TGF-β2 dictates disseminated tumour cell fate in target organs through TGF-β-RIII and p38α/β signalling

Dormancy-inducing 3D engineered matrix uncovers mechanosensitive and drug-protective FHL2-p21 signaling axis

Solid cancers frequently relapse with distant metastasis, despite local and systemic treatment. Cellular dormancy has been identified as an important mechanism underlying drug resistance enabling late relapse. Therefore, relapse from invisible, minimal residual cancer of seemingly disease-free patients call for in vitro models of dormant cells suited for drug discovery. Here, we explore dormancy-inducing 3D engineered matrices, which generate mechanical confinement and induce growth arrest and survival against chemotherapy in cancer cells. We characterized the dormant phenotype of solitary cells by P-ERKlow:P-p38high dormancy signaling ratio, along with Ki67- expression. As underlying mechanism, we identified stiffness-dependent nuclear localization of the four-and-a-half LIM domain 2 (FHL2) protein, leading to p53-independent high p21Cip1/Waf1 nuclear expression, validated in murine and human tissue. Suggestive of a resistance-causing role, cells in the dormancy-inducing matrix became sensitive against chemotherapy upon FHL2 down-regulation. Thus, our biomaterial-based approach will enable systematic screens for previously unidentified compounds suited to eradicate potentially relapsing dormant cancer cells.

Cancer-associated fibroblasts-derived CXCL1 activates DEC2-mediated dormancy in oral squamous cell carcinoma

Cancer-associated fibroblasts (CAFs) are known to play an important role in cancer progression, but their effects on tumor cell dormancy and the underlying mechanisms remain to be explored. Here, we aimed to dissect the intercellular communication between CAFs and oral squamous cell carcinoma (OSCC) cells under cellular dormancy. In this study, we investigated 61 OSCC patients and found that low expression of Differentiated Embryonic Chondrocyte gene 2 (DEC2) was closely associated with tumor recurrence, cisplatin chemotherapy administration, and infiltration of CAFs. Overexpression of DEC2 promoted the invasion and migration ability of OSCC cells but inhibited their proliferation and glucose metabolism, and characterized them as dormant and cisplatin-resistant cells. C-X-C motif ligand 1 (CXCL1) from CAFs was found to down-regulate DEC2 expression in OSCC cells, ultimately awakening dormant cells and leading to tumor recurrence, which was validated in vitro and in vivo. In conclusion, CAFs-derived CXCL1 downregulated DEC2 and "interrupted" DEC2-mediated OSCC cell dormancy, which may be a mechanism by which CAFs modulate OSCC cell dormancy and contribute to the development of new therapies for OSCC.

Autophagy is required for mammary tumor recurrence by promoting dormant tumor cell survival following therapy

BACKGROUND

Mortality from breast cancer is principally due to tumor recurrence. Recurrent breast cancers arise from the pool of residual tumor cells, termed minimal residual disease, that survive treatment and may exist in a dormant state for 20 years or more following treatment of the primary tumor. As recurrent breast cancer is typically incurable, understanding the mechanisms underlying dormant tumor cell survival is a critical priority in breast cancer research. The importance of this goal is further underscored by emerging evidence suggesting that targeting dormant residual tumor cells in early-stage breast cancer patients may be a means to prevent tumor recurrence and its associated mortality. In this regard, the role of autophagy in dormant tumor cell survival and recurrence remains unresolved, with conflicting reports of both pro-survival/recurrence-promoting and pro-death/recurrence-suppressing effects of autophagy inhibition in dormant tumor cells. Resolving this question has important clinical implications.

METHODS

We used genetically engineered mouse models that faithfully recapitulate key features of human breast cancer progression, including minimal residual disease, tumor dormancy, and recurrence. We used genetic and pharmacological approaches to inhibit autophagy, including treatment with chloroquine, genetic knockdown of ATG5 or ATG7, or deletion of BECN and determined their effects on dormant tumor cell survival and recurrence.

RESULTS

We demonstrate that the survival and recurrence of dormant mammary tumor cells following therapy is dependent upon autophagy. We find that autophagy is induced in vivo following HER2 downregulation and remains activated in dormant residual tumor cells. Using genetic and pharmacological approaches we show that inhibiting autophagy by chloroquine administration, ATG5 or ATG7 knockdown, or deletion of a single allele of the tumor suppressor Beclin 1 is sufficient to inhibit mammary tumor recurrence, and that autophagy inhibition results in the death of dormant mammary tumor cells in vivo.

CONCLUSIONS

Our findings demonstrate a pro-tumorigenic role for autophagy in tumor dormancy and recurrence following therapy, reveal that dormant tumor cells are uniquely reliant upon autophagy for their survival, and indicate that targeting dormant residual tumor cells by inhibiting autophagy impairs tumor recurrence. These studies identify a pharmacological target for a cellular state that is resistant to commonly used anti-neoplastic agents and suggest autophagy inhibition as an approach to reduce dormant minimal residual disease in order to prevent lethal tumor recurrence.

TGF-β induces an atypical EMT to evade immune mechanosurveillance in lung adenocarcinoma dormant metastasis

The heterogeneity of epithelial-to-mesenchymal transition (EMT) programs is manifest in the diverse EMT-like phenotypes occurring during tumor progression. However, little is known about the mechanistic basis and functional role of specific forms of EMT in cancer. Here we address this question in lung adenocarcinoma (LUAD) cells that enter a dormancy period in response to TGF-β upon disseminating to distant sites. LUAD cells with the capacity to enter dormancy are characterized by expression of SOX2 and NKX2-1 primitive progenitor markers. In these cells, TGF-β induces growth inhibition accompanied by a full EMT response that subsequently transitions into an atypical mesenchymal state of round morphology and lacking actin stress fibers. TGF-β induces this transition by driving the expression of the actin-depolymerizing factor gelsolin, which changes a migratory, stress fiber-rich mesenchymal phenotype into a cortical actin-rich, spheroidal state. This transition lowers the biomechanical stiffness of metastatic progenitors, protecting them from killing by mechanosensitive cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Inhibiting this actin depolymerization process clears tissues of dormant metastatic cells. Thus, LUAD primitive progenitors undergo an atypical EMT as part of a strategy to evade immune-mediated elimination during dormancy. Our results provide a mechanistic basis and functional role of this atypical EMT response of LUAD metastatic progenitors and further illuminate the role of TGF-β as a crucial driver of immune evasive metastatic dormancy.

