Mechanisms of bone loss and gain in untreated and treated osteoporosis

Aging and Bone Metabolism

Changes in bone architecture and metabolism with aging increase the likelihood of osteoporosis and fracture. Age-onset osteoporosis is multifactorial, with contributory extrinsic and intrinsic factors including certain medical problems, specific prescription drugs, estrogen loss, secondary hyperparathyroidism, microenvironmental and cellular alterations in bone tissue, and mechanical unloading or immobilization. At the histological level, there are changes in trabecular and cortical bone as well as marrow cellularity, lineage switching of mesenchymal stem cells to an adipogenic fate, inadequate transduction of signals during skeletal loading, and predisposition toward senescent cell accumulation with production of a senescence-associated secretory phenotype. Cumulatively, these changes result in bone remodeling abnormalities that over time cause net bone loss typically seen in older adults. Age-related osteoporosis is a geriatric syndrome due to the multiple etiologies that converge upon the skeleton to produce the ultimate phenotypic changes that manifest as bone fragility. Bone tissue is dynamic but with tendencies toward poor osteoblastic bone formation and relative osteoclastic bone resorption with aging. Interactions with other aging physiologic systems, such as muscle, may also confer detrimental effects on the aging skeleton. Conversely, individuals who maintain their BMD experience a lower risk of fractures, disability, and mortality, suggesting that this phenotype may be a marker of successful aging. © 2023 American Physiological Society. Compr Physiol 13:4355-4386, 2023.

Aging alters bone-fat reciprocity by shifting in vivo mesenchymal precursor cell fate towards an adipogenic lineage

Bone marrow derived mesenchymal progenitor cells (MPCs) play an important role in bone homeostasis. Age-related changes occur in bone resulting in a decrease in bone density and a relative increase in adipocity. Although in vitro studies suggest the existence of an age-related lineage switch between osteogenic and adipogenic fates, stem cell and microenvironmental contributions to this process have not been elucidated in vivo. In order to study the effects of MPC and microenvironmental aging on functional engraftment and lineage switching, transplantation studies were performed under non-myeloablative conditions in old recipients, with donor MPCs derived from young and old green fluorescent protein (GFP) transgenic mice. Robust engraftment by young MPCs or their progeny was observed in the marrow, bone-lining region and in the matrix of young recipients; however, significantly lower engraftment was seen at the same sites in old recipients transplanted with old MPCs. Differentiation of transplanted MPCs strongly favored adipogenesis over osteogenesis in old recipients irrespective of MPC donor age, suggesting that microenvironmental alterations that occur with in vivo aging are predominately responsible for MPC lineage switching. These data indicate that aging alters bone-fat reciprocity and differentiation of mesenchymal progenitors towards an adipogenic fate.

Extrinsic mechanisms involved in age-related defective bone formation

CONTEXT

Age-related bone loss is associated with progressive changes in bone remodeling characterized by decreased bone formation relative to bone resorption. Both trabecular and periosteal bone formation decline with age in both sexes, which contributes to bone fragility and increased risk of fractures. Studies in rodents and humans revealed that, independent of sex hormone deficiency, the age-related decline in bone formation is characterized by decreased osteoblast number and lifespan and reduced bone-forming capacity of individual osteoblasts. An important clinical question is to identify the mechanisms involved in the age-related defective bone formation.

EVIDENCE ACQUISITION

The mechanisms discussed in this review are based on a PubMed search and knowledge of the authors in the field.

EVIDENCE SYNTHESIS

Available basic and clinical studies indicate that multiple mechanisms are involved in the alterations of osteoblastogenesis and the resulting decline in bone formation with aging. Notably, the age-related osteoblast dysfunctions and defective bone formation are caused by a number of extrinsic clinical factors that inhibit anabolic signaling pathways in bone. Thus, targeting these pathways can abolish age-related bone loss.

CONCLUSIONS

The identification of extrinsic mechanisms involved in osteoblast dysfunctions associated with aging improves our knowledge of age-related bone loss and provides a basis for therapeutic intervention to improve bone formation and bone mass in the aging population.

Early acceleration phase and late stationary phase of remodeling imbalance in long bones of male rats exposed to long-standing acidemia: a 10-month longitudinal study using bone histomorphometry

Chronic metabolic acidosis (CMA) is known to have a detrimental effect on bone metabolism as a result of accelerated bone resorption and impaired bone formation. Typically, a number of compensatory adaptations must have occurred which may help palliate negative calcium balance and acidemia, e.g., increased intestinal calcium and phosphorus absorption. The final outcome with respect to bone remodeling after exposure to CMA for several months was, therefore, elusive. Herein, we investigated bone changes in male rats fed 1.5% NH(4)Cl in drinking water for up to 10 months to induce CMA with plasma pH of 7.2-7.3. Significant decreases in bone mineral density and content were detected by dual-energy X-ray absorptiometry after 6 months of CMA, whereas histomorphometric analysis revealed a significant decrease in bone volume already at week 2 after CMA induction. Exposure to CMA longer than 2 weeks also decreased trabecular number, trabecular thickness, osteoblast surface, mineral apposition rate, and bone formation rate, while increasing trabecular separation, osteoclast surface, and eroded surface. Bone resorption was rapid during weeks 2-16 (acceleration phase) and thereafter persisted at a slower rate (stationary phase) until week 40. Furthermore, CMA markedly reduced the total calcium content in bone and enhanced urinary calcium excretion as measured by atomic absorption spectrophotometry. It could be concluded that, after exposure to a long-standing acidemia, the enhanced bone resorption and suppressed bone formation led to osteopenia throughout the 10-month period, with accelerated bone loss seen only during the first 6 months. Thereafter, the compensatory adaptations appeared to help stabilize bone mass at a subnormal level.

