E2F activity is essential for survival of Myc-overexpressing human cancer cells

Abstract

Effective cell cycle completion requires both Myc and E2F activities. However, whether these two activities interact to regulate cell survival remains to be tested. Here we have analysed survival of inducible c-Mycoverexpressing cell lines derived from U2OS human osteosarcoma cells, which carry wild-type pRb and p53 and are deficient for p16 and ARF expression. Induced U2OS-Myc cells neither underwent apoptosis spontaneously nor upon reconstitution of the ARF-p53 axis and/or serum-starvation. However, they died massively when concomitantly exposed to inhibitors of E2F activity, including a constitutively active pRb (RbDcdk) mutant, p16, a stable p27 (p27T187A) mutant, a dominant-negative (dn) CDK2, or dnDP-1. Similar apoptotic effect was observed upon down-modulation of endogenous E2Fs through overexpression of E2F binding site oligonucleotides in U2OS-Myc cells, upon expression of RbDcdk or dnDP-1 in the Myc-amplified HL-60 (ARF7; p537) human leukemia cells, and upon cotransfection of Myc and RbDcdk in SAOS-2 (ARF+; p537) human osteosarcoma cells but not in human primary fibroblasts. Consistent with these results, a dnp53 mutant did not abrogate the Myc-induced apoptotic phenotype, which instead strictly depended on caspase-3-like proteases and on Myc transcriptional activity. Our data indicate that in contrast to normal cells, Myc-overexpressing human cancer cells need E2F activity for their survival, regardless of their ARF and p53 status, a notion that may have important implications for antineoplastic treatment strategies.

Inhibition of cell proliferation and induction of apoptosis by novel tetravalent peptides inhibiting DNA binding of E2F

“…These considerations would be compatible with a model, where a residual low level of E2F, as seen after treatment with our peptide anatagonists (Figures 5 and 6), would still allow for a transition from G 1 to S, but would be insufficient for the completion of S phase, the latter being because of the lack of an E2F-mediated antiapoptotic pathway. The nature of such a pathway remains enigmatic at present, but the existence of an E2F-mediated antiapoptotic mechanism is also suggested by other studies (Santoni-Rugiu et al, 2002). It may be possible that the inhibition of cyclin A by our peptide antagonists (Figure 6) plays a role in this context, since cyclin A may be essential for constraining the activity of E2F after S phase entry (Krek et al, 1995).…”

“…More recently, it has been described that E2F activity is essential for the survival of Myc-overexpressing tumor cells regardless of their ARF and p53 status (Santoni-Rugiu et al, 2002). It would thus appear that E2F can exert opposing functions with respect to the regulation of apoptosis, and that the precise outcome of E2F inhibition may depend on the genetic status of the cell and the loss-of-function approach, which may target either specific E2F genes (as in knockout mice) or multiple E2F members (as in the present study).…”

MIF loss impairs Myc-induced lymphomagenesis

“…65,66 Strikingly, this does not seem to be the case in MIF deficiency, as our data with DbE2F1 clearly indicate that the growth-and apoptosis-promoting E2F1 activity in MIF null cells depends on the presence of its C-terminal Rb-binding/ transactivation domain. Because Rb and its family proteins are largely inactive in Myc-or E2F-overexpressing cells, 49,50,56 we speculate that an Rb-independent inhibitor of E2F1 is lost in MIF-deficient cells, explaining why ectopic E2F1 has more effect in MIF-KO than WT fibroblasts. Clearly, further analysis is required to identify which endogenous proteins might interact with E2F1 that could determine its proor anti-apoptotic properties in the context of MIF deficiency.…”

“…25 On the other hand, the proliferative response of Myc-overexpressing cells depends on the induction and activation of E2F factors, 35,55,56 although E2F thresholds (in particular E2F1) also determine whether cells live or die. [55][56][57][58] It should be noted, however, that although both Myc and E2F1 can function as parts of multiprotein complexes that activate or repress cellular genes, neither has yet been shown to induce p53 directly. While early work suggested existence of a linear E2F1-ARF-p53 axis, more recent studies instead demonstrated that E2F1 and ARF act in parallel pathways that activate p53 in response to oncogenic signaling.…”

“…While early work suggested existence of a linear E2F1-ARF-p53 axis, more recent studies instead demonstrated that E2F1 and ARF act in parallel pathways that activate p53 in response to oncogenic signaling. 57,58 In view of the fact that continuing E2F activity is required for S-phase progression and the survival of Myc-overexpressing cells, 35,56 loss of the Ink4A/ARF locus therefore provides a selective advantage to tumor cells, while loss of E2Fs does not. Moreover, because p53 is more frequently mutated rather than deleted in Em-Myc lymphomas, 31,32 ARF loss has stronger detrimental consequences for p53 inactivation than p53 point mutations.…”