*Article* **FOXO1 Is a Key Mediator of Glucocorticoid-Induced Expression of Tristetraprolin in MDA-MB-231 Breast Cancer Cells**

**Do Yong Jeon 1,†, So Yeon Jeong 1,†, Ju Won Lee 1, Jeonghwan Kim 2, Jee Hyun Kim 3, Hun Su Chu 3, Won Jin Jeong 3, Byung Ju Lee 1, Byungyong Ahn 4, Junil Kim 2, Seong Hee Choi 3,\* and Jeong Woo Park 1,\***


**Abstract:** The mRNA destabilizing factor tristetraprolin (TTP) functions as a tumor suppressor by down-regulating cancer-associated genes. *TTP* expression is significantly reduced in various cancers, which contributes to cancer processes. Enforced expression of *TTP* impairs tumorigenesis and abolishes maintenance of the malignant state, emphasizing the need to identify a *TTP* inducer in cancer cells. To search for novel candidate agents for inducing TTP in cancer cells, we screened a library containing 1019 natural compounds using MCF-7 breast cancer cells transfected with a reporter vector containing the *TTP* promoter upstream of the luciferase gene. We identified one molecule, of which the enantiomers are betamethasone 21-phosphate (BTM-21-P) and dexamethasone 21-phosphate (BTM-21-P), as a potent inducer of *TTP* in cancer cells. We confirmed that BTM-21-P, DXM-21-P, and dexamethasone (DXM) induced the expression of *TTP* in MDA-MB-231 cells in a glucocorticoid receptor (GR)-dependent manner. To identify potential pathways linking BTM-21-P and DXM-21-P to *TTP* induction, we performed an RNA sequencing-based transcriptome analysis of MDA-MB-231 cells at 3 h after treatment with these compounds. A heat map analysis of FPKM expression showed a similar expression pattern between cells treated with the two compounds. The KEGG pathway analysis results revealed that the upregulated DEGs were strongly associated with several pathways, including the Hippo signaling pathway, PI3K-Akt signaling pathway, FOXO signaling pathway, NF-κB signaling pathway, and p53 signaling pathway. Inhibition of the FOXO pathway using a FOXO1 inhibitor blocked the effects of BTM-21-P and DXM-21-P on the induction of *TTP* in MDA-MB-231 cells. We found that DXM enhanced the binding of FOXO1 to the *TTP* promoter in a GR-dependent manner. In conclusion, we identified a natural compound of which the enantiomers are DXM-21-P and BTM-21-P as a potent inducer of *TTP* in breast cancer cells. We also present new insights into the role of FOXO1 in the DXM-21-P- and BTM-21-P-induced expression of *TTP* in cancer cells.

**Keywords:** tristetraprolin; FOXO1; dexamethasone; betamethasone; cancer cells
