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A series of experimental methods including 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide, alkaline phosphatase (ALP) activity measurement, alizarin red S stain and measurement, quantitative real-time reverse transcriptase polymerase chain reaction, and Western blot analysis were employed to assess the effects of LaCl on the proliferation, osteogenic differentiation, and mineralization of a murine preosteoblast cell line MC3T3-E1 at cell and molecular levels. The results indicated that LaCl had dual effects on the proliferation, osteogenic differentiation, and mineralization of MC3T3-E1 cells. First, LaCl promoted the proliferation, osteogenic differentiation, and mineralization of MC3T3-E1 cells at lower concentrations, then had no effects and further turned to inhibit the proliferation, osteogenic differentiation, and mineralization of MC3T3-E1 cells with increasing concentrations. The expression of runt-related transcription factor 2 (Runx2), bone morphogenetic protein 2 (BMP2), ALP, bone sialoprotein (BSP), collagen I (Col I), and osteocalcin (OCN) genes was upregulated in the presence of 0.0001 and 0.1 μM LaCl, but these genes were downregulated in the MC3T3-E1 cells treated with 1,000 μM LaCl. In addition, the expression of BMP2, Runx2, and OCN proteins was promoted by LaCl at the concentration of 0.0001 μM, but these proteins were downregulated after 1,000 μM LaCl treatment. The results suggest that LaCl likely up- or downregulates the expression of Runx2, which subsequently up- or downregulates osteoblasts marker genes Col I and BMP2 at early stages and ALP and OCN at later stages of differentiation, thus causes to promote or inhibit the proliferation, osteogenic differentiation and mineralization of MC3T3-E1 cells. The results will be helpful for understanding the mechanisms of bone metabolism and application of lanthanum-based compounds in the future. [ABSTRACT FROM AUTHOR]