Schwann cell lines derived from malignant peripheral nerve sheath tumors respond abnormally to platelet-derived growth factor-BB

Malignant peripheral nerve sheath tumor: models, biology, and translation

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive, invasive cancer that comprise around 10% of all soft tissue sarcomas and develop in about 8-13% of patients with Neurofibromatosis Type 1. They are associated with poor prognosis and are the leading cause of mortality in NF1 patients. MPNSTs can also develop sporadically or following exposure to radiation. There is currently no effective targeted therapy to treat MPNSTs and surgical removal remains the mainstay treatment. Unfortunately, surgery is not always possible due to the size and location of the tumor, thus, a better understanding of MPNST initiation and development is required to design novel therapeutics. Here, we provide an overview of MPNST biology and genetics, discuss findings regarding the developmental origin of MPNST, and summarize the various model systems employed to study MPNST. Finally, we discuss current management strategies for MPNST, as well as recent developments in translating basic research findings into potential therapies.

Systemic Options for Malignant Peripheral Nerve Sheath Tumors

OPINION STATEMENT

Malignant peripheral nerve sheath tumors (MPNSTs) are rare mesenchymal neoplasms that represent a profound therapeutic challenge due to their high proclivity for recurrence and metastasis and relatively poor response to systemic therapy regimens. While our understanding of the pathophysiology of MPNST is growing, including loss of the tumor suppressor gene neurofibromin and subsequent activation of the Ras pathway, targeted therapy to modify the poor prognosis seen in MPNST patients has thus far been without success. Correspondingly, MPNST patients are treated as per soft tissue sarcoma treatment algorithms with anthracycline-based therapy as the front-line therapy of choice for patients with unresectable, locally advanced, or metastatic MPNST. Beyond first-line anthracycline-based therapy, other standard cytotoxic chemotherapy agents used in advanced MPNST include the alkylating agent ifosfamide and the topoisomerase II inhibitor etoposide. Notably, soft tissue sarcoma regimens are used in MPNST despite distinct systemic therapy sensitivity and prognosis. This is particularly notable for neurofibromatosis type 1 (NF1)-associated MPNST, which is associated with poorer response to systemic therapy and prognosis than sporadic MPNST. As such, NF1-associated MPNST is a particular area in need of novel therapeutic strategies. Given the lack of benefit in the targeting of unique aspects of MPNST disease biology thus far, pre-clinical studies to identify novel rational therapies are critical to inform future clinical trials.

Diagnosis and management of malignant peripheral nerve sheath tumors: Current practice and future perspectives

One of the most common malignancies affecting adults with the neurofibromatosis type 1 (NF1) cancer predisposition syndrome is the malignant peripheral nerve sheath tumor (MPNST), a highly aggressive sarcoma that typically develops from benign plexiform neurofibromas. Approximately 8-13% of individuals with NF1 will develop MPNST during young adulthood. There are few therapeutic options, and the vast majority of people with these cancers will die within 5 years of diagnosis. Despite efforts to understand the pathogenesis of these aggressive tumors, the overall prognosis remains dismal. This manuscript will review the current understanding of the cellular and molecular progression of MPNST, diagnostic workup of patients with these tumors, current treatment paradigms, and investigational treatment options. Additionally, we highlight novel areas of preclinical research, which may lead to future clinical trials. In summary, MPNST remains a diagnostic and therapeutic challenge, and future work is needed to develop novel and rational combinational therapy for these tumors.

