Melatonin Regulates the Synthesis and Secretion of Several Proteins by Pars Tuberalis Cells of the Ovine Pituitary

The pars tuberalis (PT) of the pituitary may be an important target for melatonin action, but the secretory output of the melatonin-responsive cells is unknown. Using [S-35]methionine, protein synthesis and secretion have been studied in primary cultures of ovine PT cells, and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Only 4% of the labelled proteins appeared in the medium with the majority retained in the cells. Stimulation of the cells with 10 muM forskolin increased the accumulation of several labelled proteins in the medium without corresponding changes in the cell (72, 62, 44, 39, 29, 24, 23, 18 and 14 kd). Two-dimensional gel electrophoresis showed the proteins to have mildly acidic isoelectric points. Melatonin (1 muM) counteracted the stimulatory effect of forskolin on all but one (23 kd) of these secreted proteins. Immunoprecipitation showed this to be prolactin. Furthermore, melatonin alone appeared to have an inhibitory effect on the synthesis and release of proteins into the medium. The synthesis and secretion of the melatonin-responsive proteins was not inhibited by actinomycin D (1 mug/ml), indicating control at the translational level. This contrasts with the regulation of prolactin which is actinomycin D-sensitive. Pulse-chase experiments demonstrated that it requires 30 min for the secretory proteins to appear in the medium, consistent with intracellular processing and packaging prior to secretion. The secretory proteins labelled in the ovine PT, and responsive to melatonin, did not appear to be specific to the PT, as a similar profile of labelled secretory proteins was produced in primary cultures of pars distalis cells. However, melatonin had no effect on the synthesis and secretion of proteins by the pars distalis. These results demonstrate that in the ovine PT melatonin regulates the synthesis and export of several secretory proteins. These are possibly packaging proteins of secretory granules, similar to the granin family of proteins. Thus, the results confirm that melatonin-responsive cells are secretory cells and further imply that the PT-specific product is not a protein.