There is increasing evidence that tumors are heterogeneous and that a subset of cells act as cancer stem cells. Several protooncogenes and tumor suppressors control key aspects of stem cell function, suggesting that similar mechanisms control normal and cancer stem cell properties. We show here that the prototypical tumor suppressor p53, which plays an important role in brain tumor initiation and growth, is expressed in the neural stem cell lineage in the adult brain. p53 negatively regulates proliferation and survival, and thereby self-renewal, of neural stem cells. Analysis of the neural stem cell transcriptome identified the dysregulation of several cell cycle regulators in the absence of p53, most notably a pronounced downregulation of p21 expression. These data implicate p53 as a suppressor of tissue and cancer stem cell self-renewal.

The family of platelet-derived growth factors (PDGFs) plays a number of critical roles in normal embryonic development, cellular differentiation, and response to tissue damage. Not surprisingly, as it is a multi-faceted regulatory system, numerous pathological conditions are associated with aberrant activity of the PDGFs and their receptors. As we and others have shown, human gliomas, especially glioblastoma, express all PDGF ligands and both the two cell surface receptors, PDGFR-α and -β. The cellular distribution of these proteins in tumors indicates that glial tumor cells are stimulated via PDGF/PDGFR-α autocrine and paracrine loops, while tumor vessels are stimulated via the PDGFR-β. Here we summarize the initial discoveries on the role of PDGF and PDGF receptors in gliomas and provide a brief overview of what is known in this field.

Glioblastomas are the most common and malignant astrocytic brain tumors in human adults. The tumor suppressor gene TP53 is commonly mutated and/or lost in astrocytic brain tumors and the TP53 alterations are often found in combination with excessive growth factor signaling via PDGF/PDGFRalpha. Here, we have generated transgenic mice over-expressing human PDGFB in brain, under control of the human GFAP promoter. These mice showed no phenotype, but on a Trp53 null background a majority of them developed brain tumors. This occurred at 2-6 months of age and tumors displayed human glioblastoma-like features with integrated development of Pdgfralpha+ tumor cells and Pdgfrbeta+/Nestin+ vasculature. The transgene was expressed in subependymal astrocytic cells, in glia limitans, and in astrocytes throughout the brain substance, and subsequently, microscopic tumor lesions were initiated equally in all these areas. With tumor size, there was an increase in Nestin positivity and variability in lineage markers. These results indicate an unexpected plasticity of all astrocytic cells in the adult brain, not only of SVZ cells. The results also indicate a contribution of widely distributed Pdgfralpha+ precursor cells in the tumorigenic process.

Malignant glioma is the most common brain tumor in adults and is associated with a very poor prognosis. Mutations in the p53 tumor suppressor gene are frequently detected in gliomas. p53 is well-known for its ability to induce cell cycle arrest, apoptosis, senescence, or differentiation following cellular stress. That the guardian of the genome also controls stem cell self-renewal and suppresses pluripotency adds a novel level of complexity to p53. Exactly how p53 works in order to prevent malignant transformation of cells in the central nervous system remains unclear, and despite being one of the most studied proteins, there is a need to acquire further knowledge about p53 in neural stem cells. Importantly, the characterization of glioma cells with stem-like properties, also known as brain tumor stem cells, has opened up for the development of novel targeted therapies. Here, we give an overview of what is currently known about p53 in brain tumors and neural stem cells. Specifically, we review the literature regarding transformation of adult neural stem cells and, we discuss how the loss of p53 and deregulation of growth factor signaling pathways, such as increased PDGF signaling, lead to brain tumor development. Reactivation of p53 in brain tumor stem cell populations in combination with current treatments for glioma should be further explored and may become a viable future therapeutic approach.