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G. Chandrasekher, G. Maharaj, D. Sailaja; Changes in the Cell Cycle Proteins Expression During Lens Epithelial Cell Proliferation. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1902.
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Cell cycle is a sequence of stages that a living cell undergoes during proliferation. Cyclins, cyclin dependent kinases (CDKs) and their inhibitors (CDIs) play a vital role in the progression and arrest of the cell cycle. In this study we have determined the changes in the expression of various cyclins, CDKs and CDIs during lens epithelial cell proliferation and the regulatory role of the PI-3K/Akt pathway on their expression.
Epithelial cells originated from rabbit lens capsules were grown in DMEM+10% FCS for various times (0-96 h). DNA content distribution in cell cycle compartments was analyzed by flow cytometry. To study the effect of IGF-1 or PI-3K/Akt pathway inhibitors wortmannin (200 nM) or LY294002 (15 µM), cultures were partially synchronized by serum deprivation and treated with these factors. To overexpress active Akt, epithelial cells were transfected with eukaryotic Akt cDNA plasmid. Expression of various cell cycle components under different experimental conditions was analyzed by immunoblotting.
We have identified significant differences in the expression of cyclins A, D, E; CDK2, CDK4, and CDIs p21cip and p27kip in proliferating lens epithelial cells. Cyclin A, D, CDK2 and p21 levels were very high in proliferating cultures (between 24-40 h after seeding). Their levels decreased considerably by 96 h when the cells reached near quiescent state. Expression of CDK4 remained steady at all time points. In non proliferating or quiescent cultures cyclin E and p27 expression was very high. Flow cytometry analysis revealed significant rise in DNA content distribution in S phase (from 1% at 0 h to 12% at 24 h) and G2/M phase (7% to 19%) and corresponding decrease in G0/G1 phase (90% to 67%). IGF-1 upregulated the expression of cyclin A, E, CDK2 and p21, and suppressed p27 expression. PI-3K inhibitors attenuated the effect of IGF-1 on the regulation of cyclin A, CDK2, p21 and p27. In cells transfected with Akt, the expression of cyclins and CDKs is not altered significantly, but p21 level was upregulated whereas p27 expression was suppressed.
Increased expression of cyclins A and D, CDK2 and p21 during growth phase (S, when DNA is replicated) and mitotic phase (M, when cell division occurs), suggest that these proteins could be involved in cell cycle progression. Further, cyclin E and p27 expression could be important for cell cycle arrest. In pathological situations such as secondary cataract (PCO) epithelial cell growth suppression may be achieved by controlling the signaling cascades which promote the expression of specific cell cycle promoting proteins.
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