title: C/EBPβ-LAP*/LAP Expression Is Mediated by RSK/eIF4B-Dependent Signalling and Boosted by Increased Protein Stability in Models of Monocytic Differentiation.
abstract: The transcription factor C/EBPβ plays a key role in monocytic differentiation and inflammation. Its small isoform LIP is associated with proliferation at early premonocytic developmental stages and regulated via mTOR-dependent signalling. During later stages of (pre)monocytic differentiation there is a considerable increase in the large C/EBPβ isoforms LAP*/LAP which inhibit proliferation thus supporting terminal differentiation. Here, we showed in different models of monocytic differentiation that this dramatic increase in the LAP*/LAP protein and LAP/LIP ratio was accompanied by an only modest/retarded mRNA increase suggesting an important role for (post)translational mechanisms. We found that LAP*/LAP formation was induced via MEK/RSK-dependent cascades, whereas mTOR/S6K1 were not involved. Remarkably, LAP*/LAP expression was dependent on phosphorylated eIF4B, an acceleratory protein of RNA helicase eIF4A. PKR inhibition reduced the expression of eIF4B and C/EBPβ in an eIF2α-independent manner. Furthermore, under our conditions a marked stabilisation of LAP*/LAP protein occurred, accompanied by reduced chymotrypsin-like proteasome/calpain activities and increased calpastatin levels. Our study elucidates new signalling pathways inducing LAP*/LAP expression and indicates new alternative PKR functions in monocytes. The switch from mTOR- to RSK-mediated signalling to orchestrate eIF4B-dependent LAP*/LAP translation, accompanied by increased protein stability but only small mRNA changes, may be a prototypical example for the regulation of protein expression during selected processes of differentiation/proliferation.
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: René
authors.initials: R
authors.lastname: Huber
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Panterodt
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Bastian
authors.initials: B
authors.lastname: Welz
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Christmann
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Judith
authors.initials: J
authors.lastname: Friesenhagen
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Westphal
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Pietsch
authors.affiliation: Institute of Clinical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Korbinian
authors.initials: K
authors.lastname: Brand
mesh_terms: D000818:Animals; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000378:metabolism*; D002154:Calpain / Q000037:antagonists & inhibitors; D002454:Cell Differentiation; D002478:Cells, Cultured; D002918:Chymotrypsin / Q000037:antagonists & inhibitors; D039642:Eukaryotic Initiation Factors / Q000502:physiology*; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D008954:Models, Biological; D009000:Monocytes / Q000166:cytology*; D010766:Phosphorylation; D055550:Protein Stability; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D022782:CCAAT-Enhancer-Binding Protein-beta; D039642:Eukaryotic Initiation Factors; C031799:eIF-4B; D002918:Chymotrypsin; D002154:Calpain