PhD

dissertationNeuroblastoma is the most common extracranial solid tumor in children. Despite increases in the intensity of treatment of this tumor over the past several decades, the long-term survival of patients with advanced neuroblastoma remains dismal. This lack of progress has sparked interest in the use of alternative forms of therapy. One promising alternative is the use of chemical agents that induce terminal differentiation of tumor cells. The best candidate agent for this form of treatment is retinoic acid, because it is by far the most effective at inducing differentiation in neuroblastoma cell lines. Retinoic acid mediates its effect on differentiation by modulating basal transcription rates of a specific set of target genes. Therefore, to better understand the molecular basis of retinoic acid- induced differentiation in these cells, cDNA representational difference analysis was used to identify retinoic acid-responsive genes in a neuroblastoma cell line. Four genes that are induced in response to retinoic acid treatment were identified, one of which encodes the autocrine tumor motility-stimulating factor, autotaxin (ATX). Because ATX can induce motility of tumor cells and thus may play a role in metastasis, this finding presents potential implications for the use of differentiation therapy. Based on this, subsequent studies focused on the retinoic acid-mediated regulation of ATX expression in neuroblastoma cells. Interestingly, ATX expression was specifically induced after treatment with the differentiation agent retinoic acid in cell lines that carry amplification of the N-myc oncogene. Consistent with the established mechanism of action of retinoic acid, ATX induction was due to an increase in transcription rate. Since ATX induction in response to retinoic acid treatment strongly correlated with N-myc amplification, the role of N-Myc in regulating retinoic acid-mediated ATX induction was investigated. Experiments indicated that N-Myc does not function downstream of retinoic acid to regulate ATX expression, nor does it function in parallel with retinoic acid to synergistically induce ATX expression. Thus, it is likely that N-Myc itself does not play a direct role in the regulation of ATX expression. Rather, a gene that is coamplified with N-myc may be responsible for retinoic acid-inducible ATX expression that occurs in N-myc amplified cell lines