Ectopic TAL-1/SCL expression in phenotypically normal or leukemic myeloid precursors: proliferative and antiapoptotic effects coupled with a differentiation blockade.
The TAL-1 gene specifies a basic helix-loop-helix domain (bHLH) transcription factor, which heterodimerizes with E2A gene family proteins. tal-1 protein is abnormally expressed in the majority of T-cell acute lymphoblastic leukemias (T-ALLs). tal-1 is expressed and plays a significant role in normal erythropoietic differentiation and maturation, while its expression in early myeloid differentiation is abruptly shut off at the level of late progenitors/early differentiated precursors (G. L. Condorelli, L. Vitelli, M. Valtieri, I. Marta, E. Montesoro, V. Lulli, R. Baer, and C. Peschle, Blood 86:164-175, 1995). We show that in late myeloid progenitors (the phenotypically normal murine 32D cell line) and early leukemic precursors (the human HL-60 promyelocytic leukemia cell line) ectopic tal-1 expression induces (i) a proliferative effect under suboptimal culture conditions (i.e., low growth factor and serum concentrations respectively), via an antiapoptotic effect in 32D cells or increased DNA synthesis in HL-60 cells, and (ii) a total or marked inhibitory effect on differentiation, respectively, on granulocyte colony-stimulating factor-induced granulopoiesis in 32D cells or retinoic acid- and vitamin D3-induced granulo- and monocytopoiesis in HL-60 cells. Furthermore, experiments with 32D temperature-sensitive p53 cells indicate that aberrant tal-1 expression at the permissive temperature does not exert a proliferative effect but causes p53-mediated apoptosis, i.e., the tal-1 proliferative effect depends on the integrity of the cell cycle checkpoints of the host cell, as observed for c-myc and other oncogenes. tal-1 mutant experiments indicate that ectopic tal-1 effects are mediated by both the DNA-binding and the heterodimerization domains, while the N-terminally truncated tal-1 variant (M3) expressed in T-ALL malignant cells mimics the effects of the wild-type protein. Altogether, our results (i) indicate proliferative and antidifferentiative effects of ectopic tal-1 expression, (ii) shed light on the underlying mechanisms (i.e., requirement for the integrity of the tal-1 bHLH domain and cell cycle checkpoints in the host cell, particularly p53), and (iii) provide new experimental models to further investigate these mechanisms.
Condorelli
G L
GL
Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania 19107-5541, USA. condore1@jeflin.tju.edu
Tocci
A
A
Botta
R
R
Facchiano
F
F
Testa
U
U
Vitelli
L
L
Valtieri
M
M
Croce
C M
CM
Peschle
C
C
D017209:Apoptosis; D051792:Basic Helix-Loop-Helix Proteins; D002454:Cell Differentiation / Q000187:drug effects; D002455:Cell Division; D002762:Cholecalciferol / Q000494:pharmacology; D004268:DNA-Binding Proteins / Q000096:biosynthesis*; D016179:Granulocyte Colony-Stimulating Factor / Q000494:pharmacology; D018922:HL-60 Cells; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D007377:Interleukin-3 / Q000494:pharmacology; D007951:Leukemia, Myeloid / Q000378:metabolism; D010641:Phenotype; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis*; D000073942:T-Cell Acute Lymphocytic Leukemia Protein 1; D014157:Transcription Factors / Q000096:biosynthesis*; D014212:Tretinoin / Q000494:pharmacology
D051792:Basic Helix-Loop-Helix Proteins; D002762:Cholecalciferol; D004268:DNA-Binding Proteins; D016179:Granulocyte Colony-Stimulating Factor; D007377:Interleukin-3; D011518:Proto-Oncogene Proteins; D000073942:T-Cell Acute Lymphocytic Leukemia Protein 1; D014157:Transcription Factors; D014212:Tretinoin; C067143:TAL1 protein, human