Fibroblast activation protein (FAP) is upregulated in myelomatous bone and supports myeloma cell survival

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.

Growth and dormancy control of myeloma cells by mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) may have contrasting impacts on the progression of multiple myeloma (MM). Priming normal MSCs, by culturing them with MM cells, mimics the MSC-induced MM growth. We studied the contrasting effects of conditioned medium (CM) from unprimed or primed MSCs on growth of MM cells from newly diagnosed cases. We elucidated potential molecular pathways using global gene expression profiling and focused on the role of the mTOR2 component, RICTOR, as a novel mediator of dormancy in MM. Primed MSCs CM consistently increased proportions of proliferating cells and supported MM growth in 3-day (n = 20) and 10-day (n = 12) cultures, effects that were partially mediated through the IGF1 axis. In contrast, unprimed MSCs CM inhibited growth of MM cells in cases mainly from stages I/II MM. The genes most overexpressed in MM cells treated with primed MSCs CM were associated with cell cycle, DNA-damage repair, and proliferation; genes most overexpressed in MM cells treated with unprimed MSCs CM were associated with dormancy pathways including RICTOR (mTOR2 pathway), CXCR4, and BCL2. RICTOR protein level was induced by unprimed MSCs CM and was lower in KI67+ proliferating MM cells treated with primed MSCs CM. RICTOR was underexpressed in clinical relapse samples compared with baseline samples of the same patients. Inhibiting RICTOR expression in primary MM cells promoted their growth, and enforced expression of RICTOR in MM cell lines inhibited their growth. Our findings suggest that, after prolonged interactions with MM cells, bone marrow MSCs shift from MM-repressive to MM-permissive. AVAILABILITY OF DATA AND MATERIALS: Our institutional GEP data of MM cells from newly diagnosed patients used to show RICTOR expression have been deposited at Gene Expression Omnibus (GEO: GSE2658, https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE2658).

Translational assessment of a DATA-functionalized FAP inhibitor with facile 68Ga-labeling at room temperature

PURPOSE

The present study aims at evaluating the preclinical and the clinical performance of [68Ga]Ga-DATA5m.SA.FAPi, which has the advantage to be labeled with gallium-68 at room temperature.

METHODS

[68Ga]Ga-DATA5m.SA.FAPi was assessed in vitro on FAP-expressing stromal cells, followed by biodistribution and in vivo imaging on prostate and glioblastoma xenografts. Moreover, the clinical assessment of [68Ga]Ga-DATA5m.SA.FAPi was conducted on six patients with prostate cancer, aiming on investigating, biodistribution, biokinetics, and determining tumor uptake.

RESULTS

[68Ga]Ga-DATA5m.SA.FAPi is quantitatively prepared in an instant kit-type version at room temperature. It demonstrated high stability in human serum, affinity for FAP in the low nanomolar range, and high internalization rate when associated with CAFs. Biodistribution and PET studies in prostate and glioblastoma xenografts revealed high and specific tumor uptake. Elimination of the radiotracer mainly occurred through the urinary tract. The clinical data are in accordance with the preclinical data concerning the organ receiving the highest absorbed dose (urinary bladder wall, heart wall, spleen, and kidneys). Different to the small-animal data, uptake of [68Ga]Ga-DATA5m.SA.FAPi in tumor lesions is rapid and stable and tumor-to-organ and tumor-to-blood uptake ratios are high.

CONCLUSION

The radiochemical, preclinical, and clinical data obtained in this study strongly support further development of [68Ga]Ga-DATA5m.SA.FAPi as a diagnostic tool for FAP imaging.

New Frontiers in Cancer Imaging and Therapy Based on Radiolabeled Fibroblast Activation Protein Inhibitors: A Rational Review and Current Progress

Over the past decade, the tumor microenvironment (TME) has become a new paradigm of cancer diagnosis and therapy due to its unique biological features, mainly the interconnection between cancer and stromal cells. Within the TME, cancer-associated fibroblasts (CAFs) demonstrate as one of the most critical stromal cells that regulate tumor cell growth, progression, immunosuppression, and metastasis. CAFs are identified by various biomarkers that are expressed on their surfaces, such as fibroblast activation protein (FAP), which could be utilized as a useful target for diagnostic imaging and treatment. One of the advantages of targeting FAP-expressing CAFs is the absence of FAP expression in quiescent fibroblasts, leading to a controlled targetability of diagnostic and therapeutic compounds to the malignant tumor stromal area using radiolabeled FAP-based ligands. FAP-based radiopharmaceuticals have been investigated strenuously for the visualization of malignancies and delivery of theranostic radiopharmaceuticals to the TME. This review provides an overview of the state of the art in TME compositions, particularly CAFs and FAP, and their roles in cancer biology. Moreover, relevant reports on radiolabeled FAP inhibitors until the year 2021 are highlighted-as well as the current limitations, challenges, and requirements for those radiolabeled FAP inhibitors in clinical translation.

Fibroblast Activation Protein-α as a Target in the Bench-to-Bedside Diagnosis and Treatment of Tumors: A Narrative Review

Fibroblast activation protein-α (FAP) is a type II integral serine protease that is specifically expressed by activated fibroblasts. Cancer-associated fibroblasts (CAFs) in the tumor stroma have an abundant and stable expression of FAP, which plays an important role in promoting tumor growth, invasion, metastasis, and immunosuppression. For example, in females with a high incidence of breast cancer, CAFs account for 50–70% of the cells in the tumor’s microenvironment. CAF overexpression of FAP promotes tumor development and metastasis by influencing extracellular matrix remodeling, intracellular signaling, angiogenesis, epithelial-to-mesenchymal transition, and immunosuppression. This review discusses the basic biological characteristics of FAP and its applications in the diagnosis and treatment of various cancers. We review the emerging basic and clinical research data regarding the use of nanomaterials that target FAP.

