FACILITATION AND INHIBITION OF G-PROTEIN REGULATED PROTEIN SECRETION BY MELATONIN

Melatonin has been found to inhibit or enhance the constitutive secret ion of proteins from the cultured melanoma cells at nanomolar concentr ations (0.5-10 nM), in a dose dependent manner. The amplitude and dire ction of the response were found to depend on cell density: melatonin inhibited the release early after plating or at low cell density, but facilitated the release later on, or at high cell density. To elucidat e the involvement of G-proteins in these responses, the effects of gua nosine 5'-O-(3-thiotriphosphate) (GTP tau S; which was introduced into the cells during the process of permeabilization and resealing with A TP), aluminum fluoride, pertussis and cholera toxins on protein secret ion from the cells were assessed in the absence and presence of melato nin. At low cell density, melatonin inhibited release, but paradoxical ly enhanced it when GTP hydrolysis was blocked (by GTP tau S or choler a toxin treatment). Aluminum fluoride and melatonin inhibited protein release in the absence or presence of GTP tau S. At high cell density, melatonin facilitated the release and so did GTP tau S, aluminum fluo ride, their combination, and cholera toxin treatment. However, in the presence of the combination of GTP tau S, aluminium fluoride and melat onin, protein release was paradoxically inhibited. Similar treatment o f the cells with pertussis toxin, did not affect the melatonin-mediate d inhibition or facilitation. These results indicate that the effects of melatonin on protein secretion are mediated by at least one heterot rimeric G protein which belongs to the Gs class. In addition, melatoni n can facilitate secretion via a cholera and pertussis toxins-insensit ive mechanism which can be inhibited by aluminum fluoride. This effect is manifested when Gs is permanently activated (by GTP tau S or chole ra toxin).