Leukotriene B4 increases intracellular calcium concentration and phosphoinositide metabolism in mouse osteoblasts via cyclic adenosine 3',5'-monophosphate-independent pathways

Regulation of BMP2-induced intracellular calcium increases in osteoblasts

Although bone morphogenetic protein-2 (BMP2) is a well-characterized regulator that stimulates osteoblast differentiation, little is known about how it regulates intracellular Ca²⁺ signaling. In this study, intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) upon BMP2 application, focal adhesion kinase (FAK) and Src activities were measured in the MC3T3-E1 osteoblast cell line using fluorescence resonance energy transfer-based biosensors. Increase in [Ca²⁺ ]_i , FAK, and Src activities were observed during BMP2 stimulation. The removal of extracellular calcium, the application of membrane channel inhibitors streptomycin or nifedipine, the FAK inhibitor PF-573228 (PF228), and the alkaline phosphatase (ALP) siRNA all blocked the BMP2-stimulated [Ca²⁺ ]_i increase, while the Src inhibitor PP1 did not. In contrast, a gentle decrease of endoplasmic reticulum calcium concentration was found after BMP2 stimulation, which could be blocked by both streptomycin and PP1. Further experiments revealed that BMP2-induced FAK activation could not be inhibited by PP1, ALP siRNA or the calcium channel inhibitor nifedipine. PF228, but not PP1 or calcium channel inhibitors, suppressed ALP elevation resulting from BMP2 stimulation. Therefore, our results suggest that BMP2 can increase [Ca²⁺ ]_i through extracellular calcium influx regulated by FAK and ALP and can deplete ER calcium through Src signaling simultaneously. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1725-1733, 2016.

Leukotriene inhibition in hamster periodontitis. A histochemical and morphometric study

The effects of leukotriene (LT) inhibition on gingival and adjacent bone compartments were assessed by using phenidone (100 mg/kg/d) and ketoconazole (50 mg/kg/d) given for 4 weeks to periodontitis-affected hamsters. In the gingiva the two agents significantly decreased PMNL recruitment and migration and increased the vascular lumen. At the bone level, they reduced significantly preosteoclast and osteoclast numbers but did not affect osteoclast activity. Phenidone had no action on periodontitis induced inhibition of bone formation; in contrast ketoconazole enhanced formation. As both phenidone and ketoconazole are unspecific LT inhibitors it cannot be ascertained that the effects observed were actually due to LT inhibition. However, phenidone and ketoconazole induced changes different from indomethacin used in previous studies to inhibit the cyclooxygenase pathway. These discrepancies suggest that LT inhibition occurred in the present study and that they participate in gingival inflammation and osteoclastic destruction during hamster periodontitis.

Establishment of an osteoid preosteocyte-like cell MLO-A5 that spontaneously mineralizes in culture

The mechanisms controlling the initiation of mineralization of bone matrix are not clear. To examine this process, we established a cell line called MLO-A5 that mineralizes in sheets, not nodules, within 3 days of culture in the presence of beta-glycerophosphate (beta-GP) and ascorbic acid and within 7 days in the absence of beta-GP and ascorbic acid. The mineral formed in both cases was shown to be bonelike apatite by Fourier transformed infrared (FTIR) spectroscopy. Mineral-to-matrix ratios (min/matrix) calculated from the FTIR data, which are related directly to ash weight, were approximately 0.4 in the absence of beta-GP and ascorbic acid and approximately 1.2 in the presence of beta-GP and ascorbic acid. By comparison, these ratios in fetal rat calvarial cells without beta-GP equal 0 and with beta-GP 1.9. This cell line and three others (MLO-A2, -D1, and -D6) were isolated from the long bones of transgenic mice expressing the large T-antigen driven by the osteocalcin promoter, the same mice from which the osteocyte-like cell line MLO-Y4 was isolated.(1) The cell lines were selected based on a dendritic or stellate morphology. MLO-A5 cells express high alkaline phosphatase, collagen type 1, parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor, bone sialoprotein (BSP), and osteocalcin (767 ng/10(6) cells compared with <1-2.2 ng/10(6) cell for primary mouse osteoblasts and five osteoblast cell lines). The single unique feature of the MLO-A5 cells compared with the other three nonmineralizing cell lines is the high expression of messenger RNA (mRNA) for BSP. These cell lines may represent stages of osteocyte differentiation and the MLO-A5 cells represent the postosteoblast, preosteocyte responsible for triggering mineralization of osteoid.

