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 eIF2a-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.