Abrogation of fibroblast activation protein enzymatic activity attenuates tumor growth

Expression of FAP in Oral Leukoplakia and Oral Squamous Cell Carcinoma

OBJECTIVE

This study aimed to investigate the potential of fibroblast activation protein (FAP) as a biomarker in the progression of oral leukoplakia (OLK) carcinogenesis. This was achieved by evaluating FAP expression at different levels of the organisation, namely oral normal mucosa (NM), OLK, and oral squamous cell carcinoma (OSCC).

MATERIALS AND METHODS

Altogether, 88 paraffin-embedded tissue samples were examined, including 55 cases of OLK, 13 cases of OSCC, and 20 cases of NM (control group). An exhaustive investigation was performed to examine FAP expression in NM, OLK, and OSCC tissues via immunohistochemistry (IHC). The relationship between FAP expression and clinical pathologic characteristics was analysed. Reverse transcription-polymerase chain reaction (RT-PCR) and western blot (WB) also proved the expression of FAP in NM, OLK, and OSCC cells. Aberrant FAP expression in OLK and OSCC was explored using in vitro experiments.

RESULTS

Immunohistochemical results showed that high FAP expression was significantly correlated with histopathologic grade (P = .038) but not correlated with age, sex, or region (P = .953, .622, and .108, respectively). The expression level of FAP in NM tissues (0.15 ± 0.01) was minimal, whereas it was observed in OLK (0.28 ± 0.04) and OSCC (0.39 ± 0.02) tissues with a noticeable increase in expression levels (P < .001). The expression level of FAP in OLK with severe abnormal hyperplasia (S-OLK) tissues (0.33 ± 0.04) was significantly higher than in OLK with mild abnormal hyperplasia (MI-OLK, 0.26 ± 0.02) and OLK with moderate abnormal hyperplasia (MO-OLK, 0.28 ± 0.03) tissues (P < .001 and P = .039, respectively). The results of RT-PCR illustrated that the relative expression of FAP mRNA in OLK cells (2.63 ± 0.62) was higher than in NM cells (0.87 ± 0.14), but lower than in OSCC cells (5.63 ± 1.06; P = .027 and .012, respectively). FAP expression was minimal in NM cells (0.78 ± 0.06), modest in OLK cells (1.04 ± 0.06), and significantly elevated in OSCC cells (1.61 ± 0.09) based on the results of WB (P < .001).

CONCLUSIONS

Significant variations in FAP expression were observed in NM, OLK, and OSCC tissues and cells. These findings revealed that FAP may be a reliable biomarker for the early diagnosis and evaluation of OLK carcinogenesis.

Development, preclinical evaluation and preliminary dosimetry profiling of SB03178, a first-of-its-kind benzo[h]quinoline-based fibroblast activation protein-α-targeted radiotheranostic for cancer imaging and therapy

Fibroblast activation protein-α (FAP) is a marker of cancer-associated fibroblasts (CAFs) that constitute a significant portion of most carcinomas. Since it plays a critical role in tumor growth and metastasis, its timely detection to identify tumor lesions in early developmental stages using targeted radiopharmaceuticals has gained significant impetus. In the present work, two novel FAP-targeted precursors SB03178 and SB04033 comprising of an atypical benzo[h]quinoline construct were synthesized and either chelated to diagnostic radionuclide gallium-68 or therapeutic radionuclide lutetium-177, with ≥90% radiochemical purities and 22-76% decay-corrected radiochemical yields. natGa-labeled complexes displayed dose-dependent FAP inhibition, with binding potency of natGa-SB03178 being ∼17 times higher than natGa-SB04033. To evaluate their pharmacokinetic profiles, PET imaging and ex vivo biodistribution analyses were executed in FAP-overexpressing HEK293T:hFAP tumor-bearing mice. While both tracers displayed clear tumor visualization that was primarily FAP-arbitrated, with negligible uptake in most peripheral tissues, [68Ga]Ga-SB03178 demonstrated higher tumor uptake and superior tumor-to-background contrast ratios than [68Ga]Ga-SB04033. 177Lu-labeled SB03178 was subjected to tumor retention studies, mouse dosimetry profiling and mouse-to-human dose extrapolations also using the HEK293T:hFAP tumor model. [177Lu]Lu-SB03178 exhibited a combination of high and sustained tumor uptake, with excellent tumor-to-critical organ uptake ratios resulting in a high radiation absorbed dose to the tumor and a low estimated whole-body dose to humans. Our preliminary findings are considerably encouraging to support clinical development of [68Ga]Ga-/[177Lu]Lu-SB03178 theranostic pair for use in a vast majority of FAP-overexpressing neoplasms, particularly carcinomas.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Synthesis and Preclinical Evaluation of Novel 68Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Fibroblast activation protein (FAP) is a membrane-tethered serine protease overexpressed in the reactive stromal fibroblasts of >90% human carcinomas, which makes it a promising target for developing radiopharmaceuticals for the imaging and therapy of carcinomas. Here, we synthesized two novel (R)-pyrrolidin-2-yl-boronic acid-based FAP-targeted ligands: SB02055 (DOTA-conjugated (R)-(1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)glycyl)pyrrolidin-2-yl)boronic acid) and SB04028 (DOTA-conjugated ((R)-1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)-D-alanyl)pyrrolidin-2-yl)boronic acid). ^natGa- and ⁶⁸Ga-complexes of both ligands were evaluated in preclinical studies and compared to previously reported ^natGa/⁶⁸Ga-complexed PNT6555. Enzymatic assays showed that FAP binding affinities (IC₅₀) of ^natGa-SB02055, ^natGa-SB04028 and ^natGa-PNT6555 were 0.41 ± 0.06, 13.9 ± 1.29 and 78.1 ± 4.59 nM, respectively. PET imaging and biodistribution studies in HEK293T:hFAP tumor-bearing mice showed that while [⁶⁸Ga]Ga-SB02055 presented with a nominal tumor uptake (1.08 ± 0.37 %ID/g), [⁶⁸Ga]Ga-SB04028 demonstrated clear tumor visualization with ~1.5-fold higher tumor uptake (10.1 ± 0.42 %ID/g) compared to [⁶⁸Ga]Ga-PNT6555 (6.38 ± 0.45 %ID/g). High accumulation in the bladder indicated renal excretion of all three tracers. [⁶⁸Ga]Ga-SB04028 displayed a low background level uptake in most normal organs, and comparable to [⁶⁸Ga]Ga-PNT6555. However, since its tumor uptake was considerably higher than [⁶⁸Ga]Ga-PNT6555, the corresponding tumor-to-organ uptake ratios for [⁶⁸Ga]Ga-SB04028 were also significantly greater than [⁶⁸Ga]Ga-PNT6555. Our data demonstrate that (R)-(((quinoline-4-carbonyl)-d-alanyl)pyrrolidin-2-yl)boronic acid is a promising pharmacophore for the design of FAP-targeted radiopharmaceuticals for cancer imaging and radioligand therapy.

Advanced Fibroblast Activation Protein-Ligand Developments: FAP Imaging Agents: A Review of the Structural Requirements

Fibroblast activation protein-α (FAP) has attracted increasing attention as a selective marker of cancer-associated fibroblasts (CAFs) and more broadly, of activated fibroblasts in tissues undergoing remodeling of their ECM due to chronic inflammation, fibrosis, or wound healing. Since FAP is critical to the initiation of metastatic growth, its expression will serve as a molecular marker to detect tumors at an earlier stage of development compared to currently available methods. The design of high affinity small molecule FAP inhibitor will allow for noninvasive imaging of activated fibroblast in cancer patients. Small molecule inhibitors of FAP are being developed for targeted radiotherapy of tumors.

