Melatonin binding sites are functionally coupled to phosphoinositide hydrolysis in Syrian hamster RPMI 1846 melanoma cells.
Recent pharmacological studies demonstrate that Syrian hamster melanoma (RPMI 1846) cells possess a nanomolar-affinity binding site for melatonin. Inhibition of 2-[125I]-iodomelatonin binding to RPMI membranes by melatonergic ligands exhibit a rank order relationship similar to that observed in hamster hypothalamus. Biochemical studies indicate that the melatonin binding site in RPMI 1846 cells is not coupled in either a stimulatory or inhibitory fashion to adenylate cyclase as a second messenger. We now report that stimulation of RPMI 1846 melanoma cells with melatonergic agonists induces phosphoinositide hydrolysis in a concentration-dependent manner (EC50: N-acetylserotonin = 0.29 microM; 2-I-Melatonin = 0.39 microM; 6-Cl-Melatonin = 0.38 microM; Melatonin = 0.45 microM). Further, phosphoinositide hydrolysis induced by 2-I-melatonin and N-acetylserotonin was blocked by pre-incubation with the melatonin antagonist N-acetyltryptamine and prazosin, an antagonist which exhibits potency at the nanomolar affinity melatonin binding site. 2-I-melatonin and N-acetylserotonin-induced phosphoinositide hydrolysis were not blocked by the serotonin type 2 antagonist ketanserin or alpha-adrenergic antagonist, phentolamine. These data suggest that melatonin binding sites on RPMI 1846 cells are linked to phosphoinositide hydrolysis as a second messenger.
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CNS Special Projects, Bristol-Myers Squibb Company, Pharmaceutical Research Institute, Wallingford, Connecticut 06492.
Mullins
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D000818:Animals; D006224:Cricetinae; D006868:Hydrolysis; D008546:Melanoma, Experimental; D008550:Melatonin / Q000378:metabolism* / Q000494:pharmacology; D008647:Mesocricetus; D010716:Phosphatidylinositols / Q000378:metabolism*; D011869:Radioligand Assay; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000502:physiology*; D044094:Receptors, Melatonin; D014407:Tumor Cells, Cultured
D010716:Phosphatidylinositols; D011956:Receptors, Cell Surface; D044094:Receptors, Melatonin; D008550:Melatonin