Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a tumor suppressor that is lost in many human tumors and encodes a phosphatidylinositol phosphate phosphatase specific for the 3-position of the inositol ring. Here we report a novel mechanism of PTEN regulation. Binding of di-C8-phosphatidylinositol 4,5-P 2 (PI(4,5)P 2 ) to PTEN enhances phosphatase activity for monodispersed substrates, PI(3,4,5)P 3 and PI(3,4)P 2 . PI(5)P also is an activator, but PI(4)P, PI(3,4)P 2 , and PI(3,5)P 2 do not activate PTEN. Activation by exogenous PI(4,5)P 2 is more apparent with PI(3,4)P 2 as a substrate than with PI(3,4,5)P 3 , probably because hydrolysis of PI(3,4)P 2 yields PI(4)P, which is not an activator. In contrast, hydrolysis of PI(3,4,5)P 3 yields a potent activator, PI(4,5)P 2 , creating a positive feedback loop. In addition, neither di-C4-PI(4,5)P 2 nor inositol trisphosphateactivated PTEN. Hence, the interaction between PI(4,5)P 2 and PTEN requires specific, ionic interactions with the phosphate groups on the inositol ring as well as hydrophobic interactions with the fatty acid chains, likely mimicking the physiological interactions that PTEN has with the polar surface head groups and the hydrophobic core of phospholipid membranes. Mutations of the apparent PI(4,5)P 2 -binding motif in the PTEN N terminus severely reduced PTEN activity. In contrast, mutation of the C2 phospholipid-binding domain had little effect on PTEN activation. These results suggest a model in which a PI(4,5)P 2 monomer binds to PTEN, initiates an allosteric conformational change and, thereby, activates PTEN independent of membrane binding.Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) 1 (also known as MMAC and TEP) was originally identified as a tumor suppressor for gliomas (1-3). We now know that PTEN is deleted or inactivated in many tumor types, including endometrial, melanoma, and prostate (4). The PTEN protein is a phosphatidylinositol phosphate (PIP) phosphatase specific for the 3-position of the inositol ring (5). By lowering levels of phosphatidylinositol 3,4,5-P 3 (PI(3,4,5)P 3 ), PTEN inhibits cell proliferation, induces apoptosis and decreases cell motility (6, 7). Knock-out mice lacking PTEN are embryonic lethal, whereas mice with one copy of PTEN have a diminished Fas-mediated apoptosis and a high incidence of cancer (8).Neural precursor cells from PTEN ϩ/Ϫ mice show enhanced cell motility and invasiveness and are resistant to hydrogen peroxide-induced apoptosis (9). In response to some mitogens, PTEN ϩ/Ϫ lymphocytes proliferate faster than ϩ/ϩ lymphocytes (8, 10). These results demonstrate that cells are highly quantitatively dependent on PTEN levels, and hence, understanding regulation of PTEN activity is critical. Here we report a novel mechanism of PTEN regulation in which binding of phosphatidylinositol 4,5-P 2 (PI(4,5)P 2 ) to PTEN activates the phosphatase for monodispersed substrates, PI(3,4,5)P 3 and PI(3,4)P 2 , likely by an allosteric conformational change. EXPERIMENTAL PROCEDURESPhos...

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Activating transcription factor 5 (ATF5) is highly expressed in malignant glioma and plays an important role in promoting cell survival. Here we perform a genome-wide RNA interference (RNAi) screen to identify transcriptional regulators of ATF5. Our results reveal an essential survival pathway in malignant glioma, whereby activation of a RAS/MAPK or PI3K signaling cascade leads to induction of the transcription factor CREB3L2, which directly activates ATF5 expression. ATF5, in turn, promotes survival by stimulating transcription of MCL1, an anti-apoptotic BCL2 family member. Analysis of human malignant glioma samples indicates that ATF5 expression inversely correlates with disease prognosis. The RAF inhibitor sorafenib suppresses ATF5 expression in glioma stem cells and inhibits malignant glioma growth in cell culture and mouse xenografts. Our results demonstrate that ATF5 plays an essential role in malignant glioma genesis, and reveal that the ATF5-mediated survival pathway described here provides potential therapeutic targets for treatment of malignant glioma.

A cancer stem cell is defined as an undifferentiated cell with the ability to self-renew, differentiate to multiple lineages and initiate tumors that mimic the parent tumor. In this review, we focus on glioblastomas, describing recent progress and problems in characterizing these cells. There have been advances in cancer stem cell culture, but tumor cell heterogeneity has made purification of cancer stem cells difficult. Indeed, it may be that cancer stem cells significantly vary from tumor to tumor. We also discuss the proposal that cancer stem cells are resistant to radiotherapy and chemotherapy and play a major role in repopulating tumors following treatment. To overcome their resistance to conventional therapies, we may be able to use our extensive knowledge of the signaling pathways essential for stem cells during development. These pathways have potential as targets for new glioblastoma therapies. Hence, although there is an ongoing debate on the nature of cancer stem cells, the theory continues to suggest new ideas for both the lab and the clinic.