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A.E. Griep, R.K. Hyde, A.B. Savage; Differential Requirements for Rb and p57KIP2 in the Epithelium and Fiber Cells of the Mouse Ocular Lens . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2405.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In the lens, cell cycle regulation is critical for establishing and maintaining the proper balance of quiescent, proliferating and differentiated cells. The retinoblastoma protein, pRb, and the cyclin dependent kinase inhibitor, p57KIP2, are known to be required for regulating cell cycle withdrawal of mouse lens epithelial cells as they differentiate into fibers. Recent data suggest that collectively the pRb family is important for regulating proliferation in the epithelium. However, it is not known if the individual pRb family members or p57KIP2 have specific roles in the epithelium or if their roles are different in epithelium than in the fibers. Methods: To address these questions, we generated E13.5 mouse embryos carrying one or more of the following: germline or lens–specific deletions in Rb, germline mutation in p57KIP2, or αAE7 transgenic mice, which express the viral oncoprotein E7, a dominant repressor of pRb and pRb family members, in the fiber cells. The rates of proliferation in the epithelium and fiber cell compartments of the lens of these various embryos were quantified using BrdU incorporation assays and phosphophistone H3 immunohistochemistry. Results: In the epithelium, loss of Rb resulted in a lens autonomous decreased rate of proliferation as compared to controls, suggesting that these cells overcompensate for loss of Rb. While loss of p57KIP2 alone had no effect on proliferation in the epithelium, the combined loss of Rb and p57KIP2, showed a rate of proliferation indistinguishable from controls, indicating that the loss of p57KIP2 rescued the Rb mutant defect. By contrast, in the fiber cells loss of Rb or p57KIP2 resulted in increased proliferation. Their combined loss resulted in proliferation rates that were indistinguishable from the rates in fibers with loss of Rb alone, indicating that fiber cells have lost the capacity to compensate for loss of Rb and that p57KIP2 is an upstream regulator specifically of Rb in the fiber cells. Expression of E7 resulted in increased proliferation rates that were indistinguishable from that of Rb mutation, indicating that there is little, if any, contribution of other pRb family members to regulating cell cycle withdrawal during fiber cell differentiation. Conclusions: These data show that both Rb and p57KIP2 are required to maintain normal cell cycle regulation in epithelial and fiber cells. However, while the same cell cycle regulatory factors are used in both cell types, there are important differences in the regulatory circuits governing cell cycle control between the proliferation–competent and postmitotic, differentiated cells.
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