Abstract
The CCAAT/enhancer-binding protein β (C/EBPβ) is expressed as three isoforms (LAP*, liver-enriched activating protein (LAP), and liver-enriched inhibitory protein (LIP)) that differentially regulate gene expression. The interplay between LAP*, LAP, and LIP in regulating cellular processes is largely unknown, and LIP has been largely regarded to repress transcription through a passive heterodimerization-dependent mechanism. Recently, we have shown that p300/GCN5 and mSin3A/HDAC1 differentially regulate the ability of C/EBPβ to stimulate preadipocyte differentiation through activation of C/ebpα transcription. Here, we have mapped requirements for binding of mSin3A/HDAC1 to LAP/LAP* and LIP to a 4-amino acid motif in the central region of LAP/LAP* (residues 153-156) and the N terminus of LIP. Reducing mSin3A/HDAC1 binding to LAP/LAP* and LIP through deletion of this motif reduced the recruitment of HDAC1 to the C/ebpα promoter and increased preadipocyte differentiation stimulated by insulin and 1-methyl-3-isobutylxanthine. Additional studies showed that the interaction of HDAC1 with LIP provides for active repression of C/ebpα transcription and is largely responsible for the ability of LIP and HDAC1 to repress preadipocyte differentiation. Thus, although mSin3A/HDAC1 interacted readily with LAP/LAP* in addition to LIP and that expression of LAP/LAP* was sufficient to recruit HDAC1 to the C/ebpα promoter, mutations in C/ebpβ that abrogated HDAC1 association to LAP/LAP* in the absence of LIP provided no additional stimulation of differentiation or transcription beyond the deletion of LIP alone. The implication of these results for the interaction between p300/GCN5 and mSin3A/HDAC1 in regulating C/EBPα transcription and preadipocyte differentiation are discussed.
Highlights
The CCAAT/enhancer-binding protein  (C/EBP)2 is a basic leucine zipper transcription factor that plays key roles in cellular differentiation, proliferation, stress responses, and inflammation
Compromising histone deacetylase 1 (HDAC1) association with LAP/LAP* had little further effect on their transcriptional activation potential or ability to stimulate preadipocyte differentiation beyond what was obtained by deletion of LIP. These results provide the first evidence that transcriptional repression through LIP occurs through an active HDAC1-dependent mechanism and indicate that the effects of mSin3A/HDAC1 on C/EBP and preadipocyte differentiation are primarily dependent on LIP
Amino Acids 153–156 of C/EBP Are Required for mSin3A/ HDAC1 Binding—Previously, we have shown that mSin3A directly interacts with C/EBP, with HDAC1 association to C/EBP mediated through its binding to mSin3A [5]
Summary
The CCAAT/enhancer-binding protein  (C/EBP)2 is a basic leucine zipper (bZip) transcription factor that plays key roles in cellular differentiation, proliferation, stress responses, and inflammation. Dex treatment was required for these experiments, as the low level of differentiation obtained by treating the cells with insulin and MIX alone was completely abrogated by both LIP and LIP6C (data not shown), making it difficult to determine a specific effect for mSin3A/HDAC1 association.
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