Ras-Erk signaling induces phosphorylation of human TLE1 and downregulates its repressor function
T Zahavi1, A Maimon2, T Kushnir1
1Department of Developmental Biology and Cancer Research, IMRIC, Faculty of Medicine, The Hebrew University, Jerusalem, Israel.
Abstract:
Signaling mediated by the Ras-extracellular signal-regulated kinase (Erk) pathway often leads to the phosphorylation of transcriptional regulators, thereby modulating their activity and causing concerted changes in gene expression. In Drosophila, the induction of multiple Ras-Erk pathway target genes depends on prior phosphorylation of the general co-repressor Groucho, a modification that downregulates its repressive function. Here, we show that TLE1, one of the four human Groucho orthologs, is similarly phosphorylated in response to Ras-Erk pathway activation, and that this modification attenuates its capacity to repress transcription. Specifically, unphosphorylated TLE1 dominantly suppresses the induction of Ras-Erk pathway target genes in cultured human cells, and the expression of an unphosphorylatable TLE1 derivative causes severe phenotypes in a transgenic Drosophila model system, whereas a phosphomimetic variant of TLE1 exerts only negligible effects. We present data indicating that TLE1 is rapidly excluded from the nucleus following epidermal growth factor receptor pathway activation, an effect that likely accounts for its inability to mediate effective repression under such conditions. Significantly, we find that unphosphorylated TLE1 blocks oncogenic phenotypes induced by mutated H-Ras in human mammary cells, both in vitro and following their implantation in mice. Collectively, our data strongly indicate that phosphorylation of TLE family members and the consequent downregulation of their repressor function is a key conserved step in the transcriptional responses to Ras-Erk signaling, and possibly a critical event in the tumorigenic effects caused by excessive Ras-Erk pathway activity.
Insights
Phosphorylation of TLE1, a Groucho ortholog, by the Ras-Erk pathway reduces its repressive function. This conserved mechanism impacts gene expression and blocks oncogenic phenotypes driven by mutated H-Ras.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- The Ras-extracellular signal-regulated kinase (Erk) pathway regulates gene expression via transcriptional regulators.
- In Drosophila, Groucho phosphorylation by Ras-Erk signaling is crucial for target gene induction.
- Groucho's phosphorylation downregulates its gene-repressive function.
Purpose of the Study:
- To investigate if human Groucho orthologs, specifically TLE1, are phosphorylated by the Ras-Erk pathway.
- To determine the functional consequences of TLE1 phosphorylation on transcriptional repression and oncogenic signaling.
- To explore the role of TLE1 phosphorylation in Ras-Erk pathway-mediated gene expression changes.
Main Methods:
- Cell culture experiments with human cells to study TLE1 phosphorylation and function.
- Transgenic Drosophila models to assess the in vivo effects of TLE1 phosphorylation.
- In vitro and in vivo assays using human mammary cells to evaluate TLE1's role in oncogenic phenotypes.
Main Results:
- Human TLE1 is phosphorylated upon Ras-Erk pathway activation, reducing its transcriptional repressor activity.
- Unphosphorylatable TLE1 suppresses Ras-Erk target gene induction, while a phosphomimetic variant has minimal effect.
- TLE1 is excluded from the nucleus following epidermal growth factor receptor pathway activation.
- Unphosphorylated TLE1 inhibits oncogenic phenotypes induced by mutated H-Ras in human mammary cells.
Conclusions:
- TLE1 phosphorylation is a conserved mechanism that attenuates repressor function in response to Ras-Erk signaling.
- This phosphorylation event is critical for transcriptional responses to Ras-Erk signaling.
- Dysregulation of TLE1 phosphorylation may contribute to tumorigenesis associated with excessive Ras-Erk pathway activity.
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