Optimization and application of a flow cytometric PU.1 assay for murine immune cells

KDM6A Lysine Demethylase Directs Epigenetic Polarity of MDSCs during Murine Sepsis

Sepsis-induced myeloid-derived suppressor cells (MDSCs) increase mortality risk. We previously identified that long non-coding RNA Hotairm1 supports myeloid precursor shifts to Gr1+CD11b+ MDSCs during mouse sepsis. A major unanswered question is what molecular processes control Hotairm1 expression. In this study, we found by a genetic deletion that a specific PU.1-binding site is indispensable in controlling Hotairm1 transcription. We then identified H3K4me3 and H3K27me3 at the PU.1 site on the Hotairm1 promoter. Controlling an epigenetic switch of Hotairm1 transcription by PU.1 was histone KDM6A demethylase for H3K27me3 that derepressed its transcription with possible contributions from Ezh2 methyltransferase for H3K27me3. KDM6A knockdown in MDSCs increased H3K27me3, decreased H3K4me3, and inhibited Hotairm1 transcription activation by PU.1. These results enlighten clinical translation research of PU.1 epigenetic regulation as a potential sepsis immune-checkpoint treatment site.

PU.1 regulates Ccr7 gene expression by binding to its promoter in naïve CD4+ T cells

C‐C chemokine receptor type 7 (CCR7) is expressed on naïve T cells, B cells, and activated dendritic cells (DCs). We previously demonstrated that the transcription factor PU.1/Spi1 positively regulates the expression of CCR7 in DCs. In the present study, we investigated the role of PU.1 in CCR7 expression in T cells. To confirm whether PU.1 is involved in the expression of CCR7, we conducted a ChIP assay in various T cells purified from splenocytes and thymocytes and found that PU.1 binds to the Ccr7 promoter‐proximal region in spleen naïve CD4⁺ T cells, but not in thymocytes. Small interfering RNA‐mediated PU.1 knockdown resulted in decreased CCR7 expression in spleen naïve CD4⁺ T cells. Compared to naïve CD4⁺ T cells, Spi1 and Ccr7 mRNA levels decreased in Th1 and Th2 cells, in which PU.1 did not bind to the Ccr7 promoter, suggesting that CCR7 expression decreases due to the dissociation of PU.1 from the Ccr7 promoter during the development of effector T cells from naïve T cells. Collectively, we concluded that CCR7 expression level correlates with the binding level of PU.1 to the Ccr7 promoter and PU.1 acts as a transcriptional activator of the Ccr7 gene in naïve CD4⁺ T cells.

Aggressive acute myeloid leukemia in PU.1/p53 double-mutant mice

PU.1 downregulation within hematopoietic stem and progenitor cells (HSPCs) is the primary mechanism for the development of acute myeloid leukemia (AML) in mice with homozygous deletion of the upstream regulatory element (URE) of PU.1 gene. p53 is a well-known tumor suppressor that is often mutated in human hematologic malignancies including AML and adds to their aggressiveness; however, its genetic deletion does not cause AML in mouse. Deletion of p53 in the PU.1(ure/ure) mice (PU.1(ure/ure)p53(-/-)) results in more aggressive AML with shortened overall survival. PU.1(ure/ure)p53(-/-) progenitors express significantly lower PU.1 levels. In addition to URE deletion we searched for other mechanisms that in the absence of p53 contribute to decreased PU.1 levels in PU.1(ure/ure)p53(-/-) mice. We found involvement of Myb and miR-155 in downregulation of PU.1 in aggressive murine AML. Upon inhibition of either Myb or miR-155 in vitro the AML progenitors restore PU.1 levels and lose leukemic cell growth similarly to PU.1 rescue. The MYB/miR-155/PU.1 axis is a target of p53 and is activated early after p53 loss as indicated by transient p53 knockdown. Furthermore, deregulation of both MYB and miR-155 coupled with PU.1 downregulation was observed in human AML, suggesting that MYB/miR-155/PU.1 mechanism may be involved in the pathogenesis of AML and its aggressiveness characterized by p53 mutation.