Recent Advances in the Diagnosis and Pathogenesis of Neurofibromatosis Type 1 (NF1)-associated Peripheral Nervous System Neoplasms

The diagnosis of a neurofibroma or a malignant peripheral nerve sheath tumor (MPNST) often raises the question of whether the patient has the genetic disorder neurofibromatosis type 1 (NF1) as well as how this will impact the patient's outcome, what their risk is for developing additional neoplasms and whether treatment options differ for NF1-associated and sporadic peripheral nerve sheath tumors. Establishing a diagnosis of NF1 is challenging as this disorder has numerous neoplastic and non-neoplastic manifestations which are variably present in individual patients. Further, other genetic diseases affecting the Ras signaling cascade (RASopathies) mimic many of the clinical features of NF1. Here, we review the clinical manifestations of NF1 and compare and contrast them with those of the RASopathies. We also consider current approaches to genetic testing for germline NF1 mutations. We then focus on NF1-associated neurofibromas, considering first the complicated clinical behavior and pathology of these neoplasms and then discussing our current understanding of the genomic abnormalities that drive their pathogenesis, including the mutations encountered in atypical neurofibromas. As several neurofibroma subtypes are capable of undergoing malignant transformation to become MPNSTs, we compare and contrast patient outcomes in sporadic, NF1-associated and radiation-induced MPNSTs, and review the challenging pathology of these lesions. The mutations involved in neurofibroma-MPNST progression, including the recent identification of mutations affecting epigenetic regulators, are then considered. Finally, we explore how our current understanding of neurofibroma and MPNST pathogenesis is informing the design of new therapies for these neoplasms.

Malignant Peripheral Nerve Sheath Tumors State of the Science: Leveraging Clinical and Biological Insights into Effective Therapies

Malignant peripheral nerve sheath tumor (MPNST) is the leading cause of mortality in patients with neurofibromatosis type 1. In 2002, an MPNST consensus statement reviewed the current knowledge and provided guidance for the diagnosis and management of MPNST. Although the improvement in clinical outcome has not changed, substantial progress has been made in understanding the natural history and biology of MPNST through imaging and genomic advances since 2002. Genetically engineered mouse models that develop MPNST spontaneously have greatly facilitated preclinical evaluation of novel drugs for translation into clinical trials led by consortia efforts. Continued work in identifying alterations that contribute to the transformation, progression, and metastasis of MPNST coupled with longitudinal follow-up, biobanking, and data sharing is needed to develop prognostic biomarkers and effective prevention and therapeutic strategies for MPNST.

Appropriate modulation of autophagy sensitizes malignant peripheral nerve sheath tumor cells to treatment with imatinib mesylate

Malignant peripheral nerve sheath tumor (MPNST), very rare in childhood, is a highly aggressive soft-tissue tumor. We experienced a case of a 7-year-old boy with MPNST who was treated with imatinib mesylate (imatinib) after the identification of platelet-derived growth factor receptor expression in his tumor. We were unable to observe clinical benefits of imatinib in this patient. Therefore, cellular reactions of imatinib were investigated in vitro using 3 MPNST cell lines. Imatinib induced cytotoxicity in vitro with variable IC50 values (11.7 to >30 μM). Induction of apoptosis was not a pivotal mechanism in the inhibitory effects. We found that the treatment of MPNST cell lines with imatinib induced autophagy. Suppression of the initiation of autophagy by 3-methyladenine or small interfering RNA (siRNA) against beclin-1 attenuated the imatinib-mediated cytotoxicity. In contrast, blocking the formation of autophagosomes or the development of autolysosomes using siRNA against microtubule-associated protein light chain 3B, bafilomycin A1, chloroquine, or an MEK1/2 inhibitor (U0126) enhanced the imatinib-induced cytotoxicity in MPNST cells. Our data showed that the imatinib-mediated autophagy can function as a cytotoxic mechanism and that appropriate modulation of autophagy may sensitize MPNST cells to imatinib, which in turn may be a novel therapeutic strategy for MPNST.

