The role and mechanism of tight junctions in the regulation of salivary gland secretion.

Tight junctions (TJs) are cell-cell interactions that localize at the most apical portion of epithelial/endothelial cells. One of the predominant functions of TJs is to regulate material transport through paracellular pathway, which serves as a selective barrier. In recent years, the expression and function of TJs in salivary glands has attracted great interest. The characteristics of multiple salivary gland TJ proteins have been identified. During salivation, the activation of muscarinic acetylcholine receptor and transient receptor potential vanilloid subtype 1, as well as other stimuli, promote the opening of acinar TJs by inducing internalization of TJs, thereby contributing to increased paracellular permeability. Besides, endothelial TJs are also redistributed with leakage of blood vessels in cholinergic-stimulated submandibular glands. Furthermore, under pathological conditions, such as Sjögren's syndrome, diabetes mellitus, immunoglobulin G4-related sialadenitis, and autotransplantation, the integrity and barrier function of TJ complex are impaired and may contribute to hyposalivation. Moreover, in submandibular glands of Sjögren's syndrome mouse model and patients, the endothelial barrier is disrupted and involved in hyposecretion and lymphocytic infiltration. These findings enrich our understanding of the secretory mechanisms that link the importance of epithelial and endothelial TJ functions to salivation under both physiological and pathophysiological conditions.

Contribution of Interleukin-4-Induced Epithelial Cell Senescence to Glandular Fibrosis in IgG4-Related Sialadenitis.

OBJECTIVE: IgG4-related sialadenitis (IgG4-RS) is a chronic fibroinflammatory disease characterized by glandular fibrosis and hyposalivation. This study was undertaken to explore the role of cellular senescence in the pathogenesis of IgG4-RS-related fibrosis. METHODS: The expression of senescence markers and proinflammatory cytokines in the submandibular glands (SMGs) of IgG4-RS patients (n = 18) and controls (n = 14) was determined by proteomics, real-time polymerase chain reaction, Western blotting, and immunohistochemistry. After interleukin-4 (IL-4) treatment, high-throughput RNA sequencing was performed to identify the differentially expressed genes in SMG-C6 cells. A glandular fibrosis model was established by the intraglandular injection of IL-4 into mouse SMGs (n = 8 per group). RESULTS: Salivary acinar and ductal epithelial cells underwent senescence in IgG4-RS patients, as indicated by the elevated activity of senescence-associated ß-galactosidase, lipofuscin accumulation, enhanced expression of senescence markers (p53 and p16INK4A ), and up-regulation of senescence-associated secretory phenotype factors. Moreover, there was a significant increase in IL-4 levels in SMGs from IgG4-RS patients (P < 0.01), which positively correlated with p16INK4A expression and the fibrosis score. Incubation with IL-4 exacerbated salivary epithelial cell senescence by increasing the expression of p16INK4A through the reactive oxygen species (ROS)/p38 MAPK pathway. Supernatant collected from IL-4-induced senescent SMG-C6 cells enhanced fibroblast activation and matrix protein production (P < 0.05). Furthermore, injecting mice with IL-4 promoted fibrosis and senescence phenotypes in SMGs in vivo. CONCLUSION: The cellular senescence induced by IL-4 through the ROS/p38 MAPK-p16INK4A pathway promotes fibrogenesis in IgG4-RS. Our data suggest that cellular senescence could serve as a novel therapeutic target for treating IgG4-RS.