Metastasis

Abstract

Metastasis, the major cause of cancer death, represents one of the major challenges in oncology. Scientists are still trying to understand the biological basis underlying the dissemination and outgrowth of tumor cells, why these cells can remain dormant for years, how they become resistant to the immune system or cytotoxic effects of systemic therapy, and how they interact with their new microenvironment. We asked experts to discuss some of the unknowns, advances, and areas of opportunity related to cancer metastasis.

Targeting premetastatic niches

Recent advances in immunotherapeutics highlight the need to target molecules and cells involved in the tumor and metastatic microenvironments. Better tools are required to proactively identify those patients likely to develop metastasis and tailor first-line treatments to prevent the development of metastases. One opportunity involves therapies targeting premetastatic niches (PMNs), defined as those inflamed distant environments rendered favorable to supporting tumor metastasis under the influence of tumor-secreted soluble factors and tumor-shed extracellular vesicles (EVs).

PMN formation relies on a series of sequential events that include clot formation and vascular disruption, extracellular matrix modifications, cellular reprogramming of both resident and recruited immune cells, and upregulation of pro-inflammatory molecules, such as S100, TNF-α, and TGF-.β The hallmark of the PMN is the co-existence of multiple pathological changes even prior to the arrival of metastatic tumor cells.

Mounting clinical evidence documents the presence of PMNs in cancer patients, in lymph nodes as well as in distant tissues such as the omentum and liver. To improve patient outcomes and initiate treatments for a metastasis prevention program for newly diagnosed cancer patients, the existence of PMNs must be appreciated. The detection of PMNs will require synergistic approaches combining new imaging tools, such as SPECT/PET and ultrahigh magnetic field MRI, biopsies of lymph nodes and distant organs, and examination of tumor-secreted biomarkers, such as tissue and plasma-derived EV proteins, DNA, and RNA.

The dormant niche

“Why don’t you study tumor dormancy? It is SO important!” I’ll never forget the look on my mentor Mina Bissell’s face when she suggested I shift gears to study tumor dormancy. It appealed massively to me because the potential payoff for patients was obvious. And there was so much room to explore. I spent the next week reading anything and everything pertinent and hypothesized that the microenvironment—especially the niche around microvasculature—was the key to what puts tumor cells to sleep when they wander into foreign organs. We went on to show that it was.

But the more we’ve learned, the deeper the mystery has become. We’ve discovered that the perivascular niche also drives therapeutic resistance. That these dormant cells have a unique metabolism. We have hints on how they evade immunity. So many of the mechanisms borrow from normal tissue physiology. The question is—what are these cells doing that is unique to them? The key is understanding how disseminated tumor cells influence their microenvironment and how the niche responds in kind. I’m excited about applying new niche labeling and spatial profiling technologies to answer this question.

For years, we lacked specimens necessary to determine how to apply our findings to humans. Who wants to give away healthy tissues just because some dormant cells may be living there? Well, it turns out patients are willing. And invested. We’ve partnered with amazing collaborators to acquire tissue from early-stage patients and patients who succumb to metastatic disease so we can learn about dormant cells in the most relevant context of all—our bodies.

For these reasons, I’m tremendously optimistic that dormancy will shift from a biological fascination to a clinical target much sooner than later.

The dormancy challenge

When it comes to understanding the spread of cancer to other organs, what really puzzles me is that, in many cases, patients with cancer can spend years (or even decades) free of symptomatic disease before metastases emerge. At the root of this phenomenon are lingering disseminated tumor cells (DTCs) that found residency at distant sites under a state of dormancy, only to awaken years or even decades later and initiate metastases. From a clinical perspective, this pause in cancer progression holds a major challenge to long-lasting cure and a golden opportunity to prevent future metastases.

