Expression of albumin in bone tissues and osteoblastic cells: involvement of hormonal regulation.

The expression of albumin in bone-related cells was investigated. Bone marrow cells and femoral-diaphyseal and -metaphyseal tissues were obtained from normal rat femur. Western blot analysis for protein in the lysate of bone marrow cells showed expression of albumin. The production of albumin was found in the bone marrow cells and femoral-diaphyseal and -metaphyseal tissues obtained from rat femur. When the femoral-diaphyseal and -metaphyseal tissues were cultured for 48 h in a serum-free medium in vitro, albumin was greatly released into culture medium from the bone tissues. The expression of albumin in osteoblastic MC3T3-E1 cells was investigated by Western blot analysis. Osteoblastic cells were cultured for 48 h in a serum-free medium containing either vehicle, parathyroid hormone (1-34) (human PTH; 10(-7) M), 17beta-estradiol (10(-9) M), insulin-like growth factor-I (IGF-I; 10(-8) M) or zinc sulfate (10(-4) M). The production of albumin in osteoblastic cells was significantly enhanced in the presence of PTH, IGF-I, 17beta-estradiol or zinc sulfate. These factors stimulated significantly the release of albumin from osteoblastic cells into culture medium. The effect of IGF-I (10(-8) M) in increasing the production of albumin in osteoblastic cells was completely prevented in the presence of cycloheximide (10(-7) M), an inhibitor of protein synthesis, or 5, 6-dichloro-1-beta-D-ribofuranosyl-benzimidazole (DRB; 10(-6) M), an inhibitor of transcriptional activity. This study demonstrates that albumin is expressed in bone marrow cells and osteoblastic cells, and that the production of albumin in osteoblastic cells is enhanced by various bone anabolic factors in vitro.