4-Phenyl butyric acid prevents glucocorticoid-induced osteoblast apoptosis by attenuating endoplasmic reticulum stress

Apoptosis of osteoblasts triggered by high-dose glucocorticoids (GCs) has been identified as a major cause of osteoporosis. However, the molecular mechanisms underlying GC-induced osteoporosis remain elusive. This study was conducted to make clear the mechanism of GC-induced osteoblast apoptosis and to examine whether reduction of ER stress by 4-PBA inhibited osteoblast apoptosis. After treatment with dexamethasone (Dex) or hydrocortisone, cell viability was assessed using an MTT assay. Flow cytometry was performed to assess the apoptosis of MC3T3-E1 cells. The expression levels of ER stress-related proteins (CHOP, GRP78, eIF2α, and phospho-eIF2α) and apoptosis-related proteins (cleaved Caspase-3, Bcl-2, and Bax) in MC3T3-E1 cells were measured by Western blot analysis. We found that both Dex and hydrocortisone reduced cell proliferation and promoted apoptosis in MC3T3-E1 cells. In addition, the protein expression levels of cleaved Caspase-3 and Bax increased and the protein expression level of Bcl-2 decreased in MC3T3-E1 cells exposed to Dex. In addition, the Dex exposure also resulted in a release of cytochrome c (Cyt C) from mitochondria. The cellular ATP content was decreased following prolonged treatment with Dex. 4-PBA attenuated ER stress and mitochondrial dysfunction induced by Dex in MC3T3-E1 cells. Dex-mediated apoptosis of MC3T3-E1 cells is aggravated by ER stress. Moreover, Dex-induced apoptosis in MC3T3-E1 cells was inhibited by 4-PBA, suggesting that ER stress involved in Dex-induced apoptosis. In conclusion, inhibition of ER stress by 4-PBA could reduce GC-induced apoptosis in MC3T3-E1 cells.