Evidence for the synthesis of parathyroid hormone-related protein (PTHrP) by nontransformed clonal rat osteoblastic cells in vitro

Abstract

Parathyroid hormone-related protein (PTHrP) is synthesized by a variety of tumors and is thought to be the main cause of the clinical syndrome of Immoral hypercalcemia of malignancy (HHM). In addition to its parathyroid hormone (PTH)-like actions, novel actions of PTHrP on placental calcium transport and inhibition of in vitro osteoclast activity have been demonstrated. The fact that osteoblasts act as mediators of osteoclastic bone resorption prompted us to investigate whether nontranformed, osteoblastlike cells produce PTHrP. PTHrP has been detected in developing human fetal bones and in rat long bones in culture. For this study, osteogenic cells, CRP 5/4 and CRP 10/30, were employed. Both cell types represent clonal bone cell populations established from 1-day-old rats. While CRP 10/30 cells express the osteoblastic phenotype, CRP 5/4 cells resemble cells with preosteoblastic properties. With a radioimmunoassay (RIA), utilizing antiserum directed against the amino-terminal PTHrP(1–40), it was found that both cell types synthesize PTHrP constitutively. CRP 10/30 cells produce about twice as much as CRP 5/4 cells. Transforming growth factor-beta (TGF-β1) was shown to increase the synthesis of PTHrP in CRP 5/4 cells by about 2.5-fold, while in CRP 10/30 cells it caused an approximate 50% reduction of PTHrP. Employing the reverse transcriptase polymerase chain reaction (RTPCR) technique it was found that both bone cell types express mRNA for PTHrP and that the modulation of the PTHrP mRNA levels by TGF-β1 in CRP 5/4, and to a lesser degree in CRP 10/30 cells, was reflected in a change in the level of PTHrP protein in the culture medium. The present study reveals that nontransformed rat bone cells synthesize PTHrP in vitro, and that this synthesis appears to be regulated by TGF-β1. These findings are consistent with the possibility of a paracrine role for PTHrP in bone.