Malignant peripheral nerve sheath tumors (MPNSTs) are highly aggressive cancers that account for approximately 5%–10% of all soft tissue sarcomas [1]. MPNSTs are defined as tumors originating from or differentiating into nerve sheath cells [2], with the exception of tumors originating from the epineurium or peripheral nerve vasculature [3]. MPNSTs may arise sporadically (40%), following radiotherapy (10%), or in association with neurofibromatosis type 1 (NF1) (40%–50%) [4, 5]. NF1 is caused by the mutation of NF1 gene. The NF1 gene (located in 17q11.2) product is neurofibromin. A region of neurofibromin functions as a GTPase-activating protein shown to be involved in negative regulation of the RAS pathway, then caused the hydrolyzation and inactivation of GTP [6]. The mutations of NF1 cause neurofibromin to lose its functions as a tumor-suppressing factor. Types of NF1 mutations include frameshift, nonsense, missense, splicing alteration and deletion, and loss of heterozygosity [7]. The MPNST that develops from benign NF1 is named NF1-associated MPNST and is the main cause of death among NF1 patients [7, 8]. Compared with sporadic MPNSTs, NF1-associated MPNSTs usually develop in young adults, frequently relapse (in approximately 50% of patients), and lead to a dismal prognosis partly because of the high mutation frequency of NF1 gene [8]. Traditional treatment modalities, such as surgery, chemotherapy, and radiotherapy, have limited therapeutic effects on MPNSTs [9, 10]. Sporadic MPNSTs have some unusual characteristics. For instance, high expression of fibroblast growth factor receptor 1 (FGFR1) is an independent prognostic predictor for long overall survival (OS) in patients with sporadic MPNST, but associated with poor prognosis of several other types of epithelial cancer [11].

Chemokines are a class of small molecules that bind to G protein-coupled receptors on target cells. Chemokine-induced signaling cascades regulate leukocyte migration, hematopoietic stem cell proliferation, and cell adhesion to extracellular matrix molecules [12, 13]. C-X-C motif chemokine ligand 12 (CXCL12), also known as stromal cell-derived factor-1, is a chemokine that binds to C-X-C motif chemokine receptor 4 (CXCR4). CXCR4/CXCL12 signaling plays important roles in the maintenance of embryonic development, immune and inflammatory responses, regulation of hematopoiesis, human immunodeficiency virus infection, and induction of angiogenesis [14, 15]. Signaling triggered by the CXCR4-CXCL12 binding and subsequent regulation of Cyclin D1 expression has been shown to regulate several biological functions in cancers, such as cell proliferation, tumor invasion and metastasis, angiogenesis, and cell–microenvironment interactions [16, 17]. It is reported that CXCR4 and CXCL12 mRNA expression was reduced in hepatocellular carcinoma and renal cancer compared with non-cancerous tissues, but unchanged in most digestive tract carcinomas, such as colon esophageal or gastric cancer [18]. At the same time, high levels of CXCR4 have been associated with short OS in breast cancer patients [19, 20]. According to the divergence of CXCR4 expression in different cancers, extensive analyses of CXCR4 expression levels and functional studies will be necessary to understand the precise roles of CXCR4/CXCL12 signaling in different cancer types.

In NF1-associated MPNST, CXCR4/CXCL12 signaling stimulates Cyclin D1 expression and promotes cell cycle progression by activating the AKT/serine–threonine kinase/glycogen synthase kinase 3 beta signaling pathway, ultimately increasing MPNST cell proliferation. Suppression of CXCR4 activity decreases MPNST cell growth and inhibits tumorigenesis in allografts and in spontaneous genetic mouse models of MPNST [16]. However, the role of CXCR4 in sporadic MPNSTs is unclear. In this study, we addressed this question by measuring the expression of CXCR4, CXCL12, and Cyclin D1 proteins in tumor samples from Chinese patients with sporadic MPNST.

