Abstract C035: A FoxM1-autoregulation motif serves as an inhibitor to suppress cellular vitality of NSCLC
Authors: Hsu Chia-Chan , Wang I-Ching 
Published: March 2020

Elevated expression level of Forkhead box M1 (FoxM1) transcription factor is commonly found in cancers, which is highly associated with tumor malignancy and poor prognosis of cancer patients. Molecular mechanism and biological function studies showed that FoxM1 regulates tumor initiation and progression in many aspects. In cell proliferation, FoxM1 activates transcription network of cell-cycle regulatory genes, including Cdc25B, Cyclin B1, Skp2, JNK1 and TOPOIIa. Inhibition of FoxM1 reduces expression of cell-cycle regulatory genes, leading to cell cycle blockade in G1/S and G2/M transition. In tumor malignant conversion, FoxM1 enhances epithelial-mesenchymal transition (EMT) as well as invasion and migration of cancer cells by activating SNAIL, MMP-9 expression. Moreover, FoxM1 activates VEGF transcription, which may contribute to tumor survival and proliferation via angiogenesis. Prior research showed that nuclear translocation and activation of FoxM1 depends on subsequential phosphorylations on the C-terminal transactivation domain (TAD) by Ras/Raf/MAPK signaling and Cyclin E/A-CdK complex through S phase to G2 phase respectively. In addition, FoxM1 reduced its transactivity while the TAD interacts with its N-terminus sequence as deletion of N-terminus results in a constitutively active form of FoxM1 (FoxM1-ΔN). These suggest that an intrinsic autoregression of FoxM1 may be relieved through dissociation of N-terminal Inhibitory domain (NID) from C-terminus after phosphorylation on TAD. However, the underlying mechanism of how this interaction occurs and affects biological function remains elusive. To determine the biological functions of respective FoxM1 protein domains, we performed promoter activity assays and demonstrated that a putative ββαβ motif on FoxM1 N-terminus is the region essential for the inhibitory function, which depends on hydrophobic residues within the structure. Simultaneously, the inhibitory function of the ββαβ peptide can be counteracted by expression of PLK1 phosphomimetic mutations on C-terminus (S715D/S724D). Yet, FoxM1S715D/S724D mutant displayed a weaker transactivity than FoxM1-ΔN, indicating that PLK1-mediated phosphorylation on C-terminus is insufficient to relieve N-terminus inhibition. Protein co-immunoprecipitation and yeast-two hybrid assays proved that both ββαβ and αβα motifs are required for the interaction between FoxM1 N- and C-termini, underlying the intramolecular interaction based-autorepression of FoxM1. Additionally, expression of ββαβ peptide suppresses NSCLC A549 and H23 cell growth by inducing cell apoptosis and senescence, suggesting targeting FoxM1 by ββαβ peptide as a potential therapeutic strategy for cancer inhibition.

Citation Format: Chia-Chan Hsu, I-Ching Wang. A FoxM1-autoregulation motif serves as an inhibitor to suppress cellular vitality of NSCLC [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C035. doi:10.1158/1535-7163.TARG-19-C035