A role for NAD⁺ and cADP-ribose in melatonin signal transduction

The hormone melatonin modulates constitutive protein secretion and inhibits cGMP in melanoma M2R cells via cholera-toxin (CTX) sensitive pathways. Activation by melatonin of CTX-substrates is due to enhancement of their ADP ribosylation. The possibility that ADP ribosylation was enhanced by elevation of NAD⁺ was studied. Melatonin enhanced NAD⁺ and decreased cADP-ribose in the cells, in a CTX independent pathway, indicating inhibition of nicotinamide adenine dinucleotide glycohydrolase (NADase). Dibutyryl cGMP (db-cGMP), which obviates the melatonin-induced decrease in cGMP and prevents the modulation of protein secretion, abrogated the enhancement of NAD⁺. cADP-ribose is involved in calcium homeostasis and its decrease may reduce intracellular Ca²⁺. The intracellular Ca²⁺ chelator BAPTA/AM mimicked and Ca²⁺ ionophores prevented the melatonin-induced inhibition of protein secretion. These data indicate for the first time hormonal modulation of NADase resulting in two signals: (1) enhancement of NAD⁺ which may explain the increase in ADP ribosylation and activation of CTX substrates leading to facilitation of protein secretion; (2) suppression of cell cADP-ribose and consequently intracellular Ca²⁺ which may explain the melatonin-induced inhibition of protein secretion.