Ulk1-mediated phosphorylation of AMPK constitutes a negative regulatory feedback loop

Abstract

Unc-51-like kinase 1 (Ulk1) plays a central role in autophagy induction. It forms a stable complex with Atg13 and focal adhesion kinase (FAK) family interacting protein of 200 kDa (FIP 200). This complex is negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1) in a nutrient-dependent way. AMP-activated protein kinase (AMPK), which is activated by LKB1/Strad/Mo25 upon high AMP levels, stimulates autophagy by inhibiting mTORC1. Recently, it has been described that AMPK and Ulk1 interact and that the latter is phosphorylated by AMPK. This phosphorylation leads to the direct activation of Ulk1 by AMPK bypassing mTOR-inhibition. Here we report that Ulk1/2 in turn phosphorylates all three subunits of AMPK and thereby negatively regulates its activity. Thus, we propose that Ulk1 is not only involved in the induction of autophagy, but also in terminating signaling events that trigger autophagy. In our model, phosphorylation of AMPK by Ulk1 represents a negative feedback circuit.

Suppressed translation and ULK1 degradation as potential mechanisms of autophagy limitation under prolonged starvation

“…24 Similarly, ULK1/2 phosphorylate all 3 subunits of AMPK and negatively regulate its activity, thereby terminating signaling that initiates autophagy. 25 If so, based on our results, it could be suggested that downregulation of ULK1 under starvation conditions could lead to more efficient activation of AMPK.…”

Regulation of Autophagy by Protein Phosphorylation

“…The common theme of these reports is the starvation-induced activation of AMPK, the subsequent phosphorylation and activation of Ulk1, and in turn the induction of autophagy (reviewed in [14,69]). In contrast, our group was able to identify Ulk1-dependent phosphorylation sites in all three AMPK subunits, suggesting a rather complex level of regulation by mutual phosphorylation [68]. The direct activation of Ulk1 by AMPK represents a valuable model for mTOR-independent autophagy pathways [69,70].…”

“…However, in 2010 Behrends et al first discovered the interaction between AMPK and Ulk1/2 in a global proteomic analysis of the human autophagy network [66]. Subsequently, different groups confirmed the direct interaction between these two kinases [43,44,65,67,68]. Four of these groups additionally reported the AMPK-mediated phosphorylation of Ulk1.…”

“…The phospho-sites include S360/T368, S397, S486/T488 in the 1-subunit, S38, T39, S68 and S173 in the 2-subunit, and S260/T262 and S269 in the 1-subunit (for some peptides the exact phosphorylation-site could not be narrowed down further). Thus, we propose that Ulk1 is not only necessary for the induction of autophagy, but also is involved in its containment [68]. Finally, two reports describe the Ulk1-dependent phosphorylation of raptor [72,73].…”

“…Our group could show that all three AMPK subunits are phosphorylated by Ulk1 and Ulk2, and that thereby AMPK activity is negatively regulated [68]. The phospho-sites include S360/T368, S397, S486/T488 in the 1-subunit, S38, T39, S68 and S173 in the 2-subunit, and S260/T262 and S269 in the 1-subunit (for some peptides the exact phosphorylation-site could not be narrowed down further).…”