineered tumor models provide promising platforms to elucidate the individual and combined contributions of these cues to tumor malignancy under controlled and physiologically relevant conditions. This review will summarize current knowledge of the biological and biophysical microenvironmental cues that influence tumor development and progression, present examples of in vitro model systems that are presently used to study these interactions and highlight advancements in tumor engineering approaches to further improve these technologies.

Combined Inhibition of SHP2 and CXCR1/2 Promotes Anti-Tumor T Cell Response in NSCLC

SHP2 inhibitors (SHP2i) alone and in various combinations are being tested in multiple tumors with over-activation of the RAS/ERK pathway. SHP2 plays critical roles in normal cell signaling; hence, SHP2is could influence the tumor microenvironment. We found that SHP2i treatment depleted alveolar and M2-like macrophages, induced tumor-intrinsic CCL5/CXCL10 secretion and promoted B and T lymphocyte infiltration in Kras- and Egfr-mutant non-small cell lung cancer (NSCLC). However, treatment also increased intratumor gMDSCs via tumor-intrinsic, NF-kB-dependent production of CXCR2 ligands. Other RAS/ERK pathway inhibitors also induced CXCR2 ligands and gMDSC influx in mice, and CXCR2 ligands were induced in tumors from patients on KRASG12C-inhibitor trials. Combined SHP2(SHP099)/CXCR1/2(SX682) inhibition depleted a specific cluster of S100a8/9high gMDSCs, generated Klrg1+ CD8+ effector T cells with a strong cytotoxic phenotype but expressing the checkpoint receptor NKG2A, and enhanced survival in Kras- and Egfr-mutant models. Our results argue for testing RAS/ERK pathway/CXCR1/2/NKG2A inhibitor combinations in NSCLC patients.

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

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

CIRCULATING TUMOR CELLS: WHERE WE LEFT OFF?

Cancer metastasis and recurrence are the leading causes of cancer-related death. Tumor cells which leave the primary or secondary tumors and shed into the bloodstream are called circulating tumor cells (CTC). These cells are the key drivers of cancer dissemination to surrounding tissues and to distant organs. The use of CTC in clinical practice necessitates the deep insight into their biology, as well as into their role in cancer evasion of immune surveillance, tumor resistance to chemo- radio- and immunotherapies and metastatic dormancy. Aim. The purpose of the work was to review the current knowledge on the CTC biology, as well as the prospects for their use for the diagnosis and targeted treatment of metastatic disease. Methods. The work proposed the integrative literature review using MEDLINE, Biological Abstracts and EMBASE databases. Results. This review summarizes and discusses historical milestones and current data concerning СTС biology, the main stages of their life cycle, their role in metastatic cascade, clinical prospects for their use as markers for the diagnosis and prognostication of the disease course, as well as targets for cancer treatment. Conclusions. Significant progress in the area of CTC biology and their use in cancer theranostics convincingly proved the attractiveness of these cells as targets for cancer prognosis and therapy. The effective use of liquid biopsy with quantitative and phenotypic characteristics of CTCs is impeded by the imperfection of the methodology for taking biological material and by the lack of reliable markers for assessing the metastatic potential of CTCs of various origins. The variety of mechanisms of tumor cells migration and invasion requires the development of complex therapeutic approaches for anti-metastatic therapy targeting CTCs. Efforts to address these key issues could help developing new and effective cancer treatment strategies.

Modeling metastasis in mice: a closer look

Unraveling the multifaceted cellular and physiological processes associated with metastasis is best achieved by using in vivo models that recapitulate the requisite tumor cell-intrinsic and -extrinsic mechanisms at the organismal level. We discuss the current status of mouse models of metastasis. We consider how mouse models can refine our understanding of the underlying biological and molecular processes that promote metastasis, and we envisage how the application of new technologies will further enhance investigations of metastasis at single-cell resolution in the context of the whole organism. Our view is that investigations based on state-of-the-art mouse models can propel a holistic understanding of the biology of metastasis, which will ultimately lead to the discovery of new therapeutic opportunities.

