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

In the gilthead sea bream (Sparus auratus) 17β-estradiol (E2) 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 E2-dependent processes. Reports have shown that hypercalcemic effects of E2 are possibly mediated by another hypercalcemic factor, viz. parathyroid hormone related protein (PTHrP). To further investigate the possibility of PTHrP as a mediator of E2-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 E2. Compared to control, treatment with E2 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 E2 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.

► In gilthead sea bream PTHrP mediates E2-induced hypercalcemia.
► E2-treated sea bream become hypercalcemic and have elevated plasma PTHrP.
► mRNA’s of PTHrP and PTH1R are upregulated in gills, downregulated in hypophysis.
► PTHrP mediates in E2-induced hypercalcemia.
► We define an endocrine pituitary PTHrP system and a branchial paracrine PTHrP system.