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RK Hyde, AE Griep; Unique Roles for E2F1 in the Mouse Ocular Lens in the Absence of Functional pRB Proteins . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3884.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: For normal lens development, withdrawal from the cell cycle is required for the transition from proliferating epithelial cells to terminally differentiated fiber cells. Previously, we identified the pRB family of proteins as important in regulating this process. Expression of the HPV-16 oncoprotein E7 in differentiating fiber cells of transgenic mice results in continued proliferation and apoptosis. This phenotype is dependent on E7's ability to bind pRB family members. These pocket proteins are known to negatively regulate the E2F transcription factors, whose activity is necessary for cell cycle progression. In normal lenses, E2F1-5 are expressed in the anterior epithelium; E2F1, 3, and 5 are expressed in the fiber cells. By crossing E7 mice to E2F1-null mice, we learned that E2F1 is only partially responsible for the E7 induced defects. This implies that other factors, possibly other E2Fs, are involved in the E7 phenotype, but that certain functions are unique to E2F1 and cannot be assumed by these other factors. Methods: To address these issues, we examined the expression of the E2F family members and E2F target genes in the lenses from mice of various genotypes by in situ hybridization, Northern blot and RT-PCR. Results: We showed that both E2F3 isoforms are expressed in the epithelial and fiber cells. We also showed that, like E2F2, E2F3a levels are increased in the E7 lens. Interestingly, the increased E2F3a expression in the E7 lens is E2F1- dependent, while increased E2F2 expression is E2F1- independent. E2F3b expression, like that of E2F4 and 5 does not change with expression of E7. In addition, we have examined the expression of direct and indirect E2F target genes including cdc2, b-myb, cyclin A2, p53, N- myc and p19ARF. All these target genes show increased expression in the E7 lens. In the case of p19ARF, this increased expression is entirely dependent on E2F1. N-myc expression is entirely independent of E2F1 status. The remaining target genes show at least a partial E2F1- dependency. Conclusion: These results indicate that E2F2 is a potential mediator of the E2F1- independent effects of E7 expression in the lens. These results also indicate that E2F1 has unique roles in regulating the expression of at least two target genes, E2F3a and p19ARF.
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