Raloxifene: its ossification-promoting effect on female mesenchymal stem cells

Pyk2 deficiency potentiates osteoblast differentiation and mineralizing activity in response to estrogen or raloxifene

Bone remodeling is controlled by the actions of bone-degrading osteoclasts and bone-forming osteoblasts (OBs). Aging and loss of estrogen after menopause affects bone mass and quality. Estrogen therapy, including selective estrogen receptor modulators (SERMs), can prevent bone loss and increase bone mineral density in post-menopausal women. Although investigations of the effects of estrogen on osteoclast activity are well advanced, the mechanism of action of estrogen on OBs is still unclear. The proline-rich tyrosine kinase 2 (Pyk2) is important for bone formation and female mice lacking Pyk2 (Pyk2-KO) exhibit elevated bone mass, increased bone formation rate and reduced osteoclast activity. Therefore, in the current study, we examined the role of estrogen signaling on the mechanism of action of Pyk2 in OBs. As expected, Pyk2-KO OBs showed significantly higher proliferation, matrix formation, and mineralization than WT OBs. In addition we found that Pyk2-KO OBs cultured in the presence of either 17β-estradiol (E2) or raloxifene, a SERM used for the treatment of post-menopausal osteoporosis, showed a further robust increase in alkaline phosphatase (ALP) activity and mineralization. We examined the possible mechanism of action and found that Pyk2 deletion promotes the proteasome-mediated degradation of estrogen receptor α (ERα), but not estrogen receptor β (ERβ). As a consequence, E2 signaling via ERβ was enhanced in Pyk2-KO OBs. In addition, we found that Pyk2 deletion and E2 stimulation had an additive effect on ERK phosphorylation, which is known to stimulate cell differentiation and survival. Our findings suggest that in the absence of Pyk2, estrogen exerts an osteogenic effect on OBs through altered ERα and ERβ signaling. Thus, targeting Pyk2, in combination with estrogen or raloxifene, may be a novel strategy for the prevention and/or treatment of bone loss diseases.

Improving Combination Osteoporosis Therapy in a Preclinical Model of Heightened Osteoanabolism

Combining anticatabolic agents with parathyroid hormone (PTH) to enhance bone mass has yielded mixed results in osteoporosis patients. Toward the goal of enhancing the efficacy of these regimens, we tested their utility in combination with loss of the transcription factor Nmp4 because disabling this gene amplifies PTH-induced increases in trabecular bone in mice by boosting osteoblast secretory activity. We addressed whether combining a sustained anabolic response with an anticatabolic results in superior bone acquisition compared with PTH monotherapy. Additionally, we inquired whether Nmp4 interferes with anticatabolic efficacy. Wild-type and Nmp4-/- mice were ovariectomized at 12 weeks of age, followed by therapy regimens, administered from 16 to 24 weeks, and included individually or combined PTH, alendronate (ALN), zoledronate (ZOL), and raloxifene (RAL). Anabolic therapeutic efficacy generally corresponded with PTH + RAL = PTH + ZOL > PTH + ALN = PTH > vehicle control. Loss of Nmp4 enhanced femoral trabecular bone increases under PTH + RAL and PTH + ZOL. RAL and ZOL promoted bone restoration, but unexpectedly, loss of Nmp4 boosted RAL-induced increases in femoral trabecular bone. The combination of PTH, RAL, and loss of Nmp4 significantly increased bone marrow osteoprogenitor number, but did not affect adipogenesis or osteoclastogenesis. RAL, but not ZOL, increased osteoprogenitors in both genotypes. Nmp4 status did not influence bone serum marker responses to treatments, but Nmp4-/- mice as a group showed elevated levels of the bone formation marker osteocalcin. We conclude that the heightened osteoanabolism of the Nmp4-/- skeleton enhances the effectiveness of diverse osteoporosis treatments, in part by increasing hyperanabolic osteoprogenitors. Nmp4 provides a promising target pathway for identifying barriers to pharmacologically induced bone formation.

