CELL CYCLE-DEPENDENT EXPRESSION OF ESTROGEN-RECEPTOR AND EFFECT OF ESTROGEN ON PROLIFERATION OF SYNCHRONIZED HUMAN OSTEOBLAST-LIKE OSTEOSARCOMA CELLS

Dual fluoroimmunohistochemical staining of estrogen receptor (ER) and bromodeoxyuridine was performed in a human osteoblastic osteosarcoma c ell line, HOS TE85 cells. ER immunoreactivity was observed preferentia lly in the nuclei of the cells that were bromodeoxyuridine positive. E R expression at various phases of the cell cycle was investigated in H OS TE85 cells, which were synchronized at the G1/S phase boundary by i ntermittent exposure to thymidine and hydroxyurea. ER immunoreactivity became detectable in the S phase, decreased in the G2/M and G1 phases , and then reappeared in the S phase of the next cell cycle. Western b lot analysis also showed that ER protein exists in these cells and inc reases in the S phase. Moreover, Northern blot analysis demonstrated t hat the expression of ER messenger RNA increases in the early S phase, gradually decreases during the progress of the cell cycle, and increa ses again in the S phase of the subsequent cell cycle. Interestingly, 17 beta-estradiol (10(-5) M) increased cell number and [H-3]thymidine incorporation into DNA in the synchronized HOS TE85 cells, whereas thi s effect was not observed in the nonsynchronized HOS TE85 cells. The p resent studies suggest that the cell cycle-dependent regulation may co ntribute to the heterogeneity of ER expression in osteoblastic cells.