Identification of Rac guanine nucleotide exchange factors promoting Lgl1 phosphorylation in glioblastoma.

The protein Lgl1 is a key regulator of cell polarity. We previously showed that Lgl1 is inactivated by hyperphosphorylation in glioblastoma as a consequence of PTEN tumour suppressor loss and aberrant activation of the PI 3-kinase pathway; this contributes to glioblastoma pathogenesis both by promoting invasion and repressing glioblastoma cell differentiation. Lgl1 is phosphorylated by atypical protein kinase C that has been activated by binding to a complex of the scaffolding protein Par6 and active, GTP-bound Rac. The specific Rac guanine nucleotide exchange factors that generate active Rac to promote Lgl1 hyperphosphorylation in glioblastoma are unknown. We used CRISPR/Cas9 to knockout PREX1, a PI 3-kinase pathway-responsive Rac guanine nucleotide exchange factor, in patient-derived glioblastoma cells. Knockout cells had reduced Lgl1 phosphorylation, which was reversed by re-expressing PREX1. They also had reduced motility and an altered phenotype suggestive of partial neuronal differentiation; consistent with this, RNA-seq analyses identified sets of PREX1-regulated genes associated with cell motility and neuronal differentiation. PREX1 knockout in glioblastoma cells from a second patient did not affect Lgl1 phosphorylation. This was due to overexpression of a short isoform of the Rac guanine nucleotide exchange factor TIAM1; knockdown of TIAM1 in these PREX1 knockout cells reduced Lgl1 phosphorylation. These data show that PREX1 links aberrant PI 3-kinase signaling to Lgl1 phosphorylation in glioblastoma, but that TIAM1 is also to fill this role in a subset of patients. This redundancy between PREX1 and TIAM1 is only partial, as motility was impaired in PREX1 knockout cells from both patients.

Vitamin D Receptor Activation Attenuates Hippo Pathway Effectors and Cell Survival in Metastatic Neuroblastoma.

Survival for high-risk neuroblastoma remains poor. Most patients who recur, present with metastatic disease, and few targetable pathways that govern spread to distant sites are currently known. We previously developed a metastatic mouse model to select cells with enhanced ability to spread to the bone and brain and identified a signature based on differentially expressed genes, which also predicted patient survival. To discover new neuroblastoma therapies, we utilized the Connectivity Map to identify compounds that can reverse this metastatic transcriptional signature and found calcipotriol, a vitamin D3 analog, to be a compound that selectively targets cell lines with enhanced metastatic potential. Calcipotriol treatment of enhanced metastatic, but not parental, cells reduces proliferation and survival via vitamin D receptor (VDR) signaling, increases the expression of RASSF2, a negative regulator of the Hippo signaling pathway, and reduces the levels of the Hippo pathway effectors YAP and TAZ. RASSF2 is required for the effects of calcipotriol and for the reduction of levels and nuclear localization of YAP/TAZ. Migration of the enhanced metastatic cells and YAP/TAZ levels are reduced after calcipotriol treatment and YAP overexpression reduces calcipotriol sensitivity. Furthermore, metastatic cells that overexpress VDR also showed lower tumor burden in vivo. IMPLICATIONS: This newly identified link between VDR signaling and the Hippo pathway could inform treatment strategies for metastatic neuroblastoma.

Extracellular domain shedding of the ALK receptor mediates neuroblastoma cell migration.

Although activating mutations of the anaplastic lymphoma kinase (ALK) membrane receptor occur in ∼10% of neuroblastoma (NB) tumors, the role of the wild-type (WT) receptor, which is aberrantly expressed in most non-mutated cases, is unclear. Both WT and mutant proteins undergo extracellular domain (ECD) cleavage. Here, we map the cleavage site to Asn654-Leu655 and demonstrate that cleavage inhibition of WT ALK significantly impedes NB cell migration with subsequent prolongation of survival in mouse models. Cleavage inhibition results in the downregulation of an epithelial-to-mesenchymal transition (EMT) gene signature, with decreased nuclear localization and occupancy of ß-catenin at EMT gene promoters. We further show that cleavage is mediated by matrix metalloproteinase 9, whose genetic and pharmacologic inactivation inhibits cleavage and decreases NB cell migration. Together, our results indicate a pivotal role for WT ALK ECD cleavage in NB pathogenesis, which may be harnessed for therapeutic benefit.

