Aerobic glycolysis in bone: lactate production and gradients in calvaria

Metabolic reprogramming in skeletal cell differentiation

The human skeleton is a multifunctional organ made up of multiple cell types working in concert to maintain bone and mineral homeostasis and to perform critical mechanical and endocrine functions. From the beginning steps of chondrogenesis that prefigures most of the skeleton, to the rapid bone accrual during skeletal growth, followed by bone remodeling of the mature skeleton, cell differentiation is integral to skeletal health. While growth factors and nuclear proteins that influence skeletal cell differentiation have been extensively studied, the role of cellular metabolism is just beginning to be uncovered. Besides energy production, metabolic pathways have been shown to exert epigenetic regulation via key metabolites to influence cell fate in both cancerous and normal tissues. In this review, we will assess the role of growth factors and transcription factors in reprogramming cellular metabolism to meet the energetic and biosynthetic needs of chondrocytes, osteoblasts, or osteoclasts. We will also summarize the emerging evidence linking metabolic changes to epigenetic modifications during skeletal cell differentiation.

Lipolysis supports bone formation by providing osteoblasts with endogenous fatty acid substrates to maintain bioenergetic status

Bone formation is a highly energy-demanding process that can be impacted by metabolic disorders. Glucose has been considered the principal substrate for osteoblasts, although fatty acids are also important for osteoblast function. Here, we report that osteoblasts can derive energy from endogenous fatty acids stored in lipid droplets via lipolysis and that this process is critical for bone formation. As such, we demonstrate that osteoblasts accumulate lipid droplets that are highly dynamic and provide the molecular mechanism by which they serve as a fuel source for energy generation during osteoblast maturation. Inhibiting cytoplasmic lipolysis leads to both an increase in lipid droplet size in osteoblasts and an impairment in osteoblast function. The fatty acids released by lipolysis from these lipid droplets become critical for cellular energy production as cellular energetics shifts towards oxidative phosphorylation during nutrient-depleted conditions. In vivo, conditional deletion of the ATGL-encoding gene Pnpla2 in osteoblast progenitor cells reduces cortical and trabecular bone parameters and alters skeletal lipid metabolism. Collectively, our data demonstrate that osteoblasts store fatty acids in the form of lipid droplets, which are released via lipolysis to support cellular bioenergetic status when nutrients are limited. Perturbations in this process result in impairment of bone formation, specifically reducing ATP production and overall osteoblast function.

Mitochondrial β-oxidation of adipose-derived fatty acids by osteoblasts fuels parathyroid hormone-induced bone formation

The energetic costs of bone formation require osteoblasts to coordinate their activities with tissues, like adipose, that can supply energy-dense macronutrients. In the case of intermittent parathyroid hormone (PTH) treatment, a strategy used to reduce fracture risk, bone formation is preceded by a change in systemic lipid homeostasis. To investigate the requirement for fatty acid oxidation by osteoblasts during PTH-induced bone formation, we subjected mice with osteoblast-specific deficiency of mitochondrial long-chain β-oxidation as well as mice with adipocyte-specific deficiency for the PTH receptor or adipose triglyceride lipase to an anabolic treatment regimen. PTH increased the release of fatty acids from adipocytes and β-oxidation by osteoblasts, while the genetic mouse models were resistant to the hormone's anabolic effect. Collectively, these data suggest that PTH's anabolic actions require coordinated signaling between bone and adipose, wherein a lipolytic response liberates fatty acids that are oxidized by osteoblasts to fuel bone formation.

Glucose metabolism in skeletal cells

The mammalian skeleton is integral to whole body physiology with a multitude of functions beyond mechanical support and locomotion, including support of hematopoiesis, mineral homeostasis and potentially other endocrine roles. Formation of the skeleton begins in the embryo and mostly from a cartilage template that is ultimately replaced by bone through endochondrial ossification. Skeletal development and maturation continue after birth in most species and last into the second decade of postnatal life in humans. In the mature skeleton, articular cartilage lining the synovial joint surfaces is vital for bodily movement and damages to the cartilage are a hallmark of osteoarthritis. The mature bone tissue undergoes continuous remodeling initiated with bone resorption by osteoclasts and completed with bone formation from osteoblasts. In a healthy state, the exquisite balance between bone resorption and formation is responsible for maintaining a stable bone mass and structural integrity, while meeting the physiological needs for minerals via controlled release from bone. Disruption of the balance in favor of bone resorption is the root cause for osteoporosis. Whereas osteoclasts pump molar quantities of hydrochloric acid to dissolve the bone minerals in a process requiring ATP hydrolysis, osteoblasts build bone mass by synthesizing and secreting copious amounts of bone matrix proteins. Thus, both osteoclasts and osteoblasts engage in energy-intensive activities to fulfill their physiological functions, but the bioenergetics of those and other skeletal cell types are not well understood. Nonetheless, the past ten years have witnessed a resurgence of interest in studies of skeletal cell metabolism, resulting in an unprecedented understanding of energy substrate utilization and its role in cell fate and activity regulation. The present review attempts to synthesize the current findings of glucose metabolism in chondrocytes, osteoblasts and osteoclasts. Advances with the other relevant cell types including skeletal stem cells and marrow adipocytes will not be discussed here as they have been extensively reviewed recently by others (van Gastel and Carmeliet, 2021). Elucidation of the bioenergetic mechanisms in the skeletal cells is likely to open new avenues for developing additional safe and effective bone therapies.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Molecular conformations and dynamics in the extracellular matrix of mammalian structural tissues: Solid-state NMR spectroscopy approaches

