Autophagy relieves the function inhibition and apoptosis-promoting effects on osteoblast induced by glucocorticoid

Autophagy may be a major mechanism by which osteoblasts (OBs) protect against the negative effects of chronic glucocorticoid (GC) usage. OBs are closely associated with the remodeling that occurs in GC-induced osteoporosis (GIO). In osteocytes, in response to stress induced by GCs, several pathways are activated, including cell necrosis, apoptosis and autophagy. However, the role of autophagy in OBs following treatment with excess GCs has not been addressed. In the current study, confocal microscopy observation of green fluorescent protein-microtubule-associated protein 1 light chain 3 beta (LC3) punctuate, and western blotting for LC3II and Beclin 1 were performed for detection of autophagy in the MC3T3-E1 osteoblastic cell line. Flow cytometry and western blotting were used for the examination of apoptosis and expression of BAX apoptosis regulator (Bax)/apoptosis regulator Bcl-2 (Bcl-2). The expression of genes associated with osteoblastic function, runt-related transcription factor 2, alpha-1 type 1 collagen and osteocalcin, were measured by reverse transcription-quantitative polymerase chain reaction. The results indicated that autophagy was induced in OBs during dexamethasone (Dex) treatment in a dose-dependent manner. The level of autophagy did not continue to increase over time, but peaked at 48 h and then decreased gradually. Subsequently, flow cytometry was used to demonstrate that inhibition of autophagy induced apoptosis in OBs under Dex treatment, and was associated with the upregulation of Bax and the downregulation of Bcl-2 protein expression. Furthermore, the data suggested that the inhibition of autophagy also suppressed the expression of osteoblastic genes. By contrast, the stimulation of autophagy maintained the gene expression level under Dex treatment. The data revealed that autophagy is an important regulator of osteoblastic apoptosis through its interaction with Bax/Bcl-2, and maintains the osteoblastic function of MC3T3-E1 cells following GC exposure. In addition, these results indicated that the suppression of autophagy in OBs under chronic GC therapy may increase the prevalence of GIO and fragility fractures.