Distinct capacities of individual E2Fs to induce cell cycle re-entry in postmitotic lens fiber cells of transgenic mice.
OBJECTIVE
Inactivation of the retinoblastoma gene in human retinoblasts or mouse lens fiber cells causes inappropriate cell cycle entry, presumably as a consequence of elevated activity of the E2F transcription factors. Although E2Fs are known to be critical regulators of the cell cycle, it is still unclear whether family members E2F3a, E2F4 or E2F5 are individually capable of inducing cell cycle entry in vivo. In this study, we designed experiments to test whether lens-specific expression of these E2F family members would induce postmitotic fiber cells to re-enter the cell cycle.


METHODS
Transgenic mice were generated by microinjection of constructs that contained E2F cDNAs (E2F3a, E2F4 or E2F5) linked to the mouse lens-specific alphaA-crystallin promoter. The mice were characterized by histology, in situ hybridization, immunohistochemistry, BrdU incorporation, TUNEL assay and Western blots.


RESULTS
E2F3a expression was sufficient to induce cell cycle entry in lens fiber cells. Cell cycle re-entry was accompanied by apoptotic cell death resulting in microphthalmia. E2F4 expression stimulated a modest level of cell cycle re-entry, but the transgenic lenses remained normal in size and did not show significant apoptosis. Transgenic mice expressing E2F5 did not show lens defects. In both the E2F3a and E2F4 transgenic lenses, cyclin A2 and cyclin B1 expression were upregulated. Phosphorylated histone H3, a marker for mitosis, was detected in the E2F3a fiber cells. Western blots showed that both p53 and p73alpha were upregulated in the E2F3a lenses. However, expression of p21, a well-known p53 target gene, was not activated, suggesting that p73alpha might be responsible for inducing apoptosis and blocking unregulated proliferation in lens cells overexpressing E2F3a.


CONCLUSIONS
E2F3a and E2F4, but not E2F5, function to induce cell cycle entry, although E2F4 has more modest activity. E2F3a may induce cell death primarily through activation of p73alpha.
Chen
Qin
Q
College of Optometry, University of Houston, Houston, Texas, USA.
Liang
Dongcai
D
Yang
Tao
T
Leone
Gustavo
G
Overbeek
Paul A
PA
D000818:Animals; D017209:Apoptosis / Q000235:genetics; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D019925:Cyclin A / Q000235:genetics / Q000378:metabolism; D056751:Cyclin A2; D019926:Cyclin B / Q000235:genetics / Q000378:metabolism; D056744:Cyclin B1; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D050684:E2F Transcription Factors; D050699:E2F3 Transcription Factor; D050704:E2F4 Transcription Factor; D050700:E2F5 Transcription Factor; D005124:Eye Abnormalities / Q000235:genetics / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental / Q000502:physiology; D016147:Genes, Tumor Suppressor; D006801:Humans; D007908:Lens, Crystalline / Q000002:abnormalities* / Q000166:cytology / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009687:Nuclear Proteins / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D000072160:Tumor Protein p73; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D025521:Tumor Suppressor Proteins
C503721:CCNA2 protein, human; C534699:CCNB1 protein, human; C534700:Ccnb1 protein, mouse; D018797:Cell Cycle Proteins; D019925:Cyclin A; D056751:Cyclin A2; D019926:Cyclin B; D056744:Cyclin B1; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050699:E2F3 Transcription Factor; D050704:E2F4 Transcription Factor; C493762:E2F4 protein, human; D050700:E2F5 Transcription Factor; C490840:E2f3 protein, mouse; C493763:E2f4 protein, mouse; C490490:E2f5 protein, mouse; D009687:Nuclear Proteins; D014157:Transcription Factors; C000606368:Trp73 protein, mouse; D000072160:Tumor Protein p73; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins