[The PTH/PTHrP receptor: biological implications]

Abstract

Since its discovery in 1923, the parathyroid hormone (PTH), was thought to be the sole hormone capable of stimulating bone resorption, renal tubular calcium reabsorption, calcitriol synthesis, and urinary excretion of phosphate. However, in 1987, the PTHrP (PTH-related peptide), was demonstrated to share most of the biological actions of PTH through the activation of the same receptor. This receptor was cloned in 1992 and named PTH/PTHrP receptor or PTH-R1. Both, PTH and PTHrP bind with great affinity to PTH-R1 and stimulate a signal transduction system involving different G-proteins, phospholipase C, and adenylate cyclase. A third member of the PTH family, the TIP-39 (tuberoinfundibular peptide), binds and activates another PTH receptor (PTH-R2). There is evidence for other PTH receptors, a PTH-R3, probably specific for PTHrP in keratinocytes, kidney, placenta and a PTH-R4 specific for C-terminal PTH fragments. Activating mutations in the PTH-R1 gene cause Jansen type metaphyseal chondrodysplasia, whereas inactivating mutations are responsible for Blomstrand type rare chondrodysplasia and enchondromatosis. The renal and bone PTH-R1 expression is upregulated in vitamin D deficient rats and by endotoxin, interleukin-2, dexamethasone, T3, and TGF beta. On the contrary, PTH, PTHrP, angiotensin-II, IGF-1, PGE2, vitamin D, and chronic renal failure decrease its expression. In conclusions, the biological implications of the identification and cloning of different PTH receptors are at their beginning. The almost ubiquitous distribution of PTHrP and PTH-R1, the numerous PTHrP and PTH fragments, let us suppose the existence of other PTH-related receptors, and a great complexity of the bone and mineral metabolism.