Ubiquitination modulates expression of the serine arginine-rich splicing factor 1 (SRSF1) in activated T cells and in SLE patients (HUM7P.305)

Abstract

Abstract T cells from patients with systemic lupus erythematosus (SLE) are poor producers of the vital cytokine interleukin (IL)-2. We recently showed that SLE T cells express lower levels of the serine arginine- rich splicing factor 1 (SRSF1), more so in patients with active disease. Forced expression of SRSF1 restores IL-2 production, indicating that SRSF1 is important in T cell function, and its reduced expression is a potential molecular defect in SLE T cells. The goal of this study was to determine the mechanism/s regulating SRSF1 expression in human T cells and in SLE patients. T cell activation increased mRNA expression of SRSF1 by 3- to 4-fold within 24 hours, while protein expression did not alter much. This discrepancy between mRNA and protein expression was not due to reduced RNA stability. Activating T cells in presence of specific proteolysis inhibitors revealed that proteasome- but not lysosome-mediated mechanisms contribute to reduced SRSF1 expression. Accordingly, immunoprecipitation (IP) studies showed increased ubiquitination of SRSF1 in activated T cells compared with resting cells. More interestingly, SRSF1 showed increased ubiquitination in SLE patients compared to healthy controls and rheumatoid arthritis patients. Our results identify a novel mechanism of SRSF1 protein regulation in human T cells and suggest that increased ubiquitination and proteasome degradation represent a molecular mechanism that contributes to reduced SRSF1 expression in SLE T cells.