Intermittent PTH treatment can delay the transformation of mature osteoblasts into lining cells on the periosteal surfaces

Mature osteoblasts have three fates: as osteocytes, quiescent lining cells, or osteoblasts that undergo apoptosis. However, whether intermittent parathyroid hormone (PTH) can modulate the fate of mature osteoblasts in vivo is uncertain. We performed a lineage-tracing study using an inducible gene system. Dmp1-CreERt2 mice were crossed with Rosa26R reporter mice to obtain targeted mature osteoblasts and their descendants, lining cells or osteocytes, which were detected using X-gal staining. Rosa26R:Dmp1-CreERt2(+) mice were injected with 0.25 mg 4-OH-tamoxifen (4-OHTam) on postnatal days 5, 7, 9, 16, and 23. In a previous study, at 22 days after the last 4-OHTam, most LacZ+ cells on the periosteal surface were inactive lining cells. On day 25 (D25), the mice were challenged with an injection of human PTH (1-34, 80 μg/kg) or vehicle daily for 10 (D36) or 20 days (D46). We evaluated the number and thickness of LacZ+ osteoblast descendants in the calvaria and tibia. In the vehicle group, the number and thickness of LacZ+ osteoblast descendants at both D36 and D46 significantly decreased compared to D25, which was attenuated in the PTH group. In line with these results, PTH inhibited the decrease in the number of LacZ+/osteocalcin-positive cells compared to vehicle at both D36 and D46. As well, the serum levels of sclerostin decreased, as did the protein expression of sclerostin in the cortical bone. These results suggest that intermittent PTH treatment can increase the number of periosteal osteoblasts by preventing mature osteoblasts from transforming into lining cells in vivo.