Introduction
Neuroblastoma is the most common extracranial childhood cancer and the most common tumor occurring during infancy. It is an embryonal malignancy of the sympathetic nervous system arising from neuroblasts (pluripotent sympathetic cells). In the developing embryo, these cells invaginate, migrate along the neuraxis, and populate the sympathetic ganglia, adrenal medulla, and other sites. The pattern of distribution of these cells correlates with the sites of primary disease presentation. There are three distinct neuroblastoma cell types- I-type stem cells, N-type neuroblastic/ neuroendocrine precursors, and S-type schwannian cell/ melanoblastic precursors (Ross et.al., 2003). The S- type cells form monolayers in culture and show contact inhibition, whereas, the N-type cells grow as mutilayers with focal aggregates in culture (Ross et.al., 2003).
Crucial events for the invasion and spread of tumoral cells are the ability to digest the extracellular matrix, migrate, cross blood vessel walls, and reach the circulation (Kramer et.al., 2001). It is known that many growth factors can produce some of the characteristic features of invasive growth like proliferation, motility, and increased survival. Among these, platelet-derived growth factor (PDGF), a mitogen mostly for cells of mesenchymal origin, was shown to induce chemotaxis and actin reorganization and prevent cells from dying by apoptosis (Heldin et.al., 1998). Like a variety of other neural crest-derived tumor cell lines, neuroblastoma cells express specific transcripts of PDGF genes and both ά and ß PDGF receptor genes (Matsui et.al., 1993). It is thought that the simultaneous presence of PDGF and its receptor genes contributes to tumor cell growth and motility (Matsui et.al., 1993). The biological behavior of neuroblastoma can vary, in that, some tumors regress spontaneously, whereas others progress despite aggressive treatment. Infants diagnosed during their first year of life have a good prognosis, even in the presence of metastatic disease, whereas older patients with metastatic disease fare poorly, even when treated with aggressive therapy (Ribatti et.al., 2004). Unfortunately, approximately 70-80% of patients older than 1 year are diagnosed with metastatic disease, usually to lymph nodes, liver, bone, and bone marrow (Ribatti et.al., 2004). Fewer than half of these patients are cured, even with the use of high-dose therapy followed by bone marrow or stem cell rescue (Ribatti et.al., 2004).
Many chromosomal and molecular abnormalities have been identified in neuroblastoma. These biologic markers have been evaluated to determine their value in assigning prognosis, and some of these have been incorporated into the strategies used for risk-assignment. The most important of these biologic markers is MYCN. MYCN is an oncogene often expressed in neuroblastoma (Guo et.al., 2000). This gene is amplified in approximately 25% of de novo cases and is more common in patients with advanced-stage disease (Guo et.al., 2000). Patients whose tumors have MYCN amplification tend to have rapid tumor progression and a poor prognosis (Spitz et.al., 2004). Studies have also shown that neuroblastomas contain multiple cell phenotypes and this feature is often used to determine the prognosis of the disease. The phenotypically distinct neuroblastoma cell lines have been reported to show varying levels and differential expression of the MYCN gene. Higher expression of MYCN corresponds to a poorer prognosis. Most N and I type cell lines have higher levels of MYCN and some show overexpression of the MYCN gene. In the S-type cell line MYCN expression is reported to be down regulated (Spengler et.al., 1997). New therapeutic approaches are needed to improve the prognosis of neuroblastoma patients with high risk disease. Receptor tyrosine kinases have been proposed as potential targets for antitumor therapy because tyrosine kinase receptors play a role in angiogenesis, an essential step for tumor growth and metastasis (Beppu et. al., 2004). Anti-angiogenic cancer therapies are attracting increasing attention. Neuroblastoma cells express platelet- derived growth factor (PDGF), stem cell factor (SCF), and vascular endothelial growth factor (VEGF) and their respective receptors, PDGFR, c-Kit, and Flk-1. Interest has predominantly focused on interfering with endothelial mitogens such as VEGF and FGF. However, increasing evidence also implicate PDGF receptor signaling in tumor angiogenesis. The aim of this project is to explore PDGFR ß as a potential target for antitumor therapy in neuroblastoma patients. By investigating whether or not there are differences in the expression of platelet derived growth factor receptor-beta between phenotypically distinct neuroblastoma cell lines and then determining if these differences can be seen on the protein level we can begin to consider that this gene may play a role in the generation of neuroblastoma.