Prostate Cancer Dormancy and Reactivation in Bone Marrow

Prostate cancer has a variable clinical course, ranging from curable local disease to lethal metastatic spread. Eradicating metastatic cells is a unique challenge that is rarely met with the available therapies. Thus, targeting prostate cancer cells in earlier disease states is a crucial window of opportunity. Interestingly, cancer cells migrate from their primary site during pre-cancerous and malignant phases to seed secondary organs. These cells, known as disseminated cancer cells (DCCs), may remain dormant for months or decades before activating to form metastases. Bone marrow, a dormancy-permissive site, is the major organ for housed DCCs and eventual metastases in prostate cancer. The dynamic interplay between DCCs and the primary tumor microenvironment (TME), as well as that between DCCs and the secondary organ niche, controls the conversion between states of dormancy and activation. Here, we discuss recent discoveries that have improved our understanding of dormancy signaling and the role of the TME in modulating the epigenetic reprogramming of DCCs. We offer potential strategies to target DCCs in prostate cancer.

The Influence of Ligand Density and Degradability on Hydrogel Induced Breast Cancer Dormancy and Reactivation

The role of hydrogel properties in regulating the phenotype of triple negative metastatic breast cancer is investigated using four cell lines: the MDA-MB-231 parental line and three organotropic sublines BoM-1833 (bone-tropic), LM2-4175 (lung-tropic), and BrM2a-831 (brain-tropic). Each line is encapsulated and cultured for 15 days in three poly(ethylene glycol) (PEG)-based hydrogel formulations composed of proteolytically degradable PEG, integrin-ligating RGDS, and the non-degradable crosslinker N-vinyl pyrrolidone. Dormancy-associated metrics including viable cell density, proliferation, metabolism, apoptosis, chemoresistance, phosphorylated-ERK and -p38, and morphological characteristics are quantified. A multimetric classification approach is implemented to categorize each hydrogel-induced phenotype as: 1) growth, 2) balanced tumor dormancy, 3) balanced cellular dormancy, or 4) restricted survival, cellular dormancy. Hydrogels with high adhesivity and degradability promote growth. Hydrogels with no adhesivity, but high degradability, induce restricted survival, cellular dormancy in the parental line and balanced cellular dormancy in the organotropic lines. Hydrogels with reduced adhesivity and degradability induce balanced cellular dormancy in the parental and lung-tropic lines and balanced tumor mass dormancy in bone- and brain-tropic lines. The ability to induce escape from dormancy via dynamic incorporation of RGDS is also presented. These results demonstrate that ECM properties and organ-tropism synergistically regulate cancer cell phenotype and dormancy.

What Is the Storage Effect, Why Should It Occur in Cancers, and How Can It Inform Cancer Therapy?

Intratumor heterogeneity is a feature of cancer that is associated with progression, treatment resistance, and recurrence. However, the mechanisms that allow diverse cancer cell lineages to coexist remain poorly understood. The storage effect is a coexistence mechanism that has been proposed to explain the diversity of a variety of ecological communities, including coral reef fish, plankton, and desert annual plants. Three ingredients are required for there to be a storage effect: (1) temporal variability in the environment, (2) buffered population growth, and (3) species-specific environmental responses. In this article, we argue that these conditions are observed in cancers and that it is likely that the storage effect contributes to intratumor diversity. Data that show the temporal variation within the tumor microenvironment are needed to quantify how cancer cells respond to fluctuations in the tumor microenvironment and what impact this has on interactions among cancer cell types. The presence of a storage effect within a patient’s tumors could have a substantial impact on how we understand and treat cancer.

Autophagy and Cancer Dormancy

Metastasis and relapse account for the great majority of cancer-related deaths. Most metastatic lesions are micro metastases that have the capacity to remain in a non-dividing state called “dormancy” for months or even years. Commonly used anticancer drugs generally target actively dividing cancer cells. Therefore, cancer cells that remain in a dormant state evade conventional therapies and contribute to cancer recurrence. Cellular and molecular mechanisms of cancer dormancy are not fully understood. Recent studies indicate that a major cellular stress response mechanism, autophagy, plays an important role in the adaptation, survival and reactivation of dormant cells. In this review article, we will summarize accumulating knowledge about cellular and molecular mechanisms of cancer dormancy, and discuss the role and importance of autophagy in this context.

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

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

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.

