TGF-beta induces degradation of TAL1/SCL by the ubiquitin-proteasome pathway through AKT-mediated phosphorylation.

T-cell acute lymphoblastic leukemia 1 (TAL1), also known as stem cell leukemia (SCL), plays important roles in differentiation of hematopoietic and endothelial cells and is deregulated in a high percentage of T-cell acute lymphoblastic leukemia (T-ALL). In this report we show that the intracellular concentration of TAL1 is regulated by transforming growth factor beta (TGF-beta), which triggers its polyubiquitylation and degradation by the proteasome. This effect is mediated by AKT1, which phosphorylates TAL1 at threonine 90. Immunoprecipitation experiments showed that this event increases association of TAL1 with the E3 ubiquitin ligase CHIP. The E47 heterodimerization partner of TAL1 hinders this association. Our observations indicate that activation of the TGF-beta and phosphatidylinositol 3-kinase/AKT pathways might reverse overexpression of TAL1 in leukemic cells by inducing proteolysis of this important oncogene.