Systemic prostaglandin E2 increases cancellous bone formation and mass in aging rats and stimulates their bone marrow osteogenic capacity in vivo and in vitro

Abstract

Prostaglandin E 2 (PGE 2 ) has been shown to exert a bone anabolic effect in young and adult rats. In this study we tested whether it possesses a similar effect on bone formation and bone mass in aging rats. Fifteen-month-old rats were injected daily with either PGE 2 at 5 mg/kg or vehicle for 14 days. PGE 2 treatment stimulated the rate of cancellous bone formation (a 5·5-fold increase in bone formation rate), measured by the incorporation of calcein into bone-forming surfaces at the tibial proximal metaphysis. This effect resulted in increased cancellous bone area (+54%) at the same site. Since PGE 2 treatment resulted in a much higher proportion of bone surface undergoing bone formation and thus lined with osteoblasts, we tested the hypothesis that PGE 2 stimulates osteoblast differentiation from bone marrow precursor cells both in vivo and in vitro. We found that ex vivo cultures of bone marrow stromal cells from rats injected for 2 weeks with PGE 2 at 5 mg/kg per day yielded more (4-fold) mineralized nodules and exhibited a greater (by 30-40%) alkaline phosphatase activity compared with cultures from vehicle-injected rats, attesting to a stimulation of osteoblastic differentiation by PGE 2 .We also compared the osteogenic capacity of bone marrow from aging (15-month-old) versus young (5-week-old) rats and its regulation by PGE 2 in vitro. Bone marrow stromal cell cultures from aging rats exhibited a greatly diminished osteogenic capacity, reflected in reduced nodule formation (6% of young animals) and lower alkaline phosphatase activity (60% of young animals). However, these parameters could be stimulated in both groups of animals by incubation with 10-100 nM PGE 2 . The magnitude of this stimulation was greater in cultures from aging rats (+550% vs +70% in nodule formation of aging compared with young rats).In conclusion, we demonstrate here that PGE 2 exerts a bone anabolic effect in aging rats, similar to the effect we and others have reported in young, growing rats. The PGE 2 -stimulated bone formation, which augments bone mass, most likely results from recruitment of osteoblasts from their bone marrow stromal precursors.

Osteogenic differentiation of mesenchymal stem cells is regulated by osteocyte and osteoblast cells in a simplified bone niche

“…Authors: The authors believe this is a very exciting area of study with many possible factors at work. An interesting molecule to focus on could be PGE2, which has been shown to have anabolic effects for cancellous bone formation in aged rats (Keila et al, 2001). The same study found that when PGE2 was applied to bone marrow MSCs in vitro there was an increase in the number of mineral nodules.…”

“…Cbfa1 (Runx2) and osteocalcin are expressed in higher quantities by osteocytes than osteoblasts (Mikuni-Takagaki et al, 1995;Ducy et al, 1997) and might be responsible for the quicker www.ecmjournal.org E Birmingham et al Osteogenesis of MSCs in a simplifi ed bone niche osteogenic response in MSCs exposed to osteocyte factors. Certain pro-osteogenic signalling molecules, such as nitric oxide (NO) and prostaglandin E2 (PGE2), are released in large amounts from mechanically stimulated osteocytes in vitro (Klein-Nulend et al, 1995a;Klein-Nulend et al, 1995b;Keila et al, 2001), but are also expressed in static conditions (McAllister et al, 2000;Cheng et al, 2001). These signals could provide the stimuli for MSC osteogenic differentiation in vivo.…”

Cyclooxygenase-2 regulates mesenchymal cell differentiation into the osteoblast lineage and is critically involved in bone repair

“…The finding that the COX-2 metabolite PGE 2 can reverse the decrease in osteoblastogenesis observed in COX-2 -/-marrow cell culture is not surprising given the effect of PGE 2 on bone formation both in vitro and in vivo (6,7,42,47,48). However, the finding that BMP-2 complements the deficiency in COX-2 and enhances bone nodule formation to a degree similar to that observed in wild-type cultures is particularly interesting and suggest that BMP signaling events may be downstream of COX-2 activity and PGE 2 effects.…”

“…Furthermore, systemic or local injection of PGE 2 stimulates bone formation in vivo and appears to induce osteoblastogenesis from bone marrow precursors (7,47). Ex vivo cultures of bone marrow stromal cells from rats injected with PGE 2 for 2 weeks yield four times more mineralized bone nodules compared with cultures from vehicleinjected rats (47 Schematic model representing the potential mechanism of COX-2 regulation of mesenchymal cell differentiation in bone repair. In the proposed model, COX-2 is induced in the early phase of the bone reparative process and produces increased amounts of PGE 2 in the local milieu.…”

Mechanotransduction and Osteogenesis

“…It is also possible that calcium concentration determines, indirectly, the cellular response to mechanical stimulation (osteogenesis or bone resorption) because this ion is responsible for 83% of PGE 2 synthesis. Additionally, the anabolic effects of PGE 2 are mediated by EP2 prostaglandin receptor and, mainly, by EP4 prostaglandin receptor, both located at the nuclear membrane (Cherian et al, 2005;Fortier et al, 2001;Genetos et al, 2005;Ke et al, 2003;Keila et al, 2001;KleinNulend et al, 1997;Li et al, 2005;Machwate et al, 2001;Mullender et al, 2004;Nomura & Takano-Yamamoto, 2000;Reich et al, 1997;Watanuki et al, 2002;Xu et al, 2007).…”