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Solid-state NMR spectroscopy probes molecular conformation and dynamics in intact ECM.

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Collagen conformational dynamics has roles in mechanical properties of fibrils and cell adhesion.

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Solid-state NMR spectroscopy has shed new light on  the chemical structure of bone mineral.

Solid-state NMR spectroscopy has played an important role in multidisciplinary studies of the extracellular matrix. Here we review how solid-state NMR has been used to probe collagen molecular conformations, dynamics, post-translational modifications and non-enzymatic chemical changes, and in calcified tissues, the molecular structure of bone mineral and its interface with collagen. We conclude that NMR spectroscopy can deliver vital information that in combination with data from structural imaging techniques, can result in significant new insight into how the extracellular matrix plays its multiple roles.

The investigation of energy metabolism in osteoblasts and osteoclasts

The maintenance of bone homeostasis is critical for bone health. It is vulnerable to cause bone loss, even severely osteoporosis when the balance between bone formation and absorption is interrupted. Growing evidence has shown that energy metabolism disorders, such as abnormal glucose metabolism, irregular amino acid metabolism, and aberrant lipid metabolism, can damage bone homeostasis, causing or exacerbating bone mass loss and osteoporosis-related fractures. Here, we summarize the studies of energy metabolism in osteoblasts and osteoclasts and provide a better appreciation of how energy metabolism, especially glucose metabolism maintains bone homeostasis. With this knowledge, new avenues will be unraveled to understand and cue bone-related diseases such as osteoporosis.

Dual Effects of Lipid Metabolism on Osteoblast Function

The skeleton is a dynamic and metabolically active organ with the capacity to influence whole body metabolism. This newly recognized function has propagated interest in the connection between bone health and metabolic dysfunction. Osteoblasts, the specialized mesenchymal cells responsible for the production of bone matrix and mineralization, rely on multiple fuel sources. The utilization of glucose by osteoblasts has long been a focus of research, however, lipids and their derivatives, are increasingly recognized as a vital energy source. Osteoblasts possess the necessary receptors and catabolic enzymes for internalization and utilization of circulating lipids. Disruption of these processes can impair osteoblast function, resulting in skeletal deficits while simultaneously altering whole body lipid homeostasis. This article provides an overview of the metabolism of postprandial and stored lipids and the osteoblast's ability to acquire and utilize these molecules. We focus on the requirement for fatty acid oxidation and the pathways regulating this function as well as the negative impact of dyslipidemia on the osteoblast and skeletal health. These findings provide key insights into the nuances of lipid metabolism in influencing skeletal homeostasis which are critical to appreciate the extent of the osteoblast's role in metabolic homeostasis.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Solid state NMR - An indispensable tool in organic-inorganic biocomposite characterization; refining the structure of octacalcium phosphate composites with the linear metabolic di-acids succinate and adipate

Octacalcium phosphate (OCP; Ca8(HPO4)2(PO4)4. 5H2O) is a plausible precursor phase of biological hydroxyapatite, which composites with a number of biologically relevant organic metabolites. Widely used material science physicochemical structure determination techniques successfully characterize the mineral component of these composites but leave details of the structure, and interactions with mineral, of the organic component almost completely obscure. The metabolic linear di-acids succinate (SUC) and adipate (ADI) differentially expand the hydrated (100) layer of OCP. 13C13C correlation (proton driven spin diffusion, PDSD) experiments on OCP composited with (U-13C4)-SUC, and (U13C6)-ADI, show that the two di-acids per unit cell adopt non-centrosymmetric but mutually identical structures. 13C{31P}, rotational echo double resonance (REDOR) shows that one end of each linear di-acid is displaced further from the surface of the apatitic OCP layer relative to the other end. Overall the results indicate two di-acids per unit cell disposed perpendicularly across the OCP hydrated layer with one carboxylate of each di-acid substituting a hydrated surface OCP phosphate group. This study re-affirms the unique advantages of ssNMR in elucidating structural details of organic-inorganic biocomposites, and thereby mechanisms underlying the roles of small metabolites in influencing biomineralization mechanisms and outcomes.

