Glutamate is a determinant of cellular proliferation through modulation of nuclear factor E2 p45-related factor-2 expression in osteoblastic MC3T3-E1 cells

Metabolic features of myeloma cells in the context of bone microenvironment: Implication for the pathophysiology and clinic of myeloma bone disease

Multiple myeloma (MM) is a hematological malignancy characterized by the accumulation of malignant plasma cells (PCs) into the bone marrow (BM). The complex interaction between the BM microenvironment and MM PCs can lead to severe impairment of bone remodeling. Indeed, the BM microenvironment exerts a critical role in the survival of malignant PCs. Growing evidence indicates that MM cells have several metabolic features including enhanced glycolysis and an increase in lactate production through the upregulation of glucose transporters and enzymes. More recently, it has been reported that MM cells arehighly glutamine addicted. Interestingly, these metabolic changes in MM cells may affect BM microenvironment cells by altering the differentiation process of osteoblasts from mesenchymal stromal cells. The identification of glutamine metabolism alterations in MM cells and bone microenvironment may provide a rationale to design new therapeutic approaches and diagnostic tools. The osteolytic lesions are the most frequent clinical features in MM patients, often characterized by pathological fractures and acute pain. The use of the newer imaging techniques such as Magnetic Resonance Imaging (MRI) and combined Positron Emission Tomography (PET) and Computerized Tomography (CT) has been introduced into clinical practice to better define the skeletal involvement. Currently, the PET/CT with ¹⁸F-fluorodeoxyglucose (FDG) is the diagnostic gold standard to detect active MM bone disease due to the high glycolytic activity of MM cells. However, new tracers are actively under investigation because a portion of MM patients remains negative at the skeletal level by ¹⁸F-FDG. In this review, we will summarize the existing knowledge on the metabolic alterations of MM cells considering their impact on the BM microenvironment cells and particularly in the subsequent formation of osteolytic bone lesions. Based on this, we will discuss the identification of possible new druggable targets and the use of novel metabolic targets for PET imaging in the detection of skeletal lesions, in the staging and treatment response of MM patients.

Leucine reduces the proliferation of MC3T3-E1 cells through DNA damage and cell senescence

Leucine (Leu) is an essential branched-chain amino acid, present in dairy products, which has been investigated for its important role in cell signaling. The effects of Leu on several kinds of cells have been studied, altough little is known on its action upon bone cells and cell proliferation. Thus, the aim of this study is to investigate the effects of Leu supplementation on the proliferation of pre-osteoblasts from MC3T3-E1 lineage. MC3T3-E1 cells were kept in Alpha medium supplemented with 10% fetal bovine serum and 1% antibiotic-antimitotic. Cells were treated during 48h by adding 50μM of Leu, which corresponds to a 12.5% increase of the amino acid in the culture medium. The evaluation of viability and proliferation of cultured cells was performed using Trypan Blue dye. In order to identify the mechanisms related to the decreased cellular proliferation, assays were performed to assess cytotoxicity, apotosis, oxidative stress, inflammation, autophagy, senescence and DNA damage. Results showed that Leu supplementation decreased cell proliferation by 40% through mechanisms not related to cell necrosis, apoptosis, oxidative stress, autophagy or inhibition of the mTORC1 pathway. On the other hand, Leu supplementation caused DNA damage. In conclusion, Leu caused a negative impact on bone cell proliferation by inducing cell senescence through DNA damage.

Succinate and its G-protein-coupled receptor stimulates osteoclastogenesis

The mechanism underlying bone impairment in patients with diabetes mellitus, a metabolic disorder characterized by chronic hyperglycaemia and dysregulation in metabolism, is unclear. Here we show the difference in the metabolomics of bone marrow stromal cells (BMSCs) derived from hyperglycaemic (type 2 diabetes mellitus, T2D) and normoglycaemic mice. One hundred and forty-two metabolites are substantially regulated in BMSCs from T2D mice, with the tricarboxylic acid (TCA) cycle being one of the primary metabolic pathways impaired by hyperglycaemia. Importantly, succinate, an intermediate metabolite in the TCA cycle, is increased by 24-fold in BMSCs from T2D mice. Succinate functions as an extracellular ligand through binding to its specific receptor on osteoclastic lineage cells and stimulates osteoclastogenesis in vitro and in vivo. Strategies targeting the receptor activation inhibit osteoclastogenesis. This study reveals a metabolite-mediated mechanism of osteoclastogenesis modulation that contributes to bone dysregulation in metabolic disorders.

