Contribution of endoplasmic reticulum stress response to the mechanosensitivity alteration in osteocytes under simulated microgravity

The molecular and cellular mechanism of microgravity-induced osteoporosis is unclear. Our previous study showed that it may be due to the decreased mechanosensitivity of osteocytes. Some research indicated that endoplasmic reticulum stress (ERS) could affect the mechanical response of cells. This study investigated the role of ERS in the altered mechanosensitivity of osteocytes under simulated microgravity (SMG). MLO-Y4 cells (osteocyte-like cell line) were cultured under SMG for 48 h and then applied 15 dyn/cm2 fluid shear stress (FSS) in the flow chamber for 1 h. In addition, melatonin (MT, 200 nmol/L), an ERS antagonist, was added to inhibit ERS. The morphology of endoplasmic reticulum was examined by immunofluorescence, ERS marker (GRP78) by Western blot and qPCR, unfolded protein pathway (UPR) related factors (IRE1, XBP1s, PERK, ATF4, ATF6) and apoptosis-related factor (CHOP) by qPCR and the mechanical sensitive factors (NO, PGE2 and ATP) by ELISA were determined. The results showed that under SMG, the endoplasmic reticulum enlarged as the endoplasmic reticulum/nucleus area ratio increased significantly and the morphology was irregular; the expression of GRP78 and UPR related factors increased significantly; NO, PGE2 and ATP induced by FSS decreased. Furthermore, MT could counter above SMG-induced effect. ERS may play an important role in the mechanosensitivity alteration in osteocytes under microgravity.