Abstract 310: Enhanced stability of D-type cyclins correlates with glioblastoma resistance to ionizing radiation

Abstract

Abstract Transition from G1 to S phase of the cell cycle is promoted by D-type cyclins (D1, D2 and D3), regulatory subunits critical for the activation of cyclin dependent kinases 4 and 6 (CDK4/6), which phosphorylate and inactivate the retinoblastoma protein (pRb) and activate the transcription factor E2F1 to promote the expression of genes essential for DNA synthesis. Deregulated expression of D-type cyclins is frequently observed in human malignancies and correlates with increased proliferation. Cyclin Ds overexpression is also associated with radioresistance, leading to radiotherapy failure and disease recurrence. Cyclin D1 undergoes ubiquitin-dependent degradation following the exposure of cancer cells to ionizing radiation (IR), and this has been shown to be critical for IR-induced G1 growth arrest and sensitivity of cancer cells to IR. While the role of cyclin D1 overexpression in drug resistance is already well documented, less is known about the role of cyclins D2 and D3 in radio-resistance in human cancer cells, particularly in brain neoplasms. In the present study, we investigated the effect of IR on different D-type cyclins (D1, D2, D3) in human glioblastoma cell lines as well as in tumor-derived mouse oligodendrocytes progenitor cells (OPCs). Here, we show that exposure of human astrocytes to increasing levels of IR led to a rapid proteasomal degradation of all three D-type cyclins. On the other hand, IR failed to induce significant cyclin Ds degradation in a number of human glioblastoma cells (U87, A172, U251, Snb19). Analysis of cell cycle profile of all glioblastoma cells tested and exposed to IR showed prominent cell cycle arrest in G1 starting at 24 hours following exposure. Furthermore, we show that the steady-state levels of D-type cyclin is higher in OPCs compared to mouse NIH3T3 cells and fail to undergo proteasomal degradation following IR exposure. Collectively, these results suggest that the degradation of D-type cyclins is not critical for IR-induced G1 cell cycle arrest in GMB and may underlie their resistance to IR. Understanding the role of cyclin Ds in promoting radioresistance may lead to a useful pharmacological strategy to enhance radiotherapy outcome and help implement a more effective treatment modality. Citation Format: Fadila Guessous, Mouadh Benamar, Hui Zhong, Tarek Abbas. Enhanced stability of D-type cyclins correlates with glioblastoma resistance to ionizing radiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 310. doi:10.1158/1538-7445.AM2017-310