The Role of Fibroblast Activation Protein Ligands in Oncologic PET Imaging

Fibroblast activation protein inhibitor emerges as a novel and highly promising agent for diagnostic and possibly theranostic application in various malignant and non-malignant diseases. FAPI impresses with its selective expression in several pathologies, ligand induced internalization, and presence in a large variety of malignancies. Current studies indicate that FAPI is equal or even superior to the current standard oncological tracer fluorodeoxyglucose in several oncological diseases. It seems to present lower background activity, stronger uptake in tumorous lesions and thus sharper contrasts. For improved comprehension of fibroblast activation, protein expression and clinicopathologic conditions, further studies are of essence.

The role of fibroblast activation protein in health and malignancy

Fibroblast activation protein-α (FAP) is a type-II transmembrane serine protease expressed almost exclusively to pathological conditions including fibrosis, arthritis, and cancer. Across most cancer types, elevated FAP is associated with worse clinical outcomes. Despite the clear association between FAP and disease severity, the biological reasons underlying these clinical observations remain unclear. Here we review basic FAP biology and FAP's role in non-oncologic and oncologic disease. We further explore how FAP may worsen clinical outcomes via its effects on extracellular matrix remodeling, intracellular signaling regulation, angiogenesis, epithelial-to-mesenchymal transition, and immunosuppression. Lastly, we discuss the potential to exploit FAP biology to improve clinical outcomes.

Endothelial, pericyte and tumor cell expression in glioblastoma identifies fibroblast activation protein (FAP) as an excellent target for immunotherapy

Objectives

Targeted immunotherapies such as chimeric antigen receptor (CAR)-T cells are emerging as attractive treatment options for glioblastoma, but rely on identification of a suitable tumor antigen. We validated a new target antigen for glioblastoma, fibroblast activation protein (FAP), by undertaking a detailed expression study of human samples.

Methods

Glioblastoma and normal tissues were assessed using immunostaining, supported by analyses of published transcriptomic datasets. Short-term cultures of glioma neural stem (GNS) cells were compared to cultures of healthy astrocytes and neurons using flow cytometry. Glioblastoma tissues were dissociated and analysed by high-parameter flow cytometry and single-cell transcriptomics (scRNAseq).

Results

Compared to normal brain, FAP was overexpressed at the gene and protein level in a large percentage of glioblastoma tissues, with highest levels of expression associated with poorer prognosis. FAP was also overexpressed in several paediatric brain cancers. FAP was commonly expressed by cultured GNS cells but absent from normal neurons and astrocytes. Within glioblastoma tissues, the strongest expression of FAP was around blood vessels. In fact, almost every tumor vessel was highlighted by FAP expression, whereas normal tissue vessels and cultured endothelial cells (ECs) lacked expression. Single-cell analyses of dissociated tumors facilitated a detailed characterisation of the main cellular components of the glioblastoma microenvironment and revealed that vessel-localised FAP is because of expression on both ECs and pericytes.

Conclusion

Fibroblast activation protein is expressed by multiple cell types within glioblastoma, highlighting it as an ideal immunotherapy antigen to target destruction of both tumor cells and their supporting vascular network.

The role of fibroblast activation protein in progression and development of osteosarcoma cells

To investigate the expression levels of fibroblast activation protein (FAP) in human osteosarcoma tissues and its possible correlations with clinical pathological characteristics of patients with osteosarcoma, and to explore the potential effects of FAP on progression and development of osteosarcoma. Immunohistochemistry (IHC) assay was initially performed to detect the expression levels of FAP in 66 tumor tissues and adjacent non-tumor tissues. Patients were sequentially divided into two groups based on different expression levels of FAP. The correlations between the expression levels of FAP and the clinical pathological characteristics were investigated, and the role of FAP in proliferation, migration, and invasion of osteosarcoma cells was assessed via colony formation, MTT, wound healing, and transwell assays, respectively. The possible effects of FAP on tumor growth and metastasis were evaluated in vivo. We further attempted to reveal the underlying mechanism of FAP involved in tumor growth through bioinformatics and IHC assays. High expression levels of FAP were noted in human osteosarcoma tissues. It also was unveiled that FAP was significantly associated with the tumor size (P = 0.005*) and clinical stage (P = 0.017*). Our data further confirmed that knockdown of FAP remarkably blocked proliferation, migration, and invasion of osteosarcoma cells in vitro, and suppressed tumor growth and metastasis in mice via AKT signaling pathway. The possible role of FAP in progression and development of osteosarcoma could be figured out. Our data may be helpful to develop a novel therapeutic target for the treatment of osteosarcoma.

CD26 is a potential therapeutic target by humanized monoclonal antibody for the treatment of multiple myeloma

CD26, a 110-kDa transmembrane glycoprotein that is expressed on several tumor cells including malignant lymphoma, has been implicated in tumorigenesis: however, little is known regarding its role in multiple myeloma (MM). Recently, we identified CD26 expression on human osteoclasts (OCs) and demonstrated that humanized IgG₁ monoclonal antibody targeting CD26, huCD26mAb, inhibits human OC differentiation. Herein, we show that CD26 expression was present on plasma cells in the bone marrow tissues of MM patients. In vitro immunostaining studies revealed that although CD26 expression was low or absent on MM cell lines cultured alone, it was intensely and uniformly expressed on MM cell lines co-cultured with OCs. The augmented CD26 expression in MM cells was exploited to enhance anti-MM efficacy of huCD26mAb via a substantial increase in antibody-dependent cytotoxicity (ADCC) but not complement-dependent cytotoxicity (CDC). Moreover, huCD26mAb in combination with novel agents synergistically enhanced huCD26mAb induced ADCC activity against CD26+ MM cells compared with each agent alone. huCD26mAb additionally reduced the ratio of the side population (SP) fraction in CD26+ MM cells by ADCC. Finally, huCD26mAb significantly reduced the MM tumor burden and OC formation in vivo. These results suggest that CD26 is a potential target molecule in MM and that huCD26mAb could act as a therapeutic agent.

