Role of endoplasmic reticulum stress pathway in hydrostatic pressure-induced apoptosis in rat mandibular condylar chondrocytes

Excessive mechanical loads induce chondrocyte apoptosis and irreversible cartilage degeneration, but the underlying molecular mechanism is poorly understood. The aim of this study was to investigate the possible role of endoplasmic reticulum (ER) stress pathway in hydrostatic pressure (HP)-induced apoptosis in rat mandibular condylar chondrocytes. Chondrocytes were isolated from rat mandibular condylar cartilage and subjected to HP. Cell viability and apoptosis were assessed by Cell Counting Kit-8 and flow cytometry assay. Expression of ER stress-associated molecules was detected by quantitative real-time PCR and western blot analysis. In addition, expression of apoptosis-related proteins (bax, bcl-2, and cleaved-caspase-3) was assessed by western blot. To explore ER stress function, chondrocytes were pretreated with salubrinal before exposure to HP. Expression of type II collagen, aggrecan, MMP-13, and ADAMTS-5 was evaluated by real-time PCR. The results indicated that HP reduced cell viability in a magnitude- and time-dependent manner. HP-induced activation of ER stress pathway by increasing expression of GRP78, CHOP, caspase-12, PERK, and peIF2α in chondrocytes. Moreover, the expression of bax and cleaved-caspase-3 was increased, while the expression of bcl-2 was decreased in response to HP as the stress time prolonged. In addition, salubrinal suppressed HP-induced apoptosis, upregulated type II collagen and aggrecan mRNA expression, and downregulated MMP-13 and ADAMTS-5 mRNA expression in response to HP. These results demonstrate that HP induces apoptosis in mandibular condylar chondrocytes through ER stress-mediated apoptotic pathway. Suppression of ER stress by salubrinal prevents chondrocytes from undergoing apoptosis and matrix degradation induced by HP.