Specific In Vivo Roles for E2Fs in Differentiation and Development

E2Fs have been historically considered as key interacting factors for the retinoblastoma (Rb) family of pocket proteins, acting as universal regulators of cell cycle progression. Often exhibiting overlapping function, deregulated E2F activity is thought to cancer or cell death. While early reports hypothesized that E2Fs may be capable of regulating distinct functions beyond proliferation, several recent reports have characterized increasingly diverse, context dependent functions for different E2Fs in vivo, often in what appears to a manner beyond traditional cell cycle regulation. Ironically, many of these new functions are still mediated through the classical cell cycle regulatory Rb family of interacting factors. Here we review the recent advances, focusing on differentiation and development, to emphasize that E2F function is likely more complex than the simple model suggests, capable of exhibiting both specificity of function, and roles beyond cell cycle progression in vivo.     

Defective gene expression,S phase progression,and maturation during hematopoiesis in E2F1/E2F2 mutant mice

E2F plays critical roles in cell cycle progression by regulating the expression of genes involved in nucleotide synthesis, DNA replication, and cell cycle control. We show that the combined loss of E2F1 and E2F2 in mice leads to profound cell-autonomous defects in the hematopoietic development of multiple cell lineages. E2F2 mutant mice show erythroid maturation defects that are comparable with those observed in patients with megaloblastic anemia. Importantly, hematopoietic defects observed in E2F1/E2F2 double-knockout (DKO) mice appear to result from impeded S phase progression in hematopoietic progenitor cells. During DKO B-cell maturation, differentiation beyond the large pre-BII-cell stage is defective, presumably due to failed cell cycle exit, and the cells undergo apoptosis. However, apoptosis appears to be the consequence of failed maturation, not the cause. Despite the accumulation of hematopoietic progenitor cells in S phase, the combined loss of E2F1 and E2F2 results in significantly decreased expression and activities of several E2F target genes including cyclin A2. Our results indicate specific roles for E2F1 and E2F2 in the induction of E2F target genes, which contribute to efficient expansion and maturation of hematopoietic progenitor cells. Thus, E2F1 and E2F2 play essential and redundant roles in the proper coordination of cell cycle progression with differentiation which is necessary for efficient hematopoiesis.     

SAPCD2 promotes neuroblastoma progression by altering the subcellular distribution of E2F7

Recent studies uncovered the emerging roles of SAPCD2 (suppressor anaphase-promoting complex domain containing 2) in several types of human cancer. However, the functions and underlying mechanisms of SAPCD2 in the progression of neuroblastoma (NB) remain elusive. Herein, through integrative analysis of public datasets and regulatory network of GSK-J4, a small-molecule drug with anti-NB activity, we identified SAPCD2 as an appealing target with a high connection to poor prognosis in NB. SAPCD2 promoted NB progression in vitro and in vivo. Mechanistically, SAPCD2 could directly bind to cytoplasmic E2F7 but not E2F1, alter the subcellular distribution of E2F7 and regulate E2F activity. Among the E2F family members, the roles of E2F7 in NB are poorly understood. We found that an increasing level of nuclear E2F7 was induced by SAPCD2 knockdown, thereby affecting the expression of genes involved in the cell cycle and chromosome instability. In addition, Selinexor (KTP-330), a clinically available inhibitor of exportin 1 (XPO1), could induce nuclear accumulation of E2F7 and suppress the growth of NB. Overall, our studies suggested a previously unrecognized role of SAPCD2 in the E2F signaling pathway and a potential therapeutic approach for NB, as well as clues for understanding the differences in subcellular distribution of E2F1 and E2F7 during their nucleocytoplasmic shuttling.Subject terms: Cancer, Cancer     

Role of E2F1 in Apoptosis: A case Study in Feedback Loops

E2F1 is involved in both cell cycle and apoptosis, but we still fail to understand whether these are distinct E2F1 states and what controls the transition between states. Studies in cell cultures are often contradictory showing opposite effects of E2F1; even further, some phenotypes in animals lacking or overexpressing E2F1 would not be predicted from in vitro findings. In this review, we analyze current literature on the role of E2F1 in apoptosis and use engineering concepts and a systems biology approach to align the overwhelming amount of contradictory data on this gene.     

Constitutive E2F1 overexpression delays endochondral bone formation by inhibiting chondrocyte differentiation

Longitudinal bone growth results from endochondral ossification, a process that requires proliferation and differentiation of chondrocytes. It has been shown that proper endochondral bone formation is critically dependent on the retinoblastoma family members p107 and p130. However, the precise functional roles played by individual E2F proteins remain poorly understood. Using both constitutive and conditional E2F1 transgenic mice, we show that ubiquitous transgene-driven expression of E2F1 during embryonic development results in a dwarf phenotype and significantly reduced postnatal viability. Overexpression of E2F1 disturbs chondrocyte maturation, resulting in delayed endochondral ossification, which is characterized by reduced hypertrophic zones and disorganized growth plates. Employing the chondrogenic cell line ATDC5, we investigated the effects of enforced E2F expression on the different phases of chondrocyte maturation that are normally required for endochondral ossification. Ectopic E2F1 expression strongly inhibits early- and late-phase differentiation of ATDC5 cells, accompanied by diminished cartilage nodule formation as well as decreased type II collagen, type X collagen, and aggrecan gene expression. In contrast, overexpression of E2F2 or E2F3a results in only a marginal delay of chondrocyte maturation, and increased E2F4 levels have no effect. These data are consistent with the notion that E2F1 is a regulator of chondrocyte differentiation.