Tristan W. Fowler

Bone turnover across the menopause transition The role of gonadal inhibins

Accumulating evidence demonstrates increasing bone turnover and bone loss in women prior to menopause and decreases in serum estradiol levels. Increased follicle‐stimulating hormone levels have been correlated with some of these peri‐menopausal changes. However, decreases in gonadal inhibins of the transforming growth factor (TGF)‐β superfamily strongly correlate with increases in bone formation and resorption markers across the menopause transition and predict lumbar bone mass in peri‐menopausal women, likely as a result of direct inhibin suppression of osteoblastogenesis and osteoclastogenesis. Inhibins bind specifically to cells during osteoblastogenesis and osteoclastogenesis. They can block bone morphogenetic protein (BMP)‐stimulated osteoblast and osteoclast development as well as BMP‐stimulated SMAD1 phosphorylation, likely via inhibin–β‐glycan sequestration of BMP Type II receptor (BMPRII). Interestingly, continuous in vivo exposure to inhibin A is anabolic and protective against gonadectomy‐induced bone loss in mice, suggesting that inhibins contribute to the endocrine regulation of bone metabolism via a bimodal mechanism of action whereby cycling inhibin exposure suppresses bone turnover and continuous exposure to inhibins is anabolic.

Reproductive Hormones and Bone

Hypothalamic gonadotropin-releasing hormone (GnRH) stimulates secretion of pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which directly regulate ovarian function. Pituitary FSH can modulate osteoclast development, and thereby influence bone turnover. Pituitary oxytocin and prolactin effects on the skeleton are not merely limited to pregnancy and lactation; oxytocin stimulates osteoblastogenesis and bone formation, whereas prolactin exerts skeletal effects in an age-dependent manner. Cyclic levels of inhibins and estrogen suppress FSH and LH, respectively, and also suppress bone turnover via their suppressive effects on osteoblast and osteoclast differentiation. However, continuous exposure to inhibins or estrogen/androgens is anabolic for the skeleton in intact animals and protects against gonadectomy-induced bone loss. Alterations of one hormone in the hypothalamic-pituitary-gonadal (HPG) axis influence other bone-active hormones in the entire feedback loop in the axis. Thus, we propose that the action of the HPG axis should be extended to include its combined effects on the skeleton, thus creating the HPG skeletal (HPGS) axis.

LIS1 Regulates Osteoclast Formation and Function through Its Interactions with Dynein/Dynactin and Plekhm1

Microtubule organization and lysosomal secretion are both critical for the activation and function of osteoclasts, highly specialized polykaryons that are responsible for bone resorption and skeletal homeostasis. Here, we have identified a novel interaction between microtubule regulator LIS1 and Plekhm1, a lysosome-associated protein implicated in osteoclast secretion. Decreasing LIS1 expression by shRNA dramatically attenuated osteoclast formation and function, as shown by a decreased number of mature osteoclasts differentiated from bone marrow macrophages, diminished resorption pits formation, and reduced level of CTx-I, a bone resorption marker. The ablated osteoclast formation in LIS1-depleted macrophages was associated with a significant decrease in macrophage proliferation, osteoclast survival and differentiation, which were caused by reduced activation of ERK and AKT by M-CSF, prolonged RANKL-induced JNK activation and declined expression of NFAT-c1, a master transcription factor of osteoclast differentiation. Consistent with its critical role in microtubule organization and dynein function in other cell types, we found that LIS1 binds to and colocalizes with dynein in osteoclasts. Loss of LIS1 led to disorganized microtubules and aberrant dynein function. More importantly, the depletion of LIS1 in osteoclasts inhibited the secretion of Cathepsin K, a crucial lysosomal hydrolase for bone degradation, and reduced the motility of osteoclast precursors. These results indicate that LIS1 is a previously unrecognized regulator of osteoclast formation, microtubule organization, and lysosomal secretion by virtue of its ability to modulate dynein function and Plekhm1.

Low bone turnover and low BMD in Down syndrome: effect of intermittent PTH treatment.

