Abstract 3718: FOXM1 switches protein conformations upon C-terminal domain-mediated-autoregulation dictating its transcriptional activity in cancer cell proliferation
Authors: Hsu Chia-Chan , Yao Xiang , Wang I-Ching 
Published: April 2023

Abstract
Transcription factor Forkhead box M1 (FOXM1) participates in tumor initiation, tumor development and tumor metastasis through regulating transcriptional network of its target genes. Activated FOXM1 promotes G1/S and G2/M transition via manipulating Cdc25B, Cyclin B1, TOPOIIa, Aurora and Polo-like kinases, as well as cancer cell survival and invasion through upregulating Survivin, SNAIL and MMP-9. The depletion of Foxm1 repressed the oncogenic Kras (G12D) driven mice lung cancer, Rb/Trp53/Myc triple knockout mice small cell lung cancer and carcinogen induced mice hepatocellular carcinoma. Given the roles of FOXM1 in tumorigenesis and its relevance to clinical outcomes, deciphering the mechanism underlying the regulation of FOXM1 activity might provide new therapeutic target for cancer study. It is known that FOXM1 activity drops when the C-terminal transcription activation domain (TAD) is blocked by the N-terminal repression domain (NRD), which is called autorepression. Furthermore, the NRD-deleted FOXM1 displays extraordinarily high transactivity. Compared to the NRD, the TAD is relatively longer and consists of around 420 amino acid with most of them being intrinsically disordered region (IDR). Previous studies showed that the NRD binds to a relatively ordered small region on the TAD. However, it is poorly understood about whether the IDR of TAD partakes in the FOXM1 self-regulated activity and how FOXM1 protein structure switches in living cells. Our co-immunoprecipitation (co-IP) and dual-luciferase reporter assay (DLR) suggested that FOXM1 TAD self-interacts via its IDR and the C-terminal αβα motif, which promotes FOXM1 transactivation. The IDR-driven FOXM1 autoactivation was mimicked by the chemically-induced dimerization system (CID) that forces IDR-αβα proximity intermolecularly. Using fluorescence resonance energy transfer (FRET) microscopy, we observed that the IDR-αβα interaction was elevated when cell cycle progression from G1/S to G2/M in living cells. The IDR-αβα interaction and FOXM1 activity were both suppressed by PLK1 inhibition. Furthermore, the characteristics of IDR-αβα interaction did not resemble NRD-TAD interaction as revealed by FRET data where we showed the NRD-TAD interaction dropped after G1/S. The NRD-TAD interaction underlying FOXM1 autorepression was recapitulated by CID. Through co-IP and yeast-two hybrid assays (Y2H), we further identified the minimal yet biological functional unit ββαβ on NRD that suppressed FOXM1 activity and lung adenocarcinoma A549 cells in vitro and in vivo. Collectively, our work suggested that FOXM1 might be switched from the NRD-TAD mediated autorepression toward the IDR-αβα mediated activation by Plk1 during S-G2 phase. And ββαβ peptide treatment might be applied to lung cancer as a potential therapeutic strategy.
Citation Format: Chia-Chan Hsu, Xiang Yao, I-Ching Wang. FOXM1 switches protein conformations upon C-terminal domain-mediated-autoregulation dictating its transcriptional activity in cancer cell proliferation. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3718.