The Wilms' tumor suppressor WT1 enhances CD95L expression and promotes activation-induced cell death in leukemic T cells.

The role of Wilms' tumor suppressor 1 (WT1) in leukemogenesis has been investigated mostly in acute (AML) and chronic (CML) myeloid leukemias. So far, its oncogenic role has been controversially discussed because both overexpression and inactivating mutations are found. A recent study on primary samples from patients with acute T-cell leukemia (T-ALL) revealed that most of them do not express WT1 proteins although they express WT1 mRNA. In our study, we investigated WT-1 expression in ten T-ALL cell lines established from leukemia/lymphoma patients. We show that consistent with the finding in primary T-ALL cells, most of the leukemic T-cell lines tested do not overexpress WT1 proteins. We found that leukemic T-cells overexpressing WT1 protein produce higher levels of CD95L and show elevated CD95L-mediated activation-induced cell death (AICD) compared to cells lacking or expressing low levels of WT1. Ectopic expression of WT1 in the WT1-nonexpressing leukemic T-cell line increases CD95L expression and elevates activation-induced apoptosis, whereas silencing WT1 expression in the WT1-overexpressing leukemic T-cell line by siRNA confers reduced CD95L expression and reduction in AICD. Chromatin immunoprecipitation and luciferase-promoter reporter analysis demonstrate that WT1 binds to and enhances CD95L promoter activity through the Egr-binding sites. Our study provides a new role of WT1 in regulation of CD95L-mediated cell death.