Harnessing this enormous potential for intervention requires discovering how DTCs become dormant, what causes them to awaken, and how dormant DTCs can be targeted. New models and tools developed in recent years have substantially advanced our understanding of the biology of DTCs and their interactions with the unique microenvironment in each distant site. This has resulted in the identification of DTC vulnerabilities and tissue antimetastatic barriers that may be explored therapeutically to either sustain DTCs in a dormant state or eradicate them altogether. For example, blocking integrin-mediated adhesions between DTCs and the perivasculature sensitizes dormant DTCs to chemotherapy. Another prospect is the use of adjuvant natural killer cell immunotherapy to maintain long-term dormancy. Testing these concepts in practice in clinical trials is eagerly awaited.

Going forward, I marvel at a multidisciplinary approach that continues to explore how tissue-specific DTC-microenvironment interactomes diversify metastasis presentation but also probes the dynamics and plasticity of said crosstalk in response to individual’s exposures, such as aging, inflammation, and lifestyle choices. A joint effort between basic researchers, clinician-scientists, technology entrepreneurs, and patient advocates is key to meeting the grand challenge of dormancy and creating a curative paradigm for metastasis.

Targeting cancer cell dormancy

Clinical data have shown that upon diagnosis of solid cancer, disseminated disease, detected for instance as bone marrow (BM) or lymph node disseminated cancer cells (DCCs), is highly prevalent. “Warm autopsy” programs confirmed that solitary DCCs persisted systemically in metastatic patients. Importantly, detection of even a single DCC is associated with higher relapse risk. While daunting, these findings offer a discovery space and a window of opportunity to target DCCs in all their states.

Many years or decades can lapse between primary lesion treatment and metastatic disease detection, as DCCs carrying driver mutations can spontaneously or in response to therapies (persisters) pause progression and undergo a process globally defined as dormancy. Sequencing studies of “normal” tissues revealed abundant driver mutations that do not manifest until the tissue niches or systemic alterations (e.g., inflammation, aging) perturb homeostasis. It appears that dormant DCCs can also fit in this model where microenvironment-driven epigenetic changes override genetic driver function.

Mouse models have revealed how DCCs from early or late evolved cancers can enter dormancy in different niches. Other studies have identified mechanisms to induce DCC dormancy, vulnerabilities of dormant non-proliferative DCCs, and immune-evasive strategies of quiescent cancer cells. For instance, epigenetic drugs like azacytidine and retinoic acid, and small-molecule agonists of NR2F1, induce dormancy programs and suppress metastasis. Also, targeting metabolic and stress signaling adaptive pathways, like NRF2, unique to dormant DCCs leads to their eradication. Finally, immune-evasive mechanisms found in dormant cancer cells may allow tuning the immune system for their elimination. A new therapeutics toolkit to target dormant DCCs could complement in the adjuvant but also stage IV phases current anti-proliferative and immune-oncology strategies. These therapies could prevent dormant DCC awakenings and/or eradicate them. Such approaches would greatly improve patient outcomes, and fortunately clinical studies are emerging.

Intratumor bacteria in metastasis

Metastasis is an extremely inefficient process constituted by consecutive harsh steps. The mobilized cancer cells have to cope with various kinds of stresses to successfully translocate from the primary site to distal organs and colonize there. To achieve that, cancer cells often coordinate their intrinsic gene expression network with the extrinsic cellular microenvironment to confront the physical, chemical, and biological challenges during the metastatic journey. Recently, the presence of intratumor microbiota has been identified in various tumor types, where it persists with relatively low abundance and predominant intracellular localization. These microbes are not only indicators of cancer properties and status but also have the capacity of mediating cancer progression functionally: they can reorganize the cytoskeleton to help circulating cancer cells survive against physical stress, metabolize chemotherapy drugs to protect cancer cells from chemical stress, and modulate immune cells to influence biological stress. Intriguingly, they can act both outside and inside of cancer cells to modulate the extrinsic and intrinsic environments. Understanding the exact roles of intratumor microbiota may change the blueprint of cancer metastasis, cancer immunity, and cancer ecosystem. Future studies need to figure out what determines the profile, abundance, and origin of the microbiota in tumor tissues; how the intracellular bacteria crosstalk with host cancer cells; clinically how much the intratumor microbiota impinge on tumor progression; and what strategy can we develop to modulate intratumor microbiota as a therapeutic tool in the clinics.

