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Negative feedback regulation of the tumor suppressor PTEN by phosphoinositide-induced serine phosphorylation
Diana Birle1, Nunzio Bottini, Scott Williams
1Program of Signal Transduction, Burnham Institute, La Jolla, CA 92037, USA.
Abstract:
The PTEN tumor suppressor phosphatase directly counteracts the multiple functions of phosphatidylinositol 3-kinase by removing phosphate from the D3 position of inositol phospholipids. Like many lymphomas and leukemias, the Jurkat T cell line lacks PTEN protein due to frame-shift mutations in both PTEN alleles and therefore survives in long-term cell culture. We report that PTEN reintroduced into Jurkat was highly phosphorylated on serines 380 and 385 in its C terminus, particularly the former site. Phosphate was also detected at Ser(380) in PTEN in untransformed human T cells. Treatments that reduced the levels of D3-phospholipids in the cells resulted in reduced phosphorylation and accelerated degradation of PTEN. In contrast, expression of inactive PTEN-C124G or coexpression of a constitutively active protein kinase B led to increased phosphorylation and slower degradation of PTEN. These results suggest that PTEN normally is subjected to a feedback mechanism of regulation aimed at maintaining homeostatic levels of D3-phosphoinositides, which are crucial for T cell survival and activation.
Insights
The phosphatase and tensin homolog (PTEN) protein is phosphorylated on its C-terminus, particularly at Serine 380, in T cells. This phosphorylation regulates PTEN levels, maintaining crucial D3-phosphoinositide homeostasis for T cell function.
Area of Science:
- Molecular Biology
- Cell Biology
- Immunology
Background:
- The PTEN tumor suppressor phosphatase inhibits phosphatidylinositol 3-kinase (PI3K) signaling by dephosphorylating phosphoinositides at the D3 position.
- The Jurkat T cell line, common in lymphomas and leukemias, lacks functional PTEN due to mutations, contributing to its survival.
- Phosphatidylinositol 3-kinase (PI3K) pathway is critical for T cell survival and activation.
Purpose of the Study:
- To investigate the regulation of PTEN protein, particularly its phosphorylation and degradation.
- To understand the role of PTEN phosphorylation in maintaining D3-phosphoinositide homeostasis in T cells.
Main Methods:
- Reintroduction of PTEN into PTEN-deficient Jurkat T cells.
- Analysis of PTEN phosphorylation at specific C-terminal serine residues (Ser380, Ser385).
- Treatment of cells to alter D3-phospholipid levels and expression of protein kinase B (PKB/Akt).
Main Results:
- Reintroduced PTEN in Jurkat cells was highly phosphorylated on Ser380 and Ser385.
- Phosphorylation at Ser380 was also observed in normal human T cells.
- Reduced D3-phospholipid levels led to decreased PTEN phosphorylation and faster degradation.
- Inactive PTEN or active PKB/Akt increased PTEN phosphorylation and slowed its degradation.
Conclusions:
- PTEN undergoes feedback regulation through C-terminal phosphorylation, particularly at Ser380.
- This regulatory mechanism helps maintain homeostatic levels of D3-phosphoinositides.
- Maintaining D3-phosphoinositide balance is essential for T cell survival and activation.