The role of albumin in osteoblastic cells was investigated. Osteoblastic MC3T3-E1 cells with subconfluent monolayers were cultured for 24 to 72 h in a medium without fetal bovine serum (FBS) in the presence or absence of albumin (0.25, 0.5, or 1.0 mg/ml of medium). The number of osteoblastic cells was significantly increased by culture for 24 to 72 h in the presence of albumin (1.0 mg/ml). The effect of albumin in increasing cell number was completely abolished in the presence of PD98049 (10−7 M), staurosporine (10−7 M), or dibucaine (10−7 M), which is an inhibitor of various protein kinases. Also, the stimulating effect of albumin on cell proliferation was completely prevented in the presence of cycloheximide (10−7 M), an inhibitor of protein synthesis, or 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB), an inhibitor of transcriptional activity. DNA content was significantly increased in the cells cultured with albumin (1.0 mg/ml) addition for 24 to 72 h. Alkaline phosphatase activity in the cells, which participates in osteoblastic calcification, was significantly decreased by the culture with albumin (0.25, 0.5, or 1.0 mg/ml) for 24 to 72 h. The expression of insulin-like growth factor-I (IGF-I) and transforming growth factor-β1 mRNAs using transcription-polymerase chain reaction (RT-PCR) analysis in osteoblastic cells was not significantly altered by culture for 48 h with albumin (1.0 mg/ml). This study demonstrated that albumin had a role in the regulation of osteoblastic cell function.

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