Unveiling the role of hypoxia-inducible factor 2alpha in osteoporosis: Implications for bone health.

BACKGROUND: Osteoporosis (OP) has become a major public health problem worldwide. Most OP treatments are based on the inhibition of bone resorption, and it is necessary to identify additional treatments aimed at enhancing osteogenesis. In the bone marrow (BM) niche, bone mesenchymal stem cells (BMSCs) are exposed to a hypoxic environment. Recently, a few studies have demonstrated that hypoxia-inducible factor 2alpha (HIF-2α) is involved in BMSC osteogenic differentiation, but the molecular mechanism involved has not been determined. AIM: To investigate the effect of HIF-2α on the osteogenic and adipogenic differentiation of BMSCs and the hematopoietic function of hematopoietic stem cells (HSCs) in the BM niche on the progression of OP. METHODS: Mice with BMSC-specific HIF-2α knockout (Prx1-Cre;Hif-2αfl/fl mice) were used for in vivo experiments. Bone quantification was performed on mice of two genotypes with three interventions: Bilateral ovariectomy, semilethal irradiation, and dexamethasone treatment. Moreover, the hematopoietic function of HSCs in the BM niche was compared between the two mouse genotypes. In vitro, the HIF-2α agonist roxadustat and the HIF-2α inhibitor PT2399 were used to investigate the function of HIF-2α in BMSC osteogenic and adipogenic differentiation. Finally, we investigated the effect of HIF-2α on BMSCs via treatment with the mechanistic target of rapamycin (mTOR) agonist MHY1485 and the mTOR inhibitor rapamycin. RESULTS: The quantitative index determined by microcomputed tomography indicated that the femoral bone density of Prx1-Cre;Hif-2αfl/fl mice was lower than that of Hif-2αfl/fl mice under the three intervention conditions. In vitro, Hif-2αfl/fl mouse BMSCs were cultured and treated with the HIF-2α agonist roxadustat, and after 7 d of BMSC adipogenic differentiation, the oil red O staining intensity and mRNA expression levels of adipogenesis-related genes in BMSCs treated with roxadustat were decreased; in addition, after 14 d of osteogenic differentiation, BMSCs treated with roxadustat exhibited increased expression of osteogenesis-related genes. The opposite effects were shown for mouse BMSCs treated with the HIF-2α inhibitor PT2399. The mTOR inhibitor rapamycin was used to confirm that HIF-2α regulated BMSC osteogenic and adipogenic differentiation by inhibiting the mTOR pathway. Consequently, there was no significant difference in the hematopoietic function of HSCs between Prx1-Cre;Hif-2αfl/fl and Hif-2αfl/fl mice. CONCLUSION: Our study showed that inhibition of HIF-2α decreases bone mass by inhibiting the osteogenic differentiation and increasing the adipogenic differentiation of BMSCs through inhibition of mTOR signaling in the BM niche.

[Effects of drynaria total flavonoid on osteogenic differentiation of bone marrow mesenchymal stem cells at different glucose concentrations: experiment with rats].

OBJECTIVE: To study the effects of drynaria total flavonoid on osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) at different glucose concentrations. METHODS: BMSCs of SD rats were isolated, cultivated in vitro, and divided into 6 groups to be induced to differentiate into osteoblasts under different conditions: (1) low glucose control group, (2) high glucose control group, (3) low glucose classical induction group (sodium glycerophosphate+vitamin C+dexamethasone), (4) high glucose classical induction group (sodium glycerophosphate+vitamin C+dexamethasone), (5) low glucose+drynaria total flavonoid group, and (6) high glucose with drynaria total flavonoid group. Alkaline phosphate (ALP) test kit was used to examine the level of ALP. The ALP staining positive rate was examined with modified calcium cobalt method. Alizarin red staining was adopted to observe the number of calcium nodes. Immunohistochemistry was used to detect type I collagen level. Advanced glycosylation end products (AGEs) were tested by ELISA. RESULTS: The A value indicating the ALP activity, ALP staining positive rate, calcium node number, and type I collagen expression score of the low glucose+drynaria total flavonoid group were (0.439±0.024), 48.7%, (9.75±1.71) nodes/HP, and (2.21±0.07) respectively, all significantly higher than those of the sodium glycerophosphate+vitamin C+dexamethasone [(0.385±0.029), 35.0%, (6.25±0.96) nodes/HP, and (1.93±0.13) respectively, all P<0.05]. The A value, ALP staining positive rate, calcium node number, and type I collagen expression score of the high glucose with drynaria total flavonoid group were (0.352±0.022), 25.3%, (4.50±1.29)/HP, and (1.70±0.03) respectively, all significantly higher than those of the sodium glycerophosphate+vitamin C+dexamethasone [(0.139±0.013), 22.7%, (3.25±1.50)/HP, and (1.28±0.27) respectively, all P<0.05]. The AGE expression levels of the high glucose classical induction group and high glucose+drynaria total flavonoid group were both significantly higher than those of the low glucose classical induction group and low glucose+drynaria total flavonoid group (both P<0.05). There were no significant differences in the AGE level among the low glucose control, low glucose classical induction, and low glucose+drynaria total flavonoid groups (all P<0.05); and among the high glucose control, high glucose classical induction, and high glucose+drynaria total flavonoid groups (all P<0.05). However, the AGE levels of the high glucose groups were all significantly higher than those of the corresponding low glucose groups (all P<0.05). Glucose increased the AGE levels dose- and time-dependently. The concentrations of AGEs were significantly negatively correlated with the expression of type I collagen (r=-0.410, P<0.05). CONCLUSIONS: Drynaria total flavonoid promotes the osteogenic differentiation of BMSCs and relieves the inhibitory effect of osteogenic differentiation by glucose at high concentration. Thus drynaria total flavonoid may provide a potential therapy for diabetic osteoporosis.

