Estrogens and selective estrogen receptor modulators differentially antagonize Runx2 in ST2 mesenchymal progenitor cells

Estrogen and estrogen receptors mediate the mechanobiology of bone disease and repair

It is well understood that the balance of bone formation and resorption is dependent on both mechanical and biochemical factors. In addition to cell-secreted cytokines and growth factors, sex hormones like estrogen are critical to maintaining bone health. Although the direct osteoprotective function of estrogen and estrogen receptors (ERs) has been reported extensively, evidence that estrogen signaling also has a role in mediating the effects of mechanical loading on maintenance of bone mass and healing of bone injuries has more recently emerged. Recent studies have underscored the role of estrogen and ERs in many pathways of bone mechanosensation and mechanotransduction. Estrogen and ERs have been shown to augment integrin-based mechanotransduction as well as canonical Wnt/b-catenin, RhoA/ROCK, and YAP/TAZ pathways. Estrogen and ERs also influence the mechanosensitivity of not only osteocytes but also osteoblasts, osteoclasts, and marrow stromal cells. The current review will highlight these roles of estrogen and ERs in cellular mechanisms underlying bone mechanobiology and discuss their implications for management of osteoporosis and bone fractures. A greater understanding of the mechanisms behind interactions between estrogen and mechanical loading may be crucial to addressing the shortcomings of current hormonal and pharmaceutical therapies. A combined therapy approach including high-impact exercise therapy may mitigate adverse side effects and allow an effective long-term solution for the prevention, treatment, and management of bone fragility in at-risk populations. Furthermore, future implications to novel local delivery mechanisms of hormonal therapy for osteoporosis treatment, as well as the effects on bone health of applications of sex hormone therapy outside of bone disease, will be discussed.

ERK-estrogen receptor α signaling plays a role in the process of bone marrow mesenchymal stem cell-derived exosomes protecting against ovariectomy-induced bone loss

BACKGROUND

Exosomes derived from bone marrow mesenchymal stem cells (BMSC-Exos) are considered as candidates for osteoporosis (OP) therapy. Estrogen is critical in the maintenance of bone homeostasis. However, the role of estrogen and/or its receptor in BMSC-Exos treatment of OP, as well as its methods of regulation during this process remain unclear.

METHODS

BMSCs were cultured and characterized. Ultracentrifugation was performed to collect BMSC-Exos. Transmission electron microscopy, nanoparticle tracking analysis, and western blotting were used to identify BMSC-Exos. We examined the effects of BMSC-Exos on the proliferation, osteogenic differentiation, mineralization, and cell cycle distribution of MG-63 cells. The protein expression of estrogen receptor α (ERα) and the phosphorylation of ERK were investigated through western blotting. We determined the effects of BMSC-Exos on the prevention of bone loss in female rats. The female Sprague-Dawley rats were divided into three groups: the sham group, ovariectomized (OVX) group, and the OVX + BMSC-Exos group. Bilateral ovariectomy was performed in the OVX and OVX + BMSC-Exos groups, while a similar volume of adipose tissue around the ovary was removed in the sham group. The rats in OVX group and OVX + BMSC-Exos group were given PBS or BMSC-Exos after 2 weeks of surgery. Micro-CT scanning and histological staining were used to evaluate the in vivo effects of BMSC-Exos.

RESULTS

BMSC-Exos significantly enhanced the proliferation, alkaline phosphatase activity, and the Alizarin red S staining in MG-63 cells. The results of cell cycle distribution demonstrated that BMSC-Exos increased the proportion of cells in the G2 + S phase and decreased the proportion of cells in the G1 phase. Moreover, PD98059, an inhibitor of ERK, inhibited both the activation of ERK and the expression of ERα, which were promoted by administration of BMSC-Exos. Micro-CT scan showed that in the OVX + BMSC-Exos group, bone mineral density, bone volume/tissue volume fraction, trabecular number were significantly upregulated. Additionally, the microstructure of the trabecular bone was preserved in the OVX + BMSC-Exos group compared to that in the OVX group.

CONCLUSION

BMSC-Exos showed an osteogenic-promoting effect both in vitro and in vivo, in which ERK-ERα signaling might play an important role.