5-Lipoxygenase metabolites inhibit bone formation in vitro

The leukotrienes and peptido-leukotrienes are 5-lipoxygenase (5-LO) metabolites of arachidonic acid that appear to have unique effects on bone, distinct from those of the prostaglandins. Application of exogenous leukotrienes in vitro and in vivo results in increased osteoclast formation and bone resorption. While 5-LO metabolites of arachidonic acid clearly stimulate osteoclastic bone resorption, little is known concerning their effects on osteoblastic bone formation. We examined the effects of the 5-LO metabolites 5-HETE, the leukotriene LTB4 and, as representative of the peptido-leukotrienes, LTD4 on the formation of mineralized nodules of fetal rat calvarial cells in the presence of dexamethasone and recombinant human bone morphogenetic protein-2 (rhBMP-2). We also examined the effects of these 5-LO metabolites on alkaline phosphatase activity and cell proliferation in these cultures and the effects of 5-HETE and LTB4 on cultured explants of neonatal murine calvariae. We found that the bone-forming capacity of osteoblasts was impaired when cells were cultured in the presence of 5-LO metabolites. These data indicate that metabolites of the 5-LO pathway are negative regulators of bone formation. The continued presence of these metabolites in the bone environment might account, in part, for the bone loss associated with chronic inflammatory conditions.

Signal pathways that transduce growth factor-stimulated mitogenesis in bone cells

This investigation examined which signal pathways are of relevance in growth factor-stimulated bone cell mitogenesis. Platelet-derived growth factor (PDGF) and insulin-like growth factor-II (IGF-II) were potent mitogens for both the MG-63 osteoblast cell line and for primary cultures of human osteoblasts (HObs). The mitogenic action of both IGF-II and PDGF was attenuated by pertussis toxin (Ptx), by indomethacin, and by the lipoxygenase inhibitors BW755C74 and BW4AC. A combination of Ptx and indomethacin caused much greater inhibition but failed to abolish mitogenesis completely. PDGF significantly elevated inositol phosphates levels in both cell types; IGF-II had no effect on this pathway. In MG-63 cells, we demonstrated tyrosine phosphorylation of high-molecular-weight substrates elicited by both PDGF and IGF-II. Genistein inhibited the phosphorylation and mitogenic response to PDGF, but had no effect on IGF-II-induced tyrosine phosphorylation or mitogenesis. Another inhibitor of tyrosine kinases, methyl 2,5-dihydroxycinnamate, (MDHC), inhibited PDGF-stimulated mitogenesis effectively in both cell types but only blocked IGF-II-induced mitogenesis in MG-63 cells. The specificity of these inhibitors suggests that particular tyrosine kinases may regulate growth factor-induced stimulation of bone cells.

Inositol trisphosphate receptor gene expression and hormonal regulation in osteoblast-like cell lines and primary osteoblastic cell cultures

