Potential disease biomarkers: dipeptidyl peptidase 4 and fibroblast activation protein

FAPi-Based Agents in Thyroid Cancer: A New Step towards Diagnosis and Therapy? A Systematic Review of the Literature

(1) Background: Thyroid cancer (TC) is often treated with surgery followed by iodine-131. Up to 50% of the instances of TC lose their avidity to 131I, becoming more aggressive. In this scenario, [18F]FDG PET/CT imaging is used for evaluating the widespread nature of the disease, despite its low sensitivity and a false negative rate of 8-21.1%. A novel class of PET agents targeting the fibroblast activation protein inhibitor (FAPi) has emerged, studied particularly for their potential application to theranostics. (2) Methods: A search of the literature was performed by two independent authors (P.G. and L.E.) using the PubMed, Scopus, Web of Science, Cochrane Library, and EMBASE databases. The following terms were used: "FAP" or "FAPi" or "Fibroblast activating protein" and "thyroid" or "thyroid cancer", in different combinations. The included papers were original articles, clinical studies, and case reports in the English language. No time limits were used. Editorials, conference papers, reviews, and preclinical studies were excluded. (3) Results: There were 31 papers that were selected. Some studies reported a low or absent FAPi uptake in TC lesions; others reported promising findings for the detection of metastases. (4) Conclusions: The preliminary results are encouraging. FAPI agents are an alternative to [18F]FDG and a promising theranostic tool. However, further studies with a larger population are needed.

Head-to-Head Comparison of FDG and Radiolabeled FAPI PET: A Systematic Review of the Literature

FAPI-based radiopharmaceuticals are a novel class of tracers, mainly used for PET imaging, which have demonstrated several advantages over [18F]FDG, especially in the case of low-grade or well-differentiated tumors. We conducted this systematic review to evaluate all the studies where a head-to-head comparison had been performed to explore the potential utility of FAPI tracers in clinical practice. FAPI-based radiopharmaceuticals have shown promising results globally, in particular in detecting peritoneal carcinomatosis, but studies with wider populations are needed to better understand all the advantages of these new radiopharmaceuticals.

Soluble DPP4 can act as a diagnostic biomarker in Hashimoto's thyroiditis with thyroid papillary carcinoma

Background

Hashimoto's thyroiditis (HT) is an independent risk factor for papillary thyroid carcinoma (PTC), but the underlying mechanism remains unknown. The incidence of PTC in patients with HT is significantly elevated, and the presence of both HT and PTC contributes to a higher rate of misdiagnosis.

Materials and Methods

Gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed on the thyroid nodule gene chip dataset from GEO Datasets. Serum and clinical data from 191 patients with thyroid nodules at the affiliated hospital were collected for analysis. Experimental techniques, including real-time quantitative PCR, ELISA, immunohistochemistry (IHC), and enzyme activity detection, were used to measure the level of dipeptidyl peptidase 4 (DPP4) in thyroid nodule tissues and serum.

Results

Thyroid nodules in patients with HT and PTC exhibit high levels of DPP4, along with elevated concentrations of soluble DPP4 in the serum. These findings demonstrate the potential predictive value of soluble DPP4 for PTC diagnosis.

Conclusions

The concentration and enzymatic activity of soluble DPP4 in serum can serve as diagnostic biomarkers for patients with HT-associated PTC.

Activity-based bioluminescence probes for in vivo sensing applications

Bioluminescence imaging is a highly sensitive technique commonly used for various in vivo applications. Recent efforts to expand the utility of this modality have led to the development of a suite of activity-based sensing (ABS) probes for bioluminescence imaging by 'caging' of luciferin and its structural analogs. The ability to selectively detect a given biomarker has presented researchers with many exciting opportunities to study both health and disease states in animal models. Here, we highlight recent (2021-2023) bioluminescence-based ABS probes with an emphasis on probe design and in vivo validation experiments.

Role and mechanism of fibroblast-activated protein-α expression on the surface of fibroblast-like synoviocytes in rheumatoid arthritis

Fibroblast-activated protein-α (FAP) is a type II integrated serine protease expressed by activated fibroblasts during fibrosis or inflammation. Fibroblast-like synoviocytes (FLSs) in rheumatoid arthritis (RA) synovial sites abundantly and stably overexpress FAP and play important roles in regulating the cellular immune, inflammatory, invasion, migration, proliferation, and angiogenesis responses in the synovial region. Overexpression of FAP is regulated by the initial inflammatory microenvironment of the disease and epigenetic signaling, which promotes RA development by regulating FLSs or affecting the signaling cross-linking FLSs with other cells at the local synovium and inflammatory stimulation. At present, several treatment options targeting FAP are in the process of development. This review discusses the basic features of FAP expressed on the surface of FLSs and its role in RA pathophysiology and advances in targeted therapies.

Mesenchymal stromal cells in the thymus

The microenvironment of the thymus is composed of a group of stromal cells that include endoderm-derived thymic epithelial cells (TECs) and mesenchymal stromal cells such as fibroblasts and serves as a site for the development of T cells. TECs are known to play an essential role in T cell differentiation and selection. Mesenchymal stromal cells have been less studied in terms of their immunological significance compared to TECs. Recently, new technologies have made it possible to identify and characterize mesenchymal stromal cells in the thymus, revealing their unique functions in thymic organogenesis and T cell development. This review outlines the current views on mesenchymal stromal cells in the thymus, particularly highlighting the newly discovered function of thymic fibroblasts in T cell repertoire selection.

