Opposing effects of the basic helix-loop-helix transcription factor SCL on erythroid and monocytic differentiation

Abstract

The SCL gene (also called Tal-1 or TCL5) was identified because of its association with chromosomal translocations in childhood T-cell lymphoid leukemias. SCL codes for a basic helix-loop-helix (bHLH) factor that can function as a transcriptional activator or repressor. In the adult, SCL expression is restricted to hematopoietic cells and tissues, but its function in the process of lineage commitment is unknown. The present study was designed to address the role of SCL in hematopoietic cell differentiation. SCL expression was determined in primary hematopoietic cells through the screening of cDNA samples obtained by reverse transcription-polymerase chain reaction (RT-PCR) from single cells at different stages of differentiation. SCL RNA expression was highest in bipotential and committed erythroid precursors and diminished with subsequent maturation to proerythroblasts and normoblasts. In contrast, SCL mRNA was low to undetectable in precursors of granulocytes and monocytes and their maturing progeny. The same pattern of expression was observed after erythroid or monocytic differentiation of a bipotent cell line, TF-1, in that SCL mRNA levels remained elevated during erythroid differentiation and were downregulated with monocytic differentiation. Accordingly, TF-1 was chosen as a model to investigate the functional significance of this divergent pattern of SCL expression in the two lineages. Four independent clones stably transfected with an SCL expression vector exhibited enhanced spontaneous and delta-aminolevulinic acid-induced erythroid differentiation as measured by glycophorin expression and hemoglobinization, consistent with the view that SCL is a positive regulator of erythroid differentiation. Furthermore, constitutive SCL expression interfered with monocytic differentiation, as assessed by the generation of adherent cells and the expression of Fc gamma RII in response to TPA. These results suggest that the downregulation of SCL may be required for monocytic differentiation.

ETO2 coordinates cellular proliferation and differentiation during erythropoiesis

“…Through unbiased mass spectrometry analysis, we identify here these proteins as components of the core TAL‐1 complex, in which TAL‐1 acts as a nucleation platform (Lecuyer et al , 2002). In this complex, TAL‐1 is limiting since elevating TAL‐1 dosage in the cell increases GPA expression (Hoang et al , 1996), while E2A is not, as shown here for E12. Given that E2A is partitioned in TAL‐1 and non‐TAL‐1 complexes, these observations are not surprising.…”

“…NIH 3T3, TF‐1, UT‐7 and MEL cells were cultured and stimulated to differentiate as described previously (Hoang et al , 1996; Lahlil et al , 2004). Human CD34 + cells were purified and differentiated as described (Freyssinier et al , 1999; Ravet et al , 2004).…”

Lineage plasticity and commitment in T‐cell development

“…This consideration suggests that the availability of regulators that can drive cells to other fates is a necessary side‐effect of the initial growth factor requirements of T‐cell precursors, before they can switch over to TCR dependence. Correct regulation of the levels of these regulators is probably vital to channel their activities correctly; PU.1 and SCL effects, and Id2 effects by their nature, are dose dependent (76, 80–83). Whether or not the levels expressed in early T‐cell precursors account for the lineage plasticity observed in these populations is discussed later.…”

Erythropoietin-induced erythroid differentiation of the human erythroleukemia cell line TF-1 correlates with impaired STAT5 activation.

“…TF-l cells have been derived from a patient with erythroleukemia (Kitamura et al, 1989) and express high levels of an abnormal truncated EpoR (Winkelmann et al, 1995). The growth of these cells is dependent on the presence of granulocyte-macrophage colony stimulating factor (GM-CSF) or interleukin (IL)-3, while they proliferate only transiently in response to Epo which induces an erythroid differentiation program after 1 week of culture (Hoang et al, 1996).…”