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Patrick R Cammarata, Sudha Neelam, Morgan M Brooks; Inactivation of Glycogen Synthase Kinase-3beta (GSK-3β) Elicits a Disadvantageous Relationship Between EMT and Lens Mitoprotection via Nuclear β-Catenin. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3570.
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Inhibition of GSK-3β blocks mitochondrial membrane permeability transition (mMPT) for HLE-B3 cells in atmospheric oxygen. GSK-3β, in a multifactorial complex, also regulates nuclear levels of β-catenin, a known coordinator of cell survival and adhesion. Here, we show a novel, but likely unfavorable, link between nuclear β-catenin as an inducer of epithelial to mesenchymal transition (EMT) and β-catenin’s influence on the expression of the prosurvival protein, vascular endothelial growth factor (VEGF), resulting in enhanced lens epithelial cell mitoprotection.
HLE-B3 cells were treated with SB216763, a specific inhibitor of GSK-3β catalytic activity. Western blot analysis was employed to detect the levels of β-catenin, phospho-β-catenin, Bcl-2 and pBcl-2. ELISA was used to measure the levels of VEGF in cell culture supernatants. Axitinib, which inhibits lenticular cell autophosphorylation of its VEGF receptor, was employed to demonstrate a role for the VEGF-VEGFR2 receptor complex in regulating pBcl-2.
SB216763-treated HLE-B3 caused a marked inhibition of phosphorylation of glycogen synthase (downstream substrate of GSK-3β) prompting a significant increase in nuclear translocation of β-catenin. The enhancement of nuclear β-catenin resulted in significant increase of basal expression of VEGF. The increase in the VEGF levels was associated with an increase in pBcl-2, a protein we have shown to afford resistance to mMPT. When cells were treated with Axitinib, there was a significant loss of the pBcl-2. We have previously shown that loss of pBcl-2 leads to mMPT. At the same time, inhibition of GSK-3β also resulted in expression of α-smooth muscle actin and fibronectin in the nucleus relative to control.
Inactivation of GSK-3β resulted in the translocation of activated β-catenin to the lens cell nucleus. VEGF expression was elevated which enhanced the levels of pBcl-2. Augmented nuclear translocation of β-catenin also induced expression of epithelial-mesenchymal markers, likely indicating an EMT event. Taken together, our data establishes a heretofore unknown relationship implying that inhibition of GSK-3β activity not only promotes EMT, but at the same time elicits an unwanted situation whereby the newly generated “mesenchymal-like” cell population is likely to be more resistant to mitochondrial depolarization.
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