Unfavorable growth conditions such as nutrient starvation, cellular stress and limited availability of growth factors induce macroautophagy (hereafter autophagy) in eukaryotic cells. Through degradation of cellular components, autophagy provides energy for cell survival or for cellular restructuring during differentiation.4–8
Autophagy is under the control of the mTORC1 pathway that regulates cell growth.9–11
In favorable growth conditions, mTORC1 stimulates protein synthesis and cell growth and suppresses autophagy. Under unfavorable growth conditions, mTORC1 is inhibited resulting in suppression of cell growth and induction of autophagy.3,11–15
Although the dependence of autophagy on mTORC1 implies intimate crosstalk between autophagy and cell growth, the exact mechanism and the functional significance of the crosstalk remain unclear. Defining this mechanism is important for understanding the role of autophagy in cell growth and cancer, which will lead to the development of better strategies to prevent or treat cancer.
mTORC1 inhibits autophagy via phosphorylation of ULK1.3,12–14,16
ULK1 and its homolog ULK2 are key molecules involved in the induction of autophagy.3,13,14,16–20
Although the exact function of ULK in autophagy induction remains unresolved, it has been shown that ULK1 participates upstream of phosphatidyl-inositol-3-kinase class III (PI3KC3), another key mediator of autophagy induction, to regulate the formation of the phagophores, the precursor form of the autophagosome.3,12–14,19–21
Thus, ULK1 is considered as a key molecule that mediates mTORC1 signaling to the PI3KC3 autophagy machinery. ULK1 interacts with Atg13, FIP200 and Atg101 to form a protein complex.3,12–14,19,20,22,23
ULK2 also binds to the same proteins to form a distinct protein complex. Recent studies revealed that mTORC1 and 5′ AMP-activated protein kinase (AMPK) bind and phosphorylate ULK1.16,18,24
mTORC1 and AMPK have opposing effects on ULK1 activity. AMPK phosphorylation of ULK1 stimulates ULK1 and autophagy induction, whereas mTORC1 phosphorylation of ULK1 prevents AMPK from binding to ULK1 thus inhibiting autophagy induction.16
Although ULK1 is largely known to regulate autophagy induction, recent studies have shown that ULK1 and its homolog Atg1 in Drosophila melanogaster
and Caenorhabditis elegans
can negatively regulate mTORC1 signaling.1–3
ULK1 deficiency or ULK2 knockdown in mammalian cells and deletion of Atg1 gene in flies were shown to increase the phosphorylation of S6 kinase (S6K1), the key downstream target of mTORC1.1,3
Consistently, overexpression of Atg1 in Drosophila fat body reduced S6K1 phosphorylation and cell size dramatically.2
The regulation of cell growth by Atg1 and other autophagy elements has also been shown in C. elegans
These results suggest that ULK1/Atg1 negatively regulates mTORC1 activity and cell growth through a mechanism that may be conserved in C. elegans
, Drosophila and mammals.
In this study, we sought to determine the mechanism underlying the regulation of mTORC1 by ULK1. We found that either deficiency or knockdown of ULK1, ULK2 or Atg13, but not deficiency of Atg5, enhanced mTORC1 signaling, cell proliferation rates and accumulation of cell mass. The stimulatory effect of ULK1 deficiency on mTORC1 signaling occurred independently of tuberous sclerosis complex 2 (TSC2), an upstream negative regulator of mTORC1. We found that ULK1 binds raptor, induces its phosphorylation and inhibits the kinase activity of mTORC1. These findings suggest that there is a reciprocal relationship between ULK1 and mTORC1. This regulation likely depends on the direct interaction between ULK1 and mTORC1 and is independent of Atg5-mediated autophagy. The reciprocal regulation between mTORC1 and ULK1 may be important for coordinate regulation of cell growth and autophagy in response to cellular energy and stress condition.