Acquisition of a Unique Mesenchymal Precursor-like Blastema State Underlies Successful Adult Mammalian Digit Tip Regeneration.

Here, we investigate the origin and nature of blastema cells that regenerate the adult murine digit tip. We show that Pdgfra-expressing mesenchymal cells in uninjured digits establish the regenerative blastema and are essential for regeneration. Single-cell profiling shows that the mesenchymal blastema cells are distinct from both uninjured digit and embryonic limb or digit Pdgfra-positive cells. This unique blastema state is environmentally determined; dermal fibroblasts transplanted into the regenerative, but not non-regenerative, digit express blastema-state genes and contribute to bone regeneration. Moreover, lineage tracing with single-cell profiling indicates that endogenous osteoblasts or osteocytes acquire a blastema mesenchymal transcriptional state and contribute to both dermis and bone regeneration. Thus, mammalian digit tip regeneration occurs via a distinct adult mechanism where the regenerative environment promotes acquisition of a blastema state that enables cells from tissues such as bone to contribute to the regeneration of other mesenchymal tissues such as the dermis.

Engineering PTEN-L for Cell-Mediated Delivery.

The tumor suppressor PTEN is frequently inactivated in glioblastoma. PTEN-L is a long form of PTEN produced by translation from an alternate upstream start codon. Unlike PTEN, PTEN-L has a signal sequence and a tract of six arginine residues that allow PTEN-L to be secreted from cells and be taken up by neighboring cells. This suggests that PTEN-L could be used as a therapeutic to restore PTEN activity. However, effective delivery of therapeutic proteins to treat CNS cancers such as glioblastoma is challenging. One method under evaluation is cell-mediated therapy, where cells with tumor-homing abilities such as neural stem cells are genetically modified to express a therapeutic protein. Here, we have developed a version of PTEN-L that is engineered for enhanced cell-mediated delivery. This was accomplished by replacement of the native leader sequence of PTEN-L with a leader sequence from human light-chain immunoglobulin G (IgG). This version of PTEN-L showed increased secretion and an increased ability to transfer to neighboring cells. Neural stem cells derived from human fibroblasts could be modified to express this version of PTEN-L and were able to deliver catalytically active light-chain leader PTEN-L (lclPTEN-L) to neighboring glioblastoma cells.

Cotargeting Ephrin Receptor Tyrosine Kinases A2 and A3 in Cancer Stem Cells Reduces Growth of Recurrent Glioblastoma.

Glioblastoma (GBM) carries a dismal prognosis and inevitably relapses despite aggressive therapy. Many members of the Eph receptor tyrosine kinase (EphR) family are expressed by GBM stem cells (GSC), which have been implicated in resistance to GBM therapy. In this study, we identify several EphRs that mark a therapeutically targetable GSC population in treatment-refractory, recurrent GBM (rGBM). Using a highly specific EphR antibody panel and CyTOF (cytometry by time-of-flight), we characterized the expression of all 14 EphR in primary and recurrent patient-derived GSCs to identify putative rGBM-specific EphR. EPHA2 and EPHA3 coexpression marked a highly tumorigenic cell population in rGBM that was enriched in GSC marker expression. Knockdown of EPHA2 and EPHA3 together led to increased expression of differentiation marker GFAP and blocked clonogenic and tumorigenic potential, promoting significantly higher survival in vivo Treatment of rGBM with a bispecific antibody against EPHA2/A3 reduced clonogenicity in vitro and tumorigenic potential of xenografted recurrent GBM in vivo via downregulation of AKT and ERK and increased cellular differentiation. In conclusion, we show that EPHA2 and EPHA3 together mark a GSC population in rGBM and that strategic cotargeting of EPHA2 and EPHA3 presents a novel and rational therapeutic approach for rGBM.Significance: Treatment of rGBM with a novel bispecific antibody against EPHA2 and EPHA3 reduces tumor burden, paving the way for the development of therapeutic approaches against biologically relevant targets in rGBM. Cancer Res; 78(17); 5023-37. ©2018 AACR.

