5,7,3′,4′-Tetramethoxyflavone protects chondrocytes from ER stress-induced apoptosis through regulation of the IRE1α pathway

Aim of the study: To investigate the roles of endoplasmic reticulum (ER) transmembrane sensor inositol-requiring enzyme-1 (IRE1)alpha signaling in ER stress-induced chondrocyte apoptosis, and to determine the molecular mechanisms underlying chondroprotective activity of 5,7,3', 4'-tetra-methoxyflavone (TMF) from Murraya exotica. Materials and methods: IRE1 alpha was knocked down by siRNA transfection in chondrocytes, which were harvested from rats' knee cartilages. Chondrocytes with IRE1 alpha deficiency were administrated with tunicamycin (TM) and TMF. Chondrocyte apoptosis was quantified by flow cytometry and DAPI/TUNEL staining. Expression of mRNA and proteins was quantified by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western-blot, respectively. Results: IRE1 alpha deficiency significantly increased the rate of TM-induced chondrocyte apoptosis, down-regulated the expression of pro-survival factors XBP1S and Bcl-2, and up-regulated pro-apoptotic factors CHOP, p-JNK, and caspase-3. TMF suppressed TM-induced chondrocyte apoptosis by activating the expression of IRE1 alpha, which reversed the expression patterns of downstream pro-survival and pro-apoptotic factors due to IRE1 alpha deficiency. Conclusion: The mechanism of TMF in protecting chondrocytes against ER stress-induced apoptosis might be associated with regulating the activity of ER sensor IRE1 alpha and its downstream pathway.