Thrombin stimulates RPE cell proliferation by promoting c-Fos-mediated cyclin D1 expression

The retinal pigment epithelium (RPE) plays an essential role in the maintenance and normal functioning of the neural retina. Alterations in RPE function are involved in several ocular pathologies involving the breakdown of the blood-retina barrier (BRB), which exposes RPE to serum components, thrombin among them. Our previous work has shown that thrombin stimulates the proliferation of RPE cells. We here analyzed the molecular pathways leading to this outcome, in order to support thrombin involvement in proliferative vitreoretinopathy (PVR), a major cause of retinal surgery failure. We demonstrated that thrombin activation of PAR-1 promotes cyclin D1 expression at the transcriptional level by stimulating c-Fos expression, mediated by PI3K, MAPK ERK1/2, and conventional PKC activity. Our results show that ERK activation is necessary but not sufficient for the induction of cyclin D1 expression and proliferation, since the inhibition of PI3K or cPKC prevents this outcome. Analysis of thrombin-activated PAR-1 downstream effectors demonstrated that c-Fos expression by the sustained activation of ERK and c-fos transcription triggers the expression and nuclear translocation of cyclin D1, a key regulator of cell cycle G1/S phase progression leading to proliferation. Evidence here provided contributes to the understanding of the mechanisms involved in proliferative eye diseases and enhances the possibility of controlling pathologies such as proliferative PVR, which eventually lead to blindness.