Role of the Mitogen-Activated Protein Kinases (MAPKs), ERK1 and 2, and the Stress-Activated Protein Kinases (SAPKs), JNK-1 and P38 Kinase, in Retinal Pigment Epithelial (RPE) Cell Death and Proliferation

Abstract

Abstract: : Purpose: Retinal pigment epithelial (RPE) cell death and proliferation are important steps in the pathogenesis of ocular diseases. Mitogen-Activated Protein kinases (MAPKs) and Stress-Activated Protein kinases (SAPKs) are involved in the control of cell death and proliferation. We therefore investigated the involvement of the MAPKs, ERK1 and 2, and the SAPKs, JNK-1 and P38 kinase during the induction of RPE cell death and proliferation. Methods: human RPE cell proliferation was stimulated with 10% fetal calf serum (FCS) and cell death was induced by FCS depletion. Activation of the ERK1/2, JNK-1 and P38 kinase signaling pathways was detected by western blotting. Pharmacological inhibitors and activators, and antisens (AS) oligonucleotides (ODN) directed against the MAPKs and SAPKs were used to analyze the signaling involved in cell death and proliferation. Results: After FCS-induced RPE cell proliferation, the Ras/ERK signaling pathway was strongly activated. The inhibition of the activation of ERK by pharmacological and AS ODN strategies inhibited RPE cell proliferation and cyclin D1 induction. Moreover, inhibition of cyclin D1 by AS ODN decreased RPE cell proliferation, whereas blockade of RPE cell proliferation by cAMP inhibited both the activation of ERK and the induction of cyclin D1. These data demonstrated that RPE cell proliferation is under the strict control of the induction of cyclin D1 through the activation of ERK. FCS-stimulated RPE cell proliferation induced weakly and transiently the Ras/JNK-1 and the Rho/P38 kinase signaling pathways, suggesting cross-talk between ERK and JNK-1 pathways. Inhibition of JNK-1 or P38 kinase did not affect FCS-stimulated RPE cell proliferation. In contrast, long-term activation of JNK-1 and P38 kinase occurred in FCS depletion-induced RPE cell death. Inhibition of JNK-1 and P38 kinase overactivation partially reversed cell death induced by FCS depletion, suggesting that the kinetics of activation of JNK-1 and P38 kinase play a key role in transmitting cell death signaling. Conclusions: Parrallel upstream signaling pathways and cross-talks between MAPKs and SAPKs regulate the equilibrium between cell death and proliferation. This may have important implications for the development of more selective methods for the inhibition of cell death and proliferation in human RPE cells .

Keywords: retinal pigment epithelium • signal transduction • cell death/apoptosis