CELL CYCLE TRANSCRIPTION CONTROL BY UBIQUITIN SIGNALING

Abstract

The cell cycle is a tightly regulated series of molecular events which dictates proliferation. Both the timely activation of genes through transcription and destruction of proteins through the ubiquitin-proteasome system are integral to normal cell cycles. Dysregulation of these networks often underlie a variety of malignant diseases such as cancer. Forkhead box protein M1 (FOXM1) is an essential cell cycle transcription factor. FOXM1 regulates a transcriptional network that controls the G2/M transition and G1/S transition. Additionally, aberrant upregulation of the FOXM1 transcriptional network is linked to a variety of cancers. The kinases which activate FOXM1 are well explored, but the influence of the ubiquitin-proteasome system on FOXM1 remains unclear. Here, I described the role that two such enzymes, the E3 ubiquitin ligase CUL4-VPRBP and the deubiquitinating enzyme (DUB) USP21, have on the stability and activity of FOXM1 in both normal and dysregulated cell cycles. First, I demonstrate that FOXM1 degradation is enhanced by association with CUL4-VPRBP. Depletion of VPRBP enhances FOXM1 stability and causes mitotic entry defects. Interestingly, overexpression of VPRBP enhances both FOXM1 ubiquitination and transcriptional activity by a process that occurs independent of CUL4. Finally, VPRBP and FOXM1 levels are assessed in high-grade serous ovarian cancer (HGSOC) patient tumors, demonstrating a plausible mechanism for FOXM1 activation. Second, I demonstrate that FOXM1 is protected from degradation through association with the DUB USP21. Knockdown or overexpression of USP21 is able to destabilize or stabilize FOXM1, respectively, through deubiquitination of FOXM1. USP21 is able to influence mitotic entry and proliferation through regulating the FOXM1 transcriptional network. Furthermore, USP21 and FOXM1 are both significantly amplified in basal-like breast cancer with the knockdown of both sensitizing cells to the chemotherapy paclitaxel thus describing a novel combination treatment for this disease. Taken together, these results contribute to our understanding of how the ubiquitin-proteasome system positively and negatively regulates the abundance and activity of FOXM1. The research presented here further extends our understanding of the network of interactions regulating normal cell cycle dynamics and provide mechanistic and novel therapeutic insights into the promotion and treatment of cancer.Doctor of Philosoph