It is well appreciated that high-density lipoprotein (HDL) and bone physiology and pathology are tightly linked. Studies, primarily in mouse models, have shown that dysfunctional and/or disturbed HDL can affect bone mass through many different ways. Specifically, reduced HDL levels have been associated with the development of an inflammatory microenvironment that affects the differentiation and function of osteoblasts. In addition, perturbation in metabolic pathways of HDL favors adipoblastic differentiation and restrains osteoblastic differentiation through, among others, the modification of specific bone-related chemokines and signaling cascades. Increased bone marrow adiposity also deteriorates bone osteoblastic function and thus bone synthesis, leading to reduced bone mass. In this review, we present the current knowledge and the future directions with regard to the HDL–bone mass connection. Unraveling the molecular phenomena that underline this connection will promote the deeper understanding of the pathophysiology of bone-related pathologies, such as osteoporosis or bone metastasis, and pave the way toward the development of novel and more effective therapies against these conditions.
Nicholaos I Papachristou, Harry C Blair, Kyriakos E Kypreos and Dionysios J Papachristou
Russell T Turner, Kenneth A Philbrick, Amida F Kuah, Adam J Branscum and Urszula T Iwaniec
Leptin, critical in regulation of energy metabolism, is also important for normal bone growth, maturation and turnover. Compared to wild type (WT) mice, bone mass is lower in leptin-deficient ob/ob mice. Osteopenia in growing ob/ob mice is due to decreased bone accrual, and is associated with reduced longitudinal bone growth, impaired cancellous bone maturation and increased marrow adipose tissue (MAT). However, leptin deficiency also results in gonadal dysfunction, disrupting production of gonadal hormones which regulate bone growth and turnover. The present study evaluated the role of increased estrogen in mediating the effects of leptin on bone in ob/ob mice. Three-month-old female ob/ob mice were randomized into one of the 3 groups: (1) ob/ob + vehicle (veh), (2) ob/ob + leptin (leptin) or (3) ob/ob + leptin and the potent estrogen receptor antagonist ICI 182,780 (leptin + ICI). Age-matched WT mice received vehicle. Leptin (40 µg/mouse, daily) and ICI (10 µg/mouse, 2×/week) were administered by subcutaneous injection for 1 month and bone analyzed by X-ray absorptiometry, microcomputed tomography and static and dynamic histomorphometry. Uterine weight did not differ between ob/ob mice and ob/ob mice receiving leptin + ICI, indicating that ICI successfully blocked the uterine response to leptin-induced increases in estrogen levels. Compared to leptin-treated ob/ob mice, ob/ob mice receiving leptin + ICI had lower uterine weight; did not differ in weight loss, MAT or bone formation rate; and had higher longitudinal bone growth rate and cancellous bone volume fraction. We conclude that increased estrogen signaling following leptin treatment is dispensable for the positive actions of leptin on bone and may attenuate leptin-induced bone growth.
CP Carron, DM Meyer, VW Engleman, JG Rico, PG Ruminski, RL Ornberg, WF Westlin and GA Nickols
Osteoclasts are actively motile on bone surfaces and undergo alternating cycles of migration and resorption. Osteoclast interaction with the extracellular matrix plays a key role in the osteoclast resorptive process and a substantial body of evidence suggests that integrin receptors are important in osteoclast function. These integrin receptors bind to the Arg-Gly-Asp (RGD) sequence found in a variety of extracellular matrix proteins and it is well established that the interaction of osteoclast alpha v beta 3 integrin with the RGD motif within bone matrix proteins is important in osteoclast-mediated bone resorption. In this study, we characterized the effects of two synthetic peptidomimetic antagonists of alpha v beta 3, SC-56631 and SC-65811, on rabbit osteoclast adhesion to purified matrix proteins and bone, and on bone resorption in vitro. SC-56631 and SC-65811 are potent inhibitors of vitronectin binding to purified alpha v beta 3. Both SC-56631 and SC-65811 inhibited osteoclast adhesion to osteopontin- and vitronectin-coated surfaces and time-lapse video microscopy showed that osteoclasts rapidly retract from osteopontin-coated surfaces when exposed to SC-56631 and SC-65811. SC-56631 and SC-65811 blocked osteoclast-mediated bone resorption in a dose-responsive manner. Further analysis showed that SC-65811 and SC-56631 reduced the number of resorption pits produced per osteoclast and the average pit size. SC-65811 was a more potent inhibitor of bone resorption and the combination of reduced pit number and size led to a 90% inhibition of bone resorption. Surprisingly, however, osteoclasts treated with SC-65811, SC-56631 or the disintegrin echistatin, at concentrations that inhibit bone resorption did not inhibit osteoclast adhesion to bone. These results suggest that alphavbeta3 antagonists inhibited bone resorption by decreasing osteoclast bone resorptive activity or efficiency but not by inhibiting osteoclast adhesion to bone per se.