Bone loss is common in patients with diabetes, but the underlying molecular and cellular mechanisms are unclear. Here the authors show high succinate levels in mice with type 2 diabetes and that succinate can signal through succinate receptor 1 on osteoclasts to induce bone resorption.

Metformin Improves Diabetic Bone Health by Re-Balancing Catabolism and Nitrogen Disposal

Objective

Metformin, a leading drug used to treat diabetic patients, is reported to benefit bone homeostasis under hyperglycemia in animal models. However, both the molecular targets and the biological pathways affected by metformin in bone are not well identified or characterized. The objective of this study is to investigate the bioengergeric pathways affected by metformin in bone marrow cells of mice.

Materials and Methods

Metabolite levels were examined in bone marrow samples extracted from metformin or PBS -treated healthy (Wild type) and hyperglycemic (diabetic) mice using liquid chromatography-mass spectrometry (LC-MS)-based metabolomics. We applied an untargeted high performance LC-MS approach which combined multimode chromatography (ion exchange, reversed phase and hydrophilic interaction (HILIC)) and Orbitrap-based ultra-high accuracy mass spectrometry to achieve a wide coverage. A multivariate clustering was applied to reveal the global trends and major metabolite players.

Results

A total of 346 unique metabolites were identified, and they are grouped into distinctive clusters that reflected general and diabetes-specific responses to metformin. As evidenced by changes in the TCA and urea cycles, increased catabolism and nitrogen waste that are commonly associated with diabetes were rebalanced upon treatment with metformin. In particular, we found glutamate and succinate whose levels were drastically elevated in diabetic animals were brought back to normal levels by metformin. These two metabolites were further validated as the major targets of metformin in bone marrow stromal cells.

Conclusion

Overall using limited sample size, our study revealed the metabolic pathways modulated by metformin in bones which have broad implication in our understanding of bone remodeling under hyperglycemia and in finding therapeutic interventions in mammals.

[Regulatory mechanisms of skeletal tissues by amino acid signaling]

In this review, we would outline the possible signaling system for three types of amino acids including glutamate (Glu), γ-aminobutyric acid (GABA) and D-serine (D-Ser) to play a role as an extracellular signal mediator in mechanisms underlying maintenance of the cellular homeostasis in skeletal tissues. Although Glu and GABA has been thought to be an excitatory/inhibitory amino acid neurotransmitter in the mammalian central nervous system, our molecular biological analyses give rise to a novel function for Glu and GABA as an autocrine and/or paracrine factor in three types of distinct cell types including osteoblasts, osteoclasts and chondrocytes in bone tissues. Moreover, D-Ser plays a pivotal role in osteoclastogenesis through a mechanism related to the incorporation of serine enantiomers in osteoclasts after the synthesis and subsequent release from adjacent osteoblasts. Accordingly, bone formation and maintenance seems to be under control by amino acid signaling in skeletal tissues as seen with neurotransmission in the brain.

The S349T mutation of SQSTM1 links Keap1/Nrf2 signalling to Paget's disease of bone

Mutations affecting the Sequestosome 1 (SQSTM1) gene commonly occur in patients with the skeletal disorder Paget's disease of bone (PDB), a condition characterised by defective osteoclast differentiation and function. Whilst most mutations cluster within the ubiquitin-associated (UBA) domain of the SQSTM1 protein, and are associated with dysregulated NFκB signalling, several non-UBA domain mutations have also been identified. Keap1 is a SQSTM1-interacting protein that regulates the levels and activity of the Nrf2 transcription factor. This in turn controls the expression of numerous cytoprotective genes that contribute to the cell's capacity to defend itself against chemical and oxidative stress, through binding to the antioxidant response element (ARE). The PDB-associated S349T mutation maps to the Keap1-interacting region (KIR) of SQSTM1, however the effects of PDB mutant SQSTM1 on Keap1 function have not been investigated. Here we show that unlike other SQSTM1 mutations, the S349T mutation results in neither impaired ubiquitin-binding function in pull-down assays, nor dysregulated NFκB signalling in luciferase reporter assays. Keap1 is expressed in differentiating osteoclast-like cells and the S349T mutation selectively impairs the SQSTM1-Keap1 interaction in co-immunoprecipitations, which molecular modelling indicates results from effects on critical hydrogen bonds required to stabilise the KIR-Keap1 complex. Further, S349T mutant SQSTM1, but not other PDB-associated mutants, showed reduced ability to activate Nrf2 signalling as assessed by ARE-luciferase reporter assays. Thus, SQSTM1-mediated dysregulation of the Keap1-Nrf2 axis, which could potentially lead to aberrant production of oxidative response genes, may contribute to disease aetiology in a subset of PDB patients.

