Date of Award

7-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Sciences

Track

Cancer and Developmental Biology

Research Advisor

Gerard P. Zambetti, Ph.D.

Committee

Andrew M. Davidoff, M.D. Peter J. McKinnon, Ph.D. Joseph T. Opferman, Ph.D. Lawrence M. Pfeffer, Ph.D.

Abstract

Neuroblastoma (NB), the most common extracranial solid tumor in children, accounts for 15% of cancer-related deaths in pediatric patients. Caspase-8 (casp8), a proapoptotic protein, is silenced in approximately, 50-70% of neuroblastoma patient samples. Loss of casp8 has been suggested to increase NB metastasis and correlated, in some studies, with advanced-stage NB. Furthermore, decreased casp8 expression may facilitate neuroblastoma tumorigenesis by protecting cells from cell death mediated by either integrins or chemotherapeutics. Paradoxically, casp8 expression is maintained in 30-50% of NB patient samples giving rise to the possibility that casp8 may provide selective advantages for NB tumorigenesis. Caspase-8 is shown to increase tumor cell adhesion, migration and growth. This drives the question as to how a single protein can promote very opposing functions. One possible explanation is that post- translational modifications may decrease the catalytic activity of casp8 and shift its role towards survival. Caspase-8 has been shown to be phosphorylated by SRC, thus decreasing its apoptotic function and increasing ERK signaling, which supports our hypothesis. Ataxia telangiectasia mutated kinase (ATM), an essential mediator in the DNA double strand break repair mechanism, is activated by chemotherapeutic treatments, radiation, and cellular stress. Caspase-8 has one consensus phosphorylation site that has been reported to be phosphorylated in a genome-wide screen for ATM substrates. Here, we tested whether casp8 expression enhanced primary NB tumorigenesis and whether modulation of casp8 by phosphorylation altered its biological function in NB. For the first time, this study demonstrates that casp8 enhances orthotopic xenograft tumor establishment in low tumorigenic neuroblastoma cells; which, may explain why casp8 expression is maintained in some NB tumors. Furthermore, exposure to DNA damaging agents suppresses the apoptotic function of casp8 via ATM-mediated phosphorylation, thereby shifting the balance between the proapoptotic and prosurvival functions towards cell survival. This outlines a possible mechanism by which tumor cells may avoid cell death when exposed to chemotherapeutic agents.

DOI

10.21007/etd.cghs.2013.0327

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