Lung-resident alveolar macrophages regulate the timing of breast cancer metastasis

Breast disseminated cancer cells (DCCs) can remain dormant in the lungs for extended periods, but the mechanisms limiting their expansion are not well understood. Research indicates that tissue-resident alveolar macrophages suppress breast cancer metastasis in lung alveoli by inducing dormancy. Through ligand-receptor mapping and intravital imaging, it was found that alveolar macrophages express transforming growth factor (TGF)-β2. This expression, along with persistent macrophage-cancer cell interactions via the TGF-βRIII receptor, maintains cancer cells in a dormant state. Depleting alveolar macrophages or losing the TGF-β2 receptor in cancer cells triggers metastatic awakening. Aggressive breast cancer cells are either suppressed by alveolar macrophages or evade this suppression by avoiding interaction and downregulating the TGF-β2 receptor. Restoring TGF-βRIII in aggressive cells reinstates TGF-β2-mediated macrophage growth suppression. Thus, alveolar macrophages act as a metastasis immune barrier, and downregulation of TGF-β2 signaling allows cancer cells to overcome macrophage-mediated growth suppression.

From rest to repair: Safeguarding genomic integrity in quiescent cells

Quiescence is an important non-pathological state in which cells pause cell cycle progression temporarily, sometimes for decades, until they receive appropriate proliferative stimuli. Quiescent cells make up a significant proportion of the body, and maintaining genomic integrity during quiescence is crucial for tissue structure and function. While cells in quiescence are spared from DNA damage associated with DNA replication or mitosis, they are still exposed to various sources of endogenous DNA damage, including those induced by normal transcription and metabolism. As such, it is vital that cells retain their capacity to effectively repair lesions that may occur and return to the cell cycle without losing their cellular properties. Notably, while DNA repair pathways are often found to be downregulated in quiescent cells, emerging evidence suggests the presence of active or differentially regulated repair mechanisms. This review aims to provide a current understanding of DNA repair processes during quiescence in mammalian systems and sheds light on the potential pathological consequences of inefficient or inaccurate repair in quiescent cells.

Single-Cell Analysis of Bone-Marrow-Disseminated Tumour Cells

Metastasis frequently targets bones, where cancer cells from the primary tumour migrate to the bone marrow, initiating new tumour growth. Not only is bone the most common site for metastasis, but it also often marks the first site of metastatic recurrence. Despite causing over 90% of cancer-related deaths, effective treatments for bone metastasis are lacking, with current approaches mainly focusing on palliative care. Circulating tumour cells (CTCs) are pivotal in metastasis, originating from primary tumours and circulating in the bloodstream. They facilitate metastasis through molecular interactions with the bone marrow environment, involving direct cell-to-cell contacts and signalling molecules. CTCs infiltrate the bone marrow, transforming into disseminated tumour cells (DTCs). While some DTCs remain dormant, others become activated, leading to metastatic growth. The presence of DTCs in the bone marrow strongly correlates with future bone and visceral metastases. Research on CTCs in peripheral blood has shed light on their release mechanisms, yet investigations into bone marrow DTCs have been limited. Challenges include the invasiveness of bone marrow aspiration and the rarity of DTCs, complicating their isolation. However, advancements in single-cell analysis have facilitated insights into these elusive cells. This review will summarize recent advancements in understanding bone marrow DTCs using single-cell analysis techniques.

A SRC-slug-TGFβ2 signaling axis drives poor outcomes in triple-negative breast cancers

BACKGROUND

Treatment options for the Triple-Negative Breast Cancer (TNBC) subtype remain limited and the outcome for patients with advanced TNBC is very poor. The standard of care is chemotherapy, but approximately 50% of tumors develop resistance.

METHODS

We performed gene expression profiling of 58 TNBC tumor samples by microarray, comparing chemosensitive with chemoresistant tumors, which revealed that one of the top upregulated genes was TGFβ2. A connectivity mapping bioinformatics analysis predicted that the SRC inhibitor Dasatinib was a potential pharmacological inhibitor of chemoresistant TNBCs. Claudin-low TNBC cell lines were selected to represent poor-outcome, chemoresistant TNBC, for in vitro experiments and in vivo models.

RESULTS

In vitro, we identified a signaling axis linking SRC, AKT and ERK2, which in turn upregulated the stability of the transcription factors, Slug and Snail. Slug was shown to repress TGFβ2-antisense 1 to promote TGFβ2 signaling, upregulating cell survival via apoptosis and DNA-damage responses. Additionally, an orthotopic allograft in vivo model demonstrated that the SRC inhibitor Dasatinib reduced tumor growth as a single agent, and enhanced responses to the TNBC mainstay drug, Epirubicin.

CONCLUSION

Targeting the SRC-Slug-TGFβ2 axis may therefore lead to better treatment options and improve patient outcomes in this highly aggressive subpopulation of TNBCs.

