Polo-Like Kinase 1 Inhibition Kills Glioblastoma Multiforme Brain Tumor Cells in Part Through Loss of SOX2 and Delays Tumor Progression in Mice

Abstract

Glioblastoma multiforme (GBM) ranks among the deadliest types of cancer and given these new therapies are urgently needed. To identify molecular targets, we queried a microarray profiling 467 human GBMs and discovered that polo-like kinase 1 (PLK1) was highly expressed in these tumors and that it clustered with the proliferative subtype. Patients with PLK1-high tumors were more likely to die from their disease suggesting that current therapies are inactive against such tumors. This prompted us to examine its expression in brain tumor initiating cells (BTICs) given their association with treatment failure. BTICs isolated from patients expressed 110-470 times more PLK1 than normal human astrocytes. Moreover, BTICs rely on PLK1 for survival because the PLK1 inhibitor BI2536 inhibited their growth in tumorsphere cultures. PLK1 inhibition suppressed growth, caused G 2 /M arrest, induced apoptosis, and reduced the expression of SOX2, a marker of neural stem cells, in SF188 cells. Consistent with SOX2 inhibition, the loss of PLK1 activity caused the cells to differentiate based on elevated levels of glial fibrillary acidic protein and changes in cellular morphology. We then knocked glial fibrillary acidic protein (GFAP) down SOX2 with siRNA and showed that it too inhibited cell growth and induced cell death. Likewise, in U251 cells, PLK1 inhibition suppressed cell growth, downregulated SOX2, and induced cell death. Furthermore, BI2536 delayed tumor growth of U251 cells in an orthotopic brain tumor model, demonstrating that the drug is active against GBM. In conclusion, PLK1 level is elevated in GBM and its inhibition restricts the growth of brain cancer cells.

Personalizing the Treatment of Pediatric Medulloblastoma: Polo-like Kinase 1 as a Molecular Target in High-Risk Children

“…None of these trials have specifically addressed the possibility that PLK1 inhibitors may be beneficial for the treatment of brain tumors. Our group has demonstrated that PLK1 inhibitors could be used to target glioblastoma (18,40) and another group has published data agreeing with our findings in medulloblastoma (41,42). We illustrate that PLK1 is a promising drug target for medulloblastoma because (i) it is highly expressed in tumors relative to normal brain tissues and (ii) there are small molecule inhibitors that suppress primary medulloblastoma cells and cell lines in vitro and in vivo.…”

“…Primary brain tumor cells were isolated from BTX001 (SHH), BT006 (SHH), BT007 (SHH), BT008 (PNET), BT014 (Group 4), BT274 (SHH), and BT025 (Group 4) and were grown as neurospheres as previously described (17) using Neurocult media (StemCell Technologies; ref. 18). All primary cells were obtained through informed consent in abidance with the respective research ethics board guidelines at British Columbia Children's Hospital and The Hospital for Sick Children.…”

“…All primary cells were obtained through informed consent in abidance with the respective research ethics board guidelines at British Columbia Children's Hospital and The Hospital for Sick Children. A single-cell suspension of Daoy and patient-derived primary medulloblastoma cells (BTX001, BT274, BT014, and BT025) were plated in tumorsphere assays as previously described (18). Tumorspheres, plated in triplicate more than 50 mm, were quantified and photomicrographs were taken after 6 days of culture.…”

“…Cell-cycle analysis was done following 24-hour siRNA or BI2536 treatment using flow cytometry as described by Lee and colleagues (2012) (18). Histone H3 phosphorylation was tested as previously described using P-histone H3 ser10 (Cell Signaling Technology; ref.…”

Asymmetric cell division: Implications for glioma development and treatment

“…Plk1 expression is increased in a manner correlated with degree of anaplasia in human glioma cell lines [187]. Similarly, astrocytoma TPC show an increase in the expression of Plk1 more than 100 fold above that evident for non-neoplastic astrocytes [188]. Furthermore, inhibition of Plk1 in these TPC decreased proliferation and caused G2/M arrest, ultimately leading to apoptosis [188].…”