Comparison of parathyroid hormone and calcium ionophore A23187 effects on bone resorption and nucleic acid synthesis in cultured fetal rat bone

Abstract

It has recently been demonstrated that calcium ionophore A23187 mimics certain of the effects of parathyroid hormone (PTH) on bone in vitro, including stimulation of 45Ca release and cAMP formation. To further examine the relative effects of these two agents on bone cell metabolism, we compared the effects of synthetic PTH 1-34 (50 ng/ml) and calcium ionophore A23187 (0.5 micrograms/ml) on 45Ca release, DNA concentration, and nucleic acid synthesis in fetal rat forelimb rudiments cultured for periods up to 120 h. Both agents stimulated 45Ca release; however, the effects of PTH were apparent after a shorter period of exposure. Bone DNA concentration (expressed as microgram DNA/mg bone) was not affected by PTH but was significantly increased relative to control values by exposure to A23187 for 8-120 h of incubation. PTH increased the incorporation of 3H-thymidine into DNA at 30 and 48 h, and increased the incorporation of 14C-uridine into RNA at 48 h, time points which corresponded to a period of accelerated PTH stimulation of 45Ca release. In contrast, 3H-thymidine and 14C-uridine incorporation were both uniformly suppressed by A23187 at all time points examined. Thus the increased DNA concentration observed in A23187-treated rudiments appeared to be the result of a decreased rate of bone maturation and mineralization. The markedly different patterns of nucleic acid synthesis in response to PTH and A23187 suggest that these agents differ significantly in their mechanisms of action on bone cell metabolism.

Parathyroid Hormone and Parathyroid Hormone-related Protein Exert Both Pro- and Anti-apoptotic Effects in Mesenchymal Cells

“…Our data suggest that functional PTH receptors, likely through affecting proliferation or apoptosis, contribute to the increase of mesenchymal cell numbers. These results agree with studies reporting that PTH stimulates osteoblast proliferation (2,4) and increases osteoblast number (44) and that a constitutively active PTH-1R increases osteoblast number in trabecular bone in transgenic mice (15). Additionally, Watson et al (45) suggest that the PTH-1R localizes to the nucleus and may be associated with ligand-independent proliferation.…”

“…The mechanisms underlying the PTH anabolic effect in bone are not fully understood. The increased bone formation has been attributed to activation of growth factors (2,3), osteoblast precursor cell proliferation (2,4), and mature osteoblast function (2). Recent studies suggest that suppression of osteoblast apoptosis might play a major role in PTH anabolic action (5); however, the impact of cell differentiation and the pathways operating in this process have not been well characterized.…”

DNA synthesis is not necessary for osteoclastic responses to parathyroid hormone in cultured fetal rat long bones.

“…Parathyroid hormone (PTH)' is an important stimulator of bone resorption both in vivo and in vitro, and increases the number of osteoclasts in bone (8)(9)(10). PTH also increases DNA synthesis in bone cells (2,11,12,13). However, definitive evidence demonstrating that the cells that replicate in response to PTH also differentiate into osteoclast precursor cells is lacking.…”

Effects of transient PTH on early proliferation, apoptosis, and subsequent differentiation of osteoblast in primary osteoblast cultures

“…Depending on the culture systems, animal models, and experimental conditions, different studies have shown variable results with respect to the anabolic effect of PTH as it relates to osteoblast proliferation. For instance, in vitro organ cultures of embryonic mouse radii and rat calvarie showed that PTH increases osteoblast number (15) and stimulates osteoblast replication (5,8) but decreases the collagen synthesis (10,40). PTH stimulated cell proliferation in cultures of the human osteosarcoma cell line TE-85 (12,30); however, it inhibited cell proliferation in UMR-106 osteosarcoma cell cultures (33).…”