Hemgn Upregulation By Gfi1 through PU.1 Repression Contributes to Gfi1's Anti-Apoptotic Activity

Gfi1 is a transcriptional repressor that plays a critical role in hematopoiesis. The repressive activity of Gfi1 is mediated mainly by its SNAG domain that interacts with and thereby recruits the histone demethylase LSD1 to its target genes. An important function of Gfi1 is to protect hematopoietic cells against stress-induced apoptosis, which has been attributed to its participation in the posttranscriptional modifications of p53 protein, leading to suppression of p53 activity. In this study, we show that Gfi1 upregulated the expression of Hemgn, a nuclear protein, through a 16-bp promoter region spanning from +47 to +63 bp relative to the transcription start site (TSS), which was dependent on its interaction with LSD1. We further demonstrate that Gfi1, Ikaros and PU.1 bound to this 16-bp region. However, while Ikaros activated Hemgn and collaborated with Gfi1 to augment Hemgn expression, it was not required for Gfi1-mediated Hemgn upregulation. In contrast, PU.1 repressed Hemgn and inhibited Hemgn upregulation by Gfi1. Notably, PU.1 knockdown and deficiency, while augmenting Hemgn expression, abolished Hemgn upregulation by Gfi1. PU.1 (Spi-1) has been shown to be repressed by Gfi1. We show here that PU.1 repression by Gfi1 preceded and correlated well with Hemgn upregulation. Hemgn has been shown to be induced in response to DNA damage and inhibit DNA damage-induced apoptosis. Our data further demonstrate that Gfi1 and DNA damage acted in collaboration to augment Hemgn expression in a p53-independent manner. Notably, Hemgn knockdown partially abolished the protective effect of Gfi1 on DNA damage-induced apoptosis in both p53-expressing and p53-deficient cells. In contrast, partial restoration of Hemgn expression in the lineage negative (Lin^-) bone marrow cells from Gfi1^-/- mice diminished their hypersensitivity to DNA damage. Together, our date strongly suggest that Gfi1 upregulates Hemgn by repressing PU.1 and that Hemgn upregulation contributes to the anti-apoptotic activity of Gfi1 in a p53-independent manner.

No relevant conflicts of interest to declare.