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S. Tell, H. Yi, M. Jockovich, T.G. Murray, A.S. Hackam; The Canonical Wnt Signaling Pathway Regulates the Viability of Retinoblastoma Tumor Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2806.
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© ARVO (1962-2015); The Authors (2016-present)
Retinoblastoma (RB) tumors, caused by mutations in the Rb1 gene, originate from malignant transformation of a multipotent retinal progenitor cell. Evidence suggests that the Wnt pathway, a critical cellular signaling pathway is involved in RB tumorgenesis, including the expression of Wnt pathway genes in retinal progenitors and the biochemical interaction of the Rb1 and Wnt pathways. In an ongoing study to identify new therapies for RB, we have characterized Wnt signaling in human tissue and in mouse and cellular RB models.
Immunolocalization of beta–catenin was used as a marker of active Wnt signaling in humans and in LHBETATAG mice, which have functional inactivation of Rb1 from transgenic expression of SV40 large T antigen. The Wst–1 assay was used to measure viability of the RB cell line Y79, and RT–PCR was used for gene expression analyses.
In contrast to other cancers, very few RB tumor cells exhibited nuclear beta–catenin in mouse and human, indicating that Wnt signaling is suppressed. Western blotting for beta–catenin showed that Y79 cells also had low levels of Wnt signaling, which was associated with reduced expression of Wnt regulatory genes. To develop a hypothesis of the importance of the Wnt pathway to RB, we exogenously activated Wnt signaling in Y79 cells. Incubation with Wnt3a ligand, which acts upstream in the Wnt pathway, decreased viability by 75% (n=4, p<0.01). In contrast, activation of Wnt signaling using SB216763, which acts downstream by inhibiting GSK3beta, increased Y79 viability by 61%. Furthermore, SB216763 treatment conferred resistance to the pro–apoptotic agents staurosporine (82% higher than control, n=4, p<0.001) and etoposide (72% higher than control, n=3).
Wnt signaling is markedly reduced in RB tumors, suggesting that it has a negative effect on tumor survival. We show that the Wnt pathway controls viability in a cellular RB model. Notably, different Wnt signaling inducers have opposite effects, possibly due to altered expression of key pathway mediator proteins. These data suggest that Wnt ligands should be explored for their potential as a novel therapeutic strategy for retinoblastoma.
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