Fibroblast activation protein restrains adipogenic differentiation and regulates matrix-mediated mTOR signaling

Development and Evaluation of DOTA-FAPI-Maleimide as a Novel Radiotracer for Tumor Theranostic with Extended Circulation

This study aimed to evaluate a novel albumin-binding strategy for addressing the challenge of insufficient tumor retention of fibroblast activation protein inhibitors (FAPIs). Maleimide, a molecule capable of covalent binding to free thiol groups, was modified to conjugate with FAPI-04 in order to enhance its binding to endogenous albumin, resulting in an extended blood circulation half-life and increased tumor uptake. DOTA-FAPI-maleimide was prepared and radiolabeled with Ga-68 and Lu-177, followed by cellular assays, pharmacokinetic analysis, PET/CT, and SPECT/CT imaging to assess the probe distribution in various tumor-bearing models. Radiolabeling of the modified probe was successfully achieved with a radiochemical yield of over 99% and remained stable for 144 h. Cellular assays showed that the ligand concentration required for 50% inhibition of the probe was 1.20 ± 0.31 nM, and the Kd was 0.70 ± 0.07 nM with a Bmax of 7.94 ± 0.16 fmol/cell, indicative of higher specificity and affinity of DOTA-FAPI-maleimide compared to other FAPI-04 variants. In addition, DOTA-FAPI-maleimide exhibited a persistent blood clearance half-life of 7.11 ± 0.34 h. PET/CT images showed a tumor uptake of 2.20 ± 0.44%ID/g at 0.5 h p.i., with a tumor/muscle ratio of 5.64 in HT-1080-FAP tumor-bearing models. SPECT/CT images demonstrated long-lasting tumor retention. At 24 h p.i., the tumor uptake of [177Lu]Lu-DOTA-FAPI-maleimide reached 5.04 ± 1.67%ID/g, with stable tumor retention of 3.40 ± 1.95%ID/g after 4 days p.i. In conclusion, we developed and evaluated the thiol group-attaching strategy, which significantly extended the circulation and tumor retention of the adapted FAPI tracer. We envision its potential application for clinical cancer theranostics.

FAP deficiency enhances thermogenesis and attenuates metabolic inflammation in diet-induced obesity

OBJECTIVE

Fibroblast activation protein α (FAP) is expressed in normal adipose tissue and related to some pleiotropic metabolic regulators. However, the exact role and mechanism of FAP in obesity and related metabolic disorders are not well understood.

METHODS

FAP knockout mice were fed a normal diet or a high-fat diet (HFD) for 12 weeks. FAP knockout mice or wild-type mice treated with an FAP inhibitor were subjected to cold stress for 5 days.

RESULTS

FAP deficiency protected mice against HFD-induced obesity and obesity-associated metabolic dysfunction, including glucose intolerance, insulin resistance, hyperglycemia, hyperinsulinemia, and hyperlipidemia. Notably, FAP deficiency largely reversed obesity-induced adipose tissue macrophage accumulation and M1-M2 imbalance in white adipose tissue (WAT). Moreover, energy expenditure was significantly higher in FAP-deficient mice fed an HFD. Both FAP deficiency and inhibition increased cold tolerance through enhancing WAT beiging.

CONCLUSIONS

This study demonstrated that FAP deficiency protects mice against diet-induced obesity and related metabolic dysfunction. Furthermore, the protective effects are probably mediated via the promotion of WAT beiging and suppression of inflammation.

Histone demethylase KDM7A regulates bone homeostasis through balancing osteoblast and osteoclast differentiation

Histone methylation plays a crucial role in various cellular processes. We previously reported the in vitro function of histone lysine demethylase 7 A (KDM7A) in osteoblast and adipocyte differentiation. The current study was undertaken to investigate the physiological role of KDM7A in bone homeostasis and elucidate the underlying mechanisms. A conditional strategy was employed to delete the Kdm7a gene specifically in osterix-expressing osteoprogenitor cells in mice. The resulting mutant mice exhibited a significant increase in cancellous bone mass, accompanied by an increase in osteoblasts and bone formation, as well as a reduction in osteoclasts, marrow adipocytes and bone resorption. The bone marrow stromal cells (BMSCs) and calvarial pre-osteoblastic cells derived from the mutant mice exhibited enhanced osteogenic differentiation and suppressed adipogenic differentiation. Additionally, osteoclastic precursor cells from the mutant mice exhibited impaired osteoclast differentiation. Co-culturing BMSCs from the mutant mice with wild-type osteoclast precursor cells resulted in the inhibition of osteoclast differentiation. Mechanistic investigation revealed that KDM7A was able to upregulate the expression of fibroblast activation protein α (FAP) and receptor activator of nuclear factor κB ligand (RANKL) in BMSCs through removing repressive di-methylation marks of H3K9 and H3K27 from Fap and Rankl promoters. Moreover, recombinant FAP attenuated the dysregulation of osteoblast and adipocyte differentiation in BMSCs from Kdm7a deficient mice. Finally, Kdm7a deficiency prevented ovariectomy-induced bone loss in mice. This study establish the role of KDM7A in bone homeostasis through its epigenetic regulation of osteoblast and osteoclast differentiation. Consequently, inhibiting KDM7A may prove beneficial in ameliorating osteoporosis. KDM7A suppresses osteoblast differentiation and bone formation through. upregulating FAP expression and inactivating canonical Wnt signaling, and conversely promotes osteoclast differentiation and bone resorption through upregulating RANKL expression. These are based on its epigenetic removal of the repressive H3K9me2 and H3K27me2 marks from Fap and Rankl promoters. As a result, the expression of KDM7A in osteoprogenitor cells tends to negatively modulate bone mass.