Induction of senescence in primary glioblastoma cells by serum and TGFß.

Glioblastoma is the most common type of adult brain tumour and has a median survival after diagnosis of a little more than a year. Glioblastomas have a high frequency of mutations in the TERT promoter and CDKN2A locus that are expected to render them resistant to both replicative and oncogene-induced senescence. However, exposure of PriGO8A primary glioblastoma cells to media with 10% serum induced a senescence-like phenotype characterized by increased senescence-associated ß galactosidase activity, PML bodies and p21 and morphological changes typical of senescence. Microarray expression analysis showed that 24 h serum exposure increased the expression of genes associated with the TGFß pathway. Treatment of PriGO8A cells with TGFß was sufficient to induce senescence in these cells. The response of PriGO8A cells to serum was dependent on basal expression of the TGFß activator protein thrombospondin. Primary glioblastoma cells from three additional patients showed a variable ability to undergo senescence in response to serum. However all were able to undergo senescence in response to TGFß, although for cells from one patient this required concomitant inhibition of Ras pathway signalling. Primary glioblastoma cells therefore retain a functional senescence program that is inducible by acute activation of the TGFß signalling pathway.

PREX1 integrates G protein-coupled receptor and phosphoinositide 3-kinase signaling to promote glioblastoma invasion.

A defining feature of the brain cancer glioblastoma is its highly invasive nature. When glioblastoma cells are isolated from patients using serum free conditions, they accurately recapitulate this invasive behaviour in animal models. The Rac subclass of Rho GTPases has been shown to promote invasive behaviour in glioblastoma cells isolated in this manner. However the guanine nucleotide exchange factors responsible for activating Rac in this context have not been characterized previously. PREX1 is a Rac guanine nucleotide exchange factor that is synergistically activated by binding of G protein αγ subunits and the phosphoinositide 3-kinase pathway second messenger phosphatidylinositol 3,4,5 trisphosphate. This makes it of particular interest in glioblastoma, as the phosphoinositide 3-kinase pathway is aberrantly activated by mutation in almost all cases. We show that PREX1 is expressed in glioblastoma cells isolated under serum-free conditions and in patient biopsies. PREX1 promotes the motility and invasion of glioblastoma cells, promoting Rac-mediated activation of p21-associated kinases and atypical PKC, which have established roles in cell motility. Glioblastoma cell motility was inhibited by either inhibition of phosphoinositide 3-kinase or inhibition of G protein ßγ subunits. Motility was also inhibited by the generic dopamine receptor inhibitor haloperidol or a combination of the selective dopamine receptor D2 and D4 inhibitors L-741,626 and L-745,870. This establishes a role for dopamine receptor signaling via G protein ßγ subunits in glioblastoma invasion and shows that phosphoinositide 3-kinase mutations in glioblastoma require a context of basal G protein-coupled receptor activity in order to promote this invasion.

Inhibition of glioblastoma malignancy by Lgl1.

lethal giant larvae (lgl) was first identified as a tumor suppressor in Drosophila, where its loss repressed the differentiation and promoted the invasion of neuroblasts, the Drosophila equivalent of the neural stem cell. Recently we have shown that a human homolog of Lgl, Lgl1 (LLGL1), is constitutively phosphorylated and inactivated in glioblastoma cells; this occurs as a downstream consequence of PTEN loss, one of the most frequent genetic events in glioblastoma. Here we have investigated the consequences of this loss of functional Lgl1 in glioblastoma in vivo. We used a doxycycline-inducible system to express a non-phosphorylatable, constitutively active version of Lgl1 (Lgl3SA) in either a glioblastoma cell line or primary glioblastoma cells isolated under neural stem cell culture conditions from patients. In both types of cells, expression of Lgl3SA, but not wild type Lgl1, inhibited cell motility in vitro. Induction of Lgl3SA in intracerebral xenografts markedly reduced the in vivo invasion of primary glioblastoma cells. Lgl3SA expression also induced the differentiation of glioblastoma cells in vitro and in vivo along the neuronal lineage. Thus the central features of Lgl function as a tumor suppressor in Drosophila are conserved in human glioblastoma.

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

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.