Trisomy 21 affects virtually every organ system and results in the complex clinical presentation of Down syndrome (DS). Patterns of differences are now being recognized as patients’ age and these patterns bring about new opportunities for disease prevention and treatment. Low bone mineral density (BMD) has been reported in many studies of males and females with DS yet the specific effects of trisomy 21 on the skeleton remain poorly defined. Therefore we determined the bone phenotype and measured bone turnover markers in the murine DS model Ts65Dn. Male Ts65Dn DS mice are infertile and display a profound low bone mass phenotype that deteriorates with age. The low bone mass was correlated with significantly decreased osteoblast and osteoclast development, decreased bone biochemical markers, a diminished bone formation rate and reduced mechanical strength. The low bone mass observed in 3 month old Ts65Dn mice was significantly increased after 4 weeks of intermittent PTH treatment. These studies provide novel insight into the cause of the profound bone fragility in DS and identify PTH as a potential anabolic agent in the adult low bone mass DS population.

Circulating interleukin-8 levels explain breast cancer osteolysis in mice and humans.

Skeletal metastases of breast cancer and subsequent osteolysis connote a dramatic change in the prognosis for the patient and significantly increase the morbidity associated with disease. The cytokine interleukin 8 (IL-8/CXCL8) is able to directly stimulate osteoclastogenesis and bone resorption in mouse models of breast cancer bone metastasis. In this study, we determined whether circulating levels of IL-8 were associated with increased bone resorption and breast cancer bone metastasis in patients and investigated IL-8 action in vitro and in vivo in mice. Using breast cancer patient plasma (36 patients), we identified significantly elevated IL-8 levels in bone metastasis patients compared with patients lacking bone metastasis (p<0.05), as well as a correlation between plasma IL-8 and increased bone resorption (p<0.05), as measured by NTx levels. In a total of 22 ER+ and 15 ER- primary invasive ductal carcinomas, all cases examined stained positive for IL-8 expression. In vitro, human MDA-MB-231 and MDA-MET breast cancer cell lines secrete two distinct IL-8 isoforms, both of which were found to stimulate osteoclastogenesis. However, the more osteolytic MDA-MET-derived full length IL-8(1-77) had significantly higher potency than the non-osteolytic MDA-MB-231-derived IL-8(6-77), via the CXCR1 receptor. MDA-MET breast cancer cells were injected into the tibia of nude mice and 7days later treated daily with a neutralizing IL-8 monoclonal antibody. All tumor-injected mice receiving no antibody developed large osteolytic bone tumors, whereas 83% of the IL-8 antibody-treated mice had no evidence of tumor at the end of 28days and had significantly increased survival. The pro-osteoclastogenic activity of IL-8 in vivo was confirmed when transgenic mice expressing human IL-8 were examined and found to have a profound osteopenic phenotype, with elevated bone resorption and inherently low bone mass. Collectively, these data suggest that IL-8 plays an important role in breast cancer osteolysis and that anti-IL-8 therapy may be useful in the treatment of the skeletal related events associated with breast cancer.

Glucocorticoid suppression of osteocyte perilacunar remodeling is associated with subchondral bone degeneration in osteonecrosis

Through a process called perilacunar remodeling, bone-embedded osteocytes dynamically resorb and replace the surrounding perilacunar bone matrix to maintain mineral homeostasis. The vital canalicular networks required for osteocyte nourishment and communication, as well as the exquisitely organized bone extracellular matrix, also depend upon perilacunar remodeling. Nonetheless, many questions remain about the regulation of perilacunar remodeling and its role in skeletal disease. Here, we find that suppression of osteocyte-driven perilacunar remodeling, a fundamental cellular mechanism, plays a critical role in the glucocorticoid-induced osteonecrosis. In glucocorticoid-treated mice, we find that glucocorticoids coordinately suppress expression of several proteases required for perilacunar remodeling while causing degeneration of the osteocyte lacunocanalicular network, collagen disorganization, and matrix hypermineralization; all of which are apparent in human osteonecrotic lesions. Thus, osteocyte-mediated perilacunar remodeling maintains bone homeostasis, is dysregulated in skeletal disease, and may represent an attractive therapeutic target for the treatment of osteonecrosis.