The Role and Application of Fibroblast Activating Protein

Fibroblast activation protein-α (FAP), a type-II transmembrane serine protease, is rarely expressed in normal tissues but highly abundant in pathological diseases, including fibrosis, arthritis, and cancer. Ever since its discovery, we have deciphered its structure and biological properties and continue to investigate its roles in various diseases while attempting to utilize it for targeted therapy. To date, no significant breakthroughs have been made in terms of efficacy. However, in recent years, several practical applications in the realm of imaging diagnosis have been discovered. Given its unique expression in a diverse array of pathological tissues, the fundamental biological characteristics of FAP render it a crucial target for disease diagnosis and immunotherapy. To obtain a more comprehensive understanding of the research progress of FAP, its biological characteristics, involvement in diseases, and recent targeted application research have been reviewed. Moreover, we explored its development trend in the direction of clinical diagnoses and treatment.

FAP expression in adipose tissue macrophages promotes obesity and metabolic inflammation

Adipose tissue macrophages (ATM) are key players in the development of obesity and associated metabolic inflammation which contributes to systemic metabolic dysfunction. We here found that fibroblast activation protein α (FAP), a well-known marker of cancer-associated fibroblast, is selectively expressed in murine and human ATM among adipose tissue-infiltrating leukocytes. Macrophage FAP deficiency protects mice against diet-induced obesity and proinflammatory macrophage infiltration in obese adipose tissues, thereby alleviating hepatic steatosis and insulin resistance. Mechanistically, FAP specifically mediates monocyte chemokine protein CCL8 expression by ATM, which is further upregulated upon high-fat-diet (HFD) feeding, contributing to the recruitment of monocyte-derived proinflammatory macrophages with no effect on their classical inflammatory activation. CCL8 overexpression restores HFD-induced metabolic phenotypes in the absence of FAP. Moreover, macrophage FAP deficiency enhances energy expenditure and oxygen consumption preceding differential body weight after HFD feeding. Such enhanced energy expenditure is associated with increased levels of norepinephrine (NE) and lipolysis in white adipose tissues, likely due to decreased expression of monoamine oxidase, a NE degradation enzyme, by Fap-/- ATM. Collectively, our study identifies FAP as a previously unrecognized regulator of ATM function contributing to diet-induced obesity and metabolic inflammation and suggests FAP as a potential immunotherapeutic target against metabolic disorders.

Plasma proteomic changes in response to exercise training are associated with cardiorespiratory fitness adaptations

Regular exercise leads to widespread salutary effects, and there is increasing recognition that exercise-stimulated circulating proteins can impart health benefits. Despite this, limited data exist regarding the plasma proteomic changes that occur in response to regular exercise. Here, we perform large-scale plasma proteomic profiling in 654 healthy human study participants before and after a supervised, 20-week endurance exercise training intervention. We identify hundreds of circulating proteins that are modulated, many of which are known to be secreted. We highlight proteins involved in angiogenesis, iron homeostasis, and the extracellular matrix, many of which are novel, including training-induced increases in fibroblast activation protein (FAP), a membrane-bound and circulating protein relevant in body-composition homeostasis. We relate protein changes to training-induced maximal oxygen uptake adaptations and validate our top findings in an external exercise cohort. Furthermore, we show that FAP is positively associated with survival in 3 separate, population-based cohorts.

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.

Increased Fibrosis in White Adipose Tissue of Male and Female bGH Transgenic Mice Appears Independent of TGF-β Action

Fibrosis is a pathological state caused by excess deposition of extracellular matrix proteins in a tissue. Male bovine growth hormone (bGH) transgenic mice experience metabolic dysfunction with a marked decrease in lifespan and with increased fibrosis in several tissues including white adipose tissue (WAT), which is more pronounced in the subcutaneous (Sc) depot. The current study expanded on these initial findings to evaluate WAT fibrosis in female bGH mice and the role of transforming growth factor (TGF)-β in the development of WAT fibrosis. Our findings established that female bGH mice, like males, experience a depot-dependent increase in WAT fibrosis, and bGH mice of both sexes have elevated circulating levels of several markers of collagen turnover. Using various methods, TGF-β signaling was found unchanged or decreased-as opposed to an expected increase-despite the marked fibrosis in WAT of bGH mice. However, acute GH treatments in vivo, in vitro, or ex vivo did elicit a modest increase in TGF-β signaling in some experimental systems. Finally, single nucleus RNA sequencing confirmed no perturbation in TGF-β or its receptor gene expression in any WAT cell subpopulations of Sc bGH WAT; however, a striking increase in B lymphocyte infiltration in bGH WAT was observed. Overall, these data suggest that bGH WAT fibrosis is independent of the action of TGF-β and reveals an intriguing shift in immune cells in bGH WAT that should be further explored considering the increasing importance of B cell-mediated WAT fibrosis and pathology.

