γ-Secretase inhibitor-resistant glioblastoma stem cells require RBPJ to propagate

Abstract

Targeting glioblastoma stem cells with γ-secretase inhibitors (GSIs) disrupts the Notch pathway and has shown some benefit in both pre-clinical models and in patients during phase I/II clinical trials. However, it is largely unknown why some glioblastoma (GBM) does not respond to GSI treatment. In this issue of the JCI, Xie et al. determined that GSI-resistant brain tumor-initiating cells (BTICs) from GBM express a higher level of the gene RBPJ, which encodes a mediator of canonical Notch signaling, compared to non-BTICs. Knockdown of RBPJ in BTICs decreased propagation in vitro and in vivo by inducing apoptosis. Interestingly, RBPJ was shown to regulate a different transcription program than Notch in BTICs by binding CDK9, thereby affecting Pol II-regulated transcript elongation. Targeting CDK9 or c-MYC, an upstream regulator of RBPJ, with small molecules also decreased BTIC propagation, and prolonged survival in mice bearing orthotopic GBM xenografts. This study not only provides a mechanism for GSI treatment resistance, but also identifies two potential therapeutic strategies to target GSI-resistant BTICs.

Figure 1. Targeting tumor cells with elevated level Notch activity with γ-secretase inhibitors (GSIs) or shRBPJ.

Tumor cells with elevated levels of Notch activity can be divided into Notch signaling–dependent and –independent classes, which are based on their genetic or epigenetic background. For Notch-dependent tumor cells, GSI treatment can reduce Notch target gene expression and decrease propagation. However, shRPBJ in these same cells will release RBPJ-mediated repression of gene transcription, induce expression of Notch target genes, and increase tumor cell propagation. In contrast, in Notch-independent tumor cells, or GSI-resistant cells, GSI treatment still can block NICD formation and decrease Notch target gene expression. However, GSI treatment has no effect on propagation, because growth of Notch-independent tumor cells depends on genes that are not Notch targets. In this issue, Xie et al. demonstrate that knockdown of RBPJ in Notch-independent cells downregulates expression of genes, including FOXM1, CCNA2, and KRAS, that control brain tumor–initiating cell (BTIC) self-renewal and proliferation, and thereby decreases tumor growth through partnering with CDK9, which regulates elongation of RBPJ target genes. Furthermore, Xie et al. found that RBPJ is regulated by MYC and that blocking MYC expression, upstream of RBPJ, with JQ1 or blocking activity of CDK9 also decreases Notch-independent tumor cell propagation. Solid blue lines indicate results from Xie et al. Solid black lines indicate results from other reports.