Bioluminescence imaging of fibroblast activation protein-alpha in vivo and human plasma with highly sensitive probe

Fibroblast activation protein-alpha (FAPα) has emerged as a biomarker of tumor stromal fibroblasts. FAP was overexpressed in stromal fibroblasts of human malignancies and positively correlated with the depth of tumor invasion, lymphatic metastasis, distant metastases, high TNM stage and poor prognosis. However, the circulating FAP levels in the plasma of gastric cancer patients and the relationship between FAP levels and gastric cancer remain unknown. Moreover, probes with super selectivity, extremely high sensitivity, and excellent performance in quantitative detection are still lacking. Herein, we developed the bioluminescent probe BL-FAP for sensitive detection and imaging of endogenous FAP in gastric cancer cells and tissues and plasma from gastric cancer patients. The probe exhibited the high signal-to-noise ratio (15000∼fold), the excellent selectivity (FAP/DPP IV ratio and FAP/PREP ratio = 50000∼ fold), and the high sensitivity (18.1 pg/mL). BL-FAP facilitates monitoring of endogenous FAP in living cells and nude mice bearing MGC-803-luc tumors. More importantly, this probe was successfully applied to the measurement of FAP activity levels in plasma from gastric cancer patients for the first time. A significant enhancement in FAP levels was observed in patients with gastric cancer, suggesting that the FAP level may be a potential diagnostic parameter for gastric cancer.

CT radiomics signature: a potential biomarker for fibroblast activation protein expression in patients with pancreatic ductal adenocarcinoma

PURPOSE

To develop and validate a radiomics model to predict fibroblast activation protein (FAP) expression in patients with pancreatic ductal adenocarcinoma (PDAC).

METHODS

This retrospective study included consecutive 152 patients with PDAC who underwent MDCT scan and surgical resection from January 2017 to December 2017 (training set) and from January 2018 to April 2018 (validation set). In the training set, 1409 portal radiomic features were extracted from each patient's preoperative imaging. Optimal features were selected using the least absolute shrinkage and selection operator (LASSO) logistic regression algorithm, whereupon the extreme gradient boosting (XGBoost) was developed using the radiomics features. The performance of the XGBoost classifier performance was assessed by its calibration, discrimination, and clinical usefulness.

RESULTS

The patients were divided into FAP-low (n = 91; 59.87%) and FAP-high (n = 61; 40.13%) groups according to the optimal FAP cutoff (45.71%). Patients in the FAP-low group showed longer survival. The XGBoost classifier comprised 13 selected radiomics features and showed good discrimination in the training set [area under the curve (AUC), 0.97] and the validation set (AUC, 0.75). It also performed well in the calibration test and decision-curve analysis, demonstrating its potential clinical value.

CONCLUSIONS

The XGBoost classifier based on CT radiomics in the portal venous phase can non-invasively predict FAP expression and may help to improve clinical decision-making in patients with PDAC.

Identification of novel biomarkers to distinguish bradykinin-mediated angioedema from mast cell-/histamine-mediated angioedema

BACKGROUND

The pathophysiology of the underlying paroxysmal permeability disturbances in angioedema (AE) is not well understood.

METHODS

To identify clinical and laboratory parameters specific for a certain AE subtype, 40 AE patients were prospectively enrolled: 15 hereditary (HAE), 13 ACE-inhibitor induced (ACE-AE), and 12 mast cell-mediated without wheals in chronic spontaneous urticaria (CSU-AE). Ten healthy subjects served as controls. Serum levels of markers indicating activation of the ficolin-lectin pathway, of endothelial cells, or those indicating impairment of vascular integrity or inflammation were assessed by enzyme-linked immunosorbent assay.

RESULTS

New routine clinical diagnostic criteria could not be identified, not even for distinguishing bradykinin-mediated (BK-) AE (ie, HAE and ACE-AE) from mast cell-/histamine-mediated CSU-AE. However, FAP-α and tPA were significantly increased in all AE compared to controls. In HAE, FAP- α, tPA, uPAR, pentraxin-3, Tie-2, sE-selectin, and VE-cadherin were significantly increased compared to controls. In HAE compared to CSU-AE and ACE-AE, sE-Selectin, Tie-2, and VE-Cadherin were significantly increased, whereas for Ang-2 the difference was significant compared to CSU-AE only. Tie-2 correlated strongly negatively with C4, C1-INH activity, and C1-INH function.

CONCLUSIONS

This study is the first to compare HAE, ACE-AE, and CSU-AE. Although significance is limited by small sample size, Tie-2 was identified as a new promising biomarker candidate for HAE. FAP- α and tPA might serve as a marker for AE in general, whereas sE-selectin and Ang-2 were increased in BK-AE only. Our results add information to the role of endothelial dysfunction and serine proteases in different AE subtypes.

Circulating Dipeptidyl Peptidase Activity Is a Potential Biomarker for Inflammatory Bowel Disease

INTRODUCTION

Dipeptidyl peptidase (DPP)-4 is part of a larger family of proteases referred to as DPPs. DPP4 has been suggested as a possible biomarker for inflammatory bowel disease (IBD). Circulating DPP4 (cDPP4) enzyme activity was investigated as a potential biomarker for IBD. In addition, DPP enzyme activity and gene expression were quantified in colonic tissue of patients with IBD and non-IBD.