The effects of strength training and raloxifene on bone health in aging ovariectomized rats

The aim of this study was to investigate the effects of strength training (ST) and raloxifene (Ral), alone or in combination, on the prevention of bone loss in an aging estrogen-deficient rat model. Aging Wistar female rats were ovariectomized at 14months and allocated to four groups: (1) non-trained and treated with vehicle, NT-Veh; (2) strength training and treated with vehicle, ST-Veh; (3) non-trained and treated with raloxifene, NT-Ral; and (4) strength training and treated with raloxifene, ST-Ral. ST was performed on a ladder three times per week and Ral was administered daily by gavage (1mg/kg/day), both for 120days. Areal bone mineral density (aBMD), strength, microarchitecture, and biomarkers (osteocalcin, OCN; osteoprotegerin, OPG; and tartrate-resistant acid phosphatase, TRAP) were assessed. Immunohistochemistry was performed for runt-related transcription factor 2 (RUNX2), osterix (OSX), OCN, OPG, TRAP, and receptor activator of nuclear factor kappa-B ligand (RANKL). The rats that performed ST (ST-Veh) or were treated with Ral (NT-Ral) showed significant improvements in aBMD (p=0.001 and 0.004), bone strength (p=0.001), and bone microarchitecture, such as BV/TV (%) (p=0.001), BS/TV (mm(2)/mm(3)) (p=0.023 and 0.002), Conn.Dn (1/mm(3)) (p=0.001), Tb.N (1/mm) (p=0.012 and 0.011), Tb.Th (1/mm) (p=0.001), SMI (p=0.001 and 0.002), Tb.Sp (p=0.001), and DA (p=0.002 and 0.007); there was also a significant decrease in plasma levels of OCN (p=0.001 and 0.002) and OPG (p=0.003 and 0.014), compared with animals in the NT-Veh group. Ral, with or without ST, promoted an increased immunolabeling pattern for RUNX2 (p=0.0105 and p=0.0006) and OSX (p=0.0105), but a reduced immunolabeling pattern for TRAP (p=0.0056) and RANKL (p=0.033 and 0.004). ST increased the immunolabeling pattern for RUNX2 (p=0.0105), and association with Ral resulted in an increased immunolabeling pattern for OPG (p=0.0034) and OCN (p=0.0024). In summary, ST and Ral administration in aged, estrogen-deficient Wistar female rats is associated with a decrease in bone turnover marker plasma levels, increased activity of cells that promote osteoblastogenesis, and decreased activity of cells that promote osteoclastogenesis; these are correlated with higher aBMD, bone strength, and bone microarchitecture at the femoral neck. The results indicate that strength training and Ral are potential tools to reduce the risk of fractures at clinically relevant sites.

Teriparatide increases the maturation of circulating osteoblast precursors

This study shows that teriparatide promotes the circulating osteoblast (OB) precursor degree of maturation in patients affected by postmenopausal osteoporosis.

INTRODUCTION

Anabolic treatment with teriparatide has proven effective for the therapy of postmenopausal osteoporosis and significantly reduces the risk of non-vertebral fragility fractures. The aim of this study was to investigate the effect of teriparatide on circulating OB precursors.

METHODS

We evaluated by flow cytometry and real-time PCR the expression of OBs typical markers in peripheral blood mononuclear cells during treatment with teriparatide plus calcium and vitamin D, raloxifene plus calcium and vitamin D or calcium and vitamin D alone at various time points. Serum bone alkaline phosphatase and osteocalcin (OC) were measured as markers of bone turnover.

RESULTS

Our results show that circulating OB precursors are more numerous and more immature in patients affected by fragility fractures than in osteoporotic patients without fractures. We also show that teriparatide treatment increases the expression of alkaline phosphatase and of OC in OB precursors; thus, it increases their degree of maturation.

CONCLUSIONS

We suggest that teriparatide acts as anabolic agents also by promoting the maturation of OB precursors.