Review
The C/EBP family of transcription factors: a paradigm for interaction between gene expression and proliferation control

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In recent years, a link between the transcriptional regulators of lineage-specific gene expression and progenitor proliferation control has emerged. A main exponent of this phenomenon is the CCAAT/enhancer binding protein (C/EBP) family of basic region-leucine zipper proteins. These transcription factors control the differentiation of a range of cell types, and have key roles in regulating cellular proliferation through interaction with cell cycle proteins. More recently, their position at the crossroads between proliferation and differentiation has made them strong candidate regulators of tumorigenesis, and C/EBPs have been described as both tumor promoters and tumor suppressors.

Introduction

CCAAT/enhancer binding proteins (C/EBPs) are a family of six proteins, the most widely expressed, and most well studied, being the C/EBP-α and C/EBP-β isoforms. C/EBP-α and -β were initially identified as transcription factors expressed at high levels in liver, adipose and lung tissue. Both proteins exist as isoforms generated by regulated alternative translation initiation: C/EBP-α as 42 kDa and 30 kDa isofoms (p42 and p30, respectively) and C/EBP-β as p38, p33 and p20 (also known as Liver Activating Protein* (LAP*), LAP and Liver Inhibitory Protein (LIP)) [1]. These isoforms all contain the basic region-leucine zipper (BR-LZ) DNA-binding domain, and retain all or part of the N-terminal transactivation domain (Figure 1). Subsequent knockout experiments revealed that although loss of C/EBP-α led to lipodystrophy, lack of granulocytes and lethal perinatal hypoglycemia caused by insufficient hepatic glucose efflux, C/EBP-β knockout mice were viable but exhibited female sterility, defective mammary epithelial differentiation and impaired immune function [2]. Grossly, the phenotypes observed could be classified either as specific defects in gene expression in otherwise developmentally normal cells (hepatic gluconeogenesis, macrophage antibacterial function), or as failure of cells to achieve the terminally differentiated state (adipocytes, granulocytes, mammary epithelial cells and granulosa cells). Moreover, in the relevant progenitors, C/EBPs were shown to be instructive for adipocyte, eosinophil and neutrophil granulocyte differentiation. Although it was gratifying to identify C/EBPs as crucial regulators of cellular differentiation, the question remained as to how the same proteins instructed distinct fates in different progenitors, a property clearly different from that of the prototypic lineage instructor, MyoD, which promotes skeletal muscle fate in all cell types tested. Moreover, it raised the possibility of C/EBPs having a general property that enabled them to participate in the execution of terminal differentiation programs.

Here, I focus on recent results that illustrate how C/EBPs regulate cellular proliferation and differentiation. In particular, the interactions between C/EBPs and the cell cycle apparatus, their cooperation with other lineage-restricted transcription factors, and the way in which these functions make C/EBPs crucial for the oncogenic transformation of multiple cell types are discussed.

Section snippets

Control of differentiation by C/EBPs: specificity through collaboration

The ability of C/EBP-α to direct cellular fate in a context-specific manner is thought to depend on the presence of specific collaborating transcription factors. For adipocyte differentiation, this was convincingly shown to be the case because C/EBP-α was found to induce peroxisome proliferator activated receptor-γ, and failed to induce adipocyte differentiation in progenitors lacking this factor [3]. For the hematopoietic system, eosinophil differentiation in a cell line system was found to

Redundancy: C/EBP functions below the surface

Recently, a role for C/EBPs in the neuronal or glial cell lineage choice has been proposed, based on studies using C/EBP overexpression and dominant negative C/EBP molecules. Here, C/EBPs promoted the neuronal lineage and inhibited astrocyte differentiation both in vitro [9] and in vivo [10]. By contrast, genetic studies using loss-of-function mutations have shown that C/EBP-α and -β function together with NeuroD in terminal neuronal differentiation [11]. The different outcomes of these

C/EBPs: at the crossroads between differentiation and proliferation control

In addition to being executors of lineage commitment and terminal differentiation programs, C/EBPs have emerged as important negative regulators of cell proliferation in many cellular contexts. The central mechanisms identified are repression of the E2F complex (by both C/EBP-α and -β), and interference with Cdk2 and Cdk4 function (by C/EBP-α) 22, 23. These antiproliferative properties of C/EBP-α and -β raised the possibility that they could function as tumor suppressors. The discovery of

C/EBP-β: how to promote both differentiation and tumor formation

In situations where the function of C/EBP-α is conceptually straightforward (blocks proliferation, promotes differentiation, suppresses tumorigenesis), C/EBP-β is more complex. In many situations, it has biological properties that are difficult to distinguish from those of C/EBP-α. This is perhaps best exemplified by the finding that knockin of C/EBP-β into the C/EBP-α locus rescues most of the phenotypes associated with loss of C/EBP-α 36, 37. C/EBP-β has also been implicated as an important

A spanner in the works? C/EBP translocations in B-cell leukemias

The relevance of Id gene regulation by C/EBPs was highlighted by the recent surprising finding that the genes encoding five different C/EBP isoforms (α–ɛ) were involved in translocations involving the immunoglobulin heavy chain (IgH) locus in B-cell progenitor acute lymphoblastic leukemia (BCP-ALL) [49]. These translocations led to the expression of the corresponding C/EBP-encoding mRNAs, with no apparent alterations to the C/EBP coding sequences, consistent with the intact C/EBP molecules

The dark side of C/EBP-β: tumor promotion in epithelial cells

Another mechanism of cell cycle regulation by C/EBP-β in breast cancer is collaboration with the transforming growth factor (TGF)-β–Smad pathway in TGF-β-mediated repression of Myc expression and upregulation of the cyclin-dependent kinase inhibitors p21cdki and p15INK4b [54]. Here, modulation of the ratio between the LIP and LAP isoforms seemed to be crucial; cancer cells with a high LIP-to-LAP ratio were refractory to TGF-β, and decreasing this ratio by ectopic LAP restored the cytostatic

Perspectives

In summary, C/EBP proteins are able to regulate cellular proliferation and differentiation through a variety of mechanisms. The role of C/EBP-α seems to be purely antiproliferative, and it is likely that this protein will emerge as a general tumor suppressor in both hematopoietic and non-hematopoietic cell types. By contrast, C/EBP-β might both inhibit and promote cell cycle progression, depending on the cellular context and C/EBP-β forms present. It will clearly be of interest to explore

Disclosure statement

The author declares that he has no conflict of interest with regard to this article.

Acknowledgements

The author apologizes to those colleagues whose work could not be cited owing to space limitations. This work was supported by the Association for International Cancer Research (AICR), the Associazione Italiana per la Ricerca sul Cancro (AIRC), the Swedish Research Council (HematoLinne) and the European Commission (EuroStemCell Integrated Project and EuroCSC STREP).

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