BHLHE41, a transcriptional repressor involved in physiological processes and tumor development

BHLHE41 is a nuclear transcriptional repressor that belongs to the basic helix-loop-helix protein superfamily. BHLHE41 expression tends to be restricted to specific tissues and is regulated by environmental cues and biological events. BHLHE41 homodimerizes or heterodimerizes with various partners, influencing its transcription factor function. BHLHE41 is involved in the regulation of many physiological processes implicated in tissue/organ homeostasis, such as myogenesis, adipogenesis, circadian rhythms and DNA repair. At cellular level, BHLHE41 is involved in the regulation of mesenchymal stem cell properties, tissue-specific macrophage functions and lymphoid lineage physiology. In several cancer types, BHLHE41 modulates the expression of different transcriptional programs influencing cell cycle control, apoptosis, invasiveness, epithelial to mesenchymal transition and hypoxia response in the tumor environment. Depending on the cancer cell type, BHLHE41 can act as a tumor suppressor or an oncogene, and could be a target for innovative therapies. This review summarizes the available knowledge on BHLHE41 structure, biological functions, regulation and potential partners, as well as its role in physiological processes, and its implication in major cancer steps.

Bone niches in the regulation of tumour cell dormancy

Secondary metastases, accounting for 90 % of cancer-related deaths, pose a formidable challenge in cancer treatment, with bone being a prevalent site. Importantly, tumours may relapse, often in the skeleton even after successful eradication of the primary tumour, indicating that tumour cells may lay dormant within bone for extended periods of time. This review summarises recent findings in the mechanisms underlying tumour cell dormancy and the role of bone cells in this process. Hematopoietic stem cell (HSC) niches in bone provide a model for understanding regulatory microenvironments. Dormant tumour cells have been shown to exploit similar niches, with evidence suggesting interactions with osteoblast-lineage cells and other stromal cells via CXCL12-CXCR4, integrins, and TAM receptor signalling, especially through GAS6-AXL, led to dormancy, with exit of dormancy potentially regulated by osteoclastic bone resorption and neuronal signalling. A comprehensive understanding of dormant tumour cell niches and their regulatory mechanisms is essential for developing targeted therapies, a critical step towards eradicating metastatic tumours and stopping disease relapse.

Collagen XVII promotes dormancy of colorectal cancer cells by activating mTORC2 signaling

Tumor dormancy is the underpinning for cancer relapse and chemoresistance, leading to massive cancer-related death in colorectal cancer (CRC). However, our comprehension of the mechanisms dictating tumor dormancy and strategies for eliminating dormant tumor cells remains restricted. In this study, we identified that collagen XVII (COL17A1), a hemidesmosomal transmembrane protein, can promote the dormancy of CRC cells. The upregulation of COL17A1 was observed to prolong quiescence periods and diminish drug susceptibility of CRC cells. Mechanistically, COL17A1 acts as a scaffold, enhancing the crosstalk between mTORC2 and Akt, thereby instigating the mTORC2-mediated dormant signaling. Notably, the activation of mTORC2 is contingent upon the intracellular domain of COL17A1, regardless of its ectodomain shedding. Our findings underscore a pivotal role of the COL17A1-mTORC2 axis in CRC dormancy, suggesting that mTORC2-specific inhibitors may hold therapeutic prospects for the eradication of dormant tumor cells.

The ubiquitin-proteasome system in the regulation of tumor dormancy and recurrence

Tumor recurrence is a mechanism triggered in sparse populations of cancer cells that usually remain in a quiescent state after strict stress and/or therapeutic factors, which is affected by a variety of autocrine and microenvironmental cues. Despite thorough investigations, the biology of dormant and/or cancer stem cells is still not fully elucidated, as for the mechanisms of their reawakening, while only the major molecular patterns driving the relapse process have been identified to date. These molecular patterns profoundly interfere with the elements of cellular proteostasis systems that support the efficiency of the recurrence process. As a major proteostasis machinery, we review the role of the ubiquitin-proteasome system (UPS) in tumor cell dormancy and reawakening, devoting particular attention to the functions of its components, E3 ligases, deubiquitinating enzymes and proteasomes in cancer recurrence. We demonstrate how UPS components functionally or mechanistically interact with the pivotal proteins implicated in the recurrence program and reveal that modulators of the UPS hold promise to become an efficient adjuvant therapy for eradicating refractory tumor cells to impede tumor relapse.

Educate, not kill: treating cancer without triggering its defenses

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

Mesenchymal Stem Cell-Secreted Exosomes and Soluble Signals Regulate Breast Cancer Metastatic Dormancy: Current Progress and Future Outlook

Breast cancer is most common in women, and in most cases there is no evidence of spread and the primary tumor is removed, resulting in a 'cure'. However, in 10% to 30% of these women, distant metastases recur after years to decades. This is due to breast cancer cells disseminating to distant organs and lying quiescent. This is called metastatic dormancy. Dormant cells are generally resistant to chemotherapy, hormone therapy and immunotherapy as they are non-cycling and receive survival signals from their microenvironment. In this state, they are clinically irrelevant. However, risk factors, including aging and inflammation can awaken dormant cells and cause breast cancer recurrences, which may happen even more than ten years after the primary tumor removal. How these breast cancer cells remain in dormancy is being unraveled. A key element appears to be the mesenchymal stem cells in the bone marrow that have been shown to promote breast cancer metastatic dormancy in recent studies. Indirect co-culture, direct co-culture and exosome extraction were conducted to investigate the modes of signal operation. Multiple signaling molecules act in this process including both protein factors and microRNAs. We integrate these studies to summarize current findings and gaps in the field and suggest future research directions for this field.

