Identification of PPM1D as an essential Ulk1 phosphatase for genotoxic stress‐induced autophagy

Abstract

Autophagy is an evolutionary conserved process that degrades subcellular constituents. Unlike starvation‐induced autophagy, the molecular mechanism of genotoxic stress‐induced autophagy has not yet been fully elucidated. In this study, we analyze the molecular mechanism of genotoxic stress‐induced autophagy and identify an essential role of dephosphorylation of the Unc51‐like kinase 1 (Ulk1) at Ser 637 , which is catalyzed by the protein phosphatase 1D magnesium‐dependent delta isoform (PPM1D). We show that after exposure to genotoxic stress, PPM1D interacts with and dephosphorylates Ulk1 at Ser 637 in a p53‐dependent manner. The PPM1D‐dependent Ulk1 dephosphorylation triggers Ulk1 puncta formation and induces autophagy. This happens not only in mouse embryonic fibroblasts but also in primary thymocytes, where the genetic ablation of PPM1D reduces the dephosphorylation of Ulk1 at Ser 637 , inhibits autophagy, and accelerates apoptosis induced by X‐ray irradiation. This acceleration of apoptosis is caused mainly by the inability of the autophagic machinery to degrade the proapoptotic molecule Noxa. These findings indicate that the PPM1D–Ulk1 axis plays a pivotal role in genotoxic stress‐induced autophagy. image Genotoxic stress induces the accumulation of PPM1D, which promotes the dephosphorylation of Ulk1, the formation of Ulk1 puncta, and the induction of autophagy. The genetic ablation of PPM1D leads to increased apoptosis due to the inability of the autophagic machinery to degrade the proapoptotic protein Noxa. Ulk1 is dephosphorylated at Serine 637 by PPM1D during genotoxic stress‐induced autophagy. PPM1D‐dependent dephosphorylation of Ulk1 is required for Ulk1 puncta formation, Atg13 phosphorylation, and DFCP1 puncta formation. PPM1D‐promoted autophagy suppresses irradiation‐induced apoptosis by promoting the degradation of the proapoptotic protein Noxa.