Effects of PTH on PTHrP gene expression in human osteoblasts: up-regulation with the kinetics of an immediate early gene.

Parathyroid hormone-related peptide (PTHrP) is a local regulator of human bone turnover, which shares some sequence homology with the systemic hormone parathyroid hormone (PTH). The two proteins exert cellular effects through interaction with a common G protein-coupled PTH/PTHrP receptor on target cells. Whilst the PTH gene has a relatively simple structure the PTHrP gene is complex, alternative splicing of which can generate multiple mRNA species encoding PTHrP of 139, 141 and 173 amino acids. To date little is known regarding the extent to which PTH and PTHrP interact to modulate bone cell function. In this study we have used the quantitative technique of Real-Time polymerase chain reaction (PCR) to investigate the ability of PTH to induce PTHrP expression in SaOS-2 cells and in primary human osteoblasts. In addition, we have used the semi-quantitative techniques of PCR followed by Southern analysis and scanning densitometry to investigate the effects of PTH(1-34) on expression of mRNA species encoding the three PTHrP isoforms. We report a 50 fold increase in PTHrP mRNA expression 30 min after treatment with 100 ng/ml human recombinant PTH (1-34) in SaOS-2 cells, and a 38 fold rise in human osteoblasts 45-90 min post-PTH treatment. mRNA species encoding for PTHrP 1-139, 1-141 and 1-173 were all induced in human osteoblasts 45 min after exposure to PTH. Whilst the 1-139 mRNA species exhibited a sustained expression, both the 1-141 and 1-173 isoforms showed a biphasic induction with a second peak 6 hr post PTH treatment. These data demonstrate that PTH induces expression of the PTHrP gene in both SaOS-2 and primary human osteoblasts with the kinetics of an immediate early gene. Up-regulation of the PTHrP gene in response to PTH may be an important physiological mechanism by which this systemic factor effects a localised response in bone.