Tumor Dormancy and Reactivation: The Role of Heat Shock Proteins

Tumors are a heterogeneous group of cell masses originating in various organs or tissues. The cellular composition of the tumor cell mass interacts in an intricate manner, influenced by humoral, genetic, molecular, and tumor microenvironment cues that dictate tumor growth or suppression. As a result, tumors undergo a period of a dormant state before their clinically discernible stage, which surpasses the clinical dormancy threshold. Moreover, as a genetically imprinted strategy, early-seeder cells, a distinct population of tumor cells, break off to dock nearby or extravasate into blood vessels to secondary tissues, where they form disseminated solitary dormant tumor cells with reversible capacity. Among the various mechanisms underlying the dormant tumor mass and dormant tumor cell formation, heat shock proteins (HSPs) might play one of the most important roles in how the dormancy program plays out. It is known that numerous aberrant cellular processes, such as malignant transformation, cancer cell stemness, tumor invasion, metastasis, angiogenesis, and signaling pathway maintenance, are influenced by the HSPs. An accumulating body of knowledge suggests that HSPs may be involved in the angiogenic switch, immune editing, and extracellular matrix (ECM) remodeling cascades, crucial genetically imprinted strategies important to the tumor dormancy initiation and dormancy maintenance program. In this review, we highlight the biological events that orchestrate the dormancy state and the body of work that has been conducted on the dynamics of HSPs in a tumor mass, as well as tumor cell dormancy and reactivation. Additionally, we propose a conceptual framework that could possibly underlie dormant tumor reactivation in metastatic relapse.

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

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

Lung endothelium exploits susceptible tumor cell states to instruct metastatic latency

In metastasis, cancer cells travel around the circulation to colonize distant sites. Due to the rarity of these events, the immediate fates of metastasizing tumor cells (mTCs) are poorly understood while the role of the endothelium as a dissemination interface remains elusive. Using a newly developed combinatorial mTC enrichment approach, we provide a transcriptional blueprint of the early colonization process. Following their arrest at the metastatic site, mTCs were found to either proliferate intravascularly or extravasate, thereby establishing metastatic latency. Endothelial-derived angiocrine Wnt factors drive this bifurcation, instructing mTCs to follow the extravasation-latency route. Surprisingly, mTC responsiveness towards niche-derived Wnt was established at the epigenetic level, which predetermined tumor cell behavior. Whereas hypomethylation enabled high Wnt activity leading to metastatic latency, methylated mTCs exhibited low activity and proliferated intravascularly. Collectively the data identify the predetermined methylation status of disseminated tumor cells as a key regulator of mTC behavior in the metastatic niche.

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.

CRISPR/Cas9 screenings unearth protein arginine methyltransferase 7 as a novel essential gene in prostate cancer metastasis

Due to the limited effectiveness of current treatments, the survival rate of patients with metastatic castration-resistant prostate cancer (mCRPC) is significantly reduced. Consequently, it is imperative to identify novel therapeutic targets for managing these patients. Since the invasive ability of cells is crucial for establishing and maintaining metastasis, the aim of this study was to identify the essential regulators of invasive abilities of mCRPC cells by conducting two independent high-throughput CRISPR/Cas9 screenings. Furthermore, some of the top hits were validated using siRNA technology, with protein arginine methyltransferase 7 (PRMT7) emerging as the most promising candidate. We demonstrated that its inhibition or depletion via genetic or pharmacological approaches significantly reduces invasive, migratory and proliferative abilities of mCRPC cells in vitro. Moreover, we confirmed that PRMT7 ablation reduces cell dissemination in chicken chorioallantoic membrane and mouse xenograft assays. Molecularly, PRMT7 reprograms the expression of several adhesion molecules by methylating various transcription factors, such as FoxK1, resulting in the loss of adhesion from the primary tumor and increased motility of mCRPC cells. Furthermore, PRMT7 higher expression correlates with tumor aggressivity and poor overall survival in prostate cancer patients. Thus, this study demonstrates that PRMT7 is a potential therapeutic target and potential biomarker for mPCa.

Unveiling cancer dormancy: Intrinsic mechanisms and extrinsic forces

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

Lineage commitment pathways epigenetically oppose oncogenic Gαq/11-YAP signaling in dormant disseminated uveal melanoma

The mechanisms driving late relapse in uveal melanoma (UM) patients remains a medical mystery and major challenge. Clinically it is inferred that UM disseminated cancer cells (DCCs) persist asymptomatic for years-to-decades mainly in the liver before they manifest as symptomatic metastasis. Here we reveal using Gαq/11 mut /BAP wt human uveal melanoma models and human UM metastatic samples, that the neural crest lineage commitment nuclear receptor NR2F1 is a key regulator of spontaneous UM DCC dormancy in the liver. Using a quiescence reporter, RNA-seq and multiplex imaging we revealed that rare dormant UM DCCs upregulate NR2F1 expression and genes related to neural crest programs while repressing gene related to cell cycle progression. Gain and loss of function assays showed that NR2F1 silences YAP1/TEAD1 transcription downstream of Gαq/11 signaling and that NR2F1 expression can also be repressed by YAP1. YAP1 expression is repressed by NR2F1 binding to its promoter and changing the histone H3 tail activation marks to repress YAP1 transcription. In vivo CRISPR KO of NR2F1 led dormant UM DCCs to awaken and initiate relentless liver metastatic growth. Cut&Run and bulk RNA sequencing further confirmed that NR2F1 epigenetically stimulates neuron axon guidance and neural lineage programs, and it globally represses gene expression linked to G-protein signaling to drive dormancy. Pharmacological inhibition of Gαq/11 mut signaling resulted in NR2F1 upregulation and robust UM growth arrest, which was also achieved using a novel NR2F1 agonist. Our work sheds light on the molecular underpinnings of UM dormancy revealing that transcriptional programs driven by NR2F1 epigenetically short-circuit Gαq/11 signaling to its downstream target YAP1.