Fatty acid metabolism by the osteoblast

The emergence of bone as an endocrine organ able to influence whole body metabolism, together with comorbid epidemics of obesity, diabetes, and osteoporosis, have prompted a renewed interest in the intermediary metabolism of the osteoblast. To date, most studies have focused on the utilization of glucose by this specialized cell, but the oxidation of fatty acids results in a larger energy yield. Osteoblasts express the requisite receptors and catabolic enzymes to take up and then metabolize fatty acids, which appears to be required during later stages of differentiation when the osteoblast is dedicated to matrix production and mineralization. In this article, we provide an overview of fatty acid β-oxidation and highlight studies demonstrating that the skeleton plays a significant role in the clearance of circulating lipoproteins and non-esterified fatty acids. Additionally, we review the requirement for long-chain fatty acid metabolism during post-natal bone development and the effects of anabolic stimuli on fatty acid utilization by osteoblasts. These recent findings may help to explain the skeletal manifestations of human diseases associated with impaired lipid metabolism while also providing additional insights into the metabolic requirements of skeletal homeostasis.

Glucose metabolism in bone

The adult human skeleton is a multifunctional organ undergoing constant remodeling through the opposing activities of the bone-resorbing osteoclast and the bone-forming osteoblast. The exquisite balance between bone resorption and bone formation is responsible for bone homeostasis in healthy adults. However, evidence has emerged that such a balance is likely disrupted in diabetes where systemic glucose metabolism is dysregulated, resulting in increased bone frailty and osteoporotic fractures. These findings therefore underscore the significance of understanding the role and regulation of glucose metabolism in bone under both normal and pathological conditions. Recent studies have shed new light on the metabolic plasticity and the critical functions of glucose metabolism during osteoclast and osteoblast differentiation. Moreover, these studies have begun to identify intersections between glucose metabolism and the growth factors and transcription factors previously known to regulate osteoblasts and osteoclasts. Here we summarize the current knowledge in the nascent field, and suggest that a fundamental understanding of glucose metabolic pathways in the critical bone cell types may open new avenues for developing novel bone therapeutics.

Regulation of Osteoblast Metabolism by Wnt Signaling

Wnt/β-catenin signaling plays a critical role in the achievement of peak bone mass, affecting the commitment of mesenchymal progenitors to the osteoblast lineage and the anabolic capacity of osteoblasts depositing bone matrix. Recent studies suggest that this evolutionarily-conserved, developmental pathway exerts its anabolic effects in part by coordinating osteoblast activity with intermediary metabolism. These findings are compatible with the cloning of the gene encoding the low-density lipoprotein related receptor-5 (LRP5) Wnt co-receptor from a diabetes-susceptibility locus and the now well-established linkage between Wnt signaling and metabolism. In this article, we provide an overview of the role of Wnt signaling in whole-body metabolism and review the literature regarding the impact of Wnt signaling on the osteoblast's utilization of three different energy sources: fatty acids, glucose, and glutamine. Special attention is devoted to the net effect of nutrient utilization and the mode of regulation by Wnt signaling. Mechanistic studies indicate that the utilization of each substrate is governed by a unique mechanism of control with β-catenin-dependent signaling regulating fatty acid β-oxidation, while glucose and glutamine utilization are β-catenin-independent and downstream of mammalian target of rapamycin complex 2 (mTORC2) and mammalian target of rapamycin complex 1 (mTORC1) activation, respectively. The emergence of these data has provided a new context for the mechanisms by which Wnt signaling influences bone development.

Osteoblast-like MC3T3-E1 Cells Prefer Glycolysis for ATP Production but Adipocyte-like 3T3-L1 Cells Prefer Oxidative Phosphorylation

