Regulation of the Acetylation of C/EBP and its Functional Consequences.

Abstract

Because of the increasing incidence of obesity worldwide, understanding the molecular mechanisms controlling adipogenesis has become a critical area of research. Studies related to adipogenesis have led to the identification of genes involved sequentially in the differentiation of fat cells, such as C/EBPβ, C/EBPδ, C/EBPα, and PPARγ. This thesis focuses on the regulation of the activity of CCAAT/Enhancer Binding Protein (C/EBP) β, an early inducer of adipogenesis, which mediates the transcriptional activation of C/EBPα and PPARγ. Because C/EBPβ associates with the nuclear coactivator p300, which contains acetyltransferase activity, we hypothesized that C/EBPβ is acetylated and that acetylation of C/EBPβ may contribute to its role in adipogenesis. This work demonstrates that C/EBPβ is acetylated by p300 and P/CAF, and may be deacetylated by HDAC1, consistent with acetylation of C/EBPβ being a dynamic regulatory event. We have identified multiple acetylation sites in C/EBPβ, including a novel site at lysine (K) 39, in its transcriptional activation domain. Mutation of K39 impairs transcriptional activation of a variety of C/EBPβ target genes. Further, disruption of C/EBPβ acetylation at K39 renders it refractory to coactivation by p300. Expression of HDAC1 impairs acetylation of C/EBPβ at K39, and also decreases C/EBPβ-mediated transcription. The activation of C/EBPβ target genes involved in adipogenesis and adipocyte function, including C/EBPα, PPARγ, Glut-4 and leptin, is also compromised by mutation of K39 in C/EBPβ. Further, analysis of mouse adipose tissue and adipogenic 3T3-L1 cells indicates that endogenous C/EBPβ is acetylated at K39. Together, these findings support a model in which dynamic regulation of the acetylation of C/EBPβ contributes to its ability to mediate transcriptional activation of target genes associated with adipogenesis. Such insights suggest that acetylation at K39 contributes to adipocyte function, add to our ability to understand mechanisms contributing to obesity, and may reveal new therapeutic approaches for obesity and related health problems.