Gfi1 schematic and role in MM-induced block to OB differentiation. (A) Gfi1 is a multifunctional protein that interacts with many other proteins as indicated.⁴⁹  Nuclear localization signal domains lie within the N-terminal SNAG domain and within the region encoding the 6 C-terminal C2-H2 zinc finger domains.³⁵  Zinc fingers 3, 4, and 5 are required for sequence-specific DNA binding.⁴⁸  The SNAG domain is required for transcriptional repression by Gfi1 via binding corepressors CoREST and histone lysine demethylase LSD1 (H3K4 demethylation).¹⁸  It also binds and stabilizes GATA3, independently of the repressor binding amino acids.⁴³  The intermediate domain between the SNAG and Zn-finger domains is also involved in protein-protein interactions, such as binding the corepressors histone lysine methyltransferases G9a (H3K9 and H3K27 dimethylation/trimethylation),¹⁶  histone deacetylases HDAC1-3 (H3K9 deacetylation),^16,17  and Ajuba LIM domain protein.⁵⁰  This region also binds SUMO-E3 ligase PIAS3 (thereby enhancing STAT3 activation),⁴⁵  the splicing factor U2AF26,⁴⁶  and the p65 subunit of NF-κB (thereby inhibiting p65-p50 DNA binding).⁴⁴  The Zn-finger domains also are involved in protein-protein interactions, such as binding to the transcription factors Spi-1/PU.1,³⁸  ETS1,³⁹  PDRM5,⁴⁰  and Miz-1,⁴¹  and the corepressor ETO.¹⁷  The DNA-binding domain also interacts with the ubiquitin E3 ligases Triad1⁴²  and Mdm2.⁴³  Direct gene repression by Gfi1 requires both binding to the target gene and interaction with corepressor proteins. Mutations in Gfi1 that either block SNAG binding to corepressors (P2A)³⁵  or Zn-finger DNA-binding (N382S)³¹  both can act as dominant-negatives and relieve Gfi1 target gene repression. (B) Model for Runx2 repression in BMSCs by MM cells. The MM cells secrete TNF-α and other factors (eg, IL-7) that interact with cognate receptors on the BMSCs to increase the expression of Gfi1 and induce its translocation from the cytoplasm to the nucleus. Gfi1 tethered in the cytoplasm could down-regulate Runx2 by mechanisms involving modulation of other transcription factors. Eventual induction of Gfi1 translocation into the nucleus allows direct recruitment of corepressors to target genes, such as Runx2.