Unique anti-myeloma activity by thiazolidine-2,4-dione compounds with Pim inhibiting activity

Proviral Integrations of Moloney virus 2 (PIM2) is overexpressed in multiple myeloma (MM) cells, and regarded as an important therapeutic target. Here, we aimed to validate the therapeutic efficacy of different types of PIM inhibitors against MM cells for their possible clinical application. Intriguingly, the thiazolidine-2,4-dione-family compounds SMI-16a and SMI-4a reduced PIM2 protein levels and impaired MM cell survival preferentially in acidic conditions, in contrast to other types of PIM inhibitors, including AZD1208, CX-6258 and PIM447. SMI-16a also suppressed the drug efflux function of breast cancer resistance protein, minimized the sizes of side populations and reduced in vitro colony-forming capacity and in vivo tumourigenic activity in MM cells, suggesting impairment of their clonogenic capacity. PIM2 is known to be subject to ubiquitination-independent proteasomal degradation. Consistent with this, the proteasome inhibitors bortezomib and carfilzomib increased PIM2 protein levels in MM cells without affecting its mRNA levels. However, SMI-16a mitigated the PIM2 protein increase and cooperatively enhanced anti-MM effects in combination with carfilzomib. Collectively, the thiazolidine-2,4-dione-family compounds SMI-16a and SMI-4a uniquely reduce PIM2 protein in MM cells, which may contribute to their profound efficacy in addition to their immediate kinase inhibition. Their combination with proteasome inhibitors is envisioned.

Inhibitor-Decorated Polymer Conjugates Targeting Fibroblast Activation Protein

Proteases are directly involved in cancer pathogenesis. Expression of fibroblast activation protein (FAP) is upregulated in stromal fibroblasts in more than 90% of epithelial cancers and is associated with tumor progression. FAP expression is minimal or absent in most normal adult tissues, suggesting its promise as a target for the diagnosis or treatment of various cancers. Here, we report preparation of a polymer conjugate (an iBody) containing a FAP-specific inhibitor as the targeting ligand. The iBody inhibits both human and mouse FAP with low nanomolar inhibition constants but does not inhibit close FAP homologues dipeptidyl peptidase IV, dipeptidyl peptidase 9, and prolyl oligopeptidase. We demonstrate the applicability of this iBody for the isolation of FAP from cell lysates and blood serum as well as for its detection by ELISA, Western blot, flow cytometry, and confocal microscopy. Our results show the iBody is a useful tool for FAP targeting in vitro and potentially also for specific anticancer drug delivery.

Expression of anti-VEGF antibody together with anti-EGFR or anti-FAP enhances tumor regression as a result of vaccinia virotherapy

The tumor microenvironment plays an important role in tumor growth and progression. Here we demonstrate that vaccinia virus-mediated, constitutively expressed intratumoral antibodies against vascular endothelial growth factor (VEGF), epidermal growth factor receptor (EGFR), and fibroblast activation protein (FAP) significantly improved tumor regression and oncolytic virotherapy through suppression of angiogenesis, cell proliferation, and stromagenesis in virus-colonized tumors. In contrast to the tumor growth inhibition by the three tumor growth-inhibiting antibodies individually, when two of the three antibodies were expressed simultaneously by single vaccinia virus strains tumor regression was further enhanced. These findings strongly indicate that interference with the two tumor growth-stimulating mechanisms did in fact result in enhanced therapeutic efficacy in tumor xenograft models and may lead to an effective therapy in patients with cancer.

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.

Clinical Implications of Marker Expression of Carcinoma-Associated Fibroblasts (CAFs) in Patients with Epithelial Ovarian Carcinoma After Treatment with Neoadjuvant Chemotherapy

Cancer-associated fibroblasts (CAFs) play an important role in tumor initiation and progression. The aim of this study is to explore the role of 2 CAF markers, fibroblast activated protein (FAP) and α-smooth muscle actin (αSMA), in patients with epithelial ovarian cancer (EOC) post-neoadjuvant chemotherapy. Sixty-six patients with the diagnosis of EOC treated with debulking surgery after neoadjuvant therapy were retrieved from the archives. Immunohistochemistry for FAP and αSMA antibodies were performed on paraffin-embedded tissue. Fisher's exact test was performed to test the association between FAP and αSMA expression and disease status. Kaplan-Meier method with log-rank test was used to check the survival difference between different FAP tumor/stroma expressions. FAP stroma (pos) . expression was strongly associated with higher recurrences rate [OR: 15.95; 95 % CI: 1.521-835.206; p = 0.0072]. Cases with combined FAP stroma (pos) and FAP tumor (neg) had higher death rate [OR: 4.845; 95 % CI: 1.53-16.61; p = 0.0046] and higher recurrence rate [OR: 5.12; 95 % CI: 0.91-54.42; p = 0.0487] compared to all the others. Cases with combined FAP stroma (neg) and FAP tumor (neg) were more likely to have lower recurrence rates [OR: 0.086; 95 % CI: 0.001-0.997; p = 0.0248]. αSMA was expressed by tumor-associated stroma in 95 % of cases and by tumor cells in 9 % of cases. No statistical power was found for αSMA and disease status. Our data indicate that FAP plays an important role in predicting tumor aggressiveness in patients with EOC post-neoadjuvant therapy, and its frequent expression in this malignancy implicates that FAP targeted therapy could be a very attractive strategy.

Targeting inhibition of fibroblast activation protein-α and prolyl oligopeptidase activities on cells common to metastatic tumor microenvironments

Fibroblast activation protein (FAP), a membrane prolyl-specific proteinase with both dipeptidase and endopeptidase activities, is overexpressed by reactive stromal fibroblasts during epithelial-derived cancer growth. FAP digests extracellular matrix as tissue is remodeled during cancer expansion and may also promote an immunotolerant tumor microenvironment. Recent studies suggest that nonspecific FAP inhibitors suppress human cancer xenografts in mouse models. Prolyl oligopeptidase (POP), another prolyl-specific serine proteinase, is also elevated in many cancers and may have a regulatory role in angiogenesis promotion. FAP and POP cell-associated activities may be targets for diagnosis and treatment of various cancers, but their accessibilities to highly effective specific inhibitors have not been shown for cells important to cancer growth. Despite their frequent simultaneous expression in many cancers and their overlapping activities toward commonly used substrates, precise, separate measurement of FAP or POP activity has largely been ignored. To distinguish each of the two activities, we synthesized highly specific substrates and inhibitors for FAP or POP based on amino acid sequences surrounding the scissile bonds of their respective putative substrates. We found varying amounts of FAP and POP protein and activities on activated fibroblasts, mesenchymal cells, normal breast cells, and one breast cancer cell line, with some cells exhibiting more POP than FAP activity. Replicating endothelial cells (ECs) expressed POP but not FAP until tubulogenesis began. Targeting FAP-positive cells, especially mesenchymal stem cells and cancer-associated fibroblasts for inactivation or destruction, and inhibiting POP-producing EC may abrogate stromal invasion and angiogenesis simultaneously and thereby diminish cancer growth.