In the present study, we identified a significant increase in CXCL4, CXCL12, and Cyclin D1 expression in tumor samples from Chinese patients with sporadic MPNST. CXCR4 expression was positively correlated with that of CXCL12 and Cyclin D1. Most importantly, patients with high CXCR4 expression had longer OS than did patients with low expression. High CXCR4 expression might define a specific sporadic MPNST subtype.

Our present data about the prognostic value of CXCR4 in sporadic MPNSTs was quite different from the reported data from NF1-associated MPNSTs [16], which showed that high CXCR4 expression was associated with poor prognosis. In NF1-associated MPNSTs, it was reported that CXCR4/CXCL12 signaling stimulated Cyclin D1 expression and enhanced MPNST cell proliferation which indicated poor prognosis [16]. Suppression of CXCR4 inhibited the growth of MPNST cell lines and tumorigenesis in allografts and spontaneous genetic mouse models of NF1-associated MPNST [16]. There are at least two possible explanations for the different prognostic values of CXCR4 in different subtypes of MPNST. One is that the pathogenesis of sporadic MPNST and NF1-associated MPNST may differ as a result of the quite different frequency of NF1 mutations. While neurofibromin inhibits tumor cell proliferation through blockade of RAS-mediated signal transduction, it should also be considered a modulator of cell motility and cell adhesion [6]. Through interfacing with the cytoskeleton and membrane structures, neurofibromin acts as a negative regulator of the Rhodopsin/Rhodopsin-associated coiled-coil containing protein kinase (RHO/ROCK) signaling pathway involved in cytoskeletal dynamics that are instrumental in proper neuronal development [6]. The mutations of NF1 cause neurofibromin to lose its functions, leading to poor prognosis of NF1-associated MPNSTs. However, sporadic MPNSTs show no or less frequent NF1 mutations. Different pathogenesis might affect the role of CRCR4 in different subtypes of MPNSTs. A second possibility is that high CXCR4 expression might define a specific sporadic MPNST subtype. In this MPNST subtype, the typical phenotype is high CXCR4 expression which is associated with good prognosis. However, whether this phenotype represents a cause or a result is still unknown. Clearly, a more complete understanding of the role of CXCR4 in sporadic MPNSTs must await further study.

In the present study, we found that CXCR4 expression was positively correlated with that of CXCL12 and Cyclin D1, suggesting that CXCL12 and CXCR4 may act synergistically and interdependently to activate the downstream factor, Cyclin D1. However, although we found that CXCR4 expression was associated with OS of our patients, Cyclin D1 expression showed no apparent prognostic value. As a regulatory subunit of cyclin-dependent kinase 4 (CDK4) or cyclin-dependent kinase 6 (CDK6), Cyclin D1 activity is required for cell cycle G₁/S transition, and many factors, including CXCR4, would regulate the expression and function of Cyclin D1. It is unclear whether CXCL12/CXCR4 signaling regulates Cyclin D1 in sporadic MPNSTs, as is the case in NF1-associated MPNSTs. We will use the human sporadic MPNST cell lines ST88-14 and T265p21 to investigate the role of CXCL12/CXCR4/Cyclin D1 signaling in this disease in our future study.

There are several limitations in our study. First, we did not examine the function of the CXCR4/CXCL12 signaling pathway, which is likely to shed light on the prognostic role of CXCR4 in sporadic MPNSTs. A functional study using sporadic MPNST cell lines might supply more data and clues. Second, we did not investigate the expression pattern or clinical significance of the CXCR4/CXCL12 signaling pathway in NF1-associated or radiotherapy-induced MPNST. However, these are rare disease subtypes, and we hope to perform such an analysis in the future when more samples are collected. Third, we did not investigate the influence of genetic/epigenetic factors on CXCL12 and CXCR4 expression.

In conclusion, CXCR4 expression is positively correlated with that of CXCL12 and Cyclin D1 and associated with OS in Chinese sporadic MPNST patients. High CXCR4 expression may define a specific sporadic MPNST subtype which is associated with better prognosis as compared with NF1-assciated MPNSTs.