Ccaat-enhancer-binding proteins

CCAAT-enhancer-binding proteins (or C/EBPs) are a family of transcription factors, composed of six members called C/EBP α to C/EBP ζ. They promote the expression of certain genes through interaction with their promoter. Once bound to DNA, C/EBPs can recruit so-called coactivators (such as CBP, see ref. 2) that, in turn, can open up chromatin structure, or recruit basal transcription factors.

C/EBP proteins interact with the CCAAT (cytidine-cytidine-adenosine-adenosine-thymidine) box motif, which is present in several gene promoters. They are characterized by a highly conserved basic-leucine zipper (bZIP) domain at the C-terminus. This domain is involved in dimerization and DNA binding, like other transcription factors of the leucine zipper family like c-Fos and Jun. C/EBPs bZIP domain structure is composed of an α-helix that forms a coiled coil structure when it dimerizes. The different members of C/EBP family can form homodimers, heterodimers with another form of the C/EBPs and with other transcription factors that may or may not contain the leucine zipper domain. The dimerization is required for the activity of C/EBPs to bind specifically to DNA through a palindromic sequence in the major groove of the DNA. The C/EBP proteins also contain activation domains at the N-terminus and regulatory domains.

These proteins are found in hepatocytes, adipocytes, hematopoietic cells, spleen, kidney, brain and many others organs. C/EBPs proteins are involved in different cellular responses like in the control of cellular proliferation, growth and differentiation, metabolism, immunology and many others. All the members of the C/EBP family, except C/EBPγ that lacks transcriptional activation domain, can induce transcription, through their activation domains, by interacting with components of the basal transcription apparatus. Their expression is regulated at multiple levels through hormones, mitogens, cytokines, nutrients, etc.

The C/EBPα, -β, -γ and -δ genes are intronless and C/EBPε and -ζ have respectively two and four exons that lead in the case of C/EBP ε to four isoforms due to an alternative use of promoters and splicing. For C/EBPα and -β, different sizes of polypeptides can be produced by alternative use of initiation codons due to weak ribosome scanning mechanisms. The mRNA of C/EBPα can lead to two polypeptides and for C/EBPβ three different polypeptides are made: LAP* (38 kDa), LAP (35 kDa) and LIP (20 kDa). The most translated isoform is LAP, then LAP* and LIP; the latter can act as an inhibitor of the other C/EBPs by forming non-functional heterodimers.

This protein is expressed in the mammalian nervous system and has many implications in the nerve cells. C/EBPβ plays a role in neuronal differentiation, in learning and memory process, glial or neuronal cell functions and neurotrophic factory expression.

C/EBPβ function is regulated via multiple mechanisms: phosphorylation; acetylation; activation and repression via other transcription factors, oncogenic elements or chemokines; autoregulation, etc. C/EBPβ can interact with different proteins, such as CREB, NF-κB and others, leading to a trans-activation potential. Phosphorylation of C/EBPβ can have an activation or a repression effect. For example, phosphorylation of threonine 235 in human C/EBPβ, or of threonine 188 in mouse and rat C/EBPβ, is important for C/EBPβ trans-activation capacity; phosphorylation(s) of C/EBPβ in its regulatory domain can also modulate its function.

Role in Adipogenesis
C/EBPβ and δ are transiently induced during the early stages of adipocyte differentiation (adipogenesis), whereas C/EBPα is upregulated during the terminal stages of adipogenesis; each plays an important role in this process, as demonstrated from in vitro and in vivo studies. For example, murine embryonic fibroblasts (MEFs) from mice lacking both C/EBPβ and δ show impaired adipocyte differentiation in response to adipogenic stimuli [4]. In contrast, ectopic expression of C/EBPβ and δ in 3T3-L1 preadipocytes promotes adipogenesis, even in the absence of adipogenic stimuli [5, 6]. C/EBPβ and δ promote adipogenesis, at least in part, by inducing the expression of the 'master' adipogenic transcription factors, C/EBPα and PPARγ.

C/EBPα is required for both adipogenesis and normal adipocyte function. For example, mice lacking C/EBPα in all tissues except the liver (to avoid postnatal lethality) show abnormal adipose tissue formation [7]. Moreover, ectopic expression of C/EBPα in various fibroblast cell lines promotes adipogenesis [8]. It is likely that C/EBPα promotes adipogenesis by inducing the expression of PPARγ [9].

Role in Osteoporosis
C/EBPβ has recently been found to have a role in osteoporosis development. The full-length isoform of the C/EBPβ protein (LAP)activates the MafB gene, whereas the short isoform (LIP) suppress it; MafB gene activation suppress osteoclastogenisis (i.e., formation of osteoclasts). Thus upregulation of LAP diminishes the number of osteoclasts (which, in turn, weakens the osteoporotic process), whereas upregulation of LIP does the opposite (and increases the loss of bone mass).

The LAP/LIP balance is determined by mTOR protein. Inhibition of mTOR can stop osteoclast activity.