Vikte Lionikaite, Karin L Gustafsson, Anna Westerlund, Sara H Windahl, Antti Koskela, Juha Tuukkanen, Helena Johansson, Claes Ohlsson, H Herschel Conaway, Petra Henning and Ulf H Lerner
Excess vitamin A has been associated with decreased cortical bone thickness and increased fracture risk. While most studies in rodents have employed high dosages of vitamin A for short periods of time, we investigated the bone phenotype in mice after longer exposure to more clinically relevant doses. For 1, 4 and 10 weeks, mice were fed a control diet (4.5 µg retinyl acetate/g chow), a diet modeled from the human upper tolerable limit (UTL; 20 µg retinyl acetate/g chow) and a diet three times UTL (supplemented; 60 µg retinyl acetate/g chow). Time-dependent decreases in periosteal circumference and bone mineral content were noted with the supplemented dose. These reductions in cortical bone resulted in a significant time-dependent decrease of predicted strength and a non-significant trend toward reduced bone strength as analyzed by three-point bending. Trabecular bone in tibiae and vertebrae remained unaffected when vitamin A was increased in the diet. Dynamic histomorphometry demonstrated that bone formation was substantially decreased after 1 week of treatment at the periosteal site with the supplemental dose. Increasing amount of vitamin A decreased endocortical circumference, resulting in decreased marrow area, a response associated with enhanced endocortical bone formation. In the presence of bisphosphonate, vitamin A had no effect on cortical bone, suggesting that osteoclasts are important, even if effects on bone resorption were not detected by osteoclast counting, genes in cortical bone or analysis of serum TRAP5b and CTX. In conclusion, our results indicate that even clinically relevant doses of vitamin A have a negative impact on the amount of cortical bone.
Ola Nilsson, Elizabeth A Parker, Anita Hegde, Michael Chau, Kevin M Barnes and Jeffrey Baron
In the growth plate, stem-like cells in the resting zone differentiate into rapidly dividing chondrocytes of the proliferative zone and then terminally differentiate into the non-dividing chondrocytes of the hypertrophic zone. To explore the molecular switches responsible for this two-step differentiation program, we developed a microdissection method to isolate RNA from the resting (RZ), proliferative (PZ), and hypertrophic zones (HZ) of 7-day-old male rats. Expression of approximately 29 000 genes was analyzed by microarray and selected genes verified by real-time PCR. The analysis identified genes whose expression changed dramatically during the differentiation program, including multiple genes functionally related to bone morphogenetic proteins (BMPs). BMP-2 and BMP-6 were upregulated in HZ compared with RZ and PZ (30-fold each, P < 0.01 and 0.001 respectively). In contrast, BMP signaling inhibitors were expressed early in the differentiation pathway; BMP-3 and gremlin were differentially expressed in RZ (100- and 80-fold, compared with PZ, P < 0.001 and 0.005 respectively) and growth differentiation factor (GDF)-10 in PZ (160-fold compared with HZ, P < 0.001). Our findings suggest a BMP signaling gradient across the growth plate, which is established by differential expression of multiple BMPs and BMP inhibitors in specific zones. Since BMPs can stimulate both proliferation and hypertrophic differentiation of growth plate chondrocytes, these findings suggest that low levels of BMP signaling in the resting zone may help maintain these cells in a quiescent state. In the lower RZ, greater BMP signaling may help induce differentiation to proliferative chondrocytes. Farther down the growth plate, even greater BMP signaling may help induce hypertrophic differentiation. Thus, BMP signaling gradients may be a key mechanism responsible for spatial regulation of chondrocyte proliferation and differentiation in growth plate cartilage.