Synthesis and Evaluation of 68Ga-Labeled (2S,4S)-4-Fluoropyrrolidine-2-Carbonitrile and (4R)-Thiazolidine-4-Carbonitrile Derivatives as Novel Fibroblast Activation Protein-Targeted PET Tracers for Cancer Imaging

Fibroblast activation protein α (FAP-α) is a cell-surface protein overexpressed on cancer-associated fibroblasts that constitute a substantial component of tumor stroma and drive tumorigenesis. FAP is minimally expressed by most healthy tissues, including normal fibroblasts. This makes it a promising pan-cancer diagnostic and therapeutic target. In the present study, we synthesized two novel tracers, [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058, bearing a (2S,4S)-4-fluoropyrrolidine-2-carbonitrile or a (4R)-thiazolidine-4-carbonitrile pharmacophore, respectively. [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058 were evaluated for their FAP-targeting capabilities using substrate-based in vitro binding assays, and in PET/CT imaging and ex vivo biodistribution studies in an HEK293T:hFAP tumor xenograft mouse model. The IC₅₀ values of ^natGa-SB03045 (1.59 ± 0.45 nM) and ^natGa-SB03058 (0.68 ± 0.09 nM) were found to be lower than those of the clinically validated ^natGa-FAPI-04 (4.11 ± 1.42 nM). Contrary to the results obtained in the FAP-binding assay, [⁶⁸Ga]Ga-SB03058 demonstrated a ~1.5 fold lower tumor uptake than that of [⁶⁸Ga]Ga-FAPI-04 (7.93 ± 1.33 vs. 11.90 ± 2.17 %ID/g), whereas [⁶⁸Ga]Ga-SB03045 (11.8 ± 2.35 %ID/g) exhibited a tumor uptake comparable to that of [⁶⁸Ga]Ga-FAPI-04. Thus, our data suggest that the (2S,4S)-4-fluoropyrrolidine-2-carbonitrile scaffold holds potential as a promising pharmacophore for the design of FAP-targeted radioligands for cancer diagnosis and therapy.

FAP is critical for ovarian cancer cell survival by sustaining NF-κB activation through recruitment of PRKDC in lipid rafts

Fibroblast activation protein (FAP) is tumor-specific and plays an important role in tumorigenecity. However, agents against its enzymatic activity or extracellular presence were unsuccessful in the clinic for undefined reasons. Here we show that FAP expression is higher in advanced ovarian cancer and is only detected in invasive ovarian cancer cells. Silencing FAP induces apoptosis and FAP's enzymatic activity is dispensable for cell survival. To elucidate the cause of apoptosis, we find that NF-κB activity is diminished when FAP is depleted and BIRC5 (survivin) acts downstream of FAP-NF-κB axis to promote cell survival. To uncover the link between FAP and NF-κB activation, we reveal that PRKDC (DNA-PK, DNA-dependent protein kinase) forms complex with FAP and is required for NF-κB activation and cell survival. Remarkably, FAP-PRKDC interaction occurs only in lipid rafts, and depleting FAP prevents lipid raft localization of PRKDC. Given the known ability of PRKDC to direct NF-κB activation, these results suggest that FAP recruits PRKDC in lipid rafts for NF-κB activation. FAP's non-enzymatic role and functioning from lipid rafts for cell survival also offer an explanation on the failure of past FAP-targeted therapies. Finally, we demonstrate that EpCAM aptamer-delivered FAP siRNA impeded intraperitoneal xenograft development of ovary tumors.

Key aspects for conception and construction of co-culture models of tumor-stroma interactions

The tumor microenvironment is crucial in the initiation and progression of cancers. The interplay between cancer cells and the surrounding stroma shapes the tumor biology and dictates the response to cancer therapies. Consequently, a better understanding of the interactions between cancer cells and different components of the tumor microenvironment will drive progress in developing novel, effective, treatment strategies. Co-cultures can be used to study various aspects of these interactions in detail. This includes studies of paracrine relationships between cancer cells and stromal cells such as fibroblasts, endothelial cells, and immune cells, as well as the influence of physical and mechanical interactions with the extracellular matrix of the tumor microenvironment. The development of novel co-culture models to study the tumor microenvironment has progressed rapidly over recent years. Many of these models have already been shown to be powerful tools for further understanding of the pathophysiological role of the stroma and provide mechanistic insights into tumor-stromal interactions. Here we give a structured overview of different co-culture models that have been established to study tumor-stromal interactions and what we have learnt from these models. We also introduce a set of guidelines for generating and reporting co-culture experiments to facilitate experimental robustness and reproducibility.

Regulation of Fibroblast Activation Protein-α Expression: Focus on Intracellular Protein Interactions

The prolyl-specific peptidase fibroblast activation protein-α (FAP-α) is expressed at very low or undetectable levels in nondiseased human tissues but is selectively induced in activated (myo)fibroblasts at sites of tissue remodeling in fibrogenic processes. In normal regenerative processes involving transient fibrosis FAP-α⁺(myo)fibroblasts disappear from injured tissues, replaced by cells with a normal FAP-α^- phenotype. In chronic uncontrolled pathological fibrosis FAP-α⁺(myo)fibroblasts permanently replace normal tissues. The mechanisms of regulation and elimination of FAP-α expression in(myo)fibroblasts are unknown. According to a yeast two-hybrid screen and protein databanks search, we propose that the intracellular (co)-chaperone BAG6/BAT3 can interact with FAP-α, mediated by the BAG6/BAT3 Pro-rich domain, inducing proteosomal degradation of FAP-α protein under tissue homeostasis. In this Perspective, we discuss our findings in the context of current knowledge on the regulation of FAP-α expression and comment potential therapeutic strategies for uncontrolled fibrosis, including small molecule degraders (PROTACs)-modified FAP-α targeted inhibitors.

Research Progress on Dipeptidyl Peptidase Family: Structure, Function and Xenobiotic Metabolism

Prolyl-specific peptidases or proteases, including Dipeptidyl Peptidase 2, 4, 6, 8, 9, 10, Fibroblast Activation Protein, prolyl endopeptidase and prolyl carboxypeptidase, belong to the dipeptidyl peptidase family. In human physiology and anatomy, they have homology amino acid sequences, similarities in structure, but play distinct functions and roles. Some of them also play important roles in the metabolism of drugs containing endogenous peptides, xenobiotics containing peptides, and exogenous peptides. The major functions of these peptidases in both the metabolism of human health and bioactive peptides are of significant importance in the development of effective inhibitors to control the metabolism of endogenous bioactive peptides. The structural characteristics, distribution of tissue, endogenous substrates, and biological functions were summarized in this review. Furthermore, the xenobiotics metabolism of the dipeptidyl peptidase family is illustrated. All the evidence and information summarized in this review would be very useful for researchers to extend the understanding of the proteins of these families and offer advice and assistance in physiology and pathology studies.