METHODS

In study 1, DPP enzyme activity was quantified in plasma samples from 220 patients with IBD (Crohn's disease [CD] n = 130 and ulcerative colitis [UC] n = 90) and non-IBD controls (n = 26) using a colorimetric assay. In study 2, tissue and plasma samples were collected from 26 patients with IBD and 20 non-IBD controls. Plasma C-reactive protein (CRP) was quantified in all patients. Colonic DPP4, DPP8, DPP9, and fibroblast activation protein (FAP) gene expression was determined by quantitative polymerase chain reaction. cDPP and cFAP enzyme activity was also measured. Sensitivity and specificity were determined by receiver operating characteristic curve analysis.

RESULTS

In study 1, total cDPP activity was found to differentiate patients with CD with active disease (n = 18) from those in remission (n = 19; sensitivity 78% and specificity 63%). In study 2, total cDPP and cFAP activity was 28% and 48% lower in patients with elevated CRP (>10 mg/L), respectively, compared with patients with normal CRP. Gene expression of DPP4, FAP, and DPP8 was also significantly higher in colonic biopsies from patients with IBD compared with non-IBD patients (P < 0.05).

DISCUSSION

Our findings implicate the DPP enzyme family in intestinal inflammation and suggest future biomarker applications to differentiate the pathophysiological aspects of IBD.

Translational imaging of the fibroblast activation protein (FAP) using the new ligand [68Ga]Ga-OncoFAP-DOTAGA

PURPOSE

The fibroblast activation protein (FAP) is an emerging target for molecular imaging and therapy in cancer. OncoFAP is a novel small organic ligand for FAP with very high affinity. In this translational study, we establish [⁶⁸Ga]Ga-OncoFAP-DOTAGA (⁶⁸Ga-OncoFAP) radiolabeling, benchmark its properties in preclinical imaging, and evaluate its application in clinical PET scanning.

METHODS

⁶⁸Ga-OncoFAP was synthesized in a cassette-based fully automated labeling module. Lipophilicity, affinity, and serum stability of ⁶⁸Ga-OncoFAP were assessed by determining logD_7.4, IC₅₀ values, and radiochemical purity. ⁶⁸Ga-OncoFAP tumor uptake and imaging properties were assessed in preclinical dynamic PET/MRI in murine subcutaneous tumor models. Finally, biodistribution and uptake in a variety of tumor types were analyzed in 12 patients based on individual clinical indications that received 163 ± 50 MBq ⁶⁸Ga-OncoFAP combined with PET/CT and PET/MRI.

RESULTS

⁶⁸Ga-OncoFAP radiosynthesis was accomplished with high radiochemical yields. Affinity for FAP, lipophilicity, and stability of ⁶⁸Ga-OncoFAP measured are ideally suited for PET imaging. PET and gamma counting-based biodistribution demonstrated beneficial tracer kinetics and high uptake in murine FAP-expressing tumor models with high tumor-to-blood ratios of 8.6 ± 5.1 at 1 h and 38.1 ± 33.1 at 3 h p.i. Clinical ⁶⁸Ga-OncoFAP-PET/CT and PET/MRI demonstrated favorable biodistribution and kinetics with high and reliable uptake in primary cancers (SUV_max 12.3 ± 2.3), lymph nodes (SUV_max 9.7 ± 8.3), and distant metastases (SUV_max up to 20.0).

CONCLUSION

Favorable radiochemical properties, rapid clearance from organs and soft tissues, and intense tumor uptake validate ⁶⁸Ga-OncoFAP as a powerful alternative to currently available FAP tracers.

The Serine Protease CD26/DPP4 in Non-Transformed and Malignant T Cells

Simple Summary

The transmembrane serine protease CD26/Dipeptidylpeptidase 4 modulates T-cell activation, proliferation, and effector function. Due to their remarkable tumoricidal properties CD26-positive T cells are considered promising candidates for T cell-based immunotherapies while in cutaneous T cell lymphoma CD26/DPP4 expression patterns are established markers for diagnosis and possibly prognosis. With a focus on T cells, we review current knowledge on the regulation of CD26/DPP4 expression and release, its implication in T-cell effector function and the suitability CD26/DPP4 as a diagnostic and/or prognostic factor in T-cell malignancies.

Abstract

CD26/Dipeptidylpeptidase 4 is a transmembrane serine protease that cleaves off N-terminal dipeptides. CD26/DPP4 is expressed on several immune cell types including T and NK cells, dendritic cells, and activated B cells. A catalytically active soluble form of CD26/DPP4 can be released from the plasma membrane. Given its wide array of substrates and interaction partners CD26/DPP4 has been implicated in numerous biological processes and effects can be dependent or independent of its enzymatic activity and are exerted by the transmembrane protein and/or the soluble form. CD26/DPP4 has been implicated in the modulation of T-cell activation and proliferation and CD26/DPP4-positive T cells are characterized by remarkable anti-tumor properties rendering them interesting candidates for T cell-based immunotherapies. Moreover, especially in cutaneous T-cell lymphoma CD26/DPP4 expression patterns emerged as an established marker for diagnosis and treatment monitoring. Surprisingly, besides a profound knowledge on substrates, interaction partners, and associated signal transduction pathways, the precise role of CD26/DPP4 for T cell-based immune responses is only partially understood.

