RF11 | PSAT369 CDK7 and CDK9 Inhibition Induces RNA Polymerase II Ubiquitination and Proteasome Degradation in Thyroid Cancer Cells

"Superhenhanced" transcription of specific oncogenes by aberrant looping of upstream enhancer elements to marked transcriptional regulatory regions is a mechanism of oncogene overexpression. Therefore, compounds that target key positive regulators of RNA polymerase II (RNAPII) activity, such as CDK7 and CDK9, can relatively selectively reduce expression of such superenhanced oncogenes and are in clinical trials. We have previously reported activity of the CDK7 inhibitor THZ1 in medullary thyroid cancer (MTC) along with identification of a superenhancer in RET. CDK7 inhibitors also are reported to be active in anaplastic thyroid cancer cell lines. In the present study, we extend this work to both CDK7 and CDK9 inhibitors, and to a more extensive set of thyroid cancer cell lines. We selected 8 thyroid cancer cell lines (TPC1, FTC133, BCPAP, SW1376; K1, THJ-16T, C643, and 8505C) from a variety of histological subtypes with different driver mutations for treatment with a CDK7 (THZ1) or CDK9 (AZD4573) inhibitors. Cell viability assays were performed to determine IC50 and Western blots were performed for target validation. IC50 values ranged from 5-100 nM for both compounds for all cell lines demonstrating broad activity and robust activity. Unexpectedly, both compounds inhibited both CDK7 RNAPII phosphorylation (Ser5) and CDK9 RNAPII phosphorylation (Ser2) due to a loss of total RNAPII protein levels in all cell lines. Further time course studies two cell lines, 8505C (BRAFV600E) and TPC1 (RET/PTC) showed the loss off RNAPII occurred rapidly (1-3 hours) with stable RNAPII mRNA levels by qRTPCR. We therefore hypothesized that RNAPII stalling induced by inhibition of CDK7 may cause loss of RNAPII protein stability through ubiquitination and proteasome-mediated degradation. Immunoprecipitation-Western blots demonstrated variable baseline levels of RNAPII ubiquitination in 8505C (low) and TPC1 (high) cells. THZ1 treatment induced ubiquitination of RNAPII in 8505C cells signficantly, while a modest increase beyond the higher baseline occurred in TPC1. Bortezomib (proteasome inhibitor) treatment rescued THZ1-reduced RNAPII protein levels in both cell lines. In summary, human thyroid cancer cell lines are broadly sensitive to CDK7 and CDK9 inhibitors. CDK 7 and 9 inhibitors reduce RNAPII levels and THZ1 induces RNAPII ubiquitination and proteasome degradation demonstrating a new mode of action for the reduced RNAPII activity induced by these inhibitors. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m., Saturday, June 11, 2022 1:42 p.m. - 1:47 p.m.