mTOR-raptor binds and activates SGK1 to regulate p27 phosphorylation

Area of Science:

• Cellular Biology
• Molecular Biology
• Biochemistry

Background:

• The precise role of mTORC1 in cell cycle regulation remains incompletely understood.
• mTORC1 is a key regulator of cell growth, proliferation, and metabolism.
• Dysregulation of cell cycle control is a hallmark of cancer.

Purpose of the Study:

• To elucidate the role of mTORC1 in regulating cell cycle progression.
• To identify downstream targets of mTORC1 involved in cell cycle control.
• To investigate the mechanism by which mTORC1 influences p27 function.

Main Methods:

• Utilized cell culture models with controlled mTORC1 activation and inhibition.
• Employed shRNA to deplete specific proteins like TSC2, raptor, and SGK1.
• Performed Western blotting to assess protein phosphorylation and localization.
• Conducted in vitro kinase assays to confirm direct phosphorylation events.

Main Results:

• mTORC1 activation led to the phosphorylation and activation of SGK1 and p27 at T157.
• Inhibition of mTORC1 or SGK1 reversed these phosphorylation events.
• mTORC1 overexpression caused p27 mislocalization to the cytoplasm, conferring TGF-beta resistance.
• Direct phosphorylation of SGK1 by mTOR and p27 by SGK1 was demonstrated in vitro.
• mTORC1, raptor, and SGK1 form functional complexes within cells.

Conclusions:

• SGK1 is a direct substrate of mTORC1, mediating its effects on p27.
• mTORC1 promotes G1 phase progression partly via SGK1 activation.
• Aberrant mTORC1 signaling contributes to cancer by deregulating the cell cycle through SGK1-mediated p27 phosphorylation and mislocalization.