Distinctive CD26 Expression on CD4 T-Cell Subsets

Immune system CD4 T-cells with high cell-surface CD26 expression show anti-tumoral properties. When engineered with a chimeric antigen receptor (CAR), they incite strong responses against solid cancers. This subset was originally associated to human CD4 T helper cells bearing the CD45R0 effector/memory phenotype and later to Th17 cells. CD26 is also found in soluble form (sCD26) in several biological fluids, and its serum levels correlate with specific T cell subsets. However, the relationship between glycoprotein sCD26 and its dipeptidyl peptidase 4 (DPP4) enzymatic activity, and cell-surface CD26 expression is not well understood. We have studied ex vivo cell-surface CD26 and in vitro surface and intracellular CD26 expression and secretome's sCD26 in cultured CD4 T cells under different polarization conditions. We show that most human CD26negative CD4 T cells in circulating lymphocytes are central memory (T_CM) cells while CD26high expression is present in effector Th1, Th2, Th17, and T_EM (effector memory) cells. However, there are significant percentages of Th1, Th2, Th17, and Th22 CD26 negative cells. This information may help to refine the research on CAR-Ts. The cell surface CD45R0 and CD26 levels in the different T helper subsets after in vitro polarization resemble those found ex vivo. In the secretomes of these cultures there was a significant amount of sCD26. However, in all polarizations, including Th1, the levels of sCD26 were lower (although not significantly) compared to the Th0 condition (activation without polarization). These differences could have an impact on the various physiological functions proposed for sCD26/DPP4.

Dipeptidyl peptidase-4 inhibitor-induced autoimmune diseases: Current evidence

Dipeptidyl peptidase-4 inhibitors (DPP-4i) have an important place in the management of type 2 diabetes. The DPP-4 enzyme is ubiquitously distributed throughout the human body and has multiple substrates through which it regulates several important physiological functions. DPP-4 regulates several immune functions, including T-cell activation, macrophage function, and secretion of cytokines. Studies have reported an increase in autoimmune diseases like bullous pemphigoid, inflammatory bowel disease, and arthritis with DPP-4i use. The relationship of DPP-4i and autoimmune diseases is a complex one and warrants further research into the effect of DPP-4 inhibition on the immune system to understand the pathogenesis more clearly. Whether a particular cluster of autoimmune diseases is associated with DPP-4i use remains an important contentious issue. Nevertheless, a heightened awareness from the clinicians is required to identify and treat any such diseases. Through this review, we explore the clinical and pathophysiological characteristics of this association in light of recent evidence.

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.

The Long-Term Study of Urinary Biomarkers of Renal Injury in Spontaneously Hypertensive Rats

BACKGROUND

The age-related increase in blood pressure in spontaneously hypertensive rats (SHRs) is associated to cardiac hypertrophy, heart failure, and renal injury. Here, we investigated for the first time the urinary enzymatic activities of glutamil aminopeptidase (GluAp), alanyl aminopeptidase (AlaAp), dipeptidyl peptidase-4 (DPP4), and Klotho urinary levels, proteins that are strongly expressed in the kidney, as early biomarkers of renal injury in SHRs.

METHODS

Male SHR and Wistar Kyoto (WKY) rats were studied from 2 to 8 months old. Systolic blood pressure (SBP), the heart rate (HR), metabolic variables, and urinary markers were measured monthly. At the end of the study, a histopathological evaluation of the kidney was performed.

RESULTS

Kidneys of SHR did not develop signs of relevant histopathological changes, but showed increased glomerular area and cellularity. Plasma creatinine was decreased, and creatinine clearance was augmented in SHR at the end of the study. Urinary excretion of Klotho was higher in SHR at 5 and 8 months old, whereas plasma Klotho levels were similar to WKY. GluAp, AlaAp, and DPP4 urinary activities were increased in SHR throughout the time-course study. A positive correlation between glomerular area and cellularity with creatinine clearance was observed. Urinary GluAp, AlaAp, DPP4, and Klotho showed positive correlations with SBP.

CONCLUSIONS

GluAp, AlaAp, DPP4, and Klotho in the urine are useful tools for the evaluation of renal damage at early stages, before the whole histopathological and biochemical manifestations of renal disease are established. Moreover, these observations may represent a novel and noninvasive diagnostic approach to assess the evolution of kidney function in hypertension and other chronic diseases.

The fibroblast: An emerging key player in thymic T cell selection

Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.

Serum levels of soluble dipeptidyl peptidase-4 in type 2 diabetes are associated with severity of liver fibrosis evaluated by transient elastography (FibroScan) and the FAST (FibroScan-AST) score, a novel index of non-alcoholic steatohepatitis with significant fibrosis

AIM

To investigate the relationship in people with type 2 diabetes between serum soluble dipeptidyl peptidase-4 (sDDP-4) and degree of liver fibrosis assessed as the liver stiffness measurement (LSM) and FAST (FibroScan-AST) score, both of which were measured by transient elastography (FibroScan).

SUBJECTS AND METHODS

In this cross-sectional study, we examined 115 patients with type 2 diabetes. With transient elastography (FibroScan), we assessed the controlled attenuation parameter (CAP) and liver stiffness measurement (LSM) as measures of hepatic steatosis and liver fibrosis, respectively. We calculated the FAST score, which identifies progressive non-alcoholic steatohepatitis (NASH), from CAP, LSM, and the serum aspartate aminotransferase level. Significant hepatic steatosis was defined as CAP ≥280 dB/m; and significant liver fibrosis, as LSM ≥ 8.0 kPa. LSM was divided into 3 severity levels: significant fibrosis (8.0 to <9.7 kPa); advanced fibrosis, (9.7 to <13.0 kPa); and liver cirrhosis (≥ 13.0 kPa).