Activin A inhibits RANKL-mediated osteoclast formation, movement and function in murine bone marrow macrophage cultures

ABSTRACT The process of osteoclastic bone resorption is complex and regulated at multiple levels. The role of osteoclast (OCL) fusion and motility in bone resorption are unclear, with the movement of OCL on bone largely unexplored. RANKL (also known as TNFSF11) is a potent stimulator of murine osteoclastogenesis, and activin A (ActA) enhances that stimulation in whole bone marrow. ActA treatment does not induce osteoclastogenesis in stroma-free murine bone marrow macrophage cultures (BMM), but rather inhibits RANKL-induced osteoclastogenesis. We hypothesized that ActA and RANKL differentially regulate osteoclastogenesis by modulating OCL precursors and mature OCL migration. Time-lapse video microscopy measured ActA and RANKL effects on BMM and OCL motility and function. ActA completely inhibited RANKL-stimulated OCL motility, differentiation and bone resorption, through a mechanism mediated by ActA-dependent changes in SMAD2, AKT1 and inhibitor of nuclear factor &kgr;B (I&kgr;B) signaling. The potent and dominant inhibitory effect of ActA was associated with decreased OCL lifespan because ActA significantly increased activated caspase-3 in mature OCL and OCL precursors. Collectively, these data demonstrate a dual action for ActA on murine OCLs.

Characterization of a panel of six β2-adrenergic receptor antibodies by indirect immunofluorescence microscopy

BackgroundThe β2-adrenergic receptor (β2AR) is a primary target for medications used to treat asthma. Due to the low abundance of β2AR, very few studies have reported its localization in tissues. However, the intracellular location of β2AR in lung tissue, especially in airway smooth muscle cells, is very likely to have a significant impact on how the airways respond to β-agonist medications. Thus, a method for visualizing β2AR in tissues would be of utility. The purpose of this study was to develop an immunofluorescent labeling technique for localizing native and recombinant β2AR in primary cell cultures.MethodsA panel of six different antibodies were evaluated in indirect immunofluorescence assays for their ability to recognize human and rat β2AR expressed in HEK 293 cells. Antibodies capable of recognizing rat β2AR were identified and used to localize native β2AR in primary cultures of rat airway smooth muscle and epithelial cells. β2AR expression was confirmed by performing ligand binding assays using the β-adrenergic antagonist [3H] dihydroalprenolol ([3H]DHA).ResultsAmong the six antibodies tested, we identified three of interest. An antibody developed against the C-terminal 15 amino acids of the human β2AR (Ab-Bethyl) specifically recognized human but not rat β2AR. An antibody developed against the C-terminal domain of the mouse β2AR (Ab-sc570) specifically recognized rat but not human β2AR. An antibody developed against 78 amino acids of the C-terminus of the human β2AR (Ab-13989) was capable of recognizing both rat and human β2ARs. In HEK 293 cells, the receptors were predominantly localized to the cell surface. By contrast, about half of the native rat β2AR that we visualized in primary cultures of rat airway epithelial and smooth muscle cells using Ab-sc570 and Ab-13989 was found inside cells rather than on their surface.ConclusionAntibodies have been identified that recognize human β2AR, rat β2AR or both rat and human β2AR. Interestingly, the pattern of expression in transfected cells expressing millions of receptors was dramatically different from that in primary cell cultures expressing only a few thousand native receptors. We anticipate that these antibodies will provide a valuable tool for evaluating the expression and trafficking of β2AR in tissues.

Osteocyte-Intrinsic TGF-β Signaling Regulates Bone Quality through Perilacunar/Canalicular Remodeling

Poor bone quality contributes to bone fragility in diabetes, aging, and osteogenesis imperfecta. However, the mechanisms controlling bone quality are not well understood, contributing to the current lack of strategies to diagnose or treat bone quality deficits. Transforming growth factor beta (TGF-β) signaling is a crucial mechanism known to regulate the material quality of bone, but its cellular target in this regulation is unknown. Studies showing that osteocytes directly remodel their perilacunar/canalicular matrix led us to hypothesize that TGF-β controls bone quality through perilacunar/canalicular remodeling (PLR). Using inhibitors and mice with an osteocyte-intrinsic defect in TGF-β signaling (TβRIIocy-/-), we show that TGF-β regulates PLR in a cell-intrinsic manner to control bone quality. Altogether, this study emphasizes that osteocytes are key in executing the biological control of bone quality through PLR, thereby highlighting the fundamental role of osteocyte-mediated PLR in bone homeostasis and fragility.

Treatment with anti‐Sclerostin antibody to stimulate mandibular bone formation

Anti‐Sclerostin antibody (Scl‐Ab) is a promising new bone anabolic therapy. Although anti‐Scl‐Ab stimulates bone formation and repair in the appendicular and axial skeleton, its efficacy in the craniofacial skeleton is still poorly understood.