Epithelial-mesenchymal transition and proliferation of retinal pigment epithelial cells initiated upon loss of cell-cell contact.

PURPOSE: Molecular mechanisms that initiate epithelial-mesenchymal transition (EMT) involved in ocular fibrotic complications remain elusive. Studies were conducted to examine the role of cell-cell contact in regulating EMT and proliferation of retinal pigment epithelial (RPE) cells.
METHODS: Porcine RPE cells were isolated as sheets and cultured in vitro on lens capsules. Cell morphology was examined by microscopy. Western blot analysis and immunostaining were used to follow protein expression. Cell proliferation and RPE function were assessed by BrdU incorporation and phagocytosis assay, respectively.
RESULTS: RPE cells in the center of each sheet maintained cell-cell contacts and retained a differentiated phenotype. Disruption of cadherin function in these cells resulted in the loss of cell-cell contact and the concomitant induction of mesenchymal marker protein expression and cell proliferation. RPE cells at the edge of the sheet migrated away from the sheet, underwent EMT, and initiated proliferation, which was accompanied by a switch in cadherin expression from P-cadherin to N-cadherin. Although TGF-beta is thought to be a classic inducer of EMT, it was unable to initiate EMT in RPE cells maintaining cell-cell contact. However, change to alpha-SMA-positive myofibroblasts was induced by TGF-beta in cells that had already undergone EMT.
CONCLUSIONS: EMT and the onset of proliferation in RPE cells is initiated by loss of cell-cell contact. TGF-beta cannot initiate EMT or the proliferation of RPE cells maintaining cell-cell contact but appears to play an important secondary role downstream of EMT in inducing transition to a myofibroblast phenotype-a phenotype linked to the development of fibrotic complications.