Highlights

Quiescent solitary uveal melanoma (UM) DCCs in the liver up- and down-regulate neural crest and cell cycle progression programs, respectively.NR2F1 drives solitary UM DCC dormancy by antagonizing the Gαq/11-YAP1 pathway; small molecule Gαq/11 inhibition restores NR2F1 expression and quiescence. NR2F1 short-circuits oncogenic YAP1 and G-protein signaling via a chromatin remodeling program. Loss of function of NR2F1 in dormant UM DCCs leads to aggressive liver metastasis.

Graphical abstract

Emerging Role of Autophagy in Governing Cellular Dormancy, Metabolic Functions, and Therapeutic Responses of Cancer Stem Cells

Tumors are composed of heterogeneous populations of dysregulated cells that grow in specialized niches that support their growth and maintain their properties. Tumor heterogeneity and metastasis are among the major hindrances that exist while treating cancer patients, leading to poor clinical outcomes. Although the factors that determine tumor complexity remain largely unknown, several genotypic and phenotypic changes, including DNA mutations and metabolic reprograming provide cancer cells with a survival advantage over host cells and resistance to therapeutics. Furthermore, the presence of a specific population of cells within the tumor mass, commonly known as cancer stem cells (CSCs), is thought to initiate tumor formation, maintenance, resistance, and recurrence. Therefore, these CSCs have been investigated in detail recently as potential targets to treat cancer and prevent recurrence. Understanding the molecular mechanisms involved in CSC proliferation, self-renewal, and dormancy may provide important clues for developing effective therapeutic strategies. Autophagy, a catabolic process, has long been recognized to regulate various physiological and pathological processes. In addition to regulating cancer cells, recent studies have identified a critical role for autophagy in regulating CSC functions. Autophagy is activated under various adverse conditions and promotes cellular maintenance, survival, and even cell death. Thus, it is intriguing to address whether autophagy promotes or inhibits CSC functions and whether autophagy modulation can be used to regulate CSC functions, either alone or in combination. This review describes the roles of autophagy in the regulation of metabolic functions, proliferation and quiescence of CSCs, and its role during therapeutic stress. The review further highlights the autophagy-associated pathways that could be used to regulate CSCs. Overall, the present review will help to rationalize various translational approaches that involve autophagy-mediated modulation of CSCs in controlling cancer progression, metastasis, and recurrence.

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

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

Abscisic acid signaling through LANCL2 and PPARγ induces activation of p38MAPK resulting in dormancy of prostate cancer metastatic cells

Prostate cancer (PCa) is one the most common malignancies in men. The high incidence of bone metastasis years after primary therapy suggests that disseminated tumor cells must become dormant, but maintain their ability to proliferate in the bone marrow. Abscisic acid (ABA) is a stress response molecule best known for its regulation of seed germination, stomal opening, root shoot growth and other stress responses in plants. ABA is also synthesized by mammalian cells and has been linked to human disease. The aim of the present study was to examine the role of ABA in regulating tumor dormancy via signaling through lanthionine synthetase C‑like protein 2 (LANCL2) and peroxisome proliferator activated receptor γ (PPARγ) receptors. ABA signaling in human PCa cell lines was studied using targeted gene knockdown (KD), western blotting, quantitative PCR, cell proliferation, migration, invasion and soft agar assays, as well as co‑culture assays with bone marrow stromal cells. The data demonstrated that ABA signaling increased the expression of p21, p27 and p16, while inhibiting viability, migration, invasion and colony size in a reversable manner without toxicity. ABA also induced p38MAPK activation and NR2F1 signaling. Targeted gene KD of LANCL2 and PPARγ abrogated the cellular responses to ABA. Taken together, these data demonstrate that ABA may induce dormancy in PCa cell lines through LANCL2 and PPARγ signaling, and suggest novel targets to manage metastatic PCa growth.

Dormancy of cutaneous melanoma

Many cancer-related deaths including melanoma result from metastases that develop months or years after the initial cancer therapy. Even the most effective drugs and immune therapies rarely eradicate all tumor cells. Instead, they strongly reduce cancer burden, permitting dormant cancer cells to persist in niches, where they establish a cellular homeostasis with their host without causing clinical symptoms. Dormant cancers respond poorly to most drugs and therapies since they do not proliferate and hide in niches. It therefore remains a major challenge to develop novel therapies for dormant cancers. In this review we focus on the mechanisms regulating the initiation of cutaneous melanoma dormancy as well as those which are involved in reawakening of dormant cutaneous melanoma cells. In recent years the role of neutrophils and niche components in reawakening of melanoma cells came into focus and indicate possible future therapeutic applications. Sophisticated in vitro and in vivo melanoma dormancy models are needed to make progress in this field and are discussed.

PTHrP intracrine actions divergently influence breast cancer growth through p27 and LIFR

The role of parathyroid hormone (PTH)-related protein (PTHrP) in breast cancer remains controversial, with reports of PTHrP inhibiting or promoting primary tumor growth in preclinical studies. Here, we provide insight into these conflicting findings by assessing the role of specific biological domains of PTHrP in tumor progression through stable expression of PTHrP (-36-139aa) or truncated forms with deletion of the nuclear localization sequence (NLS) alone or in combination with the C-terminus. Although the full-length PTHrP molecule (-36-139aa) did not alter tumorigenesis, PTHrP lacking the NLS alone accelerated primary tumor growth by downregulating p27, while PTHrP lacking the NLS and C-terminus repressed tumor growth through p27 induction driven by the tumor suppressor leukemia inhibitory factor receptor (LIFR). Induction of p27 by PTHrP lacking the NLS and C-terminus persisted in bone disseminated cells, but did not prevent metastatic outgrowth, in contrast to the primary tumor site. These data suggest that the PTHrP NLS functions as a tumor suppressor, while the PTHrP C-terminus may act as an oncogenic switch to promote tumor progression through differential regulation of p27 signaling.

