Amino acid response by Halofuginone in Cancer cells triggers autophagy through proteasome degradation of mTOR.

BACKGROUND: In the event of amino acid starvation, the cell activates two main protective pathways: Amino Acid starvation Response (AAR), to inhibit global translation, and autophagy, to recover the essential substrates from degradation of redundant self-components. Whether and how AAR and autophagy (ATG) are cross-regulated and at which point the two regulatory pathways intersect remain unknown. Here, we provide experimental evidence that the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) specifically located at the lysosome level links the AAR with the autophagy pathway. METHODS: As an inducer of the AAR, we used halofuginone (HF), an alkaloid that binds to the prolyl-tRNA synthetase thus mimicking the unavailability of proline (PRO). Induction of AAR was determined assessing the phosphorylation of the eukaryotic translation initiation factor (eIF) 2α. Autophagy was monitored by assessing the processing and accumulation of microtubule-associated protein 1 light chain 3 isoform B (LC3B) and sequestosome-1 (p62/SQSTM1) levels. The activity of mTORC1 was monitored through assessment of the phosphorylation of mTOR, (rp)S6 and 4E-BP1. Global protein synthesis was determined by puromycin incorporation assay. mTORC1 presence on the membrane of the lysosomes was monitored by cell fractionation and mTOR expression was determined by immunoblotting. RESULTS: In three different types of human cancer cells (thyroid cancer WRO cells, ovarian cancer OAW-42 cells, and breast cancer MCF-7 cells), HF induced both the AAR and the autophagy pathways time-dependently. In WRO cells, which showed the strongest induction of autophagy and of AAR, global protein synthesis was little if any affected. Consistently, 4E-BP1 and (rp)S6 were phosphorylated. Concomitantly, mTOR expression and activation declined along with its detachment from the lysosomes and its degradation by the proteasome, and with the nuclear translocation of transcription factor EB (TFEB), a transcription factor of many ATG genes. The extra supplementation of proline rescued all these effects. CONCLUSIONS: We demonstrate that the AAR and autophagy are mechanistically linked at the level of mTORC1, and that the lysosome is the central hub of the cross-talk between these two metabolic stress responses.

Turmeric toxicity in A431 epidermoid cancer cells associates with autophagy degradation of anti-apoptotic and anti-autophagic p53 mutant.

The keratinocyte-derived A431 Squamous Cell Carcinoma cells express the p53R273H mutant, which has been reported to inhibit apoptosis and autophagy. Here, we show that the crude extract of turmeric (Curcuma longa), similarly to its bioactive component Curcumin, could induce both apoptosis and autophagy in A431 cells, and these effects were concomitant with degradation of p53. Turmeric and curcumin also stimulated the activity of mTOR, which notoriously promotes cell growth and acts negatively on basal autophagy. Rapamycin-mediated inhibition of mTOR synergized with turmeric and curcumin in causing p53 degradation, increased the production of autophagosomes and exacerbated cell toxicity leading to cell necrosis. Small-interference mediated silencing of the autophagy proteins BECLIN 1 or ATG7 abrogated the induction of autophagy and largely rescued p53 stability in Turmeric-treated or Curcumin-treated cells, indicating that macroautophagy was mainly responsible for mutant p53 degradation. These data uncover a novel mechanism of turmeric and curcumin toxicity in chemoresistant cancer cells bearing mutant p53.

Resveratrol-induced apoptosis depends on the lipid kinase activity of Vps34 and on the formation of autophagolysosomes.

In human colorectal DLD1 cancer cells, the dietary bioflavonoid resveratrol (RV) rapidly induced autophagy. This effect was reversible (on removal of the drug) and was associated with increased expression and cytosolic redistribution of the proteins Beclin1 and LC3 II. Supplementing the cells with asparagine (Asn) abrogated the Beclin-dependent autophagy. When applied acutely (2 h), RV was not toxic; however, reiterate chronic (48 h) exposure to RV eventually led to annexin V- and terminal deoxinucleotidyl transferase-mediated dUTP-biotin nick end labeling-positive cell death. This toxic effect was autophagy dependent, as it was prevented either by Asn, by expressing a dominant-negative lipid kinase-deficient class III phosphoinositide 3-phosphate kinase, or by RNA interference knockdown of Beclin1. Lamp2b silencing abolished the fusion of autophagosomes with lysosomes and preserved cell viability despite the ongoing formation of autophagosomes in cells chronically exposed to RV. The pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone inhibited RV-induced cell death, but not autophagy. These results uncover a novel pathway of RV cytotoxicity in which autophagy plays a dual role: (i) at first, it acts as a prosurvival stress response and (ii) at a later time, it switches to a caspase-dependent apoptosis pathway. The present data also indicate that genetic or epigenetic inactivation of autophagy proteins in cancer cells may confer resistance to RV-mediated killing.