Nutrient metabolism and cancer in the in vivo context: a metabolic game of give and take

Nutrients are indispensable resources that provide the macromolecular building blocks and energy requirements for sustaining cell growth and survival. Cancer cells require several key nutrients to fulfill their changing metabolic needs as they progress through stages of development. Moreover, both cell-intrinsic and microenvironment-influenced factors determine nutrient dependencies throughout cancer progression-for which a comprehensive characterization remains incomplete. In addition to the widely studied role of genetic alterations driving cancer metabolism, nutrient use in cancer tissue may be affected by several factors including the following: (i) diet, the primary source of bodily nutrients which influences circulating metabolite levels; (ii) tissue of origin, which can influence the tumor's reliance on specific nutrients to support cell metabolism and growth; (iii) local microenvironment, which dictates the accessibility of nutrients to tumor cells; (iv) tumor heterogeneity, which promotes metabolic plasticity and adaptation to nutrient demands; and (v) functional demand, which intensifies metabolic reprogramming to fuel the phenotypic changes required for invasion, growth, or survival. Here, we discuss the influence of these factors on nutrient metabolism and dependence during various steps of tumor development and progression.

Curcumin derivative C212 inhibits Hsp90 and eliminates both growing and quiescent leukemia cells in deep dormancy

BACKGROUND

Relapsed leukemia following initial therapeutic response and remission is difficult to treat and causes high patient mortality. Leukemia relapse is due to residual quiescent leukemia cells that escape conventional therapies and later reemerge. Eliminating not only growing but quiescent leukemia cells is critical to effectively treating leukemia and preventing its recurrence. Such dual targeting therapeutic agents, however, are lacking in the clinic. To start tackling this problem, encouraged by the promising anticancer effects of a set of curcumin derivatives in our earlier studies, we examined in this work the effects of a 4-arylmethyl curcumin derivative (C212) in eliminating both growing and quiescent leukemia cells.

METHODS

We analyzed the effects of C212 on the growth and viability of growing and quiescent leukemia cells using MTS, apoptosis, cell cycle and cell tracking assays. The effects of C212 on the quiescence depth of leukemia cells were measured using EdU incorporation assay upon growth stimulation. The mechanisms of C212-induced apoptosis and deep dormancy, particularly associated with its inhibition of Hsp90 activity, were studied using molecular docking, protein aggregation assay, and Western blot of client proteins.

RESULTS

C212, on the one hand, inhibits growing leukemia cells at a higher efficacy than curcumin by inducing apoptosis and G2/M accumulation; it, on the other hand, eliminates quiescent leukemia cells that are resistant to conventional treatments. Furthermore, C212 drives leukemia cells into and kills them at deep quiescence. Lastly, we show that C212 induces apoptosis and drives cells into deep dormancy at least partially by binding to and inhibiting Hsp90, leading to client protein degradation and protein aggregation.

CONCLUSION

C212 effectively eliminates both growing and quiescent leukemia cells by inhibiting Hsp90. The property of C212 to kill quiescent leukemia cells in deep dormancy avoids the risk associated with awaking therapy-resistant subpopulation of quiescent leukemia cells during treatments, which may lead to the development of novel therapies against leukemia relapse. Video abstract.

KISS1 in metastatic cancer research and treatment: potential and paradoxes

The significance of KISS1 goes beyond its original discovery as a metastasis suppressor. Its function as a neuropeptide involved in diverse physiologic processes is more well studied. Enthusiasm regarding KISS1 has cumulated in clinical trials in multiple fields related to reproduction and metabolism. But its cancer therapeutic space is unsettled. This review focuses on collating data from cancer and non-cancer fields in order to understand shared and disparate signaling that might inform clinical development in the cancer therapeutic and biomarker space. Research has focused on amino acid residues 68-121 (kisspeptin 54), binding to the KISS1 receptor and cellular responses. Evidence and counterevidence regarding this canonical pathway require closer look at the covariates so that the incredible potential of KISS1 can be realized.