[SP13786 Inhibits the Migration and Invasion of Lung Adenocarcinoma Cell A549 by Supressing Stat3-EMT via CAFs Exosomes]

背景与目的

成纤维细胞活化蛋白（fibroblast activation protein, FAP）是肿瘤相关成纤维细胞（cancer-associated fibroblasts, CAFs）的表面标志物之一，与CAFs的恶性表征关系密切，SP13786是FAP的特异性小分子抑制剂。本研究探讨SP13786作用于CAFs后，CAFs外泌体（exosomes, exo）对A549细胞迁移、侵袭的影响与机制。

方法

原代提取CAFs和癌旁成纤维细胞（peri-tumer fibroblasts, PTFs）；MTT实验检测不同浓度SP13786对CAFs增殖的影响；聚合物沉淀法提取PTFs-exo、CAFs-exo以及CAFs+SP13786-exo。将A549细胞设对照组、PTFs组、CAFs组及CAFs+SP13786组并分别以等体积的DMEM、PTFs-exo、CAFs-exo及CAFs+SP13786-exo孵育细胞。激光共聚焦实验检测A549细胞摄取外泌体的情况；免疫荧光、免疫组化和Western blot方法检测α平滑肌肌动蛋白（alpha-smooth muscle actin, α-SMA）、FAP在PTFs和CAFs中的表达及E-cadherin、N-cadherin、Slug、Stat3、P-Stat3在各组A549细胞中的表达；划痕实验和Transwell实验检测各组细胞的迁移和侵袭能力。

结果

免疫荧光、免疫组化和Western blot结果均显示α-SMA、FAP在CAFs中高表达，在PTFs中低表达（P < 0.05），表明从肺腺癌组织和癌旁组织中分别成功获得了CAFs和PTFs。MTT实验测得SP13786对于CAFs细胞的半数抑制浓度（50% inhibitory concentration, IC₅₀）约为3.3 nmol/L。免疫组化和Western blot结果显示与CAFs组相比，CAFs+SP13786组的α-SMA与FAP的表达显著降低（P < 0.05），说明抑制FAP可以显著降低CAFs的恶性表征。激光共聚焦结果显示外泌体能够被A549细胞所摄取。划痕实验与Transwell实验显示SP13786可抑制CAFs-exo对A549细胞迁移和侵袭的促进作用（P < 0.05）。与CAFs组比较，SP13786组A549细胞E-cadherin表达增多，N-cadherin与Slug表达降低（P < 0.05）；免疫荧光与Western blot显示SP13786组A549细胞的P-Stat3较CAFs组明显降低（P < 0.05），而总Stat3无显著差异。Stat3的特异性抑制剂WP1066明显抑制CAFs组A549细胞上皮间质转化（epithelial-mesenchymal transition, EMT），P-Stat3显著降低（P < 0.05），而加入WP1066后再加入SP13786-exo，P-Stat3未见进一步减低，EMT的抑制亦未见显著变化（P > 0.05）。

结论

FAP的小分子特异性抑制剂SP13786通过影响CAFs外泌体间接抑制A549细胞的迁移、侵袭，其可能机制是抑制Stat3的磷酸化从而影响A549细胞的EMT。

A Novel Chemiluminescence Probe for Sensitive Detection of Fibroblast Activation Protein-Alpha In Vitro and in Living Systems

Fibroblast activation protein-alpha (FAPα) is a key modulator of the microenvironment in multiple pathologies and is becoming the next pan-cancer target for cancer diagnostics and therapeutics. Chemiluminescence (CL) luminophores are considered as one of the most sensitive families of probes for detection and imaging applications due to their high signal-to-noise ratio. Until now, however, no such effective CL probe was reported for FAPα detection. Herein, we developed a novel CL probe for the detection of endogenous FAPα activity by incorporating FAPα-specific dipeptide substrates (glycine-proline) to the improved Schaap's adamantylidene-dioxetane. In this manner, we designed three CL probes (CFCL, BFCL, and QFCL) with the dipeptide substrate blocked by N-terminal benzyloxycarbonyl, N-tert-butoxycarbonyl or N-quinoline-4-carboxylic acid, respectively, which was used as the masking group to restrain the chemiexcitation energy. Probe CFCL exhibited the optimal specificity for the discrimination of FAPα from dipeptidase IV and prolyl oligopeptidase, which was elucidated by molecular docking simulation. Upon FAPα cleavage, CFCL was turned on for the highly selective and sensitive detection of FAPα with a limit of detection of 0.785 ng/mL. Furthermore, the ability of CFCL to image FAPα was effectively demonstrated in vitro, including various biological samples (plasma and tissue preparations), and in living systems (tumor cells and tumor-bearing mice). Furthermore, this newly established probe could be easily extended to evaluate FAPα inhibitors. Overall, we anticipate that probe CFCL will offer a facile and cost-effective alternative in the early detection of pathologies, individual tailoring of drug therapy, and drug screening.

In Vitro and In Situ Activity-Based Labeling of Fibroblast Activation Protein with UAMC1110-Derived Probes

Fibroblast activation protein (FAP) is a proline-selective protease that belongs to the S9 family of serine proteases. It is typically highly expressed in the tumor microenvironment (TME) and especially in cancer-associated fibroblasts, the main cell components of the tumor stroma. The exact role of its enzymatic activity in the TME remains largely unknown. Hence, tools that enable selective, activity-based visualization of FAP within the TME can help to unravel FAP’s function. We describe the synthesis, biochemical characterization, and application of three different activity-based probes (biotin-, Cy3-, and Cy5-labeled) based on the FAP-inhibitor UAMC1110, an in-house developed molecule considered to be the most potent and selective FAP inhibitor available. We demonstrate that the three probes have subnanomolar FAP affinity and pronounced selectivity with respect to the related S9 family members. Furthermore, we report that the fluorescent Cy3- and Cy5-labeled probes are capable of selectively detecting FAP in a cellular context, making these chemical probes highly suitable for further biological studies. Moreover, proof of concept is provided for in situ FAP activity staining in patient-derived cryosections of urothelial tumors.

[Research Progress of Cancer-associated Fibroblasts in Lung Cancer]

肿瘤间质在肿瘤复发和治疗抵抗中起着关键作用。肿瘤相关成纤维细胞（cancer-associated fibroblasts, CAFs）是肺癌间质中最丰富、最关键的细胞成分之一，CAFs分泌多种炎性细胞因子及细胞外基质，形成纤维增生性小生境，在肺癌发生发展的各个方面都起着促进作用。肺癌CAFs具有多种不同的起源，主要由正常肺成纤维细胞在受到肿瘤源性细胞因子作用后所转化而来。不同CAFs亚群具有较大的异质性，其功能及作用机制也具有很大差异性；这给靶向CAFs的临床转化应用带来了很大的挑战。本综述重点阐述了CAFs的特性和功能研究中的新进展，同时强调CAFs在肺癌发生、发展中起到的作用及其特异性。

Modeling chemical effects on breast cancer: the importance of the microenvironment in vitro

Accumulating evidence suggests that our ability to predict chemical effects on breast cancer is limited by a lack of physiologically relevant in vitro models; the typical in vitro breast cancer model consists of the cancer cell and excludes the mammary microenvironment. As the effects of the microenvironment on cancer cell behavior becomes more understood, researchers have called for the integration of the microenvironment into in vitro chemical testing systems. However, given the complexity of the microenvironment and the variety of platforms to choose from, identifying the essential parameters to include in a chemical testing platform is challenging. This review discusses the need for more complex in vitro breast cancer models and outlines different approaches used to model breast cancer in vitro. We provide examples of the microenvironment modulating breast cancer cell responses to chemicals and discuss strategies to help pinpoint what components should be included in a model.

