Inhibiting MT2-TFE3-dependent autophagy enhances melatonin-induced apoptosis in tongue squamous cell carcinoma.

Autophagy modulation is a potential therapeutic strategy for tongue squamous cell carcinoma (TSCC). Melatonin possesses significant anticarcinogenic activity. However, whether melatonin induces autophagy and its roles in cell death in TSCC are unclear. Herein, we show that melatonin induced significant apoptosis in the TSCC cell line Cal27. Apart from the induction of apoptosis, we demonstrated that melatonin-induced autophagic flux in Cal27 cells as evidenced by the formation of GFP-LC3 puncta, and the upregulation of LC3-II and downregulation of SQSTM1/P62. Moreover, pharmacological or genetic blockage of autophagy enhanced melatonin-induced apoptosis, indicating a cytoprotective role of autophagy in melatonin-treated Cal27 cells. Mechanistically, melatonin induced TFE3(Ser321) dephosphorylation, subsequently activated TFE3 nuclear translocation, and increased TFE3 reporter activity, which contributed to the expression of autophagy-related genes and lysosomal biogenesis. Luzindole, a melatonin membrane receptor blocker, or MT2-siRNA partially blocked the ability of melatonin to promote mTORC1/TFE3 signaling. Furthermore, we verified in a xenograft mouse model that melatonin with hydroxychloroquine or TFE3-siRNA exerted a synergistic antitumor effect by inhibiting autophagy. Importantly, TFE3 expression positively correlated with TSCC development and poor prognosis in patients. Collectively, we demonstrated that the melatonin-induced increase in TFE3-dependent autophagy is mediated through the melatonin membrane receptor in TSCC. These data also suggest that blocking melatonin membrane receptor-TFE3-dependent autophagy to enhance the activity of melatonin warrants further attention as a treatment strategy for TSCC.

Melatonin antagonizes cadmium-induced neurotoxicity by activating the transcription factor EB-dependent autophagy-lysosome machinery in mouse neuroblastoma cells.

Cadmium (Cd), a highly ubiquitous heavy metal, induces neurotoxicity. Melatonin, a major secretory product of the pineal gland, protects against Cd-induced neurotoxicity. However, the mechanism that accounts for this protection remains to be elucidated. Herein, we exposed mouse neuroblastoma cells (Neuro-2a cells) to different concentrations of cadmium chloride (CdCl2 ) (12.5, 25, and 50 µ mol L(-1) ) for 24 hours. We showed that Cd inhibits autophagosome-lysosome fusion and impairs lysosomal function, subsequently leading to nerve cell death. In addition, Cd decreases the level of transcription factor EB (TFEB) but induces the nuclear translocation of TFEB, associated with compromised lysosomal function or a compensatory effect after the impairment of the autophagic flux. Moreover, compared to the 50-µ mol L(-1) Cd group, administration of 1 µ mol L(-1) melatonin increased "TFEB-responsive genes" (P<.05) and the levels of lysosomal-associated membrane protein (0.57±0.06 vs 1.00±0.11, P<.05), preserved lysosomal protease activity (0.52±0.01 vs 0.90±0.02, P<.05), maintained the lysosomal pH level (0.50±0.01 vs 0.87±0.05, P<.01), and enhanced autophagosome-lysosome fusion (0.05±0.00 vs 0.21±0.01, P<.01). Notably, melatonin enhanced TFEB expression (0.37±0.04 vs 0.72±0.07, P<.05) and nuclear translocation (2.81±0.08 vs 3.82±0.05, P<.05). Tfeb siRNA blocked the melatonin-mediated elevation in autophagy-lysosome machinery in Cd-induced neurotoxicity (P<.01). Taken together, these results uncover a potent role for TFEB-mediated autophagy in the pathogenesis of Cd-induced neurotoxicity, suggesting that control of the autophagic pathway by melatonin might provide an important clue for exploring potential targets for novel therapeutics of Cd-induced neurotoxicity.

