Malignant peripheral nerve sheath tumors (MPNSTs) are the most common malignancy associated with neurofibromatosis type 1 (NF1). These Schwann cell lineage-derived sarcomas aggressively invade adjacent nerve and soft tissue, frequently precluding surgical resection. Little is known regarding the mechanisms underlying this invasive behavior. We have shown that MPNSTs express neuregulin-1 (NRG-1) β isoforms, which promote Schwann cell migration during development, and NRG-1α isoforms, whose effects on Schwann cells are poorly understood. Hypothesizing that NRG-1β and/or NRG-1α promote MPNST invasion, we found that NRG-1β promoted MPNST migration in a substrate-specific manner, markedly enhancing migration on laminin but not on collagen type I or fibronectin. The NRG-1 receptors erbB3 and erbB4 were present in MPNST invadopodia (processes mediating invasion), partially colocalized with focal adhesion kinase and the laminin receptor β1-integrin and coimmunoprecipitated with β1-integrin. NRG-1β stimulated human and murine MPNST cell migration and invasion in a concentration-dependent manner in three-dimensional migration assays, acting as a chemotactic factor. Both baseline and NRG-1β induced migration were erbB-dependent and required the action of MEK 1/2, SAPK/JNK, PI-3 kinase, Src family kinases and ROCK-I/II. In contrast, NRG-1α had no effect on the migration and invasion of some MPNST lines and inhibited the migration of others. While NRG-1β potently and persistently activated Erk 1/2, SAPK/JNK, Akt and Src family kinases, NRG-1α did not activate Akt and activated these other kinases with kinetics distinct from those evident in NRG-1β stimulated cells. These findings suggest that NRG-1β enhances MPNST migration and that NRG-1β and NRG-1α differentially modulate this process.

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.

Patients with neurofibromatosis type 1 (NF1) develop benign plexiform neurofibromas that frequently progress to become malignant peripheral nerve sheath tumors (MPNSTs). A genetically engineered mouse model that accurately models plexiform neurofibroma-MPNST progression in humans would facilitate identification of somatic mutations driving this process. We previously reported that transgenic mice overexpressing the growth factor neuregulin-1 in Schwann cells (P(0)-GGFβ3 mice) develop MPNSTs. To determine whether P(0)-GGFβ3 mice accurately model human neurofibroma-MPNST progression, cohorts of these animals were monitored through death and were necropsied; 94% developed multiple neurofibromas, with 70% carrying smaller numbers of MPNSTs. Nascent MPNSTs were identified within neurofibromas, suggesting that these sarcomas arise from neurofibromas. Although neurofibromin expression was maintained, P(0)-GGFβ3 MPNSTs exhibited Ras hyperactivation, as in human NF1-associated MPNSTs. P(0)-GGFβ3 MPNSTs also exhibited abnormalities in the p16(INK4A)-cyclin D/CDK4-Rb and p19(ARF)-Mdm-p53 pathways, analogous to their human counterparts. Array comparative genomic hybridization (CGH) demonstrated reproducible chromosomal alterations in P(0)-GGFβ3 MPNST cells (including universal chromosome 11 gains) and focal gains and losses affecting 39 neoplasia-associated genes (including Pten, Tpd52, Myc, Gli1, Xiap, and Bbc3/PUMA). Array comparative genomic hybridization also identified recurrent focal copy number variations affecting genes not previously linked to neurofibroma or MPNST pathogenesis. We conclude that P(0)-GGFβ3 mice represent a robust model of neurofibroma-MPNST progression useful for identifying novel genes driving neurofibroma and MPNST pathogenesis.

Aberrant epidermal growth factor receptor (EGFR) expression promotes the pathogenesis of malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy associated with neurofibromatosis type 1, but the mechanisms by which EGFR expression promotes MPNST pathogenesis are poorly understood. We hypothesized that inappropriately expressed EGFRs promote MPNST invasion and found that these kinases are concentrated in MPNST invadopodia in vitro. EGFR knockdown inhibited the migration of unstimulated MPNST cells in vitro and exogenous EGF further enhanced MPNST migration in a substrate-specific manner, promoting migration on laminin and, to a lesser extent, collagen. Thus, in this setting, EGF acts as a chemotactic factor. We also found that the 7 known EGFR ligands (EGF, betacellulin, epiregulin, heparin-binding EGF, transforming growth factor α [TGFα], amphiregulin, and epigen) variably enhanced MPNST migration in a concentration-dependent manner, with TGFα being particularly potent. With the exception of epigen, these factors similarly promoted the migration of non-neoplastic Schwann cells. Although transcripts encoding all 7 EGFR ligands were detected in human MPNST cells and tumor tissues, only TGFα was consistently overexpressed and was found to colocalize with EGFR in situ. These data indicate that constitutive EGFR activation, potentially driven by autocrine or paracrine TGFα signaling, promotes the aggressive invasive behavior characteristic of MPNSTs.

Aggressive invasion of adjacent tissues is characteristic of malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy occurring in neurofibromatosis type 1 (NF1). These neoplasms often aberrantly express epidermal growth factor receptors (EGFRs), leading us to hypothesize that EGFR ligands present in MPNSTs and/or adjacent tissues promote MPNST invasion. We have found that EGF promotes MPNST migration in a substrate‐specific manner, enhancing migration on laminin and collagen type I relative to fibronectin. Induction of EGFR shRNA expression in MPNST cells stably transfected with a tetracycline‐regulatable lentivirus reduces baseline migration 40‐60%. Comparing the effects of the known EGFR ligands [EGF, betacellulin, epiregulin, heparin‐binding EGF (HB‐EGF), TGFα, amphiregulin] on MPNST cell lines derived from NF1‐associated (ST88‐14) or sporadic (STS‐26T) neoplasms, we found that these ligands promote migration in a concentration‐dependent manner, with EGF acting in a chemotactic fashion. However, these factors differ in their relative ability to induce migration. Further, although ST88‐14 cells are responsive to all 6 ligands, the STS‐26T migration is not enhanced by HB‐EGF or amphiregulin. These findings suggest that although EGFR ligands enhance MPNST migration and invasion, MPNST responses to these growth factors vary. Supported by grants CA122804 and NS048353.