Kazuo Waga

Leukemia-Related Transcription Factor TEL Is Negatively Regulated through Extracellular Signal-Regulated Kinase-Induced Phosphorylation

ABSTRACT TEL is an ETS family transcription factor that possesses multiple putative mitogen-activated protein kinase phosphorylation sites. We here describe the functional regulation of TEL via ERK pathways. Overexpressed TEL becomes phosphorylated in vivo by activated ERK. TEL is also directly phosphorylated in vitro by ERK. The inducible phosphorylation sites are Ser213 and Ser257. TEL binds to a common docking domain in ERK. In vivo ERK-dependent phosphorylation reduces trans-repressional and DNA-binding abilities of TEL for ETS-binding sites. A mutant carrying substituted glutamates on both Ser213 and Ser257 functionally mimics hyperphosphorylated TEL and also shows a dominant-negative effect on TEL-induced transcriptional suppression. Losing DNA-binding affinity through phosphorylation but heterodimerizing with unmodified TEL could be an underlying mechanism. Moreover, the glutamate mutant dominantly interferes with TEL-induced erythroid differentiation in MEL cells and growth suppression in NIH 3T3 cells. Finally, endogenous TEL is dephosphorylated in parallel with ERK inactivation in differentiating MEL cells and is phosphorylated through ERK activation in Ras-transformed NIH 3T3 cells. These data indicate that TEL is a constituent downstream of ERK in signal transduction systems and is physiologically regulated by ERK in molecular and biological features.

Functional regulation of TEL by p38-induced phosphorylation

TEL is a nuclear phosphoprotein that belongs to a member of the ETS family transcription factors. TEL acts as a tumor suppressor and is essential for establishing hematopoiesis in neonatal bone marrow. Because TEL possesses multiple putative mitogen-activated protein (MAP) kinase phosphorylation sites, we here investigated functional regulation of TEL via stress signaling pathways. We showed that TEL becomes phosphorylated in vivo by activated p38 but not by JNK1. The constitutive and inducible phosphorylation sites were found to be Ser(22) and Ser(257), respectively. TEL bound to p38 and was directly phosphorylated in vitro by p38. In vivo p38-dependent phosphorylation reduced trans-repressional abilities of TEL through ETS-binding consensus site. These data indicate that TELs functions are potentially regulated by p38 which is activated by various kinds of stresses. TEL could be a constituent downstream of the specific MAP kinase in the signal transduction system.

Leukemia-related transcription factor TEL accelerates differentiation of Friend erythroleukemia cells.

TEL belongs to a member of the ETS family transcription factors that represses transcription of target genes such as FLI-1. Although TEL is essential for establishing hematopoiesis in neonatal bone marrow, its role in erythroid lineage is not understood. To investigate a role for TEL in erythroid differentiation, we introduced TEL into mouse erythroleukemia (MEL) cells. Overexpressing wild-type-TEL in MEL cells enhanced differentiation induced by hexamethylene bisacetamide or dimethylsulfoxide, as judged by the increased levels of erythroid-specific δ-aminolevulinate synthase and β-globin mRNAs. TEL bound to a corepressor mSin3A through the helix–loop–helix domain. A TEL mutant lacking this domain still bound to the ETS binding site, but lost its transrepressional effect. This mutant completely blocked erythroid differentiation in MEL cells. Moreover, it showed dominant-negative effects over TEL-mediated transcriptional repression and acceleration of erythroid differentiation. Endogenous TEL mRNA was found to increase during the first 3 days in differentiating MEL cells and drastically decrease thereafter. All these data suggest that TEL might play some role in erythroid cell differentiation.