E2F1 inhibits c-Myc-driven apoptosis via PIK3CA/Akt/mTOR and COX-2 in a mouse model of human liver cancer.
OBJECTIVE
Resistance to apoptosis is essential for cancer growth. We previously reported that hepatic coexpression of c-Myc and E2F1, 2 key regulators of proliferation and apoptosis, enhanced hepatocellular carcinoma (HCC) development in transgenic mice. Here, we investigated the molecular mechanisms underlying oncogenic cooperation between c-Myc and E2F1 in relationship to human liver cancer.


METHODS
Activation of pro- and antiapoptotic cascades was assessed by immunoblotting in experimental HCC models and in human HCC. Effect of antisense oligodeoxy nucleotides against c-Myc and E2F1 was studied in human HCC cell lines. Suppression of catalytic subunit p110alpha of phosphatidylinositol 3-kinase (PIK3CA)/Akt, mammalian target of rapamycin (mTOR), and cyclooxygenase (COX)-2 pathways was achieved by pharmacologic inhibitors and small interfering RNA in human and mouse HCC cell lines.


RESULTS
Coexpression with E2F1 did not increase proliferation triggered by c-Myc overexpression but conferred a strong resistance to c-Myc-initiated apoptosis via concomitant induction of PIK3CA/Akt/mTOR and c-Myb/COX-2 survival pathways. COX-2 was not induced in c-Myc and rarely in E2F1 tumors. In human HCC, PIK3CA/Akt/mTOR and c-Myb/COX-2 pathways were similarly activated, with levels of PIK3CA/Akt, mTOR, and c-Myb being inversely associated with patients' survival length. Silencing c-Myc and E2F1 reduced PIK3CA/Akt and mTOR and completely abolished c-Myb and COX-2 expression in human HCC cell lines. Finally, simultaneous inhibition of PIK3CA/Akt/mTOR and COX-2 activity in in vitro models caused massive apoptosis of neoplastic hepatocytes.


CONCLUSIONS
E2F1 may function as a critical antiapoptotic factor both in human and in rodent liver cancer through its ability to counteract c-Myc-driven apoptosis via activation of PIK3CA/Akt/mTOR and c-Myb/COX-2 pathways.
Ladu
Sara
S
Laboratory of Experimental Carcinogenesis, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-4262, USA.
Calvisi
Diego F
DF
Conner
Elizabeth A
EA
Farina
Miriam
M
Factor
Valentina M
VM
Thorgeirsson
Snorri S
SS
D000368:Aged; D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D006528:Carcinoma, Hepatocellular / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology*; D002352:Carrier Proteins / Q000378:metabolism; D002470:Cell Survival / Q000502:physiology; D051546:Cyclooxygenase 2 / Q000378:metabolism; D004195:Disease Models, Animal; D050687:E2F1 Transcription Factor / Q000235:genetics / Q000378:metabolism*; D005260:Female; D006801:Humans; D008113:Liver Neoplasms / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology*; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008822:Mice, Transgenic; D008875:Middle Aged; D046912:Multiprotein Complexes; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D011506:Proteins; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000378:metabolism; D019076:Transgenes / Q000502:physiology
D002352:Carrier Proteins; D051546:Cyclooxygenase 2; D050687:E2F1 Transcription Factor; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D011506:Proteins; D051057:Proto-Oncogene Proteins c-akt; D016271:Proto-Oncogene Proteins c-myc; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors; C493753:E2F1 protein, human; C493754:E2f1 protein, mouse; C489427:MYC protein, human; C489428:Myc protein, mouse; C077623:Ptgs2 protein, mouse; C496540:PTGS2 protein, human; C546842:MTOR protein, human; C546843:mTOR protein, mouse