Transcription factor E2F‐1 acts as a growth‐promoting factor and is associated with adverse prognosis in non‐small cell lung carcinomas

Abstract

Numerous upstream stimulatory and inhibitory signals converge to the pRb/E2F pathway, which governs cell-cycle progression, but the information concerning alterations of E2F-1 in primary malignancies is very limited. Several in vitro studies report that E2F-1 can act either as an oncoprotein or as a tumour suppressor protein. In view of this dichotomy in its functions and its critical role in cell cycle control, this study examined the following four aspects of E2F-1 in a panel of 87 non-small cell lung carcinomas (NSCLCs), previously analysed for defects in the pRb-p53-MDM2 network: firstly, the status of E2F-1 at the protein, mRNA and DNA levels; secondly, its relationship with the kinetic parameters and genomic instability of the tumours; thirdly, its association with the status of its transcriptional co-activator CBP, downstream target PCNA and main cell cycle regulatory and E2F-1-interacting molecules pRb, p53 and MDM2; and fourthly, its impact on clinical outcome. The protein levels of E2F-1 and its co-activator CBP were significantly higher in the tumour area than in the corresponding normal epithelium (p<0.001). E2F-1 overexpression was associated with increased E2F-1 mRNA levels in 82% of the cases examined. The latter finding, along with the low frequency of E2F-1 gene amplification observed (9%), suggests that the main mechanism of E2F-1 protein overexpression in NSCLCs is deregulation at the transcriptional level. Mutational analysis revealed only one sample with asomatic mutation at codon 371 (Glu-->Asp) and one carrying a polymorphism at codon 393 (Gly-->Ser). Carcinomas with increased E2F-1 positivity demonstrated a significant increase in their growth indexes (r=0.402, p=0.001) and were associated with adverse prognosis (p=0.033 by Cox regression analysis). The main determinant of the positive association with growth was the parallel increase between E2F-1 staining and proliferation (r=0.746, p<0.001), whereas apoptosis was not influenced by the status of E2F-1. Moreover, correlation with the status of the pRb-p53-MDM2 network showed that the cases with aberrant pRb expression displayed significantly higher E2F-1 indexes (p=0.033), while a similar association was noticed in the group of carcinomas with deregulation of the p53-MDM2 feedback loop. In conclusion, the results suggest that E2F-1 overexpression may contribute to the development of NSCLCs by promoting proliferation and provide evidence that this role is further enhanced in a genetic background with deregulated pRb-p53-MDM2 circuitry.

Life and death decisions by E2F-1

“…Although a genuine tumour-derived somatic cell mutation in E2F-1 has recently been described and the investigation of its activities could indeed prove rewarding, the lack of a panel of E2F-1 mutants makes discerning the role of E2F in tumour development more difficult to determine. 84 While the continued study of E2F-1-induced cell death will provide an insight on this issue, it must ultimately be kept in mind that although E2F lies at the core of very significant cell fate decisions, it forms only part of a complex matrix of cell signalling events and interactions. For example, a recent study looking at the role of E2F-1 in the response of keratinocytes to UVB irradiation in mouse skin unexpectedly showed that E2F1 À/À mice exhibit enhanced apoptosis following exposure to UVB when compared to wild-type counterparts.…”

Role of cell‐cycle regulators in lung cancer

“…All together, these studies strongly suggest an important role(s) of RB family members in lung cancer progression. Different studies showed that three genes downstream from the pRb pathway that support cell growth, E2F, DP-1, and HDAC, are overexpressed in lung adenocarcinomas (Chang and Szabo, 2002;Gorgoulis et al, 2002;Singhal et al, 2003a). The expression of these Rb-dependent genes is increased probably because upregulation of cyclin D1 and cyclin D2, key regulators of pRb phosphorylation, and downregulation of cyclin-dependent kinase inhibitors p15, p21, p19, and p57 (Kawamata et al, 1995;Rusin et al, 1996;Gazzeri et al, 1998;Keum et al, 1999;Jin et al, 2001;Shoji et al, 2002;Singhal et al, 2003a).…”

Roles of CREB-binding protein (CBP)/p300 in respiratory epithelium tumorigenesis

“…In mammals, the E2F family has six different members. The best-characterised member is E2F-1 and its over-expression has been shown to be strongly associated with lung carcinogenesis [55]. The ability of E2F-1 to stimulate transcription appears to be subjected to multiple regulations including co-activation by CBP/p300 and reversal of Rb-mediated repression through Rb phosphorylation [55].…”

“…The best-characterised member is E2F-1 and its over-expression has been shown to be strongly associated with lung carcinogenesis [55]. The ability of E2F-1 to stimulate transcription appears to be subjected to multiple regulations including co-activation by CBP/p300 and reversal of Rb-mediated repression through Rb phosphorylation [55]. At a molecular level, the Cdk-stimulated interaction of CBP/p300 with E2F-1 may be involved in irreversibly committing cells to cell-cycle progression [56].…”