Regulation of Endothelin-1 (ET-1) Synthesis and Secretion at the Outer Blood Retinal Barrier

Regulation of Endothelin-1 (ET-1) Synthesis and Secretion at the Outer Blood-Retinal Barrier. Santosh Narayan, Department of Pharmacology & Neuroscience, University of North Texas Health Science Center Fort Worth, TX 76102. Summary The retinal pigment epithelium (RPE) constitutes the outer blood retinal barrier at the posterior segment of the eye. The RPE provides metabolic support to the photoreceptors in the neural retina. A breakdown in the barrier supported by RPE is a hallmark in several retinopathies including proliferative vitreoretinopathy, choroidal neovascularization and macular edema. Characteristic to all epithelial cells, mature RPE cells display a polarized phenotype both in culture (ARPE-19 cells) and in vivo, with specific apical and basolateral domains. This provides a testable model to study the RPE in vitro. The purpose of this study was to characterize the RPE as a source for endothelin-1, using both in vitro and in situ models. Endothelins (ET-1,-2, and -3) are known regulators of vascular tone, that are produced at sites close to their target, ET-1, being a potent vasoconstrictor may be involved in regulating blood supply to the choroid and the neural retina. We identified the RPE to be a major source for endothelin-1 (ET-1) in situ in the human retina as well as in pigmented and albino rat retinas. Additionally, using a cell-culture model of mature polarized ARPE-19 cells, we studied the synthesis and expression of ET-1 in response to muscarinic receptor stimulation, TNF-α and more recently to thrombin. We have identified other components involved in the synthesis and turnover of ET-1 in ARPE-19 cells including the proprotein convertase-furin, endothelin-converting enzyme-1 and its isoforms and the endothelin receptor B subtype. ARPE-19 cells grown on collagen filters helped determine if secretion of ET-1 was polarized or discriminative towards either the apical or basolateral surface. We consistently observed changes in cell shape and tight junction disassembly in ARPE-19 cells following TNF-α and thrombin addition. Additionally, thrombin caused an increase in preproET-1 mRNA at earlier time points that was dependent on the rhokinase (ROCK1/2) pathway. We report a novel signaling mechanism for regulating preproET-1 mRNA and mature ET-1 secretion in ARPE-19 cells that involves the thrombin receptor (protease activated receptor-1/PAR-1) dependent activation of the rho/ROCK1/2 signaling pathway that may also be involved in thrombin induced changes in the cytoskeleton. In conclusion, the RPE may be an important source for ET-1 at the posterior segment of the eye, secretion of which is greatly enhanced by substances that promote breakdown of blood retinal barriers, inflammation and changes in the RPE cytoskeleton. In conclusion, the RPE may be an important source for ET-1 at the posterior segment of the eye, secretion of which is greatly enhanced by substances that promote breakdown of blood retinal barriers, inflammation and changes in the RPE cytoskeleton. ET-1 secreted by the RPE, under physiological conditions may provide an autoregulatory mechanism for controlling blood flow at the outer blood retinal barrier. Excessive ET-1 secretion following breakdown of the barrier may either promote wound repair or may mediate further damage to the retina, the substrates of which are presently unknown. Future experimental approaches are planned to address these possibilities.