Arid1a regulates cell cycle exit of transit-amplifying cells by inhibiting the Aurka-Cdk1 axis in mouse incisor

Stem cells self-renew or give rise to transit-amplifying cells (TACs) that differentiate into specific functional cell types. The fate determination of stem cells to TACs and their transition to fully differentiated progeny is precisely regulated to maintain tissue homeostasis. Arid1a, a core component of the switch/sucrose nonfermentable complex, performs epigenetic regulation of stage- and tissue-specific genes that is indispensable for stem cell homeostasis and differentiation. However, the functional mechanism of Arid1a in the fate commitment of mesenchymal stem cells (MSCs) and their progeny is not clear. Using the continuously growing adult mouse incisor model, we show that Arid1a maintains tissue homeostasis through limiting proliferation, promoting cell cycle exit and differentiation of TACs by inhibiting the Aurka-Cdk1 axis. Loss of Arid1a overactivates the Aurka-Cdk1 axis, leading to expansion of the mitotic TAC population but compromising their differentiation ability. Furthermore, the defective homeostasis after loss of Arid1a ultimately leads to reduction of the MSC population. These findings reveal the functional significance of Arid1a in regulating the fate of TACs and their interaction with MSCs to maintain tissue homeostasis.

Mechanisms by which electroacupuncture‑mediated histone acetylation mitigates bone loss in rats with ovariectomy‑induced osteoporosis

The aim of the present study was to investigate the effectiveness of electroacupuncture (EA) on ovariectomy-induced osteoporotic rats to elucidate potential mechanisms by which EA regulates acetylation of histones in caput femoris. A total of 40 female Sprague-Dawley rats were randomly allocated into four groups: Sham operation, ovariectomy-induced osteoporosis (OVX), EA and 17β-estradiol (E2) treatments. After 8 weeks of intervention, the trabecular morphology of each group was measured by micro-computed tomography. Biomarkers of bone metabolism in serum were detected. The protein expression of histone deacetylase 2 (HDAC2), histone H3, Ac-histone H3 and downstream cytokines involved in osteoblast and osteoclast differentiation were detected. The results showed that EA and E2 both prevented bone loss and improved trabecular morphology in OVX rats. EA was found to suppress the protein expression of HDAC2 and promoted the acetylation of histone H3 compared with the OVX model group. The results indicated that EA promoted the differentiation of osteoblasts, and suppressed that of osteoclasts, thereby improving the trabecular morphology. E2 was shown to regulate the expression of runt-related transcription factor 2 and receptor activator of nuclear factor-κB ligand without modulating the expression of HDAC2, and therefore diverged mechanistically from EA. Overall, the results of the present study suggested that the mechanisms through which EA improved bone mineral density and trabecular morphology may involve the modulation of histone H3 acetylation and regulation of osteoblast and osteoclast differentiation.

Puerarin alters the function of monocytes/macrophages and exhibits chondroprotection in mice

Recent studies have suggested that puerarin may impede osteoclastogenesis and facilitate bone regeneration, in addition to attenuating tissue inflammation. The present study investigated the therapeutic effects of puerarin on inflammatory responses and monocyte recruitment in in vitro and in vivo osteoarthritis (OA) models. Puerarin treatment increased the proliferation of OA chondrocytes, as determined by Cell Counting Kit‑8 assay. In addition, the present results suggested that puerarin suppressed the interleukin‑1β‑induced production of inflammatory cytokines in OA chondrocytes and monocytes/macrophages, as assessed by ELISA. In a mouse model of mono‑iodoacetate‑induced OA, the present histological analyses suggested that administration with puerarin attenuated the inflammatory profile of OA joints and reduced cartilage destruction. Using flow cytometry, a decreased number of myeloid‑derived C‑C chemokine receptor 2+/lymphocyte Ag 6C+ monocytes was identified in the blood of OA mice treated with puerarin compared with control OA mice. Furthermore, quantitative real‑time polymerase chain reaction analysis suggested that puerarin treatment decreased C‑C chemokine ligand 2 expression in arthritic tissues. Collectively, the results suggested that puerarin treatment limited the recruitment of inflammatory monocytes. In summary, the present study provided pre‑clinical evidence that puerarin may serve as a potential target in the treatment of OA.