Mesenchymal stromal cells (MSCs) are early progenitors that can differentiate into osteoblasts, chondrocytes, and adipocytes. We hypothesized that osteoblasts and adipocytes utilize distinct bioenergetic pathways during MSC differentiation. To test this hypothesis, we compared the bioenergetic profiles of preosteoblast MC3T3-E1 cells and calvarial osteoblasts with preadipocyte 3T3L1 cells, before and after differentiation. Differentiated MC3T3-E1 osteoblasts met adenosine triphosphate (ATP) demand mainly by glycolysis with minimal reserve glycolytic capacity, whereas nondifferentiated cells generated ATP through oxidative phosphorylation. A marked Crabtree effect (acute suppression of respiration by addition of glucose, observed in both MC3T3-E1 and calvarial osteoblasts) and smaller mitochondrial membrane potential in the differentiated osteoblasts, particularly those incubated at high glucose concentrations, indicated a suppression of oxidative phosphorylation compared with nondifferentiated osteoblasts. In contrast, both nondifferentiated and differentiated 3T3-L1 adipocytes met ATP demand primarily by oxidative phosphorylation despite a large unused reserve glycolytic capacity. In sum, we show that nondifferentiated precursor cells prefer to use oxidative phosphorylation to generate ATP; when they differentiate to osteoblasts, they gain a strong preference for glycolytic ATP generation, but when they differentiate to adipocytes, they retain the strong preference for oxidative phosphorylation. Unique metabolic programming in mesenchymal progenitor cells may influence cell fate and ultimately determine the degree of bone formation and/or the development of marrow adiposity. © 2018 American Society for Bone and Mineral Research.

Energy Metabolism of Bone

Biological processes utilize energy and therefore must be prioritized based on fuel availability. Bone is no exception to this, and the benefit of remodeling when necessary outweighs the energy costs. Bone remodeling is important for maintaining blood calcium homeostasis, repairing micro cracks and fractures, and modifying bone structure so that it is better suited to withstand loading demands. Osteoclasts, osteoblasts, and osteocytes are the primary cells responsible for bone remodeling, although bone marrow adipocytes and other cells may also play an indirect role. There is a renewed interest in bone cell energetics because of the potential for these processes to be targeted for osteoporosis therapies. In contrast, due to the intimate link between bone and energy homeostasis, pharmaceuticals that treat metabolic disease or have metabolic side effects often have deleterious bone consequences. In this brief review, we will introduce osteoporosis, discuss how bone cells utilize energy to function, evidence for bone regulating whole body energy homeostasis, and some of the unanswered questions and opportunities for further research in the field.

Fatty acid oxidation by the osteoblast is required for normal bone acquisition in a sex- and diet-dependent manner

Postnatal bone formation is influenced by nutritional status and compromised by disturbances in metabolism. The oxidation of dietary lipids represents a critical source of ATP for many cells but has been poorly studied in the skeleton, where the prevailing view is that glucose is the primary energy source. Here, we examined fatty acid uptake by bone and probed the requirement for fatty acid catabolism during bone formation by specifically disrupting the expression of carnitine palmitoyltransferase 2 (Cpt2), an obligate enzyme in fatty acid oxidation, in osteoblasts and osteocytes. Radiotracer studies demonstrated that the skeleton accumulates a significant fraction of postprandial fatty acids, which was equal to or in excess of that acquired by skeletal muscle or adipose tissue. Female, but not male, Cpt2 mutant mice exhibited significant impairments in postnatal bone acquisition, potentially due to an inability of osteoblasts to modify fuel selection. Intriguingly, suppression of fatty acid utilization by osteoblasts and osteocytes also resulted in the development of dyslipidemia and diet-dependent modifications in body composition. Taken together, these studies demonstrate a requirement for fatty acid oxidation during bone accrual and suggest a role for the skeleton in lipid homeostasis.

Energy Metabolism of the Osteoblast: Implications for Osteoporosis

Osteoblasts, the bone-forming cells of the remodeling unit, are essential for growth and maintenance of the skeleton. Clinical disorders of substrate availability (e.g., diabetes mellitus, anorexia nervosa, and aging) cause osteoblast dysfunction, ultimately leading to skeletal fragility and osteoporotic fractures. Conversely, anabolic treatments for osteoporosis enhance the work of the osteoblast by altering osteoblast metabolism. Emerging evidence supports glycolysis as the major metabolic pathway to meet ATP demand during osteoblast differentiation. Glut1 and Glut3 are the principal transporters of glucose in osteoblasts, although Glut4 has also been implicated. Wnt signaling induces osteoblast differentiation and activates glycolysis through mammalian target of rapamycin, whereas parathyroid hormone stimulates glycolysis through induction of insulin-like growth factor-I. Glutamine is an alternate fuel source for osteogenesis via the tricarboxylic acid cycle, and fatty acids can be metabolized to generate ATP via oxidative phosphorylation although temporal specificity has not been established. More studies with new model systems are needed to fully understand how the osteoblast utilizes fuel substrates in health and disease and how that impacts metabolic bone diseases.

