Figure 2. Schematic representation of signalling pathways to autophagy and the regulation of autophagy in osteosarcoma cells.

The steps of autophagy are including induction, vesicle nucleation, vesicle elongation, autophagosome formation, fusion and autolysosome formation. Autophagy is trigged and precisely regulated by nutrient deprivation, growth factors, hormones, intracellular energy information and cell stressors, such as hypoxia, osmotic stress, reactive oxygen species (ROS) and viral infection, etc. The autophagy- related genes (Atg) are involved in several molecular complexes and the major 5 groups are essential in the process of autophagy including: Atg1 protein kinase complex, the class III phosphatidylinositol 3 kinase(PI3K)-Beclin 1 complex, Atg12 conjugation system, Atg8 conjugation system and Atg9. Chemoresistance of osteosarcoma is still an obstacle in clinical therapy and some effective agents to induce autophagy are listed in the therapy for osteosarcoma.