Responses of tumor cells to therapeutic ionizing radiation include a number of processes largely mediated by the transcription factor p53. To explore the possibility of tumor cell radiosensitization by targeting p53 dependent mechanisms we investigated the role of transcriptional protein kinases CDK8/19 in the balance of cell death vs survival upon irradiation.
To study this process, real-time PCR, immunoblotting, flow cytometry, clonogenic analysis and beta-galactosidase assays were used.
We demonstrated that HCT116 human colon carcinoma cells (wild type p53) underwent a time- and dose dependent arrest in G2/M phase and senescence after 2-6 Gy single dose λ-irradiation. Virtually no subG1 (apoptotic) events were registered. In contrast, irradiation of the isogenic HCT116p53KO subline (non-functional p53) was lethal. These effects were observed in the mass cell culture and clonogenic assays. Irradiation of HCT116 cells induced p53, p21, PUMA and NOXA at mRNA and protein levels whereas none of these markers was up-regulated in HCT116p53KO. These results implicated p53 mediated processes, particularly p53/p21 dependent cell cycle arrest, in the escape of tumor cells from radiation induced death. To prevent the activation of p53 signaling, HCT116 cells were irradiated in the presence of selective CDK8/19 inhibitors (CDK8/19i). Alone, each of two chemically unrelated CDK8/19i (1 μM) had no effect on cell survival for as long as 14 days. However, in combination with 4 Gy, CDK8/19i potently prevented the accumulation of cells at the G2/M boundary and significantly increased the percentage of dead cells. These events were paralleled by an attenuation of p53 and p21 mRNAs and proteins. In the HCT116p53KO subline CDK8/19i did not influence the effects of irradiation.
Our results indicate that CDK8/19 inhibition in the irradiated wild type p53 cells is functionally similar to p53 KO: pharmacological attenuation of p53 responses (HCT116 cells) or genetic inactivation of p53 (HCT116p53KO cells) abrogate the G2/M checkpoint and preclude the repair of irradiation induced damage. Thus, inhibition of CDK8/19 mediated transcriptional reprogramming emerges as a therapeutically attractive approach in tumor radiosensitization.
Legal entity responsible for the study
The Russian Foundation of Basic Research (grant No 20-34-90046).
All authors have declared no conflicts of interest.