GSK3B induces autophagy by phosphorylating ULK1

Hye Young Ryu,Hyunhee Park,Eun-Kyoung Kim,Bo Kyoung Yeo,Seonghee Jung,Seong-Woon Yu,Leah Eunjung Kim,Hyun-Kyu An,Hyeonjeong Jeong,Ji-Won Lee,Heesun Cheong,Kyung Min Chung,Shinwon Ha,Byung-Hoon Lee,Jiyea Kim,Hyeri Nam

doi:10.1038/s12276-021-00570-6

Abstract

Unc-51-like autophagy activating kinase 1 (ULK1), a mammalian homolog of the yeast kinase Atg1, has an essential role in autophagy induction. In nutrient and growth factor signaling, ULK1 activity is regulated by various posttranslational modifications, including phosphorylation, acetylation, and ubiquitination. We previously identified glycogen synthase kinase 3 beta (GSK3B) as an upstream regulator of insulin withdrawal-induced autophagy in adult hippocampal neural stem cells. Here, we report that following insulin withdrawal, GSK3B directly interacted with and activated ULK1 via phosphorylation of S405 and S415 within the GABARAP-interacting region. Phosphorylation of these residues facilitated the interaction of ULK1 with MAP1LC3B and GABARAPL1, while phosphorylation-defective mutants of ULK1 failed to do so and could not induce autophagy flux. Furthermore, high phosphorylation levels of ULK1 at S405 and S415 were observed in human pancreatic cancer cell lines, all of which are known to exhibit high levels of autophagy. Our results reveal the importance of GSK3B-mediated phosphorylation for ULK1 regulation and autophagy induction and potentially for tumorigenesis.

Highlights

Macroautophagy is an evolutionarily conserved intracellular catabolic process with various context-dependent functions in cell survival or death[1]
Unlike unc-51-like autophagy activating kinase 1 (ULK1) knockdown, ULK2 knockdown did not decrease MAP1LC3B lipidation in I(−) hippocampal neural stem (HCN) cells (Fig. 1b). These data suggest that ULK1 but not ULK2 is essential for autophagy induction in HCN cells following insulin withdrawal
We previously demonstrated that cell death rate in I(−) HCN cells correlated with the level of autophagy flux[45]

Summary

Introduction

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved intracellular catabolic process with various context-dependent functions in cell survival or death[1]. Autophagy is characterized by an increase in the formation of double-membrane autophagosomes, which are mediated by autophagy-related (Atg) genes. Yeast Atg[1] and its mammalian homolog, unc-51-like autophagy activating kinase 1 (ULK1), are serine/threonine protein kinases that initiate autophagosome formation[7]. ULK1 and ULK2 share an overall 52% amino acid identity[9]. Despite this sequence identity, ULK1 appears to be more important than ULK2 for autophagy induction upon nutrient starvation[10], some studies have reported that deletion of both the Ulk[1] and Ulk[2] genes is required to block starvation-induced autophagy[10,11,12,13]. ULK1 forms a tetrameric complex, the so-called “initiator complex” containing ATG13, RB1-inducible coiled-coil 1 (RB1CC1/FIP200), and ATG10114,15

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Results

Conclusion