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L. A. Wiley, C. M. Garcia, M. Feldmeier, J. V. Barnett, D. C. Beebe; Posterior Capsular Opacification Occurs in the Absence of Canonical TGFβ Signaling. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3727. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Lens fibrosis (posterior capsular opacification; PCO) is common following cataract extraction and IOL implantation. The TGFβ pathway was hypothesized to play a major role in this process. We previously showed that fibrosis, as measured by the accumulation of smooth muscle α-actin (SMA), occurs in lens epithelial cells from TGβRII null mice in vivo and in vitro, suggesting that canonical TGFβ signaling contributes to, but is not required for lens fibrosis. This study evaluated the role of a non-canonical TGFβ receptor, TGFβRIII, in TGFβ signaling.
Wild type and TGFβRII null lens epithelial cells and wild type, TGFβRII, and TGFβRIII null mouse embryonic fibroblasts (MEFs) treated with TGFβ2 were used to study TGFβ signaling. Western blotting and immunostaining for p-Smad2, Smad2, Smad4, and TGFβRIII were performed to measure the downstream effects of TGFβ2 treatment.
Treatment of wild type lens epithelial explants with TGFβ2 increased the levels of p-Smad2 and promoted the nuclear localization of p-Smad2 and Smad4. Similar treatment of cells lacking TGFbRII caused the disappearance of Smad4 and abolished the low, basal levels of p-Smad2. The basal level of p-Smad2 was also decreased in TGFbRII null MEFs, even at low picomolar doses of TGFb2 . These results were confirmed by immunofluorescence with antibodies against p-Smad2. Time course studies showed that the decrease in p-Smad2 levels began 15 min after TGFb2 exposure. Elimination of canonical TGFb signaling by deletion of TGFbRII caused an increase in TGFbRIII protein levels.
Signaling through TGFβRIII is unlikely to account for the fibrosis of lens epithelial cells seen in the absence of TGFβRII. Future experiments will address the mechanisms by which TGFβRIII signaling suppresses Smad2 phosphorylation and nuclear localization and determine the effect on SMA accumulation. Elucidation of how TGFβRIII antagonizes canonical Smad signaling could provide new avenues for the prevention of PCO.
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