title: Diminished proteasomal degradation results in accumulation of Gfi1 protein in monocytes.
abstract: Gfi1 is a transcriptional repressor essential during myeloid differentiation. Gfi1-/- mice exhibit a block in myeloid differentiation resulting in the accumulation of an immature myelo-monocytic cell population and the complete absence of mature neutrophils. Even though mRNA levels of Gfi1 appear to be very low in monocytes, Gfi1 might play a role in the monocytic lineage as Gfi1-/- mice exhibit diminished monocyte-derived dendritic cells and disturbed cytokine production by macrophages in response to LPS. We show here that Gfi1 protein levels are mainly regulated by the ubiquitin-proteasome system. Upon forced monocytic differentiation of U937 cells, Gfi1 mRNA levels dropped but protein levels increased due to diminished proteasomal turnover. Similarly, Gfi1 mRNA levels are low in primary monocytes whereas the protein is clearly detectable. Conversely, Gfi1 mRNA levels are high in granulocytes but the protein is swiftly degraded by the proteasome in these cells. Chromatin immunoprecipitation experiments showed that Gfi1 binds to the promoter of several granulocyte-specific genes in primary monocytes, including C/EBPalpha, neutrophil elastase, and Gfi1 itself. The binding of the repressor Gfi1 to these promoters correlated with low expression of these genes in monocytes compared with granulocytes. Our data fit a model in which Gfi1 protein levels are induced in primary monocytes, due to diminished proteasomal degradation, to repress genes that play a role in granulocytic differentiation.
authors.affiliation: Central Hematology Laboratory, Radboud University Nijmegen Medical Centre for Molecular Life Sciences (NCMLS), PO Box 9101, 6500 HB Nijmegen, The Netherlands.
authors.forename: Jurgen A F
authors.initials: JA
authors.lastname: Marteijn
authors.forename: Laurens T
authors.initials: LT
authors.lastname: van der Meer
authors.forename: Liesbeth
authors.initials: L
authors.lastname: Van Emst
authors.forename: Theo
authors.initials: T
authors.lastname: de Witte
authors.forename: Joop H
authors.initials: JH
authors.lastname: Jansen
authors.forename: Bert A
authors.initials: BA
authors.lastname: van der Reijden
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000096:biosynthesis / Q000235:genetics; D019556:COS Cells; D002454:Cell Differentiation / Q000187:drug effects; D002460:Cell Line; D019070:Cell Lineage; D002522:Chlorocebus aethiops; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D006098:Granulocytes / Q000378:metabolism; D018922:HL-60 Cells / Q000378:metabolism; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007668:Kidney; D019272:Leukocyte Elastase / Q000096:biosynthesis / Q000235:genetics; D051379:Mice; D008969:Molecular Sequence Data; D009000:Monocytes / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014157:Transcription Factors / Q000378:metabolism*; D014212:Tretinoin / Q000494:pharmacology; D020298:U937 Cells / Q000187:drug effects / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D022763:CCAAT-Enhancer-Binding Protein-alpha; D004268:DNA-Binding Proteins; C498275:GFI1 protein, human; D012333:RNA, Messenger; D014157:Transcription Factors; D025801:Ubiquitin; D014212:Tretinoin; D019272:Leukocyte Elastase; D046988:Proteasome Endopeptidase Complex; D013755:Tetradecanoylphorbol Acetate