Activating transcription factor 4 (ATF4) promotes bone formation in human bone marrow mesenchymal stem cells. However, the underlying mechanisms of ATF4 in high glucose-induced injury of osteoblast still remain unclear. Small interfering RNA and plasmid targeting ATF4 were used to transfect MC3T3-E1 cells to knock down and overexpress ATF4 using Lipofectamin 3000. Cell viability, alkaline phosphatase (ALP) activity and levels were determined by MTT, ALP kit assay, quantitative real-time (qRT)-PCR and Western blot. Osteocalcin (OCN) expression was determined by ELISA, PCR and Western blot. The mRNA and protein levels of ATF4, glucose regulated protein 78 kDa (GRP78) and C/EBP homologous protein (CHOP) were detected by PCR and Western blot. In the current study, viabilities of MC3T3-E1 cells were inhibited by high glucose. Meanwhile, the mRNA and protein levels of ATF4 were effectively up-regulated in high glucose-incubated MC3T3-E1 cells. By conducting functional experiments, silencing ATF4 induced by small interfering RNA partially reversed the inhibitory effects of high glucose on viabilities of MC3T3-E1 cells. We also found that the expressions of ER stress-related proteins (ATF4, GRP78 and CHOP) were higher in high glucose-treated MC3T3-E1 cells but were inhibited by siATF4. However, overexpression of AFT4 had opposite results, and high glucose attenuated the protein levels of osteogenic marker genes ALP and OCN, which were further inhibited by ATF4 knockout gene. Thus, ATF4 was a necessary gene for high glucose to inhibit the proliferation and differentiation of MC3T3-E1 cells.
Keywords: Activating transcription factor 4; MC3T3-E1; differentiation; endoplasmic reticulum stress; proliferation.