Effect of autophagy on irradiation-induced damage in osteoblast-like MC3T3-E1 cells

Autophagy is activated under radiation stress, which serves an important role in maintaining bone homeostasis. However, the underlying mechanisms of irradiation-induced autophagy in bone homeostasis is not well understood. The present study aimed to determine the effects of radiation-activated autophagy on pre-osteoblastic MC3T3-E1 cells. X-ray irradiation activated autophagy in a dose-dependent manner, with an increased fluorescence intensity of monodansylcadaverine staining, increased ratio of microtubule-associated protein 1 light chain 3β (LC3)-II/LC3-I, decreased p62 expression, and increased ATG5 and beclin-1 expression levels in MC3T3-E1 cells 72 h after irradiation compared with those in non-irradiated MC3T3-E1 cells. Irradiation reduced colony formation and mineralization in a dose-dependent manner in MC3T3-E1 cells at 2 and 3 weeks after irradiation, respectively. Decreased levels of alkaline phosphatase activity and runt-related transcription factor 2 expression were observed at 72 h post-irradiation. In addition, irradiation-induced apoptosis was accompanied by a decreased ratio of Bcl-2/BAX protein and increased the activity of caspase-3. By contrast, doxycycline (DOX)-inhibited autophagy attenuated the decreased colony formation and mineralization, and aggravated the increased cell apoptosis in irradiated MC3T3-E1 cells. Furthermore, the ratio of phosphorylated P38/P38 was observed to be higher following DOX treatment within 1 week of irradiation, which was reversed 2 weeks post-irradiation. In conclusion, DOX-inhibited autophagy aggravated X-ray irradiation-induced apoptosis at an early stage, but maintained cell proliferation and mineralization at a late stage in irradiated MC3T3-E1 cells.