Wnt signaling and cellular metabolism in osteoblasts

The adult human skeleton is a multifunctional organ undergoing continuous remodeling. Homeostasis of bone mass in a healthy adult requires an exquisite balance between bone resorption by osteoclasts and bone formation by osteoblasts; disturbance of such balance is the root cause for various bone disorders including osteoporosis. To develop effective and safe therapeutics to modulate bone formation, it is essential to elucidate the molecular mechanisms governing osteoblast differentiation and activity. Due to their specialized function in collagen synthesis and secretion, osteoblasts are expected to consume large amounts of nutrients. However, studies of bioenergetics and building blocks in osteoblasts have been lagging behind those of growth factors and transcription factors. Genetic studies in both humans and mice over the past 15 years have established Wnt signaling as a critical mechanism for stimulating osteoblast differentiation and activity. Importantly, recent studies have uncovered that Wnt signaling directly reprograms cellular metabolism by stimulating aerobic glycolysis, glutamine catabolism as well as fatty acid oxidation in osteoblast-lineage cells. Such findings therefore reveal an important regulatory axis between bone anabolic signals and cellular bioenergetics. A comprehensive understanding of osteoblast metabolism and its regulation is likely to reveal molecular targets for novel bone therapies.

PTH Promotes Bone Anabolism by Stimulating Aerobic Glycolysis via IGF Signaling

Teriparatide, a recombinant peptide corresponding to amino acids 1-34 of human parathyroid hormone (PTH), has been an effective bone anabolic drug for over a decade. However, the mechanism whereby PTH stimulates bone formation remains incompletely understood. Here we report that in cultures of osteoblast-lineage cells, PTH stimulates glucose consumption and lactate production in the presence of oxygen, a hallmark of aerobic glycolysis, also known as Warburg effect. Experiments with radioactively labeled glucose demonstrate that PTH suppresses glucose entry into the tricarboxylic acid cycle (TCA cycle). Mechanistically, the increase in aerobic glycolysis is secondary to insulin-like growth factor (Igf) signaling induced by PTH, whereas the metabolic effect of Igf is dependent on activation of mammalian target of rapamycin complex 2 (mTORC2). Importantly, pharmacological perturbation of glycolysis suppresses the bone anabolic effect of intermittent PTH in the mouse. Thus, stimulation of aerobic glycolysis via Igf signaling contributes to bone anabolism in response to PTH.

Aerobic glycolysis in osteoblasts

Osteoblasts, the chief bone-making cells in the body, are a focus of osteoporosis research. Although teriparatide, a synthetic fragment of the human parathyroid hormone (PTH), has been an effective bone anabolic drug, there remains a clinical need for additional therapeutics that safely stimulates osteoblast number and function. Work in the past several decades has provided unprecedented clarity about the roles of growth factors and transcription factors in regulating osteoblast differentiation and activity, but whether these factors may regulate cellular metabolism to influence cell fate and function has been largely unexplored. The past few years have witnessed a resurgence of interest in the cellular metabolism of osteoblasts, with the hope that elucidation of their metabolic profile may open new avenues for developing bone anabolic agents. Here we review the current understanding about glucose metabolism in osteoblasts.

Bioenergetics during calvarial osteoblast differentiation reflect strain differences in bone mass

Osteoblastogenesis is the process by which mesenchymal stem cells differentiate into osteoblasts that synthesize collagen and mineralize matrix. The pace and magnitude of this process are determined by multiple genetic and environmental factors. Two inbred strains of mice, C3H/HeJ and C57BL/6J, exhibit differences in peak bone mass and bone formation. Although all the heritable factors that differ between these strains have not been elucidated, a recent F1 hybrid expression panel (C3H × B6) revealed major genotypic differences in osteoblastic genes related to cellular respiration and oxidative phosphorylation. Thus, we hypothesized that the metabolic rate of energy utilization by osteoblasts differed by strain and would ultimately contribute to differences in bone formation. In order to study the bioenergetic profile of osteoblasts, we measured oxygen consumption rates (OCR) and extracellular acidification rates (ECAR) first in a preosteoblastic cell line MC3T3-E1C4 and subsequently in primary calvarial osteoblasts from C3H and B6 mice at days 7, 14, and 21 of differentiation. During osteoblast differentiation in media containing ascorbic acid and β-glycerophosphate, all 3 cell types increased their oxygen consumption and extracellular acidification rates compared with the same cells grown in regular media. These increases are sustained throughout differentiation. Importantly, C3H calvarial osteoblasts had greater oxygen consumption rates than B6 consistent with their in vivo phenotype of higher bone formation. Interestingly, osteoblasts utilized both oxidative phosphorylation and glycolysis during the differentiation process although mature osteoblasts were more dependent on glycolysis at the 21-day time point than oxidative phosphorylation. Thus, determinants of oxygen consumption reflect strain differences in bone mass and provide the first evidence that during collagen synthesis osteoblasts use both glycolysis and oxidative phosphorylation to synthesize and mineralize matrix.

