Human monocyte to macrophage differentiation is accompa-nied by pronounced phenotypical changes and generally pro-ceeds in the absence of proliferation. The molecular events gov-erning this process are poorly understood. Here, we studied the regulation of the macrophage-specific chitotriosidase (CHIT1) gene promoter to gain insights into the mechanisms of tran-scriptional control during the differentiation of human blood monocytes into macrophages. We used transient transfections to define a cell type-specific minimal promoter that was mainly dependent on a proximal C/EBP motif that bound multiple C/EBP factors in gel shift assays. In depth analysis of occupied promoter elements using in vivo footprinting and chromatin immunoprecipitation analyses demonstrated the differentia-tion-associated recruitment of C/EBP␤ and PU.1 at the proxi-mal promoter in parallel with CHIT1 mRNA induction. Nota-bly, the induction of C/EBP␤ promoter binding strongly correlated with increased nuclear levels of Thr-235-phospho-rylated C/EBP␤ protein during the differentiation process, whereas C/EBP␤ mRNA and total protein expression remained relatively stable. Our data suggest an important constitutive gene regulatory function for C/EBP␤ in differentiated macro-phages but not in human blood monocytes.
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