RESULTS

Serum sDPP-4 correlated positively with liver enzymes, CAP, LSM, and FAST score. Multivariate analysis showed that LSM remained to be an independent factor for serum sDDP-4. Serum sDPP-4 was significantly higher in patients with LSM ≥ 8.0 kPa than in those with LSM <8.0 kPa and was significantly elevated in patients who are at risk for non-alcoholic steatohepatitis (NASH) with fibrosis (FAST score ≥ 035 or 0.67). Patients with both hepatic steatosis and liver fibrosis had the highest serum sDPP-4.

CONCLUSION

Serum sDPP-4 was strongly associated with severity of liver fibrosis evaluated by LSM and the FAST score and was markedly elevated in diabetic patients with LSM ≥ 13.0 kPa indicating probable cirrhosis.

Imaging of Fibroblast Activation Protein in Cancer Xenografts Using Novel (4-Quinolinoyl)-glycyl-2-cyanopyrrolidine-Based Small Molecules

Fibroblast activation protein (FAP) has become a favored target for imaging and therapy of malignancy. We have synthesized and characterized two new (4-quinolinoyl)-glycyl-2-cyanopyrrolidine-based small molecules for imaging of FAP, QCP01 and [111In]QCP02, using optical and single-photon computed tomography/CT, respectively. Binding of imaging agents to FAP was assessed in six human cancer cell lines of different cancer types: glioblastoma (U87), melanoma (SKMEL24), prostate (PC3), NSCLC (NCIH2228), colorectal carcinoma (HCT116), and lung squamous cell carcinoma (NCIH226). Mouse xenograft models were developed with FAP-positive U87 and FAP-negative PC3 cells to test pharmacokinetics and binding specificity in vivo. QCP01 and [111In]QCP02 demonstrated nanomolar inhibition of FAP at Ki values of 1.26 and 16.20 nM, respectively. Both were selective for FAP over DPP-IV, a related serine protease. Both enabled imaging of FAP-expressing tumors specifically in vivo. [111In]QCP02 showed high uptake at 18.2 percent injected dose per gram in the U87 tumor at 30 min post-administration.

Dynamic PET/CT Imaging of 68Ga-FAPI-04 in Chinese Subjects

This study aims to further explore dynamic ⁶⁸Ga-FAPI-04 PET/CT imaging of healthy Chinese subjects and lung cancer patients. Moreover, the variability of ⁶⁸Ga-FAPI-04 uptake in normal organs was measured to provide a basis for analyzing its biological distribution, interpreting auxiliary images, determining the reliability of image quantification, and monitoring treatment. Six patients (3 subjects without tumors and 3 lung cancer patients) separately underwent ⁶⁸Ga-FAPI-04 and 2-[¹⁸F]FDG PET/CT imaging within 1 week. The biodistribution and internal radiation dosimetry were reported and compared with data previously obtained from Caucasian patients. Moreover, the mean SUV (standardized uptake value) was normalized to body mass or to lean body mass (SUL), and the coefficients of variation (CVs) were calculated and compared for each volume of interest. The average whole-body effective dose was calculated to be 1.27E-02 mSv/MBq, which was comparable with previously reported results of ⁶⁸Ga-FAPI-04 probes. Furthermore, the SUVmean was slightly higher than the SULmean in most organs; however, the CV of the SULmean for most organs was higher than that of the SUVmean at later time points. In the liver, the CV of the SUVmean was lower (12.7%) than that of the SULmean and was similar to the CV for corresponding 2-[¹⁸F]FDG PET/CT value (11.8%). In addition, ⁶⁸Ga-FAPI-04 PET/CT showed good efficacy for diagnosing lung cancer patients in this study. A comparison of the radiation dosimetry obtained before from a Caucasian population demonstrated no clinically significant differences between these two populations after ⁶⁸Ga-FAPI-04 injection. The variability in most organs was slightly lower for SUVmean than for SULmean, suggesting that SUVmean may be the preferable parameter for quantifying images obtained with ⁶⁸Ga-FAPI-04. In addition, ⁶⁸Ga-FAPI-04 PET/CT imaging is expected to be a promising tool for diagnosing lung cancer.

Characterization of DPP-IV Inhibitory Peptides Using an In Vitro Cell Culture Model of the Intestine

Here, we characterize the activities of two depeptidyl peptidase-IV (DPP-IV) inhibitory peptides, VLATSGPG and LDKVFER, using the Caco-2 monolayer model for the intestine. VLATSGPG and LDKVFR inhibited the DPP-IV in the cells via a mixed-type inhibition mode, with in situ IC₅₀ values of 207.3 and 148.5 μM, respectively. Furthermore, VLATSGPG and LDKVFR were transported intact across the cells, with P_app values of 2.41 ± 0.16 and 4.23 ± 0.29 × 10^-7 cm/s, respectively. Fragmented peptides were identified in the basolateral side of the membrane. Two of these, GPG and VLA, exhibited high inhibitory activities of 83.6 ± 3.3 and 58.5 ± 2.5%, respectively, at 100 μM concentration. Although 3 mM VLATSGPG and LDKVFR were transported across the epithelium in a concentration-dependent manner, their transport did not damage the tight junction proteins, ZO-1 and occludin. This study demonstrates that the two peptides potentially regulate DPP-IV activity in the intestine.

