Overview of mTOR signaling pathwayNutrients, growth factors, and cellular energy levels are key determinants of cell growth and proliferation. mTOR, a serine/threonine kinase, is a master regulator of cellular metabolism and promotes cell growth in response to environmental cues. Deregulation of mTOR signaling has been implicated in many human diseases, including diabetes, neurodegenerative diseases, and cancer (1). mTOR forms two distinct signaling complexes, mTOR complex 1 (mTORC1) and mTORC2, by binding with multiple companion proteins (Figure 1). mLST8, DEPTOR, and the Tti1/ Tel2 complex exist in both mTORC1 and mTORC2 (2-5). On the other hand, RAPTOR and PRAS40 are specific to mTORC1 (6-11) whereas RICTOR, mSin1, and PROCTOR1/2 are specific to mTORC2 (10,(12)(13)(14)(15)(16). The two kinase complexes have specific substrate preferences and therefore elicit distinct downstream signaling events to modulate cellular function.One of the well-established roles of mTORC1 is to promote anabolic cellular metabolism to supply the necessary building blocks for cell growth and proliferation. mTORC1 integrates various stimuli and signaling networks to stimulate synthesis of protein, lipid, and nucleotides and block catabolic processes such as autophagy at the post-translational and transcriptional levels (reviewed in refs. 17, 18). The tuberous sclerosis (TSC) tumor suppressor complex (TSC1/TSC2) is arguably the most important upstream negative regulator of mTORC1. Genetic mutations in hamartin or tuberin (encoding TSC1 and TSC2, respectively) cause tumor development in various tissues such as angiofibromas, angiomyolipomas, lymphangioleiomyomatosis, and renal cell carcinoma. Loss-of-function mutations in either TSC1 or TSC2 lead to constitutive mTORC1 activation, which contributes to uncontrolled growth and underlies the TSC disease (19). These findings provide the scientific basis of using mTORC1 inhibitors for the treatment of TSC and related diseases such as cancer.A well-established upstream regulator of mTORC1 is the growth factor/PI3K/AKT signaling pathway. Growth factors such as insulin and IGF activate their cognate receptors (receptor tyrosine kinases [RTKs]) and subsequently activate the PI3K/ AKT signaling axis. Activated AKT directly phosphorylates and thereby inhibits TSC1/2, a GTPase-activating protein (GAP) for the Ras homolog enriched in brain (Rheb) GTPase (19-23). The AKT-dependent phosphorylation results in dissociation of TSC1/2 from lysosome, where Rheb is localized, promoting Rheb activation (24). Since GTP-bound Rheb is a potent mTORC1 activator, inhibition of TSC1/2 by AKT-dependent phosphorylation results in mTORC1 activation (25,26). Additionally, AKT directly phosphorylates and inhibits PRAS40, an mTORC1 component that negatively regulates the complex's kinase activity, leading to mTORC1 activation (8-11). Furthermore, the activated RTK also stimulates the Ras/Erk/p90 ribosomal S6 kinase 1 (RSK1) signaling axis, which directly phosphorylates TSC2 to inactivate its GAP activity (27,28). In co...

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) integrates environmental and intracellular signals to regulate cell growth. Amino acids stimulate mTORC1 activation at the lysosome in a manner thought to be dependent on the Rag small guanosine triphosphatases (GTPases), the Ragulator complex, and the vacuolar H+-adenosine triphosphatase (v-ATPase). We report that leucine and glutamine stimulate mTORC1 by Rag GTPase-dependent and -independent mechanisms, respectively. Glutamine promoted mTORC1 translocation to the lysosome in RagA and RagB knockout cells and required the v-ATPase but not the Ragulator. Furthermore, we identified the adenosine diphosphate ribosylation factor-1 GTPase to be required for mTORC1 activation and lysosomal localization by glutamine. Our results uncover a signaling cascade to mTORC1 activation independent of the Rag GTPases and suggest that mTORC1 is differentially regulated by specific amino acids.

Summary Autophagy is a stress response protecting cells from unfavorable conditions, such as nutrient starvation. The class III phosphatidylinositol-3 kinase, Vps34, forms multiple complexes and regulates both intracellular vesicle trafficking and autophagy induction. Here, we show that AMPK plays a key role in regulating different Vps34 complexes. AMPK inhibits the non-autophagy Vps34 complex by phosphorylating T163/S165 in Vps34, therefore suppresses overall PI(3)P production and protects cells from starvation. In parallel, AMPK activates the pro-autophagy Vps34 complex by phosphorylating S91/S94 in Beclin1 to induce autophagy. Atg14L, an autophagy essential gene present only in pro-autophagy Vps 34 complex, inhibits Vps34 phosphorylation but increases Beclin1 phosphorylation by AMPK. As such, Atg14L dictates the differential regulation (either inhibition or activation) of different Vps34 complexes in response to glucose starvation. Our study reveals an intricate molecular regulation of Vps34 complexes by AMPK in nutrient stress response and autophagy.

SUMMARY The transcriptional co-activators YAP and TAZ are key regulators of organ size and tissue homeostasis, and their dysregulation contributes to human cancer. Here we discover YAP/TAZ as bona fide downstream effectors of the alternative Wnt signaling pathway. Wnt5a/b and Wnt3a induce YAP/TAZ activation independent of canonical Wnt/β-catenin signaling. Mechanistically, we delineate the ‘alternative Wnt-YAP/TAZ signaling axis’ that consists of Wnt - FZD/ROR - Gα12/13 - Rho GTPases -Lats1/2 to promote YAP/TAZ activation and TEAD-mediated transcription. YAP/TAZ mediate the biological functions of alternative Wnt signaling including gene expression, osteogenic differentiation, cell migration, and antagonism of Wnt/β-catenin signaling. Together, our work establishes YAP/TAZ as critical mediators of alternative Wnt signaling.