Myeloma bone disease: Pathophysiology and management

Multiple myeloma bone disease is marked by severe dysfunction of both bone formation and resorption and serves as a model for understanding the regulation of osteoblasts (OBL) and osteoclasts (OCL) in cancer. Myeloma bone lesions are purely osteolytic and are associated with severe and debilitating bone pain, pathologic fractures, hypercalcemia, and spinal cord compression, as well as increased mortality. Interactions within the bone marrow microenvironment in myeloma are responsible for the abnormal bone remodeling in myeloma bone disease. Myeloma cells drive bone destruction that increases tumor growth, directly stimulates the OCL formation, and induces cells in the marrow microenvironment to produce factors that drive OCL formation and suppress OBL formation. Factors produced by marrow stromal cells and OCL promote tumor growth through direct action on myeloma cells and by increasing angiogenesis. Current therapies targeting MMBD focus on preventing osteoclastic bone destruction; however regulators of OBL inhibition in MMBD have also been identified, and targeted agents with a potential anabolic effect in MMBD are under investigation. This review will discuss the mechanisms responsible for MMBD and therapeutic approaches currently in use and in development for the management of MMBD.

Hepatocyte growth factor pathway upregulation in the bone marrow microenvironment in multiple myeloma is associated with lytic bone disease

Lytic bone disease (LBD) in multiple myeloma (MM) is caused by osteoclast hyperactivation and osteoblast inhibition. Based on in vitro studies, the hepatocyte growth factor (HGF) pathway is thought to be central in osteoblast inhibition. We evaluated the gene expression of the HGF pathway in vivo using bone marrow biopsies (BMBs) of patients with MM and monoclonal gammopathy of undetermined significance (MGUS), and healthy volunteers (HV). BMBs (N = 110) obtained at diagnosis were snap-frozen and used to evaluate gene expression by quantitative reverse transcription polymerase chain reaction. LBD was evaluated using standard radiographs. Enzyme-linked immunosorbent assay (ELISA) was performed on matched bone marrow plasma and immunohistochemistry on matched formalin-fixed paraffin-embedded biopsies. Gene expression of HGF, SDC1, and MET in BMBs were significantly altered in MM versus HV and MGUS, and HGF and MET correlated with the extent of LBD. A significant correlation between gene and protein expression levels was observed for SDC1 (Syndecan-1) and HGF. The HGF bone marrow plasma level was significantly lower in MM patients with no/limited versus advanced LBD. Our novel approach using snap-frozen BMBs seems generally applicable because it allows evaluation of gene expression independent of the extent of MM plasma-cell infiltration. Our study highlights the importance of the HGF pathway in MM LBD.

Mechanisms of multiple myeloma bone disease

Multiple myeloma is the second most common hematological malignancy and the most frequent cancer to involve the skeleton. Multiple myeloma bone disease (MMBD) is characterized by abnormal bone remodeling with dysfunction of both bone resorption and bone formation, and thus can be used as a paradigm for other inflammatory bone diseases, and the regulation of osteoclasts and osteoblasts in malignancy. Studies of MMBD have identified novel regulators that increase osteoclastogenesis and osteoclast function, repress osteoblast differentiation, increase angiogenesis, or permanently alter stromal cells. This review will discuss the current understanding of mechanisms of osteoclast and osteoblast regulation in MMBD, and therapeutic approaches currently in use and under development that target mediators of bone destruction and blockade of bone formation for myeloma patients, including new anabolic therapies.

Therapeutic effects of intrabone and systemic mesenchymal stem cell cytotherapy on myeloma bone disease and tumor growth

The cytotherapeutic potential of mesenchymal stem cells (MSCs) has been evaluated in various disorders including those involving inflammation, autoimmunity, bone regeneration, and cancer. Multiple myeloma (MM) is a systemic malignancy associated with induction of osteolytic lesions that often are not repaired even after prolonged remission. The aims of this study were to evaluate the effects of intrabone and systemic injections of MSCs on MM bone disease, tumor growth, and tumor regrowth in the severe combined immunodeficiency (SCID)-rab model and to shed light on the exact localization of systemically injected MSCs. Intrabone injection of MSCs, but not hematopoietic stem cells, into myelomatous bones prevented MM-induced bone disease, promoted bone formation, and inhibited MM growth. After remission was induced with melphalan treatment, intrabone-injected MSCs promoted bone formation and delayed myeloma cell regrowth in bone. Most intrabone or systemically injected MSCs were undetected 2 to 4 weeks after injection. The bone-building effects of MSCs were mediated through activation of endogenous osteoblasts and suppression of osteoclast activity. Although a single intravenous injection of MSCs had no effect on MM, sequential weekly intravenous injections of MSCs prevented MM-induced bone disease but had no effect on tumor burden. MSCs expressed high levels of anti-inflammatory (eg, HMOX1) and bone-remodeling (eg, Decorin, CYR61) mediators. In vitro, MSCs promoted osteoblast maturation and suppressed osteoclast formation, and these effects were partially prevented by blocking decorin. A subset of intravenously or intracardially injected MSCs trafficked to myelomatous bone in SCID-rab mice. Although the majority of intravenously injected MSCs were trapped in lungs, intracardially injected MSCs were mainly localized in draining mesenteric lymph nodes. This study shows that exogenous MSCs act as bystander cells to inhibit MM-induced bone disease and tumor growth and that systemically injected MSCs are attracted to bone by myeloma cells or conditions induced by MM and inhibit bone disease.

