phosphothreonine and benzyloxycarbonylleucyl-leucyl-leucine-aldehyde

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.

Topics: Amino Acid Substitution; Androstadienes; Basic Helix-Loop-Helix Transcription Factors; Dimerization; HeLa Cells; Humans; Jurkat Cells; Leupeptins; Neoplasm Proteins; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Phosphothreonine; Proteasome Endopeptidase Complex; Proteasome Inhibitors; Protein Interaction Mapping; Protein Processing, Post-Translational; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; T-Cell Acute Lymphocytic Leukemia Protein 1; TCF Transcription Factors; Transcription Factor 7-Like 1 Protein; Transforming Growth Factor beta1; Ubiquitin; Ubiquitin-Protein Ligases; Wortmannin

The expression level of cyclin D1 plays a vital role in the control of proliferation. This protein is reported to be degraded following phosphorylation by glycogen synthase kinase 3 (GSK3) on Thr-286. We recently showed that phosphorylation of Thr-286 is responsible for a decline in cyclin D1 levels during S phase, an event required for efficient DNA synthesis. These studies were undertaken to test the possibility that phosphorylation by GSK3 is responsible for the S phase specific decline in cyclin D1 levels, and that this event is regulated by the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway which controls GSK3.. We found, however, that neither PI3K, AKT, GSK3, nor proliferative signaling activity in general is responsible for the S phase decline in cyclin D1 levels. In fact, the activity of these signaling kinases does not vary through the cell cycle of proliferating cells. Moreover, we found that GSK3 activity has little influence over cyclin D1 expression levels during any cell cycle phase. Inhibition of GSK3 activity by siRNA, LiCl, or other chemical inhibitors failed to influence cyclin D1 phosphorylation on Thr-286, even though LiCl efficiently blocked phosphorylation of beta-catenin, a known substrate of GSK3. Likewise, the expression of a constitutively active GSK3 mutant protein failed to influence cyclin D1 phosphorylation or total protein expression level.. Because we were unable to identify any proliferative signaling molecule or pathway which is regulated through the cell cycle, or which is able to influence cyclin D1 levels, we conclude that the suppression of cyclin D1 levels during S phase is regulated by cell cycle position rather than signaling activity. We propose that this mechanism guarantees the decline in cyclin D1 levels during each S phase; and that in so doing it reduces the likelihood that simple over expression of cyclin D1 can lead to uncontrolled cell growth.

Topics: Animals; beta Catenin; Cell Cycle; Cell Division; Cell Line; Chromones; Cyclin D; Cyclins; Fibroblasts; Genes, bcl-1; Glycogen Synthase Kinase 3; Glycogen Synthase Kinase 3 beta; Humans; Leupeptins; Lithium Chloride; Mice; Morpholines; NIH 3T3 Cells; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Phosphothreonine; Protein Biosynthesis; Protein Kinases; Protein Processing, Post-Translational; Proto-Oncogene Proteins c-akt; Recombinant Fusion Proteins; S Phase; Signal Transduction; Sirolimus; TOR Serine-Threonine Kinases