Endoplasmic reticulum stress causes autophagy and apoptosis leading to cellular redistribution of the autoantigens Ro/Sjögren's syndrome-related antigen A (SSA) and La/SSB in salivary gland epithelial cells

Abstract

Summary The aim of this study was to examine the levels of endoplasmic reticulum (ER) stress in minor salivary glands, to investigate the interplay between ER stress-induced autophagy and apoptosis in human salivary gland (HSG) cells and to test the effect of ER stress-induced apoptosis on the cellular redistribution of the two major Sjögren's syndrome (SS) autoantigens Ro/Sjögren's syndrome-related antigen A (SSA) and La/Sjögren's syndrome-related antigen B (SSB). Minor salivary gland biopsies from SS patients and sicca controls were examined by immunohistochemistry for the expression of 78 kDa glucose-regulated protein/binding immunoglobulin protein (GRP78/BiP) as an indicator of unfolded protein response (UPR). HSG cells were treated with thapsigargin (TG) and cell viability, autophagy and apoptosis were assessed. Immunoblot was applied to detect the conversion of LC3I to LC3II and the protein levels of GRP78/BiP and X-box binding protein-1 (XBP-1). Apoptosis was evaluated by a single-stranded DNA enzyme-linked immunosorbent assay (ELISA). Ro/SSA and La/SSB localization was visualized using immunofluorescence. GRP78/BiP was expressed by acinar and ductal epithelial cells in salivary glands of patients and sicca controls. TG treatment induced autophagy, as indicated by enhanced protein expression of LC3II. The protein levels of UPR marker XBP-1 were increased after TG treatment, while GRP78/BiP levels were decreased. TG treatment resulted in induction of HSG apoptosis. Ro/SSA and La/SSB autoantigens were localized predominantly to the cytoplasm in resting cells, while they were redistributed to cell membrane and blebs in the apoptotic cells. In conclusion, ER stress is activated in minor salivary gland epithelial cells from SS patients and controls. ER stress-induced apoptosis in HSG cells leads to cell surface and apoptotic blebs relocalization of Ro/SSA and La/SSB autoantigens.