Up-regulation of hexokinaseII in myeloma cells: targeting myeloma cells with 3-bromopyruvate

Hexokinase II (HKII), a key enzyme of glycolysis, is widely over-expressed in cancer cells. However, HKII levels and its roles in ATP production and ATP-dependent cellular process have not been well studied in hematopoietic malignant cells including multiple myeloma (MM) cells.We demonstrate herein that HKII is constitutively over-expressed in MM cells. 3-bromopyruvate (3BrPA), an inhibitor of HKII, promptly and substantially suppresses ATP production and induces cell death in MM cells. Interestingly, cocultures with osteoclasts (OCs) but not bone marrow stromal cells (BMSCs) enhanced the phosphorylation of Akt along with an increase in HKII levels and lactate production in MM cells. The enhancement of HKII levels and lactate production in MM cells by OCs were mostly abrogated by the PI3K inhibitor LY294002, suggesting activation of glycolysis in MM cells by OCs via the PI3K-Akt-HKII pathway. Although BMSCs and OCs stimulate MM cell growth and survival, 3BrPA induces cell death in MM cells even in cocultures with OCs as well as BMSCs. Furthermore, 3BrPA was able to diminish ATP-dependent ABC transporter activity to restore drug retention in MM cells in the presence of OCs. These results may underpin possible clinical application of 3BrPA in patients with MM.

Therapeutic approaches to myeloma bone disease: an evolving story

Bone disease is a major morbidity factor in patients with multiple myeloma and significantly affects their overall survival. A complex interplay between malignant plasma cells and other marrow cells results in the generation of a microenvironment capable of enhancing both tumor growth and bone destruction. Bisphosphonates have consistently reduced the incidence of skeletal-related events in patients with multiple myeloma and other osteotropic tumors as well. However, their use is burdened with side-effects, including the risks of osteonecrosis of the jaw and kidney failure, suggesting that they should be discontinued after prolonged administration. New molecular targets of cell cross-talk in myeloma bone marrow are therefore under intensive investigation and new drugs are being explored in preclinical and clinical studies of myeloma bone disease. Compounds targeting osteoclast activation pathways, such as receptor activator of nuclear factor-κB/receptor activator of nuclear factor-κB ligand/osteoprotegerin, B-cell activating factor, mitogen-activated protein kinase and macrophage inflammatory protein-1α/chemokine receptor for macrophage inflammatory protein-1α axes, or soluble agents that improve osteoblast differentiation by modulating specific inhibitors such as Dickkopf-1 and transforming growth factor-β, as well as novel approaches of cytotherapy represent a new generation of promising drugs for the treatment of myeloma bone disease.

Pinch-1 was up-regulated in leukemia BMSC and its possible effect

Pinch-1, a widely expressed focal adhesion protein, has been demonstrated to be up-regulated in multiple solid tumor-associated stromal cells, particularly at invasive edges. It was supposed that Pinch-1 was intimately associated with development and progression of tumors. The expression of Pinch-1 in hematopoietic microenvironment in patients with leukemia remains unclear. This study focused on the expression of Pinch-1 in bone marrow stromal cells (BMSCs) from leukemia patients and its possible effect. BMSC was isolated and cultured from bone marrow in leukemia patients and normal healthy donors. RT-PCR and Western blot analysis were performed to determine Pinch-1 mRNA and protein level in BMSC, respectively. Lentiviral vector containing Pinch-1 siRNA was constructed, and the recombinant lentivirus particle was packaged in 293 cells. Effectiveness of Pinch-1 siRNA was determined by Western blot. The proliferation, apoptosis and motility of leukemia BMSC subjected to Pinch-1 knockdown using siRNA were tested by flow cytometry, TUNEL assay and Transwell system, respectively. Pinch-1 mRNA and protein were significantly up-regulated in ALL and AML BMSC compared to normal BMSC (p<0.01). Although there was no difference in Pinch-1 mRNA between ALL and AML BMSC, cellular levels of Pinch-1 protein in ALL BMSC were significantly higher than that in AML BMSC (p<0.01). Overexpressed Pinch-1 was significantly reduced in leukemia BMSC transfected with Pinch-1 siRNA evidenced by Western blot. Flow cytometry analysis showed that the percentage of cells in S+G2 phases in leukemia BMSC transfected with Pinch-1 siRNA was significantly lower than control (p<0.01). The percentage of apoptotic cells in leukemia BMSC transfected with Pinch-1 siRNA was 19.8±1.0%, significantly higher than controls (p<0.01). The number of leukemia BMSC transfected with Pinch-1 siRNA that migrated to the lower chamber after culturing for 24 h was 8.4±1.1 per field, significantly lower than controls (p<0.01). Pinch-1 mRNA and protein in leukemia BMSC were up-regulated drastically compared with BMSC from healthy donors. Leukemia BMSC displayed hypoproliferation, decreased migration and increased apoptosis after transfecting Pinch-1 siRNA.

Emerging strategies for targeting cell adhesion in multiple myeloma

Multiple myeloma (MM) is an incurable hematological cancer involving proliferation of abnormal plasma cells that infiltrate the bone marrow (BM) and secrete monoclonal antibodies. The disease is clinically characterized by bone lesions, anemia, hypercalcemia, and renal failure. MM is presently treated with conventional therapies like melphalan, doxorubicin, and prednisone; or novel therapies like thalidomide, lenalidomide, and bortezomib; or with procedures like autologous stem cell transplantation. Unfortunately, these therapies fail to eliminate the minimal residual disease that remains persistent within the confines of the BM of MM patients. Mounting evidence indicates that components of the BM-including extracellular matrix, cytokines, chemokines, and growth factors-provide a sanctuary for subpopulations of MM. This co-dependent development of the disease in the context of the BM not only ensures the survival and growth of the plasma cells but contributes to de novo drug resistance. In addition, by fostering homing, angiogenesis, and osteolysis, this crosstalk plays a critical role in the progression of the disease. Not surprisingly then, over the past decade, several strategies have been developed to disrupt this communication between the plasma cells and the BM components including antibodies, peptides, and inhibitors of signaling pathways. Ultimately, the goal is to use these therapies in combination with the existing antimyeloma agents in order to further reduce or abolish minimal residual disease and improve patient outcomes.

