Mixed Lineage Kinase 3 Promotes Dexamethasone-Induced Thymocyte Apoptosis

Abstract

Abstract Dexamethasone (DEX) is a corticosteroid known to regulate T-lymphocyte growth/differentiation and used in the clinic as an anti-inflammatory and immunosuppressant agent. The glucocorticoid hormones, including DEX, can induce thymocyte apoptosis. It has been reported that DEX-induced thymocyte apoptosis is associated with an early stage ceramide generation. Since we reported earlier that ceramide induces Mixed Lineage Kinase 3 (MLK3) activation and therefore, we probed whether MLK3 inhibition could attenuate DEX-induced thymocyte apoptosis. Here, we report that genetic loss of MLK3, a member of MAP3K family attenuates DEX-induced apoptosis in thymocytes. To understand any differences in thymic T cell surface markers at the basal level between WT and MLK3−/− mice, we determined protein expression of various surface proteins involved in T cell activation and apoptosis. Flow cytometry analyses indicated that the genetic loss of MLK3 increases CD27 expression on thymic CD4+, CD8+, and CD4+CD8+ T cells; however, CD28 expression was mainly increased on CD8+ and CD4+CD8+ T cells due to loss of MLK3. Interestingly, genetic loss of MLK3 paradoxically regulated CD70 expression on thymic and splenic T cells. Furthermore, 4–5 weeks old WT and MLK3−/− mice were challenged with DEX for 24 h, and apoptosis in thymic T cells was determined. The flow cytometry results showed a significant decrease in the DEX-induced apoptosis in thymic CD4+, CD8+, and CD4+CD8+ T cells, isolated from MLK3−/− mice. Taken together, these results suggest that MLK3 could be a potential target for DEX-induced thymocyte apoptosis and its inhibitors can be used to mitigate thymocyte apoptosis.