Platelet-derived growth factor-BB activates calcium/calmodulin-dependent and -independent mechanisms that mediate Akt phosphorylation in the neurofibromin-deficient human Schwann cell line ST88-14

Neurofibromatosis type 1-derived Schwann cells isolated from malignant peripheral nerve sheath tumors (MPNSTs) overexpress PDGF receptor-β and generate an aberrant intracellular calcium increase in response to PDGF-BB. Using the human MPNST Schwann cell line ST88-14, we demonstrate that, in addition to a transient phosphorylation of Akt, PDGF-BB stimulation produces an atypical sustained phosphorylation of Akt that is dependent on calcium and calmodulin (CaM). The sustained Akt phosphorylation did not occur in PDGF-BB-stimulated normal human Schwann cells or ST88-14 cells stimulated with stem cell factor, whose receptor is also overexpressed in ST88-14 cells. The sustained Akt phosphorylation induced by PDGF-BB was inhibited by pretreatment of the cells with either the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester (BAPTA-AM) or the CaM antagonist W7, whereas the transient portion was not inhibited. Akt also co-immunoprecipitated with CaM in a PDGF-BB-dependent manner, suggesting that direct interaction between Akt and CaM is involved in the sustained phosphorylation of Akt. Furthermore, we provide evidence that anti-apoptotic effects of PDGF-BB on serum-deprived ST88-14 cells can be inhibited by W7, implicating the PDGF-BB-induced activation of calcium/CaM in promoting cell survival, presumably through sustained Akt activation. We conclude that the activation of the calcium/CaM/Akt pathway resulting from stimulation of overexpressed PDGF receptor-β may contribute to the survival and tumorigenicity of MPNST cells.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

Neurofibromas, schwannomas and malignant peripheral nerve sheath tumors (MPNSTs) all arise from the Schwann cell lineage. Despite their common origin, these tumor types have distinct pathologies and clinical behaviors; a growing body of evidence indicates that they also arise via distinct pathogenic mechanisms. Identification of the genes that are mutated in genetic diseases characterized by the development of either neurofibromas and MPNSTs [neurofibromatosis type 1 (NF1)] or schwannomas [neurofibromatosis type 2 (NF2), schwannomatosis and Carney complex type 1] has greatly advanced our understanding of these mechanisms. The development of genetically engineered mice with ablation of NF1, NF2, SMARCB1/INI1 or PRKAR1A has confirmed the key role these genes play in peripheral nerve sheath tumorigenesis. Establishing the functions of the NF1, NF2, SMARCB1/INI1 and PRKAR1A gene products has led to the identification of key cytoplasmic signaling pathways promoting Schwann cell neoplasia and identified new therapeutic targets. Analyses of human neoplasms and genetically engineered mouse models have established that interactions with other tumor suppressors such as TP53 and CDKN2A promote neurofibroma-MPNST progression and indicate that intratumoral interactions between neoplastic and non-neoplastic cell types play an essential role in peripheral nerve sheath tumorigenesis. Recent advances have also provided new insights into the identity of the neural crest-derived populations that give rise to different types of peripheral nerve sheath tumors. Based on these findings, we now have an initial outline of the molecular mechanisms driving the pathogenesis of neurofibromas, MPNSTs and schwannomas. However, this improved understanding in turn raises a host of intriguing new questions.

Tamoxifen inhibits malignant peripheral nerve sheath tumor growth in an estrogen receptor-independent manner

Few therapeutic options are available for malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy associated with neurofibromatosis type 1 (NF1). Guided by clinical observations suggesting that some NF1-associated nerve sheath tumors are hormonally responsive, we hypothesized that the selective estrogen receptor (ER) modulator tamoxifen would inhibit MPNST tumorigenesis in vitro and in vivo. To test this hypothesis, we examined tamoxifen effects on MPNST cell proliferation and survival, MPNST xenograft growth, and the mechanism by which tamoxifen impeded these processes. We found that 1-5 μM 4-hydroxy-tamoxifen induced MPNST cell death, whereas 0.01-0.1 μM 4-hydroxy-tamoxifen inhibited mitogenesis. Dermal and plexiform neurofibromas, MPNSTs, and MPNST cell lines expressed ERβ and G-protein-coupled ER-1 (GPER); MPNSTs also expressed estrogen biosynthetic enzymes. However, MPNST cells did not secrete 17β-estradiol, exogenous 17β-estradiol did not stimulate mitogenesis or rescue 4-hydroxy-tamoxifen effects on MPNST cells, and the steroidal antiestrogen ICI-182,780 did not mimic tamoxifen effects on MPNST cells. Further, ablation of ERβ and GPER had no effect on MPNST proliferation, survival, or tamoxifen sensitivity, indicating that tamoxifen acts via an ER-independent mechanism. Consistent with this hypothesis, inhibitors of calmodulin (trifluoperazine, W-7), another known tamoxifen target, recapitulated 4-hydroxy-tamoxifen effects on MPNST cells. Tamoxifen was also effective in vivo, demonstrating potent antitumor activity in mice orthotopically xenografted with human MPNST cells. We conclude that 4-hydroxy-tamoxifen inhibits MPNST cell proliferation and survival via an ER-independent mechanism. The in vivo effectiveness of tamoxifen provides a rationale for clinical trials in cases of MPNSTs.

