Importance of serine727 phosphorylated STAT1 in IFNγ-induced signaling and apoptosis of human salivary gland cells

Aim: It is reported that in salivary glands of Sjogren's syndrome (SS), interferon gamma (IFN gamma) and IFN gamma-inducible genes containing signal transducers and activators of transcription 1 (STAT1) are upregulated and play a crucial role in the pathogenesis of SS. The aim of this study is to clarify which phosphorylation of STAT1, serine727 (Ser(727)) or tyrosine701 (Tyr(701)) of STAT1, is important for IFN gamma signaling and IFN gamma-induced apoptosis in salivary gland cells. Methods: We established STAT1 Tyr(701) variant (tyrosine to phenylalanine; Y701F) and STAT1 Ser(727) variant (serine to alanine; S727A), which were transfected into human salivary gland (HSG) cells. HSG cells transfected with these mutant-STAT1 were analyzed on the expression of IFN gamma-inducible genes and apoptosis after stimulation with IFN gamma. Results: In Y701F mutant-STAT1 transfected HSG cells (Ser(727)-dominant HSG cells), IFN gamma-inducible genes such as IP10, IRF1, and Fas expression were increased after stimulation with IFN gamma. In Ser(727)-dominant HSG cells, the induction of apoptosis after stimulation with IFN gamma was also increased compared with S727A mutant-STAT1 transfected HSG cells (Tyr(701)-dominant HSG cells). Conclusion: Phosphorylation of Ser(727) in STAT1 might be more important in IFN gamma signaling and the induction of apoptosis in HSG cells than phosphorylation of Tyr(701). Accordingly, we propose that phosphorylation of Ser(727) in STAT1 could be a potentially suitable new therapeutic target for SS patients to prevent the destruction of salivary glands.