Adrenomedullin Modulation Of Retinal Pigment Epithelial Cells And The Intracellular Signal Transduction Pathways

Background Adrenomedullin (ADM) is a vasorelaxant peptide originally isolated from the extract of human pheochromocytoma. Subsequent study revealed that ADM is rather more than simply a vasodilator. It has been shown to be a multifunctional regulatory peptide. ADM have a remarkable range of actions, from regulating cellular growth and differentiation, through modulating hormone secretion, to anti-apoptosis and anti-migration effects. But most of those effects exhibited a cell type specific manner. In Swiss 3T3 cells, ADM increased DNA synthesis in a dose-dependent manner by a mechanism involving specific ADM receptors and increased cAMP/PKA. In human normal glial cells and glial cell tumors, ADM suppressed cell growth and increased intracellular cAMP. The growth of human and rat astrocytomas and human glioblastomas, as well as cultured glioblas-toma-derived cell lines, was also inhibited by ADM. However, Moody et al. reported that ADM exerted mitogenic effects on cultured C6 gliomal cells that correlated with increases in cAMP and c-fos expression. To understand the mechanism of ADM function, a principal question is how a stimulus at the cell surface can be transmitted to the nucleus to initiate a process of gene expression that results in the creation of an appropriate physiological response to changes in the extracellular environment. There are a number of studies about the signal transduction pathways activated by ADM. cAMP has been found to be the second messenger in the majority of ADM actions. However, the effects of ADM are not fully mimicked by forskolin, possibly suggesting that a role for an additional second messenger be involved.ADM transcripts are expressed in various human tissues and cells, in-cluding the eye. In the iris sphincter isolated from cats and other mammalian species including human, ADM is a much more efficacious activator of adenylate cyclase and a much more effective relaxant than calcitonin gene-related peptide (CGRP). Udono et al. recently reported that human RPE cells produced and secreted ADM, and IFN-gamma and IL-lbeta induced ADM expression in ARPE-19 cells. Udono and others later demonstrated that hypoxia increase the expression of ADM in three human RPE cell lines, whereas the induction of endothelin-1 (ET-1) by hypoxia is found only in D407 cells. These findings indicate that ADM may be involved in modulating the roles of RPE in physiological and pathological processes.RPE lies between the sensitive retina and the choroid and plays a vital role in ocular metabolism. The apoptosis, degeneration and proliferation of RPE cells are responsible for the developments of a variety of blinding diseases such as retinitis pigmentosa, age-related macular degeneration (AMD) and proliferative vitreoretinopathy (PVR). The migration of activated RPE is an initial step in the development of PVR. Immunoreactive ADM levels in the vitreous of patients with PVR are found significantly higher than those of patients with proliferative diabetic retinopathy, AMD, and macular hole. Thus ADM may be involved in the pathophysiology of PVR. Agents that significantly increase intracellular levels of cAMP are also inhibitors of RPE cell migration. Recent studies have shown that ADM inhibited platelet-derived growth factor (PDGF)-BB and fetal calf serum (FCS)-induced smooth muscle cell (SMC) migration in a concentration-dependent manner. The findings of previous studies raise the possibility that ADM is related to the pathophysiology of some ocular diseases related with RPE cells. But we have little information about the signal transduction pathways activated by ADM in RPE cells. The physiological role(s) and mechanism of pharmacological effects of ADM on RPE cells are still unclear.Purpose Here we investigated whether ADM and ADMR mRNA are expressed in primary cultured human RPE and the specimens of epiretinal membranes surgically obtained from patients with PVR and if so, whether lipopolysaccharide (LPS) could upregulate the expression of ADM and ADMR mRNA in cultured human RPE; whether ADM...