Strategies in Developing Immunotherapy for Pancreatic Cancer: Recognizing and Correcting Multiple Immune "Defects" in the Tumor Microenvironment

With the advent of cancer immunotherapies, significant advances have been made in the treatment of many tumor types including melanoma, lung cancer, squamous cell carcinoma of the head and neck, renal cell carcinoma, bladder cancer, etc. However, similar success has not been observed with the treatment of pancreatic cancer and all other immunogenic “cold” tumors. This prompts the need for a better understanding of the complexity of the cold tumor microenvironment (TME) of pancreatic cancer and what are truly the “defects” in the TME making the cancer unresponsive to immune checkpoint inhibitors. Here we discuss four major immune defects that can be recognized in pancreatic cancer, including lack of high-quality effector intratumoral T cells, heterogeneous dense stroma as a barrier to effector immune cells infiltrating into the tumor, immunosuppressive tumor microenvironment, and failure of the T cells to accomplish tumor elimination. We also discuss potential strategies for pancreatic cancer treatment that work by correcting these immune defects.

Novel Small Molecule-Derived, Highly Selective Substrates for Fibroblast Activation Protein (FAP)

Fibroblast activation protein (FAP) is a proline-selective serine protease. It is hardly expressed in healthy adult tissue but upregulated in tissue remodeling sites associated with several diseases including epithelial cancer types, atherosclerosis, arthritis and fibrosis. Ongoing research aims at clinical implementation of FAP as a biomarker for these diseases. Several immunochemical methods that quantify FAP expression have been reported. An alternative/complementary approach focuses on quantification of FAP's enzymatic activity. Developing an activity-based assay for FAP has nonetheless proven challenging because of selectivity issues with respect to prolyl oligopeptidase (PREP). Here, we present substrate-type FAP probes that are structurally derived from a FAP-inhibitor (UAMC1110) that we published earlier. Both cleavage efficiency and FAP-selectivity of the best compounds in the series equal or surpass the most advanced peptide-based FAP substrates reported to date. Finally, proof-of-concept is provided that 4-aminonaphthol containing probes can spatially localize FAP activity in biological samples.

Therapeutic challenges and current immunomodulatory strategies in targeting the immunosuppressive pancreatic tumor microenvironment

BACKGROUND

Pancreatic cancer is one of the most lethal type of cancers, with an overall five-year survival rate of less than 5%. It is usually diagnosed at an advanced stage with limited therapeutic options. To date, no effective treatment options have demonstrated long-term benefits in advanced pancreatic cancer patients. Compared with other cancers, pancreatic cancer exhibits remarkable resistance to conventional therapy and possesses a highly immunosuppressive tumor microenvironment (TME).

MAIN BODY

In this review, we summarized the evidence and unique properties of TME in pancreatic cancer that may contribute to its resistance towards immunotherapies as well as strategies to overcome those barriers. We reviewed the current strategies and future perspectives of combination therapies that (1) promote T cell priming through tumor associated antigen presentation; (2) inhibit tumor immunosuppressive environment; and (3) break-down the desmoplastic barrier which improves tumor infiltrating lymphocytes entry into the TME.

CONCLUSIONS

It is imperative for clinicians and scientists to understand tumor immunology, identify novel biomarkers, and optimize the position of immunotherapy in therapeutic sequence, in order to improve pancreatic cancer clinical trial outcomes. Our collaborative efforts in targeting pancreatic TME will be the mainstay of achieving better clinical prognosis among pancreatic cancer patients. Ultimately, pancreatic cancer will be a treatable medical condition instead of a death sentence for a patient.

Targeting the Microenvironment in High Grade Serous Ovarian Cancer

Cancer⁻stroma interactions play a key role in cancer progression and response to standard chemotherapy. Here, we provide a summary of the mechanisms by which the major cellular components of the ovarian cancer (OC) tumor microenvironment (TME) including cancer-associated fibroblasts (CAFs), myeloid, immune, endothelial, and mesothelial cells potentiate cancer progression. High-grade serous ovarian cancer (HGSOC) is characterized by a pro-inflammatory and angiogenic signature. This profile is correlated with clinical outcomes and can be a target for therapy. Accumulation of malignant ascites in the peritoneal cavity allows for secreted factors to fuel paracrine and autocrine circuits that augment cancer cell proliferation and invasiveness. Adhesion of cancer cells to the mesothelial matrix promotes peritoneal tumor dissemination and represents another attractive target to prevent metastasis. The immunosuppressed tumor milieu of HGSOC is permissive for tumor growth and can be modulated therapeutically. Results of emerging preclinical and clinical trials testing TME-modulating therapeutics for the treatment of OC are highlighted.

Extracellular matrix directs phenotypic heterogeneity of activated fibroblasts

Activated fibroblasts are key players in the injury response, tumorigenesis, fibrosis, and inflammation. Dichotomous outcomes in response to varied stroma-targeted therapies in cancer emphasize the need to disentangle the roles of heterogeneous fibroblast subsets in physiological and pathophysiological settings. In wound healing, fibrosis, and myriad tumor types, fibroblast activation protein (FAP) and alpha-smooth muscle actin (αSMA) identify distinct, yet overlapping, activated fibroblast subsets. Prior studies established that FAPHi reactive fibroblasts and αSMAHi myofibroblasts can exert opposing influences in tumorigenesis. However, the factors that drive this phenotypic heterogeneity and the unique functional roles of these subsets have not been defined. We demonstrate that a convergence of ECM composition, elasticity, and transforming growth factor beta (TGF-β) signaling governs activated fibroblast phenotypic heterogeneity. Furthermore, FAPHi reactive fibroblasts and αSMAHi myofibroblasts exhibited distinct gene expression signatures and functionality in vitro, illuminating potentially unique roles of activated fibroblast subsets in tissue remodeling. These insights into activated fibroblast heterogeneity will inform the rational design of stroma-targeted therapies for cancer and fibrosis.

The EMT-related transcription factor snail up-regulates FAPα in malignant melanoma cells

FAPα is a cell surface serine protease, mainly expressed in tumor stromal fibroblasts in more than 90% of human epithelial carcinomas. Due to its almost no expression in normal tissues and its tumor-promoting effects, FAPα has been studied as a novel potential target for antitumor therapy. However, the regulation mechanism on FAPα expression is poorly understood. In this study, we found that overexpression of snail significantly increased the mRNA and protein expression levels of FAPα in malignant melanoma B16 and SK-MEL-28 cells. Overexpression of snail increased FAPα promoter activity remarkably. Snail could directly bind to FAPα promoter to regulate FAPα expression. Moreover, snail expression was positively correlated to FAPα expression in human cutaneous malignant melanoma. Furthermore, knockdown of FAPα markedly reduced snail-induced cell migration. Overall, our findings provide a novel regulation mechanism on FAPα expression and highlight the role of snail/FAPα axis as a novel target for melanoma treatment.

Alteration of the Tumor Stroma Using a Consensus DNA Vaccine Targeting Fibroblast Activation Protein (FAP) Synergizes with Antitumor Vaccine Therapy in Mice

Purpose: Fibroblast activation protein (FAP) is overexpressed in cancer-associated fibroblasts and is an interesting target for cancer immune therapy, with prior studies indicating a potential to affect the tumor stroma. Our aim was to extend this earlier work through the development of a novel FAP immunogen with improved capacity to break tolerance for use in combination with tumor antigen vaccines.Experimental Design: We used a synthetic consensus (SynCon) sequence approach to provide MHC class II help to support breaking of tolerance. We evaluated immune responses and antitumor activity of this novel FAP vaccine in preclinical studies, and correlated these findings to patient data.Results: This SynCon FAP DNA vaccine was capable of breaking tolerance and inducing both CD8⁺ and CD4⁺ immune responses. In genetically diverse, outbred mice, the SynCon FAP DNA vaccine was superior at breaking tolerance compared with a native mouse FAP immunogen. In several tumor models, the SynCon FAP DNA vaccine synergized with other tumor antigen-specific DNA vaccines to enhance antitumor immunity. Evaluation of the tumor microenvironment showed increased CD8⁺ T-cell infiltration and a decreased macrophage infiltration driven by FAP immunization. We extended this to patient data from The Cancer Genome Atlas, where we find high FAP expression correlates with high macrophage and low CD8⁺ T-cell infiltration.Conclusions: These results suggest that immune therapy targeting tumor antigens in combination with a microconsensus FAP vaccine provides two-fisted punch-inducing responses that target both the tumor microenvironment and tumor cells directly. Clin Cancer Res; 24(5); 1190-201. ©2018 AACR.