Targeting myeloma-osteoclast interaction with Vγ9Vδ2 T cells

Multiple myeloma (MM) cells stimulate osteoclastogenesis, and osteoclasts (OCs) in turn enhance MM growth and drug resistance, resulting in a vicious cycle. Vγ9Vδ2 T cells exert potent anti-tumor effects, making T cell-based immunotherapies using these cells attractive candidates for currently incurable malignancies, such as MM. However, the impact of such treatments on the MM-OC interaction is largely unknown. We demonstrate here that Vγ9Vδ2 T cells expanded by zoledronic acid and IL-2 exerted potent cytotoxic effects on both MM cells and OCs, even in coculture settings, but showed no such effect on bone marrow stromal cells. Vγ9Vδ2 T cells marginally affected colony formation from normal hematopoietic progenitors, and furthermore migrated toward osteopontin and MIP-1α, factors produced by the MM-OC interaction. These results suggest that Vγ9Vδ2 T cells expanded by zoledronic acid and IL-2 are able to migrate to MM bone lesions and preferentially target OCs as well as MM cells, thereby inhibiting both tumor expansion and bone destruction.

Effects of bone marrow stromal cells and umbilical cord blood-derived stromal cells on daunorubicin-resistant residual Jurkat cells

OBJECTIVE

To observe the effects of the hematopoietic inductive microenvironment (HIM) simulated by stromal cells of different origins on daunorubicin-resistant residual Jurkat cells (Jurkat/DNR cells).

METHODS

Jurkat/DNR cells were cultured and identified. Human umbilical cord blood-derived stromal cells (UCBDSCs) and normal human bone marrow stromal cells (BMSCs) were isolated and cocultured with Jurkat/DNR cells. Jurkat/DNR cells were collected after 14 days of coculture and analyzed with regard to cell proliferation and differentiation abilities, apoptosis, drug sensitivity, and MRD1 multidrug resistance gene mRNA expression.

RESULTS

UCBDSC-simulated HIM suppressed proliferation and promoted apoptosis, differentiation, and drug sensitivity of Jurkat/DNR cells more significantly than BMSC-simulated HIM.

CONCLUSIONS

Both BMSCs and UCBDSCs reconstruct the leukemic HIM and reverse drug resistance in Jurkat/DNR cells. UCBDSCs reconstruct the leukemic HIM and reverse drug resistance more significantly than BMSCs.

Advances in the understanding of myeloma bone disease and tumour growth

Advances in multiple myeloma support the notion that the associated bone disease, characterized by increased osteoclastogenesis and suppressed osteoblastogenesis, is both a consequence and necessity of tumour progression. Osteoblastogenesis is suppressed by secreted inhibitors and dysregulation of cell-surface 'coupling' factors on osteogenic cells. Osteoclastogenesis is increased as a consequence of osteoblast deactivation and of production of osteoclast-activating factors. Osteoclasts express soluble and cell-surface factors that stimulate myeloma growth, while osteoblasts produce bone-building factors that restrain growth of myeloma cells that are dependent on the microenvironment; detailed molecular mechanisms are discussed. Experimental and clinical findings indicate that pharmacological and experimental osteoblast-activating agents that effectively promote bone formation also reduce growth of myeloma cells within bone, seemingly by simultaneously stimulating osteoblastogenesis and restraining osteoclastogenesis. Unravelling mechanisms of myeloma bone disease expands horizons for developing novel interventions and also facilitates better understanding of the association between induction of osteolysis and disease progression.

The molecular characterization and clinical management of multiple myeloma in the post-genome era

Cancer-causing mutations disrupt coordinated, precise programs of gene expression that govern cell growth and differentiation. Microarray-based gene-expression profiling (GEP) is a powerful tool to globally analyze these changes to study cancer biology and clinical behavior. Despite overwhelming genomic chaos in multiple myeloma (MM), expression patterns within tumor samples are remarkably stable and reproducible. Unique expression patterns associated with recurrent chromosomal translocations and ploidy changes defined molecular classes with differing clinical features and outcomes. Combined molecular techniques also dissected two distinct, reproducible forms of hyperdiploid disease and have molecularly defined MM with high risk for poor clinical outcome. GEP is now used to risk-stratify patients with newly diagnosed MM. Groups with high-risk features are evident in all GEP-defined MM classes, and GEP studies of serial samples showed that risk increases over time, with relapsed disease showing dramatic GEP shifts toward a signature of poor outcomes. This suggests a common mechanism of disease evolution and potentially reflects preferential expansion of therapy-resistant cells. Correlating GEP-defined disease class and risk with outcomes of therapeutic regimens reveals class-specific benefits for individual agents, as well as mechanistic insights into drug sensitivity and resistance. Here, we review modern genomics contributions to understanding MM pathogenesis, prognosis, and therapy.

Advances in Immunotherapy of Multiple Myeloma: From the Discovery of Tumor-Associated Antigens to Clinical Trials

Tumors aberrantly express tumor-associated antigens that can be specifically recognized by T-cells, thereby providing a scientific rationale for the design and clinical testing of immunotherapeutic strategies targeting these antigens. Multiple myeloma is a fatal hematologic malignancy. Here, we review techniques to discover new tumor-associated antigens in multiple myeloma and the latest immunotherapeutic strategies employed in this disease.

Using substrate specificity of antiplasmin-cleaving enzyme for fibroblast activation protein inhibitor design