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

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

Long non-coding NR2F1-AS1 is associated with tumor recurrence in estrogen receptor-positive breast cancers

The tenacity of late recurrence of estrogen receptor (ER)‐positive breast cancer remains a major clinical issue to overcome. The administration of endocrine therapies within the first 5 years substantially minimizes the risk of relapse; however, some tumors reappear 10–20 years after the initial diagnosis. Accumulating evidence has strengthened the notion that long noncoding RNAs (lncRNAs) are associated with cancer in various respects. Because lncRNAs may display high tissue/cell specificity, we hypothesized this might provide new insights to tumor recurrence. By comparing transcriptome profiles of 24 clinical primary tumors obtained from patients who developed distant metastases and patients with no signs of recurrence, we identified lncRNA NR2F1‐AS1 whose expression was associated with tumor recurrence. We revealed the relationship between NR2F1‐AS1 and the hormone receptor expressions in ER‐positive breast cancer cells. Gain of function of NR2F1‐AS1 steered cancer cells into quiescence‐like state by the upregulation of dormancy inducers and pluripotency markers, and activates representative events of the metastatic cascade. Our findings implicated NR2F1‐AS1 in the dynamics of tumor recurrence in ER‐positive breast cancers and introduce a new biomarker that holds a therapeutic potential, providing favorable prospects to be translated into the clinical field.

Molecular principles of metastasis: a hallmark of cancer revisited

Metastasis is the hallmark of cancer that is responsible for the greatest number of cancer-related deaths. Yet, it remains poorly understood. The continuous evolution of cancer biology research and the emergence of new paradigms in the study of metastasis have revealed some of the molecular underpinnings of this dissemination process. The invading tumor cell, on its way to the target site, interacts with other proteins and cells. Recognition of these interactions improved the understanding of some of the biological principles of the metastatic cell that govern its mobility and plasticity. Communication with the tumor microenvironment allows invading cancer cells to overcome stromal challenges, settle, and colonize. These characteristics of cancer cells are driven by genetic and epigenetic modifications within the tumor cell itself and its microenvironment. Establishing the biological mechanisms of the metastatic process is crucial in finding open therapeutic windows for successful interventions. In this review, the authors explore the recent advancements in the field of metastasis and highlight the latest insights that contribute to shaping this hallmark of cancer.

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.

Beyond Tumor Suppression: Senescence in Cancer Stemness and Tumor Dormancy

Here, we provide an overview of the importance of cellular fate in cancer as a group of diseases of abnormal cell growth. Tumor development and progression is a highly dynamic process, with several phases of evolution. The existing evidence about the origin and consequences of cancer cell fate specification (e.g., proliferation, senescence, stemness, dormancy, quiescence, and cell cycle re-entry) in the context of tumor formation and metastasis is discussed. The interplay between these dynamic tumor cell phenotypes, the microenvironment, and the immune system is also reviewed in relation to cancer. We focus on the role of senescence during cancer progression, with a special emphasis on its relationship with stemness and dormancy. Selective interventions on senescence and dormancy cell fates, including the specific targeting of cancer cell populations to prevent detrimental effects in aging and disease, are also reviewed. A new conceptual framework about the impact of synthetic lethal strategies by using senogenics and then senolytics is given, with the promise of future directions on innovative anticancer therapies.

Mechanical Forces as Determinants of Disseminated Metastatic Cell Fate

Disseminated metastatic cancer cells represent one of the most relevant causes of disease relapse and associated death for cancer patients, and a therapeutic target of the highest priority. Still, our understanding of how disseminated cancer cells survive in the foreign metastatic environment, and eventually cause metastatic outgrowth, remains rather limited. In this review we focus on the cell microenvironment as a key regulator of cell behavior at the metastatic site, and especially on the mechanical properties of the extracellular matrix and associated integrin signaling. We discuss available evidence pointing to a pervasive role of extracellular matrix (ECM) mechanical properties in regulating cancer cell proliferation and survival after dissemination, and propose that this might represent an important bottleneck for cells invading and establishing into a novel tissue. We point to the known molecular players, how these might contribute to modulate the mechanical properties of the metastatic environment, and the response of cells to these cues. Finally, we propose that emerging knowledge on the physical interaction of disseminated metastatic cells and on the downstream mechanotransduction pathways, including YAP/TAZ (Yes-associated protein-1 and WW-domain transcription activator 1) and MRTFs (Myocardin-related transcription factors), may help to identify novel approaches for therapy.

