N-ACETYLTRANSFERASE AND PROTEIN-SYNTHESIS MODULATE MELATONIN PRODUCTION BY Y79 HUMAN RETINOBLASTOMA CELLS

Melatonin is synthesized by the vertebrate pineal gland in a circadian fashion and is involved in numerous circadian and seasonal processes in the organism. The vertebrate retina also produces melatonin rhythmically to regulate rhythmic physiological processes in the eye. In both organs, melatonin is synthesized from serotonin by the sequential action of the enzymes, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), and can be stimulated by increases in cyclic AMP through a mechanism requiring protein synthesis. The regulation of ocular melatonin biosynthesis in mammals and particularly humans, has not been well studied. Recently, we have shown that Y79 human retinoblastoma cells produce melatonin and that cAMP can stimulate melatonin production. Y79 cells, therefore, provide a model system in which to study melatonin synthesis in human tissue. We report that cAMP stimulates NAT, but not HIOMT activity in Y79 cells, and that stimulation of NAT activity is linearly related to melatonin release. In addition, the stimulation of NAT and melatonin requires protein synthesis. The turnover of NAT is rather rapid, with a half-life of about 20 min. These results suggest that the regulation of melatonin in Y79 retinoblastoma cells is similar to that found in the retina and pineal of other vertebrates.