Identification of novel drugs to target dormant micrometastases

Method to Isolate Dormant Cancer Cells from Heterogeneous Populations

Cancer recurrence is responsible for a high percentage of cancer-related deaths. Primary tumor removal, chemotherapy, and radiotherapy often leave behind cancer cells that are clinically undetectable. Recent evidence has shown that subpopulations of these residual cancer cells enter into a prolonged dormant state, remaining quiescent for months to years, and eventually lead to metastases and relapse (Sosa et al. Nat Rev Cancer 14:611-622, 2014). Identifying the presence of and isolating these dormancy-capable cells (DCCs) from resected tumors or bodily fluids may therefore provide an opportunity to understand their biology and develop personalized treatments for patients at risk for relapse. Physical confinement in a stiff and porous 3D matrix, which inhibits proliferation, migration, and growth of the immobilized cells, has been shown to isolate DCC populations (Preciado et al. Technology 05:1-10, 2017; Reátegui et al. J Mater Chem B 2:7440-7448, 2014). Isolated DCCs can then be recovered from the gel and analyzed. Here we describe this immobilization method that can be used to isolate DCCs from heterogeneous cell populations that may also include dormancy-incapable cancer cells and host cells.

Induction of dormancy by confinement: An agarose-silica biomaterial for isolating and analyzing dormant cancer cells

The principal cause of cancer deaths is the residual disease, which eventually results in metastases. Certain metastases are induced by disseminated dormancy-capable single cancer cells that can reside within the body undetected for months to years. Awakening of the dormant cells starts a cascade resulting in the patient's demise. Despite its established clinical significance, dormancy research and its clinical translation have been hindered by lack of in vitro models that can identify, isolate, and analyze dormancy-capable cells. We have previously shown that immobilization of cells in a stiff microenvironment induces dormancy in dormancy-capable cell lines. In this communication, we present a novel biomaterial and an in vitro immobilization method to isolate, analyze, and efficiently recover dormancy-capable cancer cells. MCF-7, MDA-MB-231, and MDA-MB-468 cells were individually coated with agarose using a microfluidic flow-focusing device. Coated cells were then immobilized in a rigid and porous silica gel. Dormancy induction by this process was validated by decreased Ki-67 expression, increased p38/ERK activity ratio, and reduced expression of CDK-2, cyclins D1, and E1. We showed that we can reliably and repeatedly induce dormancy in dormancy-capable MCF-7 cells and enhance the dormancy-capable sub-population in MDA-MB-231 cells. As expected, dormancy-resistant MDA-MB-468 cells did not survive immobilization. The dormant cells could be awakened on demand, by digesting the agarose gel in situ, and efficiently recovered by magnetically separating the silica gel, making the cells available for downstream analysis and testing. The awakened cells were shown to regain motility immediately, proliferating, and migrating normally.

The ATF6-EGF Pathway Mediates the Awakening of Slow-Cycling Chemoresistant Cells and Tumor Recurrence by Stimulating Tumor Angiogenesis

Slow-cycling cancer cells (SCCs) with a quiescence-like phenotype are believed to perpetrate cancer relapse and progression. However, the mechanisms that mediate SCC-derived tumor recurrence are poorly understood. Here, we investigated the mechanisms underlying cancer recurrence after chemotherapy, focusing on the interplay between SCCs and the tumor microenvironment. We established a preclinical model of SCCs by exposing non-small-cell lung cancer (NSCLC) cells to either the proliferation-dependent dye carboxyfluorescein diacetate succinimidyl ester (CFSE) or chemotherapeutic drugs. An RNA sequencing analysis revealed that the established SCCs exhibited the upregulation of a group of genes, especially epidermal growth factor (EGF). Increases in the number of vascular endothelial growth factor receptor (VEGFR)-positive vascular endothelial cells and epidermal growth factor receptor (EGFR) activation were found in NSCLC cell line- and patient-derived xenograft tumors that progressed upon chemotherapy. EGFR tyrosine kinase inhibitors effectively suppressed the migration and tube formation of vascular endothelial cells. Furthermore, activating transcription factor 6 (ATF6) induced the upregulation of EGF, and its antagonism effectively suppressed these SCC-mediated events and inhibited tumor recurrence after chemotherapy. These results suggest that the ATF6-EGF signaling axis in SCCs functions to trigger the angiogenesis switch in residual tumors after chemotherapy and is thus a driving force for the switch from SCCs to actively cycling cancer cells, leading to tumor recurrence.

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

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

Bioengineered models to study tumor dormancy

The onset of cancer metastasis is the defining event in cancer progression when the disease is considered lethal. The ability of metastatic cancer cells to stay dormant for extended time periods and reawaken at later stages leading to disease recurrence makes treatment of metastatic disease extremely challenging. The tumor microenvironment plays a critical role in deciding the ultimate fate of tumor cells, yet the mechanisms by which this occurs, including dormancy, is not well understood. This mini-review discusses bioengineered models inspired from tissue engineering strategies that mimic key aspects of the tumor microenvironment to study tumor dormancy. These models include biomaterial based three dimensional models, microfluidic based models, as well as bioreactor based models that incorporate relevant microenvironmental components such as extracellular matrix molecules, niche cells, or their combination to study microenvironmental regulation of tumor dormancy. Such biomimetic models provide suitable platforms to investigate the dormant niche, including cues that drive the dormant to proliferative transition in cancer cells. In addition, the potential of such model systems to advance research in the field of tumor dormancy is discussed.

Engineered In Vitro Models of Tumor Dormancy and Reactivation

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

Targeting dormant micrometastases: rationale, evidence to date and clinical implications

In spite of decades of research, cancer survival has increased only modestly. This is because most research is based on models of primary tumors. Slow recognition has begun that disseminated, dormant cancer cells (micrometastatic cells) that are generally resistant to chemotherapy are the culprits in recurrence, and until these are targeted effectively we can expect only slow progress in increasing overall survival from cancer. This paper reviews efforts to understand the mechanisms by which cancer cells can become dormant, and thereby identify potential targets and drugs either on the market or in clinical trials that purport to prevent metastasis. This review targets the most recent literature because several excellent reviews have covered the literature from more than two years ago. The paper also describes recent work in the authors' laboratories to develop a screening-based approach that does not require understanding of mechanisms of action or the molecular target. Success of this approach shows that targeting micrometastatic cells is definitely feasible.

The Predictive Link between Matrix and Metastasis

Cancer spread (metastasis) is responsible for 90% of cancer-related fatalities. Informing patient treatment to prevent metastasis, or kill all cancer cells in a patient's body before it becomes metastatic is extremely powerful. However, aggressive treatment for all non-metastatic patients is detrimental, both for quality of life concerns, and the risk of kidney or liver-related toxicity. Knowing when and where a patient has metastatic risk could revolutionize patient treatment and care. In this review, we attempt to summarize the key work of engineers and quantitative biologists in developing strategies and model systems to predict metastasis, with a particular focus on cell interactions with the extracellular matrix (ECM), as a tool to predict metastatic risk and tropism.