Phagocytic Activity of Cultured Retinal Pigment Epithelial Cells from Chick Embryo: Inhibition by Melatonin and Cyclic AMP, and Its Reversal by Taurine and Cyclic GMP

Abstract

Cultured chick retinal pigment epithelial cells phagocytosed polystyrene latex particles. The phagocytosis was inhibited very specifically by melatonin, which attained 50% inhibition at about 10^––16M. Other indole compounds such as 5-methoxytryptophol, 5-hydroxytryptophol, 6-hydroxymelatonin, N-acetylserotonin, 5-methoxytryptamine and serotonin were also inhibitory although their effects were less than 1/10,000 that of melatonin. Possible retinal neurotransmitters, acetylcholine, γ-aminobutyric acid, glycine, dopamine, aspartic acid and glutamic acid, had no or only a minimum inhibitory effect, and was also the case for prostaglandin D₂, E₂, F₂α, and I₂. Taurine was not inhibitory at all. Among nucleotides, cyclic AMP specifically inhibited phagocytosis, giving 50% inhibition at about 10^––11M. Melatonin inhibition was increased by copresence of isobutylmethylxanthine. Inhibition by either melatonin or cyclic AMP was reversed by dibutyryl cyclic GMP. The reversal was observed also with compounds which were expected to increase intracellular cyclic GMP. Prostaglandin D₂ reversed inhibition in both cases, but its effect was incomplete and per se it had an inhibitory effect. Melatonin derivatives reversed inhibition by melatonin alone but not inhibition by cyclic AMP. Taurine efficiently reversed both kinds of inhibition. Other possible neurotransmitters were ineffective. Taurine was thus the most effective of these compounds. We suggest the following hypothetic control mechanism of phagocytic activity of the pigment epithelial cells: melatonin and cyclic AMP are intercellular and intracellular signals, respectively, of stopping phagocytosis, while taurine and cyclic GMP are intercellular and intracellular signals, respectively of cancelling this stop signal. Phagocytic activity of chick retinal pigment epithelial cells might be regulated by the concentration ratio of melatonin to taurine in the interphotoreceptor space.

Use of 2-[125I]iodomelatonin to characterize melatonin binding sites in chicken retina.

“…Melatonin has been found in the retina of several species (6)(7)(8)(9)(10). Retinal melatonin has been implicated in photoreceptor outer segment disc shedding and phagocytosis (11,12), melanosome aggregation in pigment epithelium (13), and cone photoreceptor retinomotor movement (14). Because so many important biological processes are regulated by melatonin, the mechanism by which this hormone acts is of great interest.…”

Localization of cyclase and cyclic nucleotide phosphodiesterase activities in retinal pigment epithelial cells of the guinea pig.

“…Melatonine and taurine were claimed to be the exgenous factors which control the enzyme activities mediating the cAMP and cGMP levels. Melatonine and taurine act via specific receptors on the plasmalemma, as was shown in vitro experiments using cultured RPE from chick embryo (23).…”

“…The present investigation was initiated by a study of Ogino et al (23), which showed under in vitro conditions with cultured cells of the RPE an inhibition of phagocytosis by melatonine and a stimulation by taurine. It has been proposed that melatonine stimulates adenylate cyclase activity via a specific receptor on the plasmalemma resulting in an increase in the intracellular cAMP level as a second messenger that inhibits phagocytosis.…”

“…In invertebrate this shedding is under the control of the central nervous system (7), whereas in the higher verteblates the circadian oscillator seems of exist within the eye (13,15). A possible involvement of cyclic adenosine monophosphate (cAMP) (3,4,11,18) or cyclic guanosine monophosphate (cGMP) (23) has been discussed as intracellular second messengers of the clock system which controls phagocytosis of the RPE. Melatonine and taurine were claimed to be the exgenous factors which control the enzyme activities mediating the cAMP and cGMP levels.…”

Is there a hormonal regulation of phagocytosis at unicellular and multicellular levels? A critical review

“…Stress caused by swimming to exhaustion provoked lower melatonin peak and consequently higher phagocytic activity of macrophages [61]. Melatonin also suppressed phagocytic activity of cultured retinal pigment cells [62].…”