To understand the system of secreted proteins and receptors involved in cell-cell signaling, we produced a comprehensive set of recombinant secreted proteins and the extracellular domains of transmembrane proteins, which constitute most of the protein components of the extracellular space. Each protein was tested in a suite of assays that measured metabolic, growth, or transcriptional responses in diverse cell types. The pattern of responses across assays was analyzed for the degree of functional selectivity of each protein. One of the highly selective proteins was a previously undescribed ligand, designated interleukin-34 (IL-34), which stimulates monocyte viability but does not affect responses in a wide spectrum of other assays. In a separate functional screen, we used a collection of extracellular domains of transmembrane proteins to discover the receptor for IL-34, which was a known cytokine receptor, colony-stimulating factor 1 (also called macrophage colony-stimulating factor) receptor. This systematic approach is thus useful for discovering new ligands and receptors and assessing the functional selectivity of extracellular regulatory proteins.

The fibroblast growth factor (FGF) pathway promotes tumor growth and angiogenesis in many solid tumors. Although there has long been interest in FGF pathway inhibitors, development has been complicated: An effective FGF inhibitor must block the activity of multiple mitogenic FGF ligands but must spare the metabolic hormone FGFs (FGF-19, FGF-21, and FGF-23) to avoid unacceptable toxicity. To achieve these design requirements, we engineered a soluble FGF receptor 1 Fc fusion protein, FP-1039. FP-1039 binds tightly to all of the mitogenic FGF ligands, inhibits FGF-stimulated cell proliferation in vitro, blocks FGF- and vascular endothelial growth factor (VEGF)-induced angiogenesis in vivo, and inhibits in vivo growth of a broad range of tumor types. FP-1039 antitumor response is positively correlated with RNA levels of FGF2, FGF18, FGFR1c, FGFR3c, and ETV4; models with genetic aberrations in the FGF pathway, including FGFR1-amplified lung cancer and FGFR2-mutated endometrial cancer, are particularly sensitive to FP-1039-mediated tumor inhibition. FP-1039 does not appreciably bind the hormonal FGFs, because these ligands require a cell surface co-receptor, klotho or β-klotho, for high-affinity binding and signaling. Serum calcium and phosphate levels, which are regulated by FGF-23, are not altered by administration of FP-1039. By selectively blocking nonhormonal FGFs, FP-1039 treatment confers antitumor efficacy without the toxicities associated with other FGF pathway inhibitors.

There are many transmembrane receptor-like proteins whose ligands have not been identified. A strategy for finding ligands when little is known about their tissue source is to screen each extracellular protein individually expressed in an array format by using a sensitive functional readout. Taking this approach, we have screened a large collection (3,191 proteins) of extracellular proteins for their ability to activate signaling of an orphan receptor, leukocyte tyrosine kinase (LTK). Only two related secreted factors, FAM150A and FAM150B (family with sequence similarity 150 member A and member B), stimulated LTK phosphorylation. FAM150A binds LTK extracellular domain with high affinity (K D = 28 pM). FAM150A stimulates LTK phosphorylation in a ligand-dependent manner. This strategy provides an efficient approach for identifying functional ligands for other orphan receptors.leukocyte tyrosine kinase | extracellular protein | library screening | FAM150A | orphan receptor M any biological processes and pathogenic conditions involve ligand/receptor signaling. There are numerous transmembrane receptor-like proteins whose ligands have not been identified. Most known ligand-receptor interactions were discovered by painstaking protein purification, genetic approaches, or sequence homology. Strategies using cDNA library expressionsuch as secretion trapping (1), mammalian expression cloning (2), yeast signaling display (3), and λ phage binding display (4)-have also identified ligands for orphan receptors. However, these approaches are limited by the unknown comprehensiveness of the cDNA library and by the fact that the libraries are biased toward high-abundance transcripts and not focused on the extracellular proteome. An alternative strategy is to screen secreted factors that are individually expressed in an array format in which there is no bias in expression of each protein based on the abundance of its cDNA. The advantages of this approach for deorphanization of receptors are that it is extremely sensitive and selective and that it covers the majority of the extracellular proteome. Although arrayed proteins have been used to identify receptor partners with low-affinity, high-avidity interactions (5-7), few studies have used a cell-signaling measurement to study "classic" high-affinity ligandreceptor interactions.Leukocyte tyrosine kinase (LTK) is a receptor tyrosine kinase that was identified in 1988 (8). The extracellular domain (ECD) of LTK has no known domain structure except a glycine-rich region, leading to a proposal that LTK is not activated by a protein ligand. LTK has a close homolog named anaplastic lymphoma kinase (ALK) (9). The LTK pathway has been implicated in autoimmunity, neuronal development, and cancer. One study showed that gain-of-function polymorphism of Ltk has been associated with systemic lupus erythematosus (SLE) pathogenesis (10). Ltk is expressed throughout the adult hippocampus, and mouse knockout studies indicated that both Ltk and Alk are involved in adult neurogenesis with some functio...

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An increasing body of evidence has implicated FGF2 as one of the drivers of resistance to various inhibitors of VEGF-mediated angiogenesis. This resistance may play a role as a key limitation to the efficacy of therapies targeted at VEGF and its receptors. We investigated the potential for FP-1039/GSK3052230, a ligand trap that sequesters FGFs and inhibits their signaling, to enhance the activity of VEGF antagonist therapies in certain preclinical models of renal cell (RCC) and hepatocellular (HCC) carcinomas. First, we examined whether FP-1039/GSK3052230 has single agent efficacy against human RCC and HCC xenografts that express relatively high levels of FGF2, a profile that would mimic FGF2-driven resistance to VEGF therapy. We determined that this expression profile represents 34% of clear cell RCC (ccRCC) and 31% of HCC patients, based on the cancer genome atlas (TCGA) data. Human ccRCC xenografts with high FGF2 expression and low VEGFA expression demonstrated a significant inhibition in tumor growth when treated with FP-1039/GSK3052230 alone (TGI: 39-81%). In addition, we show that the high FGF2 expression profile is similarly predictive for the anti-tumor response of a human HCC model to single-agent FP-1039/GSK3052230 (TGI: 31-55%). In contrast, RCC models with low FGF2 expression, representing 66% of all ccRCC in the TCGA, are relatively insensitive to FP-1039/GSK3052230 as a single-agent. However, combination therapy of FP-1039/GSK3052230 with pazopanib in these tumors is significantly more effective than either agent alone. FP-1039/GSK3052230 not only slows tumor growth, but can induce ∼25% tumor regression when administered to mice bearing ccRCC xenografts that have become resistant to pazopanib. Together, our data demonstrate that FP-1039/GSK3052230 may be an effective therapy against RCC and HCC, both as a single agent in disease driven by FGF2 and in combination with VEGF antagonist therapies that represent the current standards of care for advanced disease. Citation Format: David I. Bellovin, Servando Palencia, Kevin Hestir, Ernestine Lee, M. Phillip DeYoung, Thomas Brennan, Gerrit Los, Kevin Baker. FP-1039/GSK3052230, an FGF ligand trap, enhances VEGF antagonist therapy in preclinical models of RCC and HCC. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5449. doi:10.1158/1538-7445.AM2014-5449