Circulating Tumor Cells: When a Solid Tumor Meets a Fluid Microenvironment

Abstract

Solid tumor dissemination from the primary site to the sites of metastasis involves tumor cell transport through the blood or lymph circulation systems. Once the tumor cells enter the bloodstream, they encounter a new hostile microenvironment. The cells must withstand hemodynamic forces and overcome the effects of fluid shear. The cells are exposed to immunological signaling insults from leukocytes, to collisions with erythrocytes, and to interactions with platelets or macrophages. Finally, the cells need to attach to the blood vessel walls and extravasate to the surrounding stroma to form tumor metastases. Although only a small fraction of invasive cells is able to complete the metastatic process, most cancer-related deaths are the result of tumor metastasis. Thus, investigating the intracellular properties of circulating tumor cells and the extracellular conditions that allow the tumor cells to survive and thrive in this microenvironment is of vital interest. In this chapter, we discuss the intravascular microenvironment that the circulating tumor cells must endure. We summarize the current experimental and computational literature on tumor cells in the circulation system. We also illustrate various aspects of the intravascular transport of circulating tumor cells using a mathematical model based on immersed boundary principles.

Keywords: Cell deformation; Circulating tumor cells; Computational modeling; Immersed boundary method; Metastatic cascade; Tumor microemboli.

Figure 1. Schematics of the CTC adhesive cascade

A cell floating with the blood flow need to reach an endothelial wall via margination process; once near the endothelial wall, it needs to adhere to the endothelium, undergo the transitions from rolling to crawling migration before anchoring to the endothelium and transmigrate the endothelial wall using one of the following ways: perivascular migration, transcellular migration or a mosaic process.

Figure 2. Circulating tumor cells in the vessel in the mouse skin

An image of tumor HT-1080 sarcoma cells: A. nondeformed cells within a microvessel; B. deformed cells occupying the full diameter of the vessel. Cells are labeled with green fluorescent protein in the nucleus, and red fluorescent protein in the cytoplasm. Image acquired using the Olympus OV100 system at x100 magnification. Reproduced from Yamauchi et al. Cancer Research 2005 Fig.3, with permission. [73]

Figure 3. Schematics of the CTC and endothelium structure

A schematic representation of the 2D model of the tumor cell in circulation. (a) The blood vessel (red top wall) is interpenetrated by a blood flow (grey arrows representing a velocity field); the CTC nucleus (black) is surrounded by the cell cortex (blue) both composed from cross-linked Hookean springs. (b) The cell near the endothelial wall develops adhesive connections (green links). (c) As a result of cell deformation, the springs are stretched and exert restoring forces (magenta arrows). (d) The cell boundary points are moved on a local fluid velocity (magenta arrows). This mathematical model utilizes the computational frameworks of the Immersed Boundary method.

Figure 4. IBCell model equations

Equations Eq.1–Eq.5 define the mathematical frameworks of the Immersed Boundary method.

Figure 5. Deformation of a CTC under a steady blood flow

A parameter space of final cellular morphologies when the stiffness of the cell cortex (blue) and the cell nuclear envelope (black) is varied by 7 orders of magnitude. Insets (i-iv)—the representative deformable (i-ii) and non-deformable (iii-iv) cell morphologies are shown within each parameter range.

Figure 6. CTC attachment to endothelium under a steady blood flow

(a) A parameter space of final cellular morphologies when the stiffness of the cell cortex (blue) and the cell nuclear envelope (black) is varied by 7 orders of magnitude. Insets (i-iv)—the representative cell morphologies for each category; the adhesive links between CTC and the endothelium shown in green. The time course image (b and c) shows the same cell at five different time point overimposed on the same image: (b) shows cell detachment from the endothelium, and (c) cell rolling on the endothelium. A fixed small part of the cell cortex is stained in magenta to illustrate the rolling effect.

Figure 7. CTC migrating on the endothelium

A time course shows the same cell at different time point overimposed on the same image. To illustrate cell crawling a fixed small part of the cell cortex is stained in magenta. Cell cortex is stained differently depending on its local stiffness: soft (cyan) close to cell focal adhesion with the endothelium, and stiff (blue) far from the adhesive contacts (green).

Figure 8. CTC microemboli in the blood circulation system

A time course (a-c) shows the same CTC cluster at different time points with cellular adhesions and adhesion to the endothelium indicated as green links. The cluster structure allows individual cells to withstand the blood shear stress and survive in the blood flow (showed as grey vector field).