Insulin effect on amino acid transport in bone

SLC1A5 provides glutamine and asparagine necessary for bone development in mice

Osteoblast differentiation is sequentially characterized by high rates of proliferation followed by increased protein and matrix synthesis, processes that require substantial amino acid acquisition and production. How osteoblasts obtain or maintain intracellular amino acid production is poorly understood. Here, we identify SLC1A5 as a critical amino acid transporter during bone development. Using a genetic and metabolomic approach, we show SLC1A5 acts cell autonomously to regulate protein synthesis and osteoblast differentiation. SLC1A5 provides both glutamine and asparagine which are essential for osteoblast differentiation. Mechanistically, glutamine and to a lesser extent asparagine support amino acid biosynthesis. Thus, osteoblasts depend on Slc1a5 to provide glutamine and asparagine, which are subsequently used to produce non-essential amino acids and support osteoblast differentiation and bone development.

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.

Bone Cell Bioenergetics and Skeletal Energy Homeostasis

The rising incidence of metabolic diseases worldwide has prompted renewed interest in the study of intermediary metabolism and cellular bioenergetics. The application of modern biochemical methods for quantitating fuel substrate metabolism with advanced mouse genetic approaches has greatly increased understanding of the mechanisms that integrate energy metabolism in the whole organism. Examination of the intermediary metabolism of skeletal cells has been sparked by a series of unanticipated observations in genetically modified mice that suggest the existence of novel endocrine pathways through which bone cells communicate their energy status to other centers of metabolic control. The recognition of this expanded role of the skeleton has in turn led to new lines of inquiry directed at defining the fuel requirements and bioenergetic properties of bone cells. This article provides a comprehensive review of historical and contemporary studies on the metabolic properties of bone cells and the mechanisms that control energy substrate utilization and bioenergetics. Special attention is devoted to identifying gaps in our current understanding of this new area of skeletal biology that will require additional research to better define the physiological significance of skeletal cell bioenergetics in human health and disease.

Effect of parathyroid hormone on amino acid transport by cultured neonatal mouse calvarial bone cells

The effect of synthetic bovine parathyroid hormone [bPTH-(1-34)] on amino acid uptake by confluent primary cultures of osteoblast-like cells isolated from neonatal mouse calvaria was studied. The uptake of proline and leucine by membrane transport Systems A, ASC, and L was discriminated on the basis of their sodium dependency and sensitivity to the system-specific amino acid analogs 2-(methylamino)-isobutyric acid (MeAIB) for System A and 2-amino-(2,2,1)-heptane-2-carboxylic acid (BCH) for System L. Treatment with 24 nM bPTH-(1-34) in serum-free EBSS for 4 hr increased the initial uptake rate of proline by 50-80% but had no effect on the uptake of leucine. Temporally, the increase in proline uptake was preceded by a 2-hr lag period and plateaued after 5-6 hr. A 5-min exposure to the hormone was sufficient to cause a significant increase in proline uptake measured 4 hr later. The magnitude of the increase was dose-related from 0.24 to 240 nM bPTH-(1-34), with the half-maximal effect occurring at 2.4 nM. Only the sodium-dependent, MeAIB-inhibitable component of proline uptake was elevated. Eadie-Hofstee analysis indicated that bPTH-(1-34) increased Vmax without changing the Km. Actinomycin D and cycloheximide prevented the hormone-stimulated increase, suggesting that RNA and protein synthesis were required. Treatment with either inhibitor alone caused a 30-35% decrease in proline transport that was not observed in the presence of bPTH-(1-34), indicating an effect not dependent on macromolecular synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone mass in obese, goldthioglucose-treated, hyperglycaemic mice

In contrast to insulin-dependent diabetics, bone mass in obese non-insulin dependent diabetics seems to be greater than in normal subjects. Hyperglycaemic, obese goldthioglucose mice were studied as a model for this last type of diabetes. A significantly greater cortical bone mass (cortical surface: 1.32 mm2 vs 1.15 mm2 for controls, p less than 0.01) with enhanced bone apposition was found together with a significantly greater trabecular bone mass (trabecular surface 0.17 mm2 vs 0.13 mm2, p = 0.05) increasing bone solidity. The pathogenesis is unclear but hyperinsulinism and overnutrition may be contributory factors.

Amino acid metabolism of experimental granulation tissue in vitro

1. The intracellular volume in granulation tissue was about 15% of the total urea space. 2. The experimental granuloma has a greater ability to retain amino acids during the proliferation phase than later during the synthesis of collagen. 3. The synthesis of collagen and other proteins by granulation tissue is related to the concentrations of proline and glutamic acid in the medium. 4. The rate of synthesis of proline from glutamic acid in granulation-tissue slices is greatest during collagen synthesis. It is enhanced by lactate. 5. Extracellular cations influence the synthesis of collagen and ouabain is inhibitory. Synthesis of other proteins is less sensitive in this respect. 6. It is suggested that the synthesis of collagen is related to the supply of certain amino acids, especially proline, and hence to the redox balance, and also to the function of the cell wall.