Pten and p53 converge on c-Myc to control differentiation, self-renewal, and transformation of normal and neoplastic stem cells in glioblastoma.

Glioblastoma (GBM) is a highly lethal primary brain cancer with hallmark features of diffuse invasion, intense apoptosis resistance and florid necrosis, robust angiogenesis, and an immature profile with developmental plasticity. In the course of assessing the developmental consequences of central nervous system (CNS)-specific deletion of p53 and Pten, we observed a penetrant acute-onset malignant glioma phenotype with striking clinical, pathological, and molecular resemblance to primary GBM in humans. This primary, as opposed to secondary, GBM presentation in the mouse prompted genetic analysis of human primary GBM samples that revealed combined p53 and Pten mutations as the most common tumor suppressor defects in primary GBM. On the mechanistic level, the "multiforme" histopathological presentation and immature differentiation marker profile of the murine tumors motivated transcriptomic promoter-binding element and functional studies of neural stem cells (NSCs), which revealed that dual, but not singular, inactivation of p53 and Pten promotes cellular c-Myc activation. This increased c-Myc activity is associated not only with impaired differentiation, enhanced self-renewal capacity of NSCs, and tumor-initiating cells (TICs), but also with maintenance of TIC tumorigenic potential. Together, these murine studies have provided a highly faithful model of primary GBM, revealed a common tumor suppressor mutational pattern in human disease, and established c-Myc as a key component of p53 and Pten cooperative actions in the regulation of normal and malignant stem/progenitor cell differentiation, self-renewal, and tumorigenic potential.

A transactivation-deficient mouse model provides insights into Trp53 regulation and function.

The gene Trp53 is among the most frequently mutated and studied genes in human cancer, but the mechanisms by which it suppresses tumour formation remain unclear. We generated mice with an allele encoding changes at Leu25 and Trp26, known to be essential for transcriptional transactivation and Mdm2 binding, to enable analyses of Trp53 structure and function in vivo. The mutant Trp53 was abundant, its level was not affected by DNA damage and it bound DNA constitutively; however, it showed defects in cell-cycle regulation and apoptosis. Both mutant and Trp53-null mouse embryonic fibroblasts (MEFs) were readily transformed by oncogenes, and the corresponding mice were prone to tumours. We conclude that the determining pathway for Trp53 tumour-suppressor function in mice requires the transactivation domain.

A leucine-rich nuclear export signal in the p53 tetramerization domain: regulation of subcellular localization and p53 activity by NES masking.

Appropriate subcellular localization is crucial for regulating p53 function. We show that p53 export is mediated by a highly conserved leucine-rich nuclear export signal (NES) located in its tetramerization domain. Mutation of NES residues prevented p53 export and hampered tetramer formation. Although the p53-binding protein MDM2 has an NES and has been proposed to mediate p53 export, we show that the intrinsic p53 NES is both necessary and sufficient for export. This report also demonstrates that the cytoplasmic localization of p53 in neuroblastoma cells is due to its hyperactive nuclear export: p53 in these cells can be trapped in the nucleus by the export-inhibiting drug leptomycin B or by binding a p53-tetramerization domain peptide that masks the NES. We propose a model in which regulated p53 tetramerization occludes its NES, thereby ensuring nuclear retention of the DNA-binding form. We suggest that attenuation of p53 function involves the conversion of tetramers into monomers or dimers, in which the NES is exposed to the proteins which mediate their export to the cytoplasm.

p53 regulation by post-translational modification and nuclear retention in response to diverse stresses.

p53 activation by diverse stresses involves post-translational modifications that alter its structure and result in its nuclear accumulation. We will discuss several unresolved topics regarding p53 regulation which are currently under investigation. DNA damage is perhaps the best-studied stress which activates p53, and recent data implicate phosphorylation at N-terminal serine residues as critical in this process. We discuss recent data regarding the potential kinases which modify p53 and the possible role of the resulting phosphorylation events. By contrast, much less is understood about agents which disrupt the mitotic spindle. The cell cycle phase, induction signal, and biochemical mechanism of the reversible arrest induced by microtubule disruption are currently under investigation. Finally, a key event in response to any genotoxic stress is the accumulation of p53 in the nucleus. The factors which determine the steady state level of p53 are starting to be elucidated, but the mechanisms responsible for nuclear accumulation and nuclear export remain controversial. We discuss new studies revealing a mechanism for nuclear retention of p53, and the potential contributions of MDM2 to this process.

v-Raf activates transcription of growth-responsive promoters via GC-rich sequences that bind the transcription factor Sp1.

The serine/threonine kinase, Raf-1, is a component of intracellular signaling pathways that control responses to extracellular stimuli. Previously, we have shown that serum-induced transcription from the murine rep-3b and human mdr1 promoters is Raf-dependent and that the activated Raf kinase, v-Raf, induces transcription of mdr1 via a GC-rich element. We now demonstrate that GC-rich sequences in the rep-3b promoter are both necessary and sufficient for induction by v-Raf. The GC-rich, v-Raf-responsive elements of rep-3b and mdr1 bind the general transcription factor Sp1 in electromobility shift assays. Mutation of a minimal GC-rich element abolished inducibility by v-Raf and eliminated binding by the transcription factor Sp1. However, Sp1 binding activity following serum stimulation of quiescent NIH 3T3 cells was unchanged, suggesting that mitogenic signals may stimulate the transactivation potential of prebound Sp1.