Discovery of a novel fibroblast activation protein (FAP) inhibitor, BR103354, with anti-diabetic and anti-steatotic effects

Fibroblast growth factor (FGF) 21 is a class of hepatokines that plays a protective role against obesity, insulin resistance, and liver damage. Despite this, protective effects of FGF21 in human appear to be minimal, possibly due to its proteolytic cleavage by the fibroblast activation protein (FAP). Here, we presented a novel FAP inhibitor, BR103354, and described its pharmacological activities as a potential therapeutic agent for the treatment of metabolic disorders. BR103354 inhibited FAP with an IC₅₀ value of 14 nM, showing high selectivity against dipeptidyl peptidase (DPP)-related enzymes and prolyl oligopeptidase (PREP). In differentiated 3T3/L1 adipocytes, the addition of FAP diminished hFGF21-induced Glut1 and phosphorylated levels of ERK, which were restored by BR103354. BR103354 exhibited good pharmacokinetic properties as evidenced by oral bioavailability of 48.4% and minimal hERG inhibition. Single co-administration of BR103354 with hFGF21 reduced nonfasting blood glucose concentrations, in association with increased intact form of hFGF21 in ob/ob mice. Additionally, chronic treatment of BR103354 for 4 weeks reduced nonfasting blood glucose concentrations with improved glucose tolerance and with reduced triglyceride (TG) content in liver of ob/ob mice. Consistently, BR103354 improved hepatic steatosis and fibrosis in a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD)-induced non-alcoholic steatohepatitis (NASH) mouse model. FAP inhibitory effects of BR103354 were confirmed in normal cynomolgus monkeys. Together, BR103354 acts as an effective FAP inhibitor in vitro and in vivo, thereby demonstrating its potential application as an anti-diabetic and anti-NASH agent.

Altered Tissue and Plasma Levels of Fibroblast Activation Protein-α (FAP) in Renal Tumours

Simple Summary

Malignant epithelial tumour’s behaviour in the kidney has traditionally been analysed attending to different prognostic parameters focussed on the proliferating neoplastic cell. This is the case of renal cell carcinoma (RCC), in which a large tumour diameter, high histological grade, and the presence of necrosis, among other factors, have been related with a high risk of distant metastasis and, therefore, worse survival. Recently, several elements of the tumour microenviroment, such as cancer-associated fibroblasts (CAFs), are being studied in order to develop more accurate diagnostic and therapeutic approaches. We present data that support that the fibroblast activation protein-α (FAP), a CAF biomarker, provides interesting information both in tumour tissues and in plasma from patients with RCC.

Abstract

(1) Background: Renal cell carcinoma (RCC) is a heterogeneous and complex disease with only partial response to therapy, high incidence of metastasis and recurrences, and scarce reliable biomarkers indicative of progression and survival. Cancer-associated fibroblasts (CAFs) play an important role supporting and promoting renal cancer progression. (2) Methods: In this study, we analysed fibroblast activation protein-α (FAP) immunohistochemical expression and its soluble isoform (sFAP) in tumour tissues and plasma from 128 patients with renal tumours. (3) Results: FAP is expressed in the cell surface of CAFs of the tumour centre and infiltrating front from clear cell renal cell carcinomas (CCRCC, n = 89), papillary renal cell carcinomas (PRCC, n = 21), and chromophobe renal cell carcinomas (ChRCC, n = 8), but not in the benign tumour renal oncocytoma (RO, n = 10). A high expression of FAP and low levels sFAP are significantly associated with high tumour diameter, high grade, and high pT stage, lymph node invasion, development of early metastases, and worse 5-year cancer specific survival of CCRCC patients. (4) Conclusions: These findings corroborate the potential usefulness of FAP immunohistochemistry and plasma sFAP as a biomarker of CCRCC progression and point to CAF-related proteins as promising immunohistochemical biomarkers for the differential diagnosis of ChRCC and RO.

Bulked Segregant RNA-Seq Reveals Distinct Expression Profiling in Chinese Wheat Cultivar Jimai 23 Responding to Powdery Mildew

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most destructive fungal diseases threatening global wheat production. Host resistance is well known to be the most efficient method to control this disease. However, the molecular mechanism of wheat powdery mildew resistance (Pm) is still unclear. To analyze the molecular mechanism of Pm, we used the resistant wheat cultivar Jimai 23 to investigate its potential resistance components and profiled its expression in response to powdery mildew infection using bulked segregant RNA-Seq (BSR-Seq). We showed that the Pm of Jimai 23 was provided by a single dominant gene, tentatively designated PmJM23, and assigned it to the documented Pm2 region of chromosome 5DS. 3,816 consistently different SNPs were called between resistant and susceptible parents and the bulked pools derived from the combinations between the resistant parent Jimai23 and the susceptible parent Tainong18. 58 of the SNPs were assigned to the candidate region of PmJM23. Subsequently, 3,803 differentially expressed genes (DEGs) between parents and bulks were analyzed by GO, COG and KEGG pathway enrichment. The temporal expression patterns of associated genes following Bgt inoculation were further determined by RT-qPCR. Expression of six disease-related genes was investigated during Bgt infection and might serve as valuable genetic resources for the improvement of durable resistance to Bgt.

