Regulation of autophagy by coordinated action of mTORC1 and protein phosphatase 2A

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Autophagy is a vital cellular process for maintaining homeostasis and cell viability.
• Inhibition of the mammalian target of rapamycin (mTOR) complex-1 (mTORC1) is known to activate autophagy.
• Amino acid starvation and pharmacological mTORC1 inhibition both block mTORC1 activity but induce autophagy at different rates, a discrepancy needing explanation.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the differential kinetics of autophagy induction by amino acid starvation versus mTORC1 inhibition.
• To identify novel factors involved in regulating autophagy initiation beyond mTORC1 inactivation.
• To explore the role of ULK1 phosphorylation and dephosphorylation in autophagy regulation.

Main Methods:

• Cellular assays to measure autophagy flux and mTORC1 activity.
• Biochemical experiments to assess protein phosphatase activity towards ULK1.
• Identification of the specific phosphatase involved using genetic and pharmacological approaches.
• Analysis of protein-protein interactions, including dissociation of PP2A from its inhibitor Alpha4.
• Assessment of ULK1's role in pancreatic ductal adenocarcinoma cell growth.

Main Results:

• Starvation, unlike mTORC1 inhibitors, increases phosphatase activity targeting ULK1, a key substrate for autophagy induction.
• The PP2A-B55α complex was identified as the starvation-stimulated phosphatase responsible for ULK1 dephosphorylation.
• Starvation induces dissociation of PP2A from its inhibitor Alpha4, enhancing its activity.
• Pancreatic ductal adenocarcinoma cells exhibit higher basal ULK1 phosphatase activity and depend on ULK1 for growth.

Conclusions:

• Autophagy induction is regulated by a dual mechanism involving mTORC1 inactivation and enhanced PP2A-B55α phosphatase activity towards ULK1.
• This coordinated regulation ensures efficient ULK1-dependent autophagy, critical for cell viability and homeostasis.
• Targeting this pathway may offer therapeutic strategies for cancers like pancreatic ductal adenocarcinoma, which rely on high basal autophagy.