Bcl-Xl Is Important for the Antiapoptotic Activity of Gfi1 and Is Upregulated By Gfi1 through Hemgn

Gfi1 is a transcriptional repressor that is critically involved in hematopoiesis. Gfi1 represses its target genes primarily through interacting with the histone demethylase LSD1 via its SNAG domain. A major function of Gfi1 in the hematopoietic system is to inhibit stress-induced apoptosis. Gfi1 has been shown to participate in post-translational modifications of the p53 protein leading to suppression of p53 activity and in PRMT1-dependent methylation of MRE11 and 53BP1, which is essential for their roles in DNA repair. We show here that Gfi1 inhibited apoptosis induced not only by DNA damage, but also by growth factor withdrawal, inhibitory cytokine TGFβ and Myc activation in a manner that was independent of p53. Interestingly, induction of DNA damage with doxorubicin (Doxo) resulted in a gradual decrease in the protein level of the pro-survival Bcl-2 family member Bcl-xL in murine pro-B Ba/F3 and human p53-deficient Burkitt lymphoma Ramos cells transduced with the doxycycline (Dox)-inducible Gfi1 (BaF/Gfi1 and Ramos/Gfi1). DNA damage-induced downregulation of Bcl-xL protein was attenuated upon induction of Gfi1 expression with Dox, concomitant with upregulation of Bcl-xL mRNA. In contrast, the levels of Bcl-xL mRNA and protein were reduced upon knockdown of Gfi1 in human p53-deficient leukemic HL-60 and U937 cells, and in lineage negative (Lin-) bone marrow (BM) cells from Gfi1^-/- mice. We further show that Bcl-xL overexpression partially rescued the hypersensitivity to DNA damage of Gfi1-knocked down HL-60 and U937 cells, and of Gfi1^-/- BM cells whereas Bcl-xL knockdown partially abolished the protective effect of Gfi1 on DNA damage-induced apoptosis. Notably, interaction with LSD1 was required and sufficient for Gfi1-mediaed upregulation of Bcl-xL, suggesting that Gfi1 may augment Bcl-xL expression by an indirect mechanism. We recently showed that Gfi1, via repressing PU.1, upregulated the expression of Hemgn, a nuclear protein that has been shown to augment Bcl-xL expression. Interestingly, knockdown of Hemgn in BaF/Gfi1 and Ramos/Gfi1 cells abolished Bcl-xL upregulation by Gfi1. Furthermore, Gfi1 failed to upregulate Bcl-xL in PU.1-deficient cells. Examination of the time courses of Gfi1-mediated upregulation of Hemgn and Bcl-xL revealed that Hemgn upregulation preceded and correlated well with Bcl-xL upregulation. Together, these results indicate that Bcl-xL has an important role in mediating the antiapoptotic activity of Gfi1 and that Gfi1 upregulates Bcl-xL through Hemgn, which is dependent on PU.1 repression by Gfi1.

No relevant conflicts of interest to declare.