Effects of Magnesium on Normal Human Osteoblasts
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Introduction: Magnesium (Mg) is the fourth most abundant cation in the human body. Optimum Mg levels are necessary to regulate numerous cellular functions and enzymes including ion channels, signaling pathways, and metabolic cycles. Physiologically, Mg plays an essential role in bone formation, neuromuscular coordination, and muscle contraction. A variety of studies has demonstrated the effect of magnesium ion on the growth and maturation of the osteoblasts derived from human osteosarcoma and stem cell lines. Mg both in lower and higher concentrations than the physiological levels has been shown to inhibit osteoblast growth and maturation. Osteoblastic cell growth involves an integration of multiple growth factors which regulate mitogenic activities and differentiation to induce bone repair. Platelet-derived growth factor (PDGF) plays a prominent role in this process by stimulating the proliferation of the cells. A study has shown the positive effect of the extracellular Mg on PDGF-induced effects on the osteosarcoma cell line (MG63). A study conducted by our lab demonstrated a similar effect of extracellular Mg on basal activity levels of osteosarcoma G292 cells but an effect of PDGF on cell metabolic activity was still evident with low Mg extracellular levels. Despite the recent progress, our understanding of the effect of Mg homeostasis and transport on normal human osteoblasts remains incomplete. This study investigates the effect of Mg on normal human osteoblasts by evaluating metabolic activity and differentiation of the cells in the presence and absence of added Mg in the extracellular medium. The aim of the study: (1) To evaluate the effect of the absence of extracellular Mg on the metabolic activity and differentiation of normal human osteoblasts.; (2) To evaluate the effect of extracellular Mg on the PDGF-induced activity of normal human osteoblasts. Materials and Methods: The metabolic activity of normal human osteoblasts was assessed in cells grown in Mg-free (no extracellularly added Mg; 0.0mM) medium and those grown in medium with physiological Mg concentration (0.8mM-control) by the MTT assay. PDGF was also added to the medium in some experiments. The differentiation and the mineralization potential of the osteoblasts were assessed by the alkaline phosphatase and alizarin red assay respectively. Results: The absence of extracellularly added Mg had a significant inhibitory action on the differentiation of normal human osteoblasts. These same conditions had a slight, nonsignificant inhibitory action on the basal cellular (MTT) activity of these cells. Moreover, the cells when incubated in media without extracellularly added Mg were responsive to PDGF with an increase in MTT activity but the absolute level of this activity was significantly lower compared to cells incubated with PDGF in medium with normal levels of Mg. Conclusion: Metabolic activity and differentiation of normal human osteoblasts are decreased in the absence of the extracellularly added Mg. Further studies are necessary to more fully understand the role of extracellular Mg in growth hormone responses in normal osteoblasts.