Microbiota and premetastatic niche

Organ specificity for metastasis formation is tumor dependent: colorectal cancer preferentially metastasizes to liver or lungs and breast cancer to bones, lungs, or brain, etc. There are two major routes for metastasis formation, which in most cases seem to occur in parallel; i.e., tumor cells can metastasize independently through the lymphatics and reach the lymph node, or through the blood and reach internal organs. In order for the latter to occur, tumor cells have to cross the blood vessels and be recruited at specific sites. The formation of a premetastatic niche is a prerequisite for attracting tumor cells at the final destination.

What drives vessel reshaping and the formation of a premetastatic niche at specific sites? One player is the intratumoral microbiota, which has been described in different tumors, including colorectal cancer and breast and bone tumors. Members of the microbiota can increase vessel permeability and reach internal organs favoring the recruitment of inflammatory cells and the formation of the premetastatic niche. Understanding whether specific microbes have preferential seeding properties to different organs and whether this can drive the metastasis site specificity is one of the challenges for the future.

The tangled NETs in metastasis

As the most abundant leukocytes in human peripheral blood, neutrophils play critical roles in cancer metastasis. Recent studies have unraveled a specialized function of neutrophils in the tumor microenvironment: releasing neutrophil extracellular traps (NETs), which are web-like extracellular DNA fibers enmeshed with histones and granular proteins.

Originally discovered to be microbe-restraining and killing, NETs turn out to do the opposite to tumor cells by enhancing their dissemination and secondary growth. NETs promote essentially every step of the metastasis cascade, including migration, invasion, immune evasion, seeding, dormancy reactivation, and colonization in target organs. Mechanistically, NETs function largely in two modalities. The sticky DNA can attract and entrap disseminated tumor cells to facilitate tumor lodging in distant organs and segregation from immune cytotoxicity. NET-decorating proteins, mainly proteases, remodel extracellular matrix, vasculatures, and tumor-suppressing factors to promote tumor spreading and growth or directly act on cancer cells to regulate metabolic programming and proliferation.

While evidence showing the pro-metastasis effects of NETs is rapidly accumulating, further studies are needed to delineate the crosstalk of tumor cells, NETs, and other microenvironmental components. Notably, the molecular features of cancer-induced NETs and the NETosis process are largely obscure. A comparison of cancer and infection-driven NETosis might be instructive. It seems that tumors induce NETosis more frequently in certain organs than others, but this organotropic basis is unclear. Also, is there a subset of tumor-associated neutrophils prone to NETosis? All these questions are important for understanding how tumors exploit NETs and for finding effective NET-targeting strategies in metastasis treatment.

Spatial metabolic dependencies

Metastasis formation is a highly dynamic process in which cancer cells traverse to distant organs. During this journey, cancer cells have a highly plastic phenotype that is adapted to the challenges they encounter over time. Once they reach their destination, they must adapt to the new environment in this distant space. Numerous metabolic changes have been observed in cancer cells during metastasis formation. Many of them are required and some are sufficient for the ability of cancer cells to successfully metastasize and master these challenges in time and space. Examples for the latter are the loss of the serine biosynthesis enzyme PHGDH or the gain of the of the lipid receptor CD36, which independently equip cancer cells with metastasis-initiating capacity. Moreover, while energetic, redox, and biomass dependencies certainly change and can be targeted in metastasizing cancer cells, a crucial role for metabolic signaling is also emerging. Yet, many questions remain, especially concerning the metabolic communication and cooperation of cancer cells with organ resident and recruited stromal cells resulting in intratumor and inter-metastases metabolic heterogeneity. Advances in single-cell and spatial metabolomics techniques open an unprecedented opportunity to address these questions. Thus, targeting metabolic dependencies of cancer cells dictated by time and space opens new therapeutic opportunities to prevent and treat metastatic disease. Clinical trials rigorously testing these strategies using, for example, metastasis-free survival as a clinical endpoint are needed. Thereby, the question will not be whether metabolism can be safely targeted, because this has already been shown multiple times since the introduction of antifolates to the clinicals almost 75 years ago, but whether these metabolic treatments are better than the current standard of care in protecting patients from metastasis formation.

