Thrombin signalling through PAR (protease-activated receptor)-1 is involved in cellular processes, such as proliferation, differentiation and cell survival. Following traumatic injury to the eye, thrombin signalling may participate in disorders, such as PVR (proliferative vitreoretinopathy), a human eye disease characterized by the uncontrolled proliferation, transdifferentiation and migration of otherwise quiescent RPE (retinal pigment epithelium) cells. PARs activate the Ras/Raf/MEK/ERK MAPK pathway (where ERK is extracellular-signal-regulated kinase, MAPK is mitogen-activated protein kinase and MEK is MAPK/ERK kinase) through the activation of Gα and Gβγ heterotrimeric G-proteins, and the downstream stimulation of the PLC (phospholipase C)-β/PKC (protein kinase C) and PI3K (phosphoinositide 3-kinase) signalling axis. In the present study, we examined the molecular signalling involved in thrombin-induced RPE cell proliferation, using rat RPE cells in culture as a model system for PVR pathogenesis.

 Our results showed that thrombin activation of PAR-1 induces RPE cell proliferation through Ras-independent activation of the Raf/MEK/ERK1/2 MAPK signalling cascade. Pharmacological analysis revealed that the activation of ‘conventional’ PKC isoforms is essential for proliferation, although thrombin-induced phosphorylation of ERK1/2 requires the activation of atypical PKCζ by PI3K. Consistently, thrombin-induced ERK1/2 activation and RPE cell proliferation were prevented completely by PI3K or PKCζ inhibition. These results suggest that thrombin induces RPE cell proliferation by joint activation of PLC-dependent and atypical PKC isoforms and the Ras-independent downstream stimulation of the Raf/MEK/ERK1/2 MAPK cascade. The present study is the first report demonstrating directly thrombin-induced ERK phosphorylation in the RPE, and the involvement of atypical PKCζ in this process.

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