Circulating antiplasmin-cleaving enzyme (APCE), a prolyl-specific serine proteinase, is essentially identical to membrane-inserted fibroblast activation protein (FAP) that is transiently expressed during epithelial-derived cancer growth. Human precursive alpha(2)-antiplasmin (Met-alpha(2)AP), the only known physiologic substrate for APCE, is cleaved N-terminally to Asn-alpha(2)AP that is rapidly cross-linked to fibrin and protects it from digestion by plasmin. Identifying a specific inhibitor of APCE/FAP continues to be intensely pursued. Recombinant FAP cleavage of peptide libraries of short amino acid sequences surrounding the scissile bond, -Pro(12)-Asn(13)-, indicated that P2 Gly and P1 Pro are required, just as we found for APCE. We examined cleavage of P4-P4' peptides, using 19 amino acid substitutions at each position and selected ones in P8-P5. K(m) values determined for peptide substrates showed that P7 Arg has the highest affinity for APCE. Peptide cleavage rate increased with Arg in P6 rather than P5 or native P7. Placing Arg in P4 or P8 reduced cleavage rates dramatically. Cleavage of substrates with extended peptide sequences before or after the scissile bond showed endopeptidase to be superior to dipeptidase activity for APCE. A substrate analogue inhibitor, Phe-Arg-(8-amino-3,6-dioxaoctanoic acid)-Gly-[r]-fluoropyrrolidide, inhibited APCE with a K(i) of 54 microM but not dipeptidyl peptidase IV even at 2 mM. The inhibitor also blocked cleavage of Met-alpha(2)AP with an IC(50) of 91 microM. Replacing Arg with Gly at the same distance from fluoropyrrolidide as P7 Arg is from P1 Pro reduced its inhibition of APCE approximately 10-fold. Results indicate that Arg at P5, P6, or P7 distances from P1 enhances affinity and efficiency of substrates or inhibitors toward APCE or FAP.

Inhibitor of DASH proteases affects expression of adhesion molecules in osteoclasts and reduces myeloma growth and bone disease

Dipeptidyl peptidase (DPP) IV activity and/or structure homologues (DASH) are serine proteases implicated in tumourigenesis. We previously found that a DASH protease, fibroblast activation protein (FAP), was involved in osteoclast-induced myeloma growth. Here we further demonstrated expression of various adhesion molecules in osteoclasts cultured alone or cocultured with myeloma cells, and tested the effects of DASH inhibitor, PT-100, on myeloma cell growth, bone disease, osteoclast differentiation and activity, and expression of adhesion molecules in osteoclasts. PT-100 had no direct effects on viability of myeloma cells or mature osteoclasts, but significantly reduced survival of myeloma cells cocultured with osteoclasts. Real-time PCR array for 85 adhesion molecules revealed upregulation of 17 genes in osteoclasts after coculture with myeloma cells. Treatment of myeloma/osteoclast cocultures with PT-100 significantly downregulated 18 of 85 tested genes in osteoclasts, some of which are known to play roles in tumourigenesis and osteoclastogenesis. PT-100 also inhibited osteoclast differentiation and subsequent pit formation. Resorption activity of mature osteoclasts and differentiation of osteoblasts were not affected by PT-100. In primary myelomatous severe combined immunodeficient (SCID)-hu mice PT-100 reduced osteoclast activity, bone resorption and tumour burden. These data demonstrated that DASH proteases are involved in myeloma bone disease and tumour growth.

The proteasome inhibitor, bortezomib suppresses primary myeloma and stimulates bone formation in myelomatous and nonmyelomatous bones in vivo

Multiple myeloma (MM), a hematologic malignancy of terminally differentiated plasma cells is closely associated with induction of osteolytic bone disease, induced by stimulation of osteoclastogenesis and suppression of osteoblastogenesis. The ubiquitin-proteasome pathway regulates differentiation of bone cells and MM cell growth. The proteasome inhibitor, bortezomib, is a clinical potent antimyeloma agent. The main goal of this study was to investigate the effect of bortezomib on myeloma-induced bone resorption and tumor growth in SCID-rab mice engrafted with MM cells from 16 patients. Antimyeloma response of bortezomib, which was evident in >50% of 16 experiments and resembled clinical response, was associated with significant increased bone mineral density (BMD) and osteoblast numbers, and reduced osteoclast numbers in myelomatous bones. This bone anabolic effect, which was also visualized on X-ray radiographs and confirmed by static and dynamic histomorphometric analyses, was unique to bortezomib and was not observed in hosts responding to melphalan, a chemotherapeutic drug widely used to treat MM. Bortezomib also increased BMD and osteoblasts number and reduced osteoclasts number in nonmyelomatous implanted bones. In vitro bortezomib directly suppressed human osteoclast formation and promoted maturation of osteoblasts. We conclude that bortezomib promotes bone formation in myelomatous and nonmyelomatous bones by simultaneously inhibiting osteoclastogenesis and stimulating osteoblastogenesis. As clinical and experimental studies indicate that bone disease is both a consequence and necessity of MM progression our results suggest and that bortezomib's effects on bone remodeling contribute to the antimyeloma efficacy of this drug.

Role of decorin in the antimyeloma effects of osteoblasts

Building on our previous report that osteoblasts and increased bone formation have a negative impact on myeloma cell growth in a subset of patients, we investigated the role of decorin, the main small leucine-rich proteoglycan (SLRP) expressed and produced by osteoblasts, in the antimyeloma effects of osteoblasts. In coculture experiments with osteoblasts, primary myeloma cell survival was significantly higher when decorin expression in osteoblasts was knocked down by short-hairpin RNA. Coculture experiments of myeloma cells and supporting osteoclasts in the presence of osteoblast-conditioned medium showed reduced myeloma cell survival, an effect that was attenuated by decorin-neutralizing antibody. Decorin overexpression in mesenchymal stem cells or use of recombinant decorin in coculture with osteoclasts reduced the ability of osteoclasts to support primary myeloma cell survival. The antimyeloma effect of decorin involved direct induction of apoptosis and activation of p21(WAF). Decorin also inhibited myeloma cell-induced tube formation and osteoclast differentiation. Decorin expression was insignificantly lower in patients' than donors' osteoblasts and slightly increased by bortezomib. Certain SLRPs are involved in the antimyeloma effect of osteoblasts directly and indirectly through inhibition of angiogenesis and osteoclastogenesis; therefore, increasing endogenous or exogenous SLRPs in myelomatous bone may help control myeloma.