M J Devlin, D J Brooks, C Conlon, M van Vliet, L Louis, C J Rosen and M L Bouxsein
Starvation induces low bone mass and high bone marrow adiposity in humans, but the underlying mechanisms are poorly understood. The adipokine leptin falls in starvation, suggesting that hypoleptinemia may be a link between negative energy balance, bone marrow fat accumulation, and impaired skeletal acquisition. In that case, treating mice with leptin during caloric restriction (CR) should reduce marrow adipose tissue (MAT) and improve bone mass. To test this hypothesis, female C57Bl/6J mice were fed a 30% CR or normal (N) diet from 5 to 10 weeks of age, with daily injections of vehicle (VEH), 1mg/kg leptin (LEP1), or 2mg/kg leptin (LEP2) (N=6–8/group). Outcomes included body mass, body fat percentage, and whole-body bone mineral density (BMD) via peripheral dual-energy X-ray absorptiometry, cortical and trabecular microarchitecture via microcomputed tomography (μCT), and MAT volume via μCT of osmium tetroxide-stained bones. Overall, CR mice had lower body mass, body fat percentage, BMD, and cortical bone area fraction, but more connected trabeculae, vs N mice (P<0.05 for all). Most significantly, although MAT was elevated in CR vs N overall, leptin treatment blunted MAT formation in CR mice by 50% vs VEH (P<0.05 for both leptin doses). CR LEP2 mice weighed less vs CR VEH mice at 9–10 weeks of age (P<0.05), but leptin treatment did not affect body fat percentage, BMD, or bone microarchitecture within either diet. These data demonstrate that once daily leptin bolus during CR inhibits bone marrow adipose expansion without affecting bone mass acquisition, suggesting that leptin has distinct effects on starvation-induced bone marrow fat formation and skeletal acquisition.
Yongmei Wang, Takeshi Sakata, Hashem Z Elalieh, Scott J Munson, Andrew Burghardt, Sharmila Majumdar, Bernard P Halloran and Daniel D Bikle
Parathyroid hormone (PTH) exerts both catabolic and anabolic actions on bone. Studies on the skeletal effects of PTH have seldom considered the effects of gender. Our study was designed to determine whether the response of mouse bone to PTH differed according to sex. As a first step, we analyzed gender differences with respect to bone mass and structural properties of 4 month old PTH treated (80 μg/kg per day for 2 weeks) male and female CD-1 mice. PTH significantly increased fat free weight/body weight, periosteal bone formation rate, mineral apposition rate, and endosteal single labeling surface, while significantly decreasing medullary area in male mice compared with vehicle treated controls, but induced no significant changes in female mice. We then analyzed the gender differences in bone marrow stromal cells (BMSC) isolated from 4 month old male and female CD-1 mice following treatment with PTH (80 μg/kg per day for 2 weeks). PTH significantly increased the osteogenic colony number and the alkaline phosphatase (ALP) activity (ALP/cell) by day 14 in cultures of BMSCs from male and female mice. PTH also increased the mRNA level of receptor activator of nuclear factor κB ligand in the bone tissue (marrow removed) of both females and males. However, PTH increased the mRNA levels of IGF-I and IGF-IR only in the bones of male mice. Our results indicate that on balance a 2-weeks course of PTH is anabolic on cortical bone in this mouse strain. These effects are more evident in the male mouse. These differences between male and female mice may reflect the greater response to PTH of IGF-I and IGF-IR gene expression in males enhancing the anabolic effect on cortical bone.
