S100 proteins in cancer

Abstract

In humans, the S100 protein family is composed of 21 members that exhibit a high degree of structural similarity, but are not functionally interchangeable. This family of proteins modulates cellular responses by functioning both as intracellular Ca(2+) sensors and as extracellular factors. Dysregulated expression of multiple members of the S100 family is a common feature of human cancers, with each type of cancer showing a unique S100 protein profile or signature. Emerging in vivo evidence indicates that the biology of most S100 proteins is complex and multifactorial, and that these proteins actively contribute to tumorigenic processes such as cell proliferation, metastasis, angiogenesis and immune evasion. Drug discovery efforts have identified leads for inhibiting several S100 family members, and two of the identified inhibitors have progressed to clinical trials in patients with cancer. This Review highlights new findings regarding the role of S100 family members in cancer diagnosis and treatment, the contribution of S100 signalling to tumour biology, and the discovery and development of S100 inhibitors for treating cancer.

Figure 1. S100 protein structural organization

a | Apo-S100 protein shown with one blue subunit and one yellow subunit. S100 proteins are regulated by Ca²⁺ binding (grey circles), which allows them to act as Ca²⁺ sensors that can translate alterations in intracellular Ca²⁺ levels into a cellular response. Ca²⁺ binding induces a conformational rearrangement that exposes a hydrophobic cleft, allowing the S100 protein to bind its cellular targets (green) and elicit a physiological response. b | Ribbon and surface diagrams of apo–S100B, Ca²⁺–S100B and the Ca²⁺–S100B–p53 peptide complex. Individual subunits are shown in blue and yellow, the Ca²⁺ ions are shown as dark grey spheres, and the TP53 peptide is shown in green. The conformational rearrangement that occurs upon Ca²⁺ binding is referred to as the ‘Ca²⁺ switch’, and involves the reorientation of helix 3 (H3) and subsequent exposure of hydrophobic residues that participate in target protein binding.

Figure 2. S100 signalling in breast cancer

Intracellular and extracellular S100A4 and S100A7, and extracellular S100A8 and S100A9 mediate breast tumour progression and metastasis. In cells that do not express oestrogen receptor-α(ERα), intracellular S100A7 enhances cell survival and invasion by upregulating nuclear factor-κB (NF-κB; which is encoded by NFKB) and epidermal growth factor receptor (EGFR) signalling. Tumour cell-derived extracellular S100A7 binds receptor for advanced glycosylation end products (RAGE) on macrophages to mediate their recruitment to the tumour microenvironment. In addition, extracellular S100A7 also binds RAGE on endothelial cells to promote angiogenesis. S100A4 expression in tumour cells enhances cell migration and invasion through interactions with cytoskeletal effectors such as myosin IIA and Rhotekin (RTKN). Stromal cell-derived S100A4 mediates the recruitment of myeloid cells to the tumour microenvironment and is required for metastatic colonization in the lung. Whether these responses are elicited by extracellular or intracellular S100A4 is unknown; however, extracellular S100A4 stimulates matrix metalloproteinase (MMP) production in endothelial cells. Within the tumour microenvironment, S100A8 and S100A9 have both autocrine and paracrine functions that sustain myeloid recruitment and/or immune suppression and NF-κB signalling, respectively. The release of cytokines by the primary tumour also promotes the recruitment of myeloid cells expressing S100A8 or S100A9 to the pre-metastatic lung to maintain a pro-inflammatory milieu that promotes tumour metastasis. Lastly, chemotherapy induces the release of tumour necrosis factor-α (TNFα) from endothelial and other stromal cells, which results in a CXC-chemokine ligand 1 (CXCL1)- or CXCL2-induced S100A8 or S100A9 signalling axis between tumour cells and myeloid cells that facilitates the survival of chemoresistant tumour cells both within the primary tumour and at metastatic sites. AP-1, activator protein 1; CAF, cancer-associated fibroblast.