Thrombospondin in Tumor Microenvironment

Thrombospondins (TSPs) are multifaceted proteins that contribute to physiologic as well as pathologic conditions. Due to their multiple receptor-binding domains, TSPs display both oncogenic and tumor-suppressive qualities and are thus essential components of the extracellular matrix. Known for their antiangiogenic capacity, TSPs are an important component of the tumor microenvironment. The N- and C-terminal domains of TSP are, respectively, involved in cell adhesion and spreading, an important feature of wound healing as well as cancer cell migration. Previously known for the activation of TGF-β to promote tumor growth and inflammation, TSP-1 has recently been found to be transcriptionally induced by TGF-β, implying the presence of a possible feedback loop. TSP-1 is an endogenous inhibitor of T cells and also mediates its immunosuppressive effects via induction of Tregs. Given the diverse roles of TSPs in the tumor microenvironment, many therapeutic strategies have utilized TSP-mimetic peptides or antibody blockade as anti-metastatic approaches. This chapter discusses the diverse structural domains, functional implications, and anti-metastatic therapies in the context of the role of TSP in the tumor microenvironment.

Tumor Cell Dormancy-Triggered by the Niche

Dormant cancer cells often survive treatment and increase the risk for tumor relapse, associated with dismal prognosis. Two recent papers describe mechanisms used by the bone marrow niche to regulate leukemia dormancy. The findings provide a molecular basis for niche-targeting therapies that may enable elimination of dormant tumor cells.

A new bioavailable fenretinide formulation with antiproliferative, antimetabolic, and cytotoxic effects on solid tumors

Fenretinide is a synthetic retinoid characterized by anticancer activity in preclinical models and favorable toxicological profile, but also by a low bioavailability that hindered its clinical efficacy in former clinical trials. We developed a new formulation of fenretinide complexed with 2-hydroxypropyl-beta-cyclodextrin (nanofenretinide) characterized by an increased bioavailability and therapeutic efficacy. Nanofenretinide was active in cell lines derived from multiple solid tumors, in primary spheroid cultures and in xenografts of lung and colorectal cancer, where it inhibited tumor growth independently from the mutational status of tumor cells. A global profiling of pathways activated by nanofenretinide was performed by reverse-phase proteomic arrays and lipid analysis, revealing widespread repression of the mTOR pathway, activation of apoptotic, autophagic and DNA damage signals and massive production of dihydroceramide, a bioactive lipid with pleiotropic effects on several biological processes. In cells that survived nanofenretinide treatment there was a decrease of factors involved in cell cycle progression and an increase in the levels of p16 and phosphorylated p38 MAPK with consequent block in G0 and early G1. The capacity of nanofenretinide to induce cancer cell death and quiescence, together with its elevated bioavailability and broad antitumor activity indicate its potential use in cancer treatment and chemoprevention.

Stem Cell Plasticity and Dormancy in the Development of Cancer Therapy Resistance

Cancer treatment with either standard chemotherapy or targeted agents often results in the emergence of drug-refractory cell populations, ultimately leading to therapy failure. The biological features of drug resistant cells are largely overlapping with those of cancer stem cells and include heterogeneity, plasticity, self-renewal ability, and tumor-initiating capacity. Moreover, drug resistance is usually characterized by a suppression of proliferation that can manifest as quiescence, dormancy, senescence, or proliferative slowdown. Alterations in key cellular pathways such as autophagy, unfolded protein response or redox signaling, as well as metabolic adaptations also contribute to the establishment of drug resistance, thus representing attractive therapeutic targets. Moreover, a complex interplay of drug resistant cells with the micro/macroenvironment and with the immune system plays a key role in dictating and maintaining the resistant phenotype. Recent studies have challenged traditional views of cancer drug resistance providing innovative perspectives, establishing new connections between drug resistant cells and their environment and indicating unexpected therapeutic strategies. In this review we discuss recent advancements in understanding the mechanisms underlying drug resistance and we report novel targeting agents able to overcome the drug resistant status, with particular focus on strategies directed against dormant cells. Research on drug resistant cancer cells will take us one step forward toward the development of novel treatment approaches and the improvement of relapse-free survival in solid and hematological cancer patients.

Is cell migration a selectable trait in the natural evolution of cancer development?

