The homeobox transcription factor Prox1 is related to the Drosophila prospero gene, which mediates cell fate decisions of neuroblasts. Prox1 is a regulator of lymphangiogenesis but it is also expressed in the developing central nervous system. Homeobox proteins are known to play essential role in the determination of cell fate and the development of the body plan. It has been shown in Drosophila that prospero-mediated transcriptional repression of stem cell genes and activation of differentiation genes prevents tumorigenic growth, suggesting that prospero is in fact a tumor suppressor. Due to its homology to prospero, we speculated that prox1 may play a role in glioma tumorigensis. We previously identified the transmembrane glycoprotein podoplanin (pdpn) as a prognostic marker in infiltrating gliomas, the most common and lethal of adult brain tumors, by analyses of gene expression profiling data. Subsequent functional studies revealed that pdpn identified a subpopulation of cells from primary neurophere cultures and fresh tumors that have characteristics of glioma cancer stem cells (GSCs), such as radiation resistance and tumor initiation, and that the protein played a functional role in maintaining the stem cell phenotype. Like prox1, pdpn is expressed both during lymphangiogenesis and neural development and mice null for either protein have a similar phenotype, including lymphedema and the lack of terminal alveoli formation in the lung. While other transcription factors have been implicated in cancer stem cells, such as Bmi-1, prox1 has note previously been implicated to have a role in this tumor-initiating, treatment resistant cell population. We therefore hypothesized that prox1 transcriptionally regulates PDPN expression in GSCs. To test this, we examined gene expression profiling data from a large glioma dataset for correlations between PDPN and PROX1 expression. No correlation was seen between expression of the two genes; however, PROX1 expression was predictive of patient survival independent of tumor grade (p=.0333). Next, we screened primary neurosphere cell lines for levels of prox1 and pdpn by immunoblot. Co-expression with pdpn was observed in all cell lines expressing prox1. To examine the role of prox1 in PDPN expression, we compared the levels of PDPN in cells for whom prox1 expression had been silenced. In preliminary results, real-time PCR for PDPN showed significant decreases in expression in the presence of PROX1-targeting siRNA. Ongoing promotor and functional analyses will validate the role of prox1 in the transcriptional activation of PDPN. Comparison of the expression profiles of isogenic clones differing only in their expression of PROX1 identifies additional transcriptional targets. In summary, we have identified prox1 as a candidate transcription factor in the regulation of the glioma stem cell phenotype.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5147.