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OBJECTIVE: To study the effects of progesterone (P) and 18-methyl levonorgestrel (LNG) on the expression of P receptors (PRA and PRB), c-fos, c-jun, and osteocalcin in the human osteosarcoma cell line MG-63 and human normal osteoblasts in order to understand the mechanism of progestin action on the proliferation and differentiation of osteoblasts. METHODS: Cell proliferation was tested by MTT assay. The expression of PR, c-fos, c-jun, and osteocalcin was measured by semi-quantitative RT-PCR, Western immunoblot or immunocytochemistry. RESULTS: Progesterone and LNG (10−10–10−6 M) stimulated cell proliferation in a dose-dependent manner. Progesterone and LNG did not affect the expression of PRA and PRB mRNA and protein, but c-fos and c-jun mRNA and protein were upregulated in a dose-dependent manner. The expression of osteocalcin mRNA was also increased in human osteoblasts in a dose and time-dependent manner with greater effects of LNG than P, while the expression of osteocalcin mRNA in MG-63 cells was not changed by P or LNG. CONCLUSION: Progesterone and LNG promote osteocalcin gene transcription by stimulating the expression of c-fos and c-jun, and result in osteoblast proliferation and differentiation. [ABSTRACT FROM AUTHOR]