The mTOR complex 2, mTORC2, is a critical downstream effector of PI3K that stimulates AGC kinase members, including AKT, PKC, and SGK. Liu and colleagues reported that the pleckstrin homology domain of SIN1, an essential component of mTORC2, directly binds the PI3K product PtdIns(3,4,5)P3 to promote mTORC2 kinase activation and membrane localization, thereby revealing a mechanistic link between PI3K and mTORC2. Cancer Discov; 5(11); 1127-9. ©2015 AACR.See related article by Liu and colleagues, p. 1194.

The ability of cells to respond to changes in nutrient availability is essential for the maintenance of metabolic homeostasis and viability. One of the key cellular responses to nutrient withdrawal is the upregulation of autophagy. Recently, there has been a rapid expansion in our knowledge of the molecular mechanisms involved in the regulation of mammalian autophagy induction in response to depletion of key nutrients. Intracellular amino acids, ATP, and oxygen levels are intimately tied to the cellular balance of anabolic and catabolic processes. Signaling from key nutrient-sensitive kinases mTORC1 and AMP-activated protein kinase (AMPK) is essential for the nutrient sensing of the autophagy pathway. Recent advances have shown that the nutrient status of the cell is largely passed on to the autophagic machinery through the coordinated regulation of the ULK and VPS34 kinase complexes. Identification of extensive crosstalk and feedback loops converging on the regulation of ULK and VPS34 can be attributed to the importance of these kinases in autophagy induction and maintaining cellular homeostasis.

Autophagy is a cellular defense response to stress conditions, such as nutrient starvation. The type III phosphatidylinositol (PtdIns) 3-kinase, whose catalytic subunit is PIK3C3/VPS34, plays a critical role in intracellular membrane trafficking and autophagy induction. PIK3C3 forms multiple complexes and the ATG14-containing PIK3C3 is specifically involved in autophagy induction. Mechanistic target of rapamycin (MTOR) complex 1, MTORC1, is a key cellular nutrient sensor and integrator to stimulate anabolism and inhibit catabolism. Inactivation of TORC1 by nutrient starvation plays a critical role in autophagy induction. In this report we demonstrated that MTORC1 inactivation is critical for the activation of the autophagy-specific (ATG14-containing) PIK3C3 kinase, whereas it has no effect on ATG14-free PIK3C3 complexes. MTORC1 inhibits the PtdIns 3-kinase activity of ATG14-containing PIK3C3 by phosphorylating ATG14, which is required for PIK3C3 inhibition by MTORC1 both in vitro and in vivo. Our data suggest a mechanistic link between amino acid starvation and autophagy induction via the direct activation of the autophagy-specific PIK3C3 kinase.

Ferroptosis is a non-apoptotic form of cell death characterized by the iron-dependent lipid peroxidation and is implicated in several human pathologies, such as tissue ischemia, neurodegeneration, and cancer. Ferroptosis appears to be high cell-context dependent and the regulation of ferroptosis by physiological or pathological conditions are unclear. Here, we report that tumor-derived IDH1 mutation sensitizes cells to ferroptosis. Deletion of the mutant IDH1 allele in IDH1 heterozygous tumor cells or pharmacological inhibition of mutant IDH1 to produce the oncometabolite D-2-hydroxyglutarate (D-2-HG) confers resistance to erastin-induced ferroptosis. Conversely, ectopic expression of mutant IDH1 or treatment of cells with cell-permeable D-2-HG promotes the accumulation of lipid reactive oxygen species (ROS) and subsequently ferroptosis. Mechanistically, mutant IDH1 reduces the protein level of the glutathione peroxidase 4 (GPX4), a key enzyme in removing lipid ROS and ferroptosis, and promotes depletion of glutathione. Our results uncover a new role of mutant IDH1 and 2-HG in ferroptosis.

The mechanistic target of rapamycin (mTOR) is a central cell growth controller and forms two distinct complexes: mTORC1 and mTORC2. mTORC1 integrates a wide range of upstream signals, both positive and negative, to regulate cell growth. Although mTORC1 activation by positive signals, such as growth factors and nutrients, has been extensively investigated, the mechanism of mTORC1 regulation by stress signals is less understood. In this study, we identified the Nemo-like kinase (NLK) as an mTORC1 regulator in mediating the osmotic and oxidative stress signals. NLK inhibits mTORC1 lysosomal localization and thereby suppresses mTORC1 activation. Mechanistically, NLK phosphorylates Raptor on S863 to disrupt its interaction with the Rag GTPase, which is important for mTORC1 lysosomal recruitment. Cells with Nlk deletion or knock-in of the Raptor S863 phosphorylation mutants are defective in the rapid mTORC1 inhibition upon osmotic stress. Our study reveals a function of NLK in stress-induced mTORC1 modulation and the underlying biochemical mechanism of NLK in mTORC1 inhibition in stress response.

Tuberous sclerosis complex (TSC) integrates upstream stimuli and regulates cell growth by controlling the activity of mTORC1. TSC complex functions as a GTPase-activating protein (GAP) towards small GTPase Rheb and inhibits Rheb-mediated activation of mTORC1. Mutations in TSC genes cause tuberous sclerosis. In this study, the near-atomic resolution structure of human TSC complex reveals an arch-shaped architecture, with a 2:2:1 stoichiometry of TSC1, TSC2, and TBC1D7. This asymmetric complex consists of two interweaved TSC1 coiled-coil and one TBC1D7 that spans over the tail-to-tail TSC2 dimer. The two TSC2 GAP domains are symmetrically cradled within the core module formed by TSC2 dimerization domain and central coiled-coil of TSC1. Structural and biochemical analyses reveal TSC2 GAP-Rheb complimentary interactions and suggest a catalytic mechanism, by which an asparagine thumb (N1643) stabilizes γ-phosphate of GTP and accelerate GTP hydrolysis of Rheb. Our study reveals mechanisms of TSC complex assembly and GAP activity.