Polyphenols, isothiocyanates, and carotenoid derivatives enhance estrogenic activity in bone cells but inhibit it in breast cancer cells

While exposure to estrogens is a major risk factor of breast and endometrial cancer, it well established that estrogens are beneficial for bone health. We have previously shown that carotenoids inhibit estrogen signaling in breast and endometrial cancer cells. The aim of this study was to compare the effects of various phytonutrients, (carotenoid derivatives, polyphenols, isothiocyanates) on estrogenic activity in breast cancer cells and osteoblast-like cells. All the tested phytonutrients inhibited estrogen response element (ERE) transactivation in breast cancer cells. In contrast, these compounds either did not affect or enhanced ERE activity and the expression of several bone-forming genes. These results were obtained using two osteoblast-like cell lines, MG-63 human osteosarcoma cells stably transfected with estrogen receptor-α (ERα) and MC3T3-E1 mouse calvaria-derived cells expressing endogenous ER. Phytonutrients-induced ERE inhibition in breast cancer cells, and its potentiation in osteoblast-like cells were associated both with a decrease and a rise in total and nuclear ERα levels, respectively. Phytonutrients activated the electrophile/antioxidant response element (EpRE/ARE) transcription system to various extents in both cancer and bone cell lines. Overexpression of Nrf2, the major EpRE/ARE activating transcription factor, mimicked the effects of phytonutrients, causing inhibition and enhancement of ERE transactivation in breast cancer cells and in osteoblast-like cells, respectively. Moreover, reduction in Nrf2 levels by RNAi led to a decrease in the phytonutrient potentiation of ERE activity transactivation in osteoblast-like cells. These findings suggest that the enhancement and inhibition of estrogen signaling by phytonutrients in bone-derived cells and breast cancer cells, respectively, is partially mediated by the activation of the Nrf2/ARE system.

Glutamate signaling in bone

Mechanical loading plays a key role in the physiology of bone, allowing bone to functionally adapt to its environment, however characterization of the signaling events linking load to bone formation is incomplete. A screen for genes associated with mechanical load-induced bone formation identified the glutamate transporter GLAST, implicating the excitatory amino acid, glutamate, in the mechanoresponse. When an osteogenic load (10 N, 10 Hz) was externally applied to the rat ulna, GLAST (EAAT1) mRNA, was significantly down-regulated in osteocytes in the loaded limb. Functional components from each stage of the glutamate signaling pathway have since been identified within bone, including proteins necessary for calcium-mediated glutamate exocytosis, receptors, transporters, and signal propagation. Activation of ionotropic glutamate receptors has been shown to regulate the phenotype of osteoblasts and osteoclasts in vitro and bone mass in vivo. Furthermore, glutamatergic nerves have been identified in the vicinity of bone cells expressing glutamate receptors in vivo. However, it is not yet known how a glutamate signaling event is initiated in bone or its physiological significance. This review will examine the role of the glutamate signaling pathway in bone, with emphasis on the functions of glutamate transporters in osteoblasts.

Negative regulation of osteoblastogenesis through downregulation of runt-related transcription factor-2 in osteoblastic MC3T3-E1 cells with stable overexpression of the cystine/glutamate antiporter xCT subunit

We have previously demonstrated that glutamate (Glu) suppresses cellular proliferation toward self-renewal through a mechanism associated with intracellular GSH depletion mediated by the bidirectional cystine/Glu antiporter in osteoblastic MC3T3-E1 cells cultured in the absence of differentiation inducers. To further evaluate the possible role of the antiporter in osteoblastogenesis, in this study, we have established stable transfectants of the xCT subunit of the antiporter in MC3T3-E1 cells. Stable overexpression led to a significant facilitation of cellular proliferation determined by different indices with increased GSH levels and decreased ROS generation in addition to promoted [(14)C]cystine incorporation, while Glu failed to significantly inhibit cellular proliferation in stable xCT transfectants. In stable transfectants cultured under differentiation conditions, drastic decreases were invariably seen in Ca(2+) accumulation, alkaline phosphatase activity and several osteoblastic marker gene expressions, in addition to downregulation of mRNA and corresponding protein for runt-related transcription factor-2 (Runx2). Runx2 promoter activity was significantly promoted by the introduction of Runx2 expression vector in a manner sensitive to the prevention by the co-introduction of xCT expression vector in MC3T3-E1 cells. In both MC3T3-E1 cells and murine calvarial osteoblasts cultured with differentiation inducers, transient transfection with xCT siRNA significantly increased Runx2 protein expression along with decreases in xCT mRNA expression and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide reduction. These results suggest that the cystine/Glu antiporter plays a pivotal role in cellular differentiation through a mechanism related to the regulation of transactivation of Runx2 essential for osteoblastogenesis toward maturation in osteoblastic cells.