Fiber-modified hexon-chimeric oncolytic adenovirus targeting cancer associated fibroblasts inhibits tumor growth in gastric carcinoma

Objective

To evaluate the effects of fiber-modified hexon-chimeric recombinant oncolytic adenovirus targeting cancer associated fibroblasts (CAFs) on the gastric CAFs and the transplantation tumor mice model of gastric carcinoma (GC).

Results

Compared with BJ cells and GPFs, the reproduction and infectivity of P9, P9-4C or GP adenoviruses were markedly higher in gastric CAFs. In addition, P9, P9-4C or GP had a significantly relatively more killing effect on gastric CAFs compared with GPFs, and have less oncolytic effect in BJ cells. Furthermore, in transplantation tumor mice model of GC we found significantly higher hexon protein expression in tumor tissues, more decreasing tumor growth and increasing inhibitory rates after treatment of P9, P9-4C or GP adenoviruses compared with Ad adenovirus.

Materials and Methods

Based on the construction of the recombinant oncolytic adenoviruses pRCAdHVR48-SDF1p-Ad/EGFP (Ad, as control) with the E1A gene transcription regulated by stromal-derived factor 1 (SDF1) promoter and the hexon replaced by hexon-chimeric (H5HVR48) gene, three fiber-modified hexon-chimeric oncolytic adenovirus through the modification fiber protein by insertion of different short peptides specifically binding to fibroblast activation protein (FAP), including pRCAdHVR48-SDF1p-FAP-P9/EGFP (P9), pRCAdHVR48-SDF1p-FAP-P9-4C/EGFP (P9-4C), pRCAdHVR48-SDF1p-FAP-GP/EGFP (GP), and their corresponding replication-defective adenovirus in parallel were reconstructed. Then the reproduction, infectivity and killing ability of the four above recombinant adenoviruses were evaluated in gastric CAFs compared with gastric para-mucosa fibroblasts (GPFs) and neonatal human foreskin fibroblasts (BJ). Furthermore, transplantation tumor mice model of GC was established, and then treated by the four above recombinant adenoviruses. Tumor size and tumor growth inhibitory rates were calculated, and histomorphology by HE staining and hexon expressions by immunohistochemistry were evaluated in tumor tissues.

Conclusions

The fiber-modified hexon-chimeric recombinant oncolytic adenovirus targeting CAFs can relatively specifically kill gastric CAFs and inhibit GC cells growth in vivo.

Identification of factors promoting ex vivo maintenance of mouse hematopoietic stem cells by long-term single-cell quantification

AFT024-induced HSC maintenance correlates with early survival/proliferation whereas early death is a major reason for HSC loss in culture. Dermatopontin is required for ex vivo HSC maintenance, and also improves HSC clonogenicity in stroma-based and stroma-free cultures.

Fibroblast activation protein (FAP) as a novel metabolic target

Objective

Fibroblast activation protein (FAP) is a serine protease belonging to a S9B prolyl oligopeptidase subfamily. This enzyme has been implicated in cancer development and recently reported to regulate degradation of FGF21, a potent metabolic hormone. Using a known FAP inhibitor, talabostat (TB), we explored the impact of FAP inhibition on metabolic regulation in mice.

Methods

To address this question we evaluated the pharmacology of TB in various mouse models including those deficient in FGF21, GLP1 and GIP signaling. We also studied the ability of FAP to process FGF21 in vitro and TB to block FAP enzymatic activity.

Results

TB administration to diet-induced obese (DIO) animals led to profound decreases in body weight, reduced food consumption and adiposity, increased energy expenditure, improved glucose tolerance and insulin sensitivity, and lowered cholesterol levels. Total and intact plasma FGF21 were observed to be elevated in TB-treated DIO mice but not lean animals where the metabolic impact of TB was significantly attenuated. Furthermore, and in stark contrast to naïve DIO mice, the administration of TB to obese FGF21 knockout animals demonstrated no appreciable effect on body weight or any other measures of metabolism. In support of these results we observed no enzymatic degradation of human FGF21 at either end of the protein when FAP was inhibited in vitro by TB.

Conclusions

We conclude that pharmacological inhibition of FAP enhances levels of FGF21 in obese mice to provide robust metabolic benefits not observed in lean animals, thus validating this enzyme as a novel drug target for the treatment of obesity and diabetes.

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Pharmacological inhibition of FAP reduces weight, improves glucose and lipid metabolism in obese, but not lean mice.

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FAP inhibitor Talabostat at higher doses lessens food intake, without any apparent adverse effects in short term studies.

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Obese FGF21 deficient mice did not exhibit meaningful change in metabolic regulation when treated with Talabostat.

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The mechanism of Talabostat in vivo action appears to center on an increase in total and active levels of plasma FGF21.

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FAP inhibition alone, or in combination with DPP4 is proposed as a novel approach to treat metabolic diseases.

Scaffold-Free Coculture Spheroids of Human Colonic Adenocarcinoma Cells and Normal Colonic Fibroblasts Promote Tumorigenicity in Nude Mice

The aim of this study was to form a scaffold-free coculture spheroid model of colonic adenocarcinoma cells (CACs) and normal colonic fibroblasts (NCFs) and to use the spheroids to investigate the role of NCFs in the tumorigenicity of CACs in nude mice. We analysed three-dimensional (3D) scaffold-free coculture spheroids of CACs and NCFs. CAC Matrigel invasion assays and tumorigenicity assays in nude mice were performed to examine the effect of NCFs on CAC invasive behaviour and tumorigenicity in 3D spheroids. We investigated the expression pattern of fibroblast activation protein-α (FAP-α) by immunohistochemical staining. CAC monocultures did not form densely-packed 3D spheroids, whereas cocultured CACs and NCFs formed 3D spheroids. The 3D coculture spheroids seeded on a Matrigel extracellular matrix showed higher CAC invasiveness compared to CACs alone or CACs and NCFs in suspension. 3D spheroids injected into nude mice generated more and faster-growing tumors compared to CACs alone or mixed suspensions consisting of CACs and NCFs. FAP-α was expressed in NCFs-CACs cocultures and xenograft tumors, whereas monocultures of NCFs or CACs were negative for FAP-α expression. Our findings provide evidence that the interaction between CACs and NCFs is essential for the tumorigenicity of cancer cells as well as for tumor propagation.

