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The most dangerous aspect of cancer lies in metastatic progression. Tumor cells will successfully form life-threatening metastases when they undergo sequential steps along a journey from the primary tumor to distant organs. From a biomechanics standpoint, growth, invasion, intravasation, circulation, arrest/adhesion, and extravasation of tumor cells demand particular cell-mechanical properties in order to survive and complete the metastatic cascade. With metastatic cells usually being softer than their non-malignant counterparts, high deformability for both the cell and its nucleus is thought to offer a significant advantage for metastatic potential. However, it is still unclear whether there is a finely tuned but fixed mechanical state that accommodates all mechanical features required for survival throughout the cascade or whether tumor cells need to dynamically refine their properties and intracellular components at each new step encountered. Here, we review the various mechanical requirements successful cancer cells might need to fulfill along their journey and speculate on the possibility that they dynamically adapt their properties accordingly. The mechanical signature of a successful cancer cell might actually be its ability to adapt to the successive microenvironmental constraints along the different steps of the journey.

Metastatic progression, which begins with the invasion and migration of tumor cells from a primary tumor, marks a major turning point in the evolution of cancer. Indeed, it eventually leads to the formation of secondary tumors, the metastases, which are very often responsible for the patient's death. Understanding the mechanisms controlling the different steps of this process, as well as those explaining the fundamental phenomenon of organotropism (i.e. the distribution of metastases in distant organs by a non-random and tumor-specific process), is essential to define new innovative therapeutic solutions. In this review paper, we will present how biomechanics is an essential element to this understanding, and will emphasize the importance of this orthogonal and promising angle of study as well as our laboratory's focus on the late stages of dissemination, arrest and extravasation of circulating cancer cells and factors secreted by the primary tumor such as extracellular vesicles.La progression métastatique, qui débute par l’invasion et la migration de cellules tumorales depuis une tumeur primaire, marque un tournant majeur dans l’évolution du cancer. En effet, elle conduit à terme à la formation des tumeurs secondaires, les métastases, responsables très souvent de la mort du patient. Comprendre les mécanismes contrôlant les différentes étapes de ce processus, ainsi que ceux expliquant le phénomène fondamental d’organotropisme (c’est-à-dire la distribution des métastases dans les organes distants par un processus non aléatoire et tumeur-spécifique), est essentiel pour définir de nouvelles solutions thérapeutiques innovantes. Dans ce document de synthèse, nous présenterons comment la biomécanique est un élément essentiel à cette compréhension, et insisterons sur l’importance de cet angle d’étude orthogonal et prometteur ainsi que la focalisation de notre laboratoire sur les étapes tardives de dissémination, d’arrêt et d’extravasation des cellules cancéreuses circulantes et des facteurs sécrétés par la tumeur primaire comme les vésicules extracellulaires.

Cancer metastasis is a multistep process during which tumor cells leave the primary tumor mass and form distant secondary colonies that are lethal. Circulating tumor cells (CTCs) are transported by body fluids to reach distant organs, where they will extravasate and either remain dormant or form new tumor foci. Development of methods to study the behavior of CTCs at the late stages of the intravascular journey is thus required to dissect the molecular mechanisms at play. Using recently developed microfluidics approaches, we have demonstrated that CTCs arrest intravascularly, through a two-step process: (a) CTCs stop using low energy and rapidly activated adhesion receptors to form transient metastable adhesions and (b) CTCs stabilize their adhesions to the endothelial layer with high energy and slowly activated adhesion receptors. In this methods chapter, we describe these easy-to-implement quantitative methods using commercially available microfluidic channels. We detail the use of fast live imaging combined to fine-tuned perfusion to measure the adhesion potential of CTC depending on flow velocities. We document how rapidly engaged early metastable adhesion can be discriminated from slower activated stable adhesion using microfluidics. Finally, CTC extravasation potential can be assessed within this setup using long-term cell culture under flow. Altogether, this experimental pipeline can be adapted to probe the adhesion (to the endothelial layer) and extravasation potential of any circulating cell.

Clustering of tumor cells is known to grant superior metastatic efficiency compared with single cells. However, the mechanisms involved remain elusive. Reporting in Cell, Wrenn et al. describe how sealed intercellular compartments, nanolumina, are used as growth factor reservoirs within tumor cell clusters to regulate tumor cell proliferation.

Metastases are the main cause of cancer-related deaths. The chain of events leading to their development is called "the metastatic cascade". The biological and biochemical aspects of this process have been well studied but the importance of biomechanical parameters only recently became a focus in the field. Studies have shown the biological fluids (blood, lymph and interstitial fluid) to play a key role in the metastatic cascade. These fluids participate in the transport of circulating tumor cells (CTCs) as well as the factors that they secrete, while at the same time influencing the events of the metastatic cascade through the forces that they generate. The hemodynamic properties and topological constraints of the vascular architecture control the formation of metastatic niches and the metastatic potential of tumor cells. In this review, we discuss the importance of these mechanical forces and highlight the novel questions and research avenues that they open.Influence de la mécanique des fluides sur la formation des métastases.La suite d’évènements menant à l’apparition de métastases est appelée « cascade métastatique ». L’étude récente de la composante biomécanique de cette cascade a révélé le rôle central des liquides biologiques dans la dissémination métastatique. Tout en participant au transport des cellules tumorales circulantes et des facteurs qu’elles sécrètent, ces liquides circulants influencent cette cascade par les forces mécaniques qu’ils génèrent. Les propriétés hémodynamiques et les contraintes topologiques de l’architecture vasculaire contrôlent la formation de niches métastatiques et le potentiel métastatique des cellules tumorales.