Abstract
It is well established that distinct cell metabolic alterations strongly contribute to the modulation of innate and adaptive immune responses. In the past decade the term immunometabolism has been introduced to describe the intracellular metabolic shifts of immune cells that lead to alterations of their functions. The pathogenesis of Sjögren's syndrome (SS), also referred to as autoimmune epithelitis, is not completely understood, but strong evidence supports the central role of the salivary glandular epithelial cells which are the target cells in the initiation of the autoimmune responses. Moreover, the altered epithelial functional phenotype, observed in the salivary gland lesion, may explain their disturbed secretory as well as immunoregulatory functions. From an immunometabolic perspective we have focused our studies on the endoplasmic reticulum (ER) of the salivary gland epithelial cells (SGEC) and the implication of its altered functions in the immunogenicity of these cells in SS. We showed that ER of SGEC in SS patients in situ is stressed and extensively dilated. Using salivary gland cell cultures, we studied in vitro the effect of ER stress on the metabolic behavior and viability of the cells. ER stress induced by thapsigargin increased spliced X-box binding protein-1 (XBP-1, transcription factor that increases the transcription of UPR target genes) levels in a time-dependent manner followed by autophagy and resulted to cell apoptosis. In apoptotic cells, we observed that the autoantigens Ro52 and La were redistributed in apoptotic blebs. During the induction of ER stress autophagy rescued the cells from apoptosis acting as a protective mechanism. We have also shown that adiponectin, a multifunctional hormone, is upregulated in the SGEC of SS patients acting in an autocrine or paracrine manner in the same cells. Adiponectin through activation of AMPK, the major sensor for cell energy demands, protected SGEC from apoptosis. Our results in combination with the work of others indicate that any effort of cell adaptation to ER stress may up regulate a proinflammatory milieu. This enhances the notion that metabolic alterations of the targeted epithelial cells in SS, independently of the cause, may induce an immunogenic phenotype. Therefore, SGEC have the potential to directly regulate susceptibility to and/or severity of autoimmune responses. Since adiponectin plays a vital role in the viability of SGEC through phosphorylation of AMPK, therapeutic interventions using PPAR agonists that upregulate adiponectin and concomitantly modify the energy metabolism, may be promising candidates for therapeutic intervention in SS.
