Clinical relevance of a novel EGF pathway directly linked to E2F3a in ovarian cancer

Abstract

5588 Background: Over the last decade the retinoblastoma protein and its down-stream effectors have been shown to be involved in tumorigenesis and progression in various cancer entities. Especially the E2F family of transcription factors, are regarded to be fundamental players in the deregulation of the cell cycle control. Proliferation-promoting E2F1 to E2F3a are responsible for promoting the cell cycle at the G1/S phase checkpoint, E2F3b to E2F8 seem to act as inhibitors of cell proliferation. By our knowledge, there are no data considering a direct interplay between the EGF-R dependent pathways and the E2F family. Methods: The functional relationship between E2F3 isoforms and EGF-R was evaluated in EGF stimulated ovarian cancer cell lines by the use of RT-PCR. Protein expression was estimated by immunoblot analyses. In vitro data and expression of miRNA-34a were confirmed in a collective of 128 ovarian cancer patients. Results: Herein we show that in ovarian cancer cell lines EGF and TGF-alpha treatment selectively increase the transcription of E2F3a, whereas all other members of the E2F-family remain unaffected upon EGF-R stimulation. E2F3a transcription was completely abolished by simultaneous treatment with EGF-R inhibitors such as Cetuximab, Erlotinib and Lapatinib. Furthermore, the E2F3a peak observed after 3 to 12 hrs after EGF treatment was associated with the initiation of cell proliferation. In addition, our investigations reveal, that EGF treatment also reduced the transcription of miRNA-34a, a specific negative epigenetic regulator of E2F3a, in ovarian cancer cells. E2F3a and E2F3b expression are shown to be elevated in 128 ovarian cancer tissue samples compared to healthy controls. Both isoforms were significantly associated with FIGO stage and histological grading. However, only the proliferation-promoting E2F3a was associated with immunohistochemically demonstrated phosphorylated EGF-R and turned out to be an independent prognosticator for progression free and overall survival. In line with this, E2F3a was also the only member of the E2F-family which was constitutively up-regulated in platinum resistant ovarian cancer cell lines. Conclusion: Taken together, this so far un-described EGF-E2F3a axis seems to play a pivotal role in the tumor biology of ovarian cancer. No significant financial relationships to disclose.