Akt phosphorylates insulin receptor substrate to limit PI3K-mediated PIP3 synthesis

Area of Science:

• Cellular signaling
• Molecular biology
• Biochemistry

Background:

• The phosphoinositide 3-kinase (PI3K)-Akt pathway is crucial for cellular functions and tightly regulated by feedback loops.
• Previous studies indicated negative feedback on acute timescales, evidenced by overshoot in Akt recruitment to the plasma membrane upon insulin stimulation.

Purpose of the Study:

• To elucidate the molecular mechanism by which Akt regulates its own signaling network.
• To identify the specific feedback mechanisms controlling insulin-stimulated Akt activation and phosphatidylinositol (3,4,5)-trisphosphate (PIP3) synthesis.

Main Methods:

• Investigated Akt-dependent phosphorylation of insulin receptor substrate (IRS) 1 and 2.
• Utilized mass spectrometry to identify phosphorylation sites.
• Assessed the impact of phosphorylation on IRS localization and PIP3 production.

Main Results:

• Akt directly phosphorylates IRS1 and IRS2 on specific residues, including S306/S303 and S577/S573 in IRS2.
• This phosphorylation leads to the depletion of plasma membrane-localized IRS1/2, reducing insulin receptor interaction.
• Consequently, Akt-mediated IRS phosphorylation limits PI3K activity and PIP3 synthesis at the plasma membrane.

Conclusions:

• Akt acts as a negative regulator of its own signaling pathway through direct phosphorylation of IRS proteins.
• Identified specific Akt phosphorylation sites on IRS2 as key drivers of this acute negative feedback loop.
• Established a novel mechanism for Akt-mediated control of PIP3 abundance via post-translational modification of the IRS scaffold.