Selective evolutionary pressure shapes the processes and genes that enable cancer survival and expansion in a tumour-suppressive environment. A distinguishing lethal feature of malignant cancer is its dissemination and seeding of metastatic foci. A key requirement for this process is the acquisition of a migratory/invasive ability. However, how the migratory phenotype is selected for during the natural evolution of cancer and what advantage, if any, it might provide to the growing malignant cells remain open issues. In this opinion piece, we discuss three possible answers to these issues. We will examine lines of evidence from mathematical modelling of cancer evolution that indicate that migration is an intrinsic selectable property of malignant cells that directly impacts on growth dynamics and cancer geometry. Second, we will argue that migratory phenotypes can emerge as an adaptive response to unfavourable growth conditions and endow cells not only with the ability to move/invade, but also with specific metastatic traits, including drug resistance, self-renewal and survival. Finally, we will discuss the possibility that migratory phenotypes are coincidental events that emerge by happenstance in the natural evolution of cancer. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

Defining the Hallmarks of Metastasis

Metastasis is the primary cause of cancer morbidity and mortality. The process involves a complex interplay between intrinsic tumor cell properties as well as interactions between cancer cells and multiple microenvironments. The outcome is the development of a nearby or distant discontiguous secondary mass. To successfully disseminate, metastatic cells acquire properties in addition to those necessary to become neoplastic. Heterogeneity in mechanisms involved, routes of dissemination, redundancy of molecular pathways that can be utilized, and the ability to piggyback on the actions of surrounding stromal cells makes defining the hallmarks of metastasis extraordinarily challenging. Nonetheless, this review identifies four distinguishing features that are required: motility and invasion, ability to modulate the secondary site or local microenvironments, plasticity, and ability to colonize secondary tissues. By defining these first principles of metastasis, we provide the means for focusing efforts on the aspects of metastasis that will improve patient outcomes.

Lymph node resection induces the activation of tumor cells in the lungs

Lymph node (LN) dissection is a crucial procedure for cancer staging, diagnosis and treatment, and for predicting patient survival. Activation of lung metastatic lesions after LN dissection has been described for head and neck cancer and breast cancer. Preclinical studies have reported that dissection of a tumor‐bearing LN is involved in the activation and rapid growth of latent tumor metastases in distant organs, but it is also important to understand how normal (non‐tumor‐bearing) LN resection influences secondary cancer formation. Here, we describe how the resection of tumor‐bearing and non‐tumor‐bearing LN affects distant metastases in MXH10/Mo‐lpr/lpr mice. Tumor cells were administered intravenously and/or intranodally into the right subiliac lymph node (SiLN) to create a mouse model of lung metastasis. Luciferase imaging revealed that tumor cells in the lung were activated after resection of the SiLN, irrespective of whether it contained tumor cells. No luciferase activity was detected in the lungs of mice that did not undergo LN resection (excluding the intravenous inoculation group). Our results indicate that resection of an LN can activate distant metastases regardless of whether the LN contains tumor cells. Hence, lung metastatic lesions are suppressed while metastatic LN are present but activated after LN resection. If this phenomenon occurs in patients with cancer, it is likely that lung metastatic lesions may be activated by elective LN dissection in clinical N0 cases. The development of minimally invasive cancer therapy without surgery would help to minimize the risk of activation of distant metastatic lesions by LN resection.

Cross-talk between cancer cells and their neighbors via miRNA in extracellular vesicles: an emerging player in cancer metastasis

Cancer metastasis is the major cause of mortality in cancer cases and is responsible for cancer deaths. It is known that cancer cells communicate with surrounding microenvironmental cells, such as fibroblast cells, immune cells, and endothelial cells, to create a cancer microenvironment for their progression. Extracellular vesicles (EVs) are small vesicles that can be secreted by most types of cells and play an important role in cell-to-cell communications via transferring bioactive cargos, including variable RNAs, such as microRNAs (miRNAs), to recipient cells. miRNAs are a class of small noncoding RNAs that posttranscriptionally regulate gene expression. The transfer of them to recipient cells influences the metastatic process of primary tumors. In this review, we summarize the function of miRNAs packaged in EVs in cancer metastasis and discuss the clinical utility of miRNAs in EVs.