Novel functions for the transcription factor E2F4 in development and disease

Abstract

The E2F family of transcription factors is a key determinant of cell proliferation in response to extra- and intra-cellular signals. Within this family, E2F4 is a transcriptional repressor whose activity is critical to engage and maintain cell cycle arrest in G0/G1 in conjunction with members of the retinoblastoma (RB) family. However, recent observations challenge this paradigm and indicate that E2F4 has a multitude of functions in cells besides this cell cycle regulatory role, including in embryonic and adult stem cells, during regenerative processes, and in cancer. Some of these new functions are independent of the RB family and involve direct activation of target genes. Here we review the canonical functions of E2F4 and discuss recent evidence expanding the role of this transcription factor, with a focus on cell fate decisions in tissue homeostasis and regeneration.

Keywords: E2F; E2F4; RB; cancer; cell cycle; development; differentiation; regeneration; stem cells.

Figure 1.

Schematic representation of the canonical RB/E2F pathway in cell cycle progression. In G1, cells can either enter S phase or exit the cell cycle into G0. Entry into S phase requires activation of transcriptional programs controlled by E2F activity. Binding of repressive complexes involving RB and E2F family members to the promoters of cell cycle genes silences their transcription in both G0 and G1. Repressive RB/E2F complexes consist of an RB family member, either E2F4 or E2F5, and additional chromatin modification and remodeling factors, including histone deacetylases (HDACs). In G0, repressive complexes generally contain p130 and the MuvB core complex (DREAM complex), whereas p107 predominates in G1. When cells enter S phase, Cyclin-CDK activity is upregulated and phosphorylates RB family proteins, promoting the dissociation of repressive RB/E2F complexes, and releasing “activator” E2Fs to upregulate the expression of cell cycle genes with histone acetyltransferases (HATs) and other chromatin-modifying factors.

Figure 2.

Structure of human E2F4. E2F4 is 413 amino acids long and contains a DNA binding domain (15-86), a dimerization domain that allows it to form heterodimers with a DP family protein (86-195), a transactivation domain (337-413), and within this, a pocket protein binding domain (PPBD, 390-407) that allows interactions with the RB family proteins. E2F4 shares these domains with the “activator” E2Fs and with E2F5. Yet unlike the “activator” E2Fs, E2F4 lacks a nuclear localization signal and shares with E2F5 a bipartite nuclear export signal (61-70, 91-100). E2F4 is thought to rely on the RB family proteins for nuclear localization, although post-translational modifications (such as phosphorylation sites, shown above with candidate kinases) and additional cofactors (shown below, with their interaction sites) may regulate E2F4 activity and cellular localization as well.

Figure 3.

Summary of the developmental phenotypes associated with loss of E2F4 function in worms, flies and mice. Loss of E2F4 results in defects in multiple tissues of (A) C. elegans, (B) S. mediterranea, (C) D. melanogaster, and (D) M. musculus (see text). A number of these phenotypes have been attributed to cell cycle-independent or RB family-independent changes in gene expression and cell fate specification, suggesting that E2F4 may play context-dependent roles.

Figure 4.

Potential mechanisms of action of E2F4 in stem cells. In addition to its canonical role as a repressor of cell cycle progression in G0/G1, E2F4 may have at least 3 different functions as a general transcription factor in adult stem cells and embryonic stem cells (ESCs). First, E2F4 may regulate developmental genes to establish cell fate during differentiation, either as a repressor in conjunction with RB to prevent the expression of aberrant transcripts, or as an activator to directly drive cellular differentiation. Second, rapidly cycling cells largely lack a G1/S checkpoint and may utilize E2F4 to undergo arrest in G2 in response to cellular stress, during which E2F4 represses genes involved in G2/M progression and DNA damage repair. Third, E2F4 may switch from a repressor to an activator of cell cycle genes to support heightened metabolic requirements. While the first 2 cases may involve formation of complexes containing E2F4 and a RB family member, the third may not require RB family proteins, which would inhibit E2F4 transcriptional activity. In addition, all 3 cases may involve additional cofactors that facilitate E2F4 translocation into the nucleus and binding to target genes.