YAP is the major downstream effector of the Hippo pathway, which controls cell growth, tissue homeostasis, and organ size. Aberrant YAP activation, resulting from dysregulation of the Hippo pathway, is frequently observed in human cancers. YAP is a transcription co-activator, and the key mechanism of YAP regulation is its nuclear and cytoplasmic translocation. The Hippo pathway component, LATS, inhibits YAP by phosphorylating YAP at Ser127, leading to 14-3-3 binding and cytoplasmic retention of YAP Here, we report that osmotic stress stimulates transient YAP nuclear localization and increases YAP activity even when YAP Ser127 is phosphorylated. Osmotic stress acts via the NLK kinase to induce YAP Ser128 phosphorylation. Phosphorylation of YAP at Ser128 interferes with its ability to bind to 14-3-3, resulting in YAP nuclear accumulation and induction of downstream target gene expression. This osmotic stress-induced YAP activation enhances cellular stress adaptation. Our findings reveal a critical role for NLK-mediated Ser128 phosphorylation in YAP regulation and a crosstalk between osmotic stress and the Hippo pathway.

Background

Asymmetric (ADMA) and symmetric dimethylarginine (SDMA) are putative uremic toxins that may exert toxicity by a number of mechanisms, including impaired nitric oxide synthesis and generation of reactive oxygen species. The study goal was to determine the association between these metabolites and cardiovascular outcomes in hemodialysis patients.

Study design

Post hoc analysis of the Hemodialysis (HEMO) Study.

Setting & participants

1,276 prevalent hemodialysis patients with available samples 3 to 6 months after randomization.

Predictor

ADMA and SDMA measured in stored specimens.

Outcomes

Cardiac death, sudden cardiac death, first cardiovascular event, and any-cause death. Association with predictors analyzed using Cox regression adjusted for potential confounders (including demographics, clinical characteristics, comorbid conditions, albumin level, and residual kidney function).

Results

Mean age of patients was 57±14 (SD) years, 63% were black, and 57% were women. Mean ADMA (0.9±0.2μmol/L) and SDMA levels (4.3±1.4μmol/L) were moderately correlated (r=0.418). Higher dialysis dose or longer session length were not associated with lower predialysis ADMA or SDMA concentrations. In fully adjusted models, each doubling of ADMA level was associated with higher risk (HR per 2-fold higher concentration; 95% CI) of cardiac death (1.83; 1.29-2.58), sudden cardiac death (1.79; 1.19-2.69), first cardiovascular event (1.50; 1.20-1.87), and any-cause death (1.44; 1.13-1.83). Compared to the lowest ADMA quintile (<0.745 μmol/L), the highest ADMA quintile (≥1.07μmol/L) was associated with higher risk (HR; 95% CI) of cardiac death (2.10; 1.44-3.05), sudden cardiac death (2.06; 1.46-2.90), first cardiovascular event (1.75; 1.35-2.27), and any-cause death (1.56; 1.21-2.00). SDMA level was associated with higher risk for cardiac death (HR, 1.40; 95% CI, 1.03-1.92), but this was no longer statistically significant after adjusting for ADMA level (HR, 1.20; 95% CI, 0.86-1.68).

Limitations

Single time-point measurement of ADMA and SDMA.

Conclusions

ADMA and, to a lesser extent, SDMA levels are associated with cardiovascular outcomes in hemodialysis patients.

Constructing nanoparticles into well-defined structures at mesoscale and larger to create novel functional materials remains a challenge. Inspired by atomic epitaxial growth, we propose an "epitaxial assembly" method to form two-dimensional nanoparticle arrays (2D NAs) directly onto desired materials. As an illustration, we employ a series of surfactant-capped nanoparticles as the "artificial atoms" and layered hybrid perovskite (LHP) materials as the substrates and obtain 2D NAs in a large area with few defects. This method is universal for nanoparticles with different shapes, sizes, and compositions and for LHP substrates with different metallic cores. Raman spectroscopic and X-ray diffraction data support our hypothesis of epitaxial assembly. The novel method offers new insights into the controllable assembly of complex functional materials and may push the development of materials science at the mesoscale.

BACKGROUND:Residual kidney function (RKF) is thought to exert beneficial effects through clearance of uremic toxins. However, the level of native kidney function where clearance becomes negligible is not known. METHODS:We aimed to assess whether levels of nonurea solutes differed among patients with 'clinically negligible' RKF compared with those with no RKF. The hemodialysis study excluded patients with urinary urea clearance >1.5 mL/min, below which RKF was considered to be 'clinically negligible'. We measured eight nonurea solutes from 1280 patients participating in this study and calculated the relative difference in solute levels among patients with and without RKF based on measured urinary urea clearance. RESULTS:The mean age of the participants was 57 years and 57% were female. At baseline, 34% of the included participants had clinically negligible RKF (mean 0.7 ± 0.4 mL/min) and 66% had no RKF. Seven of the eight nonurea solute levels measured were significantly lower in patients with RKF than in those without RKF, ranging from -24% [95% confidence interval (CI) -31 to -16] for hippurate, -7% (-14 to -1) for trimethylamine-N-oxide and -4% (-6 to -1) for asymmetric dimethylarginine. The effect of RKF on plasma levels was comparable or more pronounced than that achieved with a 31% higher dialysis dose (spKt/Vurea 1.7 versus 1.3). Preserved RKF at 1-year follow-up was associated with a lower risk of cardiac death and first cardiovascular event. CONCLUSIONS:Even at very low levels, RKF is not 'negligible', as it continues to provide nonurea solute clearance. Management of patients with RKF should consider these differences.