Insulin-like growth factor I rapidly enhances acid efflux from osteoblastic cells

Insulin-like growth factor I (IGF-I) is thought to stimulate bone resorption indirectly through a primary effect on osteoblasts, which in turn activate osteoclasts by as-yet-unidentified mechanisms. Small decreases in extracellular pH (pHo) dramatically increase the resorptive activity of osteoclasts. Our purpose was to characterize the effect of IGF-I on acid production by osteoblastic cells. When confluent, UMR-106 osteoblast-like cells and rat calvarial cells acidified the compartment beneath them. Superfusion with IGF-I caused a further decrease in pHo. To investigate the mechanism, we monitored acid efflux from subconfluent cultures. IGF-I rapidly increased net efflux of H+ equivalents in a concentration-dependent manner. IGF-II (10 nM) evoked a smaller response than IGF-I (10 nM). The response to IGF-I was partially dependent on extracellular Na+, but not glucose, and exhibited little if any desensitization. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, abolished the response to IGF-I but not to parathyroid hormone. Thus IGF-I enhances acid efflux from osteoblastic cells, via a signaling pathway dependent on activation of phosphatidylinositol 3-kinase. In vivo, acidification of the compartment between the osteogenic cell layer and the bone matrix may affect diverse processes, including mineralization and osteoclastic bone resorption.

Physiological Role of Brain Angiotensin III in Drinking Behavior in the Rat

We examined the physiologic role of endogenous brain angiotensin III (AIII), an active degradative product of angiotensin II, in drinking behavior. Adult, male spontaneously hypertensive (SH) and Wistar-Kyoto normotensive (WKY) rats that were instrumented with an intracerebroventricular (i.c.v.) cannula connected to an osmotic minipump for chronic infusion were used. 7-day i.c.v. infusion of the specific AIII antagonist, Ile(7)-AIII (10 or 100 pmol/min), resulted in no significant alteration in daily (24 h), diurnal (8:00 a.m.-8.00 p.m.) or nocturnal (8:00 p.m.-8:00 a.m.) basal water intake in both SH and WKY rats. Similar results were obtained with i.c.v. infusion of the aminopeptidase inhibitor, bestatin (150 or 300 pmol/min), given alone or simultaneously with Ile(7)-AIII (10 pmol/min). Rats that were water-deprived for the first 3 days of 7-day infusion of Ile(7)-AIII consumed significantly less water during the first 2 h after water became available. Furthermore, the accumulated water intake during the first 24 h was appreciably greater in SH than WKY rats. We interpret these results to suggest that the endogenous brain AIII may not be tonically involved in fluid homeostasis. Instead, it must be activated under conditions of dehydration, such as water deprivation, particularly in the SHRs, to initiate drinking behavior. Copyright 1996 S. Karger AG, Basel

Medium pH modulates matrix, mineral, and energy metabolism in cultured chick bones and osteoblast-like cells

The effects of medium pH were tested on calvariae, tibiae, and osteoblast-like cells from chick embryos. Bones and isolated cells were incubated for 5 h or 2 days in Hepes-buffered medium at pH values ranging from 6.8 to 8.2. Osteoblast function was evaluated by lactate production, oxygen consumption, alkaline phosphatase activity (AlPase), Ca and inorganic phosphate (Pi) flux, proline hydroxylation, DNA content, and thymidine incorporation. As medium pH was increased, glycolysis, collagen synthesis, and AlPase increased, while Ca efflux decreased. No effect of pH was seen on mitochondrial activity, Pi efflux, or cell number or proliferation. The importance of glycolysis as an endogenous pH regulator was demonstrated by inhibition with iodoacetic acid or glucose restriction and by adding lactate to the medium. The results suggest that the pH of bone interstitial fluid may be regulated by glycolysis and that changes in pH of this compartment may have marked effects on osteoblast function.

A distributed model of exchange processes within the osteon

In order to understand various exchange processes within the osteon, a mathematical model to describe the system has been developed which allows for concentration gradients in the axial and radial directions as well as cellular consumption and binding to bone surface. The normal values for the model parameter are discussed and the effects of the model parameters on the behaviour of the model are investigated. This model supports the idea that diffusion alone may be an inefficient mechanism in transport between blood and osteocytes.