Atacicept (TACI-Ig) inhibits growth of TACI(high) primary myeloma cells in SCID-hu mice and in coculture with osteoclasts

APRIL (a proliferation-inducing Ligand) and BLyS/BAFF (B-lymphocyte stimulator/B-cell-activating factor of the TNF (tumor necrosis factor) family have been shown to be the survival factors for certain myeloma cells in vitro. BAFF binds to the TNF-related receptors such as B-cell maturation antigen (BCMA), transmembrane activator and CAML interactor (TACI) and BAFFR, whereas APRIL binds to TACI and BCMA and to heparan sulfate proteoglycans (HSPG) such as syndecan-1. TACI gene expression in myeloma reportedly can distinguish tumors with a signature of microenvironment dependence (TACI(high)) versus a plasmablastic signature (TACI(low)). We tested the effect of atacicept (formerly TACI-Ig, which blocks APRIL and BAFF) and BAFFR-Ig (which blocks BAFF only) on primary myeloma growth in the SCID-hu model and in coculture with osteoclasts. With only few exceptions, atacicept and to a lesser extent BAFFR-Ig, inhibited growth of TACI(high) but not TACI(low) myeloma samples in vivo and ex vivo, and the response rate was inversely correlated with TACI expression. Most TACI(high) myeloma cells were molecularly classified as being low risk with our recently described 70-gene model. APRIL and BAFF were highly expressed by osteoclasts and were upregulated in myeloma cells after coculture with osteoclasts. Our findings suggest that APRIL plays an essential role in the survival of TACI(high) bone marrow-dependent myeloma cells and TACI gene expression may be a useful predictive marker for patients who could benefit from atacicept treatment.

Establishment and exploitation of hyperdiploid and non-hyperdiploid human myeloma cell lines

The establishment of clinically relevant human myeloma cell lines is central for our understanding of myeloma pathogenesis and development of novel therapies for the disease. Unfortunately, most available lines were generated from extramedullary sites, harbored multiple genetic abnormalities and categorized as non-hyperdiploid. In contrast, hyperdiploid myeloma cell lines, which represent more than 50% of patients, are rare. We established procedures for establishment of stroma-dependent myeloma lines by passaging primary myeloma cells, in severe combined immunodeficient-human (SCID-hu) or SCID-rab mice followed by maintenance in co-culture with stromal cells. We described the establishment and characterization of two hyperdiploid (LD and CF) and two non-hyperdiploid (JB and BN) cell lines. Using our animal models, we also established bortezomib-sensitive and -resistant BN lines. These cell lines were cellularly, phenotypically and molecularly characterized using flow cytometry immunophenotyping, DNA content, G-band and multicolor spectral karyotyping (SKY) and global gene expression profiling. All four cell lines were infected with lentiviral-expressing luciferase for detection of tumour cells at high sensitivity level and for monitoring myeloma growth in co-cultures and in vivo by live animal imaging. These myeloma cell lines and the procedures used for their establishment provide essential tools for studying myeloma biology and therapy.

Regulation of human umbilical cord blood-derived multi-potent stem cells by autogenic osteoclast-based niche-like structure

Stem cell niches provide the micro-environment for the development of stem cells. Under our culturing regimen, a kind of osteoclast-centralized structure supports the proliferation of MSCs, derived from human cord blood, once they reside on osteoclasts. MSCs in this structure expressed Oct4 which is a marker of embryonic stem cells. Floating daughter cells of MSCs colony showed abilities to differentiate into osteocyte, adipocyte, and neuronal progenitor cells. Compared with the easy senescence of MSCs without this niche-like structure in vitro, these results suggested that osteoclasts might play an important role the development and maintenance of Umbilical cord blood (UCB)-derived MSCs and might provide a means to expand UCB-MSCs in vitro, more easily, through a stem cell niche-like structure.

Bone marrow cells in the 'pre-metastatic niche': within bone and beyond

Metastasis, the spread of invasive carcinoma to sites distant from the primary tumor, is responsible for the majority of cancer-related deaths (Weigelt, B., Peterse, J. L., & van 't Veer, L. J. (2005). Breast cancer metastasis: Markers and models. Nature Reviews. Cancer, 5, 591-602). Despite progress in other areas of cancer therapeutics, the complexities of this process remain poorly understood. Consequently, there are few successful treatments that directly target this stage of carcinogenesis. Particularly enigmatic is the tissue-specificity of different tumor types observed in metastatic spread. One example is the predilection of colon cancer to spread to liver whereas breast, prostate, and lung carcinomas have a particular affinity to target and proliferate in bone. In 1889, Stephen Paget observed that circulating tumour cells would only "seed" where there was "congenial soil". Since then, attention has focused on explaining the dynamic adhesive and migratory capabilities intrinsic to tumor cells. Meanwhile, the earliest changes occurring within distant tissues that prime the "soil" to receive incoming cancer cells have largely been neglected. Recent work characterizing the importance of bone marrow-derived hematopoietic progenitor cells (HPC) in initiating these early changes has opened new avenues for cancer research and chemotherapeutic targeting (Kaplan, R. N., Riba, R. D., Zacharoulis, S., Bramley, A. H., Vincent, L., Costa, C., et al. (2005). VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature, 438, 820-827). This review discusses the inextricable relationship between bone stromal components, metastasizing cells, and bone marrow-derived hematopoietic cells, and their roles in carcinogenesis and metastasis. Understanding these dynamics may help explain the tissue-specific tropism seen in metastasis. Moreover, exploring the earliest events promoting circulating cancer cells to engraft and establish at secondary sites may expose new targets for diagnostic and therapeutic strategies and reduce the morbidity and mortality from metastatic disease.

Human mesenchymal stem cells signals regulate neural stem cell fate

Neural stem cells (NSCs) differentiate into neurons, astrocytes and oligodendrocytes depending on their location within the central nervous system (CNS). The cellular and molecular cues mediating end-stage cell fate choices are not completely understood. The retention of multipotent NSCs in the adult CNS raises the possibility that selective recruitment of their progeny to specific lineages may facilitate repair in a spectrum of neuropathological conditions. Previous studies suggest that adult human bone marrow derived mesenchymal stem cells (hMSCs) improve functional outcome after a wide range of CNS insults, probably through their trophic influence. In the context of such trophic activity, here we demonstrate that hMSCs in culture provide humoral signals that selectively promote the genesis of neurons and oligodendrocytes from NSCs. Cell-cell contacts were less effective and the proportion of hMSCs that could be induced to express neural characteristics was very small. We propose that the selective promotion of neuronal and oligodendroglial fates in neural stem cell progeny is responsible for the ability of MSCs to enhance recovery after a wide range of CNS injuries.