Distinct roles for E2F proteins in cell growth control and apoptosis

Abstract

E2F transcription activity is composed of a family of heterodimers encoded by distinct genes. Through the overproduction of each of the five known E2F proteins in mammalian cells, we demonstrate that a large number of genes encoding proteins important for cell cycle regulation and DNA replication can be activated by the E2F proteins and that there are distinct specificities in the activation of these genes by individual E2F family members. Coexpression of each E2F protein with the DP1 heterodimeric partner does not significantly alter this specificity. We also find that only E2F1 overexpression induces cells to undergo apoptosis, despite the fact that at least two other E2F family members, E2F2 and E2F3, are equally capable of inducing S phase. The ability of E2F1 to induce apoptosis appears to result from the specific induction of an apoptosis-promoting activity rather than the lack of induction of a survival activity, because co-expression of E2F2 and E2F3 does not rescue cells from E2F1-mediated apoptosis. We conclude that E2F family members play distinct roles in cell cycle control and that E2F1 may function as a specific signal for the initiation of an apoptosis pathway that must normally be blocked for a productive proliferation event.Various studies have led to the delineation of a pathway controlling the progression of cells from quiescence, through G 1 , and into S phase that involves the activation of G 1 cyclin-dependent kinases (cdk), inactivation of Rb and related proteins, and accumulation of E2F transcription factor activity (for reviews see refs. 1-7). It is also now clear that the disruption of components of this control pathway, either the activation of positive acting components such as the G 1 cyclins or the inactivation of negative components such as p53, Rb, and the cyclin-dependent kinase inhibitors (CKI), can lead to the loss of cell growth control underlying the development of various forms of human cancer (7,8).Like many other signal transduction activities, E2F consists of a family of related proteins that include five distinct E2F members and at least two heterodimer partners, DP1 and DP2. The complexity of the E2F activity, as generated by the formation of a variety of heterodimeric protein complexes, suggests a complexity of function whereby the individual family members might play distinct roles in cellular growth control. For instance, the individual E2F family members might integrate distinct signaling pathways within the cell to facilitate the orderly progression through the growth cycle. That is, individual E2F genes might respond to different components of a growth signaling process, either distinct extracellular growth factors or simply distinct signal transduction pathways that integrate a complex growth response. Additionally, but not exclusive of the first instance, the individual E2F proteins could activate distinct target genes, the total of which constitutes the range of activities necessary for cells to progress into and through S phase. Finally, the complexity...

Deregulated expression of cell-cycle proteins during premalignant progression in SENCAR mouse skin

“…Overexpression of cyclin D1, cyclin D2, and E2F could explain why cell proliferation becomes independent of external factors in late papillomas. In fact, cyclin D1, CDK4, and pRb have a functional interaction in which phosphorylation of pRb by the cyclin D/CDK complex releases E2F, which activates others genes involved in the G1/S phase transition (Chellappan et al, 1991;De Gregori et al, 1997;Dyson et al, 1993;Sherr, 1995). Overexpression of cyclin D1 and probably cyclin D2 may participate in this interaction, although we found no relevant di erences in the pattern of pRb phosphorylation.…”

“…The relative abundance of each protein at each stage was determined from the absorbance of each band and normalized against PCNA expression in the same sample. Statistical analysis showed signi®cant di erences in the four members of the E2F family (E2F-1 P50.0001; E2F-2 P50.03; E2F-4 P50.0001, and E2F-5 P50.0001) during promotion and between papillomas of 10 and 20 weeks of promotion (E2F-1 P50.0001; E2F-2 P40.03, E2F-4 P40.001, and E2F-5 P40.001) (b) to play distinct roles by inducing the expression of di erent genes that participate in G1/S phase transition (De Gregori et al, 1997;Zhang et al, 1997). We analysed the protein levels of E2F-1, E2F-2, E2F-4 and E2F-5 during mouse skin promotion by Western blotting.…”

Papillomavirus E2 induces p53-independent apoptosis in HeLa cells

“…Overexpression of E2F has been shown to activate apoptosis in a p53-independent manner (DeGregori et al, 1997;Field et al, 1996;Fueyo et al, 1998;Hsieh et al, 1997;Phillips et al, 1997). The mechanism of this induction seems to involve relief of repression of still unknown proapoptotic genes by the E2F/Rb complex, rather than direct activation of transcription (Hsieh et al, 1997).…”

Apoptosis induced by the myelodysplastic syndrome-associated NPM-MLF1 chimeric protein

“…Other apoptotic inducers such as c-Myc and E2F-1 also promote S phase entry (Evan et al, 1992;Askew et al, 1991;Wu and Levine, 1994;Qin et al, 1994;Shan and Lee, 1994), but accelerated initiation of DNA replication itself does not trigger apoptotic pathways (Hsieh et al, 1997;Phillips et al, 1997). For example, other E2F family members (E2F-2 and -3) induce S phase entry of serum-starved ®broblasts without evoking apoptosis (DeGregori et al, 1997). Thus, NPM-MLF1 seem to activate at least two separate pathways; one transmits signals that ultimately reach the apoptotic e ectors in the cytoplasm such as apoptotic Bcl-2-family members (Adams and Cory, 1998) and caspases (Thornberry and Lazebnik, 1998), and the other may activate the cell cycle machinery to promote cell proliferation resulting in acquisition of growth advantages.…”