[In vitro studies on the growth and proliferation characteristics of rat bone mesenchymal stem cells].

OBJECTIVE: This study was to investigate the growth and proliferation characteristics of rat bone mesenchymal stem cells (BMSCs) isolated by the method of whole bone marrow culture and to explore the effect of cell inoculation density and incubation period on cell proliferation, with an aim to provide multipotential seed cells for preventing from degenerative disease. METHODS: Bone mesenchymal stem cells were isolated by the method of whole bone marrow culture and then cultured in vitro. The cell morphologic features were observed by inverted microscope. The cell surface antigens were identified by flow cytometry. The effect of cell inoculation density and culture period on cell growth and proliferation was explored by analyzing the characteristics of a ten-day cell growth curve in 96-well plates. RESULTS: Flow cytometry results showed the detection rates for CD29, CD34 and CD45 were 97.68% (7607/7788), 7.93% (661/8340) and 2.76% (215/7788) respectively, which was consistent with the expression characteristics of BMSCs surface antigens. BMSCs became uniform after three cell passages, existing in a typical shape of whirlpool or radial colony. The senescent cells started to appear at 7(th) passage, and more senescent cells were found at 10(th) passage. The growth curve for moderate inoculation density was typically S-shaped. Lag phase was found during the first two days, and logarithm growth phase was in the following three days. Plateau phase started from the 6(th) day and cell numbers decreased slightly from the 8(th) day. CONCLUSION: The whole bone marrow culture is an effective way to obtain BMSCs. A moderate inoculation density was more advantageous to cell proliferation, by which more seed cells could be obtained.

[Depot-specific expression of retinol-binding protein 4 in human adipose tissue and their relationship with obesity and insulin resistance].

OBJECTIVE: To explore the depot-specific mRNA and protein expression of retinol-binding protein 4 (RBP4) in human subcutaneous and omental adipose tissue and investigate their relationship with the serum RBP4 levels, obesity and insulin resistance. METHODS: A total of 41 subjects with normal glucose regulation were recruited. Among them, there were 29 cases with normal body mass index (BMI) and 12 cases with BMI ≥ 24 kg/m(2). All were prepared to undergone a selective abdominal surgery. The levels of serum RBP4 and fasting insulin (FINS) were measured by ELISA (enzyme-linked immunosorbent assay) and chemiluminescence ELISA kit respectively. Fasting plasma glucose (FPG) was tested by glucose oxidase and the home model insulin resistance index (HOMA-IR) calculated. Real-time PCR (RT-PCR) and Western blot were employed to assess the relative mRNA and protein expression of RBP4 in subcutaneous and omental adipose tissues. The mRNA and protein expression of RBP4 from different fat depots were compared and their correlations with BMI, the serum RBP4 concentrations and HOMA-IR analyzed. RESULTS: (1) The serum concentrations of RBP4, FINS and HOMA-IR were significantly higher in overweight and obesity group than those in the normal BMI group [RBP4: (39.61 ± 1.57) mg/L vs (33.40 ± 1.28) mg/L, P < 0.01; FINS: (8.82 ± 3.79) mU/L vs (6.43 ± 4.38) mU/L, P < 0.05; HOMA-IR: 1.91 ± 0.85 vs 1.36 ± 0.72, P < 0.05]. (2) The expression levels of RBP4 mRNA and protein were significantly higher in omental adipose tissues than those in subcutaneous adipose tissue [mRNA: (5.88 ± 2.37) vs (2.07 ± 1.66), P < 0.01; protein: (0.76 ± 0.18 vs 0.15 ± 0.07, P < 0.05] and these depots difference in both groups (P < 0.05). Moreover, the overweight patients had the higher expression levels of RBP4 mRNA and protein in omental adipose tissue than normal BMI group (mRNA: 7.52 ± 0.58 vs 4.37 ± 0.45, P < 0.01; protein: 0.92 ± 0.23 vs 0.57 ± 0.13, P < 0.05). (3) The expressions of both RBP4 mRNA and protein in the omental adipose tissue were positively correlated with BMI, waist circumstance, serum concentrations of RBP4, FINS and HOMA-IR. The expression of was negatively correlated with HOMA-IR (r = -0.635, P < 0.05) in subcutaneous adipose tissue. No correlations were found between the expressions of RBP4 mRNA and protein in subcutaneous adipose tissue with BMI, waist circumstance, serum RBP4 and FINS concentrations. CONCLUSIONS: There is depot-specific expression of RBP4 in human subcutaneous and omental adipose tissues. A high expression of RBP4 in omental adipose tissue and an elevated level of serum RBP4 may contribute to the pathogenesis of obesity and IR.