Targeting Lysyl Oxidase Family Meditated Matrix Cross-Linking as an Anti-Stromal Therapy in Solid Tumours

Abstract

The lysyl oxidase (LOX) family of enzymes are a major driver in the biogenesis of desmoplastic matrix at the primary tumour and secondary metastatic sites. With the increasing interest in and development of anti-stromal therapies aimed at improving clinical outcomes of cancer patients, the Lox family has emerged as a potentially powerful clinical target. This review examines how lysyl oxidase family dysregulation in solid cancers contributes to disease progression and poor patient outcomes, as well as an evaluation of the preclinical landscape of LOX family targeting therapeutics. We also discuss the suitability of the LOX family as a diagnostic and/or prognostic marker in solid tumours.

Keywords: anti-stromal targeting; cancer; extracellular matrix; lysyl oxidases; tumorigenesis; tumour microenvironment.

Figure 1

The biogenesis of fibrillar collagens. (A) Pro-collagen alpha-chain subunits are synthesized intracellularly by cells (predominantly fibroblast-like cells). (B) These alpha chains are assembled into helical collagen trimers in the endoplasmic reticulum and are then secreted into the extracellular space where they undergo post-translational processing by bone morphogenetic protein 1 (BMP1) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS). (C) Cleavage results in the formation of mature tropocollagen. (D) The lysyl oxidase (LOX) family members catalyze oxidative deamination of the lysine residues on the mature tropocollagen chains. (E) Deaminated residues on adjacent chains spontaneously condense and result in a cross-link. (F) Cross-linked tropocollagens organize to form collagen fibrils. (G) Collagen fibrils are further organized and assembled into collagen fibers.

Figure 2

Remodeling of the extracellular matrix (ECM) in solid tumours. In healthy tissue, the ECM has a structured basement membrane consisting primarily of collagens IV and VI as well as a scaffolding arrangement of fibrillar collagens that are predominantly secreted by the fibroblasts. In comparison, solid tumours typically consist of more densely packed, aberrantly cross-linked fibrillar collagens resulting from the recruitment and activation of CAFs. As the level of deposition of fibrillar collagens such as collagens I, III and V increases in the tumour ECM, so too does LOX family mediated collagen cross-linking. In addition, tumour ECM results in a breakdown of the structure of the normal basement membrane.

Figure 3

Signaling pathways involved in regulation of and therapeutic avenues for targeting the LOX family. Transforming growth factor-β (TGF-β) regulates LOX family expression through the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) signaling pathway. LOX family induced ECM stiffening causes mechano-activation of mitogen activated protein kinase (MAPK) and focal adhesion kinase/proto-oncogene tyrosine-protein kinase (FAK/Src) signaling pathways which are also known to upregulate LOX family expression, whilst decreasing oxygen diffusion into the tumour resulting in hypoxia. Hypoxia stabilizes hypoxia-inducible factor 1-alpha (HIF-1α) and leads to increased LOX and LOXL2 transcription through binding the hypoxia response elements in their promoters. LOX family members can be inhibited directly with small molecule drugs targeting their active sites (1A) or humanized antibodies that may bind directly or indirectly (allosterically) to the active sites of individual family members (1B). Alternately, LOX family members can be inhibited indirectly through use of copper chelators which remove the critical copper cofactor from the lysyl oxidase active site (2A), inhibitors of copper trafficking proteins (2B), inhibitors of bone morphogenetic protein 1 (BMP1) to prevent LOX and LOXL1 proenzyme processing and activation (3) or inhibitors of the components of the pathways involved in LOX family regulation (4). Abbreviations: Mitogen-activated protein kinase kinase (MEK), extracellular signal-regulated kinase (ERK), messenger RNA (mRNA).

Figure 4

LOX family involvement in primary tumour formation and during the establishment of metastases. (A) Increased LOX cross-linking at the primary tumour site increases collagen fiber alignment as well as inducing multiple signaling cascades that result in the increased adhesion of cells to the ECM, increases in cancer cell proliferation, migration and invasion. Overall, LOX family actions result in changes in density and stiffness of the ECM that promote cancer cell dissemination to secondary sites. (B) At secondary sites, LOX family members are involved in generating a collagen rich ECM (1) and premetastatic niche formation through collagen remodeling (2). Extravasation of cancer cells to the premetastatic site and colonization of the cancer cells resulting in metastases (3) can then occur.