| Background:Osteoporosis is a common clinical disease in the postmenopausal women and aging population, caused by an imbalance between bone resorption and bone formation, leading to osteoporotic fractures and other serious complications.1Age-induced osteoporosis is mainly associated with increased bone marrow adipocytes and bone loss.2m3In osteoporosis bone, the reduced bone formation is directly correlated with the increasing adipose tissue.4Osteoblasts and marrow adipocytes are both differentiated from a common precursors:bone marrow mesenchymal stem cells (BMSCs).5An in vitro study has revealed that MSCs (Mesenchymal Stem Cells) derived from postmenopausal women with osteoporosis are more likely to differentiate into adipocytes than osteoblasts, comparing with cells isolated from healthy women.6There are virtual differentiation plasticity in both osteoblasts and adipocytes. It has been demonstrated that fully differentiated osteoblasts from human MSCs presented capablity of differentiation and transdifferentiation into adipocytes and vice versa.7 It also has been reported the differentiation of MSCs into adipocytes or osteoblasts is competitively balanced within the bone marrow. Beresford JN and his coworkers have shown the inverse relationshiop between the differentiation of adipocytic and osteogenic cells.9Moreover, Aline Clabaut et al proposed that MSC-derived adipocytes are able to induce MSC-derived osteoblasts to differentiate to an adipocyte phenotype within an in vitro co-culture system.10,n the other hand, various studies have indicated that adipocytes can secrete factors, such as hormones, leptin, adiponectin and fatty acids which affect the proliferation, apoptosis and function of neighbouring cells.11,12The evidence from the investigation conducted by Alexandre Elbaza suggest the effect of lipotoxicity from marrow adipocytes on osteoblast function.13In addition, the crosstalk between adipocytes and osteoclast has been unearthed recently. It has been reported that preadipocytes have a stonger ability of supporting osteoclast-like cell formation without PGE2than preosteoblasts, suggesting an emerging relationship between bone marrow cells adipogenesis and osteoclastogenesis.14The existing evidence delineate that bone marrow fat accumulation may play a critical role in the onset of osteoporosis, owing to its potential effects on either the commitment of BMSCs differentiation or the homeostasis of osteoclast-mediated bone resorption. Considering the notion, reversal bone marrow adipogenesis may be an effective therapeutic strategy for the prevention of age-related osteoporosis. N-3long-chain polyunsaturated fatty acids (PUFAs) are essential fatty acids for human beings, including alpha-Iinolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) which cannot be self-synthesized in mammals.15During the past decades, n-3PUFAs have been supposed to be beneficial for prevention from bone loss in both in vitro and in vivo studies. However, the underlying mechanisms of this action has not be fully defined yet.16-22Few studies have identified that inhibition of osteoclast differentiation and activation may be one of the mechanisms involved in preventing osteoporosis by n-3PUFAs.23,24Objectives:To the best of our knowledge, there are no report regarding the roles and mechanisms of n-3PUFAs on bone marrow adipogenesis. In the present study, by employing a transgenic mice model which overexpress fat-1gene from caenorhabditis elegan and convert n-6PUFAs to n-3PUFAs endogenously, we aim to firstly investigate the effect of endogenous n-3PUFAs on bone marrow adipogenesis under osteoporosis conditions, which could give rise to a novel insight to the current therapy for anti-osteoporosis.Methods:we screened fat-1gene positive C57BL6mice and fat-1gene negative C57BL/6mice by genotyping using PCR Kit and established ovariectomy-induced model. The bone micro-structure morphometries was assessed by micro-CT scanning after3months. The bone marrow adiposity was then evaluated via histological analysis. In addition, the expression of PPARy and RUNX-2, key transcription regulators in osteogenesis and adipogenesis, were detected by immunofluorescence and immunohistochemistry analysis.Results:1. The effect of endogenous n-3PUFAs on body weight, abdominal adipose tissue in OVX-induced osteoporosis modelThere is a significant difference in the weight of uterus between OVX and sham groups, which identified the well establishment of OVX-induced osteoporosis model in mice. At the beginning of this study, all mice are well matched for age and weight. There are a significance in the value for body weight and abdominal adipose tissue between fat-1and WT OVX mice (P<0.05) after3months postoperative, but there is no significant difference in body weight between fat-1and WT sham mice (P>0.05). OVX groups induced higher levels of body weight compared with sham groups (P<0.05).2. Effect of endogenous n-3PUFAs on femur bone micro-structure morphometries The BMD, BMC, BV/TV and Tb.N in OVX groups were significant lower in femoral region than sham group. And the BMD, BV/TV and BMC in the distal femur of fat-1OVX mice were significantly higher (P<0.05) compared to that in WT OVX mice. However, the Tb.N in WT OVX group mildly decreased (P>0.05) compared with fat-1OVX group. There was no statistical significance between WT and fat-1sham mice in femur bone micro-structure morphometries.3. Effect of endogenous n-3PUFAs on bone marrow adiposity The adipocyte parameters in the WT and fat-1groups were described in Fig.4. There are increasing adipocyte number in the OVX group compared with sham group (P<0.05) for both WT and fat-1mice. Adipocyte parameters such as AV/TV and AD#from histological analyses confirmed extensive fat accumulation in the WT OVX group verus fat-1OVX group (P<0.05), but the adipocyte parameters between fat-1and WT sham groups remained stable.4. Effect of endogenous n-3PUFAs on PPARy and RUNX2expression in the bone marrow PPARy is an essential transcription factor in adipogenesis,28the expression and/or activity of which determines the commitment of BMSCs to osteoblasts or adipocyte lineages.29In agreement with immunofluorescence data, the intensity of positive staining for PPARy was enhanced by OVX significantly (P<0.05), and the level of PPARy protein expression was lower significantly in fat-1OVX group as compared with that of WT OVX group (P<0.05). There was no significant difference between WT and fat-1sham mice in the intensity of PPARy positive staining. RUNX2is a key regulator of osteoblast differentiation, playing an important role in bone formation.30In immunohistochemistry data, positive immunoreaction of RUNX2appearing in osteoblasts was stronger in sham group than OVX group (P<0.05). In WT OVX group, RUNX2-positive staining was elevated significantly compared with fat-1OVX mice (P<0.05). There was no statistical significance between WT and fat-lsham mice in the level of RUNX2protein expression.Conclution:Our findings suggest that endogenous n-3PUFAs protect against ovariectomy-induced bone loss in vivo, partially via attenuating bone marrow adipogenesis. The effect of endogenous n-3PUFAs on bone marrow mesenchymal stem cells differentiation to either more osteoblasts or less adipocytes, may confer to the bone loss protection. |
PDF Full Text Request