The TWEAK-Fn14 cytokine-receptor axis: discovery, biology and therapeutic targeting

Abstract

TWEAK is a multifunctional cytokine that controls many cellular activities including proliferation, migration, differentiation, apoptosis, angiogenesis and inflammation. TWEAK acts by binding to Fn14, a highly inducible cell-surface receptor that is linked to several intracellular signalling pathways, including the nuclear factor-kappaB (NF-kappaB) pathway. The TWEAK-Fn14 axis normally regulates various physiological processes, in particular it seems to play an important, beneficial role in tissue repair following acute injury. Furthermore, recent studies have indicated that TWEAK-Fn14 axis signalling may contribute to cancer, chronic autoimmune diseases and acute ischaemic stroke. This Review provides an overview of TWEAK-Fn14 axis biology and summarizes the available data supporting the proposal that both TWEAK and Fn14 should be considered as potential targets for the development of novel therapeutics.

Figure 1. TWEAK–Fn14 axis publication summary

The number of TWEAK and/or Fn14 manuscripts published annually is shown. Publication numbers include both review articles and primary papers containing the terms TWEAK or Fn14 in the title or abstract with the exception that TWEAK was referred to as APO3L in the 1998 publication. The value shown for 2007 includes articles published online ahead of print.

Figure 2. The Fn14 receptor

a | A schematic representation of the human Fn14 receptor is shown. Mature human Fn14 is only 102 amino acids (aa) in length, with a predicted molecular mass of 10,925 daltons and a theoretical isoelectric point of 8.24. b | High-resolution crystal structures for the TWEAK receptor-binding domain, the Fn14 extracellular domain (or cysteine-rich domain) or TWEAK–Fn14 complexes are presently unavailable. A homology model of the Fn14 cysteine-rich domain based on the X-ray crystal structure of the APRIL (A proliferation-inducing ligand)–BCMA (B-cell maturation antigen) complex is shown here. The cysteine residues (yellow) and the non-cysteine residues that have been shown to be important (red) or dispensable (green) for TWEAK binding to Fn14 are labelled and rendered as sticks. This figure is reproduced with permission from REF. © (2006) The Biochemical Society.

Figure 3. The Fn14 gene is highly regulated in vivo

In most normal, healthy tissues Fn14 expression is relatively low, but Fn14 gene induction has been reported in various rodent models of tissue injury. Additionally, high levels of Fn14 expression have been detected in several human tumour types. As illustrated in the figure, it is assumed that Fn14 overexpression in injured tissues and solid tumours will promote Fn14 trimerization and multimerization, but this has not been confirmed experimentally.

Figure 4. TWEAK-dependent and -independent Fn14 signalling

a | TWEAK is initially synthesized as a type II transmembrane protein (denoted here as membrane (m) TWEAK) but this protein can undergo intracellular proteolysis to generate a smaller, biologically active form that is released into the extracellular mileau (denoted here as soluble (s) TWEAK). As mTWEAK proteolytic processing does not occur with 100% efficiency, TWEAK can be detected on the surface of most cell types by fluorescence-activated cell-sorter analysis. It is established that sTWEAK can bind the Fn14 receptor and induce cellular responses, but it has not yet been reported that mTWEAK can act in a cell contact-dependent, juxtacrine manner to activate Fn14-positive cells. TWEAK is a trimeric cytokine, and it is likely that TWEAK binding promotes Fn14 trimerization, TNFR-associated factor (TRAF) association, signal pathway activation, changes in gene expression and thus, cellular responses. b | Ectopic Fn14 overexpression studies conducted in vitro have indicated that when cellular Fn14 levels are elevated to a certain threshold value, TWEAK-independent Fn14 signalling can occur. It is likely that Fn14 overexpression on the cell surface induces monomer trimerization and trimer multimerization, which then triggers TRAF association and the subsequent molecular and cellular events. It is not presently known if TWEAK-independent Fn14 signalling occurs in cells that naturally express high levels of Fn14 in vivo (for example, certain tumour cells).

Figure 5. Hypothesis: TWEAK expression levels may dictate the predominant signalling mechanism in injured tissues and solid tumours

a | Tissues in which basal TWEAK expression levels are high and basal Fn14 expression levels are low, minimal Fn14 receptor activation is expected. However, an increase in Fn14 expression; for example, following tissue injury, would trigger TWEAK-dependent Fn14 signalling. b | Tissues in which both basal TWEAK and Fn14 expression levels are low, minimal Fn14 receptor activation is expected. In this situation, an increase in Fn14 expression; for example, in response to tumour cell derived-growth factors, would trigger TWEAK-independent Fn14 signalling.

Figure 6. Hypothesis: some tumour angiogenic factors may act, at least in part, by increasing TWEAK-dependent Fn14 signalling in sprouting vessels

Tumours produce multiple angiogenic factors, including vascular endothelial growth factor A (VEGFA), fibroblast growth factor 2 (FGF2) and TWEAK. These three factors bind and activate the VEGFR2, FGFR1 and Fn14 receptors, respectively, on capillary endothelial cells. Each receptor is shown here as a monomer, but when bound to ligands, they either dimerize (FGFR1, VEGFR2) or trimerize (Fn14). Ligand–receptor engagement would activate intracellular signalling cascades and induce the expression of numerous genes, including Fn14 (it is indicated here that all three angiogenic factors can induce Fn14 expression in endothelial cells, but to date, TWEAK-regulated Fn14 expression has only been reported in glioma cells). TWEAK levels in the tumour microenvironment are probably relatively high, so an increase in Fn14 gene expression is likely to trigger additional (and perhaps persistent) cycles of TWEAK-dependent Fn14 signalling in the capillary endothelial cells. In this way, TWEAK, acting in concert with VEGFA and FGF2, would play a significant role in pathological sprouting angiogenesis.

Figure 7. Potential therapeutic strategies for inhibiting Fn14-triggered pathological conditions

TWEAK-dependent Fn14 signalling is likely to be the predominant mechanism inducing pathological effects in humans. This pathway could be blocked using various protein- or small molecule-based TWEAK- or Fn14-targeted drugs that prevent TWEAK binding to Fn14. Indeed, the first two therapeutic strategies listed under TWEAK-dependent Fn14 signalling have shown efficacy in several rodent models of human disease. Alternatively, one could use membrane-permeable drugs that prevent Fn14–TRAF (TNFR-associated factor) association or inhibit downstream signalling events, but in this case it may prove more difficult to achieve TWEAK–Fn14 axis specificity. It is also possible that in tissues where the Fn14 receptor is overexpressed and TWEAK levels are low, TWEAK-independent Fn14 signalling could contribute to certain disease pathologies. This pathway could be inhibited using an anti-Fn14 monoclonal antibody (mAb) that blocks receptor multimerization or disassociates preformed multimers. Alternatively, Fn14-targeted toxins delivered as fusion proteins or chemical conjugates could be used to kill Fn14-overexpressing cells. One could also target Fn14–TRAF association or downstream signalling events as mentioned in the main text.