Altered expression of fibroblast activation protein-α (FAP) in colorectal adenoma-carcinoma sequence and in lymph node and liver metastases

Colorectal cancer (CRC) is a major health problem in elderly people because of its high incidence and high mortality rate. Despite early screening programs, more than half of CRC patients are diagnosed at advanced stages. Fibroblast activation protein-α (FAP) expression in cancer-associated fibroblasts (CAFs) has been associated with a higher risk of metastases and poor survival. Here, we have analyzed the immunohistochemical expression of FAP in 41 adenoma-carcinoma sequences. In addition, FAP expression was analyzed individually and in combination with β-catenin (BCAT), CD44 and Cyclin-D1 expression in primary tumors and in their corresponding lymph node and liver metastases (n=294). Finally, soluble FAP (sFAP) levels in plasma from CRC patients (n=127) were also analyzed by ELISA. FAP was expressed only in CRC tissue and its expression level was found to be higher in tumors exhibiting deeper local invasion and poorer cancer cell differentiation. FAP and concomitant nuclear BCAT expression in cancer cells at the infiltrating front of primary tumors and in lymph node metastases was independently associated with 5- and 10-year cancer specific and disease-free survival. Moreover, lower sFAP levels correlated with poorer survival. These findings support the potential importance of FAP as a biomarker of CRC development and progression.

Degranulation of human cytotoxic lymphocytes is a major source of proteolytically active soluble CD26/DPP4

Dipeptidyl peptidase 4 (DPP4, CD26) is a serine protease detected on several immune cells and on epithelial cells of various organs. Besides the membrane-bound enzyme, a catalytically active soluble form (sCD26/DPP4) is detected in several body fluids. Both variants cleave off dipeptides from the N-termini of various chemokines, neuropeptides, and hormones. CD26/DPP4 plays a fundamental role in the regulation of blood glucose levels by inactivating insulinotropic incretins and CD26/DPP4 inhibitors are thus routinely used in diabetes mellitus type 2 therapy to improve glucose tolerance. Such inhibitors might also prevent the CD26/DPP4-mediated inactivation of the T-cell chemoattractant CXCL10 released by certain tumors and thus improve anti-tumor immunity and immunotherapy. Despite its implication in the regulation of many (patho-)physiological processes and its consideration as a biomarker and therapeutic target, the cellular source of sCD26/DPP4 remains highly debated and mechanisms of its release are so far unknown. In line with recent reports that activated T lymphocytes could be a major source of sCD26/DPP4, we now demonstrate that CD26/DPP4 is stored in secretory granules of several major human cytotoxic lymphocyte populations and co-localizes with effector proteins such as granzymes, perforin, and granulysin. Upon stimulation, vesicular CD26/DPP4 is rapidly translocated to the cell surface in a Ca2+-dependent manner. Importantly, activation-induced degranulation leads to a massive release of proteolytically active sCD26/DPP4. Since activated effector lymphocytes serve as a major source of sCD26/DPP4, these results might explain the observed disease-associated alterations of sCD26/DPP4 serum levels and also indicate a so far unknown role of CD26/DPP4 in lymphocyte-mediated cytotoxicity.

Radiation Dosimetry and Biodistribution of 68Ga-FAPI-46 PET Imaging in Cancer Patients

Targeting cancer-associated fibroblasts (CAFs) has become an attractive goal for diagnostic imaging and therapy because they can constitute as much as 90% of a tumor mass. The serine protease fibroblast activation protein (FAP) is overexpressed selectively in CAFs, drawing interest in FAP as a stromal target. The quinoline-based FAP inhibitor (FAPI) PET tracer ⁶⁸Ga-FAPI-04 has been previously shown to yield high tumor-to-background ratios (TBRs) in patients with various cancers. Recent developments toward an improved compound for therapeutic application have identified FAPI-46 as a promising agent because of an increased tumor retention time in comparison with FAPI-04. Here, we present a PET biodistribution and radiation dosimetry study of ⁶⁸Ga-FAPI-46 in cancer patients. Methods: Six patients with different cancers underwent serial ⁶⁸Ga-FAPI-46 PET/CT scans at 3 time points after radiotracer injection: 10 min, 1 h, and 3 h. The source organs consisted of the kidneys, bladder, liver, heart, spleen, bone marrow, uterus, and remainder of body. OLINDA/EXM software, version 1.1, was used to fit and integrate the kinetic organ activity data to yield total-body and organ time-integrated activity coefficients and residence times and, finally, organ-absorbed doses. SUVs and TBR were generated from the contoured tumor and source-organ volumes. Spheric volumes in muscle and blood pool were also obtained for TBR (tumor SUV_max/organ SUV_mean). Results: At all time points, average SUV_max was highest in the liver. Tumor and organ SUV_mean decreased over time, whereas TBRs in all organs but the uterus increased. The organs with the highest effective doses were bladder wall (2.41E-03 mSv/MBq), followed by ovaries (1.15E-03 mSv/MBq) and red marrow (8.49E-04 mSv/MBq). The average effective total-body dose was 7.80E-03 mSv/MBq. Conclusion: ⁶⁸Ga-FAPI-46 PET/CT has a favorable dosimetry profile, with an estimated whole-body dose of 5.3 mSv for an administration of 200 MBq (5.4 mCi) of ⁶⁸Ga-FAPI-46 (1.56 ± 0.26 mSv from the PET tracer and 3.7 mSv from 1 low-dose CT scan). The biodistribution study showed high TBRs increasing over time, suggesting high diagnostic performance and favorable tracer kinetics for potential therapeutic applications.

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.