Malignant peripheral nerve sheath tumour (MPNST): the clinical implications of cellular signalling pathways

Malignant peripheral nerve sheath tumour (MPNST) is a rare malignancy accounting for 3–10% of all soft tissue sarcomas. Most MPNSTs arise in association with peripheral nerves or deep neurofibromas and may originate from neural crest cells, although the specific cell of origin is uncertain. Approximately half of MPNSTs occur in the setting of neurofibromatosis type 1 (NF1), an autosomal dominant disorder with an incidence of approximately one in 3500 persons; the remainder of MPNSTs develop sporadically. In addition to a variety of clinical manifestations, approximately 8–13% of NF1 patients develop MPNSTs, which are the leading cause of NF1-related mortality. Surgical resection is the mainstay of MPNST clinical management. However, because of invasive growth, propensity to metastasise, and limited sensitivity to chemotherapy and radiation, MPNST has a guarded to poor prognosis. Five-year survival rates of only 20–50% indicate an urgent need for improved therapeutic approaches. Recent work in this field has identified several altered intracellular signal transduction cascades and deregulated tyrosine kinase receptors, posing the possibility of personalised, targeted therapeutics. However, expanded knowledge of MPNST molecular pathobiology will be needed to meaningfully apply such approaches for the benefit of afflicted patients.

Imatinib mesylate inhibits cell invasion of malignant peripheral nerve sheath tumor induced by platelet-derived growth factor-BB

Malignant peripheral nerve sheath tumor (MPNST) is rare, highly aggressive, resistant to radiochemotherapy, and associated with poor prognosis. Basic research to develop new treatment regimes is critically needed. This study was designed to identify motogenic factor(s) involved in MPNST cell invasion and inhibitor(s) of such invasive activity. We profiled the invasion-inducing activities of eight motogenic growth factors on two human MPNST cell lines, FU-SFT8611 and 9817, using in vitro Matrigel invasion assays. Platelet-derived growth factor-BB (PDGF-BB) was identified as the most effective MPNST cell invasion-inducing factor. Epidermal growth factor (EGF) and hepatocyte growth factor (HGF) also stimulated invasion in one MPNST cell line. Expressions of PDGF-BB and EGF receptors (PDGFR-beta and EGFR) mRNAs were detected more frequently and their proteins were expressed at higher levels in MPNST tissues than benign peripheral nerve sheath tumors (schwannomas and neurofibromas). In both MPNST cell lines, PDGF-BB induced tyrosine phosphorylation of PDGFR-beta but not of PDGFR-alpha, and specific PDGFR-beta inhibition by small interfering RNA to the receptor inhibited PDGF-BB-stimulated MPNST cell invasion, suggesting the predominant role of PDGFR-beta. Inhibition of PDGFR-beta phosphorylation by pretreatment with herbimycin A and imatinib mesylate effectively suppressed basement membrane invasion and cell growth in vitro. No mutations were present in exons 12 and 18 of PDGFR-beta in both MPNST cell lines and 10 human MPNST tissues examined. Our results indicated that PDGF-BB enhanced the invasive activity of MPNST cells through PDGFR phosphorylation and that imatinib inhibited such activity. The results provide the ground for further assessment of the therapeutic potential of imatinib in suppressing the invasion and growth of MPNST.