The inositol trisphosphate receptor (IP3R) is an intracellular calcium channel that mediates the cellular actions of a wide variety of hormones, growth factors, and cytokines. In osteoblastic cell cultures, many bone resorbing hormones increase phosphoinositide turnover, inositol trisphosphate production, mobilization of intracellular calcium, and the secretion of osteoclast recruitment and activating factors. In this study, the effects of 17 beta-estradiol, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), phrobol ester, and serum on IP3R mRNA levels were evaluated in osteogenic-osteosarcoma cells and in primary osteoblastic cultures derived from neonatal rat calvaria. Type-specific reverse transcription polymerase chain reaction (RT-PCR) indicated that all cell types evaluated (G-292, U-2 OS, Saos-2, MC3T3-E1, UMR-106, and calvarial osteoblastic cells) express IP3R mRNA type I; G-292, U-2 OS, MC3T3-E1, and calvarial osteoblastic cells also express type II IP3R mRNA; and UMR-106 and the calvarial osteoblastic cells express type III IP3R mRNA. Northern blot and RT-PCR analyses of human G-292 osteosarcoma cells and rat calvarial osteoblastic cells showed that phorbol ester and serum increase IP3R mRNA levels, whereas 17 beta-estradiol and 1,25(OH)2D3 decrease these levels. In G-292 cells, the effect of 17 beta-estradiol was not due to accelerated IP3R mRNA degradation and required continued protein synthesis. The results show that multiple IP3R types are expressed in osteoblasts and osteoblastic osteosarcoma cells and that this expression is regulated by 17 beta-estradiol and other osteoporotic and antiosteoporotic hormones. These findings indicate that hormonal control of IP3R expression may be relevant in the chronic regulation of osteoblast secretory activity.

Leukotriene B4 stimulates osteoclastic bone resorption both in vitro and in vivo

Upon activation, the enzyme 5-lipoxygenase converts arachidonic acid into principally three products, the peptidoleukotrienes, 5-hydroperoxyeicosatetraenoic acid (5-HETE) or the leukotriene B4. We have shown that the peptido-leukotrienes (known as LTC4, LTD4, or LTE4) and 5-HETE induce osteoclastic bone resorption and that receptors for LTD4 are present on isolated avian osteoclast-like cells. Here, we show the effects of the third metabolic product of the 5-lipoxygenase (5-LO) pathway of arachidonic acid metabolism, the leukotriene LTB4, on osteoclastic bone resorption both in vivo and in vitro. Because LTB4 production is increased in a number of inflammatory conditions, it may be an important contributor to the bone loss which occurs in these disorders. LTB4 increased osteoclastic bone resorption in vivo following local administration over the calvariae of normal mice and in vitro in organ cultures of neonatal mouse calvariae. When LTB4 was injected over the calvaria of mice, there was a significant increase in bone resorption, osteoclast numbers, and eroded surfaces. LTB4 also increased the formation of resorption lacunae by isolated neonatal rat osteoclasts. Greater potency was observed with LTB4 compared with the peptido-leukotriene LTD4. This is in contrast to prostaglandins of the E series, which are reported to inhibit isolated osteoclasts. Experiments using marrow cultures suggest that LTB4 stimulates bone resorption in part by enhancing the formation of osteoclasts.

Dietary (n-3) and (n-6) polyunsaturates and acetylsalicylic acid alter ex vivo PGE2 biosynthesis, tissue IGF-I levels, and bone morphometry in chicks

This study examined the effects of dietary (n-6) and (n-3) polyunsaturated fatty acids (PUFA) and acetylsalicylic acid (ASA) on bone ash content, morphometry, fatty acid composition, ex vivo PGE2 biosynthesis, tissue IGF-I concentration, and serum alkaline phosphatase (ALPase) activity in chicks. Newly hatched chicks were fed a semipurified diet containing soybean oil (S) or menhaden oil / safflower oil (M) at 90 g/kg. At 4 days of age, chicks were divided into four equal treatment groups receiving 0 mg [symbol: see text] or 500 mg [symbol: see text] of ASA/kg of diet: S[symbol: see text]ASA, M[symbol: see text]ASA, S[symbol: see text]ASA, and M[symbol: see text]ASA. Lipid and ASA treatments did not affect bone length, bone ash, or bone mineral content in chicks. Chicks fed M had increased fractional labeled trabecular surface and tissue level bone formation rates, independent of ASA treatment, compared with those given S. A significant fat x ASA interaction effect was found for trabecular bone volume, thickness, separation, and number. Chicks fed S had higher 20:4(n-6) but lower 20:5(n-3) concentrations in liver and bone compared with those given M. Ex vivo PGE2 biosynthesis was higher in liver homogenates and bone organ cultures of chicks fed S compared with the values for those given M at 17 days. ASA treatment decreased ex vivo PGE2 production in liver homogenates and bone organ cultures of chicks, independent of the dietary lipids. Chicks fed ASA had a lower concentration of IGF-I in tibiotarsal bone compared with those not given ASA at 19 days. Serum ALPase activity was higher in chicks given M compared with those fed S, but the values were reversed with ASA feeding. This study demonstrated that both dietary fat and ASA modulated bone PGE2 biosynthesis, and that (n-3) PUFA and fat x ASA interactions altered bone morphometry.

