The Target Cells of Parathyroid Hormone (PTH) Anabolic Effect in Bone Are Immature Cells of Osteoblastic Lineage

Abstract

A number of anabolic osteogenic factors have been investigated; however, effects of those on human skeletons remain uncertain yet. Parathyroid hormone (PTH) is a major regulator in calcium metabolism, can exert anabolic effects on net bone formation in clinical trials and its various effects have been reported in vivo and in vitro. Continuous infusion of PTH decreases bone mass whereas intermittent injection of PTH increases bone mass. These diverse effects, especially PTH's anabolic effect, have not been clearly reproduced in an in vitro system and the mechanism of these PTH effects in bone is still controversial. The purpose of this study was to establish a culture model to mimic anabolic effect of PTH and elucidate its mechanism. In primary culture system of rat bone marrow, cells were treated with PTH at different times for various periods and mineralized-nodule (MN) formation was evaluated at day 14. Osteogenic markers, such as osteopontin, osteocalcin, and bone sialoprotein were also evaluated. Continuous incubation in higher than 10 nM PTH completely inhibited the MN formation. In contrast, when the cells were treated with 100 nM PTH at an early stage (day 4), MN formation was stimulated. Furthermore, pulse incubation with PTH at the early stage stimulated ALP activity, the mRNA expression for bone matrix proteins whereas this PTH treatment did not affect 3[H] thymidine incorporation or DNA content. These results indicate that the target cells for the PTH anabolic effects are the immature cells of osteoblastic lineage and the PTH stimulates the osteoblastic cell differentiation without influencing the cellular proliferation, consequently increasing the mineralized tissue formation. These findings could partly explain the mechanism whereby PTH exerts the anabolic effect in bone and suggest that PTH could be an applicable agent for bone development in restoration of bone defect, fracture healing and cell-based tissue engineering.