Parathyroid hormone regulates the distribution and osteoclastogenic potential of hematopoietic progenitors in the bone marrow.

Parathyroid hormone (PTH) increases both the number of osteoclast in bone and the number of early hematopoietic stem cells (HSCs) in bone marrow. We previously characterized the phenotype of multiple populations of bone marrow cells with in vitro osteoclastogenic potential in mice. Here we examined whether intermittent administration of PTH influences these osteoclast progenitor (OCP) populations. C57BL/6 mice were treated with daily injections of bPTH(1-34) (80 µg/kg/day) for 7 or 14 days. We found that PTH caused a significant increase in the percentage of TN/CD115(+) CD117(high) and TN/CD115(+) CD117(int) cells (p < .05) in bone marrow on day 7. In contrast, PTH decreased the absolute number of TN/CD115(+) CD117(low) cells by 39% on day 7 (p < .05). On day 14, there was no effect of PTH on osteoclast progenitor distribution in vivo. However, PTH treatment for 7 and 14 days did increase receptor activator of NF-κB ligand (RANKL)- and macrophage colony-stimulating factor (M-CSF)-stimulated in vitro osteoclastogenesis and bone resorption in TN/CD115(+) cells. In the periphery, 14 days of treatment increased the percentage and absolute numbers of HSCs (Lin(-) CD117(+) Sca-1(+) ) in the spleen (p < .05). These data correlated with an increase in the percent and absolute numbers of HSCs in bone marrow on day 14 (p < .05). Interestingly, the effects on hematopoietic progenitors do not depend on osteoclast resorption activity. These results suggest that in vivo PTH treatment increased in vitro osteoclastogenesis and resorption without altering the number of osteoclast precursors. This implies that in vivo PTH induces sustained changes, possibly through an epigenetic mechanism, in the in vitro responsiveness of the cells to M-CSF and RANKL.