|TAL1/SCL encodes a basic helix-loop-helix transcription factor and was first identified through its involvement in a chromosomal translocation in T-cell acute lymphoblastic leukemia. The mouse Tal1 cDNA was cloned from a bone marrow (BM) macrophage cDNA library and although Tal1 has critical roles in other hematopoietic lineages, its function in monocyte-macrophage (MM) differentiation has not been studied.
Using an ex vivo system of MM differentiation we detected Tal1 expression at all stages of differentiation including activated macrophages. Over-expression of Tal1 in MM precursor cells increased differentiation and proliferation and activated immune response pathways. Gene knockout studies identified severe cell cycle and proliferative defects in Tal1-null cells but had minimal effects on cell survival and only modestly increased terminal differentiation. In conjunction with the cell cycle defect in Tal1-null cells, steady state levels of p16(Ink4a) mRNA were increased and Gata2 mRNA decreased. Quantitative chromatin immunoprecipitation (ChIP) analysis demonstrated association of Tal1 and E47, one of its E protein DNA-binding partners, with an E box-GATA element in a Gata2 enhancer and with three E boxes upstream of p16(Ink4a) at early stages of differentiation. But this association became low to non-existent in more mature cells. ChIP analysis in M1 myelo-monocytic leukemia cells detected Tal1 and E2A association in untreated cells and interleukin-6 (IL-6)-induced differentiation lowered Tal1 association only. In contrast, E2A occupancy in p16(Ink4a) sites was detected in both treated and untreated cells, consistent with relief of Tal1-directed repression of p16(Ink4a) in M1 cells upon differentiation. In summary, these studies have uncovered a critical role for Tal1 in cell cycle regulation during monocytopoiesis and suggest that TAL1 repression of p16(Ink4a) transcription and activation of Gata2 likely contributes. Additionally, wild-type Tal1, but not a DNA binding-defective mutant, rescued the proliferative defect in Tal1-null cells that underscores its requirement for DNA binding. Finally, the results in Tal1-over-expressing MM precursors may be relevant to the action of TAL1 in T-lymphoid and, in particular, myeloid leukemias.