CDK7 and CDK9 inhibition interferes with transcription, translation, and stemness, and induces cytotoxicity in GBM irrespective of temozolomide sensitivity

BACKGROUND

Glioblastoma (GBM) is refractory to current treatment modalities while side effects of treatments result in neurotoxicity and cognitive impairment. Here we test the hypothesis that inhibiting CDK7 or CDK9 would effectively combat GBM with reduced neurotoxicity.

METHODS

We examined the effect of a CDK7 inhibitor, THZ1, and multiple CDK9 inhibitors (SNS032, AZD4573, NVP2, and JSH150) on GBM cell lines, patient-derived temozolomide (TMZ)-resistant and responsive primary tumor cells and glioma stem cells (GSCs). Biochemical changes were assessed by western blotting, immunofluorescence, multispectral imaging, and RT-PCR. In vivo, efficacy was assessed in orthotopic and subcutaneous xenograft models.

RESULTS

CDK7 and CDK9 inhibitors suppressed the viability of TMZ-responsive and resistant GBM cells and GSCs at low nanomolar concentrations, with limited cytotoxic effects in vivo. The inhibitors abrogated RNA Pol II and p70S6K phosphorylation and nascent protein synthesis. Furthermore, the self-renewal of GSCs was significantly reduced with a corresponding reduction in Sox2 and Sox9 levels. Analysis of TCGA data showed increased expression of CDK7, CDK9, SOX2, SOX9, and RPS6KB1 in GBM; supporting this, multispectral imaging of a TMA revealed increased levels of CDK9, Sox2, Sox9, phospho-S6, and phospho-p70S6K in GBM compared to normal brains. RNA-Seq results suggested that inhibitors suppressed tumor-promoting genes while inducing tumor-suppressive genes. Furthermore, the studies conducted on subcutaneous and orthotopic GBM tumor xenograft models showed that administration of CDK9 inhibitors markedly suppressed tumor growth in vivo.

CONCLUSIONS

Our results suggest that CDK7 and CDK9 targeted therapies may be effective against TMZ-sensitive and resistant GBM.

Abstract 2310: Targeting transcription-associated CDKs is an effective way to combat glioblastoma and medulloblastoma with minimal effect on primary neurons

Glioblastoma multiforme (GBM) is the most prevalent and malignant brain tumor in adults. Currently there are no effective therapies to manage the disease efficiently. Medulloblastoma is the most common type of pediatric tumor and accounts for approximately 15% of all pediatric brain tumors. We aimed to determine if targeting the transcription-associated cyclin-dependent kinases, cdk7 and cdk9, using specific inhibitors, could interfere with the growth and metastatic properties of glioblastoma and medulloblastoma cell lines. We tested the effectiveness of Flavopiridol, a known inhibitor of cdk4, cdk6, pTEFb and cdk9; THZ1, a cdk7 inhibitor; and SNS032, which is known to inhibit cdk2, cdk7 and cdk9, on U87 and SNB19 glioblastoma, H4 neuroglioma and Daoy medullobastoma cells. Treatment with these pharmacological agents showed very strong cytotoxic effects on these cancer cells as measured by MTT (3-(4, 5-dimethylthiazol-2yl)-2, 5-diphenyl tetrazolium bromide) assay. Among these agents the cdk7 inhibitor was most effective and showed efficacy at nanomolar concentrations. Additionally, the inhibitors interfered with the cancer cell migration, as measured by wound healing assay, and anchorage independent growth, as measured by soft agar colony formation. In stark contrast, the inhibitors had no cytotoxic effects on primary neurons. Mechanistically, western blot analysis showed that the tumor cells treated with the inhibitors had reduced levels of Pol II C-terminal domain phosphorylation, indicative of its inhibition and suppression of transcription. Furthermore, inhibitor treatment resulted in a significant reduction in p70S6 kinase phosphorylation, suggesting that in addition to the transcriptional machinery, the translational machinery is also affected upon treatment with the aforementioned cdk inhibitors. Immunostaining analysis of the cells showed a marked reduction in the levels of P-Pol II and P-p70S6 kinase upon inhibitor treatment, confirming that the cdk inhibitors indeed interfere with both transcription and translation. These inhibitors appear to be working in a p53-dependent fashion. These novel findings shed light on potential mechanisms that can be targeted to combat both glioblastoma and medulloblastoma effectively. More specifically, our studies imply that cdk7 and cdk9 inhibitors may serve as potential therapies for effective management of brain tumor.

Citation Format: Isha Bhutada, Srikumar Chellappan, Jaya Padmanabhan. Targeting transcription-associated CDKs is an effective way to combat glioblastoma and medulloblastoma with minimal effect on primary neurons [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2310.