Abstract C1: GLI1 induces CD133 expression in xenografts derived from transformed astrocytes but not in cultured cells: Extrinsic control of CD133 expression in gliomas

Abstract

Overexpression of transcription factors is often observed in malignant gliomas and the roles of such factors in tumor malignancy have been widely investigated. However, the contribution of glioma-related transcription factors to tumor initiation are largely unclear, as the majority of brain tumor studies have employed glioma-derived cell lines, which recapitulate only a subset of late-stage human tumors. In contrast to tumor-derived cell lines, which harbor numerous undefined genetic elements, normal human cells allow us to generate experimental models of human cancers by direct manipulation with defined genetic elements. Here we demonstrate that the introduction of GLI1 is sufficient for immortalized human astrocytes to be transformed whereas FOXM1 fails to induce oncogenic transformation, suggesting that GLI1, but not FOXM1, directly contributes to gliomagenesis despite both transcription factors being overexpressed in malignant gliomas. While GLI1-transformed astrocytes have the ability to form tumors in nude mice, the oncogenic potential of GLI1 is subtle compared to the active form of H-RAS. To obtain insights into the mechanisms underlying less malignant features of GLI1-transformed astrocytes, expression of inhibitors of cyclin-dependent kinases in xenografts were analyzed by immunohistochemistry. We found that p27KIP1-positive cells were frequently observed in xenografts derived from GLI1-transformed astrocytes (GLI1-xenografts) compared to those from RAS-transformed cells (RAS-xenografts): more than 60% of cells were positive for p27KIP1-staining in GLI1-xenografts whereas only ∼20% in RAS-xenografts. We also made an interesting observation that CD133-positive cells were frequently detected in GLI1-xenografts whereas they were rarely seen in RAS-xenografts. The biological relevance and regulatory mechanisms of CD133 expression in human glioma are not completely understood although the use of CD133 as a marker to sort and enrich for brain tumor stem cells is common. Since GLI1 failed to induce CD133 expression in cultured astrocytes, it is possible that additional factors present in vivo and absent in culture might collaborate with GLI1 to modulate CD133 expression and that tumor microenvironment might contribute to maintenance of brain tumor stem cells. We propose that GLI1-tranformed astrocytes may prove to be useful tools as we strive to understand the molecular mechanisms underlying CD133 expression in gliomas as well as maintenance of brain tumor stem cells. Citation Information: Cancer Res 2009;69(23 Suppl):C1.