Dormant cancer cells and polyploid giant cancer cells: The roots of cancer recurrence and metastasis

Tumour cell dormancy is critical for metastasis and resistance to chemoradiotherapy. Polyploid giant cancer cells (PGCCs) with giant or multiple nuclei and high DNA content have the properties of cancer stem cell and single PGCCs can individually generate tumours in immunodeficient mice. PGCCs represent a dormant form of cancer cells that survive harsh tumour conditions and contribute to tumour recurrence. Hypoxic mimics, chemotherapeutics, radiation and cytotoxic traditional Chinese medicines can induce PGCCs formation through endoreduplication and/or cell fusion. After incubation, dormant PGCCs can recover from the treatment and produce daughter cells with strong proliferative, migratory and invasive abilities via asymmetric cell division. Additionally, PGCCs can resist hypoxia or chemical stress and have a distinct protein signature that involves chromatin remodelling and cell cycle regulation. Dormant PGCCs form the cellular basis for therapeutic resistance, metastatic cascade and disease recurrence. This review summarises regulatory mechanisms governing dormant cancer cells entry and exit of dormancy, which may be used by PGCCs, and potential therapeutic strategies for targeting PGCCs.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

Update on Epithelial-Mesenchymal Plasticity in Cancer Progression

Epithelial-mesenchymal transition (EMT) is a cellular process by which epithelial cells lose their characteristics and acquire mesenchymal traits to promote cell movement. This program is aberrantly activated in human cancers and endows tumor cells with increased abilities in tumor initiation, cell migration, invasion, metastasis, and therapy resistance. The EMT program in tumors is rarely binary and often leads to a series of gradual or intermediate epithelial-mesenchymal states. Functionally, epithelial-mesenchymal plasticity (EMP) improves the fitness of cancer cells during tumor progression and in response to therapies. Here, we discuss the most recent advances in our understanding of the diverse roles of EMP in tumor initiation, progression, metastasis, and therapy resistance and address major clinical challenges due to EMP-driven phenotypic heterogeneity in cancer. Uncovering novel molecular markers and key regulators of EMP in cancer will aid the development of new therapeutic strategies to prevent cancer recurrence and overcome therapy resistance.

The Role of TAM Receptors in Bone

The TAM (TYRO3, MERTK, and AXL) family of receptor tyrosine kinases are pleiotropic regulators of adult tissue homeostasis maintaining organ integrity and self-renewal. Disruption of their homeostatic balance fosters pathological conditions like autoinflammatory or degenerative diseases including rheumatoid arthritis, lupus erythematodes, or liver fibrosis. Moreover, TAM receptors exhibit prominent cell-transforming properties, promoting tumor progression, metastasis, and therapy resistance in various cancer entities. Emerging evidence shows that TAM receptors are involved in bone homeostasis by regulating osteoblastic bone formation and osteoclastic bone resorption. Therefore, TAM receptors emerge as new key players of the regulatory cytokine network of osteoblasts and osteoclasts and represent accessible targets for pharmacologic therapy for a broad set of different bone diseases, including primary and metastatic bone tumors, rheumatoid arthritis, or osteoporosis.

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

Metastasis suppressor genes and their role in the tumor microenvironment

The metastatic cascade is a complex process with multiple factors contributing to the seeding and growth of cancer cells at metastatic sites. Within this complex process, several genes have been identified as metastasis suppressors, playing a role in the inhibition of metastasis. Interestingly, some of these genes have been shown to also play a role in regulating the tumor microenvironment. In this review, we comment on the recent developments in the biology of metastasis suppressor genes and their crosstalk with the microenvironment.

How circulating tumor cluster biology contributes to the metastatic cascade: from invasion to dissemination and dormancy

Circulating tumor cells (CTCs) are known to be prognostic for metastatic relapse and are detected in patients as solitary cells or cell clusters. Circulating tumor cell clusters (CTC clusters) have been observed clinically for decades and are of significantly higher metastatic potential compared to solitary CTCs. Recent studies suggest distinct differences in CTC cluster biology regarding invasion and survival in circulation. However, differences regarding dissemination, dormancy, and reawakening require more investigations compared to solitary CTCs. Here, we review the current state of CTC cluster research and consider their clinical significance. In addition, we discuss the concept of collective invasion by CTC clusters and molecular evidence as to how cluster survival in circulation compares to that of solitary CTCs. Molecular differences between solitary and clustered CTCs during dormancy and reawakening programs will also be discussed. We also highlight future directions to advance our current understanding of CTC cluster biology.

Mutual regulation of TGFβ-induced oncogenic EMT, cell cycle progression and the DDR

TGFβ signaling and the DNA damage response (DDR) are two cellular toolboxes with a strong impact on cancer biology. While TGFβ as a pleiotropic cytokine affects essentially all hallmarks of cancer, the multifunctional DDR mostly orchestrates cell cycle progression, DNA repair, chromatin remodeling and cell death. One oncogenic effect of TGFβ is the partial activation of epithelial-to-mesenchymal transition (EMT), conferring invasiveness, cellular plasticity and resistance to various noxae. Several reports show that both individual networks as well as their interface affect chemo-/radiotherapies. However, the underlying mechanisms remain poorly resolved. EMT often correlates with TGFβ-induced slowing of proliferation, yet numerous studies demonstrate that particularly the co-activated EMT transcription factors counteract anti-proliferative signaling in a partially non-redundant manner. Collectively, evidence piled up over decades underscore a multifaceted, reciprocal inter-connection of TGFβ signaling / EMT with the DDR / cell cycle progression, which we will discuss here. Altogether, we conclude that full cell cycle arrest is barely compatible with the propagation of oncogenic EMT traits and further propose that 'EMT-linked DDR plasticity' is a crucial, yet intricate facet of malignancy, decisively affecting metastasis formation and therapy resistance.

