Ovariectomy of 12-month-old rats: effects on osteoprogenitor numbers in bone cell populations isolated from femur and on histomorphometric parameters of bone turnover in corresponding tibia

Differences in vertebral, tibial, and iliac cancellous bone metabolism in ovariectomized rats

Bone histomorphometry is usually performed on the iliac bone in humans and the tibia or vertebrae in rats. Bone metabolism differences among skeletal sites may be problematic when translating experimental results from rats to humans, but data on such differences in rats are lacking. Therefore, we examined the differences in bone structure and metabolism among skeletal sites using the lumbar vertebra (LV), tibia, and iliac bone obtained from ovariectomized or sham-operated rats preoperatively and at various times from 3 days to 26 weeks postoperatively. The trabeculae were thicker in the LV, where bone metabolism was less active than at other sites, and numerous fine trabeculae were observed in the tibia, where bone metabolism was more active. The iliac bone structure and metabolism were intermediate between those of the tibia and LV. Ovariectomy induced lower bone volume and higher bone metabolism in all skeletal sites, but the changes were greatest and occurred earliest in the tibia, followed by the iliac bone and then LV. Ovariectomy caused changes in bone metabolic markers, which occurred earlier than those in bone tissue. Activation frequency (Ac.f) increased after ovariectomy. At week 26 in ovariectomized rats, Ac.f was highest in the tibia (3.13 N/year) but similar between iliac bone (0.87 N/year) and LV (1.39 N/year). Ac.f is reportedly 0.3-0.4 N/year in the iliac bone of postmenopausal women, suggesting that bone turnover in rats is several times higher than in humans. The reference values reported here are useful for translating experimental results from rats to humans.

Changes in bone Pb accumulation: cause and effect of altered bone turnover

This paper assesses the magnitude of Pb uptake in cortical and trabecular bones in healthy animals and animals with altered balance in bone turnover, and the impact of exposure to Pb on serum markers of bone formation and resorption. The results reported herein provide physiological evidence that Pb distributes differently in central compartments in Pb metabolism, such as cortical and trabecular bones, in healthy animals and animals with altered balance in bone turnover, and that exposure to Pb does have an impact on bone resorption resulting in OC-dependent osteopenia. These findings show that Pb may play a role in the etiology of osteoporosis and that its concentration in bones varies as a result of altered bone turnover characteristic of this disease, a long standing question in the field. In addition, data collected in this study are consistent with previous observations of increased half-life of Pb in bone at higher exposures. This evidence is relevant for the necessary revision of current physiologically based kinetic models for Pb in humans.

Short-term effect of ovariectomy on osteoprogenitors in the healing rat mandibular incisor extraction socket

OVX increased the percentage of AP-positive CFU-F in healing rat mandible. The increase of the number of osteoprogenitors was not significant in rat mandible-derived cultures but was in femur-derived ones. This suggests that the effect of OVX on osteoprogenitors is either smaller or develops later in mandible relative to femur.

INTRODUCTION

Osteoprogenitors play an essential role in the regeneration process that leads to the successful integration of dental implants. However, it is unclear how systemic osteoporosis affects osteoprogenitors in oral bone. The present study was designed to determine the short-term effects of ovariectomy (OVX) on osteoprogenitors from the healing extraction socket in rat mandible.

METHODS

Six-month-old rats were ovariectomized (n=8) and control rats were left intact (n=8). Two weeks post-OVX, the right mandibular incisor was extracted. Four weeks post-extraction, the basal mandibular bone between the 1st and 3rd molar in the healing extraction socket was used to determine the number of fibroblastic progenitors (CFU-F), alkaline phosphatase-positive fibroblastic progenitors (AP-positive CFU-F), Dex-dependent osteoprogenitors (CFU-O Dex) and Prog-dependent osteoprogenitors (CFU-O Prog) using colony assays (n=5). Osteocalcin mRNA expression was evaluated using in situ hybridization (n=3). Data were analyzed using two-way ANOVA or Student's t-test.

RESULTS

OVX increased the percentage of AP-positive CFU-F in both mandible and femur. The number of CFU-O was increased only in femur. Osteocalcin mRNA expression in regenerating mandible was not statistically different between control and OVX animals.

CONCLUSION

Our results suggest that the effect of OVX on osteoprogenitors is either smaller or develops later in mandible relative to femur.

Pharmacologic targeting of a stem/progenitor population in vivo is associated with enhanced bone regeneration in mice

Drug targeting of adult stem cells has been proposed as a strategy for regenerative medicine, but very few drugs are known to target stem cell populations in vivo. Mesenchymal stem/progenitor cells (MSCs) are a multipotent population of cells that can differentiate into muscle, bone, fat, and other cell types in context-specific manners. Bortezomib (Bzb) is a clinically available proteasome inhibitor used in the treatment of multiple myeloma. Here, we show that Bzb induces MSCs to preferentially undergo osteoblastic differentiation, in part by modulation of the bone-specifying transcription factor runt-related transcription factor 2 (Runx-2) in mice. Mice implanted with MSCs showed increased ectopic ossicle and bone formation when recipients received low doses of Bzb. Furthermore, this treatment increased bone formation and rescued bone loss in a mouse model of osteoporosis. Thus, we show that a tissue-resident adult stem cell population in vivo can be pharmacologically modified to promote a regenerative function in adult animals.