Characteristics of a neutral amino acid transport system (system A) in osteoblastic rat osteosarcoma cells

SLC38A2 provides proline to fulfil unique synthetic demands arising during osteoblast differentiation and bone formation

Cellular differentiation is associated with the acquisition of a unique protein signature which is essential to attain the ultimate cellular function and activity of the differentiated cell. This is predicted to result in unique biosynthetic demands that arise during differentiation. Using a bioinformatic approach, we discovered osteoblast differentiation is associated with increased demand for the amino acid proline. When compared to other differentiated cells, osteoblast-associated proteins including RUNX2, OSX, OCN and COL1A1 are significantly enriched in proline. Using a genetic and metabolomic approach, we demonstrate that the neutral amino acid transporter SLC38A2 acts cell autonomously to provide proline to facilitate the efficient synthesis of proline-rich osteoblast proteins. Genetic ablation of SLC38A2 in osteoblasts limits both osteoblast differentiation and bone formation in mice. Mechanistically, proline is primarily incorporated into nascent protein with little metabolism observed. Collectively, these data highlight a requirement for proline in fulfilling the unique biosynthetic requirements that arise during osteoblast differentiation and bone formation.

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.

Parathyroid hormone alteration of free and tRNA-bound proline specific activities in cultured mouse osteoblast-like cells

The effects of the synthetic amino-terminal fragment of parathyroid hormone [bPTH-(1-34)] on proline uptake and on the specific activities of intracellular free proline and tRNA-bound proline were studied in confluent primary cultures of osteoblast-like cells isolated from neonatal mouse calvaria. Pretreatment of cells for 4 hours with 24 nM bPTH-(1-34) increased subsequent proline uptake by approximately 50-60%; also increased were the specific activities of both intracellular free proline and tRNA-bound proline when [3H]proline was included in the extracellular uptake solution. Specific activities of the free and tRNA-bound proline pools remained elevated after proline uptake times of as long as 30 minutes and 120 minutes, respectively. These results indicate that experiments in which radiolabeled proline is used to evaluate PTH-induced protein synthesis in bone cells must be interpreted cautiously, since apparent changes in protein synthesis might actually reflect, at least in part, PTH-induced changes in the specific activities of precursor pools.

Parathyroid hormone regulation of proline uptake by cultured neonatal mouse osteoblastlike cells

Regulation of proline uptake by the synthetic amino-terminal fragment of bovine parathyroid hormone [bPTH-(1-34)] has been studied in confluent primary cultures of osteoblastlike cells isolated from neonatal mouse calvaria. The initial velocity of proline transport was increased by 85% in cultures treated with 24 nM bPTH-(1-34) for 6 h. Cycloheximide, at a concentration that inhibited protein synthesis by 97%, did not prevent this effect. However, adding the inhibitor during the first 1-2 h of hormone treatment did significantly reduce its magnitude. Exposure of cells to the inhibitor alone caused a time-dependent decrease in the basal rate of proline uptake. In the absence of protein synthesis, the maximal velocity (Vmax) of transport was 60% greater in cultures treated with 24 nM bPTH-(1-34) than in controls. The concentration of proline at which half-maximal transport occurred (Km) was unchanged. In cultures treated with cycloheximide alone, proline transport decreased as a first-order exponential with a half-life of 250-280 min. Parathyroid hormone significantly reduced this decline, increasing the half-life of proline transport activity about fourfold. These effects were duplicated by 1 mM DBcAMP. It is concluded that bPTH-(1-34) increases proline transport in osteoblastlike cells by decreasing the degradation of amino acid transport system A proteins. The hormone may also affect the synthesis of these molecules. These effects appear to be mediated by cAMP.

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)