Colorectal Liver Metastasis: Can Cytokines Make the Difference?

Colorectal cancer (CRC) is the third leading cause of cancer-related death worldwide. Metastasis is the prime driver of CRC-related mortality, and the liver is the organ most frequently involved. Despite the overall success of current treatments, colorectal liver metastasis (CRLM) is associated with poor prognoses and a survival rate of only 14%. Recent studies have highlighted the importance of the tumor microenvironment (TME) and the crosstalk within it in determining the invasion of distant organs by circulating cancer cells. In the TME, cellular communication is mediated via soluble molecules, among which cytokines have recently emerged as key regulators, involved in every aspect of tumor progression and the metastatic cascade. Indeed, in the serum of CRC patients elevated levels of several cytokines are associated with cancer development and progression. The current review evaluates the role of different cytokines during CRLM development. Additionally, considering the increasing amount of data concerning the importance of cytokine complex networks, we outline the potential of combination treatments using targeted cytokines together with other well-established therapies, such as immune checkpoint blockades, chemotherapy, or gene therapy, to improve therapeutic outcomes.

SCFβTrCP-mediated degradation of SHARP1 in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer associated with metastasis, high recurrence rate, and poor survival. The basic helix-loop-helix transcription factor SHARP1 (Split and Hairy-related Protein 1) has been identified as a suppressor of the metastatic behavior of TNBC. SHARP1 blocks the invasive phenotype of TNBC by inhibiting hypoxia-inducible factors and its loss correlates with poor survival of breast cancer patients. Here, we show that SHARP1 is an unstable protein that is targeted for proteasomal degradation by the E3 ubiquitin ligase complex SCFβTrCP. SHARP1 recruits βTrCP via a phosphodegron encompassing Ser240 and Glu245 which are required for SHARP1 ubiquitylation and degradation. Furthermore, mice injected with TNBC cells expressing the non-degradable SHARP1(S240A/E245A) mutant display reduced tumor growth and increased tumor-free survival. Our study suggests that targeting the βTrCP-dependent degradation of SHARP1 represents a therapeutic strategy in TNBC.

A cell cycle centric view of tumour dormancy

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

Dormancy in Breast Cancer

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

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

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

Cancer quiescence: non-coding RNAs in the spotlight

Cancer quiescence reflects the ability of cancer cells to enter a reversible slow-cycling or mitotically dormant state and represents a powerful self-protecting mechanism preventing cancer cell 'damage' from hypoxic conditions, nutrient deprivation, immune surveillance, and (chemo)therapy. When stress conditions are restrained, and tumor microenvironment becomes beneficial, quiescent cancer cells re-enter cell cycle to facilitate tumor spread and cancer progression/metastasis. Recent studies have highlighted the dynamic role of regulatory non-coding RNAs (ncRNAs) in orchestrating cancer quiescence. The elucidation of regulatory ncRNA networks will shed light on the quiescence-proliferation equilibrium and, ultimately, pave the way for new treatment options. Herein, we have summarized the ever-growing role of ncRNAs upon cancer quiescence regulation and their impact on treatment resistance and modern cancer therapeutics.

Taxane chemotherapy induces stromal injury that leads to breast cancer dormancy escape

A major cause of cancer recurrence following chemotherapy is cancer dormancy escape. Taxane-based chemotherapy is standard of care in breast cancer treatment aimed at killing proliferating cancer cells. Here, we demonstrate that docetaxel injures stromal cells, which release protumor cytokines, IL-6 and granulocyte colony stimulating factor (G-CSF), that in turn invoke dormant cancer outgrowth both in vitro and in vivo. Single-cell transcriptomics shows a reprogramming of awakened cancer cells including several survival cues such as stemness, chemoresistance in a tumor stromal organoid (TSO) model, as well as an altered tumor microenvironment (TME) with augmented protumor immune signaling in a syngeneic mouse breast cancer model. IL-6 plays a role in cancer cell proliferation, whereas G-CSF mediates tumor immunosuppression. Pathways and differential expression analyses confirmed MEK as the key regulatory molecule in cancer cell outgrowth and survival. Antibody targeting of protumor cytokines (IL-6, G-CSF) or inhibition of cytokine signaling via MEK/ERK pathway using selumetinib prior to docetaxel treatment prevented cancer dormancy outgrowth suggesting a novel therapeutic strategy to prevent cancer recurrence.

Cancer cell plasticity during tumor progression, metastasis and response to therapy

Cell plasticity represents the ability of cells to be reprogrammed and to change their fate and identity, enabling homeostasis restoration and tissue regeneration following damage. Cell plasticity also contributes to pathological conditions, such as cancer, enabling cells to acquire new phenotypic and functional features by transiting across distinct cell states that contribute to tumor initiation, progression, metastasis and resistance to therapy. Here, we review the intrinsic and extrinsic mechanisms driving cell plasticity that promote tumor growth and proliferation as well as metastasis and drug tolerance. Finally, we discuss how cell plasticity could be exploited for anti-cancer therapy.

