The focus of the CMBS is pediatric neuroectodermal tumors including neuroblastoma, brain tumors and Ewings sarcoma.Differentiation and NeuroblastomaSeveral clinical and biologic attributes of pediatric neuroectodermal tumors indicate that alterations in differentiation programs contribute to tumorigenesis. We have utilized the retinoid induced differentiation of neuroblastoma tumor cell lines as a model to define the signal transduction paths that mediate growth control, differentiation and cell death. Through a variety of molecular approaches we have identified a number of genes that are important in neuroblastoma differentiation, localize to cytogenetic regions altered in Neuroblastoma and/or have prognostic significance. Our studies are aimed at assessing how these signaling paths control neuroblastoma differentiation and affect tumor cell biology. Clinically relevant strategies are employed to activate or inhibit signaling paths in order to translate these findings into new therapeutic approaches. ? ? Trks and NB Biology ? In NB tumors, Trks serve as tumor markers; TrkA is expressed in good prognosis tumors. We have found that the differential activation of these signal transduction pathways in NB may alter their growth, invasiveness, chemosensitivity and cell survival. Recently we published that NGF activation of TrkA causes a transcriptional decrease in N-myc levels via activation of the MAPkinase path. Furthermore the decrease in N-myc causes a decrease in cell cycle progression that is mediated by decreases in Cyclin E dependent kinase activity, cdk6 levels and E2F transcription factors expression as well as an increase in p27. By analyzing the key components of several mechanisms required for arresting growth of NB cells we have identified that decreases in N-myc and cdk6 and increases in p27 are key control elements in regulating NB cell cycle progression. Neuroblastoma (NB) tumors expressing high levels of BDNF and TrkB are associated with poor 5-year survival outcomes. Our previous studies indicated that BDNF blocked the cytotoxic effects of vinblastine on NB cells. We evaluated the ability of BDNF to decrease the chemosensitivity of NB cells to a number of common chemotherapeutic agents and mapped the BDNF signaling system that mediates the chemoprotective effect. Two SH-SY5Y NB cell lines (TB3, TB8) expressing TrkB under the control of a tetracycline (Tet)-repressible promoter element were isolated, and used to assess apoptosis resulting from treatment with cisplatin (Cis), doxorubicin (Doxo), etoposide (Etop), and vinblastine (Vbl). BDNF treatment of high TrkB-expressing TB8 (Tet-) and TB3 (Tet-) cells blocked drug-induced cell death in a dose-dependent manner. Only high dose BDNF blocks the effects of chemotherapy in cell with low TrkB expressioin while even low doses of BDNF affect the sensitivity of cells expressing high levels of TrkB. This indicates that the environmental level of TrkB lignads may differentially affect the chemosensitivity of NB cells. The inability of NGF to protect cells indicated that activation of p75 alone was not responsible for the chemoprotective effect. BDNF's ability to rescue the cells is TrkB dependent since the selective Trk tyrosine kinase inhibitor K252a blocks it. The PI3-kinase inhibitors LY294002 and Wortmannin but not the MEK inhibitor PD98059 or the PLC-gamma inhibitor U73122 block the ability of BDNF to rescue cells from etoposide and cisplatin induced cell death. BDNF also protects NGP and KCNR NB cells expressing endogenous TrkB receptors from chemotherapy induced death and inhibition of the PI3-kinase path abrogates this effect. These results indicate that BDNF activation of TrkB via the PI3-kinase path protects NB cells from chemotherapy and that by specifically inhibiting the TrkB TK and/or PI3-kinase paths one may improve the chemosensitivity of NB cells.