CK Lea and AM Flanagan
We tested the hypothesis that ovarian androgens play a role in protecting against cancellous bone loss in oestrogen-deficient states and that conversion of androgens to oestrogens is not catalysed by aromatase P450 in the bone or bone-marrow microenvironment. We did this by administering the anti-oestrogen, ICI 182,780, alone and in combination with the anti-androgen, Casodex, and compared the effects on the skeleton with those of ovariectomy. We found that rats subjected to ovariectomy lost significantly greater cancellous bone volume compared with those treated with ICI 182,780, but that combination anti-oestrogen and anti-androgen therapy resulted in bone loss equivalent to that in ovariectomised animals. The skeletal-protective effect of preserving the ovaries in animals which had been chemically ovariectomised was attributed to suppression of osteoclast parameters. Taken together, these data suggest that a reduction in ovarian androgens accentuates the increase in osteoclast number and the reduction in cancellous bone volume which occurs in oestrogen-deficient states. Failure to detect transcripts for aromatase cytochrome P450 in the bone and bone-marrow of rats provides supportive evidence that androgens mediate their skeletal-protective effect directly and not by peripheral conversion to oestradiol.
Guillaume Mabilleau, Marie Pereira and Chantal Chenu
Type 2 diabetes mellitus (T2DM) leads to bone fragility and predisposes to increased risk of fracture, poor bone healing and other skeletal complications. In addition, some anti-diabetic therapies for T2DM can have notable detrimental skeletal effects. Thus, an appropriate therapeutic strategy for T2DM should not only be effective in re-establishing good glycaemic control but also in minimising skeletal complications. There is increasing evidence that glucagon-like peptide-1 receptor agonists (GLP-1RAs), now greatly prescribed for the treatment of T2DM, have beneficial skeletal effects although the underlying mechanisms are not completely understood. This review provides an overview of the direct and indirect effects of GLP-1RAs on bone physiology, focusing on bone quality and novel mechanisms of action on the vasculature and hormonal regulation. The overall experimental studies indicate significant positive skeletal effects of GLP-1RAs on bone quality and strength although their mechanisms of actions may differ according to various GLP-1RAs and clinical studies supporting their bone protective effects are still lacking. The possibility that GLP-1RAs could improve blood supply to bone, which is essential for skeletal health, is of major interest and suggests that GLP-1 anti-diabetic therapy could benefit the rising number of elderly T2DM patients with osteoporosis and high fracture risk.
Rhonda D Prisby, Joshua M Swift, Susan A Bloomfield, Harry A Hogan and Michael D Delp
Osteopenia and an enhanced risk of fracture often accompany type 1 diabetes. However, the association between type 2 diabetes and bone mass has been ambiguous with reports of enhanced, reduced, or similar bone mineral densities (BMDs) when compared with healthy individuals. Recently, studies have also associated type 2 diabetes with increased fracture risk even in the presence of higher BMDs. To determine the temporal relationship between type 2 diabetes and bone remodeling structural and mechanical properties at various bone sites were analyzed during pre-diabetes (7 weeks), short-term (13 weeks), and long-term (20 weeks) type 2 diabetes. BMDs and bone strength were measured in the femora and tibiae of Zucker diabetic fatty rats, a model of human type 2 diabetes. Increased BMDs (9–10%) were observed in the distal femora, proximal tibiae, and tibial mid- shafts in the pre-diabetic condition that corresponded with higher plasma insulin levels. During short- and long-term type 2 diabetes, various parameters of bone strength and BMDs were lower (9–26%) in the femoral neck, distal femora, proximal tibiae, and femoral and tibial mid-shafts. Correspondingly, blood glucose levels increased by 125% and 153% during short- and long-term diabetes respectively. These data indicate that alterations in BMDs and bone mechanical properties are closely associated with the onset of hyperinsulinemia and hyperglycemia, which may have direct adverse effects on skeletal tissue. Consequently, disparities in the human literature regarding the effects of type 2 diabetes on skeletal properties may be associated with the bone sites studied and the severity or duration of the disease in the patient population studied.