title: Knockdown of autophagy-related protein 5, ATG5, decreases oxidative stress and has an opposing effect on camptothecin-induced cytotoxicity in osteosarcoma cells.
abstract: BACKGROUND
  Autophagy induction can increase or decrease anticancer drug efficacy. Anticancer drug-induced autophagy induction is poorly characterized in osteosarcoma (OS). In this study, we investigated the impact of autophagy inhibition on camptothecin (CPT)-induced cytotoxicity in OS.
  
  
  METHODS
  Autophagy-inhibited DLM8 and K7M3 metastatic murine OS cell lines were generated by infection with lentiviral shRNA directed against the essential autophagy protein ATG5. Knockdown of ATG5 protein expression and inhibition of autophagy was confirmed by immunoblot of ATG5 and LC3II proteins, respectively. Metabolic activity was determined by MTT assay and cell viability was determined by trypan blue exclusion. Acridine orange staining and immunoblotting for LC3II protein expression were used to determine autophagy induction. Oxidative stress was assessed by staining cells with HE and DCFH-DA followed by flow cytometry analysis. Mitochondrial membrane potential was determined by staining cells with TMRE followed by flow cytometry analysis. Immunoblotting was used to detect caspase activation, Parp cleavage and p53 phosphorylation.
  
  
  RESULTS
  Autophagy inhibition caused a greater deficit in metabolic activity and cell growth in K7M3 cells compared to DLM8 cells. K7M3 cells exhibited higher basal autophagy levels than DLM8 cells and non-transformed murine MCT3 osteoblasts. Autophagy inhibition did not affect CPT-induced DNA damage. Autophagy inhibition decreased CPT-induced cell death in DLM8 cells while increasing CPT-induced cell death in K7M3 cells. Autophagy inhibition reduced CPT-induced mitochondrial damage and CPT-induced caspase activation in DLM8 cells. Buthionine sulfoximine (BSO)-induced cell death was greater in autophagy-competent DLM8 cells and was reversed by antioxidant pretreatment. Camptothecin-induced and BSO-induced autophagy induction was also reversed by antioxidant pretreatment. Significantly, autophagy inhibition not only reduced CPT-induced oxidative stress but also reduced basal oxidative stress.
  
  
  CONCLUSIONS
  The results of this study indicate that autophagy inhibition can have an opposing effect on CPT-induced cytotoxicity within OS. The cytoprotective mechanism of autophagy inhibition observed in DLM8 cells involves reduced CPT-induced oxidative stress and not reduced DNA damage. Our results also reveal the novel finding that knockdown of ATG5 protein reduces both basal oxidative stress and drug-induced oxidative stress.
authors.affiliation: Division of Pediatrics, The University of Texas MD Anderson Cancer Center, Houston, TX 77054, USA. hollomon_mg@tsu.edu.
authors.forename: Mario G
authors.initials: MG
authors.lastname: Hollomon
authors.forename: Nancy
authors.initials: N
authors.lastname: Gordon
authors.forename: Janice M
authors.initials: JM
authors.lastname: Santiago-O'Farrill
authors.forename: Eugenie S
authors.initials: ES
authors.lastname: Kleinerman
mesh_terms: D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000633:toxicity; D001343:Autophagy / Q000187:drug effects / Q000235:genetics; D000071187:Autophagy-Related Protein 5; D001859:Bone Neoplasms / Q000235:genetics* / Q000378:metabolism*; D019328:Buthionine Sulfoximine / Q000494:pharmacology; D002166:Camptothecin / Q000633:toxicity*; D020169:Caspases / Q000378:metabolism; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects / Q000235:genetics; D004789:Enzyme Activation / Q000187:drug effects; D055785:Gene Knockdown Techniques; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000235:genetics* / Q000378:metabolism; D012516:Osteosarcoma / Q000235:genetics* / Q000378:metabolism*; D018384:Oxidative Stress / Q000187:drug effects* / Q000235:genetics*
chemical_list: D000972:Antineoplastic Agents, Phytogenic; C495773:Atg5 protein, mouse; D000071187:Autophagy-Related Protein 5; D008869:Microtubule-Associated Proteins; D019328:Buthionine Sulfoximine; D020169:Caspases; D002166:Camptothecin