Thyroid hormones; thyroxine (T4) and triiodothyronine (T3), function in many body tissues to influence growth and metabolism, but little is known about thyroid hormone function in human retinal pigment epithelium (HRPE). Recent work provided direct evidence of thyroid hormone action in cultured fetal HRPE, but the function of thyroid hormone in adult HRPE is unknown. Therefore, the long-term goal of this research is to investigate thyroid hormone function within adult HRPE. ? ? In non-ocular tissues, T3 and T4 modulate protein expression at the nuclear and mitochondria transcriptional level. T4 stimulates Ca+2-ATPase activity in multiple tissues. Plasma membrane Ca+2-ATPase (PMCA) functions to transport Ca+2 within the cytosol to the extracellular space, and has been identified in cultured HRPE with all four isoforms of PMCA expressed at the molecular level. Thyroid hormone also modulates Na+,K+-ATPase gene expression and activity in multiple tissues. The Na+,K+-ATPase is located within the apical membrane of HRPE cells, and has been studied for nearly three decades. Yet, no studies have determined thyroid hormone influence on Na+,K+-ATPase in HRPE. ? ? The Na+,K+-ATPase and PMCA are two ion transporters which function to maintain ionic equilibrium within RPE and the subretinal space. Increased intracellular Ca++ in RPE is implicated in decreased phagocytosis of shed photoreceptor discs by RPE. It is possible that decreased phagocytosis by RPE, a hallmark of retinal degenerative diseases, is at least in part caused by a decrease in PMCA and/or Na+,K+-ATPase function. ? ? The objectives of this research are to determine the effect of varying physiologic concentrations of T4 on the expression and activity of PMCA, its isoforms, and Na+,K+-ATPase in adult HRPE. This proposal will test the hypothesis that T4 modulates PMCA and Na+,K+-ATPase expression in adult HRPE.
The specific aims of this study are 1) to examine the effect of varying T4 concentrations on the up- or down-regulation of expression of PMCA and its four isoforms, 2) to examine the effect of varying T4 concentrations on PMCA function, and 3) to examine the effect of T4 on the expression and function of Na+,K+-ATPase.