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

SummaryThe 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 Sj€ ogren's syndrome (SS) autoantigens Ro/ Sj€ ogren's syndrome-related antigen A (SSA) and La/Sj€ ogren's syndromerelated 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.

Identification of KRT16 as a target of an autoantibody response in complex regional pain syndrome

“…In our model, we found ANXA2 and ENO3 in the membrane fraction of the cell in the fracture group only, which could make it more accessible to various immunoglobulins. In Sjogrens syndrome, for example, major epithelial autoantigens display cellular re-distribution into the cell membrane, therefore making them more available to auto-antibodies (Katsiougiannis et al, 2015). Re-localization of autoantigens can even occur extracellularly; The nuclear DNA sensor IFI16, elevated in various autoimmune conditions, has been shown to re-localize from the nucleus to the extracellular milieu (Sporn and Vilcek, 1996).…”

TRPC1 expression and function inhibit ER stress and cell death in salivary gland cells

“…Studies have shown that ER stress is activated in minor salivary gland epithelial cells from Sjögren's syndrome (SS) patients. Moreover, an interplay between ER stress‐induced autophagy and apoptosis has also been suggested with regard to SS autoantigens Ro/SSA and La/SSB . The ER is an important intracellular organelle that is not only important for regulating Ca 2+ homeostasis but is also essential for the synthesis and folding of proteins.…”

How stress contributes to autoimmunity—lessons from Sjögren's syndrome

“…More recently, a study from our group demonstrated that induction of apoptosis in HSG cells after triggering of ER stress with TG, showed that autoantigens Ro and La were detected in the apoptotic blebs of these cells . In addition, IL‐6 mRNA was one of the cytokines that was detected in ductal and acinar cells in salivary gland biopsies from SS patients as well as IRF1 mRNA was upregulated in the epithelial cells of SS patients .…”