Suppression of tumor growth in mice by rationally designed pseudopeptide inhibitors of fibroblast activation protein and prolyl oligopeptidase

Tumor microenvironments (TMEs) are composed of cancer cells, fibroblasts, extracellular matrix, microvessels, and endothelial cells. Two prolyl endopeptidases, fibroblast activation protein (FAP) and prolyl oligopeptidase (POP), are commonly overexpressed by epithelial-derived malignancies, with the specificity of FAP expression by cancer stromal fibroblasts suggesting FAP as a possible therapeutic target. Despite overexpression in most cancers and having a role in angiogenesis, inhibition of POP activity has received little attention as an approach to quench tumor growth. We developed two specific and highly effective pseudopeptide inhibitors, M83, which inhibits FAP and POP proteinase activities, and J94, which inhibits only POP. Both suppressed human colon cancer xenograft growth > 90% in mice. By immunohistochemical stains, M83- and J94-treated tumors had fewer microvessels, and apoptotic areas were apparent in both. In response to M83, but not J94, disordered collagen accumulations were observed. Neither M83- nor J94-treated mice manifested changes in behavior, weight, or gastrointestinal function. Tumor growth suppression was more extensive than noted with recently reported efforts by others to inhibit FAP proteinase function or reduce FAP expression. Diminished angiogenesis and the accompanying profound reduction in tumor growth suggest that inhibition of either FAP or POP may offer new therapeutic approaches that directly target TMEs.

A potent immunotoxin targeting fibroblast activation protein for treatment of breast cancer in mice

Fibroblast activation protein (FAP) is highly expressed in the tumor-associated fibroblasts (TAFs) of most human epithelial cancers. FAP plays a critical role in tumorigenesis and cancer progression, which makes it a promising target for novel anticancer therapy. However, mere abrogation of FAP enzymatic activity by small molecules is not very effective in inhibiting tumor growth. In this study, we have evaluated a novel immune-based approach to specifically deplete FAP-expressing TAFs in a mouse 4T1 metastatic breast cancer model. Depletion of FAP-positive stromal cells by FAP-targeting immunotoxin αFAP-PE38 altered levels of various growth factors, cytokines, chemokines and matrix metalloproteinases, decreased the recruitment of tumor-infiltrating immune cells in the tumor microenvironment and suppressed tumor growth. In addition, combined treatment with αFAP-PE38 and paclitaxel potently inhibited tumor growth in vivo. Our findings highlight the potential use of immunotoxin αFAP-PE38 to deplete FAP-expressing TAFs and thus provide a rationale for the use of this immunotoxin in cancer therapy.

Fibroblast activation protein-α, a stromal cell surface protease, shapes key features of cancer associated fibroblasts through proteome and degradome alterations

Cancer associated fibroblasts (CAFs) constitute an abundant stromal component of most solid tumors. Fibroblast activation protein (FAP) α is a cell surface protease that is expressed by CAFs. We corroborate this expression profile by immunohistochemical analysis of colorectal cancer specimens. To better understand the tumor-contextual role of FAPα, we investigate how FAPα shapes functional and proteomic features of CAFs using loss- and gain-of function cellular model systems. FAPα activity has a strong impact on the secreted CAF proteome ("secretome"), including reduced levels of anti-angiogenic factors, elevated levels of transforming growth factor (TGF) β, and an impact on matrix processing enzymes. Functionally, FAPα mildly induces sprout formation by human umbilical vein endothelial cells. Moreover, loss of FAPα leads to a more epithelial cellular phenotype and this effect was rescued by exogenous application of TGFβ. In collagen contraction assays, FAPα induced a more contractile cellular phenotype. To characterize the proteolytic profile of FAPα, we investigated its specificity with proteome-derived peptide libraries and corroborated its preference for cleavage carboxy-terminal to proline residues. By "terminal amine labeling of substrates" (TAILS) we explored FAPα-dependent cleavage events. Although FAPα acts predominantly as an amino-dipeptidase, putative FAPα cleavage sites in collagens are present throughout the entire protein length. In contrast, putative FAPα cleavage sites in non-collagenous proteins cluster at the amino-terminus. The degradomic study highlights cell-contextual proteolysis by FAPα with distinct positional profiles. Generally, our findings link FAPα to key aspects of CAF biology and attribute an important role in tumor-stroma interaction to FAPα.

The copper transporter 1 (CTR1) is required to maintain the stability of copper transporter 2 (CTR2)

Mammalian cells have two influx Cu transporters that form trimers in membranes. CTR1 is the high affinity transporter that resides largely in the plasma membrane, and CTR2 is the low affinity transporter that is primarily associated with vesicular structures inside the cell. The major differences between CTR1 and CTR2 are that CTR1 contains a HIS/MET-rich domain N-terminal of the METS that participate in the first two stacked rings that form the pore, and a longer C-terminal tail that includes a Cu binding HIS-CYS-HIS (HCH) motif right at the end. It has been reported that CTR1 and CTR2 are physically associated with each other in the cell. We used the CRISPR-Cas9 technology to knock out either CTR1 or CTR2 in fully malignant HEK293T and OVCAR8 human ovarian cancer cells to investigate the interaction of CTR1 and CTR2. We report here that the level of CTR2 protein is markedly decreased in CTR1 knockout clones while the CTR2 transcript level remains unchanged. CTR2 was found to be highly ubiquitinated in the CTR1 knock out cells, and inhibition of the proteasome prevented the degradation of CTR2 when CTR1 was not present while inhibition of autophagy had no effect. Re-expression of CTR1 rescued CTR2 from degradation in the CTR1 knockout cells. We conclude that CTR1 is essential to maintain the stability of CTR2 and that in the absence of CTR1 CTR2 is degraded by the proteasome. This reinforces the concept that the functions of CTR1 and CTR2 are inter-dependent within the Cu homeostasis system.

Fibroblast activation protein increases metastatic potential of fibrosarcoma line HT1080 through upregulation of integrin-mediated signaling pathways

The serine protease fibroblast activation protein (FAP) is selectively expressed on tumour-associated fibroblasts in most human epithelial tumours, as well as on some mesenchymal tumours such as sarcoma. High FAP expression is most often associated with poor outcome and increased metastasis. Here, we compare the in vitro metastatic potential of HT1080 fibrosarcoma cells with and without FAP expression in order to elucidate the mechanism by which FAP may influence metastasis. In the presence of FAP, cells were more adhesive to extracellular matrix proteins and migrated and invaded through Matrigel to a greater degree. The anti-FAP antibody ESC11, which caused internalization of FAP, decreased adhesion and migration, but only when cells expressed FAP. It was also found that blocking activity of integrins β1 and αvβ3 reduced both cell adhesion and migration and this effect was much more marked in FAP-expressing HT1080 cells than mock-transfected HT1080 cells. The expression or activation of intracellular proteins that form part of the downstream signaling of integrins, including integrin-linked kinase, Rac1 and focal adhesion kinase, was also upregulated when FAP was expressed, suggesting that FAP not only upregulates metastatic-like cell behaviours through interaction with integrins, but also influences the intracellular signaling of integrins. This was confirmed using both PI3 kinase and Src kinase inhibitors, which decreased adhesion and migration in FAP-expressing cells, but did not affect mock-transfected HT1080 cells. FAP is therefore a useful target for anti-cancer therapy, as not only is its expression tumour-selective, but its downregulation has the potential to reduce incidence of metastasis.

Deficiency of fibroblast activation protein alpha ameliorates cartilage destruction in inflammatory destructive arthritis

Introduction

Inflammatory destructive arthritis, like rheumatoid arthritis (RA), is characterized by invasion of synovial fibroblasts (SF) into the articular cartilage and erosion of the underlying bone, leading to progressive joint destruction. Because fibroblast activation protein alpha (FAP) has been associated with cell migration and cell invasiveness, we studied the function of FAP in joint destruction in RA.

Methods

Expression of FAP in synovial tissues and fibroblasts from patients with osteoarthritis (OA) and RA as well as from wild-type and arthritic mice was evaluated by immunohistochemistry, fluorescence microscopy and polymerase chain reaction (PCR). Fibroblast adhesion and migration capacity was assessed using cartilage attachment assays and wound-healing assays, respectively. For in vivo studies, FAP-deficient mice were crossed into the human tumor necrosis factor transgenic mice (hTNFtg), which develop a chronic inflammatory arthritis. Beside clinical assessment, inflammation, cartilage damage, and bone erosion were evaluated by histomorphometric analyses.