[Regulation of thirst in end-stage kidney insufficiency]

About 30% of hemodialyzed patients are suffering from chronic fluid overload despite advice to restrict the oral fluid intake. To investigate the cause of the abnormal drinking behaviour a clinical study was performed in 51 non-diabetic patients with endstage renal disease exhibiting lower interdialysis weight gain (less than 3 kg, n = 17) and increased interdialysis weight gain (greater than 3 kg, n = 34). Blood pressure, body weight self-estimated thirst intensity before and after hemodialysis were analyzed. Biochemical and behavioral variables were measured including hormonal factors of water and sodium metabolism. Significant differences of dry weight, creatinine, urea nitrogen and thirst intensity were found between the two groups. Catecholamines, renin, angiotensin II, aldosterone, vasopressin and atrial natriuretic peptide exhibited a similar pattern in both groups. Atrial natriuretic peptide decreased during hemodialysis in both groups, angiotensin II, however, and norepinephrine showed an exaggerated response to ultrafiltration rate in polydipsic patients. These results suggest that changes in serum osmolality during hemodialysis did not contribute to thirst and drinking behaviour. It seems that postdialytic hypovolaemia together with higher plasma-angiotensin II-levels is responsible for increased oral intake of fluid and excessive weight gain.

Mechanism of amphotericin B stimulation of net calcium efflux from neonatal mouse calvariae

Amphotericin B is a polyene antifungal agent that binds to membrane sterols, creating aqueous pores that permit ion fluxes sufficient to cause cell lysis. It has also been shown to alter ion transport in mammalian cells, including proton secretion from renal tubular cells. The latter effect can lead to distal renal tubular acidosis in patients treated for systemic fungal infections. Based on the understanding that osteoclast-mediated bone resorption is dependent on proton secretion, we examined the effect of amphotericin B on calcium efflux from neonatal mouse calvariae in organ culture. Amphotericin B (5 micrograms/ml) stimulated net calcium efflux from calvariae within 24 h to a level almost as great as that produced by a maximally effective concentration of parathyroid hormone. The stimulated calcium efflux was completely inhibited by both 10 ng/ml salmon calcitonin, a physiologic inhibitor of osteoclast activity, and 4 x 10(-4) M acetazolamide, a specific inhibitor of carbonic anhydrase, the enzyme necessary for substantial proton generation by osteoclasts. These results indicate a direct effect of amphotericin B on bone in vitro to stimulate osteoclast-mediated calcium efflux.

Fatty acid oxidation in bone tissue and bone cells in culture. Characterization and hormonal influences

Fatty acid oxidation and its hormonal modulation were investigated in cultured rat calvaria and in cultivated cell populations. The latter were obtained from calvaria of newborn rats by sequential time-dependent digestion with collagenase, yielding eight cell populations: the early ones containing mainly fibroblasts, the middle ones being osteoblast-like, and late ones osteoblast-osteocyte-like. In calvaria, fatty acid oxidation was increased by adding 0.1 mM- and 1.0 mM-palmitate to the medium, containing 10% (v/v) fetal-calf serum. No effect was found after parathyrin addition in vitro or when injected in vivo. All cell populations obtained by sequential digestion were found to oxidize palmitate, whereby the osteoblast-like cells showed a lower oxidation rate than the other populations. Both parathyrin and calcitonin had no effect on fatty acid oxidation. 1,25-Dihydroxycholecalciferol at 1-100 nM and 24,25-dihydroxycholecalciferol at 100 nM increased oxidation primarily in the population enriched with osteoblast-like cells. Insulin at 1.6 microM diminished it in the cell populations enriched with osteoblast-like cells and in the late bone-cell fraction. However, glucagon had no effect. The energy provided by fatty acid oxidation in this system is approx. 40-80% of glucose metabolism, suggesting that this event may be of importance in the energy metabolism of bone.

Lactic acid production in mouse calvaria in vitro with and without parathyroid hormone stimulation: lack of acetazolamide effects

The effect of acetazolamide on lactic acid production in mouse calvaria explants was examined in an attempt to explain in vivo inhibition of Ca mobilization from bone by sulfonamide inhibitors of carbonic anhydrase. Lactic acid production was evaluated in 8-10-week-old CD-1 mouse calvaria over a time period consistent with acetazolamide inhibition of both PTH-stimulated and nonstimulated Ca mobilization from bone in vivo. Labeled lactate, derived from [3,4-14C]glucose, and total lactate production were determined at 2-h intervals for up to 8 h. Simultaneous assessment of 14CO2 production and [14C]pyruvate levels established that acetazolamide produced no other related metabolic effects and that the drug did not block PTH stimulation of CO2 production. Acetazolamide (4.5 X 10(-4) M) was found to have no effect on labeled or total lactate production in mouse calvaria for up to 8 h of treatment. In addition, acetazolamide did not block PTH (10(-7) M) stimulation of lactate production. However, Cl 13,850 (10(-4) M), a structural analog of acetazolamide devoid of inhibitory activity on carbonic anhydrase or Ca mobilization from bone, was shown to significantly reduce lactate production from mouse calvaria. These results, therefore, suggest that acetazolamide does not inhibit Ca mobilization from bone through inhibition of lactic acid production and fail to support a mechanistic relationship between lactic acid production and Ca mobilization from bone.

