Mechanobiology and survival strategies of circulating tumor cells: a process towards the invasive and metastatic phenotype

Abstract

Metastatic progression is the deadliest feature of cancer. Cancer cell growth, invasion, intravasation, circulation, arrest/adhesion and extravasation require specific mechanical properties to allow cell survival and the completion of the metastatic cascade. Circulating tumor cells (CTCs) come into contact with the capillary bed during extravasation/intravasation at the beginning of the metastatic cascade. However, CTC mechanobiology and survival strategies in the bloodstream, and specifically in the microcirculation, are not well known. A fraction of CTCs can extravasate and colonize distant areas despite the biomechanical constriction forces that are exerted by the microcirculation and that strongly decrease tumor cell survival. Furthermore, accumulating evidence shows that several CTC adaptations, via molecular factors and interactions with blood components (e.g., immune cells and platelets inside capillaries), may promote metastasis formation. To better understand CTC journey in the microcirculation as part of the metastatic cascade, we reviewed how CTC mechanobiology and interaction with other cell types in the bloodstream help them to survive the harsh conditions in the circulatory system and to metastasize in distant organs.

Keywords: cancer; circulating tumor cells; mechanobiology; metastasis; survival.

FIGURE 1

Metastatic dissemination of circulating tumor cells (CTCs) to distant organs. (A) Examples of cancer types that metastasize to distant organs: breast cancer (green), lung cancer (purple), liver cancer (black), pancreatic cancer (aqua), and colon cancer (blue). (B) The different metastatic cascade steps: (1) Growth: uncontrolled proliferation and growth of a primary tumor in a healthy tissue. (2) Intravasation: cancer cells undergo substantial changes to endure the physical interactions and mechanical forces in the tissue and intravasate the bloodstream through endothelial cells. (3) Circulation: once in the bloodstream, cancer cells become CTCs (single CTCs or CTC clusters). CTC clusters are identified as micro-emboli in advanced cancer stage (homotypic clusters composed of CTCs only; heterotypic clusters composed of CTCs and other cell types, such as immune cells, cancer-associated fibroblasts, platelet). Most CTCs will die within 2 h, but a subset of metastasis-competent CTCs will survive and adapt to extremes conditions to reach the right place where they extravasate. (4) Extravasation: CTCs extravasate at specific distant organs. (5) Colonization: CTCs that arrive at a distant organ and find good conditions to interact with the extracellular matrix (seed and soil concept) will colonize and form metastatic lesions.

FIGURE 2

Mechanobiology of CTCs in the circulating system. (A) CTCs experience blood fluid shear stress. In the circulation, single CTCs and homo/hetero-typic CTC clusters are exposed to various hemodynamic shear stresses and may arrest and extravasate, or break and die. (B) CTC arrest and deformability in the capillary bed. CTCs stop in the capillary bed and undergo phenotypic changes (e.g., stiffness) due to cell deformation induced by biomechanical forces.

FIGURE 3

Mechanotransduction signaling pathways of CTCs in capillaries. CTCs experience severe cell deformation during their transit and/or stop in capillaries. This can affect the mechanotransduction of signaling pathways, such as RhoA-ROCK and YAP/TAZ, resulting in increased cell invasiveness, survival and migration.

FIGURE 4

Single CTCs and heterotypic clusters in the bloodstream. (A) CTC-neutrophil clusters enhance proliferation via crosstalk of cytokines. (B) CTC-macrophage clusters promote CTC extravasation and dissemination and confer resistance to shear stress. (C) CTC-platelet clusters promote CTC invasion, survival and metastasis as well epithelial-mesenchymal transition by secreting TGF-beta.

FIGURE 5

Different in vitro microfluidic systems that mimic the effect of the microvasculature geometry on cells. (A) Figure from (Hou et al., 2009). Illustration of the bonded polydimethylsiloxane (PDMS) microchannel (150 μm in length, square cross-section area of 10 by 10 μm) and optical microscopy images showing the entry of a single MCF-7 cell into a 10 by 10 μm microchannel. (B) Figure from (Au et al., 2016). Schematic description of a microfluidic device (16 parallel microchannels of 5 × 5, 7 × 7, or 10 × 10-µm square cross-sections) designed to mimic the capillary flow conditions and computational simulation. Micrographs show a four-cell LNCaP cluster in transit through a 5-µm capillary constriction. (C) Figure from (Nath et al., 2018). Schematic representation of the motion of aggregated HeLa cells passing through a microcapillary device. (D) Figure from (Xia et al., 2018) showing: (A) the design of a micro-constriction array with sixteen flow channels (in yellow) and the pneumatic control channels (in brown); (B), a small portion of the pneumatic control part of the device made of three layers: PDMS layer for pneumatic control, PDMS layer for flow channels, and glass substrate; (C) a single patch of the micro-constriction array; and (D) photograph of the fabricated microfluidic device including the fluidic and pneumatic channels. (E) Figure from (Cognart et al., 2020). Schematic designs of microfluidic channels (upper panels) and brightfield images of SK-BR-3 and MDA-MB-231 cells before and while residing in the three types of micro-constrictions at a constant applied pressure of 10 kPa (lower panels).