The TAL1/SCL transcription factor regulates cell cycle progression and proliferation in differentiating murine bone marrow monocyte precursors.

Monocytopoiesis involves the stepwise differentiation in the bone marrow (BM) of common myeloid precursors (CMPs) to monocytes. The basic helix-loop-helix transcription factor TAL1/SCL plays a critical role in other hematopoietic lineages, and while it had been reported to be expressed by BM-derived macrophages, its role in monocytopoiesis had not been elucidated. Using cell explant models of monocyte/macrophage (MM) differentiation, one originating with CMPs and the other from more committed precursors, we characterized the phenotypic and molecular consequences of inactivation of Tal1 expression ex vivo. While Tal1 knockout had minimal effects on cell survival and slightly accelerated terminal differentiation, it profoundly inhibited cell proliferation and decreased entry into and traversal of the G(1) and S phases. In conjunction, steady-state levels of p16(Ink4a) mRNA were increased and those of Gata2 mRNA decreased. Chromatin immunoprecipitation analysis demonstrated the association of Tal1 and E47, one of its E protein DNA-binding partners, with an E box-GATA sequence element in intron 4 of the Gata2 gene and with three E boxes upstream of p16(Ink4a). Finally, wild-type Tal1, but not a DNA binding-defective mutant, rescued the proliferative defect in Tal1-null MM precursors. These results document the importance of this transcription factor in cell cycle progression and proliferation during monocytopoiesis and the requirement for direct DNA binding in these processes.