Nuclear S6K1 Enhances Oncogenic Wnt Signaling by Inducing Wnt/β-Catenin Transcriptional Complex Formation

Abstract

Ribosomal protein S6 kinase 1 (S6K1), a key downstream effector of the mammalian target of rapamycin (mTOR), regulates diverse functions, such as cell proliferation, cell growth, and protein synthesis. Because S6K1 was previously known to be localized in the cytoplasm, its function has been mainly studied in the cytoplasm. However, the nuclear localization and function of S6K1 have recently been elucidated and other nuclear functions are expected to exist but remain elusive. Here, we show a novel nuclear role of S6K1 in regulating the expression of the Wnt target genes. Upon activation of the Wnt signaling, S6K1 translocated from the cytosol into the nucleus and subsequently bound to β-catenin and the cofactors of the Wnt/β-catenin transcriptional complex, leading to the upregulation of the Wnt target genes. The depletion or repression of S6K1 downregulated the Wnt target gene expression by inhibiting the formation of the Wnt/β-catenin transcriptional complex. The S6K1-depleted colon cancer cell lines showed lower transcription levels of the Wnt/β-catenin target genes and a decrease in the cell proliferation and invasion compared to the control cell lines. Taken together, these results indicate that nuclear S6K1 positively regulates the expression of the Wnt target genes by inducing the reciprocal interaction of the subunits of the transcriptional complex.

Human beta defensin-3 mediated activation of β-catenin during human respiratory syncytial virus infection: interaction of HBD3 with LDL receptor-related protein 5

“…For these studies, we transfected A549 cells with TOP-Flash luciferase reporter plasmid. TOP-Flash luciferase reporter contains a promoter for binding (and activation) of activated TCF/LEF transcriptional factors upstream of the luciferase gene ( Lin et al, 2016 ; Lee et al, 2022 ). Therefore, TOP-Flash luciferase reporter is widely used to study transcriptional activation by β-catenin.…”

“…To validate the loss of β-catenin activity in Δ-catenin cells, we transfected FL-catenin and Δ-catenin cells with TOP-Flash luciferase reporter plasmid. Transfected cells were then treated with lithium chloride (LiCl), a potent inducer of β-catenin-mediated transcriptional activity ( Clément-Lacroix et al, 2005 ; Lin et al, 2016 ; Lee et al, 2022 ). Luciferase assay revealed robust induction of β-catenin mediated transcriptional activity in FL-catenin cells ( Figure 3B ).…”