DPP-4 as a Possible Biomarker of Inflammation Before Abdominal Surgery for Chronic Pathology: Our Experience with Elective Cholecystectomy

Background and objectives: Dipeptidyl-Peptidase 4 (DPP-4) is a protein expressed in numerous cells and tissues. Recently it has shown its involvement as a catalyst in the inflammatory response in various pulmonary, autoimmune, intestinal and other pathologies. The objective of this study was to compare the preoperative serum levels of DPP-4 in patients with and without surgical finding of perivesicular inflammation. Materials and methods: a cross-sectional analytical study nested in a prospective cohort, including patients scheduled for elective cholecystectomy, without surgical complications, that were 18–70 years of age, with low cardiovascular risk, without a history of peritonitis, pancreatitis, or jaundice and underwent ERCP protocol, type 2 diabetes mellitus, acute inflammatory (Protein C Reactive < 3 mg/L, leucocytes < 10 1000/mm³), neoplastic, nephrologic or liver disease, the use of anti-inflammatory drugs, steroids and/or antibiotics, the use of pacemakers or metallic implants and without major amputations and whom agreed to participate by providing their informed consent. Ethical and Research register: 45–16. Prior to surgery we compiled anthropometric data and a blood sample to determine the serum levels of DPP-4. The presence of perivesicular inflammation was determined in the surgery. The data was analyzed using the statistical program Rstudio. Results: High BMI values were observed (27.8 ± 6.4); waist circumference (94.7 ± 15.1) and percentage of fat mass (34.7 ± 11.7), showing a cumulative frequency of 65.9% for overweight/obesity. In 27.3% of the interventions, intraoperative perivesicular inflammation findings were reported. The serum levels of DPP-4 were lower in the group of patients with perivesicular inflammation (3947.6 ± 1659.5 vs. 3053.2 ± 1469.6, LC95% of the difference: 160.4–1628.3), being statistically significant (p = 0.018). Conclusions: In the subacute or chronic phases of cholecystitis, there appears to be a constant consumption of DPP-4, which would modulate a better immune response that could be related to the reduction of postoperative complications, so the use of Serum levels of DPP-4 as an early biomarker could improve the diagnostic accuracy of this pathology and the surgical approach.

Development of a 13C Stable Isotope Assay for Dipeptidyl Peptidase-4 Enzyme Activity A New Breath Test for Dipeptidyl Peptidase Activity

Dipeptidyl peptidase-4 inhibitors (DPP4i) are a class of orally available, small molecule inhibitors for the management of Type-II diabetes. A rapid, real-time, functional breath test for DPP4 enzyme activity could help to define DPP4i efficacy in patients that are refractory to treatment. We aimed to develop a selective, non-invasive, stable-isotope ¹³C-breath test for DPP4. In vitro experiments were performed using high (Caco-2) and low (HeLa) DPP4 expressing cells. DPP gene expression was determined in cell lines by qRT-PCR. A DPP4 selective ¹³C-tripeptide was added to cells in the presence and absence of the DPP4 inhibitor Sitagliptin. Gas samples were collected from the cell headspace and ¹³CO₂ content quantified by isotope ratio mass spectrometry (IRMS). DPP4 was highly expressed in Caco-2 cells compared to HeLa cells and using the ¹³C-tripeptide, we detected a high ¹³CO₂ signal from Caco2 cells. Addition of Sitaglitpin to Caco2 cells significantly inhibited this ¹³CO₂ signal. ¹³C-assay DPP4 activity correlated positively with the enzyme activity detected using a colorimetric substrate. We have developed a selective, non-invasive, ¹³C-assay for DPP4 that could have broad translational applications in diabetes and gastrointestinal disease.

Lymphocytes are a major source of circulating soluble dipeptidyl peptidase 4

Dipeptidyl peptidase 4 (DPP4, CD26) is a serine protease that is expressed constitutively by many haematopoietic and non-haematopoietic tissues. It exists as a membrane-associated protein, as well as in an active, soluble form (herein called sDPP4), present at high concentrations in bodily fluids. Despite the proposed use of sDPP4 as a biomarker for multiple diseases, its cellular sources are not well defined. Here, we report that individuals with congenital lymphocyte immunodeficiency had markedly lower serum concentrations of sDPP4, which were restored upon successful treatment and restoration of lymphocyte haematopoiesis. Using irradiated lymphopenic mice and wild-type to Dpp4-/- reciprocal bone marrow chimeric animals, we found that haematopoietic cells were a major source of circulating sDPP4. Furthermore, activation of human and mouse T lymphocytes resulted in increased sDPP4, providing a mechanistic link between immune system activation and sDPP4 concentration. Finally, we observed that acute viral infection induced a transient increase in sDPP4, which correlated with the expansion of antigen-specific CD8+ T cell responses. Our study demonstrates that sDPP4 concentrations are determined by the frequency and activation state of lymphocyte populations. Insights from these studies will support the use of sDPP4 concentration as a biomarker for inflammatory and infectious diseases.

A novel fibroblast activation protein-targeted near-infrared fluorescent off-on probe for cancer cell detection, in vitro and in vivo imaging

A new hemicyanine-based fibroblast activation protein-targeted near-infrared fluorescent probe is designed and it shows high selectivity and sensitivity to cancer cell detection, and in vitro and in vivo imaging. This probe is successfully applied in fluorescence detection of living cells (with a detection limit of 1500 cells per mL). It is believed that many new functions or distributions of FAP could be discovered by this new probe later.