Sauvagine and TRH differentially stimulate proopiomelanocortin biosynthesis in the Xenopus laevis intermediate pituitary.

Abstract

In the amphibian Xenopus laevis, adaptation of the skin color to background light intensity is regulated by alpha-melanophore-stimulating hormone (alpha-MSH), a proopiomelanocortin (POMC)-derived peptide. In animals adapted to a white background, the level of POMC biosynthesis in the intermediate pituitary is much lower than in animals adapted to a black background. Release of alpha-MSH from neurointermediate lobes of white-adapted animals is stimulated in vitro by the regulatory peptides sauvagine and thyrotropin-releasing hormone (TRH), which are produced in the magnocellular nucleus of the hypothalamus. To study the role of sauvagine, cAMP, TRH and phorbol 12-myristate 13-acetate (PMA) in the regulation of POMC biosynthesis, the degree of incorporation of radioactive amino acids into the POMC protein was determined after treatment of the neurointermediate lobes with these secretagogues. When lobes of white-adapted animals are incubated in vitro, biosynthetic activity spontaneously increases because hypothalamic inhibitory control is removed by dissection. In addition to this control situation, the effects of secretagogues were tested on lobes with an inhibited level of biosynthesis, which is achieved by addition of neuropeptide Y (NPY) to the incubation medium. After 24 h of treatment, TRH stimulated POMC biosynthesis in NPY-inhibited lobes of white-adapted animals from 40.2 to 95.3% of control level. This stimulation could not be reduced by adding PMA, which indicates that protein kinase C is not involved in the stimulation of POMC biosynthesis by TRH. Sauvagine partially restored POMC biosynthesis from 27.2 to 62.5% of control level, whereas 8-Br-cAMP completely counteracted NPY inhibition from 27.8 to 97.5% of control level. After 3 days of treatment, stimulation by sauvagine and 8-Br-cAMP was maintained (sauvagine increased POMC biosynthesis in NPY-inhibited lobes from 7.4 to 36.2% of control level and 8-Br-cAMP stimulated from 6.5 to 82.5% of control level). TRH had no effect on POMC biosynthesis after 3 days of treatment, although its receptor was still functional as was shown in superfusion experiments where TRH stimulated alpha-MSH secretion. The observations indicate that the neuropeptides sauvagine and TRH differently control POMC biosynthesis in the Xenopus intermediate pituitary. This differential regulation is not only apparent with regard to time aspects (sauvagine has a sustained regulatory function, whereas TRH is only effective in the initial phase of POMC biosynthesis stimulation), but also an uncoupling of biosynthetic and release processes could be shown for TRH, which did not occur with sauvagine.