Distinct Capacities of Individual E2Fs to Induce Cell Cycle Re-Entry in Postmitotic Lens Fiber Cells of Transgenic Mice

<i>Purpose:</i> 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. <i>Methods:</i> Transgenic mice were generated by microinjection of constructs that contained E2F cDNAs (E2F3a, E2F4 or E2F5) linked to the mouse lens-specific αA-crystallin promoter. The mice were characterized by histology, in situ hybridization, immunohistochemistry, BrdU incorporation, TUNEL assay and Western blots. <i>Results:</i> 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 p73α were upregulated in the E2F3a lenses. However, expression of p21, a well-known p53 target gene, was not activated, suggesting that p73α might be responsible for inducing apoptosis and blocking unregulated proliferation in lens cells overexpressing E2F3a. <i>Conclusions:</i> 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 p73α.