Concepts of extracellular matrix remodelling in tumour progression and metastasis

Abstract

Tissues are dynamically shaped by bidirectional communication between resident cells and the extracellular matrix (ECM) through cell-matrix interactions and ECM remodelling. Tumours leverage ECM remodelling to create a microenvironment that promotes tumourigenesis and metastasis. In this review, we focus on how tumour and tumour-associated stromal cells deposit, biochemically and biophysically modify, and degrade tumour-associated ECM. These tumour-driven changes support tumour growth, increase migration of tumour cells, and remodel the ECM in distant organs to allow for metastatic progression. A better understanding of the underlying mechanisms of tumourigenic ECM remodelling is crucial for developing therapeutic treatments for patients.

Fig. 1. Mechanisms of ECM remodelling.

a ECM deposition and modification: using collagen as an example, pre-procollagen is synthesised and translocated to the Golgi, where it becomes a procollagen α-chain. This procollagen molecule undergoes several post-translational modifications (PTMs) to modify its properties. The PTMs include glycosylation, pro-peptide alignment, disulphide bond formation and hydroxylation. Lysine hydroxylation of the procollagen chains by PLODs allows for spontaneous triple helix formation within the cell and secretion into the extracellular space. Here, the pro-peptides on the C- and N-terminal are cleaved by proteases creating collagen fibrils. For further collagen fibre assembly, collagens fibrils are cross-linked by LOX. b ECM degradation: proteases including MMPs cleave the ECM proteins, which releases matrix-bound growth factors (GFs) and cytokines, and ECM fragments, including matrikines and also remove barriers for cell migration. c Force-mediated ECM remodelling: integrin binding to the ECM molecules applies forces to ECM molecules. This can change the conformation of the ECM molecule, thereby exposing binding sites to support self-assembly into fibrils that induces fibre alignment. The mechanical force applied by the integrins in this modification process can also cause non-proteolytic breaching of the basement membrane that will allow cancer cell invasion.

Fig. 2. ECM remodelling in the primary tumour.

a, b Tumour-derived factors activate stromal cells which differentiate into cancer-associated fibroblasts (CAFs) leading to the secretion and deposition of large amounts of ECM components along with the cancer cells. c ECM-modifying enzymes such as LOX expressed by tumour cells and CAFs cross-link and align collagen fibres, which increases matrix stiffness around the tumour, and e the formation of a physical barrier to evade immune surveillance by T-cells. d Increased matrix stiffness promotes the interaction between ECM components and cell-surface receptors on tumour cells that triggers mechanosignalling mediated by integrins. f To sustain a tumourigenic microenvironment, tumour cells and resident immune cells secrete cytokines, chemokines and growth factors (GFs), which differentiate and recruit bone marrow-derived cells (BMDCs). g The BMDCs, CAFs and tumour cells secrete ECM-degrading proteases, including MMPs, which are cell surface-bound (e.g., MT1-MMP) or secreted (e.g., MMP-9). h Proteolytic ECM degradation generates bioactive matrikines and i releases matrix-bound GFs. These factors induce pro-tumourigenic ECM signalling that promotes tumour proliferation, migration, invasion and angiogenesis. j These combined changes to the ECM create a hypoxic environment. Neutrophils secrete potent MMP-9 that degrades ECM and releases matrix-bound VEGF that forms a concentration gradient for new angiogenic sprouting. k Stimulated by dense ECM, the tumour cells may gain endothelial-like functions and mimic the vasculature that connects to blood vessels.

Fig. 3. ECM remodelling during tumour cell migration.

a In carcinoma in situ, tumour cells are restricted from migrating and invading into the surrounding tissue by the intact basement membrane. Tumour cells and stroma cells may be physically connected through the basement membrane using integrins. ECM in the interstitial matrix is curly. b Basement membrane breakage can be achieved through proteolytic ECM degradation by proteases secreted by tumour cells and activated stroma cells (top) and through non-proteolytic, force-mediated ECM remodelling (lower box). Integrins expressed on invadopodia, an invasive actin-rich cell structure on tumour and cancer-associated fibroblasts (CAFs), bind to ECM molecules and couple them intracellularly to contractile structures. This process pulls the ECM molecules apart and applies force to the basement membrane facilitating non-proteolytic basement membrane breaching. c In invasive tumours, the basement membrane is mostly degraded. CAFs and tumour cells secrete LOX to cross-link collagen fibres. Increased cross-linking and force-mediated ECM remodelling creates linearised ECM in the tumour-surrounding interstitial matrix. Tumour cells migrate along regions with dense, aligned collagen fibres forming migratory tracks for efficient cell migration. (lower box) Membrane-bound proteases expressed on tumour cells and CAFs, such as MT1-MMP localised on invadopodia, degrades the collagen migration barriers. Exosomes contain additional proteases to clear the ECM for tumour cell migration and are released into the interstitial matrix. Integrins on the surface of secreted exosomes bind to fibronectin, which functions as a bridging molecule connecting integrins expressed on tumour cells thereby promoting the formation of invadopodia and tumour cell migration. CAFs can function as leader cells for directed tumour cell migration and are connected to tumour cells through E-cadherin/N-cadherin adhesions.

Fig. 4. ECM remodelling in the metastatic cascade.

a Angiogenesis and high MMP activity at the primary tumour site lead to a disrupted vasculature that allows tumour cells to intravasate and enter the circulation. Circulating tumour cells (CTCs) may secrete ECM that protects them from immune surveillance. b CTCs may connect via matrix-like interactions with neutrophil extracellular traps (NETs) and NETotic neutrophils using integrins that are expressed on CTCs and neutrophils. c Endothelial cells (EC) deposit and assemble fibrillar fibronectin that promotes the attachment of CTCs to the endothelial wall at distant organs. Increased MMP activity creates a leaky vasculature allowing CTCs to extravasate into the surrounding tissue. ECs also remodel the tissue in distant organs and deposit ECM contributing to the establishment of a pre-metastatic niche. d Various factors derived from the primary tumour such as growth factors, MMPs, LOX, ECM proteins like fibronectin, and exosomes, create a pre-metastatic niche at a distant site to prime the new tissue for metastasis. Stromal cells in the pre-metastatic niche are activated by tumour-derived factors and myofibroblasts remodel the ECM, for example, by the deposition of fibronectin, tenascin C, osteopontin, and versican depending on tissue context. Bone marrow-derived cells (BMDCs) are recruited to the pre-metastatic niche, attach to the remodelled ECM via integrins and contribute to further ECM remodelling in preparation for the arrival of disseminated tumour cells. e CTCs that extravasate through the disrupted vasculature into the distant tissue may become dormant. Proteases expressed in NETs including neutrophil elastase and MMP-9 cleave laminin, generating a specific matrikine that can awaken dormant tumour cells. Together, these ECM remodelling processes support the formation of metastasis.