Figure 3. S100 signalling in melanoma

Intracellular and extracellular S100A4, S100A8 and S100A9 contribute to melanoma proliferation and metastasis. Stroma-derived extracellular S100A4 and S100A9 — by signalling through receptor for advanced glycosylation end products (RAGE) and extracellular matrix metalloproteinase inducer (EMMPRIN), respectively — activate nuclear factor-κB (NF-κB)-mediated tumour cell expression of cytokines and matrix metalloproteinases (MMPs) that promote tumour cell invasion and metastasis. Extracellular S100A8 and S100A8–S100A9 heterodimers are also released from stromal cells, but their contribution to melanoma biology and their molecular mechanisms of action have not been elucidated. Melanoma-derived exosomes promote S100A8 and S100A9 expression at metastatic niches. In tumour cells, wild-type TP53 upregulates S100B expression, which promotes proliferation via p90 ribosomal S6 kinase (RSK) and a negative feedback loop involving TP53. S100B binding to TP53 blocks oligomerization and stimulates TP53 polyubiquitylation and degradation, which decreases TP53 transcriptional activity and downregulates the expression of pro-apoptotic genes. S100B binding to RSK blocks ERK-dependent phosphorylation and cytoplasmic sequestration of phospho-RSK. CAF, cancer-associated fibroblast; TRAF2, tumour necrosis factor receptor-associated factor 2.

Figure 4. S100 protein–inhibitor complexes

a | Ribbon and surface diagrams of the Ca²⁺–S100B–pentamidine complex (RCSB Protein Data bank (PDB) identifier: 3CR4), the Ca²⁺–S100B–SEN205A complex (PDB identifier: 3HCM), the Ca²⁺–S100A13–amlexanox complex (PDB identifier: 2KOT) and the Ca²⁺–S100A4–trifluoperazine (TFP) complex (PDB identifier: 3KO0). Individual S100 subunits are shown in light and dark blue, the Ca²⁺ ions are shown as grey spheres and the inhibitors as orange sticks. Small-molecule inhibitors can bind S100 proteins in distinct orientations. TFP binding induces the assembly of five Ca²⁺–S100A4–TFP dimers into a pentameric ring. b | Ribbon diagram of Ca²⁺–S100B showing three small-molecule binding sites. Site 1 involves residues from helices 3 and 4, and loop 2 (the ‘hinge’ region); site 2 involves residues from loop 2 and helix 4; and site 3 involves residues from the carboxy-terminal loop and helix 1. c | The ‘binding and functional folding’ (BFF) model for S100 protein–protein interactions (PPIs). In the absence of a molecular target, Ca²⁺-bound S100 proteins (that is, Ca²⁺–S100B) sample a large ensemble of dynamic sub-states with a range of Ca²⁺-binding affinities (red and blue ribbon diagrams) that result in a low net apparent affinity for Ca²⁺. Target binding induces a mini-folding event that stabilizes these dynamic features and biases the ensemble towards those sub-states (blue ribbon diagram), with high affinity for Ca²⁺ and a lower (that is, more favourable) global free energy (a complete description of the model can be found in Liriano et al and Markowitz et al). Typically, complexes with full-length targets exhibit lower free energies than complexes involving target-derived peptides. This property allows for high intracellular S100 protein concentrations (>1 µM) without substantial sequestration of free Ca²⁺ or disruption of Ca²⁺ oscillations, but provides a highly responsive system that is poised to regulate cellular processes upon target binding. Drugs that also enhance S100 protein Ca²⁺ occupancy can be identified and/or better engineered by monitoring changes in the structure and dynamic properties of the S100 protein upon drug and/or target binding. Part c reprinted from J. Mol. Biol423, Liriano, M. A. et al. Target binding to S100B reduces dynamic properties and increases Ca²⁺-binding affinity for wild type and EF-hand mutant proteins, 365–385 © (2012), with permission from Elsevier.