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H. Yi, S. Tell, M. Jockovich, T. G. Murray, A. S. Hackam; The Wnt Signaling Pathway Has Tumor Suppressor Properties in Retinoblastoma. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1593.
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© ARVO (1962-2015); The Authors (2016-present)
Retinoblastoma (RB) is a pediatric retinal tumor caused by mutational inactivation of the tumor suppressor pRb. Tumor initiation is believed to require changes in additional genes. Regulators of the Wnt signaling pathway, an essential signaling cascade, are candidate genes because Wnt activating mutations are implicated in the pathogenesis of many other cancer types. In this study we explored the role of Wnt signaling in retinoblastoma.
The viability of the human RB cell lines Y79, Weri-Rb1, Rb355 and the mouse SJmRBL8 cell line was measured by Wst-1 assays. Wnt activity was measured by luciferase reporter assays in the cell lines and by immunolocalization of beta-catenin in tumors in humans and in LHbetaTag mice. Flow cytometry was used for cell cycle analysis and microarray analysis was performed by Ocean Ridge Biosciences (Jupiter, FL) using 70-mer oligo arrays. Log-transformed spot intensities were normalized by subtraction of the array mean. Differentially expressed transcripts were identified by single-factor ANOVA using a Bayesian error model.
Wnt signaling was suppressed in all four RB cell lines, a finding that differs from other tumor types in which elevated Wnt signaling is essential for tumor pathogenesis. Wnt signaling was also suppressed in human (n=4) and mouse retinoblastoma tumors (n=5). Notably, Wnt signaling activators significantly reduced the viability of the four RB cell lines by up to 70% (p<0.001) and the Wnt inhibitor Dkk1 reversed this viability decrease. The mechanism of reduced viability was identified by showing that Wnt activation lead to significant cell cycle arrest in the cell lines (p<0.01), which was associated with a 3-fold increase of p53 protein and decreased expression of genes encoding cyclin-dependent kinases, metabolic enzymes and calcium regulators.
The anti-proliferative activity of Wnt in retinoblastoma contrasts with its growth promoting effects in other types of tumors. Our studies are consistent with the Wnt pathway acting as a tumor suppressor in retinoblastoma and suggest that loss of Wnt signaling is tumorigenic in the retina. This study suggests that Wnt ligands should be explored for their potential as a novel therapeutic strategy for retinoblastoma.
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