Initial steps of metastasis: cell invasion and endothelial transmigration

Abstract

Metastasis is the leading cause of cancer mortality. The metastatic cascade represents a multi-step process which includes local tumor cell invasion, entry into the vasculature followed by the exit of carcinoma cells from the circulation and colonization at the distal sites. At the earliest stage of successful cancer cell dissemination, the primary cancer adapts the secondary site of tumor colonization involving the tumor-stroma crosstalk. The migration and plasticity of cancer cells as well as the surrounding environment such as stromal and endothelial cells are mandatory. Consequently, the mechanisms of cell movement are of utmost relevance for targeted intervention of which three different types have been reported. Tumor cells can migrate either collectively, in a mesenchymal or in an amoeboid type of movement and intravasate the blood or lymph vasculature. Intravasation by the interaction of tumor cells with the vascular endothelium is mechanistically poorly understood. Changes in the epithelial plasticity enable carcinoma cells to switch between these types of motility. The types of migration may change depending on the intervention thereby increasing the velocity and aggressiveness of invading cancer cells. Interference with collective or mesenchymal cell invasion by targeting integrin expression or metalloproteinase activity, respectively, resulted in an amoeboid cell phenotype as the ultimate exit strategy of cancer cells. There are little mechanistic details reported in vivo showing that the amoeboid behavior can be either reversed or efficiently inhibited. Future concepts of metastasis intervention must simultaneously address the collective, mesenchymal and amoeboid mechanisms of cell invasion in order to advance in anti-metastatic strategies as these different types of movement can coexist and cooperate. Beyond the targeting of cell movements, the adhesion of cancer cells to the stroma in heterotypic circulating tumor cell emboli is of paramount relevance for anti-metastatic therapy.

Fig. 1

Plasticity of invading cancer cells. (A) In collectively invading cell strands or sheets, the tumor cell at the invasive front is designated as tip cell and executes the driving force by promoting focal adhesions and ECM rearrangement. Cells following the tip cells maintain their epithelial characteristics such as desmosomes, adherence, tight and gap junctions. (B) A single cancer cell can detach from the epithelial cluster by undergoing an EMT through e.g. TGF-β induced downregulation of E-cadherin and rearrangement of the cytoskeleton. This mesenchymal-like cell is able to move freely by forming focal adhesions and rearranging the ECM by secretion of proteases. EMT-transformed cells sustain a rear–front cell polarity. (C) The fastest way of cell invasion is the amoeboid cell movement, which leads to a total loss of cell polarity and paracrine directed chemotaxis in a protease-independent fashion. Cancer cells were shown to acquire these characteristics in two ways: either by inhibition of β1-integrin leading to CAT or by inhibition of proteases which subsequently induce MAT. CAT, collective to amoeboid transition; ECM, extracellular matrix; EMT, epithelial to mesenchymal transition; MAT, mesenchymal to amoeboid transition; MMP, matrix metalloproteinase.

Fig. 2

The tumor microenvironment and its impact on transendothelial migration. (A) Passive migration of epithelial cancer cells into lymph vessels may occur through intercellular gaps and active attraction of lymph endothelial cells upon secretion of VEGF-C, VEGF-D and CCL21. (B) (Transient) TGF-β induced EMT and the secretion of VEGF-A mediates hematogeneous dissemination. (C) Once transmigrated, tumor cells can use platelets as a shield against shear forces and natural killer (NK) cell attacks. (D) Two prominent cell types of the tumor microenvironment are tumor associated macrophages (TAMs, green color) and cancer associated fibroblasts (CAFs, red color). Both TAMs and CAFs secrete components which stimulate cancer progression. (E) CAFs, also referred to as myofibroblasts, rearrange the ECM and are able to form a track through which epithelial tumor cells can migrate. (F) In close contact with tumor cells, macrophages are able to promote hematogenous transmigration. CCL, chemokine (C–C motif) ligand; CSF, colony stimulating factor; CXCL12 (SDF1), stromal cell derived factor; ECM, extracellular matrix; EGF, epidermal growth factor; MMP, matrix metalloproteinase; NF-κB, nuclear factor-kappa B; TGF, transforming growth factor; VEGF, vascular endothelial growth factor.