Experimental therapeutic approaches to peripheral nerve tumors

✓Discovery that the Schwann cell is the primary cell type responsible for both the neurofibroma as well as the schwannoma has proven to represent a crucial milestone in understanding the pathogenesis of peripheral nerve tumor development. This information and related findings have served as a nidus for research aimed at more fully characterizing this family of conditions. Recent discoveries in the laboratory have clarified an understanding of the molecular mechanisms underlying the pathogenesis of benign peripheral nerve tumors. Similarly, the mechanisms whereby idiopathic and syndromic (NF1- andNF2-associated) nerve sheath tumors progress to malignancy are being elucidated. This detailed understanding of the molecular pathogenesis of peripheral nerve tumors provides the information necessary to create a new generation of therapies tailored specifically to the prevention, cessation, or reversal of pathological conditions at the fundamental level of dysfunction. The authors review the data that have helped to elucidate the molecular pathogenesis of this category of conditions, explore the current progress toward exploitation of these findings, and discuss potential therapeutic avenues for future research.

Expression of CD1d Molecules by Human Schwann Cells and Potential Interactions with Immunoregulatory Invariant NK T Cells

CD1d-restricted NKT cells expressing invariant TCR alpha-chains (iNKT cells) produce both proinflammatory and anti-inflammatory cytokines rapidly upon activation, and are believed to play an important role in both host defense and immunoregulation. To address the potential implications of iNKT cell responses for infectious or inflammatory diseases of the nervous system, we investigated the expression of CD1d in human peripheral nerve. We found that CD1d was expressed on the surface of Schwann cells in situ and on primary or immortalized Schwann cell lines in culture. Schwann cells activated iNKT cells in a CD1d-dependent manner in the presence of alpha-galactosylceramide. Surprisingly, the cytokine production of iNKT cells stimulated by alpha-galactosylceramide presented by CD1d+ Schwann cells showed a predominance of Th2-associated cytokines such as IL-5 and IL-13 with a marked deficiency of proinflammatory Th1 cytokines such as IFN-gamma or TNF-alpha. Our findings suggest a mechanism by which iNKT cells may restrain inflammatory responses in peripheral nerves, and raise the possibility that the expression of CD1d by Schwann cells could be relevant in the pathogenesis of infectious and inflammatory diseases of the peripheral nervous system.

Neurofibroma-associated growth factors activate a distinct signaling network to alter the function of neurofibromin-deficient endothelial cells

Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.

Tumor suppressor mutations and growth factor signaling in the pathogenesis of NF1-associated peripheral nerve sheath tumors: II. The role of dysregulated growth factor signaling

Patients with neurofibromatosis type 1 (NF1), one of the most common genetic disease affecting the nervous system, develop multiple neurofibromas that can transform into aggressive sarcomas known as malignant peripheral nerve sheath tumors (MPNSTs). Studies of human tumors and newly developed transgenic mouse models indicate that Schwann cells are the primary neoplastic cell type in neurofibromas and MPNSTs and that development of these peripheral nerve sheath tumors involves mutations of multiple tumor suppressor genes. However, it is widely held that tumor suppressor mutations alone are not sufficient to induce peripheral nerve sheath tumor formation and that dysregulated growth factor signaling cooperates with these mutations to promote neurofibroma and MPNST tumorigenesis. In Part I of this review, we discussed findings demonstrating that a loss of NF1 tumor suppressor gene function in neoplastic Schwann cells is a key early step in neurofibroma formation and that progression from neurofibroma to MPNST is associated with abnormalities of additional tumor suppressor genes, including p53, INK4A, andp27(kip1). In Part II of this review, we consider evidence that dysregulated signaling by specific growth factors and growth factor receptors promotes the proliferation, migration, and survival of neoplastic Schwann cells in neurofibromas and MPNSTs.