Nuclear translocation of myocardin-related transcription factor-A during transforming growth factor beta-induced epithelial to mesenchymal transition of lens epithelial cells.
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PURPOSE: Transforming growth factor beta (TGFβ) is a known inducer of epithelial to mesenchymal transition (EMT), and studies in other systems have shown that nuclear localization of the myocardin-related transcription factor (MRTF) is downstream of TGFβ. In the following study, we investigated whether nuclear translocation of MRTF-A or MRTF-B is involved in TGFβ-induced EMT of lens epithelial cells (LECs). We further investigated the relationship between matrix metalloproteinase-2 and -9 (MMP-2/9) and MRTF in the EMT of LECs. METHODS: Rat lens explant cultures were used as the model system. Explants were treated with TGFβ, an MMP-2/9 inhibitor, or actin binding drugs and immunostained for alpha smooth muscle actin (αSMA), MRTF-A, and MRTF-B. Cytoplasmic and nuclear intensities of cells were measured using ImageJ. Production of αSMA was measured using western blot analysis and ImageJ. RESULTS: Untreated explant cells exhibited little αSMA expression, and MRTF-A and B were found to reside primarily in the cytosol. However, when stimulated with TGFβ, a significantly greater number of cells exhibited nuclear expression of MRTF-A, accompanied by an increase in αSMA expression. However, MRTF-B remained in the cytoplasm following TGFβ treatment. Cotreatment with an MMP-2/9 inhibitor and TGFβ resulted in reduced MRTF-A nuclear localization and αSMA expression compared to cells treated with TGFβ alone. CONCLUSIONS: Our results are the first to demonstrate the expression of MRTF-A in LECs and that its nuclear translocation can be stimulated by TGFβ. Our data further suggest that MMP-2 and -9 are involved in the translocation of MRTF-A in LECs during TGFβ-induced EMT.