Fibroblast activation protein-based theranostics in pancreatic cancer

Fibroblast activation protein-α (FAP) is a type II transmembrane serine protease that has specific endopeptidase activity. Given its well-established selective expression in the activated stromal fibroblasts of epithelial cancers, although not in quiescent fibroblasts, FAP has received substantial research attention as a diagnostic marker and therapeutic target. Pancreatic cancer is characterized by an abundant fibrotic or desmoplastic stroma, leading to rapid progression, therapeutic resistance, and poor clinical outcomes. Numerous studies have revealed that the abundant expression of FAP in cancer cells, circulating tumor cells, stromal cells, and cancer-associated fibroblasts (CAFs) of pancreatic adenocarcinoma is implicated in diverse cancer-related signaling pathways, contributing to cancer progression, invasion, migration, metastasis, immunosuppression, and resistance to treatment. In this article, we aim to systematically review the recent advances in research on FAP in pancreatic adenocarcinoma, including its utility as a diagnostic marker, therapeutic potential, and correlation with prognosis. We also describe the functional role of FAP-overexpressing stromal cells, particulary CAFs, in tumor immuno- and metabolic microenvironments, and summarize the mechanisms underlying the contribution of FAP-overexpressing CAFs in pancreatic cancer progression and treatment resistance. Furthermore, we discuss whether targeting FAP-overexpressing CAFs could represent a potential therapeutic strategy and describe the development of FAP-targeted probes for diagnostic imaging. Finally, we assess the emerging basic and clinical studies regarding the bench-to-bedside translation of FAP in pancreatic cancer.

Increased Expression of Fibroblast Activation Protein is Associated with Autophagy Dysregulation and Oxidative Stress in Obese Women with Uterine Fibroids

Uterine fibroids (UF) represent an immense health burden throughout the world. Obesity is considered one of the risk factors for UF development; however, the underlying mechanisms remain largely unexplored. We investigated the effect of obesity on fibroblast activation and its association with inflammation, autophagy dysfunction, and oxidative stress in UF patients. Thirty-five pre-menopausal UF patients were included in this study and classified into non-obese group (BM1 ≤ 30 kg/m², n = 15) and obese group (BMI > 30 kg/m², n = 20). Tissue samples were collected from fibroids and adjacent normal myometrium. Our results showed increased expression of fibroblast activation protein (FAP) together with markers of autophagy, inflammation, and oxidative stress in UF patients, which were all more markedly upregulated in obese compared to non-obese patients. In addition, BMI was significantly positive correlated with FAP and autophagy markers. In conclusion, the results of the present study suggest that obesity-associated autophagy dysregulation together with increased FAP expression may increase the risk of UFs in obese women.

Talabostat Alleviates Obesity and Associated Metabolic Dysfunction via Suppression of Macrophage-Driven Adipose Inflammation

OBJECTIVE

Adipose tissue macrophages (ATMs) play critical roles in obesity-associated inflammation that contributes to metabolic dysfunction. Talabostat (TB) exerts some therapeutic effects on tumors and obesity. However, it remains unknown whether the metabolic benefits of TB on obesity is dependent on ATM-mediated adipose inflammation.

METHODS

Male C57BL/6J mice were fed a normal chow diet (NCD) or a high-fat diet for 12 weeks, and mice were orally administered TB daily at a low dose (0.5 mg/kg).

RESULTS

Administration of TB to mice fed a high-fat diet significantly improved adiposity and obesity-associated metabolic dysfunction, including glucose intolerance and insulin resistance, hyperlipidemia and hepatic steatosis, which were accompanied by increased whole-body energy expenditure. RNA sequencing analysis revealed extensive alterations in the transcriptome profiles associated with lipid metabolism and immune responses in adipose tissue of obese mice. Notably, TB treatment led to a significant reduction in ATM accumulation and a shift of the activation state of ATMs from the proinflammatory M1-like to the anti-inflammatory M2-like phenotype. Moreover, depletion of ATMs significantly abolished the TB-induced metabolic benefits.

CONCLUSIONS

Our study demonstrates that TB at a low dose could increase energy expenditure and control ATM-mediated adipose inflammation in obese mice, thereby alleviating obesity and its associated metabolic dysfunction.