Results

RA synovial tissues demonstrated high expression of FAP whereas in OA samples only marginal expression was detectable. Consistently, a higher expression was detected in arthritis SF compared to non-arthritis OA SF in vitro. FAP-deficiency in hTNFtg mice led to less cartilage degradation despite unaltered inflammation and bone erosion. Accordingly, FAP^−/− hTNFtg SF demonstrated a lower cartilage adhesion capacity compared to hTNFtg SF in vitro.

Conclusions

These data point to a so far unknown role of FAP in the attachment of SF to cartilage, promoting proteoglycan loss and subsequently cartilage degradation in chronic inflammatory arthritis.

Fibroblast activation protein α in tumor microenvironment: recent progression and implications (review)

Accumulated evidence has demonstrated that the microenvironment of a given tumor is important in determining its drug resistance, tumorigenesis, progression and metastasis. These microenvironments, like tumor cells, are vital targets for cancer therapy. The cross-talk between tumor cells and cancer-associated fibroblasts (CAFs, alternatively termed activated fibroblasts) is crucial in regulating the drug resistance, tumorigenesis, neoplastic progression, angiogenesis, invasion and metastasis of a tumor. Fibroblast activation protein α (FAPα) is a transmembrane serine protease and is highly expressed on CAFs present in >90% of human epithelial neoplasms. FAPα activity, alongside that of gelatinase and type I collagenase, has become increasingly important in cancer therapy due to its effectiveness in modulating tumor behavior. In this review, recent progression in the knowledge of the role of FAPα in tumor microenvironments is discussed.

Fibroblast heterogeneity in the cancer wound

Dr. Tuveson and colleagues provide a comprehensive review on the fundamental role of cancer-associated fibroblasts in shaping the tumor microenvironment and promoting tumor initiation and progression.

Fibroblasts regulate the structure and function of healthy tissues, participate transiently in tissue repair after acute inflammation, and assume an aberrant stimulatory role during chronic inflammatory states including cancer. Such cancer-associated fibroblasts (CAFs) modulate the tumor microenvironment and influence the behavior of neoplastic cells in either a tumor-promoting or tumor-inhibiting manner. These pleiotropic functions highlight the inherent plasticity of fibroblasts and may provide new avenues to understand and therapeutically intervene in malignancies. We discuss the emerging themes of CAF biology in the context of tumorigenesis and therapy.

Targeting nucleophosmin 1 represents a rational strategy for radiation sensitization

PURPOSE

To test the hypothesis that small molecule targeting of nucleophosmin 1 (NPM1) represents a rational approach for radiosensitization.

METHODS AND MATERIALS

Wilde-type and NPM1-deficient mouse embryo fibroblasts (MEFs) were used to determine whether radiosensitization produced by the small molecule YTR107 was NPM1 dependent. The stress response to ionizing radiation was assessed by quantifying pNPM1, γH2AX, and Rad51 foci, neutral comet tail moment, and colony formation. NPM1 levels in a human-derived non-small-cell lung cancer (NSCLC) tissue microarray (TMA) were determined by immunohistochemistry. YTR107-mediated radiosensitization was assessed in NSCLC cell lines and xenografts.

RESULTS

Use of NPM1-null MEFs demonstrated that NPM1 is critical for DNA double- strand break (DSB) repair, that loss of NPM1 increases radiation sensitivity, and that YTR107-mediated radiosensitization is NPM1 dependent. YTR107 was shown to inhibit NPM1 oligomerization and impair formation of pNPM1 irradiation-induced foci that colocalized with γH2AX foci. Analysis of the TMA demonstrated that NPM1 is overexpressed in subsets of NSCLC. YTR107 inhibited DNA DSB repair and radiosensitized NSCLC lines and xenografts.

CONCLUSIONS

These data demonstrate that YTR107-mediated targeting of NPM1 impairs DNA DSB repair, an event that increases radiation sensitivity.

FAP-α (Fibroblast activation protein-α) is involved in the control of human breast cancer cell line growth and motility via the FAK pathway

BACKGROUND

Fibroblast Activation Protein alpha (FAP-α) or seprase is an integral membrane serine peptidase. Previous work has not satisfactorily explained both the suppression and promotion effects that have been observed in cancer. The purpose of this work was to investigate the role of FAP-α in human breast cancer. Expression of FAP-α was characterized in primary tumour samples and in cell lines, along with the effects of FAP-α expression on in vitro growth, invasion, attachment and migration. Furthermore the potential interaction of FAP-α with other signalling pathways was investigated.

RESULTS

FAP-α was significantly increased in patients with poor outcome and survival. In vitro results showed that breast cancer cells over expressing FAP-α had increased growth ability and impaired migratory ability. The growth of MDA-MB-231 cells and the adhesion and invasion ability of both MCF-7 cells and MDA-MB-231 cells were not dramatically influenced by FAP-α expression. Over-expression of FAP-α resulted in a reduction of phosphorylated focal adhesion kinase (FAK) level in both cells cultured in normal media and serum-free media. An inhibitor to FAK restored the reduced motility ability of both MCF-7exp cells and MDA-MB-231exp cells and prevented the change in phosphorylated FAK levels. However, inhibitors to PI3K, ERK, PLCΥ, NWASP, ARP2/3, and ROCK had no influence this.

CONCLUSIONS

FAP-α in significantly associated with poor outcome in patients with breast cancer. In vitro, FAP-α promotes proliferation and inhibits migration of breast cancer cells, potentially by regulating the FAK pathway. These results suggest FAP-α could be a target for future therapies.

Extended structure-activity relationship and pharmacokinetic investigation of (4-quinolinoyl)glycyl-2-cyanopyrrolidine inhibitors of fibroblast activation protein (FAP)

Fibroblast activation protein (FAP) is a serine protease related to dipeptidyl peptidase IV (DPPIV). It has been convincingly linked to multiple disease states involving remodeling of the extracellular matrix. FAP inhibition is investigated as a therapeutic option for several of these diseases, with most attention so far devoted to oncology applications. We previously discovered the N-4-quinolinoyl-Gly-(2S)-cyanoPro scaffold as a possible entry to highly potent and selective FAP inhibitors. In the present study, we explore in detail the structure-activity relationship around this core scaffold. We report extensively optimized compounds that display low nanomolar inhibitory potency and high selectivity against the related dipeptidyl peptidases (DPPs) DPPIV, DPP9, DPPII, and prolyl oligopeptidase (PREP). The log D values, plasma stabilities, and microsomal stabilities of selected compounds were found to be highly satisfactory. Pharmacokinetic evaluation in mice of selected inhibitors demonstrated high oral bioavailability, plasma half-life, and the potential to selectively and completely inhibit FAP in vivo.