The 5-lipoxygenase inhibitor BWA4C impairs osteoclastic resorption in a synchronized model of bone remodeling

The role of leukotrienes on bone resorption was tested in a well-standardized model of bone remodeling by inhibiting their biosynthesis with BWA4C, a specific inhibitor of 5-lipoxygenase. After extraction of their upper molars unilaterally, 30 Wistar rats were divided into three groups; the first remained untreated (control group), the second received 80 mg/kg/day of BWA4C dissolved in polyethylene glycol 300 (experimental group), and the third received only the vehicle (sham-treated group). After four days of experiment, the animals were killed and the resorption profile was assessed along the antagonist mandibular buccal cortex. The main result was a dramatic decrease in the number of TRAP-positive mononucleated preosteoclasts in the experimental group (-69%, p < 0.0005 and p < 0.003 vs. the control and sham-treated groups, respectively). This drop was related to a significant decrease in the number of osteoclasts. Neither the activation of the differentiated osteoclasts nor their mean interface with the bone surface were affected by BWA4C. Concomitantly, the mast cell population residing near the vascular network limiting the periosteum was markedly and significantly increased by the treatment. These mast cells were mostly degranulating, i.e., were in a state of activation that we previously found to related to resorption. These data suggest (1) that the leukotrienes are involved in the recruitment of osteoclast progenitors, and/or their differentiation into preosteoclasts, and (2) that mast cells responded to leukotriene inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Pertussis toxin-sensitive activation of phospholipase A2 can be resolved from phosphoinositidase C in primary cultures of mouse osteoblasts using indomethacin

Recent work has established that various bone-resorbing hormones are able to activate phosphoinositide metabolism as well as eicosanoid production in osteoblast-like cells, although the relationship between these pathways is unclear. We used pertussis toxin and indomethacin to inhibit the stimulation of [3H]arachidonic acid release and [3H]phosphoinositide turnover caused by treating primary cultures of mouse osteoblasts with fetal calf serum. We found (1) that pertussis toxin and indomethacin each inhibited both pathways and (2) that although pertussis toxin inhibited [3H]arachidonic acid release to a greater extent than indomethacin, [3H]inositol phosphate accumulation was inhibited rather more effectively by indomethacin. These data suggest that whereas ligands in fetal calf serum activate [3H]arachidonic acid release largely directly via the action of a pertussis-sensitive G protein, activation of phosphoinositidase C is indirect, being substantially dependent upon eicosanoid production. These experiments suggest that serial activation of phospholipase A2 and phosphoinositidase C may occur in osteoblasts and that only the former enzyme is regulated by a pertussis toxin-sensitive G protein.

Stimulation of bone collagen and non-collagenous protein synthesis by products of 5- and 12-lipoxygenase: determination by use of a simple quantitative assay

The influence of 5- and 12-lipoxygenase products on the rate of collagen and non-collagenous protein (NCP) synthesis by murine calvarial explants has been investigated using a new assay based on the resistance of native collagen to degradation by pepsin. The reproducibility and simplicity of this assay allows the quantitative estimation of the rate of bone formation in large numbers of cultures. Hydroxyeicosatetraenoic acids (HETEs) stimulated both the rate of collagen and NCP synthesis with maximal stimulation occurring at 10-100 pM. All leukotrienes stimulated collagen synthesis. LTB4, C4 and D4 showed similar dose-responses with maximal activity occurring at 100 pM. LTE4 was less potent only showing activity at 1-10 nM. Only LTD4 demonstrated the capacity to stimulate NCP synthesis with significant stimulation being seen at 10 nM. The extreme sensitivity of bone collagen and NCP synthesis to lipoxygenase products suggests that these mediators may play a physiological role in bone remodelling.