Blood: bone disequilibrium. V. Effects of a diphosphonate (EHDP) on phosphate fluxes in rat calvaria

Calvaria taken from rats (normal or thyroparathyroidectomized) given a diphosphonate (EHDP 10 mg/kg daily for 7 days) were placed in special Ussing chambers for the measurement of phosphate fluxes to and from bone. When compared with appropriate controls, the experimental calvaria showed a marked decrease in influx, K, a significant increase in the projected equilibrium concentration, E/K, where E is the efflux and a significant increase in the calcium concentration in the medium. It is concluded that this large increase in passive "solubility" is caused by the diphosphonate's ability to stabilize the small amount of regulator phase (brushite or brushite-apatite mixture) present in bone.

Blood:bone disequilibrium. VI. Studies of the solubility characteristics of brushite: apatite mixtures and their stabilization by noncollagenous proteins of bone

Recent evidence of the occurrence of brushite in newly formed bone mineral prompted a study of the solubility properties of brushite:apatite mixtures under physiological conditions and the influence on them of pH, lactate, pyruvate, and, particularly, noncollagenous bone proteins (NCBPs). Brushite alone was surprisingly stable in solution at pH 7.4, 37 degrees C. In the presence of increasing amounts of apatite, hydrolysis of brushite to an insoluble phase occurred. A decrease of 0.1 pH unit or the addition of 1.5 mM pyruvate or 10 mM lactate increased the ion activity product (Ca2+ x HPO4(2-) 3 or more times. However, within the loose envelope of bone such conditions so different from those in the circulation might be only local or temporary. NCBPs, on the other hand, stabilized brushite in solution alone as well as in the presence of apatite for days. They probably act by adsorbing strongly to the crystal surface and preventing nucleation by apatite. This brushite-apatite-bone protein system exhibits solubility characteristics that can resolve the old problems presented by the participation of the skeleton in extracellular calcium homeostasis on the one hand, and by the apparent insolubility of the apatite mineral of bone on the other.

Electrical potential difference across bone membrane

The control of ion fluxes to and from bone has important implications to mineralization and calcium homeostasis. Since ionic transport frequently results in an electrical potential difference across the layer of cells lining bone surfaces, knowledge of this potential is critical to understanding the means of regulation of ionic concentrations in the interior bone fluid phase. This work presents a determination of a metabolism-related electrical potential difference by a thermodynamic argument based on the distribution of charged and uncharged tracers between the bone extracellular fluid compartment and the bathing medium. For embryonic chick calvaria whose viability was assured by oxygen consumption measurements, an electrical potential of 4 mV was determined, positive with respect to the bone fluid compartment. This measurement is shown to be consistent with the active transport of K+ ion into the interior of bone; the potential developed by this transport process will then exclude from the interior phase a passively leaked Ca2+ ion.

Blood:bone disequilibrium. III. Linkage between cell energetics and Ca fluxes

Specially designed Ussing chambers were used for the quantitation of fluxes of Ca2+ and lactate ions to and from viable calvaria taken from 2- to 3-day-old rat pups and young adult mice. The latter in vitro support Ca2+ levels comparable to plasma levels in vivo. The effects of various metabolic inhibitors and parathyroid hormone were studied. The results support previous findings that Ca influx into bone is a passive concentration-dependent process not significantly affected by alterations in the energy status of the bone cells. In large measure, the efflux from bone also appears to be passive. However, significant alterations in the calcium levels supported by efflux from bone were observed as acute effects of metabolic inhibitors. Part of the metabolically derived efflux may be attributed to acid production (aerobic glycolysis). However, part must be ascribed to cellular activities not yet defined.

Aerobic glycolysis in bone: lactic acid production by rat calvaria cells in culture

Considerable data have been accumulated on aerobic glycolysis of intact bone preparations. To test whether aerobic glycolysis is a feature of the bone cells themselves or of the localized conditions within an intact bone, cells isolated from rat calvaria were cultured and the effect of several factors on lactate production was determined. These cells drastically decreased lactate production when the pH in the culture medium was lowered, changing from 100 to 20 percent for a pH shift from 7.4 to 6.75. L-lactate inhibited its own formation by 40 percent at 20 mM. Parathyroid hormone (PTH) (820 U/mg) at a concentration ranging from 0.2 to 5.0 U/ml stimulated slightly the lactate production in a log-linear response, the ratio treated over control changing from 1.1 to 1.3. The maximal stimulation is observed at pH 7.0. Isolated cells respond qualitatively the same as did intact calvaria. Quantitatively, there were significant differences: notably, a smaller response to parathyroid hormone and a higher sensitivity to lowered pH.