[Protein biomarker screening and functional analysis of salivary exosomes in patients with ulcerative colitis]

To identify protein markers that may be associated with ulcerative colitis (UC) by analyzing differential proteins in the salivary exosomes from newly diagnosed patients with active UC and healthy controls (HC), and to investigate the function of salivary exosome-specific high-expression proteins in UC patients and their potential role in the pathogenesis of UC.

All patients and healthy controls were recruited from Peking University People' s Hospital. Whole saliva was obtained from 37 patients with newly diagnosed active ulcerative colitis (n=37) and apparently healthy controls (n=10). Salivary exosomes were extracted from samples, and the proteins within the exosomes were identified by liquid chromatograph-mass spectrometer (LC-MS/MS). The differentially expressed protein genes underwent gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis using the DAVID tool. In vitro, macrophages were co-cultured with salivary exosomes from UC group and those from HC group, respectively, and real-time quantitative polymerase chain reaction (qPCR) was used to detect levels of CD80⁺ and CD86⁺. Additionally, ELISA was performed to measure secretion levels of interleukin-6 (IL-6), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) in the cell supernatant.

A total of 259 proteins were co-expressed in saliva exosomes from UC group and HC group, among which 11 proteins were highly expressed in the UC group, including PDIA4, A2M, EEF2, C3, PSMA2, PSMB6, PSMA1, IGHG1, IGHG3, IGHG4 and SERPING1, while 4 proteins were lowly expressed in UC group, including TCN1, SLPI and SERPING. Functional analysis of these 15 proteins, along with 129 specific proteins found only in the UC patients and 69 specific proteins found only in HC patients, respectively, was conducted using GO/KEGG. The results revealed that in the UC group, proteasome-related proteins such as PSMA1, PSMA2 and PSMB6 expressions were increased in salivary exosomes while many key molecules involved in complement cascade pathways, such as C3 were up-regu-lated. In vitro co-culture experiments demonstrated that compared with healthy controls, the salivary exosomes of the UC patients in active stage could play a pro-inflammatory role by promoting the transformation of macrophages into M1 type cells that secrete inflammatory factors IL-1β, IL-6 and TNF-α.

Salivary exosomes in the UC patients may have the function of promoting inflammation. Analysis of protein levels in the saliva of the UC patients and healthy controls revealed significant differences in the expression levels of 15 co-expressed proteins between the two groups. Among them, C3, PSMA2, PSMB6 and PSMA1 were found to be mainly related to immune and inflammatory reactions in the UC group. These findings suggest that proteins with high specific expression in salivary exosomes of the UC patients have the potential to be used as a disease marker for UC diagnosis and may contribute to the pathogenesis of UC.