Induction of cell cycle entry and cell death in postmitotic lens fiber cells by overexpression of E2F1 or E2F2.
OBJECTIVE
Previous studies have shown that inactivation of the retinoblastoma tumor suppressor protein (pRb) can cause lens fiber cell proliferation and apoptosis. Because pRb is thought to block cell cycle progression by inhibition of E2F transcription factors, experiments were conducted to test whether overexpression of different E2F family members would be sufficient to induce fiber cell proliferation and subsequent apoptosis. The in vivo functions of the transcription factor E2F2 have not previously been analyzed or described in transgenic mice.


METHODS
Human E2F1 and E2F2 cDNAs were linked to the alphaA-crystallin promoter. Transgenic mice were generated by microinjection. Changes in cell cycle regulation were assayed by immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) incorporation and by in situ hybridization. Cell death was assayed using the TdT-dUTP terminal nick-end labeling (TUNEL) assay.


RESULTS
At embryonic day (E)15.5, strong expression of the E2F1 and E2F2 transgenes was detected in lens fiber cells with little or no expression in epithelial cells. BrdU incorporation and TUNEL assays showed that overexpression of either E2F1 or E2F2 in lens fiber cells was sufficient to cause cell cycle entry and subsequent apoptosis. Expression of either E2F1 or E2F2 was sufficient to induce the transcription of cyclins (A2, B1, and E), as well as p53 and Bax in the lens fibercells.


CONCLUSIONS
Expression of either E2F1 or E2F2 can induce postmitotic lens fiber cells to re-enter the cell cycle. Inappropriate cell cycle entry is recognized by p53 in each case, and programmed cell death ensues.
Chen
Q
Q
Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas, USA.
Hung
F C
FC
Fromm
L
L
Overbeek
P A
PA
D000818:Animals; D017209:Apoptosis; D001973:Bromodeoxyuridine / Q000378:metabolism; D002352:Carrier Proteins; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002455:Cell Division; D016213:Cyclins / Q000378:metabolism; D017931:DNA Primers / Q000737:chemistry; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D050703:E2F2 Transcription Factor; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism; D005260:Female; D017403:In Situ Hybridization; D020287:In Situ Nick-End Labeling; D007908:Lens, Crystalline / Q000166:cytology* / Q000378:metabolism; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D011247:Pregnancy; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2; D056465:Retinoblastoma-Binding Protein 1; D016196:S Phase; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism*; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D051028:bcl-2-Associated X Protein
C533431:Arid4a protein, mouse; C494827:Bax protein, mouse; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D016213:Cyclins; D017931:DNA Primers; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D050703:E2F2 Transcription Factor; C493754:E2f1 protein, mouse; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein; D001973:Bromodeoxyuridine