AKT inhibition-mediated dephosphorylation of TFE3 promotes overactive autophagy independent of MTORC1 in cadmium-exposed bone mesenchymal stem cells.

Cadmium (Cd) is a toxic metal that is widely found in numerous environmental matrices and induces serious adverse effects in various organs and tissues. Bone tissue seems to be a crucial target of Cd contamination. Macroautophagy/autophagy has been proposed to play a pivotal role in Cd-mediated bone toxicity. However, the mechanisms that underlie Cd-induced autophagy are not yet completely understood. We demonstrated that Cd treatment increased autophagic flux and inhibition of the autophagic process using Atg7 gene silencing blocked the Cd-induced mesenchymal stem cell death. Mechanistically, Cd activated nuclear translocation of TFE3 but not that of TFEB or MITF, which contributed to the expression of autophagy-related genes and lysosomal biogenesis. Specifically, Cd decreased expression of phospho-AKT (Ser473). The reduction in AKT activity led to dephosphorylation of cytosolic TFE3 at Ser565 and promoted TFE3 nuclear translocation independently of MTORC1. Notably, Cd treatment increased the activity of PPP3/calcineurin, and pharmacological inhibition of PPP3/calcineurin with FK506 suppressed AKT dephosphorylation and TFE3 activity. These results suggest that PPP3/calcineurin negatively regulates AKT phosphorylation and is involved in Cd-induced TFE3-dependent autophagy. Modulation of the PPP3/calcineurin-AKT-TFE3 autophagic-lysosomal machinery may offer novel therapeutic approaches for the treatment of Cd-induced bone damage. Abbreviations: ACTB: actin: beta; AKT: thymoma viral proto-oncogene; AMPK: AMP-activated protein kinase; ATG: autophagy related; Baf A1: bafilomycin A1; Cd: cadmium; FOXO3: forkhead box O3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MITF: melanogenesis associated transcription factor; MSC: mesenchymal stem sell; MTORC1: mechanistic target of rapamycin kinase complex 1; RPS6KB1: ribosomal protein S6 kinase: polypeptide 1; SGK1: serum/glucocorticoid regulated kinase 1; SQSTM1/p62: sequestosome 1;TFE3: transcription factor E3; TFEB: transcription factor EB; TFEC: transcription factor EC.

From the Cover: Autophagy Induction Contributes to Cadmium Toxicity in Mesenchymal Stem Cells via AMPK/FOXO3a/BECN1 Signaling.

Mesenchymal stem cells (MSCs) are a valuable in vitro model for investigating the bone toxicity of cadmium (Cd). Autophagy has been proposed to play a pivotal role in Cd-mediated toxicity. The FOXO family proteins are important transcription factors that are essential to autophagy induction. This study investigated the role of autophagy in Cd-induced skeleton damage and its potential mechanism. We exposed MSCs to different concentrations of cadmium chloride (3.5, 7, and 14 µM) for 24 h. We demonstrated that Cd treatment increased autophagic flux, and inhibition of autophagic process using BENC1 gene silencing blocked Cd-induced cell death. Cd treatment also significantly increased mRNA levels of various essential autophagy-related genes including ATG5, ATG12, BECN1, LC3, and ULK1. Specifically, Cd increased FOXO3a and FOXO1 expression at the mRNA and protein levels, and AMPK was demonstrated to enhance FOXO3a nuclear translocation and transcriptional activity by phosphorylating FOXO3a at specific serine residues (Ser588) in Cd-treated MSCs. Notably, knockdown of FOXO3a, but not FOXO1, prevented autophagy-related genes expression and autophagosome formation after Cd treatment. Taken together, our results demonstrate that Cd-induced cell death via the overactivation of FOXO3a-dependent autophagy. Modulation of the FOXO3a autophagy pathway may offer novel therapeutic approaches for the treatment of Cd-induced bone damage.