A pentamer transcriptional complex including tal-1 and retinoblastoma protein downmodulates c-kit expression in normal erythroblasts.
Human proerythroblasts and early erythroblasts, generated in vitro by normal adult progenitors, contain a pentamer protein complex comprising the tal-1 transcription factor heterodimerized with the ubiquitous E2A protein and linked to Lmo2, Ldb1, and retinoblastoma protein (pRb). The pentamer can assemble on a consensus tal-1 binding site. In the pRb(-) SAOS-2 cell line transiently transfected with a reporter plasmid containing six tal-1 binding site, pRb enhances the transcriptional activity of tal-1-E12-Lmo2 and tal-1-E12-Lmo2-Ldb1 complexes but not that of a tal-1-E12 heterodimer. We explored the functional significance of the pentamer in erythropoiesis, specifically, its transcriptional effect on the c-kit receptor, a tal-1 target gene stimulating early hematopoietic proliferation downmodulated in erythroblasts. In TF1 cells, the pentamer decreased the activity of the reporter plasmid containing the c-kit proximal promoter with two inverted E box-2 type motifs. In SAOS-2 cells the pentamer negatively regulates (i) the activity of the reporter plasmid containing the proximal human c-kit promoter and (ii) endogenous c-kit expression. In both cases pRb significantly potentiates the inhibitory effect of the tal-1-E12-Lmo2-Ldb1 tetramer. These data indicate that this pentameric complex assembled in maturing erythroblasts plays an important regulatory role in c-kit downmodulation; hypothetically, the complex may regulate the expression of other critical erythroid genes.
Vitelli
L
L
Department of Hematology-Oncology, Istituto Superiore di Sanità, 00161 Rome, Italy.
Condorelli
G
G
Lulli
V
V
Hoang
T
T
Luchetti
L
L
Croce
C M
CM
Peschle
C
C
D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D051792:Basic Helix-Loop-Helix Proteins; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015536:Down-Regulation; D004912:Erythrocytes / Q000502:physiology*; D005786:Gene Expression Regulation; D006801:Humans; D060588:LIM Domain Proteins; D008297:Male; D008667:Metalloproteins / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins; D019009:Proto-Oncogene Proteins c-kit / Q000235:genetics / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism*; D000073942:T-Cell Acute Lymphocytic Leukemia Protein 1; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
D048868:Adaptor Proteins, Signal Transducing; D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D060588:LIM Domain Proteins; D008667:Metalloproteins; D011518:Proto-Oncogene Proteins; D019009:Proto-Oncogene Proteins c-kit; D016160:Retinoblastoma Protein; D000073942:T-Cell Acute Lymphocytic Leukemia Protein 1; D014157:Transcription Factors; C486312:LDB1 protein, human; C085117:LMO2 protein, human; C513188:Lmo2 protein, mouse; C407909:TCF3 protein, human; C489366:Tal1 protein, mouse; C067143:TAL1 protein, human