Albumin has been shown to be expressed in osteoblastic cells. The role of albumin in osteoblastic cells was investigated. Osteoblastic MC3T3-E1 cells with subconfluent monolayers were cultured for 24 or 48 h in a medium without fetal bovine serum (FBS) in the presence or absence of albumin (0.5 or 1.0 mg/ml of medium) or insulin-like growth factor-I (IGF-I; 10-9 or 10-8 M). The number of osteoblastic cells was significantly increased by culture for 48 h in the presence of albumin (0.5 or 1.0 mg/ml) or IGF-I (10-9 or 10-8 M). The effect of albumin (0.5 or 1.0 mg/ml) in increasing the cell number was not significantly modulated in the presence of IGF-I (10-9 or 10-8 M). Protein content in osteoblastic cells was significantly increased by culture with albumin (1.0 mg/ml). This effect was not significantly altered in the presence of IGF-I (10-9 or 10-8 M). Alkaline phosphatase activity was significantly decreased by culture with albumin (0.5 or 1.0 mg/ ml), while it was significantly increased in the presence of IGF-I (10-9 or 10-8 M). The effect of IGF-I in increasing the enzyme activity was not significantly altered in the presence of albumin (0.5 or 1.0 mg/ ml). The effect of albumin (0.5 mg/ ml) plus IGF-I (10-9 M) in increasing the cell number was completely prevented in the presence of PD98059, suggesting that the effect of albumin or IGF-I is partly mediated through mitogen-activated protein (MAP) kinase cascade. The expression of Runx2 (type 1) mRNA using transcription-polymerase chain reaction (RT-PCR) analysis in osteoblastic cells was significantly decreased by culture for 24 or 48 h with albumin (0.5 or 1.0 mg/ml) or IGF-I (10-9 M). α1 (I) collagen mRNA expression in osteoblastic cells was significantly increased by culture for 24 h with albumin (0.5 or 1.0 mg/ml) or IGF-I (10-9 M). Albumin (0.5, 1.0, or 2.0 mg/ml) did not have a significant effect on osteoclast-like cell formation induced by culture with parathyroid hormone (PTH; 10-7 M) in mouse marrow culture. This study demonstrates that albumin has a role in the regulation of Runx2 or α1 (I) collagen mRNA expression, which may be mediated through intracellular signaling pathway, in osteoblastic cells.

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