The Extracellular Matrix Modulates the Metastatic Journey

Abstract

The extracellular matrix is perturbed in tumors. The tumor matrix promotes the growth, survival, and invasion of the cancer and modifies fibroblast and immune cell behavior to drive metastasis and impair treatment. Here, we discuss how the tumor matrix regulates metastasis by fostering tumor cell invasion into the stroma and migration toward the vasculature. We describe the role of the tumor matrix in cancer cell intravasation and vascular dissemination. We examine the impact of the matrix on disseminated tumor cell extravasation and on tumor dormancy and metastatic outgrowth. Finally, we discuss the clinical outcome of therapeutics that normalize tumor-matrix interactions.

Keywords: angiogenesis; dormancy; extracellular matrix; extravasation; integrin; intravasation; invasion; mechanics; mechanotransduction; metastasis; migration; outgrowth; pre-metastatic niche; tissue tension.

Figure 1.

Schematic showing the steps of cancer metastasis. (A) A normal epithelial acini is surrounded by a contiguous laminin-rich basement membrane and the whole structure is embedded within an interstitial extracellular matrix (ECM) that is characterized by a preponderance of curly and loosely organized collagenous proteins. Upon transformation, the acinar lumen of an “in situ” benign carcinoma progressively fills with proliferating tumor cells, basement membrane thickness gradually decreases and laminin levels drop. Furthermore, there is evidence that the surrounding interstitial ECM collagens become remodeled, reorganized and thickened. Malignant transformation to an “invasive carcinoma” is accompanied by further metalloproteinase-mediated ECM remodeling and lysyl oxidase (LOX) and lysyl hydroxlase-mediated collagen crosslinking and stiffening that provide linearized, thickened collagen-rich fibrils upon which the tumor cells migrate and invade into the surrounding parenchyma. ECM remodeling and stiffening occur in tandem with increased proliferation and activation of stromal fibroblasts and infiltration of immune cells including macrophages and neutrophils and induction of angiogenesis. (B) (Step a) Mechanical stresses such as compression stress and ECM stiffening foster tumor cell migration through the parenchyma towards the vasculature. (Step b) ECM stiffness also facilitates tumor cell intravasation into the vasculature by compromising vascular integrity and increasing tumor cell deformability through induction of an epithelial to mesenchymal transition. (Step c) Once within the circulation, the circulating tumor cells (CTCs) encounter hemodynamic shear stress. CTC survival can be potentiated by platelets through their ability to shield the tumor cells from shear stress and through integrin-dependent adhesion signaling activation. (Step d) Primary tumor cells also secrete soluble factors, ECM proteins and exosomes that create a premetastatic niche by incorporating into secondary “distal” tissues that prime the recruitment and retention of immune cells and disseminating tumor cells that foster tumor colonization. (Step e) Tumor cells find a favorable site for extravasation. With the assistance of platelets, CTCs adhere to the endothelium and migrate across the endothelial layer (Step f). The extravasated CTCs may either undergo apoptosis (step g), enter a dormant state (step h), or proliferate to form secondary metastatic lesions (step i). The dormant cells retain their proliferative ability and may eventually re-enter cell cycle and form metastatic lesions (step J).

Figure 2.

Integrin-dependent adhesion and mechanotransduction pathways. Cells constantly sample the biochemical composition of the surrounding ECM using cell surface receptors such as integrins, discoidin domain receptors (DDRs) and syndecans and modulate intracellular signaling pathways accordingly. Integrins crosstalk with multiple transmembrane proteins including growth factor receptors (GFRs). The crosstalk between integrins and adJacent transmembrane molecules can synergize to potentiate Rho GTPase activity, focal adhesion assembly, kinase signaling and stimulate gene transcription to induce tumor cell growth, survival and motility and may even induce differentiation. (Left) When adhesion signaling is low, integrins remain in an inactive conformation. (Right) Binding of integrins to ECM ligands can trigger the recruitment and activation of talin and the subsequent association of molecules such as vinculin and paxillin and integrin-linked kinase (ILK) and the activation of focal adhesion kinase (FAK) that promote the assembly of adhesion complexes. For example, paxillin and FAK form a complex with Src kinase to activate the PI3K-Akt pro-survival signaling pathway. The crosstalk between integrins and GFR pathways can also potentiate Rho GTPase signaling. RhoA stimulates ROCK kinase activity, which increases the level of phosphorylated myosin light chain (MLC) to stimulate actomyosin contraction. Rac, Rho, and Cdc42 GTPases also promote the formation of invasive cellular protrusions, such as lamellipodia, filopodia, and invadosomes. Upon ECM ligand ligation, DDRs and syndecans can recruit diverse signaling molecules such as myosin IIA to promote cell contractibility and migration. Cells additionally interrogate the mechanical properties of the ECM using integrins through a process termed mechanotransduction. Stiff substrates promote integrin clustering, adhesion signaling, and TWIST1 and YAP translocation into the nucleus to activate transcription programs that promote epithelial mesenchymal transition. In addition, a bulky glycocalyx on the plasma membrane can directly exert stress on integrins to enhance their activation and once ligated induces a kinetic trap that physically clusters integrins to promote their assembly into focal adhesions that enhance adhesion signaling.

Figure 3.

A stiffened ECM promotes the epithelial to mesenchymal transition and invasion of PyMT breast cancer cells. Representative images showing PyMT tumor organoid expressing a SNAIL-YFP reporter embedded within a soft collagen gels or a strain-stiffened collagen gel. Magnified images show the invasive front of the tumor organoids antibody stained for the YFP-EMT-marker SNAIL (green), a transcription factor upregulated during EMT. Cells are also costained for actin (white) and nuclei (blue). Figure is modified, with permission, from (Mekhdjian et al., 2017)

Figure 4.

Overview of inhibitors that perturb cell-ECM interactions and/or adhesion signaling and may have applications for the treatment of metastatic cancers.