Differential expression of PTHrP and its receptor in pituitary gland and gills in estradiol-treated gilthead sea bream ( Sparus auratus, L.)

Abstract

In the gilthead sea bream ( Sparus auratus) 17β-estradiol (E 2) plays an important role in the synthesis of vitellogenin. During vitellogenesis, vitellogenin as a nutritional precursor protein is loaded with calcium, which requires elevated plasma calcium levels. This is accomplished via E 2-dependent processes. Reports have shown that hypercalcemic effects of E 2 are possibly mediated by another hypercalcemic factor, viz. parathyroid hormone related protein (PTHrP). To further investigate the possibility of PTHrP as a mediator of E 2-induced hypercalcemia, we investigated the local expression levels of the pthrp mRNA and of the gene coding for the PTHrP receptor, PTH1R ( pth1r) in two tissues involved in the calcium regulation (gills, pituitary gland) of the sea bream treated with E 2. Compared to control, treatment with E 2 resulted in: significantly increased total calcium and plasma PTHrP levels ( P < 0.01), a down-regulation of pthrp mRNA in the pituitary gland ( P < 0.01), and up-regulation of expression levels for both pthrp and pth1r in the branchial system ( P < 0.05). These findings provide direct evidence for a mediating role of PTHrP in E 2 induced hypercalcemia, and in addition support the idea for the presence of two independent systems, an endocrine pituitary PTHrP system and a peripheral paracrine branchial PTHrP system.