5-Azacytidine- and retinoic-acid-induced reprogramming of DCCs into dormancy suppresses metastasis via restored TGF-β-SMAD4 signaling

Disseminated cancer cells (DCCs) in secondary organs can remain dormant for years to decades before reactivating into overt metastasis. Microenvironmental signals leading to cancer cell chromatin remodeling and transcriptional reprogramming appear to control onset and escape from dormancy. Here, we reveal that the therapeutic combination of the DNA methylation inhibitor 5-azacytidine (AZA) and the retinoic acid receptor ligands all-trans retinoic acid (atRA) or AM80, an RARα-specific agonist, promotes stable dormancy in cancer cells. Treatment of head and neck squamous cell carcinoma (HNSCC) or breast cancer cells with AZA+atRA induces a SMAD2/3/4-dependent transcriptional program that restores transforming growth factor β (TGF-β)-signaling and anti-proliferative function. Significantly, either combination, AZA+atRA or AZA+AM80, strongly suppresses HNSCC lung metastasis formation by inducing and maintaining solitary DCCs in a SMAD4+/NR2F1+ non-proliferative state. Notably, SMAD4 knockdown is sufficient to drive resistance to AZA+atRA-induced dormancy. We conclude that therapeutic doses of AZA and RAR agonists may induce and/or maintain dormancy and significantly limit metastasis development.

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

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

Bibliometric Analysis of Global Research on Tumor Dormancy

Tumor dormancy continues to be a research hotspot with numerous pressing problems that need to be solved. The goal of this study is to perform a bibliometric analysis of pertinent articles published in the twenty-first century. We concentrate on significant keywords, nations, authors, affiliations, journals, and literature in the field of tumor dormancy, which will help researchers to review the results that have been achieved and better understand the directions of future research. We retrieved research articles on tumor dormancy from the Web of Science Core Collection. This study made use of the visualization tools VOSviewer, CiteSpace, and Scimago Graphica, as visualization helps us to uncover the intrinsic connections between information. Research on tumor dormancy has been growing in the 21st century, especially from 2015 to the present. The United States is a leader in many aspects of this research area, such as in the number of publications, the number of partners, the most productive institutions, and the authors working in this field. Harvard University is the institution with the highest number of publications, and Aguirre-Ghiso, Julio A. is the author with the highest number of publications and citations. The keywords that emerged after 2017 were "early dissemination", "inhibition", "mechanism", "bone metastasis", and "promotion". We believe that research on tumor dormancy mechanisms and therapy has been, and will continue to be, a major area of interest. The exploration of the tumor dormancy microenvironment and immunotherapeutic treatments for tumor dormancy is likely to represent the most popular future research topics.

The Brain Pre-Metastatic Niche: Biological and Technical Advancements

Metastasis, particularly brain metastasis, continues to puzzle researchers to this day, and exploring its molecular basis promises to break ground in developing new strategies for combatting this deadly cancer. In recent years, the research focus has shifted toward the earliest steps in the formation of metastasis. In this regard, significant progress has been achieved in understanding how the primary tumor affects distant organ sites before the arrival of tumor cells. The term pre-metastatic niche was introduced for this concept and encompasses all influences on sites of future metastases, ranging from immunological modulation and ECM remodeling to the softening of the blood-brain barrier. The mechanisms governing the spread of metastasis to the brain remain elusive. However, we begin to understand these processes by looking at the earliest steps in the formation of metastasis. This review aims to present recent findings on the brain pre-metastatic niche and to discuss existing and emerging methods to further explore the field. We begin by giving an overview of the pre-metastatic and metastatic niches in general before focusing on their manifestations in the brain. To conclude, we reflect on the methods usually employed in this field of research and discuss novel approaches in imaging and sequencing.

Radiated tumor cell-derived microparticles effectively kill stem-like tumor cells by increasing reactive oxygen species

Stem-like tumor cells (SLTCs) are thought to be the cellular entity responsible for clinical recurrence and subsequent metastasis. Inhibiting or killing SLTCs can effectively reduce recurrence and metastasis, yet little has been done to clear SLTCs because they are usually resistant to chemotherapy, radiotherapy, and even immunotherapy. In this study, we established SLTCs by low-serum culture and confirmed that the low-serum-cultured tumor cells were in a quiescent state and resistant to chemotherapy, showing features of SLTCs, consistent with the reported data. We demonstrated that SLTCs had high levels of reactive oxygen species (ROS). Based on the finding that radiated tumor cell-derived microparticles (RT-MPs) contained ROS, we used RT-MPs to kill SLTCs. We found that RT-MPs could further increase ROS levels and kill SLTCs in vivo and in vitro partially by ROS carried by the RT-MPs themselves, providing a new method for eliminating SLTCs.

Awakening of Dormant Breast Cancer Cells in the Bone Marrow

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

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

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

Out of the cycle: Impact of cell cycle aberrations on cancer metabolism and metastasis

The use of cell cycle inhibitors has necessitated a better understanding of the cell cycle in tumor biology to optimize the therapeutic approach. Cell cycle aberrations are common in cancers, and it is increasingly acknowledged that these aberrations exert oncogenic effects beyond the cell cycle. Multiple facets such as cancer metabolism, immunity and metastasis are also affected, all of which are beyond the effect of cell proliferation alone. This review comprehensively summarized the important recent findings and advances in these interrelated processes. In cancer metabolism, cell cycle regulators can modulate various pathways in aerobic glycolysis, glucose uptake and gluconeogenesis, mainly through transcriptional regulation and kinase activities. Amino acid metabolism is also regulated through cell cycle progression. On cancer metastasis, metabolic plasticity, immune evasion, tumor microenvironment adaptation and metastatic site colonization are intricately related to the cell cycle, with distinct regulatory mechanisms at each step of invasion and dissemination. Throughout the synthesis of current understanding, knowledge gaps and limitations in the literature are also highlighted, as are new therapeutic approaches such as combinational therapy and challenges in tackling emerging targeted therapy resistance. A greater understanding of how the cell cycle modulates diverse aspects of cancer biology can hopefully shed light on identifying new molecular targets by harnessing the vast potential of the cell cycle.