PTEN loss represses glioblastoma tumor initiating cell differentiation via inactivation of Lgl1

Alexander Gont,Sylvie J Lavictoire,Amin Kassam,John Woulfe,Ian J Restall,Ian Aj Lorimer,Manijeh Daneshmand,Mathieu Soucie,Vasco F Da Silva,Doris A E Parolin,Garth Nicholas,Jennifer E L Hanson

doi:10.18632/oncotarget.1164

Alexander Gont, Sylvie J Lavictoire + Show 10 more

Open Access

https://doi.org/10.18632/oncotarget.1164

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Abstract

Glioblastoma multiforme is an aggressive and incurable type of brain tumor. A subset of undifferentiated glioblastoma cells, known as glioblastoma tumor initiating cells (GTICs), has an essential role in the malignancy of this disease and also appears to mediate resistance to radiation therapy and chemotherapy. GTICs retain the ability to differentiate into cells with reduced malignant potential, but the signaling pathways controlling differentiation are not fully understood at this time. PTEN loss is a very common in glioblastoma multiforme and leads to aberrant activation of the phosphoinositide 3-kinase pathway. Increased signalling through this pathway leads to activation of multiple protein kinases, including atypical protein kinase C. In Drosophila, active atypical protein kinase C has been shown to promote the self-renewal of neuroblasts, inhibiting their differentiation along a neuronal lineage. This effect is mediated by atypical protein kinase c-mediated phosphorylation and inactivation of Lgl, a protein that was first characterized as a tumour suppressor in Drosophila. The effects of the atypical protein kinase C/Lgl pathway on the differentiation status of GTICs, and its potential link to PTEN loss, have not been assessed previously. Here we show that PTEN loss leads to the phosphorylation and inactivation of Lgl by atypical protein kinase C in glioblastoma cells. Re-expression of PTEN in GTICs promoted their differentiation along a neuronal lineage. This effect was also seen when atypical protein kinase C was knocked down using RNA interference, and when a non-phosphorylatable, constitutively active form of Lgl was expressed in GTICs. Thus PTEN loss, acting via atypical protein kinase C activation and Lgl inactivation, helps to maintain GTICs in an undifferentiated state.

Highlights

Glioblastoma multiforme is an aggressive type of adult brain tumor
To detect phosphorylated transduced Lgl1, a phospho(Ser) protein kinase C (PKC) substrate antibody was used; in total cell extracts this labeled a prominent band of the expected size in cells transduced with Lgl1, but not in cells transduced with Lgl3SA (Figure 1B)
Knockdown of PKCι reduced the intensity of this band, confirming that PKCι is responsible for Lgl1 phosphorylation in these cells (Figure 1D)

Summary

Introduction

Glioblastoma multiforme is an aggressive type of adult brain tumor. As this disease invariably exhibits extensive intracerebral dissemination at the time of diagnosis. Patients are commonly treated with radiation and the alkylating agent temozolomide; this prolongs survival but is not curative [1]. A characteristic histological feature of glioblastoma is that glioblastoma cells show considerable phenotypic heterogeneity. One factor contributing to this heterogeneity is that glioblastoma cells exist in different differentiation states [2]. A subset of glioblastoma cells are relatively undifferentiated; these are variously referred www.impactjournals.com/oncotarget

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