CNS metastases

The interplay between cancer cells and their microenvironment reflects the evolutionary biology of this disease: the cancer cell’s inherent plasticity enables the cell to overcome a variety of environmental constraints. This concept is brought to the forefront in the setting of metastasis. Away from the primary tumor, metastatic cancer cells encounter environments that may differ, strikingly, from that of the primary tumor. This is perhaps most evident in the case of solid tumor metastasis to the CNS, a site of disease with disproportionate morbidity and mortality. In the CNS, cancer cells must contend with unique neighboring cells (neurons, astrocytes, and oligodendrocytes) as well as metabolic and signaling constraints wholly unlike those found in the periphery (cerebrospinal fluid, neurotransmitters).

To overcome these severe constraints, cancer cells exploit their epigenetic freedom, accessing transcriptional programs from throughout the genome. In doing so, cancer cells from a variety of solid tumor primaries, with a variety of genetic changes, will “solve” this problem of survival in the CNS in transcriptionally similar ways. This remarkable display of evolutionary dynamics is evident in cancer cell-astrocyte interactions, cancer cell-neuron interactions, and cancer cell-choroid plexus interactions, to name a few. Cancer’s ability to live and grow in the harshest and most inhospitable environments enables it to serve as a biological probe—uncovering neurobiology not previously appreciated. The dominance of these microenvironmental effects in the setting of CNS metastasis suggests that microenvironmental therapeutic approaches might represent a powerful tool in the therapeutic arsenal.

Oligometastasis: A spectrum of disease

Metastases are the principal cause of cancer mortality and until recently have been considered to be widespread and require systemic treatment. The oligometastasis hypothesis articulated by Hellman and Weichselbaum suggested a spectrum of metastatic spread, both in metastasis number and pace of progression. The translational impact of this hypothesis is that local ablative therapies might be beneficial and curative in a subset of patients with limited number of metastases or a slow pace of spread. As a corollary to this hypothesis, Hellman and Weichselbaum suggested that excellent responders to systemic treatments might benefit from ablation of the remaining resistant tumors. These resistant tumors have since become known as oligoprogressors. In the ensuing years, a biological basis for oligometastasis has been defined from many laboratories in terms of the genetic makeup of the tumor and host, especially the host immune response. Metastases exhibit significant inter- and intratumoral heterogeneity in immune contexture. Oligometastases with immune stimulatory infiltration of the tumor microenvironment were associated with prolonged survival and favorable patterns of relapse after local therapies. Patients with an oligometastatic phenotype demonstrated distinct miRNA expression patterns in metastases, with 14q32-encoded miRNAs suppressing adhesion, invasion, and migration of tumor cells. Most recently, the metastatic spectrum has been framed in the context of branched versus punctuated evolution of tumors, with each evolutionary pattern correlating with attenuated (i.e., oligometastatic) versus rapid, disseminated progression, respectively. Multiple prospective trials in patients with oligometastatic and oligoprogressive disease have demonstrated a benefit for patients with local therapies, such as radio-therapy, surgery, and radiofrequency ablation. A recent negative, phase II trial in breast cancer has heightened the need for effective biomarkers to identify patients who might benefit from ablative treatments. Advances in systemic therapies, liquid biopsies, and imaging emphasize the need to examine the oligometastasis hypothesis and the role of local ablative therapies in the context of these basic science and clinical advances.

Biographies

David Lyden

Cyrus M. Ghajar

Ana Luísa Correia

Julio A. Aguirre-Ghiso

Shang Cai

Maria Rescigno

Guohong Hu and Peiyuan Zhang

Sarah-Maria Fendt

Adrienne Boire

Rohan R. Katipally and Ralph R. Weichselbaum