Second, a Runx input into the URE was proposed to mediate silencing as well as activation (17). factor PU.1 provides essential pleiotropic inputs regulating multiple cell fate decisions during differentiation of blood cells from hematopoietic stem cells (HSCs). Its roles all depend on tight regulation of PU.1 itself, with different levels and patterns 5,15-Diacetyl-3-benzoyllathyrol of expression distinguishing various cell lineages and different developmental phases. PU.1 is essential for the development of myeloid and lymphoid lineages (22,30), but inappropriately controlled expression can cause severe developmental defects and/or malignancy. The precise basis of PU.1 regulation is therefore important to resolve and could be a model for multifunctional transcription factor deployment in development from stem cells. PU.1 is expressed specifically in HSCs and their derivatives. Upon differentiation of HSCs, PU.1 expression is silenced in erythroid cells but elevated in macrophages, continues at moderately high levels in neutrophils and most types of dendritic cells, 5,15-Diacetyl-3-benzoyllathyrol and is fixed at lower levels in committed B cells (4,24). A particularly dramatic shift of PU.1 expression occurs in the development of T cells. Although the earliest intrathymic precursors express PU.1 at HSC-like levels, PU.1 expression is silenced during the transition to the DN3 stage of T-cell development, as the cells undergo lineage commitment (3,33,35). This silencing is crucial, as forced expression of PU.1 beyond this stage causes a developmental block. PU.1 overexpression in DN3 thymocytes or a DN3-like immature T-cell line, 5,15-Diacetyl-3-benzoyllathyrol Adh.2C2, can also cause the cells to gain myeloid characteristics (2,10,19), linking the silencing of PU.1 to exclusion of alternative fate choices during T-lineage commitment. The mechanism of this essential silencing event is not fully understood. To date, most aspects of PU.1 regulation have been explained by invoking just two regulatory elements: the promoter and an upstream regulatory element (URE) at 14 kb upstream of the transcription start site of theSfpi1gene, which encodes PU.1. Both are suggested to contribute to cell type specificity (20). Thus, differential regulation would imply roles for different combinations of transcription factors working at these same elements. TheSfpi1promoter contains octamer binding sites affecting B-cell expression (7), while PU.1 can bind its own promoter with Sp1 to regulate itself in myeloid cells (8).Sfpi1promoter activity can also be directed in myeloid cells by C/EBP and AP-1 (5). These regulatory inputs toSfpi1may be modulated by cell-type-specific DNA methylation as well (1). The promoter alone cannot drive reporter expression in a chromatin context, however, and 5,15-Diacetyl-3-benzoyllathyrol the search for added regulatory function yielded the conserved URE (around kb 14), reported to be a myeloid-specific enhancer, enhancing promoter activity in a myeloid cell line but not in a mature T-cell line (20). In myeloid cells, the URE binds C/EBP (6,38) and PU.1 and may thus contribute to autoregulation as well (26,31). Data suggest that the URE could also play a role in silencing in T cells, and two mechanisms have been offered for this. First, a TCF/LEF site in the distal URE could promote repression as long as Wnt signals are absent (28). However, this mechanism does not explain continued PU.1 repression at stages of development when T cells are known to require canonical Wnt signaling (12,37). Second, a Runx input into the URE was proposed to mediate silencing as well as activation (17). Initiation of PU.1 expression in HSCs depends on Runx1, which unfolds the chromatin structure of theSfpi1gene and primes it for expression (16,25). The proximal URE enhancer has three conserved Runx1 sites able to bind Runx1. Mice with a deletion either of Runx1 itself or of these URE Runx sites showed a decrease in PU.1 expression in myeloid and B cells. In T-lineage cells, deletion of 5,15-Diacetyl-3-benzoyllathyrol Runx1 produces a developmental block at the DN2 stage (13,18), and the surviving cells have higher PU.1 expression, consistent with Runx1 repression ofSfpi1(17). However, URE Runx sites are maintained in an open state of accessibility, with the Runx sites apparently occupied, in PU.1-expressing myeloid and B cells and PU.1-negative T-lineage cells alike (15). Thus, it remains unresolved how both the initial Rabbit Polyclonal to TEP1 activation and the T-lineage-specific silencing ofSfpi1can be mediated by the same factor binding to the same sites. However, it is not proven that all regulation.
