Role of extracellular matrix in breast cancer development: a brief update

Epithelial-mesenchymal transition (EMT)

The EMT (process of losing epithelial characteristics and gaining mesenchymal properties) is the beginning step of metastasis. About 90% deaths from BC are due to invasion and metastasis, and EMT plays a significant role involving different transcription factors (TFs) and signals^2–5. It induces metastasis through ECM disruption and metabolism reprogramming. Aberrant cancer metabolism promotes EMT which further aggravates metabolism (especially glucose metabolism) through EMT-specific TFs such as Snail and TWIST⁶. Platelets and platelet-derived TGF-β promote epithelial-mesenchymal-like transition and promote metastasis in vivo⁷. Snail is a transcriptional repressor of E-cadherin (cell-cell adhesion molecule), and E-cadherin loss is a hallmark of EMT². Snail and TWIST cooperate inducing another TF, ZEB1⁸ (significant inducer of EMT, invasion, and metastasis), which is triggered by extracellular hyaluronic acid (HA). Furthermore, ZEB1 induces HAS2 synthesis, promoting HA production in a positive feedback loop and its expression is correlated with ZEB1 expression in poor prognosis tumors. HAS2 also has a role in TGF-β-induced EMT⁹.

Enzymes in ECM remodeling

Various ECM-remodeling enzymes are induced in BC promoting stem/progenitor signaling pathways and metastasis. Major ECM proteins induced are fibrillar collagens, fibronectin, specific laminins, proteoglycans, and matricellular proteins and these could be potential drug targets for therapy¹⁰. Matrix metalloproteinases (MMPs) degrade ECM proteins promoting invasion and metastasis. The MMP-11 (stromelysin-3) seems facilitating tumor development through apoptosis inhibition. However, it suppresses metastasis in animal models, exhibiting different roles in tumor progression¹¹. β-D mannuronic acid (BDM) is a MMP inhibitor, inhibiting MMP-2 and MMP-9 involved in invasion, metastasis, and angiogenesis¹². BDM possesses anti-metastatic activity and inhibits tumor growth by suppressing inflammatory chemokine and tumor–promoting cytokines¹³. MMP-14 located on the cell surface, is a potential target to stop metastasis and a novel antibody-mediated MMP-14 blockade seems to limit hypoxia and metastasis in triple negative breast cancer (TNBC) models¹⁴. Loss of ECM integrity by plasmin facilitates cancer cell spread^15,16 and plasmin-induced ECM degradation may be controlled by lipoprotein-A (competitive inhibitor of plasminogen)¹⁷. Vitamin C seems to be very important curbing tumor growth, and metastasis as ECM integrity requires vitamin C¹⁷. The Lox (Lysil oxidase) family of genes enhances ECM fibrosis through collagen cross-linking and it seems down-regulation of LOXL4 promotes BC growth and lung metastasis in mice¹⁸. The LOXL2 protein catalyzes cross-linking of ECM components collagen and elastin and is involved in cancer progression and metastasis. The intracellular LOXL2 shows EMT induction and Snail-1 stabilization, and LOXL-2/Snail-1-mediated E-cadherin down-regulation promotes lung metastasis of BC without affecting ECM stiffness¹⁹.

The enzyme procollagen lysyl hydroxylase-2 required for collagen synthesis, increases breast tumor stiffness, promotes metastatic tumors in lymph nodes and lungs. Matrix stiffness promotes tumor progression and invasion of ER+ type BC²⁰. The hardened ECM drives invasion and metastasis through ERK1/2 signal up-regulation and JAK2/STAT5 signal down-regulation. The enzyme heparanase cleaves heparan sulfate, promoting tumor invasion and metastasis. ER stress during chemotherapy enhances the heparanase activity²¹. The MMTV-heparanase mice promoted growth and metastasis of breast tumor cells to lungs suggesting a role for heparanase in BC progression²². Elemene (extract of Curcuma erhizoma plant), is an anticarcinogenic phytochemical showing effects by down-regulating heparanase expression (potential target for heparanase)²³. The heparin and nanoheparin derivatives show their anti-cancer activities by reducing BC cell proliferation and metastasis²⁴.

Stromal cells in BC development

Tumor cells recruit tumor-associated macrophages (TAMs), which become proangiogenic by secreting VEGF-A which nourishes tumor cells and build a vessel network for their invasion. Hypoxia also induces macrophages to produce more VEGF and suppress immune response, promoting invasion²⁵. Cancer-associated fibroblasts (CAFs) are involved in tumor development, progression, inflammation, metastasis, and build resistance to cancer therapy through secretion of hormones, cytokines, growth factors, etc. and cross-talk with other stromal cells, cancer cells, and ECM. CAFs can be potential therapeutic targets in BC²⁶. Cancer cell proliferation and migration is induced by activated fibroblasts derived from endothelial-to-mesenchymal transformation²⁷. Adipocytes have a significant role in cancer progression, ECM remodeling, phenotype changes of CAFs, and resistance to cancer therapy²⁸.

Various ECM proteins in BC progression

Integrins, the primary receptors of MECs for ECM, act as sensors of epithelial microenvironment. Their altered expression seems to disorganize ECM and promotes metastasis²⁹. Increased MEC proliferation occurs due to enhanced activity of integrin signaling (b1-, b5-, and b6- integrins) by co-activating the oncogenes for enhanced growth factor signaling. Protein ECM1 is involved in angiogenesis, promoting TNBC migration and invasion³⁰. Protein Hic-5 (focal adhesion scaffold/adaptor protein) promotes mammary duct formation. Focal adhesions of cells are attached to ECM and transduce signals from ECM to cell. Hic-5 is up-regulated in CAFs of BC, involved in EMT and invadopodia formation facilitating invasion, migration and metastasis³¹. The sustained directionality of tumor cells to a vessel is promoted by a chemotactic gradient of hepatocyte growth factor (HGF) produced from vessel endothelium. This directional streaming is possible by HGF/c-Met signaling pathway between endothelial cells and tumor cells; and c-Met inhibitors could be a potential target to block tumor cell streaming and metastasis³².