A negative correlation between expression profiles of runt-related transcription factor-2 and cystine/glutamate antiporter xCT subunit in ovariectomized mouse bone

We have previously demonstrated that glutamate (Glu) suppresses cellular proliferation toward self-renewal through a mechanism associated with the depletion of intracellular GSH after promoting the retrograde operation of the bidirectional cystine/Glu antiporter in undifferentiated osteoblastic MC3T3-E1 cells. In this study, we investigated the expression profile of the xCT subunit of the antiporter as well as the master regulator of osteoblastogenesis runt-related transcription factor-2 (Runx2) in ovariectomized mouse bone. In spinal columns isolated 28 days after ovariectomy, a marked reduction was seen with the intensity of Von Kossa staining used as an index of ossification. In femurs of these ovariectomized mice, a significant decrease was seen in mRNA and protein levels of Runx2 along with increased expression of both mRNA and the corresponding protein for the xCT subunit. To evaluate the possible role of the antiporter in osteoblastogenesis, stable transfectants were established with the xCT subunit toward the culture with osteoblastic differentiation inducers in MC3T3-E1 cells. In stable xCT transfectants cultured under differentiation conditions, marked decreases were seen in nodule formation, Ca(2+) accumulation, and osteoblastic marker gene expression, in addition to downregulation of both mRNA and the corresponding protein for Runx2. Runx2 promoter activity was markedly stimulated in MC3T3-E1 cells transfected with a responsive promoter plasmid after the culture under differentiation conditions, while transient and stable transfection with xCT expression vector invariably prevented the stimulation through an activator protein-1 site. These results suggest that Runx2 expression would be negatively regulated by the cystine/glutamate antiporter expressed by osteoblastic cells at the level of gene transactivation.

A by-product of glutathione production in cancer cells may cause disruption in bone metabolic processes

Bone is a frequent site for metastasis of breast and prostate cancers, often resulting in pathologic changes in bone metabolism and severe pain. The mechanisms involved are not well understood, but tumour cells may release factors that interfere with bone homeostasis. Several observations have led us to hypothesize that the functional disruptions in bone metastasis are the result of a biological process common to many cell types. The high metabolic activity characteristic of cancer cells often upregulates oxidative stress protection mechanisms such as the antioxidant molecule glutathione. In maintaining redox balance, this normal metabolic response may result in unintended pathologic effects in certain sensitive organ sites. Malignant glioma cells kill surrounding neurons in the brain specifically by secreting the amino acid glutamate, an obligatory waste product of glutathione synthesis. We suggest that glutamate release is a plausible mechanism that may account for the pathologic changes in bone metastasis, since bone, like brain, is also highly sensitive to glutamatergic disruption. This report reviews the available evidence to draw a mechanistic connection between tumour cell oxidative stress and the pathology seen in patients with bone metastasis.

Acetylcholinesterase supports anchorage independence in colon cancer

Various roles have been attributed to Acetylcholinesterase (AChE) in cancer. Evidence exists for a pro-apoptotic function, consistent with a protective role of AChE. Because other reports suggested that upregulated AChE in some tumors may control cell adhesion, we tested the effects of AChE on anchorage independence (an essential component of metastasis) of colon tumor cells. Several AChE inhibitors dose-dependently suppressed colony formation of HTB-38 cells in soft agar. This effect of AChE was confirmed with HTB-38 cells stably overexpressing AChE. In contrast, cell proliferation was not altered by the effective doses of these chemical inhibitors or by transfected AChE. Protection from cell cycle arrest consecutive to cancer cell detachment may be conveyed by changes in cell-matrix interactions. Reflective of such changes, the AChE overexpressing cells adhered more strongly to Fibronectin than did the vector controls. The AChE-dependent adhesion was RGD-dependent and accompanied by increased c-Myb DNA-binding, suggesting that AChE upregulates an Integrin receptor via c-Myb. In support of these observations, we find AChE message and protein to be expressed in a large fraction of colon cancers and in all colon tumor cell lines analyzed, but only rarely in normal colon specimens. Our results imply a dual role for AChE in colon cancer. While the anti-apoptotic effects of AChE may be protective against early stages of tumorigenesis, this gene product may support the later stages of transformation by enhancing anchorage independent growth. The induction of Integrins could render the cells independent of microenvironmental cues and override cell cycle arrest after deadhesion.