Hypoxia and the extracellular matrix: drivers of tumour metastasis

Abstract

Of the deaths attributed to cancer, 90% are due to metastasis, and treatments that prevent or cure metastasis remain elusive. Emerging data indicate that hypoxia and the extracellular matrix (ECM) might have crucial roles in metastasis. During tumour evolution, changes in the composition and the overall content of the ECM reflect both its biophysical and biological properties and these strongly influence tumour and stromal cell properties, such as proliferation and motility. Originally thought of as independent contributors to metastatic spread, recent studies have established a direct link between hypoxia and the composition and the organization of the ECM, which suggests a new model in which multiple microenvironmental signals might converge to synergistically influence metastatic outcome.

Figure 1. Biosynthesis of fibrillar collagens

The biosynthesis of type I collagen and other fibrillar collagens can be divided into intracellular (parts a–c) and extracellular (parts d–f) steps. The first intracellular step involves the synthesis of procollagen polypeptides from any of 42 distinct collagen gene transcripts (part a). Procollagens are post-translationally modified within the cisternae of the endoplasmic reticulum (ER) by prolyl 4-hydroxylase α-subunit isoform 1 (P4HA1), P4HA2 and P4HA3 and by procollagen-lysine 2-oxyglutarate 5-dioxygenase 1 (PLOD1), PLOD2 and PLOD3 lysyl hydroxylase enzymes (part b). Hydrolysine residues can be further modified to galactosyl hydroxylysine and to glucosylgalactosyl hydroxylysine by collagen galactosyltransferase and glucosyltransferase, respectively. The carboxyl termini of three properly hydroxylated procollagen molecules will associate and spontaneously propagate a procollagen triple helix from the carboxyl terminus to the amino terminus. The triple helical procollagen will be transported from the ER to the extracellular space via the Golgi (part c). Two metalloproteinases, a procollagen N-terminal proteinase and a procollagen C-terminal proteinase, cleave the non-helical termini (part d) and the mature collagen proteins spontaneously aggregate to form a collagen fibril (part e). The final step, collagen fibre formation, is initiated by collagen crosslinking, which is catalysed by lysyl oxidase (LOX) family members and occurs via the lysine aldehyde- or hydroxylysine aldehyde-initiated pathway (part f). The number and the proportion of the various crosslinks are tissue specific and are regulated by the steric relationship between localized collagen molecules, the type of collagens co-polymerized and the glycosylation and the hydroxylation of the participating amino acid residues. For example, lysine aldehyde-initiated crosslinks are found in soft connective tissue, in contrast to hydroxylysine aldehyde-initiated crosslinks, which are found in stiff connective tissues. Many non-fibrillar collagens retain a non-collagenous N- or C-terminal, which prevents the spontaneous formation of collagen fibrils, and in these collagens cysteine crosslinks might be the only source of covalent intermolecular bonds. Enzymes highlighted in red are induced under hypoxic conditions. LOXL, LOX-like protein.

Figure 2. Hypoxia promotes ECM remodelling to facilitate metastasis

Extracellular matrix (ECM) remodelling is tightly controlled to maintain tissue integrity. Cancer cells and associated stromal cells that have been exposed to hypoxia are transcriptionally reprogrammed to produce: matrix metalloproteinases (MMPs) and other proteases, which degrade the basement membrane surrounding a tumour (part a); aligned collagen fibres within the interstitial matrix, which function as a highway for local invasion, intravasation and metastasis (part b); and growth factors, which might be retained in the fibrotic microenvironment and function as chemotactic signals that recruit and activate stromal cells to further promote cancer progression (part c).

Figure 3. Hypoxia recruits and reprogrammes cells to produce fibrillar collagen

Hypoxia-induced and hypoxia-inducible factor (HIF)-regulated growth factor secretion by tumour cells promotes the recruitment of macrophages and fibroblasts to hypoxic regions of the primary tumour. Macrophages produce growth factors such as transforming growth factor β1 (TGFβ1) and platelet-derived growth factor (PDGF) that activate recruited and resident fibroblasts to stimulate collagen deposition. Hypoxic cancer cells also signal to mesenchymal stem cells, which might participate in collagen deposition. HIFs regulate the production of collagen-modifying enzymes, including prolyl 4-hydroxylase α-subunit isoform 1 (P4HA1), P4HA2, procollagen-lysine 2-oxyglutarate 5-dioxygenase 2 (PLOD2), lysyl oxidase (LOX), LOX-like protein 2 (LOXL2) and LOXL4 to facilitate the proper maturation of collagen fibres. Together, these signalling pathways promote the production of a fibrillar collagen network (that is produced by multiple cell types), which increases the ability of cancer cells to invade blood vessels. BMDSCs, bone marrow-derived stem cells; ECM, extracellular matrix.