The effect of fluoride administration on rat serum osteocalcin expression during orthodontic movement

INTRODUCTION

Osteocalcin is a bone protein that has been used to mark bone turnover. The precise role of this protein in bone remodeling has not been fully elucidated; however, it was shown to be instrumental in the activation of osteoclastic bone resorption via its effect on osteoblasts. Fluoride influences bone growth by acting as a mitogenic agent for osteoblasts. In this study, we used a rodent model to determine the effects of fluoride administration on systemic osteocalcin during orthodontic tooth movement.

METHODS

Thirty-two 8-week-old female Wistar rats were divided into 4 groups. The first experimental group (n = 10) was fed a normal laboratory diet and distilled water (MilliQ). The second experimental group (n = 10) had the same diet with the addition of fluoridated water. All animals in both groups had 100-g nickel-titanium coil springs secured to their mandibular incisors and left first molars to instigate orthodontic tooth movement for 14 days. The third (n = 6) and fourth (n = 6) groups were the controls. These animals did not undergo orthodontic tooth movement and were given fluoridated (100 ppm sodium fluoride) and nonfluoridated water ad libitum. Phlebotomy was performed via the lateral tail vein before placement of the orthodontic appliances, and final blood samples were obtained via cardiac puncture after the rats were killed 2 weeks later. Serum was isolated from the samples after centrifuging at each stage of phlebotomy, and an enzyme-linked immunosorbent assay (ELISA) was subsequently performed to determine osteocalcin levels in the various sample groups.

RESULTS

Orthodontic tooth movement with high, continuous forces was found to have a positive, statistically significant correlation with serum osteocalcin expression in the rodent model. The use of fluoride as a variable was found to increase the mean osteocalcin concentration, but this was not statistically significant.

CONCLUSIONS

Further understanding of the biological implications of increased osteocalcin expression requires additional research into the local expression of this protein in the gingival crevice during orthodontic movement.

The effects of binge alcohol exposure on bone resorption and biomechanical and structural properties are offset by concurrent bisphosphonate treatment

BACKGROUND

Chronic alcohol consumption reduces bone mass and strength, increasing fracture risk for alcohol abusers. Mechanisms underlying this vulnerability involve modulation of bone remodeling. Direct effects of alcohol on bone formation have been documented; those on bone resorption are less well studied. Skeletal effects of exposure to high blood alcohol concentrations (BAC's) attained during binge drinking have not been studied. We examined the effects of repeated binge-like alcohol treatment on bone resorption, bone mineral density and vertebral compressive strength in adult male rats treated with the aminobisphosphonate, risedronate.

METHODS

A binge alcohol exposure model was developed using intraperitoneal (IP) injection to administer a 20% (vol/vol) alcohol/saline solution (3 g/kg, 1X/day) on four consecutive days for 1, 2 or 3 weeks in 400 g rats, with and without weekly risedronate treatment (0.5 mg/kg, 1X/week). Total serum deoxypyridinoline (Dpd) a crosslink of bone type collagen released during resorption was measured by ELISA. Bone mineral density (BMD) was measured using peripheral quantitative computed tomography (pQCT). Vertebral compressive strength was determined using an Instron materials testing machine. Trabecular integrity was analyzed by computer-aided trabecular analysis system (TAS).

RESULTS

Peak BAC's averaged 308.5 +/- 12 mg/dL; average BAC was 258.6 +/- 28.7 mg/dL at time of euthanasia. No significant effects of treatment were observed after 1 or 2 weeks of binge alcohol exposure. At 3 weeks of alcohol treatment serum Dpd was significantly increased (205%, p < 0.05) over controls. Bone mineral density (BMD) in cancellous bone of distal femur and lumbar spine were significantly decreased (34% and 21% respectively, p < 0.01) after 3 weeks of binge treatment. Vertebral (L4) compressive strength (maximum load sustained before failure) also decreased (27%, p < 0.05) after 3 binge alcohol cycles. Risedronate maintained the Dpd level (p < 0.01), BMD (p < 0.001) and vertebral structural biomechanical properties (p < 0.01) of binge-treated rats at control levels (E vs ER). Indices of trabecular architectural integrity [Trabecular bone volume/tissue volume (BV/TV), bone area (BAR) and trabecular separation (Tb.Sp)] analyzed at week 3 showed (BV/TV) and (BAR) were significantly reduced in alcohol-binged rats (p < 0.01), while (Tb.Sp) was significantly increased (p < 0.01). Risedronate also maintained the trabecular architectural indices of binge-treated rats at control levels (E versus ER, p < 0.01).

CONCLUSIONS

In adult male rats, BAC's reflective of those attained during alcoholic binge drinking may affect the skeleton in part by stimulating bone resorption, an effect mitigated by risedronate.