Insidious changes in stromal matrix fuel cancer progression

Reciprocal interactions between tumor and stromal cells propel cancer progression and metastasis. A complete understanding of the complex contributions of the tumor stroma to cancer progression necessitates a careful examination of the extracellular matrix (ECM), which is largely synthesized and modulated by cancer-associated fibroblasts. This structurally supportive meshwork serves as a signaling scaffold for a myriad of biologic processes and responses favoring tumor progression. The ECM is a repository for growth factors and cytokines that promote tumor growth, proliferation, and metastasis through diverse interactions with soluble and insoluble ECM components. Growth factors activated by proteases are involved in the initiation of cell signaling pathways essential to invasion and survival. Various transmembrane proteins produced by the cancer stroma bind the collagen and fibronectin-rich matrix to induce proliferation, adhesion, and migration of cancer cells, as well as protease activation. Integrins are critical liaisons between tumor cells and the surrounding stroma, and with their mechano-sensing ability, induce cell signaling pathways associated with contractility and migration. Proteoglycans also bind and interact with various matrix proteins in the tumor microenvironment to promote cancer progression. Together, these components function to mediate cross-talk between tumor cells and fibroblasts ultimately to promote tumor survival and metastasis. These stromal factors, which may be expressed differentially according to cancer stage, have prognostic utility and potential. This review examines changes in the ECM of cancer-associated fibroblasts induced through carcinogenesis, and the impact of these changes on cancer progression. The implication is that cancer progression, even in epithelial cancers, may be based in large part on changes in signaling from cancer-associated stromal cells. These changes may provide early prognostic indicators to further stratify patients during treatment or alter the timing of their follow-up visits and observations.

Impact of fibroblast activation protein on osteosarcoma cell lines in vitro

Fibroblast activation protein (FAP) or seprase, which belongs to the group type II integral serine proteases, is an integral membrane serine peptidase. Previous studies have demonstrated that FAP has an effect on tumor growth, proliferation and invasion. However, the cellular functional role that FAP plays in osteosarcoma (OS) remains unknown. The aim of the present study was to investigate the activities of FAP in OS cell lines. The gene expression of FAP was knocked down through a hammerhead ribozyme transgene, and the various functions between the knockdown cells and their control cells were tested using a series of functional assays in vitro. The results indicated that knockdown of FAP markedly reduced the ability of cellular growth, matrix adhesion, migration and invasion in MG-63 and HOS cell lines compared with the control cells (P<0.05). In conclusion, FAP influences OS cells and may play a role in OS tumor progression and metastasis.

Peptide-based boronates: How to achieve tissue specificity in anticancer therapy

Dipeptidyl boronic acids are suitable candidates for the design of “pro-soft” drugs because recent studies have proven that these acids undergo a pH-dependent cyclization equilibrium, generating an inactive cyclic form under physiological conditions. Dipeptidyl boronic acids possess a wide range of potential targets, and the 26S proteasome appears to be one of the main targets. This multicatalytic complex is involved in intracellular protein turnover and is overexpressed in certain pathological conditions, such as malignancies, autoimmune diseases and neurodegenerative diseases. Bortezomib is the first-in-class derivative approved by the Food and Drug Administration for the treatment of hematological malignancies (i.e., relapsed and refractory multiple myeloma and mantle cell lymphoma) but is inactive against solid tumors due to an insufficient tissue distribution. The present study suggests a possible strategy for enhancing the in vivo performance of dipeptidyl boronic acids endowed with promising proteasome-inhibiting properties and their applicability as anticancer agents. In particular, dipeptidyl boronic acids might have a fruitful application as pro-soft drugs when an appropriate recognition motif serves as a substrate for a tumor-specific protease, generating the active form of the drug in situ and preventing systemic side effects after diffusion through cells and tissues.

Selective Inhibitors of Fibroblast Activation Protein (FAP) with a (4-Quinolinoyl)-glycyl-2-cyanopyrrolidine Scaffold

Fibroblast activation protein (FAP) is a serine protease that is generally accepted to play an important role in tumor growth and other diseases involving tissue remodeling. Currently there are no FAP inhibitors with reported selectivity toward both the closely related dipeptidyl peptidases (DPPs) and prolyl oligopeptidase (PREP). We present the discovery of a new class of FAP inhibitors with a N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine) scaffold. We have explored the effects of substituting the quinoline ring and varying the position of its sp(2) hybridized nitrogen atom. The most promising inhibitors combined low nanomolar FAP inhibition and high selectivity indices (>10(3)) with respect to both the DPPs and PREP. Preliminary experiments on a representative inhibitor demonstrate that plasma stability, kinetic solubility, and log D of this class of compounds can be expected to be satisfactory.

Identification of selective and potent inhibitors of fibroblast activation protein and prolyl oligopeptidase

Fibroblast activation protein (FAP) is a serine protease selectively expressed on reactive stromal fibroblasts of epithelial carcinomas. It is widely believed to play a role in tumor invasion and metastasis and therefore to represent a potential new drug target for cancer. Investigation into its biological function, however, has been hampered by the current unavailability of selective inhibitors. The challenge has been in identifying inhibitors that are selective for FAP over both the dipeptidyl peptidases (DPPs), with which it shares exopeptidase specificity, and prolyl oligopeptidase (PREP), with which it shares endopeptidase specificity. Here, we report the first potent FAP inhibitor with selectivity over both the DPPs and PREP, N-(pyridine-4-carbonyl)-d-Ala-boroPro (ARI-3099, 6). We also report a similarly potent and selective PREP inhibitor, N-(pyridine-3-carbonyl)-Val-boroPro (ARI-3531, 22). Both are boronic acid based inhibitors, demonstrating that high selectivity can be achieved using this electrophile. The inhibitors are stable, easy to synthesize, and should prove to be useful in helping to elucidate the biological functions of these two unique and interesting enzymes, as well as their potential as drug targets.

Cancer-associated fibroblasts as targets for immunotherapy

Immunotherapy for solid tumors has shown promise in preclinical as well as early clinical studies. However, its efficacy remains limited. The hindrance to achieving objective, long-lasting therapeutic responses in solid tumors is, in part, mediated by the dynamic nature of the tumor and its complex microenvironment. Tumor-directed therapies fail to eliminate components of the microenvironment, which can reinstate a tumorigenic milieu and contribute to recurrence. Cancer-associated fibroblasts (CAFs) form the most preponderant cell type in the solid tumor microenvironment. Given their pervasive role in facilitating tumor growth and metastatic dissemination, CAFs have emerged as attractive therapeutic targets in the tumor microenvironment. In this article, we highlight the cross-talk between CAFs and cancer cells, and discuss how targeting CAFs has the potential to improve current immunotherapy approaches for cancer.

Identification of inhibitory scFv antibodies targeting fibroblast activation protein utilizing phage display functional screens

Fibroblast activation protein (FAP) is a serine protease selectively expressed on tumor stromal fibroblasts in epithelial carcinomas and is important in cancer growth, adhesion, and metastases. As FAP enzymatic activity is a potent therapeutic target, we aimed to identify inhibitory antibodies. Using a competitive inhibition strategy, we used phage display techniques to identify 53 single-chain variable fragments (scFvs) after three rounds of panning against FAP. These scFvs were expressed and characterized for binding to FAP by surface plasmon resonance and flow cytometry. Functional assessment of these antibodies yielded an inhibitory scFv antibody, named E3, which could attenuate 35% of FAP cleavage of the fluorescent substrate Ala-Pro-7-amido-4-trifluoromethylcoumarin compared with nonfunctional scFv control. Furthermore, a mutant E3 scFv was identified by yeast affinity maturation. It had higher affinity (4-fold) and enhanced inhibitory effect on FAP enzyme activity (3-fold) than E3. The application of both inhibitory anti-FAP scFvs significantly affected the formation of 3-dimensional FAP-positive cell matrix, as demonstrated by reducing the fibronectin fiber orientation from 41.18% (negative antibody control) to 34.06% (E3) and 36.15% (mutant E3), respectively. Thus, we have identified and affinity-maturated the first scFv antibody capable of inhibiting FAP function. This scFv antibody has the potential to disrupt the role of FAP in tumor invasion and metastasis.