ULK1 inhibits the kinase activity of mTORC1 and cell proliferation

Abstract

ULK1 (Unc51-like kinase, hATG1) is a Ser/Thr kinase that plays a key role in inducing autophagy in response to starvation. ULK1 is phosphorylated and negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1). Previous studies have shown that ULK1 is not only a downstream effector of mTORC1 but also a negative regulator of mTORC1 signaling. ( 1-3) Here, we investigated how ULK1 regulates mTORC1 signaling, and found that ULK1 inhibits the kinase activity of mTORC1 and cell proliferation. Deficiency or knockdown of ULK1 or its homolog ULK2 enhanced mTORC1 signaling, cell proliferation rates and accumulation of cell mass, whereas overexpression of ULK1 had the opposite effect. Knockdown of Atg13, the binding partner of ULK1 and ULK2, mimicked the effects of ULK1 or ULK2 deficiency or knockdown. Both insulin and leucine stimulated mTORC1 signaling to a greater extent when ULK1 or ULK2 was deficient or knocked down. In contrast, Atg5 deficiency did not have a significant effect on mTORC1 signaling and cell proliferation. The stimulatory effect of ULK1 knockdown on mTORC1 signaling occurred even in the absence of tuberous sclerosis complex 2 (TSC2), the negative regulator of mTORC1 signaling. In addition, ULK1 was found to bind raptor, induce its phosphorylation, and inhibit the kinase activity of mTORC1. These results demonstrate that ULK1 negatively regulates the kinase activity of mTORC1 and cell proliferation in a manner independent of Atg5 and TSC2. The inhibition of mTORC1 by ULK1 may be important to coordinately regulate cell growth and autophagy with optimized utilization of cellular energy.

Regulation of Autophagy by Protein Phosphorylation

“…Collectively, these multiple phosphorylations inhibit mTORC1 activity through hindrance of substrate docking to raptor [72]. These results are in line with the findings by Jung et al [73]. Collectively, the Ulk1-mediated phosphorylation of AMPK and raptor represent regulatory feedback loops and contribute to the positive and negative regulation of autophagy.…”

“…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]. The common theme here is that this phosphorylation leads to the inhibition of mTORC1 thus inducing autophagy.…”

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

“…Thus, phosphorylation of RPTOR by ULK1 negatively regulates MTOR complex 1 23 and has also been proposed as a mechanism connecting ULK1 activity and cell proliferation. 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.…”

Drosophila Fip200 is an essential regulator of autophagy that attenuates both growth and aging

“…17 Upon activation, ULK1 suppresses cell growth and protein synthesis by inhibiting TORC1 while promoting autophagic degradation. 18,19 Reciprocal inhibition between ULK1 and TORC1 is suggested to be an interlocked feedback loop that serves as a Autophagy-related 1 (Atg1)/Unc-51-like protein kinases (ULKs) are evolutionarily conserved proteins that play critical physiological roles in controlling autophagy, cell growth and neurodevelopment. rB1-inducible coiled-coil 1 (rB1CC1), also known as PtK2/FAK family-interacting protein of 200 kda (FiP200) is a recently discovered binding partner of ULK1.…”